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NEW AGE INTERNATIONAL (P) LIMITED, PUBLISHERS
PUBLISHING FOR ONE WORLD
New Delhi · Bangalore · Chennai · Cochin · Guwahati · Hyderabad
Jalandhar · Kolkata · Lucknow · Mumbai · Ranchi
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Copyright © 2005, 1983, New Age International (P) Ltd., Publishers
Published by New Age International (P) Ltd., Publishers
All rights reserved.
No part of this ebook may be reproduced in any form, by photostat, microfilm,
xerography, or any other means, or incorporated into any information retrieval
system, electronic or mechanical, without the written permission of the publisher.
All inquiries should be emailed to [email protected]
ISBN (13) : 978-81-224-2489-8
PUBLISHING FOR ONE WORLD
NEW AGE INTERNATIONAL (P) LIMITED, PUBLISHERS
4835/24, Ansari Road, Daryaganj, New Delhi - 110002
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To
My Wife
PARKASH
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Preface
Out of the various television systems in use in different countries, India adopted the 625-B
monochrome (black and white) and the compatible PAL-B colour systems. Most European and
many other countries are also using these standards. However, majority of the available books
on television deal with the American 525 line monochrome and NTSC colour systems. Moreover,
in the latest editions of the popular books on this subject, emphasis has totally shifted to colour
television because of its wide acceptance in all the advanced countries. Since colour television
has just been introduced in India, both monochrome and colour transmissions will co-exist for
a long time in this country. Keeping these facts in view, this text has been prepared to provide
an integrated approach with equal emphasis on both monochrome and colour systems.
The book has been designed to meet the requirements of a modern text book on ‘Television
Engineering’ for Electrical and Electronics Engineering students at the degree level. It will
also meet the needs of a comprehensive course on TV Engineering in Polytechnics and Technical
Schools. In addition the book will be of immense value to practising engineers and technicians.
Students engaged in self study will also benefit very much from this text.
The matter has been so presented that any Engineering student with a basic knowledge
of the various electronic building blocks and fundamentals of communication systems will
have no difficulty in understanding the subject.
Comprehensive design criteria for various sections of the receiver have been given in
each chapter without going into rigorous mathematical details. Due emphasis has also been
laid on TV receiver servicing and servicing equipment. Detailed charts for locating faults and
trouble shootting together with alignment procedures for the various sections of the receiver
have also been included.
Early TV receivers manufactured in India and other countries used vacuum tube circuitry.
However, with rapid advances in technology, hybrid circuitry soon came into use and transistors
replaced most vacuum tubes. With the widespread development of integrated circuits, special
ICs are now available and are fast replacing discrete circuitry employing transistors. Since
these developments have been very fast, sets employing tubes, only transistors and ICs are in
use simultaneously. In view of this fact discussion of TV circuits using tubes, hybrid circuitry
and ICs has been included in the chapter devoted to receiver circuits. The stress, however, is
more on solid state receiver circuits and design.
Because of the importance of colour transmission and reception, two comprehensive
chapters have been exclusively devoted to the techniques of colour television and various colour
television systems. All modern colour receivers use solid-state devices in most sections of the
receiver. It is natural that receivers manufactured in India will also be of this type with specially
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designed ICs for performing almost all the functions in the receiver. Therefore, the description
of colour receivers using vacuum tubes has been totally omitted. However, the functioning and
use of special instruments necessary for the manufacture, testing and servicing all types of
colour receivers has been included in corresponding chapters.
Chapter 1 gives basic principles of TV transmission and reception. Chapters 2, 3 and 4
deal with analysis and synthesis of TV pictures, composite video signal and channel bandwidth
requirements. Chapters 5 and 6 include discussion on receiver picture tubes and television
camera tubes. Chapter 7 is on TV studio equipment and transmission principles. Chapter 8
gives a block schematic approach to monochrome TV receivers. Chapter 9 explains the
propagation phenomena and antenna systems with special reference to TV transmission and
reception. Chapter 10 is devoted to various applications of television. In Chapters 11 to 24
detailed circuit analysis and design principles of the various sections of the receiver are given.
In Chapters 25 and 26 fundamentals of colour television and various colour TV systems
have been fully described, naturally with a greater emphasis on the PAL-B & G systems.
Chapter 27 is exclusively devoted to special circuits like remote control tuning, automatic
fine tuning etc. Chapter 28 deals with all types of equipment needed for testing, alignment
and servicing monochrome and colour receivers. Chapter 29 discusses in detail procedures for
alignment of various sections of the receiver. Comprehensive details for trouble shooting and
servicing are presented in Chapter 30. Diagnostic test charts are also included.
The manuscript and its various pre-drafts have been used successfully by the author for
a selective one semester course on TV Engineering at the final degree level at BITS, Pilani.
About twice the material necessary for a one semester course is included in the book. By a
judicious choice of chapters and sequencing the instructor can prepare suitable presentations
for a two semester sequence for both degree and diploma students emphasising basic principles,
overall systems or technological details.
To assist the student and the instructor a set of review questions are included at the end
of each chapter.
R.R. Gulati
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Suggested Further Reading
This book was published in 1983 and ever since its popularity has been growing. Its relevance
as a comprehensive course in Television Engineering lies in its excellent presentation of the
fundamentals of television transmission and reception.
In it, analysis and synthesis of TV pictures, generation of composite video and audio
signals, channel bandwidth requirements and design factors for various sections of the receiver
have been evolved from first principles and supported with mathematical derivations where
necessary.
However, the author wishes to point out that during the past two decades or so, television
receiver designs have gradually changed because of rapid technological advances in the field
of entertainment electronics. Therefore, the students are advised to reinforce their learning by
further reading books which describe latest techniques and circuits of modern television
receivers.
This author has contributed in this direction by writing the following three books now
published by NEW AGE INTERNATIONAL PUBLISHERS.
1. COLOUR TELEVISION–PRINCIPLES AND PRACTICE. This book describes
colour TV principles in depth and gives detailed insight of colour TV systems and
standards, frequency synthesized tuning and channel selection, chroma processing
sub-systems and matrixing, modern receiver circuits employing latest ICs and also
colour receiver alignment and servicing.
2. MODERN TELEVISION PRACTICE–PRINCIPLES, TECHNOLOGY AND
SERVICING. The main feature of this book is the side by side coverage of B&W and
colour TV transmission and reception techniques for a better grasp of the entire
field of television engineering. The 2nd edition of this book published in 2002 also
contains chapters on Satellite Television Technology, Cable Television, VCR and
Video Disc Recording and Playback, Teletext Broadcast Service and TV Games,
Digital Television and Advanced Television Systems.
3. COMPOSITE SATELLITE AND CABLE TELEVISION. This book presents basics
and systematic exposition to various equipments, devices, and circuit formulations
involved in Satellite and Cable Television. Its 2nd edition contains extended coverage
on Signal Encoding & Compression Techniques, Digital Satellite Transmission and
Reception, Conditional Access (CAS) System, Direct-to-Home Satellite Broadcasts,
High Definition TV (HDTV) and TV Home Entertainment Theatres.
R. R. Gulati
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Contents
Preface
v
Suggested Further Reading
vii
Introduction ........................................................................................................................ 1
1.
Elements of a Television System ................................................................................... 8
1.1
1.2
1.3
1.4
1.5
1.6
1.7
2.
Analysis and Synthesis of Television Pictures ........................................................ 16
2.1
2.2
2.3
2.4
2.5
2.6
3.
Gross Structure ........................................................................................................ 16
Image Continuity ...................................................................................................... 16
Number of Scanning Lines ...................................................................................... 19
Flicker ........................................................................................................................ 21
Fine Structure .......................................................................................................... 25
Tonal Gradation ........................................................................................................ 31
Composite Video Signal ................................................................................................. 36
3.1
3.2
3.3
3.4
3.5
3.6
4.
Picture Transmission ................................................................................................. 8
Sound Transmission ................................................................................................. 11
Picture Reception ..................................................................................................... 11
Sound Reception ....................................................................................................... 12
Synchronization ........................................................................................................ 12
Receiver Controls ..................................................................................................... 13
Colour Television ...................................................................................................... 13
Video Signal Dimensions ......................................................................................... 36
Horizontal Sync Details ........................................................................................... 39
Vertical Sync Details ................................................................................................ 41
Scanning Sequence Details ...................................................................................... 47
Functions of Vertical Pulse Train ........................................................................... 49
Sync Details of the 525 Line System ...................................................................... 49
Signal Transmission and Channel Bandwidth ........................................................ 54
4.1
4.2
4.3
4.4
4.5
Amplitude Modulation ............................................................................................. 54
Channel Bandwidth ................................................................................................. 56
Vestigial Sideband Transmission ............................................................................ 57
Transmission Efficiency ........................................................................................... 58
Complete Channel Bandwidth ................................................................................ 59
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4.6
4.7
4.8
4.9
4.10
4.11
5.
The Picture Tube ............................................................................................................. 74
5.1
5.2
5.3
5.4
5.5
5.6
6.
Basic Principle .......................................................................................................... 86
Image Orthicon ......................................................................................................... 89
Vidicon ....................................................................................................................... 94
The Plumbicon .......................................................................................................... 97
Silicon Diode Array Vidicon ..................................................................................... 99
Solid State Image Scanners ................................................................................... 100
Basic Television Broadcasting ................................................................................... 106
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
7.9
7.10
7.11
7.12
7.13
7.14
7.15
8.
Monochrome Picture Tube ....................................................................................... 74
Beam Deflection ....................................................................................................... 76
Screen Phosphor ....................................................................................................... 79
Face Plate .................................................................................................................. 79
Picture Tube Characteristics ................................................................................... 82
Picture Tube Circuit Controls ................................................................................. 82
Television Camera Tubes .............................................................................................. 86
6.1
6.2
6.3
6.4
6.5
6.6
7.
Reception of Vestigial Sideband Signals ................................................................ 60
Frequency Modulation ............................................................................................. 62
FM Channel Bandwidth........................................................................................... 66
Channel Bandwidth for Colour Transmission ....................................................... 67
Allocation of Frequency Bands for Television Signal Transmission .................... 68
Television Standards ................................................................................................ 68
Television Studio .................................................................................................... 106
Television Cameras ................................................................................................ 108
Programme Control Room ..................................................................................... 110
Video Switcher ........................................................................................................ 111
Synchronizing System ............................................................................................ 115
Master Control Room (MCR) ................................................................................. 117
Generation of Amplitude Modulation ................................................................... 118
Television Transmitter .......................................................................................... 119
Positive and Negative Modulation ........................................................................ 120
Sound Signal Transmission ................................................................................... 122
Merits of Frequency Modulation ........................................................................... 123
Generation of Frequency Modulation ................................................................... 125
Stabilized Reactance Modulator ............................................................................ 127
Generation of FM from PM .................................................................................... 128
FM Sound Signal .................................................................................................... 130
Television Receiver ....................................................................................................... 134
8.1
8.2
8.3
Types of Television Receivers ................................................................................ 134
Receiver Sections .................................................................................................... 135
Vestigial Sideband Correction ............................................................................... 139
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8.4
8.5
8.6
8.7
8.8
8.9
8.10
9.
Choice of Intermediate Frequencies ..................................................................... 140
Picture Tube Circuitry and Controls .................................................................... 143
Sound Signal Separation ....................................................................................... 143
Sound Section ......................................................................................................... 144
Sync Processing and AFC Circuit ......................................................................... 146
Vertical Deflection Circuit ..................................................................................... 146
Horizontal Deflection Circuit ................................................................................ 147
Television Signal Propagation and Antennas ....................................................... 150
9.1
9.2
9.3
9.4
9.5
9.6
9.7
9.8
9.9
9.10
Radio Wave Propagation ........................................................................................ 150
Television Signal Transmission ............................................................................ 152
Interference Suffered by Carrier Signals ............................................................. 153
Preference of AM for Picture Signal Transmission ............................................. 155
Antennas ................................................................................................................. 156
Television Transmission Antennas ....................................................................... 161
Television Receiver Antennas ............................................................................... 164
Colour Television Antennas .................................................................................. 170
Transmission Lines ................................................................................................ 173
Attenuation Pads .................................................................................................... 178
10. Television Applications ................................................................................................ 182
10.1
10.2
10.3
10.4
10.5
10.6
10.7
10.8
Television Broadcasting ......................................................................................... 182
Cable Television...................................................................................................... 182
Closed Circuit Television (CCTV) ......................................................................... 186
Theatre Television .................................................................................................. 188
Picture Phone and Facsimile ................................................................................. 189
Video Tape Recording (VTR) ................................................................................. 189
Television via Satellite ........................................................................................... 196
TV Games ................................................................................................................ 199
11. Video Detector ................................................................................................................ 210
11.1
11.2
11.3
11.4
11.5
11.6
11.7
Video (Picture) Signal Detection ........................................................................... 210
Basic Video Detector .............................................................................................. 211
IF Filter ................................................................................................................... 213
DC Component of the Video Signal ...................................................................... 215
Intercarrier Sound .................................................................................................. 216
Video Detector Requirements ................................................................................ 217
Functions of the Composite Video Signal ............................................................. 217
12. Video Section Fundamentals ...................................................................................... 220
12.1
12.2
12.3
12.4
Picture Reproduction ............................................................................................. 220
Video Amplifier Requirements .............................................................................. 220
Video Amplifiers ..................................................................................................... 225
Basic Video Amplifier Operation .......................................................................... 228
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12.5
12.6
Comparison of Video Signal Polarities in Tube and Transistor Circuits ........... 230
Relative Merits of Grid and Cathode Modulation of the Picture Tube .............. 233
13. Video Amplifiers—Design Principles ....................................................................... 236
13.1
13.2
13.3.
13.4
13.5
13.6
13.7
13.8
13.9
13.10
Vacuum Tube Amplifier ......................................................................................... 236
High Frequency Compensation ............................................................................. 238
Low Frequency Compensation .............................................................................. 243
Transistor Video Amplifier .................................................................................... 245
Transistor Circuit Analysis ................................................................................... 246
Guidelines for Broad-Banding ............................................................................... 250
Frequency Compensation ...................................................................................... 251
Video Driver ............................................................................................................ 254
Contrast Control Methods ..................................................................................... 254
Screen Size and Video Amplifier Bandwidth ....................................................... 257
14. Video Amplifier Circuits .............................................................................................. 260
14.1
14.2
14.3
14.4
14.5
14.6
14.7
14.8
Direct Coupled Video Amplifier ............................................................................ 260
Problems of DC Coupling ....................................................................................... 262
Partial DC Coupling ............................................................................................... 264
Consequences of AC Coupling ............................................................................... 267
DC Reinsertion ....................................................................................................... 269
AC Coupling with DC Reinsertion ........................................................................ 270
The AC Coupling .................................................................................................... 271
Video Preamplifier in an IC Chip ......................................................................... 272
15. Automatic Gain Control and Noise Cancelling Circuits ..................................... 276
15.1
15.2
15.3
15.4
15.5
15.6
15.7
15.8
15.9
15.10
Advantages of AGC ................................................................................................ 276
Gain Control of VT and FET Amplifiers ............................................................... 277
Gain Control of Transistor Amplifiers .................................................................. 278
Types of AGC .......................................................................................................... 279
Various AGC Systems ............................................................................................ 281
Merits of Keyed AGC System ................................................................................ 285
Delayed AGC ........................................................................................................... 285
Noise Cancellation .................................................................................................. 286
Typical AGC Circuits ............................................................................................. 287
AGC Adjustments ................................................................................................... 291
16. Sync Separation Circuits ............................................................................................. 296
16.1
16.2
16.3
16.4
16.5
16.6
Sync Separator—Basic Principle .......................................................................... 296
Sync Separator Employing a Pentode .................................................................. 298
Transistor Sync Separator ..................................................................................... 299
Noise in Sync Pulses .............................................................................................. 300
Typical Tube Sync Separator Circuit .................................................................... 302
Transistor Noise Gate Sync Separator ................................................................. 303
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16.7
16.8
Improved Noise Gate Sync Separator .................................................................. 304
Sync Amplifier ........................................................................................................ 305
17. Sync Processing and AFC Circuits ........................................................................... 308
17.1
17.2
17.3
17.4
17.5
17.6
17.7
17.8
Sync Waveform Separation ................................................................................... 308
Vertical Sync Separation ....................................................................................... 309
Horizontal Sync Separation ................................................................................... 311
Automatic Frequency Control ............................................................................... 312
AFC Circuit Employing Push-Pull Discriminator ............................................... 313
Single Ended AFC Circuit ..................................................................................... 316
Phase Discriminator (AFC) with Push-Pull Sawtooth ........................................ 318
DC Control Voltage ................................................................................................ 319
18. Deflection Oscillators ................................................................................................... 324
18.1
18.2
18.3
18.4
18.5
18.6
Deflection Current Waveforms .............................................................................. 325
Generation of Driving Voltage Waveforms .......................................................... 327
Blocking Oscillator and Sweep Circuits ............................................................... 329
Multivibrator Deflection Oscillators ..................................................................... 335
Complementary-Symmetry Relaxation Oscillator .............................................. 340
Sine-Wave Deflection Oscillators .......................................................................... 341
19. Vertical Deflection Circuits ........................................................................................ 346
19.1
19.2
19.3
19.4
19.5
19.6
19.7
19.8
Requirements of the Vertical Deflection Stage .................................................... 346
Vacuum Tube Vertical Deflection Stage ............................................................... 354
Transistor Multivibrator Driven Vertical Output Stage ..................................... 356
Blocking Oscillator Driven Output Stage ............................................................. 357
Transformerless Output Circuit ............................................................................ 358
Vertical Sweep Module .......................................................................................... 359
The Miller Deflection Circuit ................................................................................. 360
Integrated Circuit for the Vertical System .......................................................... 362
20. Horizontal Deflection Circuits ................................................................................... 366
20.1
20.2
20.3
20.4
20.5
20.6
20.7
20.8
20.9
20.10
20.11
20.12
Horizontal Output Stage........................................................................................ 366
Equivalent Circuit .................................................................................................. 367
Horizontal Amplifier Configurations .................................................................... 370
Vacuum Tube Horizontal Deflection Circuit ........................................................ 370
Sequence of Operations .......................................................................................... 377
Horizontal Amplifier Controls ............................................................................... 378
‘S’ Correction ........................................................................................................... 381
Improved Line Output Circuit .............................................................................. 382
Output Circuit Stabilization .................................................................................. 384
Transistor Horizontal Output Circuits ................................................................. 385
Transistor Line Output Stage ............................................................................... 386
Horizontal Combination IC-CA.920 ...................................................................... 388
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20.13 Horizontal Deflection Circuits in Colour Receivers............................................. 391
20.14 SCR Horizontal Output Circuit ............................................................................. 394
21. Sound System .................................................................................................................. 400
21.1
21.2
21.3
21.4
21.5
21.6
21.7
21.8
Sound Signal Separation ....................................................................................... 400
Sound Take-off Circuits ......................................................................................... 401
Inter-carrier Sound IF Amplifier .......................................................................... 403
AM Limiting ............................................................................................................ 404
FM Detection .......................................................................................................... 405
FM Sound Detectors ............................................................................................... 408
Sound Section Integrated Circuits ........................................................................ 420
Audio Output Stage ................................................................................................ 423
22. RF Tuner .......................................................................................................................... 428
22.1
22.2
22.3
22.4
22.5
22.6
22.7
22.8
22.9
22.10
Tuner Operation ..................................................................................................... 428
Factors Affecting Tuner Design ............................................................................ 429
Basic Coupling Circuits ......................................................................................... 430
Tuner Circuit Arrangement .................................................................................. 434
Types of Tuners ...................................................................................................... 436
Various Sections of a VHF Tuner .......................................................................... 438
Electronic Tuning ................................................................................................... 441
Vacuum Tube Tuner ............................................................................................... 444
Transistor Tuners ................................................................................................... 445
UHF Tuners ............................................................................................................ 447
23. Video IF Amplifiers ....................................................................................................... 452
23.1
23.2
23.3
23.4
23.5
23.6
23.7
23.8
Video IF Section...................................................................................................... 452
IF Amplifiers ........................................................................................................... 454
Vacuum Tube and Solid-State IF Amplifiers ....................................................... 455
Vestigial Sideband Correction ............................................................................... 460
The IF Sound Signal .............................................................................................. 460
Adjacent Channel Interference ............................................................................. 460
Video IF Amplifier Circuits ................................................................................... 463
IF Sub-systems Employing ICs ............................................................................. 465
24. Receiver Power Supplies ............................................................................................. 472
24.1
24.2
24.3
24.4
24.5
24.6
24.7
24.8
Low Voltage Power Supplies ................................................................................. 473
Types of Rectifier Circuits ..................................................................................... 475
Heater Circuits ....................................................................................................... 478
Voltage Regulators ................................................................................................. 479
Low Voltage Power Supply Circuits ..................................................................... 482
High Voltage Power Supply ................................................................................... 483
Stabilized Thyristor Power Supply ....................................................................... 484
Switch Mode Power Supply (SMPS) ..................................................................... 485
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25. Essentials of Colour Television ................................................................................. 490
25.1
25.2
25.3
25.4
25.5
25.6
25.7
25.8
25.9
25.10
25.11
25.12
25.13
25.14
25.15
25.16
25.17
25.18
25.19
25.20
Compatibility .......................................................................................................... 490
Natural Light .......................................................................................................... 491
Colour Perception ................................................................................................... 492
Three Colour Theory .............................................................................................. 493
Luminance, Hue and Saturation ........................................................................... 495
Colour Television Camera ..................................................................................... 497
The Luminance Signal ........................................................................................... 498
Values of Luminance (Y) and Colour Difference Signals on Colours ................. 502
Polarity of the Colour Difference Signals ............................................................. 503
Colour Television Display Tubes ........................................................................... 505
Delta-Gun Colour Picture Tube ............................................................................ 505
Purity and Convergence......................................................................................... 507
Precision-in-Line (P.I.L.) Colour Picture Tube .................................................... 510
The Deflection Unit ................................................................................................ 513
Purity and Static Convergence Adjustments ....................................................... 514
Dynamic Convergence Adjustments ..................................................................... 516
Trintron Colour Picture Tube ................................................................................ 519
Pincushion Correction Techniques ....................................................................... 521
Automatic Degaussing (ADG) Circuit................................................................... 522
Grey Scale Tracking ............................................................................................... 523
26. Colour Signal Transmission and Reception ........................................................... 528
26.1
26.2
26.3
26.4
26.5
26.6
26.7
26.8
26.9
26.10
26.11
26.12
26.13
26.14
26.15
26.16
Colour Signal Transmission .................................................................................. 528
Bandwidth for Colour Signal Transmission ......................................................... 530
Modulation of Colour Difference Signals.............................................................. 530
Weighting Factors .................................................................................................. 533
Formation of the Chrominance Signal ................................................................. 535
NTSC Colour TV System ....................................................................................... 537
NTSC Colour Receiver ........................................................................................... 541
Limitations of the NTSC System .......................................................................... 544
PAL Colour Television System .............................................................................. 544
Cancellation of Phase Errors ................................................................................. 546
PAL-D Colour System ............................................................................................ 547
The PAL Coder ....................................................................................................... 549
PAL-D Colour Receiver .......................................................................................... 551
Merits and Demerits of the PAL System .............................................................. 559
SECAM System ...................................................................................................... 559
Merits and Demerits of SECAM Systems ............................................................ 563
27. Remote Control and Special Circuits ....................................................................... 566
27.1
27.2
Remote Control ....................................................................................................... 566
Electromechanical Control System ....................................................................... 569
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27.3
27.4
27.5
27.6
27.7
27.8
27.9
27.10
Electronic Control Systems ................................................................................... 571
Electronic Touch Tuning ........................................................................................ 573
Frequency Synthesizer TV Tuner ......................................................................... 574
Automatic Fine Tuning (AFT) ............................................................................... 575
Booster Amplifiers .................................................................................................. 578
Automatic Brightness Control ............................................................................... 579
Instant-on Circuitry ............................................................................................... 580
Picture-tube Boosters ............................................................................................. 582
28. Alignment and Servicing Equipment ....................................................................... 586
28.1
28.2
28.3
28.4
28.5
28.6
28.7
28.8
28.9
28.10
Multimeter .............................................................................................................. 586
Vacuum Tube Voltmeter (VTVM) ......................................................................... 588
Digital Multimeters ................................................................................................ 590
Cathode Ray Oscilloscope (CRO)........................................................................... 591
Video Pattern Generator........................................................................................ 599
Sweep Generator .................................................................................................... 601
Marker Generator .................................................................................................. 603
The Colour Bar Generator ..................................................................................... 606
Vectroscope ............................................................................................................. 611
High voltage Probe ................................................................................................. 613
29. Receiver Circuits and Alignment .............................................................................. 616
29.1
29.2
29.3
29.4
29.5
Monochrome TV Receiver Circuit ......................................................................... 616
Monochrome Receiver Alignment ......................................................................... 618
Television Test Charts ........................................................................................... 626
All IC Television Receivers .................................................................................... 628
Alignment of Colour Receivers .............................................................................. 630
30. Receiver Servicing ........................................................................................................ 638
30.1 Trouble Shooting Procedure .................................................................................. 638
30.2 Trouble Shooting Monochrome Receivers ............................................................ 641
30.3 Servicing of Various Functional Blocks ................................................................ 644
30.4 Trouble Shooting Colour Receivers ....................................................................... 663
30.5 Servicing Circuit Modules ..................................................................................... 663
30.6 Safety Precautions in Televisions Servicing ........................................................ 666
Appendices ......................................................................................................................... 671
A. Conversion Factors and Prefixes ...................................................................... 670
B. Transient Response and Wave Shaping .......................................................... 671
C. Television Broadcast Channels ........................................................................ 679
D. Satellite Television ............................................................................................ 681
Index ................................................................................................................................. 685
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Introduction
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Introduction
Development of Television
Television* means ‘to see from a distance’. The desire in man to do so has been there for ages.
In the early years of the twentieth century many scientists experimented with the idea of
using selenium photosensitive cells for converting light from pictures into electrical signals
and transmitting them through wires.
The first demonstration of actual television was given by J.L. Baird in UK and C.F.
Jenkins in USA around 1927 by using the technique of mechanical scanning employing rotating
discs. However, the real breakthrough occurred with the invention of the cathode ray tube and
the success of V.K. Zworykin of the USA in perfecting the first camera tube (the iconoscope)
based on the storage principle. By 1930 electromagnetic scanning of both camera and picture
tubes and other ancillary circuits such as for beam deflection, video amplification, etc. were
developed. Though television broadcast started in 1935, world political developments and the
second world war slowed down the progress of television. With the end of the war, television
rapidly grew into a popular medium for dispersion of news and mass entertainment.
Television Systems
At the outset, in the absence of any international standards, three monochrome (i.e. black and
white) systems grew independently. These are the 525 line American, the 625 line European
and the 819 line French systems. This naturally prevents direct exchange of programme between
countries using different television standards. Later, efforts by the all world committee on
radio and television (CCIR) for changing to a common 625 line system by all concerned proved
ineffective and thus all the three systems have apparently come to stay. The inability to change
over to a common system is mainly due to the high cost of replacing both the transmitting
equipment and the millions of receivers already in use. However the UK, where initially a 415
line monochrome system was in use, has changed to the 625 line system with some modification
in the channel bandwidth. In India, where television transmission started in 1959, the 625-B
monochrome system has been adopted.
The three different standards of black and white television have resulted in the
development of three different systems of colour television, respectively compatible with the
three monochrome systems. The original colour system was that adopted by the USA in 1953
on the recommendations of its National Television Systems Committee and hence called the
NTSC system. The other two colour systems–PAL and SECAM are later modifications of the
NTSC system, with minor improvements, to conform to the other two monochrome standards.
*From the Greek tele (= far) and the Latin visionis (from videre = to see).
2
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INTRODUCTION
Regular colour transmission started in the USA in 1954. In 1960, Japan adopted the
NTSC system, followed by Canada and several other countries. The PAL colour system which
is compatible with the 625 line monochrome European system, and is a variant of the NTSC
system, was developed at the Telefunken Laboratories in the Federal Republic of Germany
(FRG). This system incorporates certain features that tend to reduce colour display errors that
occur in the NTSC system during transmission. The PAL system was adopted by FRG and UK
in 1967. Subsequently Australia, Spain, Iran and several other countries in West and South
Asia have opted for the PAL system. Since this system is compatible with the 625-B monochrome
system, India also decided to adopt the PAL system. The third colour TV system in use is the
SECAM system. This was initially developed and adopted in France in 1967. Later versions,
known as SECAM IV and SECAM V were developed at the Russian National Institute of
Research (NIR) and are sometimes referred to as the NIR-SECAM systems. This system has
been adopted by the USSR, German Democratic Republic, Hungary, some other East European
countries and Algeria. When both the quality of reproduction and the cost of equipment are
taken into account, it is difficult to definitely establish the superiority of any one of these
systems over the other two. All three systems have found acceptance in their respective
countries. The deciding factor for adoption was compatibility with the already existing
monochrome system.
Applications of Television
Impact of television is far and wide, and has opened new avenues in diverse fields like public
entertainment, social education, mass communication, newscasts, weather reports, political
organization and campaigns, announcements and guidance at public places like airport
terminals, sales promotion and many others. Though the capital cost and operational expenses
in the production and broadcasting of TV programmes are high compared to other media, its
importance for mass communication and propagation of social objectives like education are
well recognized and TV broadcasts are widely used for such purposes.
Closed Circuit Television (CCTV) is a special application in which the camera signals
are made available over cable circuits only to specified destinations. This has important
applications where viewers need to see an area to which they may not go for reasons of safety
or convenience. Group demonstrations of surgical operations or scientific experiments,
inspection of noxious or dangerous industrial or scientific processes (e.g. nuclear fuel processing)
or of underwater operations and surveillance of areas for security purposes are some typical
examples.
A special type of CCTV is what might be called wired community TV. Small communities
that fall in the ‘shadow’ of tall geographical features like hills can jointly put up an antenna at
a suitable altitude and distribute the programme to the subscribers’ premises through cable
circuits. Another potential use of CCTV that can become popular and is already technically
feasible is a video-telephone or ‘visiphone’.
Equipment
Television broadcasting requires a collection of sophisticated equipment, instruments and
components that require well trained personnel. Television studios employ extensive lighting
facilities, cameras, microphones, and control equipment. Transmitting equipment for
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MONOCHROME AND COLOUR TELEVISION
modulation, amplification and radiation of the signals at the high frequencies used for television
broadcast are complex and expensive. A wide variety of support equipment essential in broadcast
studios, control rooms and outside includes video tape recorders, telecine machines, special
effects equipment plus all the apparatus for high quality sound broadcast.
Coverage
Most programmes are produced live in the studio but recorded on video tape at a convenient
time to be broadcast later. Of course, provision for live broadcast also has to be there for VIP
interviews, sports events and the like. For remote pick-ups the signal is relayed by cable or RF
link to the studio for broadcasting in the assigned channel. Each television broadcast station is
assigned a channel bandwidth of 7 MHz (6 MHz in the American, 8 MHz in the British and 14
MHz in the French systems). In the earlier days TV broadcast was confined to assigned VHF
bands of 41 to 68 MHz and 174 to 230 MHz. Later additional channel allocations have been
made in the UHF band between 470 and 890 MHz. Because of the use of VHF-UHF frequencies
for television broadcast, reception of TV signals is limited to roughly the line of sight distance.
This usually varies between 75 and 140 km depending on the topography and radiated power.
Area of TV broadcast coverage can be extended by means of relay stations that rebroadcast
signals received via microwave links or coaxial cables. A matrix of such relay stations can be
used to provide complete national coverage. With the rapid strides made in the technology of
space and satellite communication it has now become possible to have global coverage by linking
national TV systems through satellites. Besides their use for international TV networks, large
countries can use satellites for distributing national programmes over the whole area. One
method for such national coverage is to set up a network of sensitive ground stations for receiving
signals relayed by a satellite and retelecasting them to the surrounding area. Another method
is to employ somewhat higher transmitter power on the satellite and receive the down
transmissions directly through larger dish antenna on conventional television receivers fitted
with an extra front-end converter. A combination of both the methods was successfully tested
in India where NASA’s ATS-6 satellite was used for the SITE programme trials in 1975-76.
This resulted in the launching of INSAT 1-A in April 1982.
Recent Trends
In the last decade, transistors and integrated circuits have greatly improved the quality of
performance of TV broadcasting and reception. Modern camera tubes like vidicon and plumbicon
have made TV broadcast of even dimly lit scenes possible. Special camera tubes are now used
for different specific applications. The most sensitive camera tubes available today can produce
usable signals even from the scenes where the human eye sees total darkness. With rapid
advances in solid state technology, rugged solid state image scanners may conceivably replace
the fragile camera tubes in the not-too-distant future. Experimental solid state cameras are
already in use for some special applications. Solid state ‘picture-plates’ for use in receivers are
under active development. Before long the highly vulnerable high vacuum glass envelope of
the picture tube may be a thing of the past. Since solid state charge coupled devices are scanned
by digital addressing, the camera scanner and picture plate can work in exact synchronism
with no non-linear distortions of the reconstructed picture.
An important recent technological advance is the use of pseudo-random scan. The signal
so generated requires much less bandwidth than the one for conventional method of scanning.
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INTRODUCTION
Besides all this, wider use of composite devices, made by integrated solid state technology, for
television studio and transmitter equipment as well as for receivers will result in higher quality
of reproduction, lower costs and power consumptions with increased reliability and compactness.
Special mention may be made of the surface acoustic wave filter to replace the clumsy and
expensive IF transformer. Further, large screen TV reception systems based on projection
techniques now under development will make it possible to show TV programmes to large
audience as in a theatre.
With the rapid development of large scale integrated (LSI) electronics in the last decade,
digital communication by pulse code modulation (PCM) has made immense progress. The
advantage gained is, that virtual freedom from all noise and interference is obtained by using
a somewhat larger bandwidth and a specially coded signal. Even if the final transmission in
TV is retained in its present form, so that all previous receivers remain usable, the processing
of pictures from the camera to the transmitter input is likely to change over to PCM techniques.
Unlike the case of monochrome TV standards, the International Telecommunication Union
(ITU), a UN special agency, has already adopted a single set of standards accepted by all
member countries for the production and processing of picture signals by digital methods.
Digital TV has become all the more attractive since solid state cameras compatible with digital
signal processing and deflection circuitry have also been developed and are at present in the
field testing stage.
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Elements of a Television System
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Elements of a Television System
The fundamental aim of a television system is to extend the sense of sight beyond its natural
limits, along with the sound associated with the scene being televised. Essentially then, a TV
system is an extension of the science of radio communication with the additional complexity
that besides sound the picture details are also to be transmitted.
In most television systems, as also in the C.C.I.R. 625 line monochrome system adopted
by India, the picture signal is amplitude modulated and sound signal frequency modulated
before transmission. The carrier frequencies are suitably spaced and the modulated outputs
radiated through a common antenna. Thus each broadcasting station can have its own carrier
frequency and the receiver can then be tuned to select any desired station. Figure 1.1 shows a
simplified block representation of a TV transmitter and receiver.
1.1
PICTURE TRANSMISSION
The picture information is optical in character and may be thought of as an assemblage of a
large number of bright and dark areas representing picture details. These elementary areas
into which the picture details may be broken up are known as ‘picture elements’, which when
viewed together, represent the visual information of the scene. Thus the problem of picture
transimission is fundamentally much more complex, because, at any instant there are almost
an infinite number of pieces of information, existing simultaneously, each representing the
level of brightness of the scene to the reproduced. In other words the information is a function
of two variables, time and space. Ideally then, it would need an infinite number of channels to
transmit optical information corresponding to all the picture elements simultaneously. Presently
the practical difficulties of transmitting all the information simultaneously and decoding it at
the receiving end seem insurmountable and so a method known as scanning is used instead.
Here the conversion of optical information to electrical form and its transmission are carried
out element by element, one at a time and in a sequential manner to cover the entire scene
which is to be televised. Scanning of the elements is done at a very fast rate and this process is
repeated a large number of times per second to create an illusion of simultaneous pick-up and
transmission of picture details.
A TV camera, the heart of which is a camera tube, is used to convert the optical
information into a corresponding electrical signal, the amplitude of which varies in accordance
with the variations of brightness. Fig. 1.2 (a) shows very elementary details of one type of
camera tube (vidicon) to illustrate this principle. An optical image of the scene to be transmitted
is focused by a lens assembly on the rectangular glass face-plate of the camera tube. The inner
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9
ELEMENTS OF A TELEVISION SYSTEM
RF
amplifier
Crystal
oscillator
Power
amplifier
Transmitter
antenna
Scanning and
synchronizing
circuits
Light
Television
camera
Video
amplifier
AM
modulating
amplifier
Audio
amplifier
FM
modulating
amplifier
FM
sound
transmitter
Combining
network
Microphone
Fig. 1.1 (a) Basic monochrome television transmitter.
Loudspeaker
Sound
IF
amplifier
Receiver
antenna
FM
sound
demodulator
Audio
amplifier
Picture tube
RF
tuner
Common
IF
amplifiers
Video
detector
Video
amplifier
Light
Scanning and
synchronizing
circuits
Fig. 1.1 (b) Basic monochrome television receiver.
Fig. 1.1 Simplified block diagram of a monochrome television broadcasting system.
side of the glass face-plate has a transparent conductive coating on which is laid a very thin
layer of photoconductive material. The photolayer has a very high resistance when no light
falls on it, but decreases depending on the intensity of light falling on it. Thus depending on
the light intensity variations in the focused optical image, the conductivity of each element of
the photolayer changes accordingly. An electron beam is used to pick-up the picture information
now available on the target plate in terms of varying resistance at each point. The beam is
formed by an electron gun in the TV camera tube. On its way to the inner side of the glass faceplate it is deflected by a pair of deflecting coils mounted on the glass envelope and kept mutually
perpendicular to each other to achieve scanning of the entire target area. Scanning is done in
the same way as one reads a written page to cover all the words in one line and all the lines on
the page (see Fig. 1.2 (b)). To achieve this the deflecting coils are fed separately from two
sweep oscillators which continuously generate saw-tooth waveforms, each operating at a
different desired frequency. The magnetic deflection caused by the current in one coil gives
horizontal motion to the beam from left to right at a uniform rate and then brings it quickly to
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MONOCHROME AND COLOUR TELEVISION
the left side to commence the trace of next line. The other coil is used to deflect the beam from
top to bottom at a uniform rate and for its quick retrace back to the top of the plate to start this
process all over again. Two simultaneous motions are thus given to the beam, one from left to
right across the target plate and the other from top to bottom thereby covering the entire area
on which the electrical image of the picture is available. As the beam moves from element to
element, it encounters a different resistance across the target-plate, depending on the resistance
of the photoconductive coating. The result is a flow of current which varies in magnitude as the
elements are scanned. This current passes through a load resistance RL, connected to the
conductive coating on one side and to a dc supply source on the other. Depending on the
magnitude of the current a varying voltage appears across the resistance RL and this corresponds
to the optical information of the picture.
Glass plate
Conductive coating
Focusing
lens
Cathode
Photoconductive
surface
Light
Electron gun
Electron beam
Object
to be televised
Video signal
output
Magnetic deflection and
focusing coils
RL
Power supply
Fig. 1.2 (a) Simplified cross-sectional view of a Vidicon TV camera tube.
Top
Height
W dth
Width
Trace
Retrace
Bottom
Fig. 1.2 (b) Path of scanning beam in covering picture area.
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ELEMENTS OF A TELEVISION SYSTEM
If the scanning beam moves at such a rate that any portion of the scene content does not
have time to move perceptibly in the time required for one complete scan of the image, the
resultant electrical signal contains the true information existing in the picture during the time
of the scan. The desired information is now in the form of a signal varying with time and
scanning may thus be identified as a particular process which permits the conversion of
information existing in space and time coordinates into time variations only. The electrical
information obtained from the TV camera tube is generally referred to as video signal (video is
Latin for ‘see’). This signal is amplified and then amplitude modulated with the channel picture
carrier frequency. The modulated output is fed to the transmitter antenna for radiation along
with the sound signal.
1.2
SOUND TRANSMISSION
The microphone converts the sound associated with the picture being televised into
proportionate electrical signal, which is normally a voltage. This electrical output, regardless
of the complexity of its waveform, is a single valued function of time and so needs a single
channel for its transmission. The audio signal from the microphone after amplification is
frequency modulated, employing the assigned carrier frequency. In FM, the amplitude of the
carrier signal is held constant, whereas its frequency is varied in accordance with amplitude
variations of the modulating signal. As shown in Fig. 1.1 (a), output of the sound FM transmitter
is finally combined with the AM picture transmitter output, through a combining network,
and fed to a common antenna for radiation of energy in the form of electromagnetic waves.
1.3
PICTURE RECEPTION
The receiving antenna intercepts the radiated picture and sound carrier signals and feeds
them to the RF tuner (see Fig. 1.1 (b)). The receiver is of the heterodyne type and employs two
or three stages of intermediate frequency (IF) amplification. The output from the last IF stage
Control grid
Accelerating anode
Cathode
Focusing anode
Final anode
Heater
Screen
Electron beam
Electron gun
Phosphor
coating
Base
Deflection coils
EHT
Fig. 1.3 Elements of a picture tube.
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MONOCHROME AND COLOUR TELEVISION
is demodulated to recover the video signal. This signal that carries the picture information is
amplified and coupled to the picture tube which converts the electrical signal back into picture
elements of the same degree of black and white. The picture tube shown in Fig. 1.3 is very
similar to the cathode-ray tube used in an oscilloscope. The glass envelope contains an electrongun structure that produces a beam of electrons aimed at the fluorescent screen. When the
electron beam strikes the screen, light is emitted. The beam is deflected by a pair of deflecting
coils mounted on the neck of the picture tube in the same way and rate as the beam scans the
target in the camera tube. The amplitudes of the currents in the horizontal and vertical deflecting
coils are so adjusted that the entire screen, called raster, gets illuminated because of the fast
rate of scanning.
The video signal is fed to the grid or cathode of the picture tube. When the varying
signal voltage makes the control grid less negative, the beam current is increased, making the
spot of light on the screen brighter. More negative grid voltage reduces the brightness. if the
grid voltages is negative enough to cut-off the electron beam current at the picture tube there
will be no light. This state corresponds to black. Thus the video signal illuminates the fluorescent
screen from white to black through various shades of grey depending on its amplitude at any
instant. This corresponds to the brightness changes encountered by the electron beam of the
camera tube while scanning the picture details element by element. The rate at which the spot
of light moves is so fast that the eye is unable to follow it and so a complete picture is seen
because of the storage capability of the human eye.
1.4
SOUND RECEPTION
The path of the sound signal is common with the picture signal from antenna to the video
detector section of the receiver. Here the two signals are separated and fed to their respective
channels. The frequency modulated audio signal is demodulated after at least one stage of
amplification. The audio output from the FM detector is given due amplification before feeding
it to the loudspeaker.
1.5
SYNCHRONIZATION
It is essential that the same coordinates be scanned at any instant both at the camera tube
target plate and at the raster of the picture tube, otherwise, the picture details would split and
get distorted. To ensure perfect synchronization between the scene being televised and the
picture produced on the raster, synchronizing pulses are transmitted during the retrace, i.e.,
fly-back intervals of horizontal and vertical motions of the camera scanning beam. Thus, in
addition to carrying picture detail, the radiated signal at the transmitter also contains
synchronizing pulses. These pulses which are distinct for horizontal and vertical motion control,
are processed at the receiver and fed to the picture tube sweep circuitry thus ensuring that the
receiver picture tube beam is in step with the transmitter camera tube beam.
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ELEMENTS OF A TELEVISION SYSTEM
1.6
RECEIVER CONTROLS
The front view of a typical monochrome TV receiver, having various controls is shown in Fig.
1.4. The channel selector switch is used for selecting the desired channel. The fine tuning
control is provided for obtaining best picture details in the selected channel. The hold control
is used to get a steady picture in case it rolls up or down. The brightness control varies the
beam intensity of the picture tube and is set for optimum average brightness of the picture.
The contrast control is actually the gain control of the video amplifier. This can be varied to
obtain the desired contrast between the white and black contents of the reproduced picture.
The volume and tone controls form part of the audio amplifier in the sound section, and are
used for setting the volume and tonal quality of the sound output from the loudspeaker.
Vertical hold
Channel selector
Fine tuning
Contrast
Tone
Brightness
Volume and On - off
Fig. 1.4 Television receiver controls
1.7
COLOUR TELEVISION
Colour television is based on the theory of additive colour mixing, where all colours including
white can be created by mixing red, green, and blue lights. The colour camera provides video
signals for the red, green, and blue information. These are combined and transmitted along
with the brightness (monochrome) signal.
Each colour TV system* is compatible with the corresponding monochrome system.
Compatibility means that colour broadcasts can be received as black and white on monochrome
receivers. Conversely colour receivers are able to receive black and white TV broadcasts. This
is illustrated in Fig. 1.5 where the transmission paths from the colour and monochrome cameras
are shown to both colour and monochrome receivers.
At the receiver, the three colour signals are separated and fed to the three electron guns
of colour picture tube. The screen of the picture tube has red, green, and blue phosphors arranged
in alternate dots. Each gun produces an electron beam to illuminate the three colour phosphors
separately on the fluorescent screen. The eye then integrates the red, green and blue colour
information and their luminance to perceive the actual colour and brightness of the picture
being televised.
* The three compatible colour television systems are NTSC, PAL and SECAM.
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MONOCHROME AND COLOUR TELEVISION
Colour Receiver Controls
NTSC colour television receivers have two additional controls, known as Colour and Hue
controls. These are provided at the front panel along with other controls. The colour or saturation
control varies the intensity or amount of colour in the reproduced picture. For example, this
control determines whether the leaves of a tree in the picture are dark green or light green,
and whether the sky in the picture is dark blue or light blue. The tint or hue control selects the
correct colour to be displayed. This is primarily used to set the correct skin colour, since when
flesh tones are correct, all other colours are correctly reproduced.
It may be noted that PAL colour receivers do not need any tint control while in SECAM
colour receivers, both tint and saturation controls are not necessary. The reasons for such
differences are explained in chapters exclusively devoted to colour television.
Optical filters
R
Object
G
B
Combining matrix
vR R1
Red, green and
blue guns
Red, green
and blue
phosphors
R
Colour G
receiver
B
vG R2
vB R3
Colour camera
Transmission
path
v
Colour picture
tube
R
v = 0.3 vR + 0.59 vG + 0.11 vB
Electron gun
Phosphor
screen
Monochrome
receiver
Object
Monochrome
camera
Black and white
picture tube
Fig. 1.5. Signal transmission paths illustrating compatibility between colour
and monochrome TV systems. R, G and B represent three camera tubes which develop
video signals corresponding to the red, green and blue contents of the scene being televised.
Review Questions
1. Why is scanning necessary in TV transmission ? Why is it carried out at a fast rate ?
2. What is the basic principle of operation of a television camera tube ?
3. What is a raster and how is it produced on the picture tube screen ?
4. Why are synchronizing pulses transmitted along with the picture signal ?
5. Why is FM preferred to AM for sound signal transmission ?
6. Describe briefly the functions of various controls provided on the front panel of a TV receiver.
7. Describe the basic principle of colour television transmission and reception.
8. Describe the function of saturation and hue controls in a NTSC colour TV receiver.
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2
Analysis and Synthesis of
Television Pictures
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2
Analysis and Synthesis of
Television Pictures
The basic factors with which the television system must deal for successful transmission
and reception of pictures are:
(a) Gross Structure: Geometric form and aspect ratio of the picture.
(b) Image Continuity: Scanning and its sequence.
(c) Number of Scanning Lines: Resolution of picture details.
(d) Flicker: Interlaced scanning.
(e) Fine Structure: Vertical and horizontal resolution.
(f) Tonal Gradation: Picture brightness transfer characteristics of the system.
2.1
GROSS STRUCTURE
The frame adopted in all television systems is rectangular with width/height ratio, i.e., aspect
ratio = 4/3. There are many reasons for this choice. In human affairs most of the motion occurs
in the horizontal plane and so a larger width is desirable. The eyes can view with more ease
and comfort when the width of a picture is more than its height. The usage of rectangular
frame in motion pictures with a width/height ratio of 4/3 is another important reason for adopting
this shape and aspect ratio. This enables direct television transmission of film programmes
without wastage of any film area.
It is not necessary that the size of the picture produced on the receiver screen be same
as that being televised but it is essential that the aspect ratio of the two be same, otherwise the
scene details would look too thin or too wide. This is achieved by setting the magnitudes of the
current in the deflection coils to correct values, both at the TV camera and receiving picture
tube. Another important requirement is that the same coordinates should be scanned at any
instant both by the camera tube beam and the picture tube beam in the receiver. Synchronizing
pulses are transmitted along with the picture information to achieve exact congruence between
transmitter and receiver scanning systems.
2.2
IMAGE CONTINUITY
While televising picture elements of the frame by means of the scanning process, it is necessary
to present the picture to the eye in such a way that an illusion of continuity is created and any
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ANALYSIS AND SYNTHESIS OF TELEVISION PICTURES
motion in the scene appears on the picture tube screen as a smooth and continuous change. To
achieve this, advantage is taken of ‘persistence of vision’ or storage characteristics of the human
eye. This arises from the fact that the sensation produced when nerves of the eye’s retina are
stimulated by incident light does not cease immediately after the light is removed but persists
for about 1/16th of a second. Thus if the scanning rate per second is made greater than sixteen,
or the number of pictures shown per second is more than sixteen, the eye is able to integrate
the changing levels of brightness in the scene. So when the picture elements are scanned
rapidly enough, they appear to the eye as a complete picture unit, with none of the individual
elements visible separately.
In present day motion pictures twenty-four still pictures of the scene are taken per
second and later projected on the screen at the same rate. Each picture or frame is projected
individually as a still picture, but they are shown one after the other in rapid succession to
produce the illusion of continuous motion of the scene being shown. A shutter in the projector
rotates in front of the light source and allows the film to be projected on the screen when the
film frame is still, but blanks out any light from the screen during the time when the next film
frame is being moved into position. As a result, a rapid succession of still-film frames is seen on
the screen. With all light removed during the change from one frame to the next, the eye sees
a rapid sequence of still pictures that provides the illusion of continuous motion.
Scanning. A similar process is carried out in the television system. The scene is scanned
rapidly both in the horizontal and vertical directions simultaneously to provide sufficient number
of complete pictures or frames per second to give the illusion of continuous motion. Instead of
the 24 as in commercial motion picture practice, the frame repetition rate is 25 per second in
most television systems.
Horizontal scanning. Fig. 2.1 (a) shows the trace and retrace of several horizontal
lines. The linear rise of current in the horizontal deflection coils (Fig. 2.1 (b)) deflects the beam
across the screen with a continuous, uniform motion for the trace from left to right. At the
peak of the rise, the sawtooth wave reverses direction and decreases rapidly to its initial value.
This fast reversal produces the retrace or flyback. The start of the horizontal trace is at the left
W
Start
of a line
End
of a line
Trace
Retrace
H
Raster
Fig. 2.1 (a) Path of scanning beam in covering picture area (Raster).
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MONOCHROME AND COLOUR TELEVISION
edge of raster. The finish is at the right edge, where the flyback produces retrace back to the
left edge.
i(H)
i(H) max
Trace
Raster of 625 lines
Retrace
Right
t
1st line
2nd line
Trace
Retrace
3rd line
Left
Trace period
Retrace
t c period
One cycle of
deflection current
Fig. 2.1 (b) Waveform of current in the horizontal deflection coils producing
linear (constant velocity) scanning in the horizontal direction.
Note, that ‘up’ on the sawtooth wave corresponds to horizontal deflection to the right.
The heavy lines in Fig. 2.1 (a) indicate the useful scanning time and the dashed lines correspond
to the retrace time.
Vertical scanning. The sawtooth current in the vertical deflection coils (see Fig. 2.2)
moves the electron beam from top to bottom of the raster at a uniform speed while the electron
beam is being deflected horizontally. Thus the beem produces complete horizontal lines one
below the other while moving from top to bottom.
As shown in Fig. 2.2 (c), the trace part of the sawtooth wave for vertical scanning deflects
the beam to the bottom of the raster. Then the rapid vertical retrace returns the beam to the
top. Note that the maximum amplitude of the vertical sweep current brings the beam to the
bottom of the raster. As shown in Fig. 2.2 (b) during vertical retrace the horizontal scanning
continues and several lines get scanned during this period. Because of motion in the scene
being televised, the information or brightness at the top of the target plate or picture tube
screen normally changes by the time the beam returns to the top to recommence the whole
process. This information is picked up during the next scanning cycle and the whole process is
repeated 25 times to cause an illusion of continuity. The actual scanning sequence is however
a little more complex than that just described and is explained in a later section of this chapter.
It must however be noted, that both during horizontal retrace and vertical retrace intervals
the scanning beams at the camera tube and picture tube are blanked and no picture information
is either picked up or reproduced. Instead, on a time division basis, these short retrace intervals
are utilized for transmitting distinct narrow pulses to keep the sweep oscillators of the picture
tube deflection circuits of the receiver in synchronism with those of the camera at the
transmitter. This ensures exact correspondence in scanning at the two ends and results in
distortionless reproduction of the picture details.
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ANALYSIS AND SYNTHESIS OF TELEVISION PICTURES
Top
Vertical
retrace
Linear
vertical
trace
Bottom
(a) Trace
(b) Retrace
i(V)
i(V) max
Top
One cycle of vertical
Top
deflection current
Trace period
W
Bottom
vertical
trace
Retrace
Raster
1st fframe
a e
H
t
2nd fframe
a e
Retrace period
t
Bottom
(c) Current waveform in vertical deflection coils
Fig. 2.2 Vertical deflection and deflection current waveform.
2.3
NUMBER OF SCANNING LINES
Most scenes have brightness gradations in the vertical direction. The ability of the scanning
beam to allow reproduction of electrical signals according to these variations and the capability
of the human eye to resolve these distinctly, while viewing the reproduced picture, depends on
the total number of lines employed for scanning.
It is possible to arrive at some estimates of the number of lines necessary by considering
the bar pattern shown in Fig. 2.3 (a), where alternate lines are black and white. If the thickness
of the scanning beam is equal to the width of each white and black bar, and the number of
scanning lines is chosen equal to the number of bars, the electrical information corresponding
to the brightness of each bar will be correctly reproduced during the scanning process. Obviously
the greater the number of lines into which the picture is divided in the vertical plane, the
better will be the resolution.However, the total number of lines that need be employed is
limited by the resolving capability of the human eye at the minimum viewing distance.
Beam path
Beam spot
Dark
White
D
W
D
W
D
W
Fig. 2.3 (a) Scanning spot perfectly aligned with black and white lines.
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MONOCHROME AND COLOUR TELEVISION
Angle subtended at the eye
by picture elements at critical
viewing distance (a)
n th bar
(n + 1) th bar
H
Eye of the
observer
Two adjacent
black picture
elements just resolved
D = 4H
Fig. 2.3 (b) Critical viewing distance as determined by the ability of the eye to
resolve two separate picture elements.
Adjacent
black and white lines
of resolution
Beam path
Beam spot
Fig. 2.3 (c) Scanning beam focused on the junction of black and white lines.
The maximum number of alternate light and dark elements (lines) which can be resolved
by the eye is given by
1
Nv = αρ
where Nv = total number of lines (elements) to be resolved in the vertical direction, α = minimum
resolving angle of the eye expressed in radians, and ρ = D/H = viewing-distance/picture height.
For the eye this resolution is determined by the structure of the retina, and the brightness
level of the picture. it has been determined experimently that with reasonable brightness
variations and a minimum viewing distance of four times the picture height (D/H = 4), the
angle that any two adjacent elements must subtend at the eye for distinct resolution is
approximately one minute (1/60 degree). This is illustrated in Fig. 2.3 (b). Substituting these
values of α and ρ we get
1
≈ 860
(π / 180 × 1 / 60) × 4
Thus if the total number of scanning lines is chosen close to 860 and the scanning beam as
illustrated in Fig. 2.3 (a) just passes over each bar (line) separately while scanning all the lines
from top to bottom of the picture frame, a distinct pick up of the picture information results
and this is the best that can be expected from the system. This perhaps explains the use of 819
lines in the original French TV system.
Nv =
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ANALYSIS AND SYNTHESIS OF TELEVISION PICTURES
21
In practice however, the picture elements are not arranged as equally spaced segments
but have random distribution of black, grey and white depending on the nature of the picture
details or the scene under consideration. Statistical analysis and subjective tests carried out to
determine the average number of effective lines suggest that about 70 per cent of the total
lines or segments get separately scanned in the vertical direction and the remaining 30 per
cent get merged with other elements due to the beam spot falling equally on two consecutive
lines. This is illustrated in Fig. 2.3 (c). Thus the effective number of lines distinctly resolved,
i.e., Nr = Nv × k, where k is the resolution factor whose value lies between 0.65 to 0.75. Assuming
the value of k = 0.7 we get, Nr = Nv × k = 860 × 0.7 = 602.
However, there are other factors which also influence the choice of total number of lines
in a TV system. Tests conducted with many observers have shown that though the eye can
detect the effective sharpness provided by about 800 scanning lines, but the improvement is
not very significant with line numbers greater than 500 while viewing pictures having motion.
Also the channel bandwidth increases with increase in number of lines and this not only adds
to the cost of the system but also reduces the number of television channels that can be provided
in a given VHF or UHF transmission band. Thus as a compromise between quality and cost,
the total number of lines inclusive of those lost during vertical retrace has been chosen to be
625 in the 625-B monochrome TV system. In the 525 line American system, the total number
of lines has been fixed at 525 because of a somewhat higher scanning rate employed in this
system.
2.4
FLICKER
Although the rate of 24 pictures per second in motion pictures and that of scanning 25 frames
per second in television pictures is enough to cause an illusion of continuity, they are not rapid
enough to allow the birghtness of one picture or frame to blend smoothly into the next through
the time when the screen is blanked between successive frames. This results in a definite
flicker of light that is very annoying to the observer when the screen is made alternately
bright and dark.
This problem is solved in motion pictures by showing each picture twice, so that 48
views of the scene are shown per second although there are still the same 24 picture frames
per second. As a result of the increased blanking rate, flicker is eliminated.
Interlaced scanning. In television pictures an effective rate of 50 vertical scans per second
is utilized to reduce flicker. This is accomplished by increasing the downward rate of travel of
the scanning electron beam, so that every alternate line gets scanned instead of every successive
line. Then, when the beam reaches the bottom of the picture frame, it quickly returns to the
top to scan those lines that were missed in the previous scanning. Thus the total number of
lines are divided into two groups called ‘fields’. Each field is scanned alternately. This method
of scanning is known as interlaced scanning and is illustrated in Fig. 2.4. It reduces flicker to
an acceptable level since the area of the screen is covered at twice the rate. This is like reading
alternate lines of a page from top to bottom once and then going back to read the remaining
lines down to the bottom.
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MONOCHROME AND COLOUR TELEVISION
Beginning of 2nd field
Beginning of
1st field
1
2
3
4
5
313
314
315
316
Retrace liness not shown
310
311
312
313
622
623
624
625
End of 1st field
End of 2nd field
Fig. 2.4 Principle of interlaced scanning. Note that the vertical retrace
time has been assumed to be zero.
In the 625 lime monochrome system, for successful interlaced scanning, the 625 lines of
each frame or picture are divided into sets of 312.5 lines and each set is scanned alternately to
cover the entire picture area. To achieve this the horizontal sweep oscillator is made to work at
a frequency of 15625 Hz (312.5 × 50 = 15625) to scan the same number of lines per frame
(15625/25 = 625 lines), but the vertical sweep circuit is run at a frequency of 50 instead of
25 Hz. Note that since the beam is now deflected from top to bottom in half the time and the
horizontal oscillator is still operating at 15625 Hz, only half the total lines, i.e., 312.5 (625/2 =
312.5) get scanned during each vertical sweep. Since the first field ends in a half line and the
second field commences at middle of the line on the top of the target plate or screen (see
Fig. 2.4), the beam is able to scan the remaining 312.5 alternate lines during its downward
journey. In all then, the beam scans 625 lines (312.5 × 2 = 625) per frame at the same rate of
15625 lines (312.5 × 50 = 15625) per second. Therefore, with interlaced scanning the flicker
effect is eliminated without increasing the speed of scanning, which in turn does not need any
increase in the channel bandwidth.
It may be noted that the frame repetition rate of 25 (rather than 24 as used in motion
pictures) was chosen to make the field frequency equal to the power line frequency of 50 Hz.
This helps in reducing the undesired effects of hum due to pickup from the mains, because
then such effects in the picture stay still, instead of drifting up or down on the screen. In the
American TV system, a field frequency of 60 was adopted because the supply frequency is
60 Hz in USA. This brings the total number of lines scanned per second ((525/2) × 60 = 15750)
lines to practically the same as in the 625 line system.
Scanning periods. The waveshapes of both horizontal and vertical sweep currents are
shown in Fig. 2.5. As shown there the retrace times involved (both horizontal and vertical) are
due to physical limitations of practical scanning systems and are not utilized for transmitting
or receiving any video signal. The nominal duration of the horizontal line as shown in Fig. 2.5 (a)
is 64 µs (106/15625 = 64 µs), out of which the active line period is 52 µs and the remaining 12 µs
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ANALYSIS AND SYNTHESIS OF TELEVISION PICTURES
is the line blanking period. The beam returns during this short interval to the extreme left side
of the frame to start tracing the next line.
Similarly with the field frequency set at 50 Hz, the nominal duration of the vertical
trace (see Fig. 2.5(b)) is 20 ms (1/50 = 20 ms). Out of this period of 20 ms, 18.720 ms are spent
in bringing the beam from top to bottom and the remaining 1.280 ms is taken by the beam to
return back to the top to commence the next cycle. Since the horizontal and vertical sweep
oscillators operate continuously to achieve the fast sequence of interlaced scanning, 20 horizontal
lines
FG 1280 µs = 20 linesIJ get traced during each vertical retrace interval. Thus 40 scanning
H 64 µs
K
lines are lost per frame, as blanked lines during the retrace interval of two fields. This leaves
the active number of lines, Na, for scanning the picture details equal to 625 – 40 = 585, instead
of the 625 lines actually scanned per frame.
1(H)
f = 15625 Hz
Trace
Retrace
t
52 s
64 s
12 s
Fig. 2.5 (a) Horizontal deflection current.
i(v)
f = 50 Hz
Trace
Retrace
18.720 ms
20 ms
t
1.280 ms
Fig. 2.5 (b) Vertical deflection current.
Scanning sequence. The complete geometry of the standard interlaced scanning pattern
is illustrated in Fig. 2.6. Note that the lines are numbered in the sequence in which these are
actually scanned. During the first vertical trace actually 292.5 lines are scanned. The beam
starts at A, and sweeps across the frame with uniform velocity to cover all the picture elements
in one horizontal line. At the end of this trace the beam then retraces rapidly to the left side of
the frame as shown by the dashed line in the illustration to begin the next horizontal line.
Note that the horizontal lines slope downwards in the direction of scanning because the vertical
deflecting current simultaneously produces a vertical scanning motion, which is very slow
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MONOCHROME AND COLOUR TELEVISION
compared with horizontal scanning. The slope of the horizontal trace from left to right is greater
than during retrace from right to left. The reason is that the faster retrace does not allow the
beam so much time to be deflected vertically. After line one, the beam is at the left side ready
to scan line 3, omitting the second line. However, as mentioned earlier it is convenient to
number the lines as they are scanned and so the next scanned line skipping one line, is numbered
two and not three. This process continues till the last line gets scanned half when the vertical
motion reaches the bottom of the raster or frame. As explained earlier skipping of lines is
accomplished by doubling the vertical scanning frequency from the frame or picture repetition
rate of 25 to the field frequency of 50 Hz. With the field frequency of 50 Hz the height of the
raster is so set that 292.5 lines get scanned as the beam travels from top to bottom and reaches
C
A
B
B
1st vertical trace
(292.5 lines)
Inactive lines during
1st vertical retrace
(20 lines)
1 to 29
292.5
292.5 to
o 312.5
C
D
A
2nd vertical trace
(292.5 lines)
312.5 tto 605
1st field = 3
312.5 lines
D
Inactive lines during
2nd vertical retrace
(20 lines)
605 to
o 625
2nd field = 3
312.5 lines
One frame or picture = 625 lines
Fig. 2.6 Odd line interlaced scanning procedure.
point B. Now the retrace starts and takes a period equal to 20 horizontal line periods to reach
the top marked C. These 20 lines are known as inactive lines, as the scanning beam is cut-off
during this period. Thus the second field starts at the middle of the raster and the first line
scanned is the 2nd half of line number 313. The scanning of second field, starting at the middle
of the raster automatically enables the beam to scan the alternative lines left unscanned during
the first field. The vertical scanning motion otherwise is exactly the same as in the previous
field giving all the horizontal lines the same slope downwards in the direction of scanning. As
a result 292.5 lines again get scanned and the beam reaches the bottom of the frame when it
has completed full scanning of line number 605. The inactive vertical retrace again begins and
brings the beam back to the top at point A in a period during which 20 blanked horizontal lines
(605 to 625) get scanned. Back at point A, the scanning beam has just completed two fields or
one frame and is ready to start the third field covering the same area (no. of lines) as scanned
during the first field. This process (of scanning fields) is continued at a fast rate of 50 times a
second, which not only creates an illusion of continuity but also solves the problem of flicker
satisfactorily.
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ANALYSIS AND SYNTHESIS OF TELEVISION PICTURES
2.5
FINE STRUCTURE
The ability of the image reproducing system to represent the fine structure of an object is
known as its resolving power or resolution. It is necessary to consider this aspect separately in
the vertical and horizontal planes of the picture.
Vertical resolution. The extent to which the scanning system is capable of resolving
picture details in the vertical direction is referred to as its vertical resolution. It has already
been explained that the vertical resolution is a function of the scanning lines into which the
picture is divided in the vertical plane. Based on that discussion the vertical resolution in the
625 lines system can then be expressed as
Vr = Na × k
where Vr is the vertical resolution expressed in number of lines, Na is the active number of
lines and k is the resolution factor (also known as Kell factor).
Assuming a reasonable value of k = 0.69,
Vr = 585 × 0.69 = 400 lines
It is of interest to note that the corresponding resolution of 35 mm motion pictures is
about 515 lines and thus produces greater details as compared to television pictures.
Alternate black and white
lines of resolution
Scanning
spot
e(t)
Peak white level
Black level
Vertical bar pattern for determination
of amplifier requirements
t
Output video waveform
Fig. 2.7 (a) Determination of horizontal resolution.
W
H
Fig. 2.7(b) Chess-board pattern for studying
vertical and horizontal resolution.
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MONOCHROME AND COLOUR TELEVISION
Horizontal resolution. The capability of the system to resolve maximum number of picture
elements along the scanning lines determines horizontal resolution. This can be evaluated by
considering a vertical bar pattern as shown in Fig. 2.7(a). It would be realistic to aim at equal
vertical and horizontal resolution and as such the number of alternate black and white bars
that should be considered is equal to
Na × aspect ratio = 585 × 4/3 = 780
Before proceeding further it must be recognised that as all lines in the vertical plane are
not fully effective, in a similar way all parts of an individual line are not fully effective all the
time. As explained earlier, it ultimately depends on the random distribution of black and white
areas in the picture. Thus for equal vertical and horizontal resolution, the same resolution
factor may be used while determining the effective number of distinct picture elements in a
horizontal line. Therefore, the effective number of alternate black and white segments in one
horizontal line for equal vertical and horizontal resolution are :
N = Na × aspect ratio × k = 585 × 4/3 × 0.69 = 533
To resolve these 533 squares or picture elements the scanning spot must develop a video
signal of square wave nature switching continuously along the line between voltage levels
corresponding to black and peak white. This is shown along the bar pattern drawn in Fig. 2.7(a).
Since along one line there are 533/2 ≈ 267 complete cyclic changes, 267 complete square wave
cycles get generated during the time the beam takes to travel along the width of the pattern.
Thus the time duration th of one square wave cycle is equal to
th =
active period of each horizontal line
number of cycles
52 × 10 −6
seconds
267
thg frequency of the periodic wave
=
∴
fh =
1 267 × 10 6
=
= 5 MHz
th
52
Since the consideration of both vertical and horizontal resolutions is based on identical
black and white bars in the horizontal and vertical planes of the picture frame, it amounts to
considering a chessboard pattern as the most stringent case and is illustrated in Fig. 2.7(b).
Here each alternate black and white square element takes the place of bars for determining
the capability of the scanning system to reproduce the fine structure of the object being televised.
The actual size of each square element in the chess pattern is very small and is equal to
thickness of the scanning beam. It would be instructive to know as an illustration that the size
of such a square element on the screen of a 51 cm picture tube is about 0.5 mm2 only.
Since the spacing of these small elements in the above consideration corresponds to the
limiting resolution of the eye, it will distinguish only the alternate light and dark areas but not
the shape of the variations along the scanning line. Thus the eye will fail to distinguish the
difference between a square wave of brightness variation and a sine wave of brightness variation
in the reproduced picture. Therefore, if the amplifier for the square-wave signal is capable of
reproducing a sine-wave of frequency equal to the repetition frequency of the rectangular
wave, it is satisfactory for the purpose of TV signals. It may be mentioned that even otherwise
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ANALYSIS AND SYNTHESIS OF TELEVISION PICTURES
27
to handle a 5 MHz square wave would necessitate reproduction up to 11th harmonic of a
periodic sinusoidal wave of 5 MHz by the associated electronic circuitry. This would mean a
bandwidth of atleast up to 5 × 11 = 55 MHz which is excessive and almost impossible to provide
in practice. Another justification for restricting the bandwidth up to 5 MHz is that in practice
it is rare when alternate picture elements are black and white throughout the picture width
and height, and a bandwidth up to 5 MHz has been found to be quite adequate to produce most
details of the scene being televised.
Therefore, the highest approximate modulating frequency ‘fh’ that the 625 line television
system must be capable of handling for successful transmission and reception of picture details
is
No. of active lines × aspect ratio × resolution factor
2 × time duration of one active line
585 × 4 / 3 × 0.69
=
2 × 52 × 10 −6
≈ 5 MHz
fh =
In the second (525 line) widely used television system, where the active number of lines
is 485 and the duration of one active line is 57 µs, the highest modulating frequency fh ≈ 4
MHz.
This explains the allocation of 6 MHz as the channel bandwidth in USA and other
countries employing the 525 line system in comparison to a channel bandwidth allocation of 7
MHz in countries that have adopted the 625 line system. Similarly in the French 819 TV
system where the highest modulating frequency comes to 10.4 MHz a channel bandwidth of
14 MHz is allowed.
Colour resolution and bandwidth. As explained above a bandwidth of 5 MHz (4 MHz in
the American system) is needed for transmission of maximum horizontal detail in monochrome.
However, this bandwidth is not necessary for the colour video signals. The reason is that the
human eye’s colour response changes with the size of the object. For very small objects the eye
can perceive only the brightness rather than the colours in the scene. Perception of colours by
the eye is limited to objects which result in a video frequency output up to about 1.5 MHz. Thus
the colour information needs much less bandwidth than monochrome details and can be easily
accommodated in the channel bandwidth allotted for monochrome transmission.
Low-frequency requirements. The analysis of the signals produced by the bar pattern
gives no information regarding the low-frequency requirement of a video amplifier used to
handle such signals. This requirement may be determined from consideration of a pattern
shown in Fig. 2.8(a). The signal output during vertical excursions of the beam would be a
square wave (see Fig. 2.8(b)) at vertical field frequency. It is apparent then, that any amplifier
capable of reproducing this waveform would be required to have good square-wave response at
50 Hz. Any degradation in response as shown in Fig. 2.8(c) would result in brightness distortion.
In order to have satisfactory square-wave response at field frequency, an amplifier must have
good sine-wave response with negligible phase distortion down to a much lower frequency
than the field frequency. In addition, to correct phase and amplitude response at the field
frequency, it is necessary to preserve the dc component of the brightness signal. Thus a good
frequency response from dc to about 5 MHz becomes necessary for true reproduction of the
brightness variations and find details of any scene.
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MONOCHROME AND COLOUR TELEVISION
e(t)
V
t
t(v)
Fig. 2.8(a) Single bar pattern.
Fig. 2.8(b) Ideal response to scanning
of single bar pattern.
e(t)
t
Fig. 2.8(c) Distorted response due to poor
low frequency response of the system.
Influence of number of lines on bandwidth. As the number of lines employed in a television
picture is increased, the bandwidth necessary for a given quality of definition also increases.
This is due to the fact that increasing the number of lines per picture decreases the time
duration of each line. This means that the spot travels across the screen at a higher velocity
and results in increase of the highest modulating frequency. For example doubling the number
of lines per frame would very much improve the vertical resolution, infact it would get doubled
but would need increasing the bandwidth in the same ratio. If now, it is required to increase
the horizontal resolution so that it again equals the vertical resolution it would be necessary to
scan double the number of alternate black and white signal elements in a line, and this would
necessitate multiplying the original highest video frequency by a factor of four. The conclusion
is that, if the number of lines employed in a television system is increased, it is necessary to
increase the video frequency bandwidth in direct proportion to the increase in number of lines
to maintain the same degree of vertical definition (as before), and in order to increase horizontal
definition in the same proportion as the increase in vertical resolution the video frequency
bandwidth must increase as the square of the increase in number of lines.
Effect of interlaced scanning on bandwidth. As already explained, interlaced scanning
reduces flicker. However, scanning 50 complete frames of 625 lines in a progressive manner
would also eliminate flicker in the picture but this would need double the scanning speed
which in turn would double the video frequencies corresponding to the picture elements in a
line. This would necessitate double the channel bandwidth of that required with interlaced
scanning. It should be noted that by employing interlaced scanning, the basic concept of
interchangeability of time and bandwidth is not violated, because more time in allowed for
transmission and this results in decrease of bandwidth needed for each TV channel. Thus
interlaced scanning reduces flicker and conserves bandwidth.
Effect of field frequency on bandwidth. With increase in field frequency the time available
for each field decreases and this results in a proportionate decrease of the active line period.
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ANALYSIS AND SYNTHESIS OF TELEVISION PICTURES
Hence, bandwidth increases in direct proportion to the increase in the field frequency.
Bandwidth requirement for transmission of synchronising pulses. The equalizing pulses
to be discussed later have a pulse width of 2.3 µs with an allowed rise time of 0.2 µs. The
highest sinusoidal frequency which must lie in the pass band of the system for effective
transmission of these pulses is given by the expression :
10 6
1
=
= 2.5 MHz
2 × 0.2
2 × allowed rise time
It is then clear that all sync pulses are safely preserved in the video circuitry where, as
has been shown, a frequency bandwidth considerably in excess of this figure has to be maintained
in order to preserve the required picture definition.
Highest necessary frequency =
Interlace error. As explained earlier interlaced scanning provides a means of decreasing
the effect of flicker in the TV picture without increasing the system bandwidth. The selection
of 2 : 1 as the interlace ratio is the simplest with least circuit complications. Here, by selecting
an odd number of lines, the symmetry in frame blanking pulses is achieved and this enables
perfect interlaced scanning. Any error in scanning timings and sequence would leave a large
number of picture elements unresolved and thus the quality of the reproduced picture gets
impaired. Fig. 2.9 shows various cases of interlace error. For convenience of explanation the
retrace time has been assumed to be zero. Interlace error occurs due to the time difference in
starting the second field. For perfect interlace the second field should start from point ‘b’ (see
Fig. 2.9 (a)), i.e., 32 µs away from ‘a’, the starting point of the first field. If it starts early or late
interlace error will be there. For a 16 µs delay in the start of the second field (Fig. 2.9 (b)),
starting points of the two fields will be 48 µs apart instead of the desired 32 µs. Then the
percentage interlace error
48 − 32
× 100 = 50%
32
if the second field starts 16 µs early even then the error would be 50%. For a delay of 32 µs the
two fields will overlap (Fig. 2.9 (c)) and the interlace error would be 100%, i.e., half the picture
area will go unscanned.
=
Assumed starting point of one scanning field
Start of alternate field
b
a
32 s
b
a
3 s
32
(a) Perfect interlace
16 s
The two fields overlap
a, b
32 s
(b) 50% interlace error
(c) 100% interlace error
Fig. 2.9 Examples of interlace error.
The above examples demonstrate that incorrect start of any field produces vertical
displacement between the lines of the two fields. This brings these lines closer leaving gaps
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MONOCHROME AND COLOUR TELEVISION
between the pairs thus formed. The result is a deterioration of the picture’s vertical resolution
because certain areas do not get scanned at all.
For perfect interlaced scanning it is essential that the starting points at the top of the
frame is separated exactly one half line between first and second fields. To achieve this it is
necessary to feed two regularly spaced synchronising pulses to the field time base during each
frame period. One of these pulses must arrive in the middle of a line and the next at the end of
a line. This is shown in Fig. 2.10. Thus the vertical time base must be triggered 50 times per
second in the manner explained above. For half line separation between the two fields only the
topmost and the extreme bottom lines are then half lines whereas the remaining lines are all
full lines. If there are x number of full lines per field, where x may be even or odd, the total
number of full lines per frame is then 2x, an even number. To this, when the two half lines get
added the total number of lines per frame becomes odd. Thus for interlaced scanning the total
number of lines in any TV system must be odd. With an even number of lines the two fields are
bound to fall on each other and interlaced scanning would not take place.
W
H
A
B
Trigger pulse instances
Fig. 2.10 Vertical trigger pulse instances.
A—after 1st field, B—after 2nd field.
Further for correct interlacing it becomes necessary that at the transmitter automatic
frequency control must be utilized to maintain a horizontal scanning frequency that is exactly
312.5 times as great as the field frequency, i.e., 50 Hz. This is accomplished by generating a
stable frequency at 15625 Hz by crystal controlled oscillator circuits. A frequency doubling
circuit produces a frequency of 31250 Hz, which is utilized to control the correct generation of
equalizing and vertical sync pulses. Four frequency division circuits each with a ratio of 5 : 1
are employed to derive 50 Hz, the vertical scanning frequency (31250 = 5 × 5 × 5 × 5 × 50). Thus
all the required frequencies are derived from a common stable source and they automatically
remain interlocked in the correct ratios. To achieve this, i.e., frequency division, the total
number of lines per frame must be a product of small whole numbers. The frame frequency of
625 satisfies all the above requirements. Similarly 525 lines in the American system and 819
lines in the French system also meet these requirements.
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ANALYSIS AND SYNTHESIS OF TELEVISION PICTURES
31
Comparison of various TV systems. Picture and sound signal standards for the principal
monochrome television systems are given at the end of chapter 4. The CCIR 625-B monochrome
system used in most parts of Europe and adopted by India has a video bandwidth of 5 MHz,
whereas the British 625 line system has a video bandwidth of 5.5 MHz. Obviously, here 0.73
has been used as the resolution factor instead of the 0.69 used in our system. So the British
system is marginally better than the European system. The French TV system employs 819
lines with a video bandwidth of 10.4 MHz. This system therefore has both much improved
vertical resolution and a better horizontal resolution.
The American 525 line system employs a frame frequency of 30 as compared to 25 in the
CCIR 625-B monochrome system. Thus, the line frequency in this system is 15750, which
compares very closely to our system where the line frequency is 15625. However, the American
system employs a bandwidth of 4 MHz which suggests that the horizontal resolution of this
system is less than all other systems in use. It must be noted that the number of lines employed
by a given TV system is not in itself, a guide to the quality of resolution available from the
system. It is true that greater the number of lines the better the vertical resolution, but an
assessment of the horizontal resolution, i.e., the bandwidth employed by the system is a better
overall guide to the quality of definition.
2.6
TONAL GRADATION
In addition to proper bandwidth required to produce the details allowed by the scanning system
at the transmitting end and the picture tube at the receiving end, the signal-transmission
system should have proper transfer characteristics to preserve same brightness gradation as
the eye would perceive when viewing the scene directly. Any non-linearity in the pick-up and
picture tube should also be corrected by providing inverse nonlinearities in the channel circuitry
to obtain overall linear characteristics. Note that the sensation in the eye to detect changes or
brightness is logarithmic in nature and this must be taken into account while designing the
overall channel.
Various other factors that influence the tonal quality of the reproduced picture are :
(a) Contrast. This is the difference in intensity between black and white parts of the
picture over and above the brightness level.
(b) Contrast ratio. The ratio of maximum to minimum brightness relative to the original
picture is called contrast ratio. In broad daylight the variations in brightness are very wide
with ratio as high as 10000 : 1, whereas the picture tube, because of certain limitations, cannot
produce a contrast with variations more than 50 : 1 or atmost 100 : 1. Ratio of brightness
variations in the reproduced picture on the screen of the picture tube, to the brightness variations
in the original scene is known as Gamma of the picture. Its value is close to 0.5. In studios,
under controlled conditions of light, the variations are less wide than outside and so the
brightness variations that can be reproduced by the picture tube are not very much different
than that of the scene. Realism is still maintained because the viewer does not actually see the
scene being televised. Another factor which makes stringent demands from the system
unnecessary is the fact that our eye can accommodate not more than 10 : 1 variations of light
intensity at any time. Too bright a representation of the bright areas in a picture would make
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MONOCHROME AND COLOUR TELEVISION
grey areas appear as dark in comparison. This is true at all levels of light intensity with
brightness variations in relative ratios of 10 : 1. When a TV receiver is off, there is no beam
impinging on the fluorescent screen of the picture tube and no light gets emitted. Then with
normal light in the room the screen appears as dull white. But when the receiver is no, and a
TV programme is being received the bright portions of the scene appear quite bright because
the corresponding amplitude of the video signal makes the control-grid of the picture tube
much less negative and the consequent increased beam current causes more light on the screen.
However, for a very dark portion of the scene the corresponding video signal makes the grid
highly negative with respect to the cathode and thus cuts-off the beam current and no light is
emitted on the corresponding portions on the screen. These areas appear to the eye as dark in
comparison with the high light areas of the screen, whereas the same area in the absence of
beam current when the set was off appeared close to a white shade. This as explained earlier is
due to the logarithmic response of the human eye and its inability to accommodate light intensity
variations greater than 10 : 1.
(c) Viewing distance. The viewing distance from the screen of the TV receiver should not
be so large that the eye cannot resolve details of the picture. The distance should also not be so
small that picture elements become separately visible. The above conditions are met when the
vertical picture size subtends an angle of approximately 15° at the eye. The distance also
depends on habit, varies from person to person, and lies between 3 to 8 times the picture
height. Most people prefer a distance close to five times the picture height. While viewing TV,
a small light should be kept on in the room to reduce contrast. This does not strain the eyes
and there is less fatigue.
Review Questions
1.
Justify the choice of rectangular frame with width to height ratio = 4/3 for television transmission and reception.
2.
How is the illusion of continuity created in television pictures ? Why has the frame reception
rate been chosen to be 25 and not 24 as in motion pictures ?
3.
What do you understand by interlaced scanning ? Show that it reduces flicker and conserve
bandwidth.
4.
What do you understand by active and blanking periods in horizontal and vertical scanning ?
Give the periods of nominal, active and retrace intervals of horizontal and vertical scanning as
used in the 625 line system.
5.
How many horizontal lines get traced during each vertical retrace ? What is the active number of
lines that are actually used for picture information pick up and reception ?
6.
Draw a picture frame chart showing the total number of active and inactive lines during each
field and establish the need for terminating the first field in a half line and the beginning the
second at the middle of a line at the top.
7.
Justify the choice of 625 lines for TV transmission. Why is the total number of lines kept odd in
all television systems ? What is the significance of choosing the number of lines as 625 and not
623 or 627 ?
8.
What do you understand by resolution or Kell-factor ? How does it affect the vertical resolution
of a television picture ? Show that the vertical resolution increases with increase in number of
scanning lines.
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ANALYSIS AND SYNTHESIS OF TELEVISION PICTURES
9.
33
What is meant by equal vertical and horizontal ‘resolution ?’ Derive an expression for the highest modulating frequency in a television system and show that it is nearly 5 MHz. in the 625-B
monochrome system.
10. Show that if the number of lines employed in a TV system is increased then the highest video
frequency must increase as the square of the increase in number of lines for equal improvement
in vertical and horizontal resolution.
11. Show that the 625-B TV system is only marginally superior to the 525 line American system.
12. What do you understand by interlace error and how does it affect the quality of the picture ?
Calculate the percentage interlace error when the second field is delayed by 8 µs. Retrace time
may be assumed to be negligible.
13. In the British 625 lines system the resolution factor employed is 0.73 instead of 0.69 as used in
the 625-B monochrome system. All other scanning details remaining the same, calculate the
highest modulating frequency used in the British system.
14. Explain the need for providing very good low frequency response and phase characteristics in
amplifiers used in any TV link, for proper reproduction of brightness variations.
15. The relevant data for a closed circuit TV system is given below. Calculate the highest modulating frequency that will be generated while scanning the most stringent case of alternate black
and white dots for equal vertical and horizontal resolution.
No. of lines
= 250
Interlace ratio
=1:1
Picture repetition rate
= 50/sec
Aspect ratio
= 4/3
Vertical retrace time
= 10% of the picture frame time
Horizontal retrace time
= 20% of the total line time
Assume resolution factor = 0.8
Ans
≈ 2 MHz
16. Explain the meaning of terms-tonal gradation, contrast, contrast ratio and gamma of the picture.
When a TV receiver is off, no electron beam strikes the picture tube screen and the screen face
looks a dull white. With the set on and a black and white picture showing on the screen, no
electron beam impinges on the darker area of the reproduced picture. But these areas now appear quite black instead of the dull white of the switched-off set. Explain the reason for this
difference in appearance.
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Composite Video Signal
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Composite Video Signal
Composite video signal consists of a camera signal corresponding to the desired picture
information, blanking pulses to make the retrace invisible, and synchronizing pulses to
synchronize the transmitter and receiver scanning. A horizontal synchronizing (sync) pulse is
needed at the end of each active line period whereas a vertical sync pulse is required after each
field is scanned. The amplitude of both horizontal and vertical sync pulses is kept the same to
obtain higher efficiency of picture signal transmission but their duration (width) is chosen to
be different for separating them at the receiver. Since sync pulses are needed consecutively
and not simultaneously with the picture signal, these are sent on a time division basis and
thus form a part of the composite video signal.
3.1
VIDEO SIGNAL DIMENSIONS
Figure 3.1 shows the composite video signal details of three different lines each corresponding
to a different brightness level of the scene. As illustrated there, the video signal is constrained
to vary between certain amplitude limits. The level of the video signal when the picture detail
being transmitted corresponds to the maximum whiteness to be handled, is referred to as
peak-white level. This is fixed at 10 to 12.5 percent of the maximum value of the signal while
the black level corresponds to approximately 72 percent. The sync pulses are added at 75 percent
level called the blanking level. The difference between the black level and blanking level is
known as the ‘Pedestal’. However, in actual practice, these two levels, being very close, tend to
merge with each other as shown in the figure. Thus the picture information may vary between
10 percent to about 75 percent of the composite video signal depending on the relative brightness
of the picture at any instant. The darker the picture the higher will be the voltage within those
limits.
Note that the lowest 10 percent of the voltage range (whiter than white range) is not
used to minimize noise effects. This also ensures enough margin for excessive bright spots to
be accommodated without causing amplitude distortion at the modulator.
At the receiver the picture tube is biased to ensure that a received video voltage
corresponding to about 10 percent modulation yields complete whiteness at that particular
point on the screen, and an analogous arrangement is made for the black level. Besides this,
the television receivers are provided with ‘brightness’ and ‘contrast’ controls to enable the
viewer to make final adjustments as he thinks fit.
D.C. component of the video signal. In addition to continuous amplitude variations for
individual picture elements, the video signal has an average value or dc component
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37
COMPOSITE VIDEO SIGNAL
corresponding to the average brightness of the scene. In the absence of dc component the
receiver cannot follow changes in brightness, as the ac camera signal, say for grey picture
elements on a black background will then be the same as a signal for white area on a grey
back-ground. In Fig. 3.1, dc components of the signal for three lines have been identified, each
representing a different level of average brightness in the scene. It may be noted that the
break shown in the illustration after each line signal is to emphasize that dc component of the
video signal is the average value for complete frames rather than lines since the background
information of the picture indicates the brightness of the scene. Thus Fig. 3.1 illustrates the
concept of change in the average brightness of the scene with the help of three lines in separate
frames because the average brightness can change only from frame to frame and not from line
to line.
20
12.5
Active line
per od
period
52 s
Pedestal height
Ped.
height
Peak white
level
Picture
details
D.C.. level
40
0.4
Blanking
level
D.C.
D
C. level
evel
60
64 s
One line
duration
64 s
Horz sync
pulses
Ped. height
D.C.. level
e
80
75
Dark level
Composite video signal (percent of max)
100
Darker than
n dark
v/v max%
S
0.2
0
0.2
0.4
P
0.6
0.8
1.0
t
0
Fig. 3.1 Arbitrary picture signal details of three scanning lines with different average
brightness levels. Note that picture to sync ratio P/S = 10/4.
Pedestal height. As noted in Fig. 3.1 the pedestal height is the distance between the
pedestal level and the average value (dc level) axis of the video signal. This indicates average
brightness since it measures how much the average value differs from the black level. Even
when the signal loses its dc value when passed through a capacitor-coupled circuit the distance
between the pedestal and the dc level stays the same and thus it is convenient to use the
pedestal level as the reference level to indicate average brightness of the scene.
Setting the pedestal level. The output signal from the TV camera is of very small amplitude
and is passed through several stages of ac coupled high gain amplifiers before being coupled to
a control amplifier. Here sync pulses and blanking pulses are added and then clipped at the
correct level to form the pedestals. Since the pedestal height determines the average brightness
of the scene, any smaller value than the correct one will make the scene darker while a larger
pedestal height will result in higher average brightness. The video control operator who observes
the scene at the studio sets the level for the desired brightness in the reproduced picture which
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MONOCHROME AND COLOUR TELEVISION
he is viewing on a monitor receiver. This is known as dc insertion because this amounts to
adding a dc component to the ac signal. Once the dc insertion has been acomplished the pedestal
level becomes the black reference and the pedestal height indicates correct relative brightness
for the reproduced picture. However, the dc level inserted in the control amplifier is usually
lost in succeeding stages because of capacitive coupling, but still the correct dc component can
be reinserted when necessary because the pedestal height remains the same.
The blanking pulses. The composite video signal contains blanking pulses to make the
retrace lines invisible by raising the signal amplitude slightly above the black level (75 per
cent) during the time the scanning circuits produce retraces. As illustrated in Fig. 3.2, the
composite video signal contains horizontal and vertical blanking pulses to blank the
corresponding retrace intervals. The repetition rate of horizontal blanking pulses is therefore
equal to the line scanning frequency of 15625 Hz. Similarly the frequency of the vertical blanking
pulses is equal to the field-scanning frequency of 50 Hz. It may be noted that though the level
of the blanking pulses is distinctly above the picture signal information, these are not used as
sync pulses. The reason is that any occasional signal corresponding to any extreme black portion
in the picture may rise above the blanking level and might conceivably interfere with the
synchronization of the scanning generators. Therefore, the sync pulses, specially designed for
triggering the sweep oscillators are placed in the upper 25 per cent (75 per cent to 100 per cent
of the carrier amplitude) of the video signal, and are transmitted along with the picture signal.
100
90
Horz sync
n pulse
added here
r (25%)
Vertical
sync pulse added
de here (25%)
Blanking level
80
72% 75
70
Amplitude %
60
50
40
Black
level
64 s
160 s
Vertical blanking pulse period
(No picture information
during this interval)
Horz
blanking
pulses
Picture
information
30
20
12.5%
10
Peak white level
t
0
Fig. 3.2 Horizontal and vertical blanking pulses in video signal. Sync pulses are added above
the blanking level and occupy upper 25% of the composite video signal amplitude.
Sync pulse and video signal amplitude ratio. The overall arrangement of combining the
picture signal and sync pulses may be thought of as a kind of voltage division multiplexing
where about 65 per cent of the carrier amplitude is occupied by the video signal and the upper
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COMPOSITE VIDEO SIGNAL
25 per cent by the sync pulses. Thus, as shown in Fig. 3.1, the final radiated signal has a
picture to sync signal ratio (P/S) equal to 10/4. This ratio has been found most satisfactory
because if the picture signal amplitude is increased at the expense of sync pulses, then when
the signal to noise ratio of the received signal falls, a point is reached when the sync pulse
amplitude becomes insufficient to keep the picture locked even though the picture voltage is
still of adequate amplitude to yield an acceptable picture. On the other hand if sync pulse
height is increased at the expense of the picture detail, then under similar conditions the
raster remains locked but the picture content is of too low an amplitude to set up a worthwhile
picture. A ratio of P/S = 10/4, or thereabout, results in a situation such that when the signal to
noise ratio reaches a certain low level, the sync amplitude becomes insufficient, i.e., the sync
fails at the same time as the picture ceases to be of entertainment value. This represents the
most efficient use of the television system.
3.2
HORIZONTAL SYNC DETAILS
The horizontal blanking period and sync pulse details are illustrated in Fig. 3.3. The interval
between horizontal scanning lines is indicated by H. As explained earlier, out of a total line
Front porch (blanked)
Back porch (blanked)
Picture
e space
on
a
the raster
Horz sync
pulse = 4.7 ms
P cture
Picture
H = 64 ms
Front porch = 1.5 ms
Amplitude %
100
Back porch
= 5.8 ms
80
75
Picture
c
information = 52 ms
60
Horz blanking
pulse = 12 ms
40
20
12.5
t
0
Retrace
begins
Blanking begins
Trace
Horz
deflection 0
sawtooth
t
Retrace
Retrace ends
Blanking ends
Fig. 3.3 Horz line and sync details compared to horizontal deflection sawtooth
and picture space on the raster.
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MONOCHROME AND COLOUR TELEVISION
period of 64 µs, the line blanking period is 12 µs. During this interval a line synchronizing
pulse is inserted. The pulses corresponding to the differentiated leading edges of the sync
pulses are actually used to synchronize the horizontal scanning oscillator. This is the reason
why in Fig. 3.3 and other figures to follow, all time intervals are shown between sync pulse
leading edges.
The line blanking period is divided into three sections. These are the ‘front porch’, the
‘line sync’ pulse and the ‘back porch’. The time intervals allowed to each part are summarized
below and their location and effect on the raster is illustrated in Fig. 3.3.
Details of Horizontal Scanning
Period
Time (µs)
Total line (H)
64
Horz blanking
12 ± .3
Horz sync pulse
4.7 ± 0.2
Front porch
1.5 ± .3
Back porch
5.8 ± .3
Visible line time
52
Front porch. This is a brief cushioning period of 1.5 µs inserted between the end of the
picture detail for that line and the leading edge of the line sync pulse. This interval allows the
receiver video circuit to settle down from whatever picture voltage level exists at the end of the
picture line to the blanking level before the sync pulse occurs. Thus sync circuits at the receiver
are isolated from the influence of end of the line picture details. The most stringent demand is
made on the video circuits when peak white detail occurs at the end of a line. Despite the
existence of the front porch when the line ends in an extreme white detail, and the signal
amplitude touches almost zero level, the video voltage level fails to decay to the blanking level
before the leading-edge of the line sync pulse occurs. This results in late triggering of the time
base circuit thus upsetting the ‘horz’ line sync circuit. As a result the spot (beam) is late in
arriving at the left of the screen and picture information on the next line is displaced to the
left. This effect is known as ‘pulling-on-whites’.
Line sync pulse. After the front proch of blanking, horizontal retrace is produced when
the sync pulse starts. The flyback is definitely blanked out because the sync level is blacker
than black. Line sync pulses are separated at the receiver and utilized to keep the receiver line
time base in precise synchronism with the distant transmitter. The nominal time duration for
the line sync pulses is 4.7 µs. During this period the beam on the raster almost completes its
back stroke (retrace) and arrives at the extreme left end of the raster.
Back porch. This period of 5.8 µs at the blanking level allows plenty of time for line
flyback to be completed. It also permits time for the horizontal time-base circuit to reverse
direction of current for the initiation of the scanning of next line. Infact, the relative timings
are so set that small black bars (see Fig. 3.3) are formed at both the ends of the raster in the
horizontal plane. These blanked bars at the sides have no effect on the picture details reproduced
during the active line period.
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COMPOSITE VIDEO SIGNAL
The back porch* also provides the necessary amplitude equal to the blanking level
(reference level) and enables to preserve the dc content of the picture information at the
transmitter. At the receiver this level which is independent of the picture details is utilized in
the AGC (automatic gain control) circuits to develop true AGC voltage proportional to the
signal strength picked up at the antenna.
3.3
VERTICAL SYNC DETAILS
The vertical sync pulse train added after each field is somewhat complex in nature. The reason
for this stems from the fact that it has to meet several exacting requirements. Therefore, in
order to fully appreciate the various constituents of the pulse train, the vertical sync details
are explored step by step while explaining the need for its various components.
The basic vertical sync added at the end of both even add odd fields is shown in Fig. 3.4.
Its width has to be kept much larger than the horizontal sync pulse, in order to derive a
suitable field sync pulse at the receiver to trigger the field sweep oscillator.
The standards specify that the vertical sync period should be 2.5 to 3 times the horizontal
line period. If the width is less than this, it becomes difficult to distinguish between horizontal
and vertical pulses at the receiver.
End of second (even) field
623
H
624
H
625
H
Beginning of first (odd) field
Lines 1, 2, 3rd
1 t half
1st
ha f
3.5 to 17
18
8
19
Vertical pulse
inte val
interval
Vertical
ert ca blanking
blank ng interval
in erval
(a)
End of first field
i
310
H
311
H
Beginning of second field
312
H
3 3 314
313.5
3 315
3 5
313.5
H/2
3 to 330
316
33
331
332
2.5 H
(160 s)
20 line
e period
er od = (20 × 64 s = 1280 s))
(b)
Fig. 3.4 Composite video waveforms showing horizontal and basic vertical sync pulses at
the end of (a) second (even) field, (b) first (odd) field. Note, the widths of
horizontal blanking intervals and sync pulses are exaggerated.
*In colour TV transmission a short sample (8 to 10 cycles) of the colour subcarrier oscillator
output is sent to the receiver for proper detection of colour signal sidebands. This is known as colour
burst and is located at the back porch of the horizontal blanking pedestal.
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MONOCHROME AND COLOUR TELEVISION
If the width is greater than this, the transmitter must operate at peak power for an
unnecessarily long interval of time. In the 625 line system 2.5 line period (2.5 × 64 = 160 µs)
has been allotted for the vertical sync pulses. Thus a vertical sync pulse commences at the end
of 1st half of 313th line (end of first field) and terminates at the end fo 315th line. Similarly
after an exact interval of 20 ms (one field period) the next sync pulse occupies line numbers—
1st, 2nd and 1st half of third, just after the second field is over. Note that the beginning of
these pulses has been aligned in the figure to signify that these must occur after the end of
vertical stroke of the beam in each field, i.e., after each 1/50th of a second. This alignment of
vertical sync pulses, one at the end of a half-line period and the other after a full line period
(see Fig. 3.4), results in a relative misalignment of the horizontal sync pulses and they do not
appear one above the other but occur at half-line intervals with respect to each other. However,
a detailed examination of the pulse trains in the two fields would show that horizontal sync
pulses continue to occur exactly at 64 µs intervals (except during the vertical sync pulse periods)
throughout the scanning period from frame to frame and the apparent shift of 32 µs is only due
to the alignment of vertical sync instances in the figure.
As already mentioned the horizontal sync information is extracted from the sync pulse
train by differentiation, i.e., by passing the pulse train through a high-pass filter. Indeed pulses
corresponding to the differentiated leading edges of sync pulses are used to synchronise the
horizontal scanning oscillator. The process of deriving these pulses is illustrated in Fig. 3.5.
Furthermore, receivers often use monostable multivibrators to generate horizontal scan, and
so a pulse is required to initiate each and every cycle of the horizontal oscillator in the receiver.
Sync pulses
R2
Composite
video signal
Sync
separator
L.P.F.
C1
H.P.F.
C2
R1
Integrated output
Differentiated output
Fig. 3.5 Sync pulse separation and generation of vertical and horizontal sync pulses.
This brings out the first and most obvious shortcoming of the waveforms shown in Fig. 3.4.
The horizontal sync pulses are available both during the active and blanked line periods but
there are no sync pulses (leading edges) available during the 2.5 line vertical sync period. Thus
the horizontal sweep oscillator that operates at 15625 Hz, would tend to step out of synchronism
during each vertical sync period. The situation after an odd field is even worse. As shown in
Fig. 3.4, the vertical blanking period at the end of an odd field begins midway through a
horizontal line. Consequently, looking further along this waveform, we see that the leading
edge of the vertical sync pulse comes at the wrong time to provide synchronization for the
horizontal oscillator. Therefore, it becomes necessary to cut slots in the vertical sync pulse at
half-line-intervals to provide horizontal sync pulses at the correct instances both after even
and odd fields. The technique is to take the video signal amplitude back to the blanking level
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COMPOSITE VIDEO SIGNAL
4.7 µs before the line pulses are needed. The waveform is then returned back to the maximum
level at the moment the line sweep circuit needs synchronization. Thus five narrow slots of
4.7 µs width get formed in each vertical sync pulse at intervals of 32 µs. The trailing but rising
edges of these pulses are actually used to trigger the horizontal oscillator. The resulting
waveforms together with line numbers and the differentiated output of both the field trains is
illustrated in Fig. 3.6. This insertion of short pulses is known as notching or serration of the
broad field pulses.
Note that though the vertical pulse has been broken to yield horizontal sync pulses, the
effect on the vertical pulse is substantially unchanged. It still remains above the blanking
voltage level all of the time it is acting. The pulse width is still much wider than the horizontal
pulse width and thus can be easily separated at the receiver. Returning to Fig. 3.6 it is seen
that each horizontal sync pulse yields a positive spiked output from its leading edge and a
negative spiked pulse from its trailing edge. Time-constant of the differentiating circuit is so
chosen, that by the time a trailing edge arrives, the pulse due to the leading edge has just
about decayed. The negative-going triggering pulses may be removed with a diode since only
the positive going pulses are effective in locking the horizontal oscillator.
End of 2nd field
623
624
625
1
2
1
2
3
4.7 s
623
624
4
5
27.3 s
1
625
3
2
3
4
5
6
(a)
End of 1st field
311
312
313
1
311
312
313
314
2
315
3
314
4
315
316
5
316
317
318
(b)
Fig. 3.6 Differentiating waveforms (a) pulses at the end of even (2nd) field and the
corresponding output of the differentiator (H.P.F.) (b) pulses at the end of odd (1st)
field and the corresponding output of the differentiator (H.P.F.) Note, the differentiated pulses
bearing line numbers are the only ones needed at the end of each field.
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MONOCHROME AND COLOUR TELEVISION
However, the pulses actually utilized are the ones that occur sequentially at 64 µs
intervals. Such pulses are marked with line numbers for both the fields. Note that during the
intervals of serrated vertical pulse trains, alternate vertical spikes are utilized. The pulses not
used in one field are the ones utilized during the second field. This happens because of the
half-line difference at the commencement of each field and the fact that notched vertical sync
pulses occur at intervals of 32 µs and not 64 µs as required by the horizontal sweep oscillator.
The pulses that come at a time when they cannot trigger the oscillator are ignored. Thus the
requirement of keeping the horizontal sweep circuit locked despite insertion of vertical sync
pulses is realized.
Now we turn to the second shortcoming of the waveform of Fig. 3.4. First it must be
mentioned that synchronization of the vertical sweep oscillator in the receiver is obtained
from vertical sync pulses by integration. This is illustrated in Fig. 3.5 where the time-constant
R2C2 is chosen to be large compared to the duration of horizontal pulses but not with respect to
width of the vertical sync pulses. The integrating circuit may equally be looked upon as a lowpass filter, with a cuit-off frequency such that the horizontal sync pulses produce very little
output, while the vertical pulses have a frequency that falls in the pass-band of the filter. The
voltage built across the capacitor of the low-pass filter (integrating circuit) corresponding to
the sync pulse trains of both the fields is shown in Fig. 3.7. Note that each horizontal pulse
causes a slight rise in voltage across the capacitor but this is reduced to zero by the time the
next pulse arrives. This is so, because the charging period for the capacitor is only 4.7 µs and
the voltage at the input to the integrator remains at zero for the rest of the period of 59.3 µs.
Hence there is no residual voltage across the vertical filter (L.P. filter) due to horizontal syncpulses. Once the broad serrated vertical pulse arrives the voltage across the output of the filter
starts increasing. However, the built up voltage differs for each field. The reason is not difficult
to find. At the beginning of the first field (odd field) the last horz sync pulse corresponding to
the beginning of 625th line is separated from the 1st vertical pulse by full one line and any
voltage developed across the filter will have enough time to return to zero before the arrival of
the first vertical pulse, and thus the filter output voltage builds up from zero in response to the
five successive broad vertical sync pulses. The voltage builds up because the capacitor has
more time to charge and only 4.7 µs to discharge. The situation, however, is not the same for
the beginning of the 2nd (even) field. Here the last horizontal pulse corresponding to the
beginning of 313th line is separated from the first vertical pulse by only half-a-line. The voltage
developed across the vertical filter will thus not have enough time to reach zero before the
arrival of the first vertical pulse, which means that the voltage build-up does not start from
zero, as in the case of the 1st field. The residual voltage on account of the half line discrepancy
gets added to the voltage developed on account of the broad vertical pulses and thus the
voltage developed across the output filter is some what higher at each instant as compared to
the voltage developed at the beginning of the first-field. This is shown in dotted chain line in
Fig. 3.7.
The vertical oscillator trigger potential level marked as trigger level in the diagram
(Fig. 3.7) intersects the two filter output profiles at different points which indicates that in the
case of second field the oscillator will get triggered a fraction of a second too soon as compared
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45
COMPOSITE VIDEO SIGNAL
to the first field. Note that this inequlity in potential levels for the two fields continues during
the period of discharge of the capacitor once the vertical sync pulses are over and the horizontal
sync pulses take-over. Though the actual time difference is quite short it does prove sufficient
to upset the desired interlacing sequence.
End of 2nd field
1st field
625
1
(a)
End of 1st field
3 2
312
2nd field
3 3
313
(b)
After end of 2nd field
v0
Trigger level
After end of 1st field
Time
error
Trigger pulse
for the vertical oscillator
(c)
t
0
Fig. 3.7 Integrating waveforms (a) pulses at the end of 2nd (even) field (b) pulses at the end of
1st (odd) field (c) integrator output. Note the above sync pulses have purposely been drawn
without equalizing pulses.
Equalizing pulses. To take care of this drawback which occurs on account of the halfline discrepancy five narrow pulses are added on either side of the vertical sync pulses. These
are known as pre-equalizing and post-equalizing pulses. Each set consists of five narrow pulses
occupying 2.5 lines period on either side of the vertical sync pulses. Pre-equalizing and postequalizing pulse details with line numbers occupied by them in each field are given in Fig. 3.8.
The effect of these pulses is to shift the half-line discrepancy away both from the beginning
and end of vertical sync pulses. Pre-equalizing pulses being of 2.3 µs duration result in the
discharge of the capacitor to essentially zero voltage in both the fields, despite the half-line
discrepancy before the voltage build-up starts with the arrival of vertical sync pulses. This is
illustrated in Fig. 3.9. Post-equalizing pulses are necessary for a fast discharge of the capacitor
to ensure triggering of the vertical oscillator at proper time. If the decay of voltage across the
capacitor is slow as would happen in the absence of post-equalizing pulses, the oscillator may
trigger at the trailing edge which may be far-away from the leading edge and this could lead to
an error in triggering.
Thus with the insertion of narrow pre and post equalizing pulses, the voltage rise and
fall profile is essentially the same for both the field sequences (see Fig. 3.9) and the vertical
oscillator is triggered at the proper instants, i.e., exactly at an interval of 1/50th of a second.
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46
MONOCHROME AND COLOUR TELEVISION
This problem could possibly also be solved by using an integrating circuit with a much larger
time constant, to ensure that the capacitor remains virtually uncharged by the horizontal
pulses. However, this would have the effect of significantly reducing the integrator output for
vertical pulses so that a vertical sync amplifier would have to be used. In a broadcasting
situation, there are thousands of receivers for every transmitter. Consequently it is much
more efficient and economical to cure this problem in one transmitter than in thousands of
receivers. This, as explained above, is achieved by the use of pre and post equalizing pulses.
The complete pulse trains for both the fields incorporating equalizing pulses are shown in
Fig. 3.10.
29.7 s
1st
2nd field ending
2.3 + 0.1 s
2nd
2nd half
ha f
of 623
3rd
624
5h
5th
625
311
1st field ending
4th
312
1st half
ha f of
313
(a) Pre-sync equalizing pulses (five)
1st
s
2nd
3rd
4th
5th
1st field
2nd
2
d half
a
of 3rd
4
5
2nd field
316
317
1st half
ha f of
318
(b) Post-sync equalizing pulses (five)
Fig. 3.8 Pre-sync equalizing and Post-sync equalizing pulses.
From the comparison of the horizontal and vertical output pulse forms shown in Figs. 3.7
and 3.9 it appears that the vertical trigger pulse (output of the low-pass filter) is not very
sharp but actually it is not so. The scale chosen exaggerates the extent of the vertical pulses.
The voltage build-up period is only 160 µs and so far as the vertical synchronizing oscillator is
concerned this pulse occurs rapidly and represents a sudden change in voltage which decays
very fast.
The polarity of the pulses as obtained at the outputs of their respective fields may not be
suitable for direct application in the controlled synchronizing oscillator and might need inversion
depending on the type of oscillator used. This aspect will be fully developed in the chapter
devoted to vertical and horizontal oscillators.
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47
COMPOSITE VIDEO SIGNAL
Active lines
Pre-equalizing pulses
Field sync pulses
Post-equalizing pulses Blanking lines
End of 2nd field
621
622
309
623
End of 1st field
310
v0
Identical vertical
r
sync
tbuilt-up
Trigger level
(no time error)
0
t
Fig. 3.9 Identical vertical sync voltage built-up across the integrating capacitor.
3.4 SCANNING SEQUENCE DETAILS
A complete chart giving line numbers and pulse designations for both the fields (corresponding
to Fig. 3.10) is given below :
First Field (odd field)
Line numbers : one to 1st-half of 313th line (312.5 lines)
1, 2 and 3rd 1st-half, lines
2.5 lines—Vertical sync pulses
3rd 2nd-half, 4, and 5
2.5 lines—Post-vertical sync equalizing pulses.
6 to 17, and 18th 1st-half
12.5 lines—Blanking retrace pulses
18th 2nd-half to 310
292.5 lines—Picture details
311, 312, and 313th 1st-half
2.5 lines—Pre-vertical sync equalizing pulses
for the 2nd field.
Total number of lines = 312.5
Second field (even field)
Line numbers : 313th 2nd-half to 625 (312.5 lines)
313th 2nd-half, 314, 315
2.5 lines—Vertical sync pulses
316, 317, 318th 1st-half
2.5 lines—Post-vertical sync equalizing pulses
318th 2nd-half-to 330
12.5 lines—Blanking retrace pulses
331 to 1st-half of 623rd
292.5 line—Picture details
623 2nd-half, 624 and 625
2.5 lines—Pre-vertical sync equalizing pulses
for the 1st field
Total number of lines = 312.5
Total Number of Lines per Frame = 625
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309
310
End of 1st
field
End of 2nd
field
312
5 narrow
equalizing
pulses
311
1
2
2.5 lines
l nes
315
3
5
317
318
7
8
9
319
320
321
10
12
323
324
325
14
326
13
12.5
1
. blanked lines
11
12.5 lines
l nes
Blanking level
322
(b) Pulse train at the end of 1st field
5 narrow
equalizing pulses
316
6
(a) Pulse train at the end of 2nd field
5 Post
s
equalizing
pulses
4
2.5 lines
l nes
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Fig. 3.10 Field synchronizing pulse trains of the 625 lines TV system.
5 broad field
sync pulses
314
5 Vertical
a
sync pulses
313
625
5 Preequalizing
pulses
624
2.5 lines
l nes
Field
F
d blanking
b an ing per
period
od o
of 30 llines
nes
327
15
328
329
16
330
17
19
331
18
332
20
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MONOCHROME AND COLOUR TELEVISION
49
COMPOSITE VIDEO SIGNAL
Approximate location of line numbers. The serrated vertical sync pulse forces the vertical
deflection circuity to start the flyback. However, the flyback generally does not begin with the
start of vertical sync because the sync pulse must build up a minimum voltage across the
capacitor to trigger the scanning oscillator. If it is assumed that vertical flyback starts with
the leading edge of the fourth serration, a time of 1.5 lines passes during vertical sync before
vertical flyback starts. Also five equalizing pulses occur before vertical sync pulse train starts.
Then four lines (2.5 + 1.5 = 4) are blanked at the bottom of the pricture before vertical retrace
begins. A typical vertical retrace time is five lines. Thus the remaining eleven (20 – (4 + 5) =
11) lines are blanked at the top of the raster. These lines provide the sweep oscillator enough
time to adjust to a linear rise for uniform pick-up and reproduction of the picture.
3.5
FUNCTIONS OF VERTICAL PULSE TRAIN
By serrating the vertical sync pulses and the providing pre- and post-equalizing pulses the
following basic requirements necessary for successful interlaced scanning are ensured.
(a) A suitable field sync pulse is derived for triggering the field oscillator.
(b) The line oscillator continues to receive triggering pulses at correct intervals while the
process of initiation and completion of the field time-base stroke is going on.
(c) It becomes possible to insert vertical sync pulses at the end of a line after the 2nd
field and at the middle of a line at the end of the 1st field without causing any interlace
error.
(d) The vertical sync build up at the receiver has precisely the same shape and timing on
odd and even fields.
3.6
SYNC DETAILS OF THE 525 LINE SYSTEM
In the 525 line American TV system where the total number of lines scanned per second is
15750, the sync pulse details are as under :
Details of Horz Blanking
Period
Time (µs)
Field line (H)
63.5
Horz blanking
9.5 to 11.5
Horz sync pulse
4.75 ± 0.5
Front porch
1.26 (minimum)
Back porch
3.81 (minimum)
Visible line
52 to 54
Details of vertical Blanking
Period
Time
Total field (V) period
= 1/60 sec. = 16.7 ms
Visible field time
= 15 to 16 ms
Vertical blanking
= 0.8 to 1.3 ms
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50
MONOCHROME AND COLOUR TELEVISION
Total duration of six (serrated)
vertical sync pulses
= 3H = 190.5 µs
Each serrated pulse
= H/2 = 31.75 µs
Each equalizing pulse
(Six pre- and six post-equailzing pulses
are provided at H/2 intervals)
= 0.04 H = 2.54 µs
Review Questions
1.
Sketch composite video signal waveform for at least three three successive lines and indicate :
(i) extreme white level, (ii) blanking level, (ii) pedestal height and (iv) sync pulse level. Justify
the choice of P/S ratio = 10/4 in the composite signal. Why is the combining of picture signal and
sync pulses called a voltage division multiplex ?
2.
Sketch composite video signal waveforms for the picture information shown in Fig. P 3.1.
W
H
(a)
(b)
Shaded areas indicate darkness
Fig. P3.1
3.
Show picture information on a raster for the video signals drawn in Fig. P3.2.
6 s
64
64 s
White level
(a)
(b)
Fig. P3.2
4.
Sketch the details of horizontal blanking and sync pulses. Label on it (i) front porch, (ii) horizontal sync pulse, (iii) back porch and (iv) active line periods. Why are the front porch and back
porch intervals provided before and after the horizontal sync pulse ? Explain why the blanking
pulses are not used as sync pulses.
5.
Enumerate the basic requriments that must be satisfied by the pulse train added after each
field. Why is it necessary to serrate the broad vertical sync pulse ?
6.
Sketch the pulse trains that follow after the second and first field of active scanning. Why are
the vertical sync pulses notched at 32 µs interval and not at 64 µs interval to provide horizontal
sync pulses ?
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51
COMPOSITE VIDEO SIGNAL
7.
Explain how the horizontal and vertical sync pulses are separated and shaped at the receiver.
For a time constant of 5 µs for the differentiating circuit, and 100 µs for the integrating circuit,
plot the output waveforms from both the circuits for the entire vertical period. Calculate the
error in timing for successive vertical fields in the absence of equalizing pulses.
8.
Sketch the complete pulse trains that follow at the end of both odd and even fields. Fully label
them and explain how the half line discrepancy is removed by insertion of pre-equalizing pulses.
9.
Justify the need for pre and post equalizing pulses. Why it is necessary to keep their duration
equal to the half-line period ?
10. Justify the need for a blanking period corresponding to 20 complete lines after each active field
of scanning. Why does the vertical retrace not begin with the incoming of the first serrated
vertical sync pulse ?
11. Sketch the complete pulse trains that follow at the end of odd and even fields in the 525 line
television system. Justify the need for six instead of five pre and post equalizing pulses.
12. Show by any suitable means approximate correspondence between line numbers and the location
of the electron beam on the screen, both for odd and even fields.
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4
Signal TransmiIIion and
Channel Bandwidth
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4
Signal Transmission and
Channel Bandwidth
In most television systems as also in the C.C.I.R 625 line, the picture signal is amplitude
modulated and sound signal frequency modulated before transmission. The channel bandwidth
is determined by the highest video frequency required for proper picture reception and the
maximum sound carrier frequency deviation permitted in a TV system.
Need for modulation. The need for modulation stems from the fact that it is impossible
to transmit a signal by itself. The greatest difficulty in the use of unmodulated wave is the
need for long antennas for efficient radiation and reception. For example, a quarter-wavelength
antenna for the transmitting frequency of 15 kHz would be 5000 meters long. A vertical antenna
of this size is unthinkable and in fact impracticable.
Another important reason for not transmitting signal frequencies directly is that both
picture and sound signals from different stations are concentrated within the same range of
frequencies. Therefore, radiation from different stations would be hopelessly and inextricably
mixed up and it would be impossible to separate one from the other at the receiving end. Thus
in order to be able to separate the intelligence from different stations, it is necessary to translate
them all to different portions of the electromagnetic spectrum depending on the carrier frequency
assigned to each station. This also overcomes the difficulties of poor radiation at low frequencies.
Once signals are translated before transmission, a tuned circuit provided in the RF section of
the receiver can be used to select the desired station.
4.1 AMPLITUDE MODULATION
In amplitude modulation the intelligence to be conveyed is used to vary the amplitude of the
carrier wave. As an illustration, an amplitude modulated signal is shown in Fig. 4.1 (a) where
ec = Ec cos ωct is the carrier wave and
em = Em cos ωmt is the modulating signal.
Note that the camera signal is actually complex in nature but a single modulating
frequency has been chosen for convenience of analysis.
The equation of the modulated wave is :
e = A cos ωct
where A = (Ec + kEm cos ωmt) when k is a constant of the modulator.
54
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55
SIGNAL TRANSMISSION AND CHANNEL BANDWIDTH
+
ec = Ec Cos c t
Ec
t
0
–
Unmodulated carrier wave
em = Em Cos m t
Em
t
Modulating signal
+
Ec + Em Cos m t
Ec
0
t
Carrier
– Ec
–
LSB
– (Ec + Em Cos m t)
Modulated wave
Fig. 4.1(a) Modulation of R.F. carrier
with a signal frequency.
USB
fm
fc – f m
fm
fc
fc + f m
Fig. 4.1(b) Frequency spectrum of AM wave.
On substituting the value of A we get :
e = (Ec + kEm cos ωmt) cos ωct = Ec (1 + m cos ωmt) cos ωct
where m =
...(4.1)
kEm
is the modulation index.
Ec
It may be noted that at kEm = Ec, m = 1 and the corresponding depth of modulation is
then termed as 100%.
Equation (4.1) may be expanded by the use of trigonometrical identities and expressed
as :
mEc
mEc
cos (ωc – ωm) t –
cos (ωc + ωm)t
...(4.2)
2
2
This result shows that if a carrier wave having frequency equal to fc is amplitude modulated
with a single frequency fm, the resultant wave consists of the carrier (fc) and the sum and
difference components (fc ± fm) of the carrier frequency and the modulating frequency. However,
if the modulating signal consists of more than a single frequency, as it would be for a video
signal, the equation can be extended to include the sum and difference of the carrier and all
frequency components of the modulating signal. This is illustrated in Fig. 4.1 (b) where fm has
been shown to be the highest modulating frequency. The region between fc and (fc + fm) is
called the upper sideband (USB) and that between fc and (fc – fm) the lower sideband (LSB).
e = Ec cos ωct +
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56
MONOCHROME AND COLOUR TELEVISION
Therefore if the modulated wave is to be transmitted without distortion by this method, the
transmission channel must be atleast of width 2fm centred on fc.
4.2 CHANNEL BANDWIDTH
In the 625 line TV system where the frequency components present in the video signal extend
from dc (zero Hz) to 5MHz, a double sideband AM transmission would occupy a total bandwidth
of 10 MHz. The actual band space allocated to the television channel would have to be still
greater, because with practical filter characteristics it is not possible to terminate the bandwidth
of a signal abruptly at the edges of the sidebands. Therefore, an attenuation slope of 0.5 MHz
is provided at each edge of the two sidebands. This adds 1 MHz to the required total band
space. In addition to this, each television channel has its associated FM (frequency modulated)
sound signal, the carrier frequency of which is situated just outside the upper limit of 5.5 MHz
of the picture signal. This, together with a small guard band, adds another 0.25 MHz to the
channel width, so that a practical figure for the channel bandwidth would be 11.25 MHz. This
is illustrated in Fig. 4.2.
Amplitude
Total channel
c
width = 11.25 MHz
P
5 5 MHz
5.5
M z
5 5 MHz
5.5
M z
S
Guard band
0.25 MHz
Picture carrier
Lower sideband (LSB)
5.5 5
4
3
2
1
Attenuation slope
Upper sideband (USB)
0
1
2
3
4
5
5.5
f(MHz)
5.75
Frequency relative to picture carrier
Fig. 4.2 Total channel bandwidth using double sideband picture signal.
P is picture carrier and S is sound carrier.
Such a bandwidth is too large, and if used, would limit the number of channels in a
given high frequency spectrum allocated for TV transmission. Therefore, to ensure spectrum
conservation, some saving in the bandwidth allotted to each channel is desirable.
Single sideband transmission (SSB). A careful look at eqn. (4.2) reveals that the carrier
component conveys no information because its amplitude and frequency remain constant no
matter what the amplitude of the modulating voltage is. However, the presence of the carrier
frequency is necessary at the receiver for recovering the modulating frequency fm, from the
upper sideband by taking (fc + fm) – fc or from the lower sideband by taking fc – (fc – fm).
Therefore, though superfluous from the point of view of transmission of intelligence, the carrier
frequency is radiated along with the sideband components in all radio-broadcast and TV systems.
Such an arrangement results in simpler transmitting equipment and needs a very simple and
inexpensive diode detector at the receiver for recovering the modulation components without
undue distortion.
From eqn. (4.2) it is also obvious that the two sidebands are images of each other, since
each is equally affected by changes in the modulating voltage amplitude via the component
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57
SIGNAL TRANSMISSION AND CHANNEL BANDWIDTH
mEc
. Also, any change in the frequency of the modulating signal results in identical changes
2
in the band spread of the two sidebands. It is seen, therefore, that all the information can be
conveyed by the use of one sideband only and this results in a saving of 5 MHz per channel. It
may, however, be noted that the magnitude of the detected signal in the receiver will be just
half of that obtained when both the sidebands are transmitted. This is no serious drawback
because the IF (intermediate frequency) amplifier stages of the receiver provide enough gain
to develop reasonable amplitude of the video signal at the output of video detector.
4.3 VESTIGIAL SIDEBAND TRANSMISSION
In the video signal very low frequency modulating components exist along with the rest of the
signal. These components give rise to sidebands very close to the carrier frequency which are
difficult to remove by physically realizable filters. Thus it is not possible to go to the extreme
and fully suppress one complete sideband in the case of television signals. The low video
frequencies contain the most important information of the picture and any effort to completely
suppress the lower sideband would result in objectionable phase distortion at these frequencies.
This distortion will be seen by the eye as ‘smear’ in the reproduced picture. Therefore, as a
compromise, only a part of the lower sideband, is suppressed, and the radiated signal then
consists of a full upper sideband together with the carrier, and the vestige (remaining part) of
the partially suppressed lower sideband. This pattern of transmission of the modulated signal
is known as vestigial sideband or A5C transmission. In the 625 line system, frquencies up to
0.75 MHz in the lower sideband are fully radiated. The net result is a normal double sideband
transmission for the lower video frequencies corresponding to the main body of picture
information.
As stated earlier, because of fillter design difficulties it is not possible to terminate the
bandwidth of a signal abruptly at the edges of the sidebands. Therefore, an attenuation slope
covering approximately 0.5 MHz is allowed at either end. Any distortion at the higher frequency
end, if attenuation slope were not allowed, would mean a serious loss in horizontal detail,
since the high frequency components of the video modulation determine the amount of horizontal
detail in the picture. Fig. 4.3 illustrates the saving of band space which results from vestigial
sideband transmission. The picture signal is seen to occupy a bandwidth of 6.75 MHz instead
to 11 MHz.
4.25 MHz
Saving
Sav g in band
ba d space
s a e
To a channel
Total
chan e width
w dth = 7 MHz
P
S
5.5 MHz
1.25 MHz
0.25 MHz
Amplitude
0.5 MHz
Guard edge
Part of LSB removed
by filter
5.5 5
4
2
0.75
MHz
LSB
LS
.75
0
Full USB
2
4
1.25
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f(MHz)
5.5
5.75
Fig. 4.3 Total channel bandwidth using vestigial’ lower sideband.
58
MONOCHROME AND COLOUR TELEVISION
4.4 TRASMISSION EFFICIENCY
Though the total power that is developed and radiated at the transmitter has no direct bearing
on bandwidth requirements, the saving in power that can be effected by suppressing the carrier
and one of the sidebands cannot be totally ignored. This can be demonstrated by considering
the power relations in the modulated wave. Based on eqn. (4.2) the total power Pt, in the
modulated wave is the sum of the carrier power Pc, and the power in the two sidebands. This
can be expressed as
Pt = Pc + PUSB + PLSB =
where
Ec
2
Ec2 m2 Ec2 m 2 Ec2
+
+
2R
8R
8R
...(4.3)
is the r.m.s. value of the sinusoidal carrier wave, and R is the resistance in which
the power is dissipated. Equation (4.3) can be simplified to read as
Pt = Pc +
F
GH
m2
m2
m2
Pc = Pc 1 +
Pc +
4
4
2
I
JK
...(4.4)
Note from the above expression that Pc remains constant but Pt depends on the value of
the modulation index m. Also note that when several frequency components of different
amplitudes modulate the carrier wave, which in fact is the rule rather than an exception, the
carrier power Pt is unaffected but the total sideband power gets distributed in the individual
sideband component powers. This is so because the total modulating voltage is equal to the
square root of the sum of the squares of individual modulating voltages.
It can be seen from eqn. (4.4), that at 100% modulation (m = 1) the transmitted power
attains its maximum possible value. Pt(max) = 1.5 Pc, where the power contained in the two
sidebands has a maximum value of 50% of the carrier power. It is clear then, that the carrier
component that is redundent, so far as the transmission of intelligence is concerned, constitutes
about 72% of the total power that is radiated in the double sideband, full carrier (better known
as A3 modulation) AM system. Therefore, a lot of economy can be effected if the carrier power
is suppressed and not transmitted. Furthermore, suppression of one sideband results in more
economy and also halves the bandwidth requirements for transmission as compared to A3. In
practice SSB is used to save power and bandwidth in mobile communication systems, telemetry,
radio navigation, military and several other such applications. However, such a system needs
the generation of carrier frequency at the receiver for detection and this necessitates the
transmission of a low level pilot carrier along with either of the two sidebands. In addition to
this, a single sideband with suppressed carrier requires excellent frequency stability on the
part of both transmitter and receiver. Any deviation in frequency and phase of the generated
carrier at the receiver would severely impair the quality of the picture when used for television
signal transmission. Such difficulties are not unsurmountable, but this tends to make the
receiver circuitry more complicated, which in turn adds to the cost of the receiver. In point to
point communication systems, where only one receiver is necessary, the additional expense is
justifiable and infact SSB is now the accepted mode of communication for such applications.
However, in television and radio broadcast systems, where a very large number of receivers
simultaneously receive programme from one transmitter, additional cost of receivers is not
justified and as such SSB cannot be recmmended. Therefore, as stated earlier, in all TV systems,
full carrier is radiated and vestigial sideband transmission is used. In radio broadcast where
the channel bandwidth is only 100 kHz, both the sidebands are transmitted along with full
carrier.
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59
SIGNAL TRANSMISSION AND CHANNEL BANDWIDTH
4.5 COMPLETE CHANNEL BANDWIDTH
The sound carrier is always positioned at the extremity of the fully radiated upper sideband
and hence is 5.5 MHz away from the picture carrier. This is its logical place since it makes for
minimum interference between the two signals. The FM sound signal occupies a frequency
spectrum of about ± 75 KHz around the sound carrier. However, a guard band of 0.25 MHz is
allowed on the sound carrier side of the television channel to allow for adequate inter-channel
separation. The total channel bandwidth thus occupies 7 MHz and this represents a bandspace
saving of 4.25 MHz per channel, when compared with the 11.25 MHz space, which would be
required by the corresponding double sideband signal. Figure 4.4 show the complete channel.
The frequency axis is scaled ralative to the picture carrier, which is marked as 0 MHz. This
makes the diagram very informative, since details such as the widths of the upper and lower
sidebands and the relative position of the sound carrier are easily read off.
Total channel width = 7 MHz
5 5 MHz
5.5
Amplitude
1.25
C
P
s
0.25 MHz
4.433 M
MHz
z
Sound signal
sidebands 150 KHz
Colour subcarrier
.75
0
1
2
3
4
Upper picture sideband
s
f(MHz)
5 5.5
0.75 MHz
1.25 MHz
Fig. 4.4 C.C.I.R. (Indian and European) TV channel sideband spectrum.
C is colour subcarrier frequency.
Fig. 4.5 (a) show television channel details of the British 625 line system, where the
highest modulating frequency employed is 5.5 MHz and the lower sideband up to 1.25 MHz
Tota channel
Total
chan e width
w th = 8 MHz
M z
1.25
MHz
0.5 MHz
0.25 MHz
6 MHz
M z
4.433 MHz
Amplitude
P
S
C
150 KHz
Sound signal
spectrum
Full upper sideband
Vestigial LSB
1
1.25
0
1
2
3
4
Upper picture sideband
s de a d
5.5 MHz
Lower Picture sideband
1.75 MHz
Guard edge
5
5.5
6
0.75
MHz
Fig. 4.5(a) U.K. TV channel standards.
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f(MHz)
6.25
60
Amplitude
MONOCHROME AND COLOUR TELEVISION
Tota channel
Total
channe width
w th = 6 MHz
M z
4.5 MHz
C
1.25 P
3.58 MHz
MHz
S
.75
1.25
0
1
2
3
0.25 MHz
Sound signal
spectrum
75 KHz
f(MHz)
4
4.5 4.75
Fig. 4.5(b) American TV channel standards.
is allowed to be radiated. The total bandwidth per channel is 8 MHz. Fig. 4.5 (b) illustrates
channel details of 525 line American system, where the highest allowed modulating frequency
is 4 MHz with a total bandwidth of 6 MHz. In the French 819 line system where the highest
modulating frequency is 10.4 MHz a channel bandwidth equal to 14 MHz is allowed. The
diagram in Fig. 4.6 shows how two adjacent C.C.I.R. 625 line channels in the VHF Band-I are
disposed one after the other.
Amplitude
P = 55.25
MHz
54
55
56
S = 60.75 P = 62.25
MHz
MHz
S = 67.75
MHz
5.5 MHz
5.5 MHz
Band I
Channel III
Band I
Channel IV
57
58
54 to 61
(7 MHz)
59
60
61
62
63
64
65
66
67
68
f(MHz)
61 to 68
(7 MHz)
Fig. 4.6 Sideband spectrum of two adjacent channels of the lower VHF
band of television station allocations.
4.6 RECEPTION OF VESTIGIAL SIDEBAND SIGNALS
In principle an SSB signal with carrier cannot be demodulated by an envelope detector. Either
synchronous demodulation or a square law device to produce effective multiplication of the
carrier with the sideband is required. However, it can be shown that if the sideband amplitude
is small compared to the carrier, then the envelope of the SSB with carrier signal nearly
corresponds to the modulating signal. In that case, envelope detection can be used and is the
normal practice in television receivers. With vestigial sideband however, the relative amplitude
of the frequencies for which both sidebands exist is double that of the true SSB component at
the envelope detector output. In the video signal it would be so for the low frequency content of
the picture signal, and in effect, amounts to distortion in terms of relative amplitudes for
different frequencies and needs correction at the receiver.
This when expressed in another way means that if the picture carrier were successively
modulated to an equal depth by a series of frequencies throughout the video frequency range
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SIGNAL TRANSMISSION AND CHANNEL BANDWIDTH
Relative video detector
output voltage
employed by the system, and the resulting voltage output from the detector recorded, the
output voltage against input frequency characteristic obtained would have the form shown
in Fig. 4.7. The vestigial sideband extends to 0.75 MHz below the carrier and thereafter this
sideband is linearly attenuated down to zero at 1.25 MHz. The detector output voltage would
thus be twice as great between 0 Hz and 0.75 MHz than between 1.25 MHz to 5 MHz.
2.0
Output without receiver
response correction
1.0
Output
ut
when receiver
response
p ns has the correct form
0
1
2
0.75 1.25
3
4
5 5.5
f(MHz)
Video modulating frequency
Fig. 4.7 Receiver video detector output vs modulating frequency characteristics
illustrating the need for specially shaped receiver IF response curves.
Relative transmitter
output
v0
7 MHz
P
5 MHz
1.25
0
.75
v0
1
.92
1
3
4
5
5.75
f(MHz)
5.5
b
Picture carrier
.6
.5
.4
a+b=1
a + b = 1
a
.2
.08
2
(a)
b
.8
Relative receiver output
S
a
0
1.25
.75
.7
.5
1
.7
2
3
4
5 5.75
f(MHz)
(b)
Fig. 4.8 Ideal characteristics of a TV transmitter and receiver. (a) transmitter output
characteristics for vestigial sideband signals. (b) desired receiver characteristics
for correct reproduction of video signals. Note that the picture carrier is
positioned half-way down the response curve.
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MONOCHROME AND COLOUR TELEVISION
Between 0.75 MHz and 1.25 MHz the output voltage would fall linearly following the sideband
attenuation slope of the transmitter. To correct this discrepancy, it is necessary to so shape
the receiver response curve, that the frequencies present ‘twice’ are afforded less amplification
than those occurring in one sideband only. The desired response is shown in Fig. 4.8. The
response curve is shaped to place the picture carrier half-way down the side corresponding to
the suppressed sideband. The width of the sloping edge on which the carrier is positioned is
twice the width of the vestigial sideband. To understand how this achieves the desired result,
refer to Fig. 4.8 and consider the treatment afforded to various frequencies within the video
bandwidth. Frequencies between 5 MHz and 0.75 MHz i.e., those present in the upper sideband
only, are seen to give unit output. Next, consider a frequency component at 0.5 MHz. This is
present in both the sidebands. The total detector output is again unity. The component in the
lower sideband gives rise to an output of a volts, while that in the upper sideband gives rise to
b volts. From the geometry of the figure we see that (a + b) = 1. As a further example consider
the response at 0.7 MHz. This component in the vestigial sideband gives rise to an output =
0.08 V, whilst in the upper sideband, it gives rise to 0.92 V. Again the sum of the two is unity,
so that the same output is achieved for frequencies between 0.75 MHz and 5 MHz. Note that at
0.75 MHz the output in the vestigial sideband is zero, and that in the upper sideband it is
equal to one. The necessary correction detailed above is carried out at the Intermediate
Frequency (IF) amplifier stages of the television receiver by suitably shaping the passband
characteristics of the tuned amplifiers. This matter is fully dealt with in Chapter 8.
Demerits of Vestigial Sideband Transmission
(a) A small portion of the transmitter power is wasted in the vestigial sideband filters
which remove the remaining lower sideband.
(b) The attenuation slope of the receiver to correct the boost at lower video frequencies
places the carrier at 50 per cent output voltage which amounts to introducing a loss
of about 6 db in the signal to noise voltage ratio relative to what be available if double
sideband transmission is used.
(c) Some phase and amplitude distortion of the picture signal occurs despite careful filter design at the transmitter. Also, it is very difficult to tune IF stages of the receiver
to correspond exactly with the ideal desired response as shown in Fig. 4.8 and this too
introduces some phase and amplitude distortion.
(d) More critical tuning at the receiver becomes necessary because for a given amount of
local oscillator mismatch or drift after initial tuning, the degeneration of picture quality
is less with wider lower sideband than with narrow lower sideband. In this respect
the British 625 line system is superior because it allows 1.25 MHz unattenuated
lower sideband transmission as compared to 0.75 MHz in most other systems.
Despite these demerits of vestigial sideband transmission it is used in all television
systems because of the large saving it effects in the bandwidth required for each channel.
4.7 FREQUENCY MODULATION
The sound signal is frequency modulated because of its inherent merits of interference-free
reception. Here the amplitude of the modulated carrier remains constant, whereas its frequency
is varied in accordance with variations in the modulating signal. The variation in carrier
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SIGNAL TRANSMISSION AND CHANNEL BANDWIDTH
frequency is made proportional to the instantaneous value of the modulating voltage. The rate
at which this frequency variation takes place is equal to the modulating frequency. It is assumed
that the phase relations of a complex modulating signal will be preserved. However, for
simplicity, it is again assumed that the modulating signal is sinusoidal. The situation is
illustrated in Fig. 4.9 which shown the modulating voltage, and the resulting frequency
modulated wave. Fig. 4.9 also shows the frequency variation with time, which is seen to be
identical to the variations with time of the modulating voltage.
+v
t
0
–v
Modulating signal
+ v¢
t
0
– v¢
Frequency modulation
(deviation in frequency greatly exaggerated)
+f
d
t
0
–f
Frequency vs time in FM
Fig. 4.9 Basic FM modulation waveforms.
Analysis of Frequency-Modulated (FM) Wave. In order to understand clearly the meaning
of instantaneous frequency fi and the associated instan-taneous angular velocity ωi = 2πfωi,
the equation of an ac wave in the generalized form may first be written as :
e = A sin φ(t)
where e = instantaneous amplitude
A = peak amplitude
φ(t) = total angular displacement at time t.
The instantaneous angular velocity ωt is, by definition, the instantaneous rate of change
dφ(t)
of angular displacement φ(t).
dt
Thus
ωt =
dφ(t)
dt
...(4.5)
A sinusoidal wave of constant frequency say fc(ωc = 2πfc) is a special case of eqn. (4.5) and then
φ(t) = ωct + θ where θ is the angular position at t = 0. Application of eqn. (4.5) yields the result
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MONOCHROME AND COLOUR TELEVISION
ωt =
dφ(t)
= ωc
dt
A frequency modulated wave with sinusoidal modulation can now be expressed as:
ωi = ωc + 2π∆f cos ωmt
...(4.6)
where ωi = instantaneous angular velocity
ωc = angular velocity of carrier wave
(average angular velocity).
ωm = 2π times the modulating frequency fm.
∆f = maximum deviation of instantaneous frequency from the average value.
It may be emphasized that the frequency deviation ∆f is proportional to the peak
amplitude (cos ωm t = ± 1) of the modulating signal and is independent of the modulating
frequency.
The equation of the FM wave can now be obtained by combining eqn. (4.5) and (4.6)
to give the value of φ(t). The steps involved are as follows :
ωi =
dφ(t)
= ωc + 2π∆f cos ωm t
dt
Integration gives :
φ(t) = ωc t +
FG 2π∆f IJ sin ω
Hω K
mt
m
+θ
where the constant of integration θ defines the angular position at time t = 0.
Substituting the above value of φ(t) into the generalized form e = A sin φ(t) yields :
FG
H
e = A sin ω c t +
2π∆f
sin ω m t
ωm
IJ
K
...(4.7)
where for the sake of simplicity angle θ has been assumed to be equal to zero.
Equation (4.7) is commonly written in the form
e = A sin (ωct + mf sin ωmt)
...(4.8)
where mf is termed the ‘modulation index’ of the FM wave and is defined as :
mf = modulation index =
∆f
frequency deviation
=
fm
modulating frequency
It may be noted that for a given frequency deviation, the modulation index varies inversely
as the modulating frequency. Also mf is defined only for sinusoidal modulation unlike m of AM
which is defined for any modulating signal.
Frequency Spectrum of the FM Wave. The eqn. (4.8) is of the form, sine of a sine, and can
be expressed as :
e = A [(sin ωc t cos (mf sin ωm t) + cos ωc t sin (mf sin ωm t)]
The term cos (mf sin ωm t) can be expanded into
J0(mf) + 2J2(mf) cos 2ωm t + 2J4(mf) cos 4ωm t + ......
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...(4.9)
65
SIGNAL TRANSMISSION AND CHANNEL BANDWIDTH
and sin (mf sin ωm t) into :
2J1 (mf) sin ωm t + 2J3(mf) sin 3ωm t + ......
Substitution of these results into eqn. (4.9) and some manipulation yields :
e = A{J0(mf) sin ωc t
+ J1(mf)[sin (ωc + ωm)t – sin (ωc – ωm)t]
+ J2(mf)[sin (ωc + 2ωm)t – sin (ωc – 2ωm)t]
+ J3(mf)[sin (ωc + 3ωm)t – sin (ωc – 3ωm)t] + ......
+ Jn(mf) ..........} + ......
...(4.10)
where Jn* (mf) are Bessel functions of the first kind and nth order with argument mf.
The final expression obtained in eqn. (4.10) yields the following information :
(a) FM has infinite number of sidebands besides the carrier. The sidebands are separated
from the carrier by integer multiples of fm.
(b) For a given mf , Jn coefficients eventually decrease to negligible values as n increases
and the values for different n may be positive, negative or zero.
(c) The sidebands on either side of the carrier at equal distance from fc have equal
amplitudes so that the sideband distribution is symmetrical about the carrier
frequency.
(d) For a given modulating frequency, increase in the amplitude of the modulating signal
results in an increase of ∆f and therefore of mf causing larger number of sidebands to
acquire significant amplitudes. Thus higher amplitude signals would need more
sidebands for transmission without distortion. However, the total transmitted power
stays constant.
(e) The way the number of significant J coefficients increase with mf is illustrated in the
table below.
Table 1. Bessel Functions of the 1st Kind
X
(mf )
n or order
J0
J1
J2
J3
J4
J5
J6
J7
J8
0.00
1.0
—
—
—
—
—
—
—
—
0.25
0.98
0.12
—
—
—
—
—
—
—
0.5
0.94
0.24
0.03
—
—
—
—
—
—
1.0
0.77
0.44
0.11
0.02
—
—
—
—
—
2.0
0.22
0.58
0.35
0.13
0.03
—
—
—
—
3.0
– 0.26
0.34
0.49
0.31
0.13
0.04
—
—
—
4.0
– 0.40
– 0.07
0.36
0.43
0.28
0.13
0.05
—
—
5.0
– 0.18
– 0.33
0.05
0.36
0.39
0.26
0.13
0.05
—
6.0
0.15
– 0.28
– 0.24
0.11
0.36
0.36
0.25
0.13
0.06
7.0
0.30
0.00
– 0.30
– 0.17
0.16
0.55
0.34
0.23
0.13
*Theory of Bessel’s functions is not necessary for us. Tabulated values of Bessel’s function are
widely available.
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MONOCHROME AND COLOUR TELEVISION
(f) As seen in the table when the modulation index (mf) is less than 0.5, i.e., when the
frequency deviation is less than half the mdoulating frequency, the second and higherorder sideband components are relatively small and the frequency band required to
acommodate the essential part of the signal is the same as in amplitude modulation.
On the other hand when mf exceeds unity, there are important higher-order sideband
components contained in the wave and this results in increased bandwidth requirements.
(g) The modulation index actually depends on both the amplitude and frequency of the
modulating tone. It is higher in FM systems that permit large frequency deviation for
a maximum amplitude of the modulating tone. This is turn results in higher order
significant J coefficients and a larger bandwidth is required for reasonably distortion
free transmission.
(h) Since a lot of the higher sidebands have insignificant relative amplitudes, their exclusion will not distort the modulated wave unduly, and while calculating channel
bandwidth J coefficients having values less than 0.05 for a calculated value of mf can
be neglected.
4.8 FM CHANNEL BANDWIDTH
Based on the above discussion the channel bandwidth
BW = 2nfm
...(4.11)
where fm is the frequency of the modulating wave and n is the number of the significant sidefrequency components. The value of n is determined from the modulation index.
Though the higher frequencies in speech or music have much less amplitude as compared
to lower audio frequencies, we shall estimate the channel bandwidth for the worst case where
even the highest frequency to be transmitted causes maximum permitted frequency deviation.
The maximum frequency deviation of commercial FM is limited to 75 kHz, and the
modulating frequencies typically cover 25 Hz to 15 kHz.
If a 15 kHz tone has unit amplitude, i.e., equal to the maximum allowed amplitude, then
mf =
75
= 5. From the Bessel function table, for mf = 5, the significant (0.05) value of Jn = 7,
15
i.e., n = 7. Therefore
BW = 2 × 7 × 15 = 210 kHz
Had the amplitude been less, the maximum frequency deviation would not be developed,
and the bandwidth would be smaller. This brings out an interesting observation that in
frequency modulation (with fixed ∆f) the bandwidth depends on the tone amplitude, whereas
in amplitude modulation, the bandwidth depends on the tone frequency.
Similarly in the 625-B television system where the standards specify that the maximum deviation (∆f) should not exceed ± 50 kHz for the highest modulating frequency of 15
kHz, mf =
50
≈3
15
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SIGNAL TRANSMISSION AND CHANNEL BANDWIDTH
This gives a value of n = 5 as seen in the given chart.
BW = 2 × 5 × 15 = 150 kHz.
∴
The bandwidth can also be estimated from ‘Carson’s Rule’ which states that to a good
approximation, the bandwidth required to pass an FM wave is equal to twice the sum of the
deviation and the highest modulating frequency. Thus, for the standard FM transmission the
required bandwidth = 2(75 + 15) = 180 kHz. This nearly checks with the value of *210 kHz
estimated earlier.
Similarly for the 625 line system the Carson’s Rule yields a bandwidth requirement of
2(50 + 15) = 130 kHz and this is close to the value calculated earlier. The resultant deviation of
± 75 kHz around the sound carrier is very much within the guard-band edge and reasonably
away from any significant video sideband components.
It may be noted that in the American television system where the maximum permissible
deviation is ± 25 kHz around the sound carrier, a bandwidth of about 100 kHz is enough for
sound signal transmission.
4.9 CHANNEL BANDWIDTH FOR COLOUR TRANSMISSION
As explained in the chapter devoted to the analysis and synthesis of TV pictures the colour
video signal does not extend beyond about 1.5 MHz. Therefore, the colour information can be
transmitted with a restricted bandwidth much less than 5 MHz. This feature allows the narrow
band chrominance (colour) signal to be multiplexed with the wideband luminance (brightness)
signal in the standard 7 MHz television channel. This is achieved by modulating the colour
signal with a carrier frequency which lies within the normal channel bandwidth. This is called
colour subcarrier frequency and is located towards the upper edge of the video frequencies to
avoid interference with the monochrome signal.
7 MHz
Amplitude
P
1.25 0.75
C
1
2
Sound signal
spectrum
Colourr s
signal
spectrum
tr
Video signal spectrum
0
S
3
4
5
4 433 M
4.433
MHz
z
5.5 MHz
5.5
f(MHz)
(relative to picture
carrier)
5.75
Fig. 4.10 C.C.I.R. 625 lines monochrome and the compatible PAL Colour channel bandwidth details.
In the PAL colour system which is compatible with the C.C.I.R. 625 line monochrome
system the colour subcarrier frequency is located 4.433 MHz way from the picture carrier. The
*In commercial FM broadcast a frequency spectrum of 200 kHz is allotted for each channel.
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MONOCHROME AND COLOUR TELEVISION
bandwidth of colour signals is restricted to about ± 1.2 MHz around the subcarrier. Fig. 4.10
gives necessary details of the location of monochrome (picture), colour and sound signal
spectrums all within the same channel bandwidth of 7 MHz. It may be noted that in the
American television system where the channel bandwidth is 6 MHz, the colour subcarrier is
located 3.58 MHz away from the picture carrier.
4.10 ALLOCATION OF FREQUENCY BANDS FOR TELEVISION SIGNAL
TRANSMISSION
For effective amplitude modulation and better selectivity at the RF and IF tuned amplifiers in
the receiver, it is essential that the carrier frequency be chosen about ten times that of the
highest modulating frequency. Since the highest modulating frequency for picture signal
transmission is 5 MHz, the minimum carrier frequency that can be employed, cannot be much
less than 40 MHz. As an illustration consider a carrier frequency fc = 10 MHz. With the highest
video modulating frequency = 5 MHz, a deviation of 50 per cent from the centre frequency
would be necessary in any tuned circuit to accommodate the lower and upper sideband
frequencies. However, if the carrier frequency is fixed at, say 50 MHz, the percentage deviation
required to pass the upper and lower sideband frequencies for the same modulating frequency
would be only 10 per cent. It is obvious from these observations that selectivity is bound to be
poor at the receiver tuned amplifiers with a carrier frequency of 10 MHz. The 3 db down points
with a carrier frequency of 50 MHz are within 5 per cent deviation from the carrier frequency
and thus the selectivity is bound to be much better. Further, each television channel occupies
about 7 MHz. In order to accommodate several TV channels, the carrier frequencies have to be
in the region of the spectrum above about 40 MHz. This explains why television transmission
has to be carried out at very high frequencies in the VHF and UHF bands. In radio broadcast
where the highest modulating frequency is only 5 kHz, lower carrier frequencies can be used,
and accordingly transmission is carried out in the medium wave band (550 kHz to 1600 kHz)
and short wave bands extending up to about 30 MHz. Transmission at very high frequencies
has its own problems and limitations for long distance transmission and these are discussed in
another chapter.
4.11 TELEVISION STANDARDS
After having learnt about various aspects of television transmission and reception, it would be
instructive to review, in detail, the picture and sound signal standards as specified by the
International Radio Consulative Committee (C.C.I.R) for the 625-B monochrome system and
also to compare its main characteristics with that of other principal television system. This is
detailed in Tables 2 and 3.
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SIGNAL TRANSMISSION AND CHANNEL BANDWIDTH
Table 2. Television Signal Standards
Vision and Sound Signal Standards for the 625-B Monochrome System Adopted by India as
Recommended by the International Radio Consultative Committee (C.C.I.R.)
Characteristics of the 625-B Monochrome TV System
No. of lines per picture (frame)
625
Field frequency (Fields/second)
50
Interlace ratio, i.e., No. of fields/picture
2/1
Picture (frame) frequency, i.e., Pictures/second
25
Line frequency and tolerance in lines/second,
15625 ± 0.1%
(when operated non-synchronously)
Aspect Ratio (width/height)
4/3
Scanning sequence
(i) Line : Left to right
(ii) Field : Top to bottom
System capable of operating independently of power
supply frequency
YES
Approximate gamma of picture signal
0.5
Nominal video bandwidth, i.e., highest video modulating frequency (MHz)
5
Nominal Radio frequency bandwidth, i.e., channel bandwidth (MHz)
7
Sound carrier relative to vision carrier (MHz)
+ 5.5
Sound carrier relative to nearest edge of channel (MHz)
– 0.25
Nearest edge of channel relative to picture carrier (MHz)
– 1.25
Fully radiated sideband
Upper
Nominal width of main sideband (upper) (MHz)
5
Width of end-slope of full (Main) sideband (MHz)
0.5
Nominal width of vestigial sideband (MHz)
0.75
Vestigial (attenuated) sideband
Lower
Min : attenuation of vestigial sideband in db,
(below the ideal demodulated curve)
(at 1.25 MHz) 20 db
(at 4.43 MHz) 30 db
Width of end-slope of attenuated (vestigial) sideband (MHz)
Type and polarity of vision modulation
0.5
(A5C) Negative
Synchronizing level as a percentage of peak carrier
Blanking level as percentage of peak carrier
Difference between black and blanking level as a percentage of peak carrier
Peak white level as a percentage of peak carrier
Type of sound modulation
100
72.5 to 77.5
0 to 7
10 to 12.5
FM, ± 50 KHz
Pre-emphasis
50 µs
Resolution
400 max
Ratio of effective radiated powers of vision and sound
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MONOCHROME AND COLOUR TELEVISION
The values to be considered are : (i) the r.m.s. value of the carrier at the peak of modulation
envelope for the vision signal. (ii) the r.m.s. value of the unmodulated carrier for amplitude
modulated and frequency modulated sound transmissions.
Details of Line-Blanking Intervals
Nominal duration of a horizontal line
= 64 µs = H
Line blanking interval
= 12 ± 0.3 µs
Front porch
= 1.5 ± 0.3 µs
Sync pulse width
= 4.7 ± 0.2 µs
Back porch
= 5.8 ± 0.3 µs
Build up time (10% to 90%) of line blanking edges
= 0.3 ± 0.1 µs
Interval between datum level and black edge of line blanking
signal (average calculated time for information)
= 10.5 µs
Details of Field-Blanking Intervals
Field blanking period = 20 H (20 lines)
= 1280 µs
Pre sync equalizing pulses, 5 pulses of duration 1/2 H, i.e.,
32 µs, total time
= 160 µs
Equalizing pulses are narrow pulses with pulse width
Field sync pulses at
1
2
= 2.35 ± 0.1 µs
H intervals,
5 such pulse, each pulsewidth
= 27.3 µs
Interval between field sync pulses
= 4.7 ± 0.2 µs
Interval between equalizing pulses
= 29.65 µs
Post-sync equalizing pulses, 5 pulses
same as for presync. eq. pulses
Build up time of field blanking edges.
= 0.3 ± 0.1 µs
Build up time for field sync pulses
= 0.2 ± 0.1 µs
Table 3. Principal Television System
Particulars
Western Europe,
North and South
Middle East,
America including England
India and most US, Canada, Mexico
Asian countries
and Japan
USSR
France
Lines per frame
625
525
625
625
625
Frames per second
25
30
25
25
25
Field frequency (Hz)
50
60
50
50
50
Line frequency (Hz)
15,625
15,750
15,625
15,625
15,625
Video bandwidth (MHz)
5 ot 6
4.2
5.5
6
6
(Contd.) ...
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SIGNAL TRANSMISSION AND CHANNEL BANDWIDTH
Channel bandwidth (MHz)
7 or 8
6
8
8
8
Video modulation
Negative
Negative
Negative
Negative
Positive
Picture modulation
AM
AM
AM
AM
FM
Sound signal modulation
FM
FM
FM
FM
AM
Colour system
PAL
NTSC
PAL
SECAM
SECAM
(i) England earlier used 405 line system in the 5 MHz channel.
(ii) France earlier used 819 line system with a channel bandwidth of 14 MHz.
Review Questions
1.
Why is it necessary to mdoulate the picture and sound signals before transmission ?
Why is TV transmission carried out in the UHF and VHF bands ?
2.
Show that in the 625-B system, a total channel bandwidth of 11.25 MHz would be necessary if
both the sidebands of the amplitude mdoulated picture signal are fully radiated along with the
frequency modulated picture signal.
3.
Why is an attenuation slope of 0.5 MHz allowed at both the edges of the AM picture signal
sidebands ? Why is a guard band provided at the sound signal edge of the television channel ?
4.
Why is it necessary to affect economy in channel bandwidth ? Why SSB is not used for picture
signal transmission ?
5.
What is vestigial sideband transmission and why it is used for transmission of TV picture signals ?
6.
Why is a portion of the lower sideband of the AM picture signal transmitted along with the
carrier and full USB ? Does it need any correction somewhere in the television link ? If so where
is it carried out ?
7.
Sketch and fully label the desired response of a TV receiver that includes necessary correction on
account of the discrepancy caused by VSB transmission. Comment on the response curve drawn
by you.
8.
Show that a total channel bandwidth of 7 MHz is necessary for successful transmission of both
picture and sound signals in the 625 line TV system. Sketch frequency distribution of the channel and mark the location of picture and sound signal carrier frequencies. Why is the sound
carrier located 5.5 MHz away from the picture carrier ?
9.
Justify the allocation of 8 MHz in the British TV system and 6 MHz in the American system for
each TV channel. What is the separation between picture and sound carriers in each of these
systems ?
10. What is ‘modulation index’ in FM transmission and how does it affect the bandwidth required
for each FM channel ?
11. Explain how you would proceed to determine the channel bandwidth for transmission of sound
signals (highest modulating frequency = 15 kHz) by frequency modulation. How does the permitted
maximum deviation affect the bandwidth requirements ?
12. Show that in the 625-B system where the maximum allowed frequency deviation is ± 50 kHz, a
bandwidth of 150 kHz is necessary for almost distortion free transmission by frequency modulation, the highest modulating frequency being 15 kHz, Repeat this for the American system where
the maximum allowed deviation is ± 25 kHz. Verify the results by ‘Carson’s Rule’ of determining
channel bandwidth.
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5
The Picture Tube
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5
The Picture Tube
The picture tube or ‘kinescope’ that serves as the screen for a television receiver is a specialized
from of cathode-ray tube. It consists of an evacuated glass bulb or envelope, inside the neck of
which is rigidly supported an electron gun that supplies the electron beam. A luminescent
phosphor coating provided on the inner surface of its face plate produces light when hit by the
electrons of the fast moving beam.
A monochrome picture tube has one electron gun and a continuous phosphor coating
that produces a picture in black and white. For colour picture tubes the screen is formed of
three different phosphors and there are three electron beams, one for each colour phosphor.
The three colours—red, green and blue produced by three phosphors combine to produce
different colours. More details of colour picture tubes are given in chapters devoted to colour
television.
5.1
MONOCHROME PICTURE TUBE
Modern monochrome picture tubes employ electrostatic focussing and electromagnetic
deflection. A typical black and white picture tube is shown in Fig. 5.1. The deflection coils are
Neck
Bell
Tension band
Screen (face plate)
Envelope or bulb
Fig. 5.1. A rectangular picture tube.
mounted externally in a specially designed yoke that is fixed close to the neck of the tube. The
coils when fed simultaneously with vertical and horizontal scanning currents deflect the beam
at a fast rate to produce the raster. The composite video signal that is injected either at the
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75
THE PICTURE TUBE
grid or cathode of the tube, modulates the electron beam to produce brightness variations of
the tube, modulates the electron beam to produce brightness variations on the screen. This
results in reconstruction of the picture on the raster, bit by bit, as a function of time. However,
the information thus obtained on the screen is perceived by the eye as a complete and continuous
scene because of the rapid rate of scanning.
Electron Gun
The various electrodes that constitute the electron gun are shown in Fig. 5.2. The cathode is
indirectly heated and consists of a cylinder of nickel that is coated at its end with thoriated
tungsten or barium and strontium oxides. These emitting materials have low work-function
External
conductive coating
Deflection windings
Deflection yoke
1
Glass envelope
K
G2
G1
Base
Inner
aquadag
conductive
coating (18 KV)
Electrostatic
focusing
G3
Glass face
plate
S
P
Electron beam
Control grid
Accelerating anode
Final anode
18 KV
2
Phosphor
coating
Aluminized
coating
Focusing anode
Centering magnets (two)
Pincushion error magnets (two)
HV connector
+
EHT
18 KV
Fig. 5.2. Elements of a picture tube employing low voltage
electrostatic focusing and magnetic deflection.
and when heated permit release of sufficient electrons to form the necessary stream of electrons
within the tube. The control grid (Grid No. 1) is maintained at a negative potential with respect
to cathode and controls the flow of electrons from the cathode. However, instead of a wiremesh
structure, as in a conventional amplifier tube, it is a cylinder with a small circular opening to
confine the electron stream to a small area. The grids that follow the control grid are the
accelerating or screen grid (Grid No. 2) and the focusing grid (Grid No. 3). These are maintained
at different positive potentials with respect to the cathode that vary between + 200 V to + 600 V.
All the elements of the electron gun are connected to the base pins and receive their rated
voltages from the tube socket that is wired to the various sections of the receiver.
Electrostatic Focussing
The electric field due to the positive potential at the accelerating grid (also known as 1st anode)
extends through the opening of the control grid right to the cathode surface. The orientation of
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MONOCHROME AND COLOUR TELEVISION
this field is such that besides accelerating the electrons down the tube, it also brings all the
electrons in the stream into a tiny spot called the crossover. This is known as the first
electrostatic lens action. The resultant convergence of the beam is shown in Fig. 5.2. The
second lens system that consists of the screen grid and focus electrode draws electrons from
the crossover point and brings them to a focus at the viewing screen. The focus anode is larger
in diameter and is operated at a higher potential than the first anode. The resulting field
configuration between the two anodes is such that the electrons leaving the crossover point at
various angles are subjected to both convergent and divergent forces as they more along the
axis of the tube. This in turn alters the path of the electrons in such a way that they meet at
another point on the axis. The electrode voltages are so chosen or the electric field is so varied
that the second point where all the electrons get focused is the screen of the picture tube.
Electrostatic focusing is preferred over magnetic focusing because it is not affected very much
by changes in the line voltage and needs no ion-spot correction.
Beam Velocity
In order to give the electron stream sufficient velocity to reach the screen material with proper
energy to cause it to fluoresce, a second anode is included within the tube. This is a conductive
coating with colloidal graphite on the inside of the wide bell of the tube. This coating, called
aquadag, usually extends from almost half-way into the narrow neck to within 3 cm of the
fluorescent screen as shown in Fig. 5.2. It is connected through a specially provided pin at the
top or side of the glass bell to a very high potential of over 15 kV. The exact voltage depends on
the tube size and is about 18 kV for a 48 cm monochrome tube. The electrons that get accelerated
under the influence of the high voltage anode area, attain very high velocities before they hit
the screen. Most of these electrons go straight and are not collected by the positive coating
because its circular structure provides a symmetrical accelerating field around all sides of the
beam. The kinetic energy gained by the electrons while in motion is delivered to the atoms of
the phosphor coating when the beam hits the screen. This energy is actually gained by the
outer valence electrons of the atoms and they move to higher energy levels. While recturning
to their original levels they give out energy in the form of electromagnetic radiation, the
frequency of which lies in the spectral region and is thus perceived by the eye as spots of light
of varying intensity depending on the strength of the electron beam bombarding the screen.
Because of very high velocities of the electrons which hit the screen, secondary emission
takes place. If these secondary emitted electrons are not collected, a negative space charge
gets formed near the screen which prevents the primary beam from arriving at the screen.
The conductive coating being at a very high positive potential collects the secondary emitted
electrons and thus serves the dual purpose of increasing the beam velocity and removing
unwanted secondary electrons. The path of the electron current flow is thus from cathode to
screen, to the conductive coating through the secondary emitted electrons and back to the
cathode through the high voltage supply. A typical value of beam current is about 0.6 mA with
20 kV applied at the aquadag coating.
5.2
BEAM DEFLECTION
Both electric and magnetic fields can be employed for deflecting the electron beam. However,
in television picture tubes electromagnetic deflection is preferred for the following reasons :
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THE PICTURE TUBE
(a) As already stated the electron beam must attain a very high velocity to deliver enough
energy to the atoms of the phosphor coating. Because of this the electrons of the beam remain
under the influence of the deflecting field for a very short time. This necessitates application of
high deflecting fields to achieve the desired deflection. For example with an anode voltage of
about 1 kV, as would be the case in most oscilloscopes, some 10 V would be necessary for 1 cm
deflection of the beam on the screen, whereas in a picture tube with 15 kV at the final anode,
about 7500 V would be necessary to get full deflection on a 50 cm screen. It is very difficult to
generate such high voltages at the deflection frequencies. On the other hand with magnetic
deflection it is a large current that would be necessary to achieve the same deflection. Since it
is more convenient to generate large currents than high voltages, all picture tubes employ
electromagnetic deflection.
(b) With electrostatic deflection the beam electrons gain energy. Thus larger deflection
angles tend to defocus the beam. Further, the deflection plates need to be placed further apart
as the deflection angle is made larger, thus requiring higher voltages to produce the same
deflection field. Magnetic deflection is free from both these shortcomings and much larger
deflection angles can be achieved without defocusing or nonlinearities with consequent saving
in tube length and cabinet size.
(c) For electrostatic deflection two delicate pairs of deflecting plates, are needed inside
the picture tube, whereas for magnetic deflection two pairs of deflecting coils are mounted
outside and close to the neck of the tube. Such a provision is economical and somewhat more
rugged.
Deflection Yoke
The physical placement of the two pairs of coils around the neck of the picture tube is illustrated
in Fig. 5.3 and the orientation of the magnetic fields produced by them is shown in Fig. 5.4. In
combination, the vertical and horizontal deflection coils are called the ‘Yoke’. This yoke is fixed
outside and close to the neck of the tube just before it begins to flare out (see Fig. 5.2).
Vertical raster plane
Horizontal deflection
windings
Top
p
Vertical deflection
windings
Electron
beam
H
Horz raster
plane
V
Bottom
Fig. 5.3. Cross-sectional view of a yoke showing location of vertical and
horizontal deflection windings about the neck of the picture tube.
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MONOCHROME AND COLOUR TELEVISION
H–
Horizontal
deflection coil
Neck of the picture tube
V+
V–
Magnetic
field lines
Vertical
deflection coil
Vertical
deflection coil
Electron
beam
Magnetic field of
electron beam
Horizontal
deflection coil
H+
Fig. 5.4. Horizontal and Vertical deflecting coils (pairs) around the neck of the picture tube.
Note that the location of the beam on the picture tube screen will depend on the strength
and direction of currents in the two pairs of coils. For the directions of current shown
the beam will be deflected upwards and to the left.
The magnetic field of the coils reacts with the electron beam to cause its deflection. The
horizontal deflection coil which sweeps the beam across the face of the tube from left to right is
split into two sections and mounted above and below the beam axis. The vertical deflection coil
is also split into two sections and placed left and right on the neck in order to pull the beam
gradually downward as the horizontal coils sweep the beam across the tube face. Each coil gets
its respective sweep input from the associated sweep circuits, and together they form the
raster upon which the picture information is traced. It may be noted that a perpendicular
displacement results because the magnetic field due to each coil reacts with the magnetic field
of the electron beam to produce a force that deflects the electrons at right angles to both the
beam axis and the deflection field.
Deflection Angle
This is the maximum angle through which the beam can be deflected without striking the side
of the bulb. Typical values of deflection angles are 70°, 90°, 110° and 114°. As shown in Fig. 5.5,
it is the total angle that is specified. For instance a deflection angle of 110° means the electron
beam can be deflected 55° from the centre. The advantage of a large deflection angle is that for
equal picture size the picture tube is shorter and can be installed in a smaller cabinet. However,
a large deflection angle requires more power from the deflection circuits. For this reason the
tubes are made with a narrow neck to put the deflection yoke closer to the electron beam. A
110° yoke has a smaller hole diameter (about 3 cm) compared with neck diameters for tubes
with lesser deflection angles. Different screen sizes can be filled with the same deflection
angle, because bigger tubes have larger axial lengths.
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THE PICTURE TUBE
Short
S
o p
picture
u tube
u length
110° deflection angle
Electron gun
Longer
o
p
picture
u tube
u length
55° deflection angle
Electron gun
110°
10
55° deflection angle
Point of deflection
(a)
(b)
Fig. 5.5. Effect of deflection angle on picture tube length for the same face plate size.
(a) Picture tube rated for 110° deflection angle, (b) Picture tube rated for 55° deflection angle
Note : The nominal deflection angle that is listed for picture tubes
is usually the diagonal deflection angle.
Cosine Winding
With increased deflection angles it becomes necessary to use a special type of winding to generate
uniform magnetic fields for linear deflection. In this arrangement the thickness of the deflection
winding varies as the cosine of the angle from a central reference line. Such a winding is
known as ‘Cosine winding’ and its appearance in a deflection yoke is shown in Fig. 5.3. Nearly
all present day yokes are wound in this manner to ensure linear deflection.
5.3
SCREEN PHOSPHOR
The phosphor chemicals are generally light metals such as zinc and cadmium in the form of
sulphide, sulphate, and phosphate compounds. This material is processed to produce very fine
particles which are then applied on the inside of the glass plate. As already explained the high
velocity ellectrons of the beam on hitting the phosphor excite its atoms with the result that the
corresponding spot fluoresces and emits light. The phosphorescent characteristics of the
chemicals used are such that an afterglow remains on the screen for a short time after the
beam moves away from any screen spot. This afterglow is known as persistence. Medium
persistence is desirable to increase the average brightness and to reduce flicker. However, the
persistence must be less than 1/25 second for picture tube screens so that one frame does not
persist into the next and cause blurring of objects in motion. The decay time of picture tube
phosphors is approximately 5 ms, and its persistence is referred to as P4 by the industry.
5.4
FACE PLATE
A rectangular image on a circular screen is wasteful of screen area. Therefore, all present day
picture tubes have rectangular face plates, with a breadth to height ratio of 4 : 3. A rectangular
tube with 54 cm screen means that the distance between the two diagonal points is 54
centimeters. Approximately 1.5 cm thickness provides the strength required for the large face
plate to withstand the air pressure on the evacuated glass envelope. In older receivers special
glass or plastic shields were placed in the cabinet in front of the picture tube to prevent any
glass from hitting the viewer in case of an implosion. Modern picture tubes incorporate integral
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implosion protection. There are a number of different systems in use. In one arrangement,
known as kimcode, a metal rim band (see Fig. 5.1) is held around the tube by a tension strap or
with a layer of epoxy cement. In another system called Panoply, a special faceplate is held in
front of the tube by epoxy cement. In all cases it is essential to check for implosion proofing
while replacing any picture tube.
Yoke and Centering Magnets
The yoke on all black and white tubes is positioned right up against the flare of the tube in
order to achieve complete coverge of the full screen area. If the yoke is not moved as far forward
as possible, the electron beam will strike the neck of the picture tube and cause a shadow near
the corners of the face plate. The mounting system permits positioning of the yoke against the
tube funnel and allows rotation of the yoke to ensure that horizontal lines run parallel to the
natural horizontal axis.
Electrical centering of the beam can be accomplished by supplying direct current through
the horizontal and vertical deflection coils. However, this method is not used now because of
the added current drain on the low voltage power supply. Modern tubes have a pair of permanent
magnets (see Fig. 5.2) for centering, in the form of rings usually mounted on the yoke cover.
Poles of both the magnets can be suitably shifted with a pair of projecting tabs provided on the
magnetic rings. When the two tabs (one from each ring) coincide with each other, the strongest
field is achieved; that is, the beam will be pushed furthest off centre. When the two tabs are
180° apart (on opposite sides) the field is minimum and so is the decentering. The two rings
are rotated together to change the direction in which decentering occurs. This is illustrated in
Fig. 5.6.
Yoke frame
Windings terminal board
Horz deflection
windings
Movable centering
magnets (two)
Adjustable
pin-cushion
magnets (two)
Vertical-deflection
windings
Vertical-deflection windings
(a) Back view
(b) Front view
Fig. 5.6. Deflection yoke details.
The edge of the yoke linear (see Fig. 5.2) is used to hold small permanent magnets. As
shown in Fig. 5.6 these are positioned to correct any ‘pincushion error’.
Screen Brightness
It is estimated that about 50 per cent of the light emitted at the screen, when the electron
beam strikes it, travels back into the tube. Another 20 percent or so is lost in the glass of the
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THE PICTURE TUBE
tube because of internal reflections and only about 20 percent of it reaches the viewer. Image
contrast is also impaired because of interference caused by the light which is returned to the
screen after reflection from some other points.
Also any ions is the beam, which do exist despite best precautions while degassing,
damage the phosphor material on hitting it and thus cause a dark brownish patch on the
screen. This area usually centers around the middle of the screen because the greater mass of
the ions prevents any appreciable deflection during their transit, with the result, that they
arrive almost at the centre of the screen.
To overcome these serious drawbacks practically all modern picture tubes employ a
very thin coating of aluminium on the back surface of the screen phosphor. The aluminized
coating is very thin and with a final anode voltages of 10 kV or more, the electrons of the beam
have enough velocity to penetrate this coating and excite the phosphor. Thus most of the light
that would normally travel back and get lost in the tube is now reflected back to the screen by
the metal backing and this results in a much improved brilliancy. The aluminized coating is
connected to the high voltage anode coating and thus helps in draining off the secondary emitted
electrons at the screen. This further improves the brightness.
Ion-trap
In older picture tubes a magnetic beam, bender commonly known as ‘ion-trap’ was employed
to deflect the heavy ions away from the screen. In present day picture tubes having a thin
metal coating on the screen, it is no longer necessary to provide an ion-trap. This is because
the ions on account of their greater mass fail to penetrate the metal backing and do not reach
the phosphor screen.
Thus an aluminized coating when provided on the phosphor screen, not only improves
screen brightness and contrast but also makes the use of ‘ion-traps’ unnecessary.
High Voltage Filter Capacitor
A grounded coating is provided on the outer surface of the picture tube. This provides shielding
from stray fields and also acts as one plate of the capacitor, the other plate being the inner
anode coating with the glass bulb serving as the insulator between the two. The capacitor thus
formed (see Fig. 5.2) serves as a filter capacitor for the high voltage supply. This capacitor can
hold charge for a long time after the anode voltage is switched off and so before handling the
picture tube the capacitor must be discharged by shorting the anode button to the grounded
wall coating.
Spark-gap Protection
On account of close spacing between the various-electrodes and the use of very high voltages,
arcing of flashover can occur in the electron gum especially at the control grid. This arcing
causes voltage surges, which result in damage to the associated circuit components. Therefore
for protection of the receiver circuit, due to any arcing, metallic spark-gaps are provided as
shunt paths for the surge currents. In some designs neon bulbs are used as spark gaps. The
gas in the neon tube ionizes when the potential exceeds a certain limit and thus provides a
shunt path for the high voltage arc current.
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5.5
MONOCHROME AND COLOUR TELEVISION
PICTURE TUBE CHARACTERISTICS
As shown in Fig. 5.7 the transfer characteristics of picture tubes are similar to the grid-plate
characteristics of vacuum tubes. The grid of the picture tube has a fixed bias that is set with
the brightness control for optimum average brightness of the picture on the screen. The video
signal that finally controls the brightness variations on the screen may be applied either at the
grid or cathode of the picture tube. Each method has its own merits and demerits and are
discussed in another chapter. This method of varying the beam current to control the
instantaneous screen brightness is called intensity or ‘Z’ axis modulation. The peak-to-peak
amplitude of the ac video signal determines the contrast in the picture, between peak white
with maximum beam current and black at cut-off. The contrast control is in the video amplifier,
which controls the peak-to-peak amplitude of the video signal applied to the picture tube.
Anode current
mA
Peak white
1.6
1.2
0.8
Cut-off bias
(black)
0.4
VG1K
– 80
– 60
– 40
– 20
0
D.C. bias
Video signal
Fig. 5.7. Transfer characteristics of a picture tube. Note the
alignment of blanking level with cut-off bias.
At cut-off the grid voltage is negative enough to reduce the beam current to a value low
enough to extinguish the beam, and this corresponds to the black level in the picture. The
parts of the screen without any luminescence look black in comparison with the adjacent white
areas.
5.6
PICTURE TUBE CIRCUIT CONTROLS
Manufacturers usually recommend a sufficiently high voltage to the second anode of the picture
tube to produce adequate screen brilliancy for normal viewing. This voltage is always obtained
from the output of the horizontal deflection circuit. The dc voltages to the screen grid and focus
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THE PICTURE TUBE
grid are also taken from the horizontal stage and adjusted to suitable values by resistive potential
divider networks. This is shown in Fig. 5.8.
+ Vcc
V VH H
Contrast control
+
K
+ 300 V F
F
From video
detector
G1
G2 G 3
Picture tube
+ 600V
VG1K = – 50
R1
R2
Video amplifier
R3
+ 300 V
Brightness control
R4
EHT
2KV
(Boosted B + Supply)
Fig. 5.8. Picture tube circuit and associated controls.
A variable bias control either in the cathode circuit or control grid lead is provided to
control the electron density, which in turn controls the brightness on the screen. This control,
known as the ‘brightness control’, is brought out at the front panel of the receiver to enable the
viewer to adjust brightness.
As discussed earlier most modern picture tubes do not require critical focus adjustment.
Therefore no focus control is normally provided and instead dc voltage at the focus electrode is
carefully set as explained above.
The contrast control through not strictly a part of the picture tube circuit forms part of
cathode or control grid circuit. This control is also provided at the front panel of the receiver
and its variation enables adjustment of contrast in the reproduced picture.
Picture Tube Handling
The very high vacuum in a modern picture tube means that there is a danger of implosion if
the tube is struck with a hard object or if it is made to rest on its neck. Because of the large
volume of the tube, there is a very high pressure on the glass shell.
In case it breaks the resulting implosion will often cause tube fragments to fly in all
directions at high speed. This may cause severe injury to the persons hit by the tube fragments.
Manufacturers recommend the use of protective goggles and gloves whenever picture tubes
are handled and such precautions should be observed. The tube neck is particularly fragile
and must be handled with care.
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MONOCHROME AND COLOUR TELEVISION
Review Questions
1.
Sketch the sectional view of a picture tube that employs electrostatic focusing and electromagnetic deflection and label all the electrodes.
2.
Explain briefly, how the electron beam is focused on the tube screen. What is meant by crossover
point in the electron gun ?
3.
What type of phosphor is employed for picture tube screens ? Why is a medium persistence
phosphor preferred ?
4.
What is the function of aquadag coating on the inner side of the tube bell ? Why is a grounded
coating provided on the outer surface of the picture tube ?
5.
Why is an aluminized coating provided on the phosphor screen ? How are any stary ions prevented from hitting the screen ?
6.
What do you understand by a 54 cm picture tube ? Why is it necessary to employ implosion
protection in picture tubes ?
7.
What precautions must be observed while handling a picture tube ? Why is it necessary to provide spark-gap protection between the various electrodes ?
8.
Discuss the merits of electromagnetic deflection over electrostatic deflection in television picture tubes. Why is ‘cosine winding’ used for deflection coils ?
9.
What is meant by the deflection angle of a picture tube ? What is the advantage of providing a
large deflection angle yoke ?
10. Explain how the yoke is mounted on the tube neck. Describe how the centering of the electron
beam is accomplished with the help of centering magnets. Why are small permanent magnets
provided at the edges of the yoke liner ?
11. Show with a circuit diagram how dc potentials are supplied to the various electrodes of the
picture tube.
12. What are the functions of ‘brightness’ and ‘contrast’ controls ? Explain their action with suitable
circuit diagrams.
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6
Television Camera Tubes
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6
Television Camera Tubes
A TV camera tube may be called the eye of a TV system. For such an analogy to be correct the
tube must possess characteristic that are similar to its human counterpart. Some of the more
important functions must be (i) sensitivity to visible light, (ii) wide dynamic range with respect
to light intensity, and (iii) ability to resolve details while viewing a multielement scene.
During the development of television, the limiting factor on the ultimate performance
had always been the optical-electrical conversion device, i.e., the pick-up tube. Most types
developed have suffered to a greater or lesser extent from (i) poor sensitivity, (ii) poor resolution,
(iii) high noise level, (iv) undesirable spectral response, (v) instability, (vi) poor contrast range
and (vii) difficulties of processing.
However, development work during the past fifty years or so, has enabled scientists and
engineers to develop image pick-up tubes, which not only meet the desired requirements but
infact excel the human eye in certain respects. Such sensitive tubes have now been developed
which deliver output even where our eyes see complete darkness. Spectral response has been
so perfected, that pick-up outside the visible range (in infra-red and ultraviolet regions) has
become possible. Infact, now there is a tube available for any special application.
6.1
BASIC PRINCIPLE
When minute details of a picture are taken into account, any picture appears to be composed of
small elementary areas of light or shade, which are known as picture elements. The elements
thus contain the visual image of the scene. The purpose of a TV pick-up tube is to sense each
element independently and develop a signal in electrical form proportional to the brightness of
each element. As already explained in Chapter 1, light from the scene is focused on a
photosensitive surface known as the image plate, and the optical image thus formed with a
lens system represents light intensity variations of the scene. The photoelectric properties of
the image plate then convert different light intensities into corresponding electrical variations.
In addition to this photoelectric conversion whereby the optical information is transduced to
electrical charge distribution on the photosensitive image plate, it is necessary to pick-up this
information as fast as possible. Since simultaneous pick-up is not possible, scanning by an
electron beam is resorted to. The electron beam moves across the image plate line by line, and
field by field to provide signal variations in a successive order. This scanning process divides
the image into its basic picture elements. Through the entire image plate is photoelectric, its
construction isolates the picture elements so that each discrete small area can produce its own
signal variations.
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TELEVISION CAMERA TUBES
Photoelectric Effects
The two photoelectric effects used for converting variations of light intensity into electrical
variations are (i) photoemission and (ii) photoconductivity. Certain metals emit electrons when
light falls on their surface. These emitted electrons are called photoelectrons and the emitting
surface a photocathode. Light consists of small bundles of energy called photons. When light is
made incident on a photocathode, the photons give away their energy to the outer valence
electrons to allow them to overcome the potential-energy barrier at the surface. The number of
electrons which can overcome the potential barrier and get emitted, depends on the light
intensity. Alkali metals are used as photocathode because they have very low work-function.
Cesium-silver or bismuth-silver-cesium oxides are preferred as photoemissive surfaces because
they are sensitive to incandescent light and have spectral response very close to the human
eye.
The second method of producing an electrical image is by photoconduction, where the
conductivity or resistivity of the photosensitive surface varies in proportion to the intensity of
light focused on it. In general the semiconductor metals including sel nium, tellurium and lead
with their oxides have this property known as photoconductivity. The variations in resistance
at each point across the surface of the material is utilized to develop a varying signal by scanning
it uniformly with an electron beam.
Image Storage Principle
Television cameras developed during the initial stages of development were of the non-storage
type, where the signal output from the camera for the light on each picture element is produced
only at the instant it is scanned. Most of the illumination is wasted. Since the effect of light on
the image plate cannot be stored, any instantaneous pick-up has low sensitivity. Image disector
and flying-spot camera are examples of non-storage type of tubes. These are no longer in use
and will not be discussed. High camera sensitivity is necessary to televise scenes at low light
levels and to achieve this, storage type tubes have been developed. In storage type camera
tubes the effect of illumination on every picture element is allowed to accumulate between the
times it is scanned in successive frames. With light storage tubes the amount of photoelectric
signal an be increased 10,000 times approximately compared with the earlier non-storage
type.
The Electron Scanning Beam
As in the case of picture tubes an electron gun produces a narrow beam of electrons for scanning.
In camera tubes magnetic focusing is normally employed. The electrons must be focused to a
very narrow and thin beam because this is what determines the resolving capability of the
camera. The diameter of the beam determines the size of the smallest picture element and
hence the finest detail of the scene to which it can be resolved. Any movement of electric
charge is a flow of current and thus the electron beam constitutes a very small current which
leaves the cathode in the electron gun and scans the target plate. The scanning is done by
deflecting the beam with the help of magnetic fields produced by horizontal and vertical coils
in the deflection yoke put around the tubes. The beam scans 312.5 lines per field and 50 such
fields are scanned per second.
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MONOCHROME AND COLOUR TELEVISION
Video Signal
In tubes employing photoemissive target plates the electron beam deposits some charge on the
target plate, which is proportional to the light intensity variations in the scene being televised.
The beam motion is so controlled by electric and magnetic fields, that it is decelerated before it
reaches the target and lands on it with almost zero velocity to avoid any secondary emission.
Because of the negative acceleration the beam is made to move back from the target and on its
return journey, which is very accurately controlled by the focusing and deflection coils, it strikes
an electrode which is located very close to the cathode from where it started. The number of
electrons in the returning beam will thus vary in accordance with the charge deposited on the
target plate. This in turn implies that the current which enters the collecting electrode varies
in amplitude and represents brightness variations of the picture. This current is finally made
to flow through a resistance and the varying voltage developed across this resistance constitutes
the video signal. Figure 6.1 (a) illustrates the essentials of this technique of developing video
signal.
Faceplate
Faceplate
Target
Light
image
Scanning
beam
Photoconductive
coating
Electron gun
Electron
image
v0
RL
Camera
lens
+
Photoemissive
coating
Fig. 6.1(a). Production of video
signal by photoemission.
Light
image
v0
RL
+
Fig. 6.1(b). Production of video signal
by photoconduction.
In camera tubes employing photoconductive cathodes the scanning electron beam causes
a flow of current through the photoconductive material. The amplitude of this current varies
in accordance with the resistance offered by the surface at different points. Since the conductivity
of the material varies in accordance with the light falling on it, the magnitude of the current
represents the brightness variations of the scene. This varying current completes its path
under the influence of an applied dc voltage through a load resistance connected in series with
path of the current. The instantaneous voltage developed across the load resistance is the
video signal which, after due amplification and processing is amplitude modulated and
transmitted. Figure 6.1 (b) shows a simplified illustration of this method of developing video
signal.
Electron Multiplier
When the surface of a metal is bombarded by incident electrons having high velocities, secondary
emission takes place. Aluminium, as an example, can release several secondary electrons for
each incident primary electron. Camera tubes often include an electron multiplier structure,
making use of the secondary emission effect to amplify the small amount of photoelectric current
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TELEVISION CAMERA TUBES
that is later employed to develop video signal. The electron multiplier is a series of cold anodecathode electrodes called dynodes mounted internally, with each at a progressively higher
positive potential as illustrated in Fig. 6.2. The few electrons emitted by the photocathode are
accelerated to a more positive dynode. The primary electrons can then force the ejection of
secondary emission electrons when the velocity of the incident electrons is large enough. The
secondary emission ratio is normally three or four, depending on the surface and the potential
applied. The number of electrons available is multiplied each time the secondary electrons
strike the emitting surface of the next more positive dynode. The current amplification thus
obtained is noise free because the electron multiplier does not have any active device or resistors.
Since the signal amplitude is very low any conventional amplifier, if used instead of the electron
multiplier, woul cause serious S/N ratio problems.
Glass envelope
Dynode 4
+ (400 V)
Dynode 3
(+ 300 V)
Dynode 2
(+ 200 V)
Secondary
electrons
Dynode 1
(+ 100 V)
Anode
(+ 600 V)
Photoelectrons
Dynode 5
(+ 500 V)
Incident light
Photo cathode
(0 V)
Fig. 6.2. Illustration of an electron-multiplier structure.
Types of Camera Tubes
The first developed storage type of camera tube was ‘Iconoscope’ which has now been replaced
by image-orthicon because of its high light sensitivity, stability and high quality picture
capabilities. The light sensitivity is the ratio of the signal output to the incident illumination.
Next to be developed was the vidicon and is much simpler in operation. Similar to the vidicon
is another tube known as plumbicon. The latest device in use for image scanning is the solid
state image scanner.
6.2
IMAGE ORTHICON
This tube makes use of the high photoemissive sensitivity obtainable from photocathodes,
image multiplication at the target caused by secondary emission and an electron multiplier. A
sectional view of an image orthicon is shown in Fig. 6.3. It has three main sections: image
section, scanning section and electron gun-cum-multiplier section.
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Photoelectrons
(electron image)
Image accelerator
Grid no. 6
– 300 V
Grid no. 4 and
wall coating, 80 V
Grid no. 2, and
dynode no. 1,
Grid no. 3,
300 V
80 V
Electron gun
Decelerator
Grid no. 5, 40 V
Wiremesh
screen
Scanning beam
Return beam
Camera
lens
Photocathode
– 600 V
Target
– 3 to – 5 V
Secondary electrons
Image
section
Glass plate
Alignment
coil
Deflection
coil
Focusing
coil
Scanning section
Five stage
electron
multiplier
Electron gun
and
multiplier section
Fig. 6.3. Principle of operation of Image Orthicon (non-field mesh type).
(i) Image Section
The inside of the glass face plate at the front is coated with a silverantimony coating sensitized
with cesium, to serve as photocathode. Light from the scene to be televised is focused on the
photocathode surface by a lens system and the optical image thus formed results in the release
of electrons from each point on the photocathode in proportion to the incident light intensity.
Photocathode surface is semitransparent and the light rays penetrate it to reach its inner
surface from where electron emission takes place. Since the number of electrons emitted at
any point in the photocathode has a distribution corresponding to the brightness of the optical
image, an electron image of the scene or picture gets formed on the target side of the photocoating
and extends towards it. Through the convertion efficiency of the photocathode is quite high, it
cannot store charge being a conductor. For this reason, the electron image produced at the
photocathode is made to move towards the target plate located at a short distance from it. The
target plate is made of a very thin sheet of glass and can store the charge received by it. This
is maintained at about 400 volts more positive with respect to the photocathode, and the
resultant electric field gives the desired acceleration and motion to the emitted electrons towards
it. The electrons, while in motion, have a tendency to repel each other and thin can result in
distortion of the information now available as charge image. To prevent this divergence effect
an axial magnetic field, generated in this region by the ‘long focus coil’ is employed. This
magnetic field imparts helical motion of increasing pitch and focuses the emitted electrons on
the target into a well defined electron image of the original optical image. The image side of
the target has a very small deposit of cesium and thus has a high secondary emission ratio.
Because of the high velocity attained by the electrons while in motion from photocathode to
the target plate, secondary emission results, as the electrons bombard the target surface. These
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secondary electrons are collected by a wire-mesh screen, which is located in front of the target
on the image side and is maintained at a slightly higher potential with respect to the target.
The wire-mesh screen has about 300 meshes per cm2 with an open area of 50 to 75 per cent, so
that the screen wires do not interfere with the electron image. The secondary electrons leave
behind on the target plate surface, a positive charge distribution, corresponding to the light
intensity distribution on the original photocathode.
For storage action this charge on the target plate should not spread laterally over its
surface, during the storage time, since this would destory the resolution of the device. To
achieve this the target is made out of extremely thin sheet of glass. The positive charge
distribution builds up during the frame storage time (40 ms) and thus enhances the sensitivity
of the tube. It should be clearly understood, that the light from the scene being televised
continuously falls on the photocathode, and the resultant emitted electrons on reaching the
target plate cause continuous secondary emission. This continuous release of electrons results
in the building up of positive charge on the target plate.
Because of the high secondary emission ratio, the intensity of the positive charge
distribution is four to five times more as compared to the charge liberated by the photocathode.
This increase in charge density relative to the charge liberated at the photocathode is known
as ‘image multiplication’ and contributes to the increased sensitivity of image orthicon. As
shown in Fig. 6.3, the two-sided target has the charge image on one side while an electron
beam scans the opposite side. Thus, while the target plate must have high resistivity laterally
for storage action, it must have low resistivity along its thickness, to enable the positive charge
to conduct to the other side which is scanned. It is for this reason that the target plate is very
thin, with thickness close to 0.004 mm. Thus, whatever charge distribution builds up on one
side of the target plate due to the focused image, appears on the other side, which is scanned,
and it is from here that the video signal is obtained.
(ii) Scanning Section
The electron gun structure produces a beam of electrons that is accelerated towards the target.
As indicated in the figure, positive accelerating potentials of 80 to 330 volts are applied to grid
2, grid 3, and grid 4 which is connected internally to the metalized conductive coating on the
inside wall of the tube. The electron beam is focused at the target by magnetic field of the
external focus coil and by voltage supplied to grid 4. The alignment coil provides magnetic
field that can be varied to adjust the scanning beam’s position, if necessary, for correct location.
Deflection of electron beam’s to scan the entire target plate is accomplished by magnetic fields
of vertical and horizontal deflecting coils mounted on yoke external to the tube. These coils are
fed from two oscillators, one working at 15625 Hz, for horizontal deflection, and the other
operating at 50 Hz, for vertical deflection.
The target plate is close to zero potential and therefore electrons in the scanning beam
can be made to stop their forward motion at its surface and then return towards the gun
structure. The grid 4 voltage is adjusted to produce uniform deceleration of electrons for the
entire target area. As a result, electrons in the scanning beam are slowed down near the target.
This eliminates any possibility of secondary emission from this side of the target plate. If a
certain element area on the target plate reaches a potential of, say, 2 volts during the storage
time, then as a result of its thinness the scanning beam ‘sees’ the charge deposited on it, part
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of which gets diffused to the scanned side and deposits an equal number of negative charges on
the opposite side. Thus out of the total electrons in the beam, some get deposited on the target
plate, while the remaining stop at its surface and turn back to go towards the first electrode of
the electron multiplier. Because of low resistivity across the two sides of the target, the deposited
negative charge neutralizes the existing positive charge in less than a frame time. The target
can again become charged as a result of the incident picture information, to be scanned during
the successive frames. As the target is scanned element by element, if there are no positive
charges at certain points, all the electrons in the beam return towards the electron gun and
none gets deposited on the target plate. The number of electrons, leaving cathode of the gun, is
practically constant, and out of this, some get deposited and remaining electrons, which travel
backwards provide signal current that varies in amplitude in accordance with the picture
information. Obviously then, the signal current is maximum for black areas on the picture,
because absence of light from black areas on the picture does not result in any emission on the
photocathode, and there is no secondary emission at the corresponding points on the target,
and no electrons are needed from the beam to neutralize them. On the contrary for high light
areas, on the picture, there is maximum loss of electrons from the target plate, due to secondary
emission, and this results in large deposits of electrons from the beam and this reduces the
amplitude of the returning beam current. The resultant beam current that turns away from
the target, is thus, maximum for black areas and minimum for bright areas on the picture.
High intensity light causes large charge imbalance on the glass target plate. The scanning
beam is not able to completely neutralize it in one scan. Therefore the earlier impression
persists for several scans.
Image Resolution. It may be mentioned at this stage that since the beam is of low velocity
type, being reduced to near zero velocity in the region of the target it is subjected to stray
electric fields in its vicinity, which can cause defocusing and thus loss of resolution. Also on
contact with the target, the electrons would normally glide along its surface tangentially for a
short distance and the point of contact becomes ill defined. The beam must strike the target at
right angle at all points of the target, for better resolution. These difficulties are overcome in
the image-orthicon by the combined action of electrostatic field because of potential on grid 4,
and magnetic field of the long focusing coil. The interaction of two fields gives rise to cycloidal
motion to the beam in the vicinity of target, which then hits it at right angle no matter which
point is being scanned. This very much improves the resolving capability of the picture tube.
(iii) Electron Multiplier
The returning stream of electrons arrive at the gun close to the aperture from which electron
beam emerged. The aperture is a part of a metal disc covering the gun electrode. When the
returning electrons strike the disc which is at a positive potential of about 300 volts, with
respect to the target, they produce secondary emission. The disc serves as first stage of the
electron multiplier. Successive stages of the electron multiplier are arranged symmetrically
around and back of the first stage. Therefore secondary electrons are attracted to the dynodes
at progressively higher positive potentials. Five stages of multiplication are used, details of
which are shown in Fig. 6.4. Each multiplier stage provides a gain of approximately 4 and thus
a total gain of (4)5 ≈ 1000 is obtained at the electron multiplier. This is known as signal
multiplication. The multiplication so obtained maintains a high signal to noise ratio. The
secondary electrons are finally collected by the anode, which is connected to the highest supply
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voltage of + 1500 volts in series with a load resistance RL. The anode current through RL has
the same variations that are present in the return beam from the target and amplified by the
electron multiplier. Therefore voltage across RL is the desired video signal; the amplitude of
which varies in accordance with light intensity variations of scene being televised. The output
across RL is capacitively coupled to the camera signal amplifier. With RL = 20 K-ohms and
typical dark and high light currents of magnitudes 30 µA and 5 µA respectively, the camera
output signal will have an amplitude of 500 mV peak-to-peak.
Grid no. 2 and
dynode no. 1,
300 V
Dynode
no. 3,
880 V
Dynode
no. 5,
1450 V
Scanning beam
Electron gun
Return beam
(signal current)
Secondary
electrons
Cc
0.03 F
Dynode
no. 2,
600 V
Dynode
no. 4,
1160 V
Anode
Signal output
to amplifier
RL
20 K
1500 V
Fig. 6.4. Electron-multiplier section of the Image Orthicon.
Field Mesh Image Orthicon. The tube described above is a non-field mesh image orthicon.
In some designs an additional pancake-shaped magnetic coil is provided in front of the face
plate. This is connected in series with the main focusing coil. The location of the coil results in
a graded magnetic field such that the optically focused photocathode image is magnified by
about 1.5 times. Thus the charge image produced on the target plate is bigger in size and this
results in improved resolution and better overall performance. Such a camera tube is known
as a field mesh Image Orthicon.
Light Transfer Characteristics and Applications—During the evolution of image orthicon
tubes, two separate types were developed, one with a very close target-mesh spacing (less than
0.001 cm) and the other with somewhat wider spacing. The tube, with very close target mesh
spacing, has very high signal to noise ratio but this is obtained at the expense of sensitivity
and contrast ratio. This is a worthwhile exchange where lighting conditions can be controlled
and picture quality is of primary importance. This is generally used for live shows in the
studios. The other type with wider target-mesh spacing has high sensitivity and contrast ratio
with more desirable spectral response. This tube has wider application for outdoor or other
remote pickups where a wide range of lighting conditions have to be accommodated. More
recent tubes with improved photocathodes have sensitivities several times those of previous
tubes and much improved spectral response. Overall transfer characteristics of such tubes are
drawn in Fig. 6.5. Tube ‘A’ is intended primarily for outdoor pick-ups where as tube ‘B’ is much
suited for studio use and requires strong illumination. The knee of the transfer characteristics
is reached when the illumination causes the target to be fully charged with respect to the mesh
between successive scans by the electron beam. The tube is sometimes operated slightly above
the knee, to obtain the black border effect (also known as Halo effect) around the high light
areas of the target.
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MONOCHROME AND COLOUR TELEVISION
Output current (A)
10.0
A
1.0
B
0.1
0.0001
0.001
0.01
0.1
1.0
Illumination on photocathode
(ft – candles)
Fig. 6.5. Light transfer characteristics of two different Image Orthicons.
6.3
VIDICON
The Vidicon came into general use in the early 50’s and gained immediate popularity because
of its small size and ease of operation. It functions on the principle of photoconductivity, where
the resistance of the target material shows a marked decrease when exposed to light. Fig. 6.6
Target connection
30 to 60 V
Alignment coil
Grid no. 2 (accelerator)
300 V
Grid no. 1,
0 to 100 V
Target
Light
image
Glass
face plate
Cathode, 0V
Grid no. 4
(decelerator)
275 V
Focusing
coil
Grid no. 3 (beam focus)
275 to 300 V
Horizontal and vertical
deflecting coils
Fig. 6.6. Vidicon camera tube cross-section.
illustrates the structural configuration of a typical vidicon, and Fig. 6.7 shows the circuit
arrangement for developing camera signal output. As shown there, the target consists of a
thin photo conductive layer of either selenium or anti-mony compounds. This is deposited on a
transparent conducting film, coated on the inner surface of the face plate. This conductive
coating is known as signal electrode or plate. Image side of the photolayer, which is in contact
with the signal electrode, is connected to DC supply through the load resistance RL. The beam
that emerges from the electron gun is focused on surface of the photo conductive layer by
combined action of uniform magnetic field of an external coil and electrostatic field of grid No
3. Grid No. 4 provides a uniform decelerating field between itself, and the photo conductive
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layer, so that the electron beam approaches the layer with a low velocity to prevent any
secondary emission. Deflection of the beam, for scanning the target, is obtained by vertical and
horizontal deflecting coils, placed around the tube.
Signal plate
I = 0.3 mA
Cc
Camera
signal output
Photolayer
White
Black
R = 2 M R = 20 M
Scanning beam
RL
50 K
+ 40 V
Electron
gun
Fig. 6.7. Circuit for output signal from a Vidicon camera tube.
Charge Image
The photolayer has a thickness of about 0.0001 cm, and behaves like an insulator with a
resistance of approximately 20 MΩ when in dark. With light focused on it, the photon energy
enables more electrons to go to the conduction band and this reduces its resistivity. When
bright light falls on any area of the photoconductive coating, resistance across the thickness of
that portion gets reduces to about 2 MΩ. Thus, with an image on the target, each point on the
gun side of the photolayer assumes a certain potential with respect to the DC supply, depending
on its resistance to the signal plate. For example, with a B + source of 40 V (see Fig. 6.7), an
area with high illumination may attain a potential of about + 39 V on the beam side. Similarly
dark areas, on account of high resistance of the photolayer may rise to only about + 35 volts.
Thus, a pattern of positive potentials appears, on the gun side of the photolayer, producing a
charge image, that corresponds to the incident optical image.
Storage Action
Though light from the scene falls continuously on the target, each element of the photocoating
is scanned at intervals equal to the frame time. This results in storage action and the net
change in resistance, at any point or element on the photoconductive layer, depends on the
time, which elapses between two successive scannings and the intensity of incident light. Since
storage time for all points on the target plate is same, the net change in resistance of all
elementary areas is proportional to light intensity variations in the scene being televised.
Signal Current
As the beam scans the target plate, it encounters different positive potentials on the side of the
photolayer that faces the gun. Sufficient number of electrons from the beam are then deposited
on the photolayer surface to reduce the potential of each element towards the zero cathode
potential. The remaining electrons, not deposited on the target, return back and are not utilized
in the vidicon. However, the sudden change in potential on each element while the beam
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scans, causes a current flow in the signal electrode circuit producing a varying voltage across
the load resistance RL. Obviously, the amplitude of current and the consequent output voltage
across RL are directly proportional to the light intensity variations on the scene. Note that,
since, a large current would cause a higher voltage drop across RL, the output voltage is most
negative for white areas. The video output voltage, that thus develops across the load resistance
(50 K-ohms) is adequate and does not need any image or signal multiplication as in an image
orthicon. The output signal is further amplified by conventional amplifiers before it leaves the
camera unit. This makes the vidicon a much simpler picture tube.
Leaky Capacitor Concept
Another way of explaining the development of ‘charge image’ on the photolayer is to consider
it as an array of individual target elements, each consisting of a capacitor paralleled with a
light dependent resistor. A number of such representations are shown in Fig. 6.8. As seen
there, one end of these target elements is connected to the signal electrode and the other end
is unterminated facing the beam.
C
Target element
Glass
faceplate
Scanning beam
R
Electron gun
Light
C
R
Cc
v0
Light dependent
resistor
RL
+
40V
Fig. 6.8. Schematic representation of a Vidicon target area.
In the absence of any light image, the capacitors attain a charge almost equal to the B +
(40 V) voltage in due course of time. However, when an image is focused on the target the
resistors in parallel with the capacitors change in value depending on the intensity of light on
each unit element. For a high light element, the resistance across the capacitor drops to a
fairly low value, and this permits lot of charge from the capacitor to leak away. At the time of
scanning, more electrons are deposited, on the unterminated end of this capacitor to recharge
it to the full supply voltage of + 40 V. The consequent flow of current that completes its path
through RL develops a signal voltage across it. Similarly for black areas of the picture, the
resistance across the capacitors remains fairly high, and not much charge is allowed to leak
from the corresponding capacitors. This in turn needs fewer number of electrons from the
beam to recharge the capacitors. The resultant small current that flows, develops a lower
voltage across the load resistance.
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The electron beam thus ‘sees’ the charge on each capacitor, while scanning the target,
and delivers more or less number of electrons to recharge them to the supply voltage. This
process is repeated every 40 ms to provide the necessary video signal corresponding to the
picture details at the upper end of the load resistor. The video signal is fed through a blocking
capacitor to an amplifier for necessary amplification.
Light Transfer Characteristics
Vidicon output characteristics are shown in Fig. 6.9. Each curve is for a specific value of ‘dark’
current, which is the output with no light. The ‘dark’ current is set by adjusting the target
voltage. Sensitivity and dark current both increase as the target voltage is increased. Typical
output for the vidicon is 0.4 µA for bright light with a dark current of 0.02 µA. The
photoconductive layer has a time lag, which can cause smear with a trail following fast moving
objects. The photoconductive lag increases at high target voltages, where the vidicon has its
highest sensitivity.
Dark current = 0.2 A
Output current A
1.0
0.1
A
C
0.02 A
0.01
0.001
0.01
B
0.004 A
0.
0.1
1.0
10
100
1000
Illumination on tube face,
(ft-candles)
Fig. 6.9. Light transfer characteristics of Vidicon.
Applications
Earlier types of vidicons were used only where there was no fast movement, because of inherent
lag. These applications included slides, pictures, closed circuit TV etc. The present day improved
vidicon finds wide applications in education, medicine, industry, aerospace and oceanography.
It is, perhaps, the most popular tube in the television industry. Vidicon is a short tube with a
length of 12 to 20 cm and diameter between 1.5 and 4 cm. Its life is estimated to be between
5000 and 20,000 hours.
6.4
THE PLUMBICON
This picture tube has overcome many of the less favourable features of standard vidicon. It has
fast response and produces high quality pictures at low light levels. Its smaller size and light
weight, together with low-power operating characteristics, makes it an ideal tube for
transistorized television cameras.
Except for the target, plumbicon is very similar to the standard vidicon. Focus and
deflection are both obtained magnetically. Its target operates effectively as a P–I–N semiconductor diode. The inner surface of the faceplate is coated with a thin transparent conductive
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layer of tin oxide (SnO2). This forms a strong N type (N+) layer and serves as the signal plate of
the target. On the scanning side of this layer is deposited a photoconductive layer of pure lead
monoxide (PbO) which is intrinsic or ‘I’ type. Finally the pure PbO is doped to form a P type
semiconductor on which the scanning beam lands. The details of the target are shown in Fig. 6.10
(a). The overall thickness of the target is 15 × 10– 6 m. Figure 6.10 (b) shows necessary circuit
details for developing the video signal. The photoconductive target of the plumbicon functions
similar to the photoconductive target in the vidicon, except for the method of discharging each
storage element. In the standard vidicon, each element acts as a leaky capacitor, with the
leakage resistance decreasing with increasing light intensity. In the plumbicon, however, each
element serves as a capacitor in series with a reverse biased light controlled diode. In the
signal circuit, the conductive film of tin oxide (SnO2), is connected to the target supply of 40
volts through an external load resistance RL to develop the camera output signal voltage.
Light from the scene being televised is focussed through the transparent layer of tin-oxide on
the photoconductive lead monoxide. Without light the target prevents any conduction because
of absence of any charge carriers and so there is little or no output current. A typical value of
dark current is around 4 nA (4 × 10– 9 Amp). The incidence of light on the target results in
photoexcitation of semiconductor junction between the pure PbO and doped layer. The resultant
decrease in resistance causes signal current flow which is proportional to the incident light on
each photo element. The overall thickness of the target is 10 to 20 µm.
Signal plate
SnO2
n-type layer
(SnO2)
Glass
faceplate
p-type layer
(doped PbO)
Light
Scanning beam
Camera
signal
output
I0 = 0.3 A
Intrinsic
PbO
I (A)
Cc
1
Doped PbO
Scanning
beam
RL
50 K
0.1
+ 40V
Cathode of
electron gun
Intrinsic layer
(Pure PbO)
(a)
0.01
1
10
100
Target illumination
(lumens)
(b)
(c)
Fig. 6.10. Plumbicon camera tube (a) target details
(b) output signal current and (c) characteristics.
Light Transfer Characteristics
The current output versus target illumination response of a plumbicon is shown in Fig. 6.1 (c).
It is a straight line with a higher slope as compared to the response curve of a vidicon. The
higher value of current output, i.e., higher sensitivity, is due to much reduced recombination
of photogenerated electrons and holes in the intrinsic layer which contains very few
discontinuities. For target voltages higher than about 20 volts, all the generated carriers are
swept quickly across the target without much recombinations and thus the tube operates in a
photosaturated mode. The spectral response of the plumbicon is closer to that of the human
eye except in the red colour region.
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6.5
SILICON DIODE ARRAY VIDICON
This is another variation of vidicon where the target is prepared from a thine n-type silicon
wafer instead of deposited layers on the glass faceplate. The final result is an array of silicon
photodiodes for the target plate. Figure 6.11 shows constructional details of such a target. As
shown there, one side of the substrate (n-type silicon) is oxidized to form a film of silicon
dioxide (SiO2) which is an insulator. Then by photomasking and etching processes, an array of
fine openings is made in the oxide layer. These openings are used as a diffusion mask for
producing corresponding number of individual photodiodes. Boron, as a dopent is vapourized
through the array of holes, forming islands of p-type silicon on one side of the n-type silicon
substrate. Finally a very thin layer of gold is deposited on each p-type opening to form contacts
for signal output. The other side of the substrate is given an antiflection coating. The resulting
p-n photodiodes are about 8 µm in diameter. The silicon target plate thus formed is typically
0.003 cm thick, 1.5 cm square having an array of 540 × 540 photodiodes. This target plate is
mounted in a vidicon type of camera tube.
Antireflection
coating
Substrate
(n-type silicon)
Gold coating for
signal output
Gold overlay
p-type silicon
Light
Scanning beam
Depletion region
Silicon dioxide
(insulator)
n + layer
Fig. 6.11. Constructional details (enlarged) of a
silicon diode array target plate.
Scanning and Operation
The photodiodes are reverse biased by applying +10 V or so to the n + layer on the substrate.
This side is illuminated by the light focused on to it from the image. The incidence of light
generates electron-hole pairs in the substrate. Under influence of the applied electric field,
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holes are swept over to the ‘p’ side of the depletion region thus reducing reverse bias on the
diodes. This process continues to produce storage action till the scanning beam of electron gun
scans the photodiode side of the substrate. The scanning beam deposits electrons on the p-side
thus returning the diodes to their original reverse bias. The consequent sudden increase in
current across each diode caused by the scanning beam represents the video signal. The current
flows through a load resistance in the battery circuit and develops a video signal proportional
to the intensity of light falling on the array of photodiodes. A typical value of peak signal
current is 7 µA for bright white light.
The vidicon employing such a multidiode silicon target is less susceptible to damage or
burns due to excessive high lights. It also has low lag time and high sensitivity to visible light
which can be extended to the infrared region. A particular make of such a vidicon has the trade
name of ‘Epicon’. Such camera tubes have wide applications in industrial, educational and
CCTV (closed circuit television) services.
6.6
SOLID STATE IMAGE SCANNERS*
The operation of solid state image scanners is based on the functioning of charge coupled
devices (CCDs) which is a new concept in metal-oxide-semiconductor (MOS) circuitry. The
CCD may be thought of to be a shift register formed by a string of very closely spaced MOS
capacitors. It can store and transfer analog charge signals—either electrons or holes—that
may be introduced electrically or optically.
The constructional details and the manner in which storing and transferring of charge
occurs is illustrated in Fig. 6.12. The chip consists of a p-type substrate, the one side of which
is oxidized to form a film of silicon dioxide, which is an insulator. Then by photolithographic
processes, similar to those used in miniature integrated circuits an array of metal electrodes,
known as gates, are deposited on the insulator film. This results in the creation of a very large
number of tiny MOS capacitors on the entire surface of the chip.
(a)
3
2
1
1
2
S1 substrate p-type
S1O2 3
Surface potential
t1
(b)
t1
t3
(c)
t2
t3
t2
Electron
energy
Fig. 6.12. A three phase n-channel MOS charge coupled device. (a) construction (b) transfer
of electrons between potential wells (c) different phases of clocking voltage waveform.
* For more details on Solid State Image Scanners refer to IEEE Trans on ‘Charge Coupled
Devices—Technology and Applications’ Edited by Roger Melen and Dennis Buss.
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The application of small positive potentials to the gate electrodes results in the
development of depletion regions just below them. These are called potential wells. The depth
of each well (depletion region) varies with the magnitude of the applied potential. As shown in
Fig. 6.12 (a), the gate electrodes operate in groups of three, with every third electrode connected
to a common conductor. The spots under them serve as light sensitive elements. When any
image is focused onto the silicon chip, electrons are generated within it, but very close to the
surface. The number of electrons depends on the intensity of incident light. Once produced
they collect in the nearby potential wells. As a result the pattern of collected charges represents
the optical image.
Charge Transfer
The charge of one element is transferred along the surface of the silicon chip by applying a
more positive voltage to the adjacent electrode or gate, while reducing the voltage on it. The
minority carriers (electrons in this case) while accumulating in the so called wells reduce their
depths much like the way a fluid fills up in a container. The acumulation of charge carries
under the first potential wells of two consecutive trios is shown in Fig. 6.12 (b) where at instant
t1 a potential φ1 exists at the corresponding gate electrodes. In practice the charge transfer is
effected by multiphase clock voltage pulses (see Fig. 6.12 (c)) which are applied to the gates in
a suitable sequence. The manner in which the transition takes place from potential wells under
φ1 to those under φ2 is illustrated in Fig. 6.12 (b). A similar transfer moves charges from φ2 to φ3
and then from φ3 to φ1 under the influence of continuing clock pulses. Thus, after one complete
clock cycle, the charge pattern moves one stage (three gates) to the right. The clocking sequence
continues and the charge finally reaches the end of the array where it is collected to form the
signal current.
Scanning of Television Pictures
A large number of CCD arrays are packed together to form the image plate. It does not need an
electron gun, scanning beam, high voltage or vacuum envelope of a conventional camera tube.
The potential required to move the charge is only 5 to 10 volt. The spot under each trio serves
as the resolution cell. When light image is focused on the chip, electrons are generated in
proportion to the intensity of light falling on each cell.
Out
1 2 3
123
1
2
3
Address
register
Driving phases
Readout
register
Fig. 6.13. Basic organization of line addressed charge transfer area imaging devices.
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The principle of one-dimensional charge transfer as explained above can be integrated
in various ways to render a solid-state area image device. The straightforward approach consists
of arranging a set of linear imaging structures so that each one corresponds to a scan line in
the display. The lines are then independently addressed and read into a common output diode
by application of driving pulses through a set of switches controlled by an address register as
shown in Fig. 6.13. To reduce capacitance, the output can be simply a small diffused diode in
one corner of the array. The charge packets emerging from any line are carried to this diode by
an additional vertical output register. In such a line addressed structure (Fig. 6.13) where the
sequence of addressing the lines is determined by the driving circuitry, interlacing can be
accomplished in a natural way.
Cameras Employing Solid-State Scanners
CCDs have a bright future in the field of solid state imaging. Full TV line-scan arrays have
already been constructed for TV cameras. However, the quality of such sensors is not yet
suitable for normal TV studio use. RCA SID 51232 is one such 24 lead dual-in-line image
senser. It is a self-scanned senser intended primarily for use in generating standard interlaced
525 line television pictures. The device contains 512 × 320 elements and is constructed with a
3 phase n-channel, vertical frame transfer organization using a sealed silicon gate structure.
Its block diagrams is shown in Fig. 6.14 (a). The image scanner’s overall picture performance
is comparable to that 2/3 inch vidicon camera tubes but undesirable characteristics such as lag
and microphonics are eliminated.
Output
circuit
Bias charge
circuit
IG1 IG2
fIS
fH1
fH2
fH3
fOG
OS
Horz register
Optical glass window
OD
fVB1
fVB2
Storage area
RD fR
fVB3
fVA1
fVA2
(b)
Image area
fVA3
(a)
OD
OS
RD
fR
fOG
Output transistor drain
Output transistor source
Output reset transistor drain
Output reset transistor gate clock
Output gate clock
IG1, IG2 Input gates
fVA1 fVA2 fVA3 fVB1
fVB2 fVB3
Vertical register
clocks
Horz register clocks
fH1, fH2, fH3
fIS Output register source clock
Fig. 6.14. A 512 × 320 element senser (RCA SID 51232) for very compact TV cameras,
(a) chip’s block diagram, (b) view with optical glass window.
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TELEVISION CAMERA TUBES
The SID 51232 is supplied in a hermetic, edge contacted, 24-connection ceramic dual-inline package. The package contains an optical glass window (see Fig. 6.14 (b)) which allows an
image to be focussed into senser’s 12.2 mm image diagonal.
Review Questions
1.
What is the basic principle of a camera pick-up tube ? Describe the two photoelectric effects used
for converting variations of light intensity into electrical signals.
2.
What do you understand by image storage capability of a modern television pick-up tube ? Explain
why storage type tubes have must higher sensitivity as compared to the earlier non-storage
type.
3.
Draw cross-sectional view of an image orthicon camera tube and explain how it develops video
signal when light from any scene is focused on its face plate.
4.
What do you understand by image multiplication and signal multiplication in an image orthicon
camera tube ? Why is an electron multiplier preferred over conventional amplifiers for amplifying the video signal at the output of the camera tube ?
5.
In an image orthicon, what is the function of the wire-mesh screen and why is it located very
close to the target plate ? Explain with the help of transfer characteristics the effect of targetmesh spacing on the overall performance of the tube.
6.
In an image orthicon :
(a) Why is the electron beam given a cycloidal motion before it hits the target plate ?
(b) Why is the electron beam velocity brought close to zero on reaching the target plate ?
(c) What is the function of the decelerator grid ?
7.
Explain with the help of suitable sketches, how video signal is developed in a vidicon camera
tube ? How is the vidicon different from an image-orthicon and what are its special applications.
8.
What do you understand by ‘dark current’ in a vidicon ? Explain how the inherent smear effect in
a vidicon is overcome in a Plumbicon. Explain with a suitable sketch the mechanism by which
the video siganl is developed from the P-I-N structure of its target.
9.
Give constructional details of the vidicon target prepared from a thin n-type silicon wafer which
operates as an array of photodiodes. Explain how the signal voltage is developed from such a
target.
10. Explain with suitable sketches the basic principle of a solid state image scanner. Describe briefly
the manner in which the CCD array is scanned to provide interlaced scanning.
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7
Basic Television Broadcasting
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7
Basic Television Broadcasting
The composite video signal generated by camera and associated circuitry is processed in the
control room before routing it to the transmitter. At transmitter, picture carrier frequency
assigned to the station is generated, amplified and later amplitude modulated with the incoming
video signal. The sound output associated with the scene is simultaneously processed and
frequency modulated with channel sound carrier frequency. The two outputs, one from picture
signal transmitter and the other from sound signal transmitter are combined in a suitable
network and then fed to a common antenna network for transmission. As is obvious, the picture
and sound signals, though generated and processed simultaneously pass through two
independent transmitters at the broadcasting station. Thus, it is logical, that the two
transmitting arrangements be studied separately. The first part of this chapter deals with
television studio setup and picture signal transmission, while the later part is devoted to
frequency modulation and sound signal transmission.
7.1
TELEVISION STUDIO
A TV studio is an acoustically treated compact anechoic room. It is suitably furnished and
equipped with flood lights for proper light effects. The use of dimmerstats with flood lights
enables suitable illumination level of any particular area of the studio depending on the scene
to be televised. Several cameras are used to telecast the scene from different angles. Similarly
a large number of microphones are provided at different locations to pick up sound associated
with the programme.
The camera and microphone outputs are fed into the control room by coaxial cables. The
control room has several monitors to view pictures picked up by different cameras. A monitor
is a TV receiver that contains no provision for receiving broadcast signals but operates on a
direct input of unmodulated signal. A large number of such monitors are used to keep a check
on the content and quality of pictures being telecast. Similarly, headphones are used to monitor
and regulate sound output received from different microphones through audio mixers.
In addition to a live studio, video tape recording and telecine machine rooms are located
close to the control room. In most cases, programmes as enacted in the studio are recorded on
a video tape recorder (VTR) through the control room. These are later broadcast with the VTR
output passing through the same control room. Figure 7.1 illustrates a typical layout of a
television studio setup. As shown, the telecine machines together with a slide scanner are
installed next to the control room. Such a facility enables telecasting of cinematograph films
and advertisement slides. All the rooms are interconnected by coaxial cables and shielded
wires. In large establishments, there are several such studio units with their outputs feeding
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BASIC TELEVISION BROADCASTING
Rewinding room
VTR
M
Control
panel
VTR
C
C
VTR
MON
VTR room
35 mm
C
PROJ
D
35 mm
MON
MON MON MON
Equipment racks
PROJ
16 mm
Slide
C
MON
VIP studio
D
Telecine and slide
scanner room
Passage
D
D
D
Camera
apparatus room
D
MON
MON
C
M
Captions
M
Main studio
Ca
me
Boom
ra
Camera
Glass panel
MON
Announcer
Light DIST
controls
Ca
MON
C
M
ra
me
Lights
News reader
D
D
MON
C
Corridor
MON
Camera control and
video mixer panels
Video
SW
DIR
Glass panel
C
M
D
ASST
DIR
Engineer
MON
Sync pulse
generator
Stage
Corridor
Remote
C.C. Units
Audio
console
Sound
engineer
MON
Test pattern
generator
Light
director
MON
Special
effects
generator
Light
controls
Production control room
Visitors
Power room
Corridor
Fig. 7.1 Plan of a typical television studio (Abbreviations used are :
MON-Monitor, C-Camera, D-Door, PROJ-Projector, ASST DIR- Assistant Director,
DIR-Director, SW-Switcher, C.C. Units Camera Control Units, M-Microphone,
VTR Video Tape Recorder, DIST-Distribution).
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D
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MONOCHROME AND COLOUR TELEVISION
the transmitter through a switcher in the master control room, which selects one programme
at a time. Even in studio set-ups with only one control room, there are several studio rooms, all
connected to the same control room. This enables preparation of different programmes in other
studios while a programme is being telecast or recorded from one studio.
7.2
TELEVISION CAMERAS
Television cameras may assume different physical and electrical configurations. However, in
general they may be divided into two basic groups—self contained cameras and two-unit systems
that employ separate camera heads driven by remote camera control equipment located in the
central apparatus room (see Fig. 7.1). A self contained camera has all the elements necessary
to view a scene and generate a complete television signal. Such units are employed for outdoor
locations and normally have a VTR and baby flood lights as an integral part of the televising
setup. The remote camera head usually contains only photosensitive pick-up tube, its associated
deflection circuitry, video preamplifier and a video monitor. Thus the bulk of the circuitry is
contained in the camera control unit, which is connected to the camera head by means of a
multiconductor cable. This cable not only carries video, deflection and sync signals but also
feeds high voltage supplies necessary for the camera tube. The remote camera control unit
contains most of the electrical operating and set-up controls. For this reason, it is usually
located near a viewing monitor so that the results of any adjustments may be easily viewed on
the monitor screen. All camera controls are available on a panel in the production control
room.
Camera Lenses
Television cameras can produce images to different scales depending on the focal length (viewing
angle) of the lens employed. Lenses of longer focal length are narrow angle lenses while those
of shorter focal length are wide angle lenses. Narrow angle lenses (below 20°) are suitable for
closeups of distant objects because of the magnifying effect due to their longer focal length.
Lenses with angles over 60° are most suited for location shots which cover large areas. Medium
angle lenses (20 to 60°) are called universal lenses and are used for televising normal scenes.
All lenses consist of a combination of simple lens elements to minimize spherical aberration
and other optical distortions.
Lens Turret
A judicious choice of lens can considerably improve the quality of image, depth of field and the
impact which is intended to be created on the viewer. Accordingly a number of lenses with
different viewing angles are provided. Their focal lengths are slightly adjustable by movement
of the front element of the lens located on the barrel of the lens assembly. This lens compliment
of the TV camera is mounted on a turret. The lens turret is screwed in the front of the camera
and rotation of the turret brings the desired lens in front of the camera tube. An image orthicon
turret assembly holds four lenses of focal lengths 35 mm, 50 mm, 150 mm and a zoom lens of 40
to 400 mm. Figure 7.2 shows such a lens turret mounted in front of a television camera.
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BASIC TELEVISION BROADCASTING
View finder
TV camera
Lens turret
Stand
Fig. 7.2. A television camera with lens turret and view finder.
Zoom Lens
A zoom lens has a variable focal length with a range of 10 : 1 or more. In this lens the viewing
angle and field view can be varied without loss of focus. This enables dramatic close-up control.
The smooth and gradual change of focal length by the cameraman while televising a scene
appears to the viewer as it he is approaching or receding from the scene.
The variable focal length is obtained by moving individual lens elements of a compound
lens assembly. A zoom lens can in principle simulate any fixed lens which has a focal length
within the zoom range. It may, however, be noted that the zoom lens is not a fast lens. The
speed of a lens is determined by the amount of light it allows to pass through it. Thus under
poor lighting conditions, faster fixed focal length lenses mounted on the turret are preferred.
In many camera units only a zoom lens is provided instead of the turret lens assembly.
This alone enables the camera operator to have close-ups, wide coverage of the scene and
distant shots without loss of focus. This is particularly so in colour TV cameras where the
scene is often well defined and suitably illuminated for proper reproduction of colour details.
Camera Mountings
As shown in Fig. 7.2, studio cameras are mounted on light weight tripod stands with rubber
wheels to enable the operator to shift the camera as and when required. It is often necessary to
be able to move the camera up and down and around its central axis to pick-up different
sections of the scene. In such cases, pan-tilt units may be used which typically provide a 360°
rotational capability and allow tilting action of plus or minus 90°. In many applications, primarily
closed circuit systems, where it is desirable to be able to remotely move the camera both
horizontally and vertically, small servo motors are provided as part of the camera mount.
Small motors are also used for remote focusing of the lens unit. In exceptional cases when an
overview of a scene is necessary, a remotely controlled camera is hung from the ceiling.
View Finder
To permit the camera operator to frame the scene and maintain proper focus, an electronic
view-finder is provided with most TV cameras. This view-finder is essentially a monitor which
reproduces the scene on a small picture tube. It receives video signals from the control room
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MONOCHROME AND COLOUR TELEVISION
stabilizing amplifier. The view-finder has its own deflection circuitry as in any other monitor,
to produce the raster. The view-finder also has a built-in dc restorer for maintaining average
brightness of the scene being televised.
Studio Lightings
In a television studio it is necessary to illuminate each area of action separately besides providing
an average level of brightness over the entire scene. Lighting scheme is so designed that shadows
are prevented. As many as 50 to 100 light fittings of different types are often provided in most
studios. The light fixtures used include spot lights, broads and flood lights of 0.5 KW to 5 KW
ratings. A number of such fittings are suspended from the top so that these can be shifted
unseen by the viewer. In big studios catwalks (passages close to the ceiling) are built for ease
of changing location of the suspended light fixtures.
The brightness level in different locations of the studio is controlled by varying effective
current flow through the corresponding lamps. For a smooth current control, dimmerstats
(autotransformers) are used for low rating lamps are silicon controlled rectifiers (SCRs) for
higher power lights. The power to all the lines is fed through automatic voltage stabilizers in
order to maintain a steady voltage supply. The mains distribution boards and switches are
located in a separate room close to the studios. The dimmerstats and other light control
equipment is mounted on a separate panel in the programme control room.
Audio Pick-up
The location and placement of microphones depends on the type of programme. For panel
discussions, news-reading and musical programmes the microphones may be visible to the
viewer and so can be put on a desk or mounted on floor stands. However, for plays and many
other similar programmes the microphones must be kept out of view. For such applications
these are either hidden suitably or mounted on booms. A microphone boom is an adjustable
extended rod from a stand which is mounted on a movable platform. The booms carry
microphones close to the area of pick-up but keep them high enough to be out of the camera
range. Boom operators manipulate boom arms for distinct sound pick-up yet keeping the
microphones out of camera view.
7.3
PROGRAMME CONTROL ROOM
As explained earlier all video and audio outputs are routed through a common control room.
This is necessary for a smooth flow and effective control of the programme material. This room
is called the Programme Control Room (PCR). It is manned by the programme director, his
assistant, a camera control unit engineer, a video mixer expert, a sound engineer and a lighting
director. The programme director with the help of this staff effects overall control of the
programme while it is telecast live or recorded on a VTR. The camera and sound outputs from
the announcer’s booth and VIP studios are also routed through the programme control room.
The video and audio outputs from different studios and other ancillary sources are
terminated on separate panels in the control room. One panel contains the camera control unit
and video mixer. In front of this panel are located a number of monitors for editing and
previewing all incoming and outgoing programmes. Similarly another panel (see Fig. 7.1) houses
microphone controls and switch-in controls of other allied equipment. This panel is under
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BASIC TELEVISION BROADCASTING
111
control of the sound engineer who in consulation with the programme director selects and
controls all available sound outputs.
Fig. 7.3. A typical programme control room.
The producer and the programme assistant have in front of them a talk-back control
panel for giving instructions to the cameramen, boom operator, audio engineer and floor
manager. The producer can also talk over the intercom system to the VTR and telecine machine
operators. The lighting is controlled by switches and faders from a dimmer console which is
also located in the programme control room. Figure 7.3 shows a view of a typical programme
control room.
Camera Control Unit (C.C.U.)
The camera control unit has provision to control zoom lens action and pan-tilt movement besides
beam focus and brightness control of camera tubes. The C.C.U. engineer manipulates various
controls under directions from the producer. In broadcast stations, the video signal must be
maintained within very close tolerances of amplitudes. The C.C.U. engineer has the necessary
facilities to adjust parameters such as video gain, camera sensitivity, blanking level video
polarity etc. For live broadcast of programmes televised far away from the studios, microwave
links are used. The modulated composite video signal received over the microwave link is
demodulated and processed in the usual manner by the C.C.U. engineer for transmission on
the channel allocated to the station.
7.4
VIDEO SWITCHER
A video switcher is a multicontact crossbar switch matrix with provision for selecting any one
or more out of a large number of inputs and switching them on to outgoing circuits. The input
sources include cameras, VTRs and telecine machine outputs, besides test signals and special
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effects generators. Thus at this point the programme producer with the assistance of video
switcher may select the output of any camera, or mix the output of two or more cameras.
Similarly various effects such as fades, wipes, dissolves, supers and so on may be introduced
and controlled with a mixer. The results obtained from these switching procedures are quite
familiar to any one who has watched commercial television programmes. Through switching,
a rather restricted two-dimensional picture presented by one camera can be given additional
perspective by changing the display to another camera that views the same scene from a
somewhat different angle. Also information being viewed by a number of cameras at various
locations can be presented on a single monitor. The ultimate destination of the outputs from
the video switcher may be transmitter or a VTR. It could as well as be a string of monitors in
a closed circuit television system.
Types of Video Switchers. Broadcast switchers incorporate some method of vertical
blanking interval controlled switching. Switching in this manner, during the vertical blanking
period, eliminates any visible evidence of switching that might be observed as a disruption
during the normal vertical scan. There are three types of video switchers :
(i) Mechanical Pushbutton Switcher. In this type the signals are terminated on the
actual switch contacts. The bank of switches is interlocked to prevent simultaneous operation.
This type of switcher is used primarily for portable field units or in CCTV systems because
switching is not frequent and momentary disturbances in the picture during switching can be
tolerated.
C-1
C-2
C-3
MON 1
MON 2
MON 3
MON 4
MON 5
Fig. 7.4. A 3 × 5 Switching matrix.
(ii) Relay Switcher. The relay switcher or relay cross-bar is an electromechanical
switcher. Here magnetically activated read switch contacts are used to effect switching. The
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BASIC TELEVISION BROADCASTING
relays can be operated by remote control lines. Reed relays have fast operate time (around
1 ms) and so can be used to enable switching during the vertical blanking interval. Figure 7.4
shows a 3 × 5 switching matrix employing a reed relay switcher. This is an example of a
distribution-type switcher system where all inputs are available to all monitors. The X’s indicate
the possibility of combinations that may be achieved. If the crosspoint indicated by the ‘X’ on
the intersection of the lines from camera 3 and monitor 2 were selected, the scene viewed by
camera 3 would appear an monitor 2. Similarly any or all of the remaining monitors may be
selected to view any camera that is desired. Isolation amplifiers, though not shown, are used
in-between the cameras and monitors.
(iii) Electronic Switcher. These are all electronic switchers and use solid state devices
that provide transition times of the order of a few micro-seconds. Their size is generally very
small and due to inherent reliability need much less maintenance. Almost all present day
switchers employed in broadcasting are electronic switchers.
Camera-1
100%
% Brightness
0%
Camera-2
(a)
v0
Mixing
network
Amp
Camera-1
output
Camera-2
Came
a-2
output
Amp
Common control
shaft
(b)
Camera-1
100%
% Brightness
0%
Camera-2
(c)
Fig. 7.5. Two types of switching transitions (a) lap-dissolve
(b) circuit for affecting lap-dissolve transition, (c) fade out-fade in transition.
Types of Switching Transitions. The actual switching transition is either carried out
by a lap-dissolve operation or a fade out-fade in form of switching. Both methods are illustrated
in Fig. 7.5. The lap-dissolve switching (Fig. 7.5(a)) may be accomplished by two potentiometers
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MONOCHROME AND COLOUR TELEVISION
connected to the two signals that are to be switched. Signal amplitude from say camera number
1 is slowly reduced, while that from camera number 2 is increased at the same rate. This, as
shown in Fig. 7.5(b) may be done through variable resistors connected at the inputs of two
amplifiers. The values of these potentiometers may be changed as described above to control
the gain of amplifiers whose outputs are then combined, Fig. 7.5(c) illustrates the fade outfade in method of video transfer. It can also be carried out by two potentiometers employed in
the lap-dissolve method. However, in this case, by separate actuation of the two potentiometers,
signal number 1 is slowly reduced in amplitude until zero signal level is obtained, and then
signal No. 2 is slowly raised from 0 to 100 per cent by the second potentiometer. The amplifiers
used are commonly called mixers or faders.
Electronic Switcher Configuration
Figure 7.6 is a functional block diagram of very simple broadcast switcher-mixer. It has five
inputs out of which any two may be selected to drive the two buffer amplifiers. These, in turn
feed into a mixer amplifier.
Captions
camera
Camera
inputs
1
2
3
VTR
4
5
Bus A
Buffer
amplifiers
Mixer
amplifiers
Switched
output
Bus B
Mixer
control
Sync input
Fig. 7.6. A simple switcher for mixing outputs from two buses.
The mixer transfers video signals by fade out-fade in method. The potentiometers at the
remote mixer amplifier can be positioned to select 100 per cent output from either A or B bus.
Assume A and B inputs were at 100 per cent and 0 per cent levels respectively. If camera No.
2 is selected on the A bus, it would appear at the output. Similarly, if camera No. 3 is selected
on the B bus, it will not appear at the mixer output. However, when the levers that control the
potentiometers are moved through their full travel, the output from the mixer amplifier would
transfer from A bus to B bus at a relatively slow rate providing a transition from camera No. 2
to camera No. 3. Similarly more complex switchers can be designed to provide different switching
matrices.
Special Effects Generator
A special effects generator is normally located along with the camera control units in the
camera apparatus room. It is programmed to generate video signals for providing special effects.
Its output is available at a panel in the production control room. The special effects signals
include curtain moving effects, both horizontal and vertical. These are inserted while changing
from one scene to another. Similarly many other patterns are available which can be interposed
in-between any two programmes. Infact several options are available and can be selected while
ordering the equipment.
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BASIC TELEVISION BROADCASTING
7.5
SYNCHRONIZING SYSTEM
To generate a meaningful picture on the raster of a monitor or receiver, some means are needed
to synchronize the scanning systems of both the camera and the monitor. In a multicamera
system, as is often the case in broadcasting, it is necessary to have them all synchronized by a
single sync pulse generator. Accordingly a common sync drive circuitry is provided which controls
scanning sequence, insertion and timing of sync pulses in all the cameras. With such a control,
when the scene shifts from one camera to another, the synchronizing waveforms are in phase
so that the monitor or home receiver is not interrupted in its scanning process. In the absence
of such a provision, while switching from one camera to another, the monitor or receiver would
have to read just its scanning procedure for the incoming camera and the picture might roll
momentarily. Figure 7.7(a) shows one method of driving multiple cameras from a single sync
generator. The sync line is terminated in a 75 ohm resistor because the output of most TV
camera equipment is designed to work into a 75 ohm load.
Camera
MON 1
Sync
generator
MON 2
Distribution
amplifier
Camera
1
MON 3
Camera
2
MON 4
Camera
3
MON 5
75 W
75 W termination
resistor
Fig. 7.7(a) A sync generator driving
several camera units
Camera 1
Fig. 7.7(b) Connections to several monitors
for displaying the output of a single camera.
Camera 2
Camera 3
MON
Push buttons
Fig. 7.7(c). Switcher for selecting any camera output to one monitor.
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Distribution of Camera Outputs to Monitors
Monitors usually have well designed video amplifiers with bandwidth as large as 30 MHz.
This enables excellent reproduction of pictures. Any defects are clearly seen. This is useful
while testing and adjusting studio and other allied equipment. Several monitors may be used
to display the scene viewed by one camera. When the number of monitors is large and they are
located at a considerable distance from each other, a distribution amplifier (see Fig. 7.7(b)) is
used to route the video signal to all of them. Similarly it is often desirable to provide means for
viewing the output from different cameras on a monitor. This is simply done by selecting the
monitor inputs from one camera or another by push button switches as shown in Fig. 7.7(c).
The switches not depressed connect terminating resistors to the appropriate cameras. In
operation all switches are interlocked so that only one camera can be connected to the monitor
at any time. Depressing one switch releases all other.
From mains
(50 Hz)
Shaping
circuit
Mains lock
error
voltage
2fH
multivibrator
AFC
(phase detector
circuit)
50 Hz
Vertical drive
VD
50 Hz
Master
oscillator
31.25 KHz
2fH XTL
oscillator
31.25 KHz
Pulse
shaper
and buffer
31250
Hz
625 : 1
Divider
circuits
and gating
Vertical
drive
shaper
Distribution
amplifier
50 Hz
Ext
source
2:1
Divider
31250
Hz
Vertical
Vertical blanking
blanking
VB
shaper
15625
Hz
Horz.
blanking
shaper
15625 Hz
31250
Hz
Sync
developing
circuits
Horz
drive
shaper
Blanking
mixer
Distribution
amplifier
Mixed
blanking
Distribution
MB
amplifier
Horz drive HD
Distribution
amplifier
50 Hz
Sync
shaper
Mixed
sync
MS
Distribution
amplifier
Fig. 7.8. Block diagram of a drive and sync pulse generator (SPG).
Sync Pulse Generation (SPG) Circuitry
Figure 7.8 is a block representation of a drive and sync pulse generator which would provide
all the pulse waveforms necessary to meet C.C.I.R. standards. The generator contains (i) a
crystal controlled or mains locked timing system, (ii) pulse shapers which generate required
pulse trains for blanking, synchronizing and deflection drives and (iii) distribution amplifiers.
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BASIC TELEVISION BROADCASTING
117
A synchronized master oscillator generates an output at 2fH, viz., 2 × 15625 = 31250 Hz
which drives other circuits. As shown in the figure, this oscillator can be synchronized from
(i) a crystal controlled oscillator operating at exact 2fH, (ii) an external source, or (iii) 50 Hz
mains supply. In the mains frequency lock mode, the 2fH multivibrator is controlled by a phase
detector circuit which compares the 50 Hz square wave derived from the master oscillator
through a divider chain with the 50 Hz square wave derived from mains supply. The error
signal which develops at output of the phase detector corrects the frequency of the multivibrator
and locks it with the mains frequency. While broadcasting programme received from another
TV station, its sync pulses are processed and fed at the ‘external source’ input to slave the sync
circuitry of the station to that of the incoming station.
The master oscillator frequency is twice the horizontal frequency, and is coincident with
the frequency of the equalizing and serration pulses which are driven from its output. The
buffer amplifier isolates the master oscillator from the rest of the circuitry. The divider and
gating block serves two functions. The most important function is to accurately divide the
master oscillator frequency by a factor of 625, thus deriving a 50 Hz output that is phase
locked with the original 31.25 kHz source. The output is correctly shaped in a shaper circuit
before using it to initiate vertical drive and vertical blanking waveforms. The sync developing
circuit provides equalizing and serration pulses.
The output from the master oscillator (31.25 kHz) is also fed to a 2 : 1 divider to derive
output at 15625 Hz, the horizontal frequency. This is used to derive horizontal blanking,
horizontal drive and horizontal sync signals in appropriate circuits.
The basic building block necessary for generating and shaping the various sync and
drive sources include frequency dividers, pulse shapers or stretchers, delay circuits, adders
and logic gates. The circuitry of a modern SPG (sync pulse generator) employs ICs and
transistors. This results in a compact, accurate and reliable unit.
The various outputs from the SPG are derived through distribution amplifiers which
develop the necessary power and act as buffers between the generation and distribution points.
7.6
MASTER CONTROL ROOM (MCR)
In small broadcasting houses the PCR has a master switcher for routing the composite video
signal and allied audio output directly to the transmitter. The ancillary equipment is mostly
located in the Central apparatus room. However, in bigger establishments which have a large
number of studios and production control rooms, all outputs from various sources are routed
through the master control room. This room houses centralized video equipment like sync
pulse generators, special effects generator, test equipment, video and audio monitors besides a
master routing switcher.
Picture Signal Transmission
At the production control room video signal amplitude as received from the camera is very low
and direct coupled amplifiers are used to preserve dc component of the signal. Further on, ac
coupling is provided because it is often technically easier and less expensive to use such a
coupling. This involves loss of dc component which, however, is reinserted at the transmitter
before modulation. This is carried out by a dc restorer circuit often called a blanking level
clamp.
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MONOCHROME AND COLOUR TELEVISION
In the master control room the composite video signal is raised to about one volt P-P
level before feeding it to the cable that connects the control room to the transmitter. Though
the transmitter is located close to the studios, often in the same building, matching networks
are provided at both ends of the connecting cable to avoid unnecessary attenuation and frequency
distortion.
7.7
GENERATION OF AMPLITUDE MODULATION
In AM transmitters where efficiency is the prime requirement, amplitude modulation is effected
by making the output current of a class C amplifier proportional to the modulating voltage.
This amounts to applying a series of current pulses at the frequency of the carrier to the
output tuned (tank) circuit where the amplitude of each pulse follows the variations of the
modulating signal. The resonant frequency of the tuned circuit is set equal to the carrier
frequency. In the tank circuit each current pulse causes a complete sine wave at the resonant
frequency whose amplitude is proportional to the applied current pulse.
The accumulative effect of this flywheel action of the resonant circuit is generation of a
continuous sine wave voltage at the output of tank circuit. The frequency of this voltage is
equal to carrier frequency having amplitude variations proportional to magnitude of the
modulating signal.
RF input
ip
0
t
Total bias
t
0
Fixed bias
v0
– Vg
RF
in
v0
0
t
RL
CB(RF)
+
CN
(neutralizing
capacitor)
B+
AF
in
Fig. 7.9. Grid modulated class C amplifier.
In practice AM may be generated by applying the modulating voltage source in series
with any of the DC supplies of the class C amplifier. Thus grid (or base), plate (or collector) and
cathode (or emitter) modulation are all possible. As an illustration Fig. 7.9 shows the basic
circuit and corresponding waveforms of a grid modulated class C amplifier. The modulating
voltage is in series with the fixed negative grid bias and so the amplitude of the total bias is
proportional to the amplitude of the modulating signal, and varies at the rate of the modulating
frequency. Since the carrier RF source is also in series with the bias and modulating voltage,
these get superimposed and the total bias appears as shown in Fig. 7.9. The resulting plate
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BASIC TELEVISION BROADCASTING
current flows in pulses and the amplitude of each pulse is proportional to the instantaneous
bias and therefore to the instantaneous modulating voltage. As explained earlier the application
of these pulses to the tank circuit then yields amplitude modulation.
In an AM transmitter, amplitude modulation can be generated at any point after the
crystal oscillator. If the output stage in the transmitter is plate modulated the method is called
high level modulation. If modulation is applied at any other point, including some other electrode
of the output amplifier then so-called low-level modulation is produced. The end product,
however, of both the system is the same.
It is not practicable to use plate modulation at the output stage in a television transmitter,
because of the difficulty of generating high video powers at the large bandwidths required.
Accordingly, grid modulation of the output stage is the highest level of modulation employed
in TV transmitters. It is called ‘high-level’ modulation in TV broadcasting and anything else is
then called ‘low-level’ modulation. In recent television transmitter designs, it has become a
standard practice to affect modulation in two stages. A carrier frequency of 40 MHz is employed
in a balanced modulator configuration and a band ranging from 35 to 45 MHz is obtained as its
output. In some designs vestigial sideband correction is also carried out in the modulator
output circuit. Its output is then mixed with an appropriate high frequency to get the desired
channel carrier frequency and its sidebands as the difference product. This process is illustrated
for channel 4 (Band-1) in Fig. 7.10.
USB = 40 to 45 MHz
LSB = 35 to 40 MHz
Video input
0–5 MHz
Balanced
modulator
First local
– oscillator
40 MHz
Mixer
Second local
– oscillator
102.25 MHz
Bandpass
filter (LSB)
57.25 to
67.25 MHz
Channel 4
Channel carrier = 62.25 MHz
USB = 62.25 to 67.25 MHz
LSB = 57.25 to 62.25 MHz
Fig. 7.10. Modulation and frequency translation.
7.8
TELEVISION TRANSMITTER
A simplified functional block diagram of a television transmitter is shown in Fig. 7.11. Necessary
details of video signal modulation with picture carrier of allotted channel are shown in picture
transmitter section of the diagram. Note the inclusion of a dc restorer circuit (DC clamp) before
the modulator. Also note that because of modulation at a relatively low power level, an amplifier
is used after the modulated RF amplifier to raise the power level. Accordingly this amplifier
must be a class-B push-pull linear RF amplifier. Both the modulator and power amplifier
sections of the transmitter employ specially designed VHF triodes for VHF channels and
klystrons in transmitters that operate in UHF channels.
Vestigial Sideband Filter
The modulated output is fed to a filter designed to filter out part of the lower sideband
frequencies. As already explained this results in saving of band space.
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Composite
video signal
TV
camera
Camera
amp
HD VD MB MS
Deflection and
SP Gen
Equalization
and signal level
setting
Video
amp
Signal level
controls
M
Distributor
and
switcher
Audio
processing
unit
M – monitor
LS – Loudspeaker
Video
amp
DC
amp
Frequency
multiplier
Crystal
oscillator
Audio
amp
M
Modulated
RF amp
Power
amp
Transmitting
antenna
From other studios
LS
Audio
input
Coaxial
cable to
transmitter
VSB filter
and
combining
network
RF
amp
LS
Cable to
Distributor transmitter Audio
and
amp
switcher
Audio inputs from
other studios
Preemphasis
circuit
FM
modulator
Crystal
oscillator
AFC
Frequency
multiplier
Power
amp
Fig. 7.11. Simplified block diagram of a television transmitter.
Antenna
The filter output feeds into a combining network where the output from the FM sound
transmitter is added to it. This network is designed in such a way that while combining, either
signal does not interfere with the working of the other transmitter.
A coaxial cable connects the combined output to the antenna system mounted on a high
tower situated close to the transmitter. A turnstile antenna array is used to radiate equal
power in all directions. The antenna is mounted horizontally for better signal to noise ratio.
7.9
POSITIVE AND NEGATIVE MODULATION
When the intensity of picture brightness causes increase in amplitude of the modulated envelope,
it is called ‘positive’ modulation. When the polarity of modulating video signal is so chosen that
sync tips lie at the 100 per cent level of carrier amplitude and increasing brightness produces
decrease in the modulation envelope, it is called ‘negative modulation’. The two polarities of
modulation are illustrated in Fig. 7.12.
+v
+v
Modulating signal
t
0
t
0
Carrier
–v
–v
(a) Positive modulation
(b) Negative modulation
Fig. 7.12. RF waveforms of an amplitude modulated composite video signal.
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BASIC TELEVISION BROADCASTING
Comparison of Positive and Negative Modulation
(a) Effect of Noise Interference on Picture Signal. Noise pulses created by automobile ignition
systems are most troublesome. The RF energy contained in such pulses is spread more or less
uniformly over a wide frequency range and has a random distribution of phase and amplitude.
When such RF pulses are added to sidebands of the desired signal, and sum of signal and noise
is demodulated, the demodulated video signal contains pulses corresponding to RF noise peaks,
which extend principally in the direction of increasing envelope amplitude. This is shown in
Fig. 7.13. Thus in negative system of modulation, noise pulse extends in black direction of the
signal when they occur during the active scanning intervals. They extend in the direction of
sync pulses when they occur during blanking intervals. In the positive system, the noise extends
in the direction of the white during active scanning, i.e., in the opposite direction from the sync
pulse during blanking.
Noise pulse extends
towards black
Noise pulse
Black
White
t
(a)
Noise pulse
Black
White
t
Noise pulse extends
towards white
(b)
Fig. 7.13. Effect of noise pulses (a) with negative modulation,
(b) with positive modulation.
Obviously the effect of noise on the picture itself is less pronounced when negative
modulation is used. With positive modulation noise pulses will produce white blobs on the
screen whereas in negative modulation the noise pulses would tend to produce black spots
which are less noticeable against a grey background. This merit of lesser noise interference on
picture information with negative modulation has led to its use in most TV systems.
(b) Effect of Noise Interference on Synchronization. Sync pulses with positive modulation
being at a lesser level of the modulated carrier envelope are not much affected by noise pulses.
However, in the case of negatively modulated signal, it is sync pulses which exist at maximum
carrier amplitude, and the effect of interference is both to mutilate some of these, and to
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MONOCHROME AND COLOUR TELEVISION
produce lot of spurious random pulses. This can completely upset the synchronization of the
receiver time bases unless something is done about it. Because of almost universal use of
negative modulation, special horizontal stabilizing circuits have been developed for use in
receivers to overcome the adverse effect of noise on synchronization.
(c) Peak Power Available from the Transmitter. With positive modulation, signal
corresponding to white has maximum carrier amplitude. The RF modulator cannot be driven
harder to extract more power because the non-linear distortion thus introduced would affect
the amplitude scale of the picture signal and introduce brightness distortion in very bright
areas of the picture. In negative modulation, the transmitter may be over-modulated during
the sync pulses without adverse effects, since the non-linear distortion thereby introduced,
does not very much affect the shape of sync pulses. Consequently, the negative polarity of
modulation permits a large increase in peak power output and for a given setup in the final
transmitter stage the output increases by about 40%.
(d) Use of AGC (Automatic Gain Control) Circuits in the Receiver. Most AGC circuits in
receivers measure the peak level of the incoming carrier signal and adjust the gain of the RF
and IF amplifiers accordingly. To perform this measurement simply, a stable reference level
must be available in the signal. In negative system of modulation, such a level is the peak of
sync pulses which remains fixed at 100 per cent of signal amplitude and is not affected by
picture details. This level may be selected simply by passing the composite video signal through
a peak detector. In the positive system of modulation the corresponding stable level is zero
amplitude at the carrier and obviously zero is no reference, and it has no relation to the signal
strength. The maximum carrier amplitude in this case depends not only on the strength of the
signal but also on the nature of picture modulation and hence cannot be utilized to develop
true AGC voltage. Accordingly AGC circuits for positive modulation must select some other
level (blanking level) and this being at low amplitude needs elaborate circuitry in the receiver.
Thus negative modulation has a definite advantage over positive modulation in this respect.
The merits of negative modulation over positive modulation, so far as picture signal
distortion and AGC voltage source are concerned, have led to the use of negative modulation
in almost all TV systems now in use.
7.10 SOUND SIGNAL TRANSMISSION
The outputs of all the microphones are terminated in sockets on the sound panel in the
production control room. The audio signal is accorded enough amplification before feeding it to
switchers and mixers for selecting and mixing outputs from different microphones. The sound
engineer in the control room does so in consultation with the programme director. Some prerecorded music and special sound effects are also available on tapes and are mixed with sound
output from the studio at the discretion of programme director. All this needs prior planning
and a lot of rehearsing otherwise the desired effects cannot be produced. As in the case of
picture transmission, audio monitors are provided at several stages along the audio channel to
keep a check over the quality and volume of sound.
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BASIC TELEVISION BROADCASTING
123
Preference of FM over AM for Sound Transmission
Contrary to popular belief both FM and AM are capable of giving the same fidelity if the
desired bandwidth is allotted. Because of crowding in the medium and short wave bands in
radio transmission, the highest modulating audio frequency used in 5 kHz and not the full
audio range which extends up to about 15 kHz. This limit of the highest modulating frequency
results in channel bandwidth saving and only a bandwidth of 10 kHz is needed per channel.
Thus, it becomes possible to accommodate a large number of radio broadcast stations in the
limited broadcast band. Since most of the sound signal energy is limited to lower audio
frequencies, the sound reproduction is quite satisfactory.
Frequency modulation, that is capable of providing almost noise free and high fidelity
output needs a wider swing in frequency on either side of the carrier. This can be easily allowed
in a TV channel, where, because of very high video frequencies a channel bandwidth of 7 MHz
is allotted. In FM, where highest audio frequency allowed is 15 kHz, the sideband frequencies
do not extend too far and can be easily accommodated around the sound carrier that lies 5.5 MHz
away from the picture carrier. The bandwidth assigned to the FM sound signal is about 200 kHz
of which not more than 100 kHz is occupied by sidebands of significant amplitude. The latter
figure is only 1.4 per cent of the total channel bandwidth of 7 MHz. Thus, without encroaching
much, in a relative sense, on the available band space for television transmission all the
advantages of FM can be availed.
7.11 MERITS OF FREQUENCY MODULATION
Frequency modulation has the following advantages over amplitude modulation.
(a) Noise Reduction
The greatest advantage of FM is its ability to eliminate noise interference and thus increase
the signal to noise ratio. This important advantage stems from the fact that in FM, amplitude
variations of the modulating signal cause frequency deviations and not a change in the amplitude
of the carrier. Noise interference results in amplitude variations of the carrier and thus can be
easily removed by the use of amplitude limiters.
It is also possible to reduce noise in FM by increasing frequency deviation. This deviation
can be made as large as required without increasing the transmitter power. Higher audio
frequencies are mostly harmonics of the lower audio range. They have low amplitudes and so
cause a small deviation of the carrier frequency. Noise power interference is also generally low
in amplitude and so results in frequency deviation similar to that caused by higher audio
frequencies. Thus higher audio frequencies are most susceptible to noise effects. If these
frequencies were artificially boosted in amplitude at the transmitter and correspondingly
reduced at the receiver, improvement in noise immunity could be expected. This in fact is the
standard practice in all FM transmission and reception. In AM on the other hand, the signal
modulation can be increased relative to noise modulation only by increasing the transmitter
output power. A 20 db improvement in signal-to-noise voltage ratio requires ten-times increase
in frequency deviation in FM but an increase of 100 times in AM power output. Evidently an
AM system in this respect reaches an economical limit long before the FM system, provided
additional bandwidth is available for FM transmission.
In an FM receiver, if two signals are being received simultanesouly, the weaker signal
will be eliminated almost entirely if it possesses less than half the amplitude of the other
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MONOCHROME AND COLOUR TELEVISION
stronger signal. However, in AM the interfering signal or station can be heard or received even
when a 100 : 1 relationship exists between their amplitudes.
Pre-emphasis and De-emphasis. The boosting of higher audio modulating frequencies,
in accordance with a prearranged response curve is termed pre-emphasis, and the compensation
at the receiver is called de-emphasis. Examples of circuits used for each function are shown in
Fig. 7.14. As is obvious from these configurations, the pre-emphasis and de-emphasis networks
are high-pass and low-pass filters respectively. The time constant of the filter for pre-emphasis
at transmitter and later de-emphasis at receiver has been standardized at 50 µs in all the
CCIR systems. However, in systems employing American FM and TV standards, networks
having a time constant of 75 µs are used. A 50 µs (= RC) de-emphasis corresponds to a frequency
1
, which comes to 3180 Hz.
2πRC
Figure 7.15 shows pre-emphasis and de-emphasis curves corresponding to a time constant of
50 µs.
response curve that is 3 db down at the frequency given by
+ Vcc
+ Vcc
L/R = 50 ms
L = 0.5H
RC = 50 ms
RL
OR
R = 10 KW
C = .001 mF
v0
AF in
v0
AF in
Cc
R = 50 KW
Pre-emphasized
AF out
Fig. 7.14(a). Pre-emphasis circuits.
R = 50 K
Cc
L = 0.5 H
AF out
v.in
Pre-emphasized
AF in from
FM demodulator
Cc
AF out
v.in
L/R = 50 s
C = .001 F
R = 10 K
OR
RC = 50 s
Fig. 7.14(b). De-emphasis circuits.
db
12.6
Pre-emphasis curve
+3
0
–3
De-emphasis curve
12.6
25 Hz
3180 Hz 15 KHz
Fig. 7.15. 50 µs emphasis curves.
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BASIC TELEVISION BROADCASTING
125
(b) Transmitter Efficiency
The amplitude of the FM wave is independent of the depth of modulation, whereas in AM it is
dependent on this parameter. This means that low level modulation can be used in FM and all
succeeding amplifiers can be class ‘C’ which are more efficient. Thus, unlike AM, all amplifiers
handle constant power and this results in more economical FM transmitters.
(c) Adjacent Channel Interference
Because of the provision of a guard band in between any two TV channels, there is less
interference than in conventional AM broadcasts.
(d) Co-channel Interference
The amplitude limiter in the FM section of the receiver works on the principle of passing the
stronger signal and eliminating the weaker. In this manner, a relatively weak interfering
signal or any pick-up from a co-channel station (a station operating at the same carrier
frequency) gets eliminated in a FM system.
It may be noted that from general broadcast point of view FM needs much wider
bandwidth than AM. It is 7 to 15 times as large as that needed for AM. Besides, FM transmitters
and receivers tend to be more complex and hence are expensive. However, in TV transmission
and reception, where handling of the picture signal is equally complex, FM sound does not add
very much to the cost of equipment.
7.12 GENERATION OF FREQUENCY MODULATION
The primary requirement of an FM generator is a variable output frequency, where the
variations are proportional to the instantaneous value of the modulating voltage. In one method
of FM generation, either the inductance or capacitance of the tank circuit of an LC oscillator is
varied to change the frequency. If this variation can be made directly proportional to the
amplitude of the modulating voltage, true FM will be obtained.
A voltage-variable reactance is generally placed across the oscillator tank circuit. The
oscillator is tuned to deliver the assigned carrier frequency with the average reactance of the
variable element present in parallel with its own tank circuit. The capacitance (or inductance)
of the reactance element changes on application of modulating voltage to cause frequency
deviations in the oscillator frequency. Larger the departure of modulating voltage from zero,
greater is the reactance variation, and in turn higher is the frequency deviation.
Basic Reactance Modulator
An FET, tube or transistor when suitably biased can be used as a variable reactance element.
Similarly a varactor diode can also be used for this purpose. The basic circuit arrangements of
a reactance modulator either with an FET or with a vacuum tube are shown in Fig. 7.16.
Provided certain simple conditions are met, the impedance Z as seen at the terminals marked
A – A′ in the figures, is almost entirely reactive. The conditions which must be met are, (i) the
current ig should be negligible compared to the plate (or drain) current, and (ii) the impedance
XC >> R, preferably by more than 5 : 1.
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MONOCHROME AND COLOUR TELEVISION
A
ig
C
G
R
vg
i
D
C
Z
gm
A
ig
i
P
G
v
S
R
vg
Z
v
K
A
A
0
– Vg
(a)
(c)
(b)
Fig. 7.16. Basic reactance modulator circuits (a) employing an FET (b) employing a VT
(c) relation between bias voltage and transconductance of the device.
With these assumptions, the following analysis, which is valid for both the circuits can
be made : With a voltage v applied across the terminals A – A′, currents i and ig will flow in the
plate (or drain) circuit and bias circuit respectively. The current ig will develop a voltage vg = ig
×R=
gm × R × V
V×R
. The current i because of tube or FET action = gmvg =
, where gm is
R − jX C
R − jX C
the mutual conductance of the device.
Z=
∴
FG
H
IJ
K
1
R − jX C
X
v
1− j C .
=
=
g
gm × R
R
i
m
If XC >> R, the above equation will reduce to ;
Z=–j
XC
gm × R
This impedance is clearly a capacitive reactance, which may be written as :
Xeq =
∴
1
XC
1
=
=
2πfgm RC
2πf C eq
gm R
Ceq = gmRC
The following conclusions follow from this result :
(i) The equivalent capacitance depends on the device transconductance and can therefore be varied with bias voltage. The approximate relation between the bias voltage
and gm is illustrated in Fig. 7.16 (c).
(ii) The capacitance can be originally adjusted to any value (within reasonable limits), by
variation of the components R and C.
(iii) gmRC has the dimensions of capacitance.
(iv) From the circuits and analysis made, it is clear that if R and C are interchanged, the
impedance across A – A′ will become inductive with Leq =
RC
.
gm
(v) Similarly it can be shown that by using L and R instead of C and R in the biasing
circuit, both capacitive and inductive reactances can be obtained across the terminals
A – A′.
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BASIC TELEVISION BROADCASTING
Transistor Reactance Modulator
Figure 7.17 shows a circuit of an LC oscillator that is frequency modulated by a capacitive
reactance (RC) transistor modulator. As shown, the audio frequency voltage is applied at the
base of the transistor Q1. The amplitude variations of this driving voltage vary the forward
bias to change the transistor collector current. This changes β of the transistor which results
in a proportionate change in the equivalent capacitance across the oscillator tank circuit. Note
the use of RF chokes in the circuit. They are used to isolate various points of the circuit for ac
current while providing a dc path.
+ Vcc
RFC
Cc
RFC
R3
R1
Reactance
modulator
C
Oscillator
Q2
Q1
Tank circuit
R
R2
AF in
C(RF)
RE
Cf
CE
R4
RE
C1 L
CE
C2
FM
output
C
Fig. 7.17. Reactance modulator circuit.
Varactor Diode Modulator
The circuit of Fig. 7.18 shows such a modulator. It is seen that the varactor diode has been
back-biased to provide the junction capacitance effect. The bias is varied by the modulating
voltage which is injected in series with the dc bias source through transformer T1. The
instantaneous changes in the bias voltage cause corresponding changes in the junction
capacitance, which in turn vary the oscillator frequency accordingly. It is often used for automatic
frequency control and remote tuning.
To
oscillator
tank
circuit
Cc
Varactor
diode
RFC
T1
AF in
Cb
(RF)
– VD
Fig. 7.18. Varactor diode modulator.
7.13 STABILIZED REACTANCE MODULATOR
Although the oscillator on which the reactance modulator operates cannot be crystal controlled,
it must nevertheless have the stability of a crystal if it is to be a part of a commercial transmitter.
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MONOCHROME AND COLOUR TELEVISION
This suggests that frequency stabilization of the reactance modulator is required. The block
diagram of Fig. 7.19 shows a typical AFC (automatic frequency control) system used with the
reactance controlled FM transmitter. Here a fraction of the output is taken from the limiter
and fed to a mixer, which also receives the signal from a crystal oscillator. The resulting
difference signal at a frequency which is much less than that of the master oscillator, is amplified
and fed to a phase* discriminator. The output of the discriminator which is connected to the
reactance modulator, provides a dc control voltage to counteract rapidly any drift in the average
frequency of the master oscillator. The time constant of the discriminator load is quite large (of
the order of 100 ms). Hence the discriminator will react to slow changes in the incoming
frequency, but not to normal frequency changes due to frequency modulation, which are too
fast. Note that the discriminator must be connected to give a positive output if the input
frequency is higher than the discriminator tuned frequency and a negative output if it is lower.
Thus any drift in the oscillator frequency towards the higher side will produce a positive control
voltage and this fed in series with the input of the reactance modulator, will increase its transconductance. This increases its output capacitance (Ceq = gmRC) and thus lowers the oscillator’s
centre frequency. Analogous sequence of operations will take place to raise the oscillator
frequency when it drifts on the lower side of its centre frequency. In the transmitter block
diagram of Fig. 7.11 a reactance modulator with such a frequency control has been incorporated.
Master
oscillator
Frequency
multiplier
Buffer
Limiter
FM out
DC control voltage
Reactance
modulator
fs
AF in
Crystal
oscillator
f0
Mixer
fs – f0
RF
discriminator
IF
amplifier
Fig. 7.19. A typical transmitter AFC system.
7.14 GENERATION OF FM FROM PM
The direct modulators have the disadvantage of being based on an oscillator which is not
stable enough for communication or broadcast purposes. As explained above, it needs
stabilization which adds to circuit complexity. Note that the use of a crystal oscillator is not
possible because it cannot be successfully frequency modulated. It is possible, however, to
generate FM via Phase Modulation (PM) where a crystal oscillator can be used. It has been
dφ(t)
= ωc
dt
+ 2π∆f cos ωmt (eqn. 4.6) for a sinusoidal modulating signal. Conversely an FM signal of amplitude
shown in Chapter 4 that the instantaneous angular frequency for an FM signal ωi =
*Phase discriminator circuits are discussed in Chapter 21, which is devoted to FM sound detection.
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BASIC TELEVISION BROADCASTING
A is given by vFM(t) = A cos
LM
N
z
t
−∞
ω i dt
OP
Q
LM
O
cos ω t P
N
Q
L ∆f
O
= A cos Mω t + f sin ω t P
N
Q
= A cos ω c t + 2π∆f
c
z
t
m
−∞
m
m
In contrast a phase modulated (FM) signal
v
= A cos [ωct + ∆ φ cos ωmt]
PM(t)
where both ∆f and ∆φ are independent of fm and depend only on the modulating signal amplitude
and system constants. Hence the only difference between FM and PM is that if the modulating
signal is integrated before performing PM then we get FM. An integrator is a low-pass (bassboost) circuit. A simple R-L integrator is shown in Fig. 7.20 (a). Here R/(2πL) is set at about 30
Hz, so that in the audio range the response falls with frequency at 6 db/decade.
Armstrong FM System
Figure 7.20 (b) shows the functional block diagram of an Armstrong FM system. As explained
above the audio voltage enters the modulator, which is essentially a phase modulator, after
bass-boosting through an equalizer. The carrier frequency from the crystal oscillator after a
phase shift of 90° is fed to a balanced modulator which also receives equalized audio signal.
The two sidebands obtained from the balanced modulator are added to the unmodulated carrier
in the combining amplifier. The amplitude of carrier voltage obtained from the crystal oscillator
through the buffer stage is kept quite large in comparison with that of the sidebands. This as
usual, is essential for effective modulation.
L
AF in
= L/R
Equalized
AF out
R
Em
Em
Em Em
Ec
E out/E in R/ L
d
d
Ec
(a) RL equalizer
(c) FM phasor diagram
(d) AM phasor diagram
fc 0°
Crystal
oscillator
Carrier
frequency
(fc)
Buffer
amplifier
Combining
amplifier
Frequency
multipliers
Power
amplifier
FM out
Side bands
90° phase
shifter
Balanced
modulator
fc 90°
1/f network
(audio equalizer)
(b) Block diagram
Fig. 7.20. The Armstrong frequency-modulation system.
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AF in
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MONOCHROME AND COLOUR TELEVISION
As shown in the phasor diagram of Fig. 7.20 (c), the two sideband phasors rotate in
unison but in opposite direction, and the sum of the carrier and sidebands causes the carrier to
change phase by the angle φd. It will be noted that the phase relation of carrier and side
frequencies is at 90° with respect to the AM situation as shown in Fig. 7.20 (d) where only the
amplitude of the carrier is varied and not the phase. It may also be noted that the resultant
phasor (see Fig. 7.20 (c)) in the case of phase modulation varies in amplitude besides phase
deviation and thus a little amplitude modulation is also present in the output. However, the
change in amplitude is made very small, though at a price, by making the phase deviation very
small. A typical value is ∆f = 50 Hz at a carrier frequency of 1 MHz corresponding to a maximum
phase deviation of 5 × 10– 5 radians. For such a small phase deviation, incidental amplitude
variation is negligible.
The most convenient crystal oscillator frequency is around 1 MHz, while TV broadcast
carrier frequencies are in the region of 50 to 100 MHz. Frequency multipliers are used to raise
the carrier frequency. It may be noted that in frequency multiplication, ∆f and the carrier
frequency get multiplied by the same factor. Usually to achieve the final ∆f of ± 75 kHz, a
larger multiplying factor is required than that needed to raise the carrier frequency to the
required value. This is accomplished by shifting the carrier frequency down by a hetrodyning
process at some point within the multiplier chain.
7.15 FM SOUND SIGNAL
As explained in an earlier section, audio signals from different microphones are received at
the sound panel in the production control room. After due amplification all the outputs are fed
into a switcher, where if necessary they are mixed and the desired output is selected. The final
output goes to a distributor in the master control room, where both picture and sound signals
from different studios are received. This distributor is switched to select corresponding picture
and sound signals from the desired studio. As in case of video signals the audio signals are also
routed to the sound transmitter through a cable (see Fig. 7.11) with matching networks on
either side.
At transmitter the audio signal is frequency modulated and transferred to assigned
channel sound carrier frequency by the use of multipliers. It is later amplified through several
stages of power amplifiers to raise the power output to desired level. Audio monitors are provided
at various points to keep a check on the sound quality. It is finally fed to the common antenna
array through a combining network for radiation along with the modulated picture signal.
Review Questions
1.
Draw the layout of a typical television studio and explain how the picture and sound signals are
processed in the control room. What is the role of a special effects generator ?
2.
What is the difference between a self-contained and a two-unit camera system ? What is the
function of view finder which is provided at the hood of camera ?
3.
Explain how in a multicamera system, synchronization is maintained between the cameras and
control monitor. Explain with a functional block diagram how sync and equalizing pulses are
generated and kept phase-locked with a common master oscillator.
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BASIC TELEVISION BROADCASTING
131
4.
Draw a 3 × 5 switching matrix designed to select output from different cameras on any of the five
monitors. Explain how an electronic video switcher accomplishes a smooth change-over from one
output to another through a mixer amplifier.
5.
Draw a block diagram to show how the video signal is modulated and processed at the picture
transmitter. Why is high level modulation not used in a TV transmitter ?
6.
Discuss the merits and demerits of positive and negative amplitude modulation and justify the
choise of negative modulation in most TV systems.
7.
Why is FM chosen for transmission of sound signal in TV systems ? Why are pre-emphasis and
de-emphasis circuits provided at the FM transmitter and receiver respectively ?
8.
How is frequency modulation produced ? Draw the circuit of a basic reactance modulator and
prove that its output reactance varies with changes in the amplitude of the drive voltage.
9.
Draw the block diagram of an AFC circuit that forms part of a reactance FM modulator to stabilize the centre frequency of the master oscillator. Explain its control action.
10. Explain briefly how a phase modulator can be used to generate frequency modulation. Draw the
block schematic diagram of an Armstrong modulator and explain how an FM output is obtained
from it. What is the main merit of this modulator ?
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8
Television Receiver
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8
Television Receiver
The preceding chapters were devoted to the complexities and essential requirements of
generation and transmission of composite video and sound signals associated with the televised
scene. As is logical, we should now turn our attention to the receiver. In effect, a television
receiver is a combination of an AM receiver for the picture signal and an FM receiver for the
associated sound. In addition, the receiver also provides suitable scanning and synchronizing
circuitry for reproduction of image on the screen of picture tube. We shall confine our discussion
to monochrome (black and white) receivers. The basic principle and essential details of colour
receivers are described in Chapters 25 and 26. However, it may be noted that all the circuits
for a black-and-white picture are also needed in a colour receiver. The colour television picture
is just a monochrome picture with colour added in the main areas of picture information.
8.1
TYPES OF TELEVISION RECEIVERS
The receiver may use tubes for all stages, have all solid-state devices-transistors and integrated
circuits, or combine tubes and transistors as a hybrid receiver. A typical chassis of a monochrome
receiver is shown in Fig. 8.1.
Video and sound
(intercorrier) IF section
Video and audio
output circuits
Sync and
vertical
deflection
circuits
Loudspeaker
Vertical (line)
output
transformer
Damper diode
VHF tuner
Horz output and
horizontal EHT
section
Contrast
Brightness
On/off
volume
Vertical hold
Picture tube Power
supply
Yoke
Fig. 8.1. Rear view of a black and white receiver with the back removed.
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TELEVISION RECEIVER
(a) All Tube Receivers. This type mainly applies to earlier monochrome receivers. All
the functions are provided by about 12 tubes including several multipurpose tubes with two or
three stages in one glass envelope. The dc supply for tubes is between 140 to 280 volts.
(b) Solid-State Receivers. In this type all states except the picture tube use semiconductor
diodes, transistors and integrated circuits. The dc supply is between 12 to 100 volts for various
stages. The heater power to the picture tube is supplied through a separate filament transformer.
(c) Hybrid Receivers. In this type the deflection circuits generally use power tubes, while
the signal circuits use transistor and integrated circuits. These receivers usually have a line
connected power supply, with series heaters for the tubes. Two dc sources, one for semiconductor
devices and the other for tubes are provided.
8.2
RECEIVER SECTIONS
It is desirable to have a general idea of the organization of the receiver before going into circuit
details. Figure 8.2 shows block schematic diagram of a typical monochrome TV receiver. As
shown there, the receiver has been divided into several main sections depending on their
functions and are discussed below.
UHF
antenna
Sound IF
amplifier
5.5 MHz
UHF Tuner
VHF
antenna
UHF
mixer
Local
osc
FM
sound
detector
Audio
voltage
amplifier
FM intercarrier 5.5 MHz
sound signal trap cct
Audio
power
amplifier
Speaker
Volume control
Fine tuning
RF
amplifier
Mixer
Local
osc
Fine tuning
VHF tuner
Video IF
amplifier
2/3 stages
Last
video IF
amplifier
AGC line
P.IF = 38.9 MHz
S.IF = 33.4 MHz
R1
C1
Sync
separator
Video
detector
AGC
circuit
F
F
Contrast
Keying pulses from
control
horz output stage
Brightness
control
50 Hz
Vertical
Vertical
output
(field) osc
amplifier
50 Hz
15625 Hz
Vertical hold
C2
AFC
AC filament voltage
R2
AC mains
220 V, 50 Hz
Low voltage
power supply
B+
Video
amplifier
Horz
(line) osc
15625 Hz.
Horz
output
amplifier
Pulses from horz
output stage
Yoke
Pulses for
keyed AGC
and AFC
Damper
Fig. 8.2. Block diagram of a monochrome television receiver.
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Picture
tube
EHT
rectifier
Boosted B+
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MONOCHROME AND COLOUR TELEVISION
Antenna System
Strongest signal is induced in the antenna if it has same polarization as the transmitting
antenna. All TV antennas are mounted in horizontal position for better reception and favourable
signal to noise ratio. The need for good signal strength has led to the use of tuned antennas.
For channels located in the VHF band, a half wave-length antenna is most widely used. Such
antennas behave like low ‘Q’ tuned circuits and a single antenna tuned to the middle frequency
of various channels of interest can serve the purpose. Various antennas in use are of dipole
type with reflectors and directors. A folded dipole with a reflector is used because its response
is more uniform over a band of frequencies. A Yagi antenna, i.e., a dipole with one reflector and
two or more directors, is a compact high gain directional array, and is often used in fringe
areas. In areas where signal strength is very low, booster amplifiers with suitable matching
network are used. On the other hand, in areas situated close to a transmitting antenna, where
signal strength is quite high, various types of indoor antennas are frequently employed.
Since it is not possible for one dipole antenna to cover both upper and lower VHF band
channels effectively; high and low band dipoles are mounted together and connected to a common
transmission line. For channels in the UHF bound, where the attenuation is very high and the
signals reaching the antenna are weaker, special antennas like fan dipole, rhombic and parabolic
reflector type are often used.
A transmission line connects antenna to the receiver input terminals for the RF tuner.
A twin-lead is generally used. This type is an unshielded balanced line with characteristic
impedance equal to 300 ohms. When there is a problem of interference, a shielded coaxial
cable is used. This cable has high attenuation, especially at UHF channel frequencies. It has a
characteristic impedance of 75 ohms.
The current practice is to design input circuit of the TV receiver for a 300 ohm
transmission line. It has been found that a 300 ohm transmission line used with a half-wave
dipole produces a broad frequency response without too large a loss due to mismatching. A
folded dipole has an impedance close to 300 ohms at its resonant frequency, and a much uniform
response is obtained with this antenna. Receivers designed to receive UHF channels have two
inputs; one to match a 300 ohm transmission line and the other for a 75 ohm coaxial cable. A
signal strength of the order of 500 µV to 1 mV and a signal to noise ratio of 30 : 1 are considered
adequate for satisfactory reception of both picture details and sound output.
RF Section
This section consists of RF amplifier, mixer and local oscillator and is normally mounted on a
separate sub-chassis, called the ‘Front End’ or ‘RF Tuner’. Either tubes or transistors can be
used. With tubes, local oscillator and mixer functions are usually combined in one stage called
the ‘frequency converter’. The purpose of the tuner unit is to amplify both sound and picture
signals picked up by the antenna and to convert the carrier frequencies and their associated
bands into the intermediate frequencies and their sidebands . The receiver uses superhetrodyne
principle as used in radio receivers. The signal voltage or inoformation from various stations
modulated over different carrier frequencies is hetrodyned in the mixer with the output from
a local oscillator to transfer original information on a common fixed carrier frequency called
the intermediate frequency (IF). The setting of the local oscillator frequency enables selection
of desired station. The standard intermediate frequencies for the 625-B system are-Picture
IF = 38.9 MHz, Sound IF = 33.4 MHz.
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In principle an RF amplifier is not necessary and signal could be fed directly to the
tuned input circuit of the mixer. However, the problems of a relatively weak input signal with
low signal to noise ratio, local oscillator radiation and image rejection are such, that a stage of
amplification ahead of the mixer is desirable. The tuning for different channels is carried out
with a channel selector switch which changes resonant frequencies of the associated tuned
circuits by varying either inductance or capacitance of these circuits. The RF section is shown
separately in Fig. 8.3 (a), where the channel selector switch has been set for channel 4 (Band I),
VHF tuner
RF
amplifier
Picture IF
38.9 MHz
Mixer
IF out
Sound IF = 33.4 MHz
Local
osc
(Ganged)
Channel selector
switch
101.15 MHz
Fine
tuning
Fig. 8.3 (a). Block diagram of a VHF tuner. Selector switch
set for channel 4 band I (61-68 MHz).
Fig. 8.3 (b). Ideal response curve of the RF amplifier when set for channel 4.
i.e., 61 to 68 MHz. The picture carrier frequency in this channel is 62.25 MHz and the sound
carrier 67.75 MHz. The RF amplifier must have sufficient bandwidth to accept both the picture
and sound signals. This is illustrated in Fig. 8.3 (b). The local oscillator frequency is set at
101.15 MHz. In the mixer, both sum and difference (sideband) frequencies are generated. The
output circuit of the mixer is however, tuned to deliver difference frequencies i.e., the
intermediate frequencies and their sidebands. The required IFs are then produced as here:
(Local oscillator frequency of 101.15 MHz)–(Picture carrier frequency of 62.25 MHz) = Picture
IF of 38.9 MHz, (Local oscillator frequency of 101.15 MHz)–(Sound carrier frequency of 67.75
MHz) = 33.4 MHz. The desired output response from the mixer is shown in Fig. 8.4. Notice that
frequency changing process reverses the relative positions of the sound and picture signals.
This is obvious, since the oscillator works above the signal frequencies, and ‘difference’
frequencies produced, when the picture and sound frequencies are substracted must give a
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MONOCHROME AND COLOUR TELEVISION
higher IF from the lower frequency picture signal. It may be noted that picture and sound
signals would remain in the same relative position, i.e., with sound carreir frequency higher
than picture carrier frequency if local oscillator frequency is set below, instead of above the
carriers. The local oscillator frequency is kept higher because of ease of oscillator design and
several other merits. The ratio of highest to lowest radio frequency that the local oscillator
must generate, when the oscillator frequency is chosen to be higher than the incoming carrier
frequency, is much less than when the local oscillator frequency is kept below the incoming
channel frequency. It is much easier to design an oscillator that maintains almost a constant
output amplitude and a sinusoidal waveshape when its overall frequency range is less. This
justifies the choice of higher local oscillator frequency.
Fig. 8.4. Location of sound and picture IF frequencies at the output of mixer.
The tuning of RF and oscillator tuned circuits is pre-set for switching in different channels.
Despite the fact that modern tuner units are remarkably stable, most receiver manufacturers
provide a fine tuning control for small adjustments of local oscillator frequencies. The control
is varied to obtain best picture results on the screen.
IF Amplifier Section
A short length of coaxial cable feeds tuner output to the first IF amplifier. This section is also
called video IF amplifier since composite video signal is the envelope of the modulated picture
IF signal. Practically all the gain and selectivity of the receiver is provided by the IF section.
With tubes, 2 or 3 IF stages are used. With transistors, 3 to 4 If stages are needed. In integrated
circuits, one IC chip contains all the IF amplifier stages.
Essential Functions of the IF Section
The main function of this sections is to amplify modulated IF signal over its entire bandwidth
with an input of about 0.5 mV signal from the mixer to deliver about 4 V into the video detector.
This needs an overall gain of about 8000. This gain should be adjustable, by automatic gain
control, over a wide range to accommodate input signal variations at the antenna from 50 µV
to 0.5 V, to deliver about 4 V peak-to-peak signal at the input of the video detector. To achieve
desired gain, atleast three stages of tuned amplifiers are cascaded and to obtain desired
bandwidth the resonant frequencies of these stages are staggered. Such an arrangement
provides desired gain and selectivity.
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TELEVISION RECEIVER
8.3
VESTIGIAL SIDEBAND CORRECTION
Another important function assigned to IF section is to equalize the amplitudes of side-band
components, because of vestigial sideband transmission. The need for this was fully explained
in Chapter 4, and a reference to Fig. 4.8 will show, that for vestigial side-band correction the
picture carrier frequency gain must be 6 db down on the IF frequency response curve. It is also
necessary to shape the IF response curve around the picture IF frequency in such a way that
all lower video frequencies, which got a boost on account of partial lower side-band transmission
(besides the full upper side band), are duly attenuated and get restored to their actual level.
This is achieved by suitable tuning and shaping the response of the IF stages. This is fully
illustrated in Fig. 8.5, which shows ideal overall response of the IF section.
Required channel-3
Higher adjacent
(Band I)
channel–4
(54–61 MHz)
fcP = 62.25 MHz
fcP = 55.25 MHz
f
cS = 67.75 MHz
fcS = 60.75 MHz
Local osc frequency = 94.15 Mhz
Lower adjacent
channel–2
fcP = 48.25 MHz
fcS = 53.75 MHz
db
0
100%
Vestigial
sideband
correction
Relative gain
–6
50%
– 12
– 18
– 24
– 26
Sound signal
attenuation
– 30
Unwanted strong
adjacent channel
– 36
beat signal
94.15–62.25
– 40
= 31.9 MHz
31
Unwanted strong
adjacent channel beat
signal 94.15–53.75
= 40.4 MHz
Picture IF
38.9 MHz
5%
Sound IF
33.4 MHz
32
33
34
35
36
37
38
39
Trap circuit
40
41
f(MHz)
Trap circuit
Fig. 8.5. Overall picture IF response curve of a receiver tuned to channel 3-(Band I).
The diagram shows disposition of IF frequencies, vestigial sideband correction,
sound signal attenuation and locations of unwanted adjacent channel
interfering beat frequencies.
In the IF amplifier circuitry, provision must be made for rejection of signals from adjacent
channels. For this purpose special tuned circuits, called trap circuits, are connected in the
signal path in such a way that the offending frequencies are removed. These trap circuits are
disposed at convenient places in the IF amplifiers. Their position will vary from receiver to
receiver, but generally they are placed in the input circuit of the first IF amplifier. The way in
which these unwanted adjacent channel IF signals appear is illustrated in Fig. 8.5. As an
example, suppose that the receiver is switched to channel 3 on Band I. The local oscillator
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MONOCHROME AND COLOUR TELEVISION
frequency for channel 3 is (55.25 + 38.9) 91.15 MHz. This would beat with the channel picture
and sound carrier frequencies to give the desired picture and sound IF frequencies. Besides
these, the sound carrier of channel 2, which is close to the beginning of channel 3, will beat
with the local oscillator to give unwanted difference frequency of 40.4 MHz (94.15 – 53.75),
which would lie close to the upper skirt of desired IF response. Similarly, the picture carrier of
upper adjacent channel 4, will also beat with the local oscillator to produce another unwanted
difference frequency signal of 31.9 MHz (94.15 – 62.25). This is close to the lower skirt of IF
response. The trap circuits are designed to attenuate these two adjacent channel interfering
frequencies by about 40 db as shown in the figure. It is understood that such interference
would occur only if transmitters operating at channels 2 and 4 are located close to the transmitter
operating at channel 3.
8.4
CHOICE OF INTERMEDIATE FREQUENCIES
Since the picture and sound carriers in any channel are spaced by 5.5 MHz, it is natural that
the corresponding IF frequencies are also located at the same difference. Accordingly, if the
picture IF is fixed at a certain frequency the sound IF automatically gets fixed at a frequency 5.5
MHz less than the picture IF frequency. Therefore we shall refer mostly to picture IF frequency.
The factors which influence the choice of intermediate frequencies in TV receivers are:
(i) Image Rejection Ratio
For a desired input signal at 100 MHz the local oscillator frequency is set at 110 MHz if the IF
frequency is fixed at 10 MHz. However, for the same input signal frequency, if the IF frequency
is chosen to be 40 MHz, the local oscillator must be set to give an output at 140 MHz. This is
shown in Fig. 8.6 (a) and (b). In the first case, if another station is operating at 120 MHz, it will
Desired signal
frequency = 100 MHz
Image signal
frequency = 120 MHz
Input
Mixer
Desired signal
frequency = 100 MHz
Image signal
frequency = 180 MHz
IF out Input
10 MHz
fc = 110 MHz
Mixer
IF out
40 MHz
Antenna
RF
amplifier
Mixer
IF
out
fc = 140 MHz
Local
osc
Local
osc
(a)
(b)
Local
osc
(c)
Fig. 8.6(a) and (b). Illustration of image signal interference, (c) Local oscillator signal radiation.
also be received with equal strength because the incoming signal will beat with the local oscillator
frequency of 110 MHz to develop output at 10 MHz. Similarly in the second case, a station
operating at 180 MHz will be received equally well because the output circuit of mixer is tuned
to deliver output at 40 MHz. Note that in each case the undesired signal which gets received is
spaced at a gap of twice the IF frequency, and is known as ‘Image Signal’. The image rejection
ratio is defined as the output due to desired station divided by output due to image signal.
Without the use of an RF amplifier prior to the mixer, there is nothing that can stop the
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TELEVISION RECEIVER
reception of image signal if that is present. With RF amplifier the output due to image signal
can be very much reduced or completely eliminated. With lower IF frequency, say 10 MHz, the
image frequency at 120 MHz is not very far away from desired frequency of 100 MHz and
might pass through the pass-band of RF amplifier through somewhat attenuated. But if the IF
frequency is kept high, as shown in Fig. 8.6 (b), image signal frequency is 80 MHz away from
the desired signal and has no chance of passing through the RF amplifier. Thus the use of RF
amplifier helps in reducing interference due to image signals and a higher IF frequency results
in a very high image rejection ratio.
By choosing an IF greater than half the entire band to be covered it is possible to eliminate
image interference. For the lower VHF band (41 to 68 MHz) the IF frequency comes to 13 MHz.
In the upper VHF band (174 to 230 MHz) desired IF frequency is 28 MHz. In the UHF band
(470 to 528 MHz), where the image problem is most serious, half of the difference of entire
band results in the choice of an IF frequency of 56 MHz. But this is higher than the lowest
frequency used in the lower VHF band and because of direct pick-up problems in that band, it
cannot be used. Therefore, the IF frequency must be less than 41 MHz.
(ii) Pick-up Due to Local Oscillator Radiation from TV Receivers
If the output from the local oscillator of a TV receiver gets coupled to the antenna, it will get
radiated and may cause interference in another receiver. This is shown in Fig. 8.6 (c). Here
again advantage lies with higher IF frequency, because with higher IF there is a greater
separation between the resonant circuits of local oscillator and RF amplifier circuits. Thus
lesser signal is coupled from the local oscillator through the RF amplifier to the antenna circuit
and interference due to local oscillator radiation is reduced.
(iii) Ease of Separation of Modulating Signal from
IF Carrier at the Demodulator
For ease of filtering out the IF carrier freuency, it is desirable to have a much higher IF frequency
as compared to the highest modulating frequency. In radio receivers the IF frequency is 455
KHz and the highest audio frequency is only 5 KHz. In TV receivers, with the highest modulating
frequency of 5 MHz, an IF frequency of atleast 40 MHz is desirable.
(iv) Image Frequencies Should Not Lie in the FM Band
The FM band is from 88 MHz to 110 MHz. With IF frequency chosen close to 40 MHz, the
image frequencies of the lower VHF band fall between 121 to 148 MHz and thus cannot cause
any interference in the FM band. Higher TV channels are much above the FM band.
(v) Interference or Direct Pick-Up from Bands Assigned for other Services
Amateur and industrial applications frequency band lies between 21 to 27 MHz. If the IF
frequency is chosen above 40 MHz, even the second harmonics of this band will not cause any
serious direct pick-up problems.
(vi) Gain
It is easier to build amplifiers with large gain at relatively low frequencies. The TV sets
manufactured some 30 years back used IF frequency as low as 12 MHz. This was mostly due to
limitations of active devices available, and the poor quality of components marketed at that
time. With the rapid strides made by electronics industry during the past three decades, active
devices which can perform very well at high frequencies are now easily available. The quality
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MONOCHROME AND COLOUR TELEVISION
of components and other techniques have also considerably improved. Thus the gain criteria is
no longer a constraint in choosing higher IF frequencies. The merits of having high IF frequency
are thus obvious and this has lead to the choice of IF frequencies close of 40 MHz. In the 625B system adopted by India and several other countries the recommended IF frequencies are :
Picture IF = 38.9 MHz, Sound IF = 33.4 MHz. It will be of interest to note that sets manufactured
in USA have picture IF = 45.75 MHz and sound IF = 41.25 MHz. In the British 625 line system,
because of channel bandwidth difference, the picture IF = 39.5 MHz and sound IF = 33.5 MHz.
Video Detector. Modulated IF signals after due amplification in the IF section are fed to
the video detector. The detector is designed to recover composite video signal and to transform
the sound signal to another lower carrier frequency. This is done by rectifying the input signal
and filtering out unwanted frequency components. A diode is used, which is suitably polarized
to rectify either positive or negative peaks of the input signal. Figure 8.7 shows a simplified
circuit arrangement of a video detector. Note the use of an L-C filter instead of the usual RC
configuration employed in ratio receiver detectors. This is to avoid undue attenuation of the
video signal while filtering out carrier components. The video signal shown in Fig. 8.7 is of
correct polarity for feeding to the cathode of picture tube after one stage of video amplification.
Fig. 8.7. (a) Last IF amplifier output (modulated IF signal)
(b) Video detector and sound signal separation circuit.
Video Amplifier. The picture tube needs video signal with peak-to-peak amplitude of 80
to 100 volts for producing picture with good contrast. With an input of about 2 volts from the
detector, the video amplifier is designed to have a gain that varies between 40 to 60. A contrast
control which essentially is gain control of the video amplifier is provided on the front panel of
the receiver to adjust contrast between black and white parts of the picture. A large constrast
makes the picture hard, whereas too low a value leaves it weak or soft.
The video amplifier is dc coupled from the video detector to the picture tube, in order to
preserve the dc component for correct brightness. However, in some video amplifier designs,
on account of complexities of a direct coupled amplifier, ac coupling is instead used. The dc
component of the video signal is restored by a diode clamper before feeding it to cathode or grid
of the picture tube. In video amplifiers that employ tubes, one stage is enough to provide the
desired gain. In transistor amplifier designs, a suitable configuration of two transistors and a
driver often becomes necessary to obtain the same gain.
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Besides gain, response of the amplifier should ideally be flat from dc (zero) to 5 MHz to
include all essential video components. This needs rigorous design considerations because the
band of frequencies to be covered extends from dc through audio range to radio frequencies. A
loss in gain of high frequency components in the video signal would reduce sharpness of the
picture whereas a poor low frequency response will result in loss of boundary details of letters
etc. It is also essential that phase distortion in the amplifer is kept to a minimum. Excessive
phase distortion at low frequencies results in smear effect over picture details. Thus the video
amplifier of a television receiver needs careful design to achieve desired characteristics. Various
wide-banding techniques are employed to extend bandwidth of the amplifier.
8.5
PICTURE TUBE CIRCUITRY AND CONTROLS
The output from the video amplifier may be fed either at the cathode or control grid of the
picture tube. In either case a particular polarity of the video signal is essential for correct
reproduction of picture details. In most cases cathode drive is preferred. The grid is thus left
free to receive retrace blanking pulses to ensure that no retrace lines are seen on the screen for
any setting of the brightness control. Figure 8.8 shows the passage of video signal from video
detector to the picture tube.
Composite
video signal
Amplified video
signal
EHT
80 V P – P
vin
Modulated Video
IF signal detector
Video
amplifier
B+
Contrast
control
Brightness
control
Fig. 8.8. Passage of video signal from detector to picture tube.
8.6
SOUND SIGNAL SEPARATION
The picture and sound signals on their respective carriers are amplified together in the IF
section. On application of the two signals to the video detector, the picture IF (38.9 MHz) acts
as the carrier and beats with the sound carrier (33.4 MHz) and its associated FM side-band
frequencies, to produce difference i.e., 5.5 MHz ± 50 KHz components.This is called intercarrier beat signal and is in effect the second conversion of sound carrier frequency. The resultant
product, however, retains its original FM modulation.
If amplitude variations in the FM modulated difference signal of 5.5 MHz is to be avoided
to suppress audio signal distortion, the amplitude of sound IF carrier (33.4 MHz) together
with its side bands must be attenuated by about 20 db below the picture IF carrier level. This
is achieved by shaping the lower skirt of the IF section response in such a way that the sound
IF lies at — 26 db (5 per cent of the maximum) on the voltage gain axis of the IF response. This
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is clearly shown in Fig. 8.5, where a small pedestal has been created at 33.4 MHz to achieve
the desired objective. Note that any drift in the local oscillator frequency has no effect on the
inter-carrier sound beat frequencies. This is so, because any shift in the local oscillator frequency,
shifts both the picture and sound IFs by the same amount. The video detector circuit is modified
(see Fig. 8.7) by providing a resonant trap circuit to isolate the sound signal. In some receivers
the inter-carrier sound signal is separated after one stage of amplification in the video amplifier.
8.7
SOUND SECTION
As shown in the receiver block diagram (Fig. 8.2), the relatively weak FM sound signal, now on
a carrier frequency of 5.5 MHz is given at least one stage of amplification before feeding it to
the FM detector. This stage is a tuned amplifier, with enough bandwidth to pass the FM sound
signal. This tuned amplifier is known as sound IF. The FM detector is normally a ratio detector
or a discriminator preceded by a limiter. The characteristics of a typical FM detector are shown
in Fig. 8.9. As shown, the audio output is proportional to deviations from the carrier frequency.
The frequency of audio signal depends on the rate of frequency deviation. At the output of FM
detector, a de-emphasis circuit is provided that has the same time constant (50 µs) as that of
the pre-emphasis circuit employed at the sound transmitter. This restores the amplitude of
higher audio frequencies to their correct level. The audio signal receives atleast one stage of
amplification before it is passed on to the audio output (power) amplifier. The volume and tone
controls form part of the audio amplifiers. These are brought out at the front panel of the
receiver. The power amplifier is either a single ended or push-pull configuration employing
tubes or transistors. Special ICs have been developed which contain FM demodulator and
most parts of the audio amplifier. These are fast replacing discrete circuits hitherto used in
the sound section of the receiver. The audio amplifier feeds into one or two loudspeakers provided
at a convenient location along front panel of the receiver.
Fig. 8.9. Response curve of an FM detector.
Automatic Gain Control (AGC)
AGC circuit controls gain of RF and IF stages to deliver almost constant signal voltage to the
video detector, despite changes in the signal picked up by the antenna. The change in gain is
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TELEVISION RECEIVER
achieved by shifting the operating point of the amplifying devices (tubes or transistors) used in
the amplifiers. The operating point is changed by a bias voltage that is developed in the AGC
circuit. Any shift in the operating point changes gm (mutual conductance) of the tube or power
gain of the transistor circuit which in turn results in change of stage gain.
Sync level in the composite video signal is fixed irrespective of the picture signal details.
Hence, sync pulse tops represent truly the signal strength. A peak rectifier is used to develop
a control voltage which is proportional to the sync level. The composite video signal to the peak
rectifier in the AGC circuit is either obtained from the output of video detector or after one
stage of video amplification. The output is filtered and the dc voltage thus obtained is fed to
the input (bias) circuits of the RF and IF amplifiers to control their gain. Decoupling circuits
are used to avoid interaction between different amplifier stages. AGC is normally not applied
to the last IF stage because at that level the signal strength is quite large and any shift in the
initially chosen operating point can cause distortion because of partial operation on the nonlinear portion of the device characteristics.
Since AGC voltage is proportional to the signal strength, even weak RF signals will also
develop some control voltage. This when applied to the RF amplifier will tend to reduce its
gain, though the stage should provide maximum possible gain for weak signals to maintain
high signal to noise ratio. Therefore, the RF amplifier is not fed any AGC voltage till the signal
strength attains a certain predetermined level. This is achieved by providing a voltage delay
circuit in the AGC line. Such a provision is known as delayed AGC. The AGC control, as explained
above is illustrated in Fig. 8.10 by a block schematic circuit arrangement.
Composite video
signal (input)
To IF amplifiers
Decoupling
network
R2
To RF
amplifier
R3
C2
AGC
delay
circuit
C3
Rectified
output
R1
C1
AGC bias
line
Peak
AGC
rectifier
AGC
filter
RL
v0
Fig. 8.10. Block diagram of AGC system.
Sync Separation
The horizontal and vertical sync pulses that form part of the composite video signal are separated
in the sync separator. The composite video signal is either taken from the video detector output
or after one stage of video amplification. A sync separator is a clipper that is suitably biased to
produce output, only during sync pulse amplitude of the video signal. In some receivers, a
noise gate preceds the sync separator. This suppresses strong noise pulses if present in the
video signal. A sync pulse train as obtained from a sync separator is shown in Fig. 3.5.
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8.8
MONOCHROME AND COLOUR TELEVISION
SYNC PROCESSING AND AFC CIRCUIT
The pulse train as obtained from the sync separator is fed simultaneously to a differentiating
and an integrating circuit. The differentiated output (see Fig. 3.5) provides sharp pulses for
triggering the horizontal oscillator, while output from the integrator controls the frequency of
the vertical oscillator. As explained in Chapter 3, pre and post equalizing pulses ensure identical
vertical sync pulses both after the first and second fields.
An integrating circuit is a low-pass filter and hence sharp noise pulses do not appear at
its output. However, the differentitator, being a high-pass filter, develops output in response
to noise pulses in addition to the spiked horizontal sync pulses. This results in occasional
wrong triggering of the horizontal oscillator which results in diagonal tearing of the reproduced
picture. To overcome this difficulty, a special circuit known as automatic frequency control
(AFC) circuit (Fig. 8.11) is employed. The AFC circuit employs a discriminator arrangement
which compares the incoming horizontal sync pulses and the voltage that develops across the
output of the horizontal deflection amplifier. The AFC output is a dc control voltage that is free
of noise pulses. This control voltage is used to synchronize the horizontal oscillator with the
received horizontal sync pulses.
Sync
voltage
D.C.
control
voltage
R
Sync
discriminator
C
Filter
Horz fly back pulses
from H.O.T.
Fig. 8.11. Block diagram of AFC circuit.
8.9
VERTICAL DEFLECTION CIRCUIT
Blocking oscillators or cathode coupled multivibrators are normally employed as vertical
deflection oscillators. The controlling time constants are suitably chosen to develop output
corresponding to trace and retrace periods. The necessary sawtooth voltage is developed by
charging and discharging a capacitor with different time constants. This capacitor forms part
of the waveshaping circuit which is connected across the oscillator.
The frequency of the oscillator is controlled by varying the resistance of the RC coupling
network and is locked in synchronizm by the vertical sync pulses. A part of the coupling network
resistance is a potentiometer that is located on the fornt panel of the reciver. This is known as
‘Vertical Hold Control’ (see Fig. 8.2) and enables resetting of the vertical oscillator frequency
when it drifts far away from 50 Hz.
The output from the oscillator-cum-waveshaping circuit if fed to a power amplifier, the
output of which is coupled to the vertical deflection coils to produce vertical deflection of the
beam on picture tube screen.
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8.10 HORIZONTAL DEFLECTION CIRCUIT
The horizontal oscillator (see Fig. 8.2) is similar to the vertical oscillator and is set to develop
sweep drive voltage at 15625 Hz. However, the frequency of this oscillator is controlled by dc
control voltage developed by the AFC circuit. Since the noise pulses in the control voltage are
completely suppressed, most receivers do not provide any horizontal frequency (hold) control,
as is normally done for the vertical oscillator. The oscillator output is waveshaped to produce
linear rise of current in the horizontal deflection coils. Since the deflection coils need about one
amp of current to sweep the entire raster, the output of the oscillator is given one stage of
power amplification (as for vertical deflection) and then fed to the horizontal deflection coils.
Low Voltage Power Supply
The usual B + or low voltage power supply is obtained by rectifying and filtering the ac mains
supply. If necssary the mains voltage is stepped up or down before rectification. Silicon diodes
are normally used for rectification. In some power supply designs, which do not employ a
mains transformer, voltage doubler circuits are used to raise the dc voltage. For circuits that
employ transistors and integrated circuits, regulated low voltage power supplies are normally
provided. While branching the dc supply to various sections of the receiver, decoupling networks
are used to avoid any undue coupling between different sections of the receiver. The filament
power is supplied by either connecting all the heaters in series across the ac mains or by a low
voltage winding on the mains transformer.
High Voltage (EHT) Supply
As already stated in the chapter on picture tubes, an anode voltage of the order of 15 kV is
needed for sufficient brightness in black and white picture tubes. This is known as HV or EHT
(extra high tension) supply.
High voltage rectifier
HV winding
EHT
(15–18 kV)
From horz
osc
Horz
deflection
amplifier
To horz
deflection coils
Boosted B+
supply
Fig. 8.12. Basic circuit of E.H.T. supply.
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MONOCHROME AND COLOUR TELEVISION
To obtain such a high voltage by stepping up the mains voltage with a transformer is
almost impossible and prohibitive in cost. A novel method used for obtaining EHT source is
illustrated in Fig. 8.12. During retrace intervals of horizontal scanning, high voltage pulses of
amplitude between 6 to 9 kV are developed across the primary winding of the horizontal ouptut
transformer. As shown in the figure these are stepped up by an autotransformer winding to
about 10 to 15 kV and then fed to a high voltage rectifier. The output of the rectifier is filtered
to provide required dc voltage.
Such an arrangement does not load very much the horizontal output stage because the
current demand from this high voltage source is less than 1 mA.
The horizontal output circuit is so designed, that in addition to providing EHT source,
the energy stored in the horizontal deflection coils during retrace is tapped through a diode
called damper diode to charge a capacitor. The voltage thus developed across the capacitor,
actually adds 200 to 300 volts to normal B + voltage to give a boosted B + supply of 400 to 700
volts. This voltage is also suitable for first and second anodes of the picture tube. This
arrangement makes the horizontal deflection circuit very efficient.
Review Questions
1.
Draw block diagram of an RF Tuner (Front End) and explain how incoming signals from different
stations are translated to common picture IF and sound IF frequencies. Illustrate your answer
by choosing carrier frequencies of any channel in the VHF band.
2.
What do you understand by image rejection ratio ? Explain how by providing an RF amplifier,
image signal reception is greatly minimized. What are the other merits of using an RF amplifier
before the frequency converter ?
3.
Describe briefly the factors that influenced the choice of picture IF = 38.9 MHz and sound IF =
33.4 MHz in the 625-B monochrome television system.
4.
What are the essential functions which are assigned to IF section of the receiver ? Show by
sketching output voltage verses frequency response of the IF section, how vestigial sideband
correction is carried out. Why is the sound signal amplitude attenuated to about 5 per cent of the
maximum output voltage ?
5.
Explain how composite video signal is detected ? How is the polarity of the video output signal
decided ? Why is it dependent on the number of video amplifier stages ?
6.
What do you understand by intercarrier sound system ? Explain why any shift in the local oscillator
freuency does not effect the frequency of the intercarrier beat signal. Where and how is the
intercarrier sound signal separated from the video signal ?
7.
Draw a block diagram of the sound channel in a TV receiver. Explain briefly how the intercarrier sound signal as obtained at the video detector, is processed to produce sound output. Why
is a de-emphasis circuit provided after the FM detector ?
8.
Explain briefly how sync pulses are separated from the composite video signal and processed to
synchronize the vertical and horizontal oscillators.
9.
Describe briefly how EHT and boosted B + voltages are developed from the horizontal output
circuit of the sweep amplifier.
10. Draw block diagram of a monochrome TV receiver and label its various sections Indicate by
waveshapes the nature of signal at the input and output of each block of the receiver.
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9
Television Signal Propagation
and Antennas
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9
Television Signal Propagation
and Antennas
9.1
RADIO WAVE PROPAGATION
Radio waves are electromagnetic waves, which when radiated from transmitting antennas,
travel through space to distant places, where they are picked up by receiving antennas. Although
space is the medium through which electromagnetic waves are propagated, but depending on
their wavelengths, there are three distinctive methods by which propagation takes place. These
are: (a) ground wave or surface wave propagation, (b) sky wave propagation, and (c) space
wave propagation.
(a) Ground Wave Propagation
Vertically polarized electromagnetic waves radiated at zero or small angles with ground, are
guided by the conducting surface of the ground, along which they are propagated. Such waves
are called ground or surface waves. The attenuation of ground waves, as they travel along the
surface of the earth is proportional to frequency, and is reasonably low below 1500 kHz.
Therefore, all medium wave broadcasts and longwave telegraph and telephone communication
is carried out by ground wave propagation.
(b) Sky Wave Propagation
Ground wave propagation, above about 1600 kHz does not serve any useful purpose as the
signal gets very much attenuated within a short distance of its transmission. Therefore, most
radio communication in short wave bands up to 30 MHz (11 metres) is carried out by sky
waves. When such waves are transmitted high up in the sky, they travel in a straight line until
the ionosphere is reached. This region which begins about ‘120 km above the surface of the
earth, contains large concentrations of charged gaseous ions, free electrons and neutral
molecules. The ions and free electrons tend to bend all passing electromagnetic waves. The
angle by which the wave deviates from its straight path depends on (i) frequency of the radio
wave (ii) angle of incidence at which the wave enters the ionosphere (iii) density of the charged
particles in the ionosphere at the particular moment and (iv) thickness of the ionosphere at
the point. Figure 9.1 illustrates the path of several waves entering the ionosphere at different
incident angles. With increase in frequency, the allowable incident angle at the ionosphere
becomes smaller until finally a frequency is reached, when it becomes impossible to deflect the
beam back to earth. For ordinary ionospheric conditions this frequency occurs at about 35 to
40 MHz. Above this frequency, the sky waves cannot be used for radio communication between
distant points on the earth. This is why no frequencies beyond about 30 MHz (11 metres) are
allotted for radio communication.
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Fig. 9.1. Ray paths for different angles of incidence (φ) at the ionosphere.
(c) Space Wave Propagation
As explained above, propagation of radio waves above about 40 MHz (which is the beginning of
television transmission band) is not possible through either surface or sky wave propagation.
Thus, the only alternative for transmission in the VHF and UHF bands, despite large
attenuation, is by radio waves which travel in a straight line from transmitter to receiver. This
is known as space wave propagation. Its maximum range, because of the nature of propagation,
is limited to the line of sight distance between the transmitter and receiver.
For not too large distances, the surface of the earth can be assumed to be flat and different
rays of wave propagation can reach the receiver from transmitter as shown in Fig. 9.2(a). As
seen there, ht and hr, are the heights of transmitting and receiving antennas respectively, and
d is the distance that separates them from each other. Both the direct wave AB and reflected
wave ACB contribute to the field strength at the receiving antenna. Assuming the earth’s
surface to be perfectly reflecting, the total field strength E, due to both direct and reflected
waves, for reasonably large value of d can be expressed as:
E* =
4 πfht hr
d2
E0
Line o
f sig
ght
TR
Transmitting antenna
(T)
A
Receiving antenna
Direct
(R)
wave
B
ht
Reflected
wave
hr
C
Ground surface
ht
dissta
ance d
Earth
RE
hr
R
R
d
Fig. 9.2(a). Space wave propagation. For the
sake of clarity, the antenna heights have been
greatly exaggerated in comparison with the
distance between them.
Fig. 9.2(b). Computation of line-of-sight
distance. The height of antennas has been
greatly exaggerated in comparison with
R, the radius of earth.
2 E0
2πfht hr
sin
but when d is large, as is often the case, the sine of the angle can be
d
d
replaced by the angle and thus the above expression becomes
4πfht hr
E=
E0
d2
*E =
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MONOCHROME AND COLOUR TELEVISION
where E0 is the field strength at unit distance from the transmitter, and f is the frequency of
the transmitted signal. The field strength varies inversely as tbe square of the distance between
the two antennas but is directly proportional to their heights.
Various Aspects of Space Wave Propagation
(i) Effect of Earth’s Curvature. Earth’s curvature limits the maximum distance between
the transmitting and receiving antennas. This is depicted in Fig. 9.2(b). The maximum line of
sight distance d between the two antennas can be easily found out. Neglecting (hr)2 and (ht)2,
being very small as compared to R, the radius of the earth, the line-of-sight distance d ≈
4.22( ht +
hr ) km.
where ht and hr are expressed in metres. In reality etectromagnetic waves are bent slightly as
they glide along the surface of the earth and this increases the line-of sight distance by a small
amount. It is evident that the ground coverage will increase with increase in height of both
transmitting and receiving antennas. It is due to this reason that television transmitting and
receiving antennas are placed as high as possible, for example atop tall buildings and on high
plateaus. Another advantage is that the local noise disturbance pick-up is reduced by placing
the antennas at high altitudes.
(ii) Effect of Atmospherics. The presence of gas molecules and water vapour affects the
dielectric constant and hence the refractive index of the atmosphere. As a result, the space
waves are differently refracted or reflected under varying conditions of atmosphere. This under
certain conditions enables the propagation to reach points very much beyond the line of sight.
Similarly under adverse weather conditions the signal attenuation increases, thereby reducing
effective distance of transmission.
Occasionally the concentration of charged particles in the ionosphere increases sharply
and it becomes possible for waves up to 60 MHz to return to earth. Though this enhances the
range of sky wave propagation, but the exact time and place of occurrence of such phenomena
cannot be predicted Thus this phenomenon has little value for commercial operation, but does
explain to some extent the distant reception of high frequency and TV signals, which occurs at
times under unusual conditions.
(iii) Effect of Obstacles. Tall and massive objects like hills and buildings, will obstruct
surface waves, which travel close to ground. Consequently, shadow zones and diffraction will
result. For this reason in some areas antennas higher than those indicated by theoretical
expressions are needed. On the other hand, some areas receive such signals by reflection only.
Again, in some areas strong reflected signals are received besides direct signals. This can
cause a form of interference known as ‘ghosting’ on the screen of a television receiver because
of a phase difference between the two signals.
9.2
TELEVISION SIGNAL TRANSMISSION*
The RF carrier power output of commonly used VHF television transmitters varies from 10 to
50 kW. A satisfactory level of signal strength is said to exist when the image produced on the
*Television broadcast channels are given in Appendix C.
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TELEVISION SIGNAL PROPAGATION AND ANTENNAS
153
screen of the receiver overrides noise by an acceptable margin. Signal strength is a function of
power radiated, transmitting and receiving antenna heights, and the terrain above which the
propagation occurs. The acceptable signal to noise ratio at the picture tube screen is measured
in terms of peak-to-peak video signal voltage (half tone), injected at the grid or cathode of the
picture tube versus the r.m.s. random noise voltage at that point. A peak signal to r.m.s. noise
ratio of 45 db is generally considered adequate to produce a good quality picture.
Field strength is indicated by the amount of signal received by a receiving antenna at a
height of 10 metres from ground level, and is measured in microvolts per metre of antenna
dipole length. The field strength for very good reception in thickly populated and built-up
areas is 2500 µV/ metre for channels 2 to 4 (47 to 68 MHz), and 3550 µV/metre for channels 5
to 11 (174 to 223 MHz). For channels in the UHF band, a field strength of about 5000 µV/metre
becomes necessary. This is so because of the lower sensitivity of the receiver for higher channels.
Range of Transmission
A sample calculation shows that for a transmitting antenna height of 225 metres above ground
level the radio horizon is 60 km. If the receiving antenna height is 16 metres above ground
level the total distance is increased to 76 km. Greater distance between antennas may be
obtained by locating them on top of very tall buildings or hillocks. However, links longer than
120 km are hardly ever used for TV transmission because of limitations of radiated power,
high channel frequencies and antenna heights. Thus, depending on the transmitter power and
other factors the service area may extend up to 120 km for the channels in the VHF band but
drops to about 60 km for UHF channels.
Booster Stations
Some areas are either shadowed by mountains or are too far away from the transmitter for
satisfactory television reception. In such cases booster stations can be used. A booster station
must be located at such a place, where it can receive and rebroadcast the programme to receivers
in adjoining areas. Mussoorie (U.P.) is one such booster station. Its receiving and transmitting
antennas are located on top of a hill. The station receives Delhi TV station (channel 4)
programmes and relays them in channel 10 to the surrounding areas and regions on the other
side of the hills.
9.3
INTERFERENCE SUFFERED BY CARRIER SIGNALS
In addition to thermal and man-made noise, the carrier signal must compete with various
other forms of interfering signals originating from other television stations, radio transmitters,
industrial radiating devices and TV receivers. When the interfering signal has a frequency
that lies within the channel to which a TV receiver is tuned, the extent of interference depends
only on the relative field strengths of the desired signal and the interfering signal. If the
interfering signal frequency spectrum lies outside the desired channel, selectivity of the receiver
aids in rejecting the interference.
(a) Co-channel Interference
Two stations operating at the same carrier frequency, if located close by, will interfere with
each other. This phenomenon which is common in fringe areas is called co-channel interference.
As the two signal strengths in any area almost equidistant from the two co-channel stations
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become equal, a phenomenon known as ‘venetian-blind’ interference occurs. This takes the
form of horizontal black and white bars, superimposed on the picture produced by the tuned
channel. These bars tend to move up or down on the screen. As the strength of the interfering
signal increases, the bars become more prominent, until at a signal-to-interference ratio of 45
db or so, the interference becomes intolerable. The horizontal bars are a visible indication of
the beat frequency between the two interfering carriers. Figure 9.3 shows the bar pattern that
appears on the screen. The frequency of the beat note, which is equal to frequency separation
between the two carriers, is usually of the order of a few hertz. This is so because most
transmitters operate almost at the correct assigned frequencies. Motion of the bars, upwards
or downwards occurs whenever the beat frequency is not an exact multiple of the field frequency.
Co-channel interference was a serious problem in early days of TV transmission, when the
channel allocation was confined to VHF band only. This necessitated the repetition of channels
at distances not too far from each other. Now, when a large number of channels in the UHF
band are available such a problem does not exist. The sharing of channel numbers is carefully
planned so that within the ‘service area’ of any station, signals from the distant stations under
normal conditions of reception are so weak as to be imperceptible. However, during a period of
abnormal reception conditions (often during spring) when the signals from distant VHF stations
are received much more strongly, co-channel interference can occur in fringe areas. The use of
highly directional antennas is very helpful in etiminating co-channel interference.
Interfering
bars
Fig. 9.3. Venetian-blind interference caused by beat frequency
between picture carriers of co-channels.
(b) Adjacent Channel Interference
Stations located close by and occupying adjacent channels, present a different interference
problem. Adjacent channel interference (see Fig. 8.5) may occur as a result of beats between
any two of these frequencies or between a carrier and any sidebands. A coarse dot structure is
produced on the screen if picture carrier of the desired channel beats with sound carrier of the
lower adjacent channel. The beat pattern is more pronounced if the lower adjacent sound
carrier is relatively strong and is not sufftciently attenuated in the receiver.
The next most prominent source of interference is the one produced by picture sideband
components of the upper adjacent channel. The beat frequency between adjacent picture carrier
is 7 MHz. Since this is far beyond the video frequency range, the resultant beat pattern is not
discernible. However, the picture sidebands of the upper adjacent channel may beat with the
desired channel carrier and produce an interfering image. To prevent adjacent channel
interference, several sharply tuned band eliminator filters (trap circuits) are provided in the
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IF section of the receiver. This was explained in Chapter 8 while discussing desired IF response
characteristics of the receiver. In addition to this, the guard band between two adjacent channels
also minimizes the intensity of any adjacent channel interference. A space of about 150 km
between adjacent channel stations is enough to eliminate such an interference and is normally
allowed.
(c) Ghost Interference
Ghost interference arises as a result of discrete reflections of the signal from the surface of
buildings, bridges, hills, towers etc. Figure 9.4 (a) shows paths of direct and reflected electromagnetic waves from the transmitter to the receiver. Since reflected path is longer than the
direct path, the reflected signal takes a longer time to arrive at tke receiver.
The direct signal is usually stronger and assumes control of the synchronizing circuitry
and so the picture, due to the reflected signal that arrives late, appears displaced to the right.
Such displaced pictures are known as ‘trailing ghost’ pictures. On rare occasions, direct signal
may be the weaker of the two and the receiver synchronization is now controlled by the reflected
signal. Then the ghost picture, now caused by direct signal, appears displaced to the left and is
known ‘as leading ghost’ picture. Figure 9.4 (b) shows formation of trailing and leading ghost
pictures on the receiver screen.
Reflecting surface
or object
Ghost
image
Reflected
path
T
Direct
image
R
Direct path
Fig. 9.4 (a). Geometry of
multiple path transmission.
Trailing ghost image
Leading ghost image
Fig. 9.4 (b). Ghost interference.
The general term for the propagation condition which causes ghost pictures is ‘multipath
transmission’. Ghost pictures are particularly annoying when the relative strengths of the two
signals, vary such, that first one and then the other assume control of the receiver synchronism.
In such cases the ghost image switches over from a leading condition to a trailing one or viceversa at a very fast rate. The effect of such reflected signals (ghost images) can be minimized
by using directional antennas and by locating them at suitable places on top of the buildings.
9.4
PREFERENCE OF AM FOR PICTURE SIGNAL TRANSMISSION
At the VHF and UHF carrier frequencies there is a displacement in time between the direct
and reflected signals. The distortion which arises due to interference between multiple signals
is more objectionable in FM than AM because fhe frequency of the FM signal continuously
changes. If FM were used for picture transmission, the changing best frequency between the
multiple paths, delayed with respect to each other, would produce a bar interference pattern
in the image with a shimmering effect, since the bars continuously change as the beat frequency
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MONOCHROME AND COLOUR TELEVISION
changes. Hence, hardly any steady picture is produced. Alternatively if AM were used, the
multiple signal paths can atmost produce a ghost image which is steady. In addition to this,
circuit complexity and bandwidth requirements are much less in AM than FM. Hence AM is
preferred to FM for broadcasting the picture signal.
9.5
ANTENNAS
In the preceding sections of this chapter, while explaining various methods of propagation, it
was taken for granted that transmitters can somehow transmit and receivers have some means
of receiving what is transmitted. Actually a structure must be provided, both for effective
radiation of energy at the transmitter and efficient pick up at the receiver. An antenna is such
a structure. It is generally a metallic object, often a rod or wire, that is used to convert highfrequency current into electromagnetic waves, and vice versa. Though their functions are
different, transmitting and receiving antennas behave identically.
Radiation Mechanism
An antenna may be thought of as a short length of a transmission line. When high frequency
alternating source is applied at its one end, the resulting forward and reverse travelling waves
combine to form a standing wave pattern on the line. However, all the forward energy does not
get reflected at the open end, and a small portion escapes from the system and is thus radiated.
The electromagnetic radiation from the transmitting antenna has two components—a
magnetic field associated with current in the antenna and an electric fleld associated with the
potential. The two fields are perpendicular to each other in space and both are perpendicular
to he direction of propagation of the wave. This is illustrated in Fig. 9.5. An electromagnetic
wave is horizontally polarized if its electric field is in the horizontal direction. Thus an antenna
fixed horizontally produces horizontally polarized waves. Similarly a vertical antenna produces
vertically polarized waves.
x
z
y
E
E
x
Dipole
antenna
H
R.F. energy
z
Direction
of propagation
y
E-Electric field
H-Magnetic field
H
Fig. 9.5. Transverse electromagnetic wave in free space.
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The amount of energy that is radiated in space by a transmission line antenna is however,
extremely small, unless the wires of the line are suitably oriented and their lengths made
comparable to the wavelength, If the two wires of the transmission line are opened up, there is
less likelihood of cancellation of radiation from the two-wire tips and this improves the efficiency
of radiation. This type of radiator is called a dipole. When total length of the two wires is halfwavelength, the antenna is called a half-wave dipole. Figure 9.6 shows the evolution of such
an antenna from the basic transmission line. As shown there, the antenna is effectively a piece
of quarter-wavelength transmission line bent out and open circuited at the far ends. Such a
length has low impedance at the end connected to the main feeder transmission line. This in
turn means that a large current will flow at the input of the half-wave dipole and efficient
radiation will take place.
T.line
/2
/2
/2
(a) Transmission line
(b) Opened-out
transmission line
(c) Conductors
in line
(d) Half-wave
dipole (centrefed)
Fig. 9.6. Evolution of the dipole.
With the help of Maxwell’s equations it is possible to deduce expressions for the energy
radiated by an antenna, the direction or directions in which it propagates and the field strength
at any distance from it.* The results show that the field strength depends on the power
transmitted and is inversely proportional to the distance from the radiating source. The
coefficient of current (I2rms) in the expression for the radiated power has the dimensions of
resistance and is called radiation resistance. In effect, radiation resistance is the equivalent
resistance which dissipates the same amount of power as that radiated from the antenna
when same current flows through them. For a quarter-wave antenna the radiation resistance
is 36.5 ohms, and for a half-wave antenna it is 73 ohms.
Radiation Patterns of Resonant Antennas
A resonant antenna is a transmission line whose length is an exact multiple of wavelengths
and is open at both ends. The current distribution and radiation patterns of such resonant
wires of different wavelengths which are remote from the ground are shown in Figs. 9.7 and
9.8. As seen there, for a λ/2 dipole the radiation is maximum at right angles to it, and eventually
falls to zero in line with the antenna. This may be explained by considering that at right angles
to the short length of the antenna the distance from a remote point to any part of the antenna
wire is practically the same. Thus reinforcement of radiation will take place in this direction.
*Such analysis is beyond the scope of this book.
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L = l/2
(a)
L=l
(b)
L = 3l/2
(c)
Fig. 9.7. Current distribution on resonant dipoles.
Dipole
54°
L = l/2
(a)
42°
L=l
(b)
L = 3l/2
(c)
Fig. 9.8. Radiation patterns (Polar diagrams) of various resonant dipoles
located remote from ground.
When the distant point lies in a direction other than normal, there will be some
cancellation and finally full cancellation will take place at points that are in line with the
antenna. Thus the radiation pattern cross section, as presented in Fig. 8.8 (a) is a figure of
eight with its axis at right angles to the dipole. Moreover, exactly the same radiation pattern
will exist in any other plane, and so the three-dimensional pattern is the surface of revolution
obtained by rotating the cross section about an axis coinciding with the dipole. For an antenna
of length equal to a whole wavelength the polarity of current, (as shown in Fig. 9.7 (b)) on one
half of the antenna is opposite to that on the other half As a result, the radiation at right
angles from this antenna will be zero because the field due to one half fully cancels the field
due to the other half of the antenna. The direction of maximum radiation exists at 54° to the
antenna. The pattern acquires lobes and there are four such for this antenna. As the length of
the dipole is increased to three half-wavelengths, the current distribution is changed to that of
Fig. 9.7(c) and the radiation pattern takes the shape shown in Fig. 9.8 (c). As the length of the
aerial wire is further increased, the number of lobes in the radiation pattern increases, but the
angle of largest lobe with the direction of antenna decreases.
Nonresonant Antennas
A nonresonant antenna (Fig. 9.9(a)) is one which is correctly terminated and as such only a
forward travelling wave exists and there are no standing waves. As shown in Fig. 9.9(b) the
Voltage and current
distribution
Antenna
wire
Antenna
L
Feed
R (Terminating
resistance)
(a)
(b)
Fig. 9.9. Nonresonant antenna (a) layout and current distribution
(b) typical directional radiation pattern for L = 4λ.
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radiation pattern, though similar to the corresponding resonant antenna, is unidirectional. In
fact there are half the number of lobes compared to the resonant antenna. This is due to
absence of the reflected wave, which otherwise combines vectorially with the forward wave to
create the radiation pattern.
Ungrounded Antennas
When the antenna is very close to the ground, its radiation pattern gets modified on account of
reflections from the ground. If the ground is assumed to be a perfect conductor, a true mirror
image of the actual antenna is considered to exist below the ground. The overall radiation
pattern is then the sum of patterns caused by an array of two nearby antennas. Some typical
radiation patterns for various heights above ground are shown in Fig. 9.10.
(a) Height above
ground = /4
(b) Height above
ground = /2
(c) Height above
ground = Fig. 9.10. Vertical radiation patterns of an ungrounded half-wave horizontal dipole
with varying heights above the ground surface.
Grounded Antennas
When one end of the antenna is actually grounded, the image of the antenna behaves as if it
has been joined to the physical antenna and the combination acts as an antenna of double the
size. The current distribution and radiation patterns of different earthed vertical antennas are
shown in Fig. 9.11. As shown there, the voltage and current distribution on such a grounded
λ/4 antenna, commonly known as the basic Marconi antenna, is the same as those of the
ungrounded half-wave Hertz antenna. As is obvious Marconi antenna needs half the length as
compared to Hertz antenna to produce the same radiation pattern.
/2
/2
/4
Antenna
(a) Current distribution
Antenna length = /4
Antenna length = /2
Antenna length = (b) Radiation patterns
Fig. 9.11. Current distribution and vertical directional patterns of grounded antennas.
Antenna Gain
As explained above, all practical antennas concentrate their radiation in some direction, to a
greater or lesser extent. Thus the field (power) density in the direction is greater than what it
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would have been if the antenna were omnidirectional. This may be interpreted that the antenna
has a gain in a particular direction. The directive gain is thus defined as the ratio of the power
density in the direction of maximum radiation to the power density that would be radiated by
an isotropic antenna. The gain being a ratio of powers is expressed in decibels.
Antenna Arrays
It is clear from previous discussion that radiation from different types of antennas is not uniform
in all directions. Though an antenna can be suitably oriented to get maximum response in any
desired direction, additional directive gain in preferred directions can be obtained by using
more than one radiator arranged in a specific manner in space. Such arrangements of radiators
are known as antenna arrays. The simplest type of array consists of two antennas A1 and A2
separated by a distance d. A special case of directivity is obtained when d = λ/4 and the currents
in the two antennas have a phase difference of 90° between them. This results in a cardioid
shaped directional pattern as shown in Fig. 9.12.
d = /4
A2
A1
d
Fig. 9.12. Cardioid shaped directional pattern formed by parallel half-wave antennas.
A1 and A2 are the locations of two antennas with currents 90° out of phase.
A broadside array consists of a number of identical radiators equally spaced along a line
and carrying same amount of current in phase from the same source. As indicated in Fig. 9.13(a),
this arrangement is strongly directional at right angles to the plane of the array.
Dipoles
Radiation
pattern
Dipoles
Radiation
pattern
Support
Feed line
/4 /4
Fig. 9.13 (a). Broadside array and pattern.
Fig. 9.13 (b). End-fire array and pattern.
Another arrangement known as end-fire array consists of a number of conductors equally
spaced in a line (Fig. 9.13(b)), carrying same magnitude of current but with a progressive
phase difference between them. The directional pattern of such an antenna has directivity
along the array axis in the direction, in which antenna currents become more lagging.
It is possible to combine several different arrays to obtain highly directional radiation
patterns. Such combinations are often used in HF transmission/reception for point to point
communication. Gains, well in excess of 50, are not uncommon with such arrangements.
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Folded Dipole
As shown in Fig. 9.14 (a), the folded dipole is made of two half-wave antennas joined at the
ends with one open at the centre where the transmission line is connected. The spacing between
the two conductors is small compared with a half wave length. This antenna has the same
directional characteristics and signal pick up as that of a straight dipole but its impedance is
approximately 300 ohms. This is nearly four times that of a dipole, because for the same power
applied, the antenna now draws half the current than it would have, in the case of a dipole.
Hence the impedance (Z0) = 4 × 72 = 288 ohms for a half-wave dipole with equal diameter
arms. This is generally considered as 300 ohms.
0.5 nominal
0 48 actual
0.48
/2
I/2
I
I/2
Transmission
line (300)
Fig. 9.14 (a). Folded dipole antenna.
Fig. 9.14 (b). High impedance folded
dipole antenna.
If elements of unequal diameter are used, or an additional closed conductor of the same
diameter is added in between the two (Fig. 9.14(b)), an impedance as large as 650 ohms can be
obtained.
Parasitic Elements
It is not necessary for all the elements of an array to be connected to the output of the
transmitter. An element connected direct to the transmitter is called a driver, whereas a radiator
not directly connected is called a parasitic element. Such a parasitic radiator receives energy
through the induction field of a driven element, rather than by a direct connection to the
transmission line. In general, a parasitic element longer than the driver and close to it reduces
signal strength in its own direction, and increases it in the opposite direction. This in effect
amounts to reflection of energy towards the driver and thus, this element is called a reflector.
Again, a parasitic element shorter than the driver from which it receives energy, tends to
increase radiation in its own direction, and is therefore called a director. The number of directors
and their lengths can be varied to obtain increased directivity and broad band response.
9.6
TELEVISION TRANSMISSION ANTENNAS
As already explained, television signals are transmitted by space wave propagation and so the
height of antenna must be as high as possible in order to increase the line-of-sight distance.
Horizontal polarization is standard for television broadcasting, as signal to noise ratio is
favourable for horizontally polarized waves when antennas are placed quite high above the
surface of the earth.
Turnstile Array
To obtain an omnidirectional radiation pattern in the horizontal plane, for equal television
signal radiation in all directions, an arrangement known as ‘turnstile array’ is often used. In
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this type of antenna two crossed dipoles are used in a turnstile arrangement as shown in
Fig. 9.15(a). These are fed in quadrature from the same source by means of an extra λ/4 line.
Each dipole has a figure of eight pattern in the horizontal plane, but crossed with each other.
The resultant field in all directions is equal to the square root of the sum of the squares of
fields radiated by each conductor in that direction. Thus the resultant pattern as shown in
Fig. 9.15(b) is very nearly circular in the plane of the turnstile antenna. Fig. 9.15(c) shows
several turnstiles stacked one above the other for vertical directivity.
A
Individual
antenna
Combined
pattern
B
Patterns of
individual
antennas
90° lag
Fig. 9.15 (a). Turnstile array.
Fig. 9.15 (b). Directional pattern in
the plane of turnstile.
/2
/2
/2
/2
/2
/2
/2
Horizontal
plane
Fig. 9.15 (c). Stacked turnstile array.
Dipole Panel Antenna System
Another antenna system that is often used for band I and band III transmitters consists of
dipole panels mounted on the four sides at the top of the antenna tower as shown in Fig. 9.16.
Each panel consists of an array of full-wave dipoles mounted in front of reftectors. For obtaining
unidirectional pattern the four panels mounted on the four sides of the tower are so fed that
the current in each lags behind the previous by 90°. This is achieved by varying the field cable
length by λ/4 to the two alternate panels and by reversal of polarity of the current.
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TELEVISION SIGNAL PROPAGATION AND ANTENNAS
l
l/2
/2
Dipole 1
l/2
/2
4
Tower
2
3
l/2
/2
(a)
(b)
Fig. 9.16. Dipole panel antenna system (a) panel of dipoles (b) radiation pattern
of four tower mounted dipole antenna panels.
Combining Network
The AM picture signal and FM sound signal from the corresponding transmitters are fed to the
same antenna through a balancing unit called diplexer. As illustrated in Fig. 9.17, the antenna
combining system is a bridge configuration in which first two arms are formed by the two
radiators of the turnstile antenna and the other two arms consist of two capacitive reactances.
Under balanced conditions, video and sound signals though radiated by the same antenna, do
not interfere with the functioning of the transmitter other than their own.
Antenna load
north-south turnstile
elements
Antenna load
east-west turnstile
elements
Balun
Picture
transmitter
Reactance
Reactance
Sound
transmitter
Fig. 9.17. Equivalent bridge circuit of a diplexer for feeding picture and sound
transmitters to a common turnstile array.
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9.7
MONOCHROME AND COLOUR TELEVISION
TELEVISION RECEIVER ANTENNAS
For both VHF and UHF television channels, one-half-wave length is a practical size and
therefore an ungrounded resonant dipole is the basic antenna often employed for reception of
television signals. The dipole intercepts radiated electromagnetic waves to provide induced
signal current in the antenna conductors. A matched transmission line connects the antenna
to the input terminals of the receiver. It may be noted that the signal picked up by the antenna
contains both picture and sound signal components. This is possible, despite the 5.5 MHz
separation between the two carriers, because of the large bandwidth of the antenna. In fact a
single antenna can be designed to receive signals from several channels that be close to each
other.
Antennas for VHF Channels
Although most receivers can produce a picture with sufficient contrast even with a weak signal,
but for a picture with no snow and ghosts, the required antenna signal strength lies between
100 and 2000 µV. Thus, while a half-wave dipole will deliver satisfactory signal for receivers
located close to the transmitter, elaborate arrays become necessary for locations far away from
the transmitter.
Yagi-Uda Antenna
The antenna widely used with television receivers for locations within 40 to 60 km from the
transmitter is the folded dipole with one reflector and one director. This is commonly known as
Yagi-Uda or simply Yagi antenna. The elements of its array as shown in Fig. 9.18(a) are arranged
collinearly and close together. This antenna provides a gain close to 7 db and is relatively
unidirectional as seen from its radiation pattern drawn in Fig. 9.18(b). These characteristics
are most suited for reception from television transmitters of moderate capacity. To avoid pickup from any other side, the back lobe of the radiation pattern can be reduced by bringing the
radiators closer to each other. The resultant improvement in the front to back ratio of the
signal pick-up makes the antenna highly directional and thus can be oriented for efficient
pick-up from a particular station. However, bringing the radiators closer has the adverse effect
of lowering the input impedance of the array. The separation shown in Fig. 9.18(a) is an optimum
value.
Reflector
Director
Radiation pattern
/10
/10
0.55
55
0.45
45
Driven element
(b)
(a)
Fig. 9.18. Yagi-Uda antenna (a) antenna (b) radiation pattern.
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TELEVISION SIGNAL PROPAGATION AND ANTENNAS
165
Antenna Length
As mentioned earlier, it is not necessary to erect a separate antenna for each channel because
the resonant circuit formed by the antenna is of low ‘Q’ (quality factor) and as such has a broad
band response. For the lower VHF channels (Band I—channels 2 to 4) the length of the antenna
may be computed for a middle value. While this antenna will not give optimum results at other
frequencies, the reception will still be quite satisfactory in most cases if the stations are not
located far away.
Though the antenna length used should be as computed by the usual expression:
Wavelength (λ) =
3 × 10 8
metres, but in practice it is kept about 6 per cent less than the
f (Hz)
calculated value. This is necessary because the self-capacitance of the antenna alters the current
distribution at its ends. The small distance between the two quarter wave rods of the driver,
where the lead-in line is connected can be taken as too small and hence neglected. Note that
this gap does not affect the current distribution significantly.
Antenna Mounting
The receiving antenna is mounted horizontally for maximum pick-up from the transmitting
antenna. As stated earlier, horizontal polarization results in more signal strength, less reflection
and reduced ghost images. The antenna elements are normally made out of 1/4″ (0.625 cm) to
1/2″ (1.25 cm) dia aluminium pipes of suitable strength. The thickness of the pipe should be so
chosen that the antenna structure does not get bent or twisted during strong winds or occasional
sitting and flying off of birds. A hollow conductor is preferred because on account of skin effect,
most of the current flows over the outer surface of the conductor.
The antenna is mounted on a suitable structure at a height around 10 metres above the
ground level. This not only insulates it from the ground but results in induction of large signal
strength which is free from any interference.
The centre of the closed section of the half-wave folded dipole is a point of minimum
voltage, allowing direct mounting at this point to the grounded metal mast without shorting
the signal voltage. A necessary precaution while mounting the antenna is that it should be at
least two metres away from other antennas and large metal objects. In crowded city areas
close to the transmitter, the resultant signal strength from the antenna can sometimes be very
low on account of out of phase reflections from surrounding buildings. In such situations,
changing the antenna placement only about a metre horizontally or vertically can make a big
difference in the strength of the received signal, because of standing waves set up in such
areas that have large conductors nearby. Similarly rotating the antenna can help minimize
reception of reflected signals, thereby eliminating the appearance of ghost images.
In areas where several stations are located nearby, antenna rotators are used to turn its
direction. These are operated by a motor drive to set the broad side of the antenna for optimum
reception from the desired station. However, in installations where a rotating mechanism is
not provided, it is a good practice to connect the antenna to the receiver before it is fixed in
place permanently and proceed as detailed below:
(i) Try changing the height of the antenna to obtain maximum signal strength.
(ii) Rotate the antenna to check against ghost images and reception of signals from faroff stations.
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(iii) When more than one station is to be received, the final placement must be a compromise for optimum reception from all the stations in the area. In extreme cases, it may be
desirable to erect more than one antenna.
Indoor Antennas
In strong signal areas it is sometimes feasible to use indoor antennas provided the receiver is
sufficiently sensitive. These antennas come in a variety of shapes. Most types have selector
switches which are used for modifying the response pattern by changing the resonant frequency
of the antenna so as to minimize interference and ghost signals. Generally the switch is rotated
with the receiver on, until the most satisfactory picture is obtained on the screen. Almost all
types of indoor antennas have telescopic dipole rods both for adjusting the length and also for
folding down when not in use.
Fringe Area Antenna
In fringe areas where the signal level is very low, high-gain antenna arrays are needed. The
gain of the antenna increases with the number of elements employed. A Yagi antenna with a
large number of directors is commonly used with success in fringe areas for stations in the
VHF band. As already mentioned, a parasitic element resonant at a lower frequency than the
driven element will act as a mild reflector, and a shorter parasitic element will act as a mild
‘concentrator’ of radiation. As a parasitic element is brought closer to the driven element, then
regardless of its precise length, it will load the driven element more and therefore reduce its
input impedance. This is perhaps the main reason for invariable use of a folded dipole as the
driven element of such an array. A gain of more than 10 db with a forward to back ratio of
about 15 is easily obtained with such an antenna. Such high gain combinations are sharply
directional and so must be carefully aimed while mounting, otherwise the captured signal will
be much lower than it should be. A typical Yagi antenna for use in fringe areas is shown in
Fig. 9.19 (a). In such antennas the reflectors are usually 5 per cent longer than the dipole and
may be spaced from it at 0.15 to 0.25 wavelength depending on design requirements. Similarly
the directors may be 4 per cent shorter than the antenna element, but where broadband
characteristics are needed successive directors are usually made shorter (see Fig. 9.19 (a)) to
be resonant for the higher frequency signals of the spectrum.
2 46 m
2.46
Reflector
Antenna
2 41 m
2.41
2 12 m
2.12
1.2 m
Directors
0.5 m
Transmission
line feed point
Direction of
signal pick-up
Supporting
mast
(a)
(b)
Fig. 9.19 (a) A typical Yagi antenna, (b) Channel four antenna.
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TELEVISION SIGNAL PROPAGATION AND ANTENNAS
167
In some fringe area installations, transistorised booster amplifiers are also used along
with the Yagi antenna to improve reception. These are either connected just close to the antenna
or after the transmission line, before the signal is delivered to the receiver.
Yagi Antenna Design
The following expressions can be used as a starting point while designing any Yagi antenna
array.
Length of dipole (in metres) ≈
143
( f is the centre frequency of the channel)
f (MHz)
Length of reflector (in metres) ≈ 152/f (MHz)
Length of first director (in metres) ≈ 137/f (MHz)
Length of subsequent directors reduces progressively by 2.5 per cent.
Spacing between reflector and dipole = 0.25λ ≈ 75/f (MHz)
Spacing between director and dipole = 0.13λ ≈ 40/f (MHz)
Spacing between director and director = 0.13λ ~
− 39/f (MHz)
The above lengths and spacings are based on elements of 1 to 1.2 cm in diameter. It may
be noted that length of the folded dipole is measured from centre of the fold at one end to the
centre of the fold at the other end.
It must be remembered that the performance of Yagi arrays can only be assessed if all
the characteristics like impedance, gain, directivity and bandwidth are taken into account
together. Since there are so many related variables, the dimensions of commercial antennas
may differ from those computed with the expressions given above. However, for single channel
antennas the variation is not likely to be much. Figure 9.19 (b) shows a dimensioned sketch of
channel four (61 to 68 MHz) antenna designed for locations not too far from the transmitter. It
has an impedance = 40 + j20 Ω, a front to back pick up ratio = 30 db, and an overall length =
0.37 wavelength.
Multiband Antennas
It is not possible to receive all the channels of lower and higher VHF band with one antenna.
The main problem in using one dipole for both the VHF bands is the difficulty of maintaining
a broadside response. This is because the directional pattern of a low-band dipole splits into
side lobes at the third and fourth harmonic frequencies in the 174 to 223 MHz band. On the
other hand a high-band dipole cut for a half wavelength in the 174 to 233 MHz band is not
suitable for the 47 to 68 MHz band because of insufficient signal pick-up at the lower frequencies.
As a result, antennas for both the VHF bands generally use either separate dipoles for each
band or a dipole for the lower VHF band modified to provide broadside unidirectional response
in the upper VHF band also.
Diplexing of VNF Antennas
When it is required to combine the outputs from the lower and upper VHF band antennas to a
common lead-in wire (feeder) it is desirable to employ a filter network that not only matches
the signal sources to the common feeder but also isolates the signals in the antennas from each
other. Such a filter arrangement is called a ‘diplexer’. Its circuit with approximate component
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MONOCHROME AND COLOUR TELEVISION
values for bands I (47 to 68 MHz) and III (174 to 263 MHz) is given in Fig. 9.20 (a). The manner
in which it is connected to the two antennas is shown in Fig. 9.20 (b). Similarly a triplexer
filter can be employed when three different antennas are to feed their outputs to a common
socket in the receiver. The combining arrangement can be further modified to connect the
output from a UHF antenna to the same feeder line.
15 pF
From band
III antenna
0.16 mH
9.4 pF
Common down
lead to receiver
0.1 mH
30
pF
0.05 mH
UHF band - III
antenna
From band
I antenna
VHF band - I
antenna
Diplexer
Common
down-lead
(b)
(a)
Fig. 9.20. Diplexing antenna outputs (a) diplexer network,
H.P.-L.P.-filter combination (b) diplexer connections.
Conical Dipole Antenna
The VHF dual-band antenna pictured in Fig. 9.21 (a) is generally called a conical or fan dipole.
As shown in Fig. 9.21 (b), this antenna consists of two half-wave dipoles inclined at about 30°
from the horizontal plane, similar to a section of a cone. In some designs a horizontal dipole is
provided in between the two half-wave dipoles. The dipoles are tilted by about 30° inward
towards the wavefront of the arriving signal. This as shown in the figure results in a total
included angle of 120° between the two conical sections in the broadside direction. A straight
reflector is provided behind the conical dipoles.
Reflector
Reflector
120°
30°
Dipoles
/2
Lead wire
/4
Mast
Direction of wave travel
(a)
(b)
Fig. 9.21. VHF fan (conical) dipole with reflector (a) pictorial view,
(b) spacing of elements.
With the dipole lengths chosen for channel 2, this antenna is extensively used as a
receiving antenna to cover both the VHF bands. The antenna resistance is about 150 ohms.
The response pattern of the antenna contains only one major lobe on all the channels.
This is so because for the 174-223 MHz band, the tilting of the dipole rods shifts the direction
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of the split lobes produced at the third and fourth harmonic frequencies so that they combine
to produce a main forward lobe in the broadside direction. This is an improvement over the
conventional dipole where an element cut for the low frequencies will have a multilobed pattern
on the higher channels, and an element cut for the high frequencies will have a poor response
on the lower channels. Though, one conical antenna array may be adequate for all VHF channels,
sometimes three or four such arrays are stacked high for better and more uniform reception.
In-line Antenna
Another combination antenna which is known as in-line antenna is shown in Fig. 9.22. It
consists of a half-wave folded dipole with reflector for the lower VHF band, that is in line with
the shorter half-wave folded dipole meant for the upper VHF band. The distance between the
two folded dipoles is approximately one-quarter wavelength at the high-band dipole frequency.
This is the length of the line connecting the short dipole to the long dipole, where the
transmission line to the receiver is connected. The directivity pattern of the in-line antenna is
relatively uniform on all VHF channels, with a unidirectional broadside response. Its input
resistance is about 150 ohms.
UHF Antennas
The basic principle of antennas for picking up signals from stations that operate in the UHF
band is more or less the same as that in the VHF band. However, on account of higher
attenuation suffered by the UHF signals, it becomes necessary to have very high gain and
directive antennas. Besides this, higher gain is also necessary because receivers are less sensitive
and tend to be more noisy at these frequencies than at lower frequencies. Therefore at microwave
frequencies, some special type of antennas are used, in which the optical properties of reflection,
refraction and diffraction are utilized to concentrate the radiated waves for higher directivity
and more gain. Though a large number of microwave antennas for specific applications have
been developed, the two types that find wide application for television reception are briefly
described below.
Low-band folded
dipole
Reflector
High-band
folded dipole
Directors
Fig. 9.22. In-line YAGI antenna array for lower and upper VHF bands.
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MONOCHROME AND COLOUR TELEVISION
Bow-Tie or Di-Fan Antenna
This di-fan half-wave dipole is the simplest type of UHF antenna as the basic Yagi is for the
VHF band. As shown in Fig. 9.23, the dipoles are triangular in shape made out of metal sheet,
instead of rods. This unit has a broad band response with radiation pattern resembling the
figure of eight. When a screen reflector is placed at its back the response becomes unidirectional.
For greater gain two or four sets of dipoles can be put together to form an array. Note the use
of a mesh screen reflector instead of a rod as used in VHF antennas. Screen reflectors are more
efficient than rods but their big size and bulk makes it impossible to use them in VHF antennas.
Parabolic Reflector Antenna
In this type of antenna (see Fig. 9.24) the dipole is placed at the focal point of a parabolic
reflector. The principle is the same as that of parabolic reflectors of the headlights of a vehicle
though in an inverse way. The incoming electromagnetic waves are concentrated by the reflector
towards the dipole. This provides both high gain and directivity. Note that instead of using an
entire parabolic structure only a section is used. The use of such a reflector provides a gain of
8 db over that of a resonant half-wave dipole.
Mesh-screen
reflector
Reflector
Dipole
Specially
designed dipoles
Lead-in wire
Fig. 9.23. Fan dipole UHF antenna.
Fig. 9.24. Parabolic reflector antenna.
In areas where both VHF and UHF stations are in operation, combination antennas
serve to simplify reception problems from all the channels. Various combinations of different
VHF and UHF antennas are in use. One such combination consists of a low-band conical antenna
for VHF signals and a broad-band fan dipole for the microwave frequency region. A single
lead-in line delivers signals to the receiver through the use of a special coupling device which
is mounted directly on the antenna itself.
9.8
COLOUR TELEVISION ANTENNAS
The requirements to be met by colour television antennas are somewhat different than those
for monochrome receivers. In monochrome receiver antennas, the emphasis is on higher gain
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while it may vary from channel to channel because of the wide frequency range. This, in itself,
is no problem. In fact manufacturers generally design antennas to deliver more gain on higher
frequency channels than on lower channels in order to compensate for higher transmission
losses and lower receiver sensitivity at very high frequencies. However, in such antennas the
gain not only varies from channel to channel but also from one end of the channel to the other.
As an illustration Fig. 9.25 shows the response curve of a typical wide range monochrome
antenna for channel 4, i.e., from 61 to 68 MHz. As shown there, the gain changes by about 4 db
from beginning to end of the channel. If this antenna is used for colour TV reception from the
same channel, the colour signal spectrum, which lies around 66.68 MHz will receive almost 2
db less gain than the video carrier and most of its sidebands. While it is true that the channel
sound signal spectrum receives even lesser gain than the colour components but this does not
affect the receiver reproduction. This is so because the picture contrast and sound volume
controls provided in the receiver can be varied independently to get desired picture and sound
outputs. However, the reduction in gain of colour signal frequencies cannot be separately
compensated for and this results in poor colour picture quality. In colour television, the relative
phase angle of the combined colour difference signal phasor determines the colour in the picture.
Any change in gain is accompanied by a phase shift of the incoming signal. Thus a change in
gain in the region of colour signal spectrum tends to change the hues in the picture. For example
a slight shift of the colour signal phasor can turn red colour to orange and yellow to green. In
fact, if a large phase shift occurs, no colours may get reproduced. Therefore the most important
requirement of a colour TV antenna is a flat response over the entire channel. As labelled
along the response curve in Fig. 9.25, the antenna output should not vary by more than one db
over the frequency range of any one channel for satisfactory reproduction of colour details.
10
Permissible
variation for
colour reception
dh gain over dipole
8
6
4
2
61
62
63
62.25 Picture
carrier
64
65
66
67
68
66.68 Colour
67.75
sub carrier Sound carrier
Frequency (MHz)
Fig. 9.25. Typical response curve of a wide range antenna for channel 4.
Log Periodic Antennas
The stringent requirement of almost flat response besides high gain over any single channel
has led to the development of a relatively new class of broadband antennas. The most popular
of this type is the log periodic antenna. The name log periodic stems from the fact that the
impedance of the antenna has a periodic variation when plotted against logarithm of frequency.
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MONOCHROME AND COLOUR TELEVISION
Figure 9.26 (a) illustrates this periodic nature of the antenna impedance. Such a behaviour
results from the geometric relationship between the relative lengths of the antenna elements
and the distances which separate them from each other. This naturally results in the antenna
getting larger and larger as the distance from the smallest element increases.
The basic construction of a log periodic antenna consisting of a six element array is
illustrated in Fig. 9.26 (b). As shown there, the largest dipole is at the back and each adjacent
element is shorter by a fixed ratio typically 0.9. Also the distance between the dipoles becomes
shorter and shorter by a constant factor which is typically 35 per cent of quarter wave spacing.
As a result, the resonant frequencies for the dipoles overlap to cover the desired frequency
range. All the dipoles are active elements without parasitic reflectors or directors. The active
dipoles, as shown in the figure, are interconnected by a crossed wire net which transposes the
signal by 180°.
Impedance
Staggered feeder
system
Transmission
line
log f
Fig. 9.26 (a). Periodic nature of the impedance
of a log periodic antenna when plotted on a
logarithmic scale.
Fig. 9.26 (b). Constructional details of a log
periodic antenna.
When this antenna is pointed in the direction of the desired station, only one or two of
the dipole elements in the antenna react to that frequency and develop the necessary signal.
All the other elements remain inactive, i.e., do not develop any signal at that particular
frequency. However, for any other incoming channel some other elements will resonate to
develop the signal. Thus only one or two elements combine to deliver the signal from any one
channel. Such an arrangement results in a uniform gain response over each channel.
When the largest dipole is cut for channel 2, the array will cover all the low-band VHF
channels as antenna resonance moves towards the shorter elements at the front. However, for
the high-band VHF channels from 174 to 223 MHz the elements operate as 3 λ/2 dipoles. They
are angled in as a ‘V’ to line up with the split lobes in the directional response for third harmonic
resonance. Figure 9.27 shows such a log periodic antenna.
When the largest dipole is cut for the lowest channel in the UHF band of 470 to 890
MHz, the array can cover all the UHF channels. The ‘V’ angle is not necessary in the UHF
array because this frequency range is less than 2 : l. The UHF antenna array can be mounted
along with the VHF array where a U/V splitter, i.e., a diplexer network connects the two
antennas to a common transmission line for the downlead. It may be noted that the antenna
described above is only one type of log periodic antenna out of a wide variety, quite different in
appearance.
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TELEVISION SIGNAL PROPAGATION AND ANTENNAS
Vee’d elements
Mast
Fig. 9.27. A colour log periodic antenna. The elements are vee’d
to eliminate dual phase problems.
When colour transmission is to be received from only one channel, there is no need for a
specially designed antenna. For example, the antenna designed for monochrome reception on
channel four only can also be used with good results for receiving colour transmission from the
same channel. However, the elements of the antenna must be cut and spaced accurately to
ensure almost uniform gain over the entire channel. The antenna shown in Fig. 9.19 (b) can be
used with colour receivers for receiving colour transmission from channel four.
9.9
TRANSMISSION LINES
A transmission line is used for delivering the antenna signal to the receiver. The desirable
requirements of a transmission line are:
(i) the losses along the line should be minimum.
(ii) there should be no reflection of the signal on the line.
(iii) the line itself should not pick up any stray signals. To prevent this the line should be
balanced or shielded or both.
The two main types ef transmission lines are the two wire parallel conductor type and
the concentric (co-axial) type. Flat twin-lead, tubular twinlead, open wire line and co-axial
type transmission lines are shown in Fig. 9.28. Flat-twin lead and tubular-twin lead transmission
lines are constructed in the form of a plastic ribbon and are generally called twin-lead either
flat or tubular. These together with the open wire line though balanced are not shielded lines.
A line is balanced when each of the two conductors has the same capacitance (or voltage) to
ground. The balance corresponds to the dipole antenna itself which has balanced signals of
opposite phase in the arms. The connections of a balanced line between the antenna and receiver
are shown in Fig. 9.29 (a). The balanced line is connected to the two ends of a centre-tapped
antenna input transformer. Then any in-phase stray field cutting across both the wires of a
balanced line will induce an equal voltage in each line. The consequent currents tend to induce
voltages of opposite polarity in the centre tapped input transformer which cancel each other.
However, the antenna signal from the dipole has opposite phases in the two sides of the line
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and the voltages that are induced in the input transformer reinforce each other and consequently
a large signal voltage is delivered to the receiver from the secondary side of the input
transformer. A shielded line, i.e., the coaxial cable is completely enclosed with a metal sheath
or braid that is grounded to serve as a shield for the inner conductor. The shield prevents stray
signals from inducing current in the centre conductor. Usually the shield is grounded to the
receiver chassis. With only one conductor the line is unbalanced. The connections of a co-axial
transmission line between the antenna and receiver are shown in Fig. 9.29 (b). Though the
line is unbalanced, a balanced transformer (balun) can be used at the input of the receiver for
converting the input signal from unbalanced to balanced form, if necessary. It may be noted
that if the two inner conductors (insulated) are used within the shield then the line is both
balanced and shielded. Shielded lines generally have more capacitance and higher losses. The
attenuation is caused by I2R losses in the a.c. resistance of the line. This reduces the amplitude
of the antenna signal delivered by the line to the receiver. The longer the line and higher the
frequency, the greater is the attenuation. The characteristic impedance of the line which results
from uniform spacing between the two conductors is the same regardless of length of the line.
Conductors
Plastic insulation
Conductors
Plastic insulation
(a) Flat twin lead
(b) Tubular twin lead
Inner
conductor
Braided conductor
shield
Insulating spacer
Bare wire
Outside jacket
(c) Open wire line
Inner insulation
(d) Coaxial cable
Fig. 9.28. Transmission lines.
Antenna
Lead-in
wire
Antenna
Matching transformer
Shielded cable
To receiver
Fig. 9.29 (a). Connections from antenna to
receiver input terminals (balanced).
To receive
Fig. 9.29 (b). Unbalanced match.
Flat Twin-Lead
The flat parallel wire is one of the most popular transmission lines in use for the VHF range.
The wires are encased in a plastic ribbon of polyethylene which is strong, flexible and unaffected
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TELEVISION SIGNAL PROPAGATION AND ANTENNAS
175
by sunlight, water or cold. The characteristic impedance ranges from 75 ohms to 300 ohms. As
stated earlier this line is balanced. It is, however, unshielded and therefore not recommended
for noisy locations. It should not be run close to power lines to avoid pick-up of 50 Hz hum. It
should also be kept away from large metal structures which can alter the balance of the line.
Since most receivers have a balanced input impedance of 300 ohms, the 300 ohms twin-lead
(spacing about 1 cm, wire gauge 20 to 22) is convenient for impedance matching. The losses in
a flat twin lead are much greater when the line is wet.
Tubular Twin-Lead
In this type the two parallel conductors are embedded in a polyethylene plastic tubing with air
as dielectric for most of the inside area. Though expensive it has low losses and is especially
suited for the UHF band of frequencies. The twin line is enclosed in a strong ptastic jacket for
protection against adverse weather conditions.
Open-wire Line
As shown in Fig. 9.28 (c), this line is constructed with low loss insulating spacers between the
parallel bare-wire conductors. The open wire line causes least attenuation because air is the
dielectric between conductors. However, the characteristic impedance is relatively high. With
a spacing of about 1.5 cm the impedance of this line is about 450 ohms.
Co-axial Cable
This line consists of a central conductor in a dielectric that is completely enclosed by a metallic
shield which may be a tubing or a flexible braid of copper or aluminium. A plastic jacket
moulded over the line provides protection. Because of the grounded shield the coaxial cable is
immune to any stray pick-ups. With an outside diameter of about 1 cm the characteristic
impedance is 75 ohms. Because of higher attenuation and higher costs, a shielded line is used
only when the surrounding noise is very severe or where multiple lines must be run close to
each other. In cable distribution systems, coaxial cable is a necessity despite its high losses.
The losses in this system are compensated by the use of distribution amplifiers. Special
connectors are available for terminating coaxial lines. Foam coaxial cable is also available.
The use of foam as dielectric reduces the attenuation by about 20 per cent at 100 MHz.
Characteristic Impedance
When a transmission line has a length comparable with a wavelength of the signal, the
characteristic impedance of the line depends on the small inductance of the conductors and the
distributed capacitance between the conductors. It can be shown that the characteristic
impedance Z0 =
L / C ohms, where L is the inductance per unit length and C the capacitance
per unit length. The closer the conductor spacing, the greater is the capacitance and smaller
the Z0 of the line.
Resonant and Non-resonant Lines
When a line is terminated by a resistive load equal to Z0 of the line, there is no reflection and
maximum power transfer takes place from source to load. Such a line is non-resonant because
there are no reflections. A line terminated by Z0 then becomes effectively an infinitely long
line because there is no discontinuity at the load. The length of the line is not critical. When
such a line having Z0 = 300 ohms is connected to the 300 ohms antenna input terminals of the
receiver, there is no reflection and maximum energy is delivered to the receiver from the
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antenna. Because of correct termination the line can be cut to any length without any loss in
match of impedances. However, more length will produce more I2R losses.
When the line is not terminated by Z0, there will be reflections and standing waves will
be set up in the line. This effect makes the line resonant. The signal strength at any point on
the transmission line will now be a function of the length of the transmission line unlike the
case with non-resonant lines. The ratio of the voltage maximum to that of voltage minimum
along the line is defined as the voltage standing wave ratio, abbreviated VSWR. Note that for
a line terminated in Z0, the VSWR is one. In a resonant line, the greater the mismatch, the
higher the VSWR which is greater than one. The extreme cases of high VSWR correspond to a
short or an open terminated line. If one touches a resonant line, the added hand-capacitance
can mean much more or much less signal delivery depending on where one touches the line.
The use of transmission lines as resonant circuits is illustrated in Fig. 9.30. The maximum
impedance is at the point of highest voltage on the line, say at the open end of an equivalent
quarter-wave section of the line and minimum at the point of highest current, say at the short
I
I
Shorted
end
I
Shorted
end
I
Open
end
Open
end
4
/4
/2
(a) Quarter-wave sections
(b) Half-wave sections
Fig. 9.30. Transmission-line sections as resonant circuits.
circuited end of an equivalent quarter-wave line. Note that the impedance at any point equals
the ratio of voltage to current. As shown in the figure, a quarter-wave section shorted at the
end is equivalent to a parallel-tuned circuit at the generator side because there is a high
impedance across the terminals at the resonant frequency. For a line-length shorter than a
quarter-wave, the line is equivalent to an inductance. The open quarterwave section provides
a very low impedance at the generator side of the line. A line less than a quarter-wave makes
the line appear as a capacitance. The half-wave sections however repeat the impedances at the
end of the line to furnish the same impedance at the generator side. The main features of
quarter-wave (λ/4) and half-wave (λ/2) sections are given in the table below.
Length
Quarter-wave
Termination
Input impedance
Phase shift
shorted
Open circuit
90°
Quarter-wave
open
Short circuit
90°
Half-wave
shorted
Short circuit
180°
Hatt-wave
open
Open circuit
180°
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TELEVISION SIGNAL PROPAGATION AND ANTENNAS
Such transmission lines are often called STUBS. A stub can be used (i) for impedance
matching, (ii) as an equivalent series resonant circuit to short an interfering r.f. signal, and
(iii) for phasing signals correctly in antennas. A quarter-wave line produces a phase change of
90° whereas a halt-wave section shifts the phase by 180°. To reduce interference, an open λ/4
stub at the interfering signal frequency can be used. One side is connected across the antenna
input terminals of the receiver, while the open end produces a short at the receiver input one
quarter-wave back.
A 300 ohms twin-lead is designed to have almost a constant impedance for all the channels
in a band. This is used to connect the antenna output to the input of the receiver. Matching the
impedance of a multiband antenna to the characteristic impedance of the line is not critical
because an impedance mismatch of 2.5 to 1 results in a one db loss of the signal.
Quarter-wave Matching Section
A quarter-wave section can be used for matching two unequal impedances Z1 and Z2. The
characteristic impedance Z0 of the quarter-wave section should then be, Z0 =
Z1 Z2 . This is
iliustrated in Fig. 9.31, where an antenna with an impedance equal to 75 ohms is matched to
a 300 ohms transmission line by a quarter-wave matching section. This section should then
have an impedance equal to Z1 Z2 = 75 × 300 = 150 ohms. The required length of the quarterwave section can be estimated using the expression λ/4 (metres) = v
80
, where ‘v’ is the
f (MHz)
velocity factor which varies between 0.6 to 0.8 depending on the type of lead-in wire used.
75 Antenna
/4
/
Z0 = 150
Quarter-wave
matching section
Z0 = Z1 × Z2
Z0 = 300
Transmission
line
Fig. 9.31. Use of a quarter-wave section for
matching antenna to transmission line.
Balun (Balancing Unit)
Two quarter-wave sections of the type discussed above can be combined to make a balancing
and impedance transforming unit. This is illustrated in Fig. 9.32 (a). This is called a Balun and
is used for matching balanced and unbalanced impedances. Two quarter-wave lines each having
an impedance equal to 150 ohms are connected in parallel at one end, resulting in 75 ohms
impedance across points A and B. Either A or B can be grounded to provide an unbalanced
impedance at the ungrounded point with respect to ground. At the other end the two 150 ahms
quarter-wave sections are connected in series to provide 300 ohms impedance between points
C and D. The quarter wavelength of the line isolates the ground point from C or D, allowing a
balanced impedance with respect to ground. Either side of the Balun can be used for input
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MONOCHROME AND COLOUR TELEVISION
with the other side as output. It is usefully employed for matching a 72 ohms coaxial line to the
300 ohms receiver input or in the reverse direction, i.e., a 300 ohms twin-lead to a 72 ohms
unbalanced input. As illustrated in Fig. 9.32 (b) bifilar windings are uscd to simulate the 150
ohms transmission line sections. The windings, as illustrated in Fig. 9.32 (c), are on a ferrite
core to increase the inductance, thus making the line electrically longer.
1
A
5
300
balanced
c
5
4
Bifilar
windings
300
balanced
c
7
3
D
8
C
6
2
75
unbalanced
n
7
3
B
1
A
6
2
75
unbalanced
n
C
B
4
8
(a)
D
(b)
A
C
B
D
Twin wire
Ferrite core
(c)
Fig. 9.32. Balun to match between 75Ω unbalanced and 300Ω balanced impedances
(a) with λ/4 matching sections, (b) equivalent transformer, (c) constructional details.
9.10 ATTENUATION PADS
In cases where excessive antenna signal causes overloading, the signal can be attenuated
without introducing any mismatch by using resistive networks called pads. Two different pad
configurations are illustrated in Fig. 9.33, where component values for different attenuations
are given in separate charts. The resistors used in such pads are low wattage carbon resistances.
Wire-wound resistors are not used because of their inductance. The use of a resistance matching
pad has the advantage of providing both attenuation and impedance match that is independent
of frequency.
Attenuation
R1
R2
Attenuation
R3
R4
6 db
47Ω
390Ω
6 db
22Ω
100Ω
10 db
82Ω
220Ω
10 db
33Ω
51Ω
20 db
120Ω
68Ω
20 db
56Ω
15Ω
R1
R1
300Ω
input
300Ω
output
R2
R1
R3
R3
72Ω
input
R4
72Ω
output
R1
(a)
(b)
Fig. 9.33. Resistance pads (a) balanced ‘H’ pad for 300Ω twin lead
(b) unbalanced ‘T’ pad for 72Ω coaxial line.
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TELEVISION SIGNAL PROPAGATION AND ANTENNAS
179
Review Questions
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
Describe briefly the different methods by which radio waves of different frequencies are propagated. Why space wave propagation is the only effective mode of radiation above about 40 MHz ?
What do you understand by line-of-sight distance in space wave propagation ? What are the
effects of atmospherics and obstacles on space waves ? Why is it necessary to keep both the
transmitting and receiving antennas as high as possible for television ?
What do you understand by wave polarization ? Why is horizontal polarization preferred for
television and FM broadcasts ? Why is TV transmission limited to about 100 km ? What are
booster stations and under what conditions are they employed ?
Describe briefly co-channel and adjacent channel interference effects. Discuss the techniques
employed to eliminate such interference in fringe areas.
What is a ghost image and what causes it to appear on the receiver screen along with the reproduced picture ? Differentiate between leading and trailing ghost pictures. Why is AM preferred
over FM for picture signal transmission ?
Describe briefly radiation mechanism from an antenna. Explain the evolution of a dipole for
effective radiation. Sketch approximate radiation patterns for ungrounded resonant antennas of
lengths λ/4, λ/2 and 3λ/2 and justify them.
Define directional gain and front to back ratio as applied to receiving antennas. What is an
antenna array ? Sketch radiation patterns for (i) a broadside array and (ii) end-fire array. How
are these patterns modified when the antennas are very close to the ground ?
What is the function of a reflector and a director in a Yagi antenna. Explain why such antenna
configurations are highly directional. What is the effect of increasing the number of director
elements ?
What are the special requirements of a fringe area television antenna and how are these achieved ?
Give constructional details of a typical fringe area antenna and explain the precautions that
must be taken while mounting it.
Give constructional details of a turnstile antenna and explain by drawing radiation pattern its
suitability for television transmission. Draw the circuit of a diplexer arrangement employed for
feeding the output from both picture and sound signal transmitters to the same antenna.
Why is it not possible to use the same antenna for reception for both lower and upper VHF
channels ? Describe any one type of multiband array commonly employed to cover all the channels in the VHF band.
Describe briefly the basic principle of bow-tie (di-fan) and parabolic reflector type of antennas
commonly employed for reception from UHF television channels.
Explain why an antenna used for colour TV reception must deliver almost constant output voltage over any one channel.
Sketch a typical log periodic antenna and explain its special characteristics. Why are its elements
bent in a ‘V’ shape ?
Describe with suitable sketches various types of lead-in wires used for connecting the antenna
to the TV receiver. What is the essential difference betwecn balanced and unbalanced lines and
how are they connected to the receiver ? Why is a coaxial cable preferred for connecting a UHF
antenna ?
What is a stub ? Explain how quarter-wave line sections can be used for providing an impedance
match between a low impedance antenna and 300 ohms lead-in line.
What is a ‘Balun’ ? Give its constructional details and explain how it can be used as an impedance matching network between two different impedances at high frequencies.
Under what conditions does it become necessary to use an attenuator pad between the transmission line and receiver ? Draw ‘H’ and ‘T’ pad configurations and explain how besides providing a
match between the line and receiver the desired attenuation is also achieved.
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10
Television Applications
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10
Television Applications
Television, by its use in broadcasting has opened broad new avenues in the fields of
entertainment and dissemination of information. The not-so-well-known applications are in
the area of science, industry and education, where the television camera has contributed
immeasurably to man’s knowledge of his environment and of himself. The television camera is
probably best described as an extension of the human eye because of its ability to relay
information instantaneously. Its capability to view events occurring in extremely hazardous
locations has led to its use in areas of atomic radiation, underwater environments and outer
space. Some of its important applications which are of direct interest to our society are described
briefly in this chapter.
10.1 TELEVISION BROADCASTING
Broadcasting means transmission in all directions by electromagnetic waves from the
transmitting station. Broadcasting, that deals mostly with entertainment and advertising, is
probably the most familiar use of television. Millions of television sets in use around the world
attest to its extreme popularity. Most programmes produced live in the studio are recorded on
video tape at a convenient time to be shown later. Initially television transmission was confined
to the VHF band only but later a large number of channel allocations were made in the UHF
band also. The distance of transmission, as explained earlier, is confined to line of the sight
between the transmitting and receiving antennas. The useful service range is up to 120 km for
VHF stations and about 60 km for UHF stations. Television broadcasting initially started with
monochrome picture but around 1952 colour transmission was introduced. Despite its complexity
and higher cost, colour television has become such a commercial success that it is fast
superseding the monochrome system.
10.2 CABLE TELEVISION
In recent years master antenna (MATV) and community antenna (CATV) television systems
have gained widespread popularity. The purpose of a MATV system is to deliver a strong
signal (over 1 mV) from one or more antennas to every television receiver connected to the
system. Typical applications of a MATV system are hotels, motels, schools, apartment buildings
and so on.
The CATV system is a cable system which distributes good quality television signal to a
very large number of receivers throughout an entire community. In general, this system feeds
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TELEVISION APPLICATIONS
increased TV programmes to subscribers who pay a fee for this service. A CATV system may
have many more active (VHF and UHF) channels than a receiver tuner can directly select.
This requires use of a special active converter in the head-end.
(a) MATV
The block diagram of a basic MATV system is shown in Fig. 10.1 (a). One or more antennas are
usually located on roof top, the number depending on a available telecasts and their direction.
Each antenna is properly oriented so that all stations are received simultaneously. In order to
allow a convenient match between the coaxial transmission line and components that make up
the system, MATV systems are designed to have a 75 Ω impedance. Since most antennas have
a 300 Ω impedance, a balun is used to convert the impedance to 75 ohms. As shown in the
figure, antenna outputs feed into a 4-way hybrid. A hybrid is basically a signal combining
linear mixer which provides suitable impedance matches to prevent development of standing
waves. The standing waves, if present, result in ghosts appearing in an otherwise good TV
picture.
The output from the hybrid feeds into a distribution amplifier via a preamplifier. The
function of these amplifiers is to raise the signal amplitude to a level which is sufficient to
overcome the losses of the distribution system while providing an acceptable signal to every
receiver in the system. The output from the distribution amplifier is fed to splitters through
coaxial trunk lines. A splitter is a resistive-inductive device which provides trunk line isolation
and impedance match.
Coaxial distribution lines carry television signals from the output of splitters to points
of delivery called subscriber tap-offs. The subscriber taps, as shown in Fig. 10.1 (b), can be
either transformer coupled, capacitive coupled or in the form of resistive pads. They provide
isolation between receivers on the same line thus preventing mutual interference. The taps
look like ac outlets and are normally mounted in the wall. Wall taps may be obtained with
300 Ω output 75 Ω output and a dual output. The preferred method is to use a 75 Ω type with
a matching transformer. The matching transformer is usually mounted at the antenna terminals
of the receiver and will have a VHF output and a UHF output. Since improperly terminated
lines will develop standing waves, the end of each 75 Ω distribution cable is terminated with a
75 Ω resistor called a terminator.
(b) CATV
Formerly CATV system were employed only in far-fringe areas or in valleys surrounded by
mountains where reception was difficult or impossible because of low level signal conditions.
However, CATV systems are now being used in big cities where signal-level is high but all
buildings render signals weak and cause ghosts due to multipath reflections. In either case,
such a system often serves an entire town or city. A single antenna site, which may be on top
of a hill, mountain or sky-scraper is chosen for fixing antennas. Several high gain and properly
oriented antennas are employed to pick up signals from different stations. In areas where
several signals are coming from one direction, a single broad based antenna (log-periodic) may
be used to cover those channels. Most cable television installations provide additional services
like household, business and educational besides commercial TV and FM broadcast programmes.
These include news, local sports and community programmes, burgler and fire alarms, weather
reports, commercial data retrieval, meter reading, document reproduction etc. Educational
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services include computer aided instructions, centralized library services and so on. Many of
the above options require extra subscription fee from the subscriber.
Balun
Balun
Balun
Balun
Antenna
n
system
t
4 way hybrid
(linear mixer)
Preamplifier
Coaxial
cable
Signal
n
processing
s
Distribution
amplifier
Splitter
Tap-off points
Cable
R
TR
TR
Cable
TR
TR
R
TR
TR
TR
Distribution
b
TR-Television
receiver
Cable
TR
TR
R
Termination
resistance 75
Fig. 10.1 (a) Block diagram of an MATV system.
(i) Transformer
(ii) Capacitive
(iii) Resistive
Fig. 10.1 (b) Subscriber taps of different types.
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Since several of the above mentioned service need two-way communication between the
subscriber and a central processor, the coaxial distribution network has a large number of
cable pairs, usually 12 or 24. This enables the viewer to choose any channel or programme out
of the many that are available at a given time.
CATV Plan. Figure 10.2 shows the plan of a typical CATV system. The signals from
various TV channels are processed in the same manner as in a MATV system. In fact, a CATV
system can be combined with a MATV set-up. When UHF reception is provided in addition to
VHF, as often is the case, the signal from each UHF channel is processed by a translator. A
translator is a frequency converter which hterodynes the UHF channel frequencies down to a
VHF channel. Translation is advantageous since a CATV system necessarily operates with
lengthy coaxial cables and the transmission loss through the cable is much greater at UHF
than at VHF frequencies. As in the case of MATV, various inputs including those from
translators are combined in a suitable mixer. The set-up from the antennas to this combiner is
called a head-end.
FM channels
Head
a
end
n
FM band
amplifier
From VHF and UHF
antenna systems
TV
TV
TV
TV
Local programme
TV
TV
modulator
Amplifiers
and
translators
Combining
network
Coaxial
cable
Amplifier
Equalizer
Level
adjustment
Distribution
amplifier
Trunk lines to
distant locations
Trunk lines to
distant locations
Amplifiers
Splitter
Splitter
Tap-off points
R
R
R
R
R
R
Termination resistor 75
Fig. 10.2. A simplified block diagram of a CATV system.
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Further, as shown in the figure the CATV outputs from the combiner network are fed to
a number of trunk cables through a broadband distribution amplifier. The trunk cables carry
signals from the antenna site to the utilization site (s) which may be several kilometres away.
Feeder amplifiers are provided at several points along the line to overcome progressive signal
attenuation which occurs due to cable losses. Since cable losses are greater at higher frequencies
it is evident that high-band attenuation will be greater than low-band attenuation. Therefore,
to equalize this the amplifiers and signal splitters are often supplemented by equalizers. An
equalizer or tilt control consists of a bandpass filter arrangement with an adjustable frequency
response. It operates by introducing a relative low-frequency loss so that outputs from the
amplifiers or splitters have uniform relative amplitude response across the entire VHF band.
The signal distribution from splitters to tap-off points is done through multicore coaxial
cables in the same way as in a MATV system. In any case the signal level provided to a television
receiver is of the order of 1.5 mV. This level provides good quality reception without causing
acompanying radiation problems from the CATV system, which could cause interference to
other installations and services.
10.3 CLOSED CIRCUIT TELEVISION (CCTV)
Closed circuit television is a special application in which camera signals are made available
only to a limited number of monitors or receivers. The particular type of link used depends on
distance between the two locations, the number and dispersion of receivers and mobility of
either camera or receiver. Figure 10.3 illustrates various link arrangements which are often
used. The simplest link is a cable where video signal from the camera is connected directly
through a cable to the receiver. A television monitor, which is a receiver, without RF and IF
circuits, is only required for reception in such a link arrangement. About one volt peak-to-peak
signal is required by the monitor. Since the video signal is normally delivered via cables and
even when transmitted, it is over a limited region and for restricted use, CCTV neede not
follow television broadcast standards.
Video
amplifier
Camera
Monitor
Cable
M
(a) Direct camera link to one monitor
Equalizer
Distribution
amplifier
M
P
M
Camera
P – PAD
P
M
(b) Direct camera link to several monitors
(Figure 10.3 Contd. .....)
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TR
P
TR
Transmitter
Camera
P
TR
(c) Wireless link to several receivers
Microwave link
TR
Camera
CCU
& T–R
CU
&T
Camera
control unit
and trans-receiver
Control unit
and transmitter
P
TR
P
TR
(d) Output of a remotely controlled camera feeding several
TV receivers located at a distance
Fig. 10.3. Commonly used closed-circuit television (CCTV) systems.
CCTV Applications
There are numerous applications of CCTV and a few are briefly described here.
(i) Education. One instructor may lecture to a large number of students sitting at different locations. Similarly close-ups of demonstration experiments and other aids can be shwon
on monitors during these lectures.
(ii) Medicine. Several monitors and camera units can be installed to observe seriously
ill patients in intensive care units. In medical institutions, operations when performed can be
shown to medical students without their actually gathering around the oepration table.
(iii) Business. Television cameras can be installed at different locations in big departmental stores to keep an eye over customers and sales personnel.
(iv) Surveillance. In banks, railway yards ports, traffic points and several other similar
locations, closed circuit TV can be effectively used for surveillance.
(v) Industry. In industry CCTV has applications in remote inspection of materials. Observance of nuclear reactions and other such phenomena would have been impossible without
television. Similarly television has played a great role in the scanning of earth’s surface and
probing of other planets.
(vi) Home. In homes a CCTV monitor finds its application in seeing the caller before
opening the door.
(vii) Aerospace and Oceanography. Here a wireless link is used between the transmitter
and receiver. In some applications camera is remotely controlled over a microwave radio link.
As shown in Fig. 10.3 (c), for aerospace and oceanography a carrier is used for transmitting the
signal and a complete receiver is then necessary for reception.
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10.4 THEATRE TELEVISION
Television programmes can be shown to a large audience in theatres. Similarly cinematographic
films can be telecast for viewing on television receivers. Some examples of such applications
are as follows.
(i) TV Programmes in Theatres
Special programmes can be shown on a large screen by optical projection in a theatre where
spherical mirrors and reflectors are used to enlarge the image. With about 80 KV on the final
anode of the picture tube, there is enough light to show pictures on a standard theatre screen.
The same idea can be used for projecting TV programmes at home on a small screen.
(ii) Film Recorders
Film recorders produce a cinematographic film by photographing a television picture displayed
on the screen of a picture tube. For doing so the film has to be pulled down frame by frame
during successive blanking intervals. A video tap recording can only be rebroadcast in countries
where the same TV standards are in use, whereas film recordings offer a ready means of
exporting programmes to other countries using different standards.
(iii) Telecine Machines and Slide Projectors
Many television programmes originate from 35 mm and 16 mm photographic cinema films.
Slides are also often used in TV programmes. Therefore, telecine machines and slide projectors
form part of the television studio equipment for transmitting motion pictures and advertisement
slides. Telecine machines are cinema projectors equipped with mirror or prism reflector
arrangement for focusing pictures, as produced by them, on the face of a TV camera. Slide
scanners also have a similar optical arrangement for transmitting still from different slides.
For high utilization of the projectorcamera chain, an optical multiplexer is often used. This
switches or directs one of the several optical image sources to the lens of a single camera, thus
enabling the use of one TV camera for receiving programmes from three or four film and slide
projectors. For the accompanying sound signal pick-up, the usual optical or magnetic track
playback facility is incorporated in the multiplexer setup.
An additional problem of using telecine projectors is the difference in frame rate of
motion pictures and television scanning. Motion pictures are taken at the rate of 24 frames/sec
but while screening, each frame is projected twice to reduce flicker. This amounts to an effective
frame rate of 48/sec. However, in TV transmission, while the frame rate is 25, the field rate is
50 on account of interlaced scanning. Thus, there is a difference of one picture frame/sec between
the two and if not corrected, would cause a rolling bar on the raster besides loss of some signal
output. In order to overcome this discrepancy, the film in the telecine projector is pulled down
by the shutter mechanism at the rate of 25 frames/sec. It is achieved by a suitable speed
correction in the drive mechanism. This naturally results in a little faster movement of scenes
and objects on the television screen but the distortion caused is so small that it is hardly
noticeable. The corresponding small increase in the pitch of reproduced sound also goes
undetected.
Similarly in the 525 line system where the frame rate is 30/sec, it becomes necessary to
suitably modify the film rate of 24 to achieve compatibility between the two in order to prevent
rolling and loss of any picture information. The necessary correction is carried out by projecting
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one frame three times and the following frame two times. This sequence is repeated alternately
by means of an intermittent shutter mechanism of 3 : 2 pull-down cycle for the film. The pulldown is carried out by a shutter which operates at the rate of 60 pulls per second. Thus, out of
a total of 24 frames, 12 are projected three times while the rest only two times. This then
makes the film rate of 24 frames equal to the scanning rate of (12 × 3 + 12 × 2 = 60) 60 fields per
second.
The speed alteration and sequencing explained above makes direct scanning of motion
pictures possible through any TV network. While slides are used for stills, small advertisement
films are recorded beforehand on video tapes for telecasting when required.
(iv) Pay Television
Special programmes like first-run films, sports events and cultural programmes that are
normally not broadcast on the usual TV channels because of their higher cost are made available
to television subscribers either through the cable television network or on special channels. At
the receiver a special decoder is used to receive the picture. The decoder also has the provision
to register an extra charge for such special screenings. This service is optional but a fixed
charge is made for initial installation.
10.5 PICTURE PHONE AND FACSIMILE
This is another fascinating application of television where two people can see each other while
talking over the telephone line. A picture phone installation includes a unit that contains a
small picture tube and a miniature TV camera. The highest modulating frequency in picturephone services is normally limited to about 1.5 MHz.
Facsimile is another application of electronic transmission of visual information, usually
a still picture, over telephone lines. Since there is no motion, a slow scanning rate is employed.
Facsimile is employed for sending copies of documents over telephone lines.
10.6 VIDEO TAPE RECORDING (VTR)
Video tape recording was introduced in 1956 and it proved to be a vast improvement over the
earlier method of recording motion pictures taken from the screen of the television receiver.
Video tape retains the ‘live’ quality of broadcasting and has the capability of being edited and
duplicated without any delay. The other advantages are (i) immediate playback capability, (ii)
convenience of repeating the recorded material as many times as the viewer wishes, and (iii)
ease of duplication for distribution to a large number of users.
The video signal can be recorded on a magnetic tape for picture reproduction in a similar
way as the audio tape is used for reproduction of sound. In an audio recorder, the plastic tape
(mylar) that has a very fine coating of ferric oxide is made to move in physical contact with the
tape head. Any electrical signal applied to the tape head magnetizes the magnetic particles on
the tape, as it passes across the head. For each cycle of the signal, two tiny bar magnets are
produced on the tape and the length of bar magnets is inversely proportional to the frequency
of applied signal. Thus, on a recorded tape these bar magnets form a chain with like poles
adjacent to each other. When a recorded tape moves across the playback head, there is a
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change in flux linkages with the head and hence a voltage is developed across its coil terminals.
This technique of recording and reproducing audio signals is illustrated in Fig. 10.4.
Tape
width
Gap length
Output
Bar magnets
H
N N
S
Tape head
S
N
v
One wave
Recording length
head
v
0
N
S
Tape
motion
t
N N
S S
N
v0
0
t
Output signal
Input signal
Fig. 10.4(a) Electrical signal recorded in the
form of bar magnets on the magnetic tape.
Fig. 10.4(b) Development of signal
during playback.
Audio Range
Each head has a certain gap length. As the recording frequency increases, the length of the bar
magnets decreases. A limiting frequency is reached when at a given tape speed, total length of
the two adjacent bar magnets becomes equal to the gap length. At this frequency, output of the
playback head will be zero since each bar magnet will produce equal and opposite voltage in
the coil, with the result that no net flux passes through core of the head. For a tape speed of 19
cm (7.5″) per second and with a gap length of 6.3 microns (0.00025″) the usable frequency
comes to about 15 KHz and is enough for audio recording.
Audio Signal Dynamic Range
Since output voltage from a playback head is directly proportional to the rate of change of flux,
for every doubling of frequency the output voltage will become twice. In other words, every
time the frequency gets halved (one octave lower), the output falls by 6 db. Assuming that the
entire audio range occupies 9 octaves, output at frequencies that lie in the lowest octave will be
54 db below the output in the highest octave. This discrepancy is got over by providing equalizing
circuits in the playback amplifier. The equalizing network is designed to have characteristics
where the output voltage falls by 6 db per octave to allow for the rising response at the playback
head.
AC Bias
If the signal to be recorded is applied directly to the record head, the output will be highly
distorted on account of non-linearity of B-H curve of the core material around its zero axis.
This difficulty is solved by superimposing the recording signal on a high frequency ac voltage.
The amplitude of the high frequency bias is so chosen that its positive and negative peaks lie
within the linear portions of the B-H curve. As shown in Fig. 10.5, the two outputs (marked X
and Y) add up to give linear output with improved signal to noise ratio. The ac bias frequency
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is kept fairly high so that the beat signal between the highest signal frequency and bias frequency
does not fall in the audio range. An ac bias frequency close to 60 KHz is considered adequate
and is normally employed in audio tape recorders.
B
(a+b)
+ Output
t
(X)
H
(Y)
– Output
(a+b)
Mixed input
signal
Sum of
X and Y
outputs
t High frequency
biasing signal
(b)
t
Recording
signal
(a)
Fig. 10.5. Effect of a.c. (ultrasonic) biasing in audio tape recording.
Video Recording
The above introduction to audio recording will enable a better appreciation of the special
problems of recording video signals on a magnetic tape.
Video Frequencies
For recording up to the highest video frequency of 5 MHz if the head gap is kept at 0.00025″
(6.3 microns) a tape speed of the order of 39 metres (1300 inches) per second would be necessary.
With the head fixed as in audio recording and moving the tape at such a high speed would
result in excessive wear and tear besides mechanical instability. Decreasing the gap below
about 6 microns to lower the tape speed is not possible because of technological problems.
However, it is not essential that only tape should move and head remains stationary. It is the
relative speed of tape and head which is responsible for the output voltage. Hence the relative
speed is increased by moving the tape head in opposite direction to the tape movement. In
practice tape heads are mounted on the periphery of a drum which rotates around an axis
relative to the direction of tape movement. This enables reduction in tape speed to as low as
7.5 inches (≈ 20 cm) per second.
Video Signal Dynamic Range
As described earlier, output voltage from a playback head is directly proportional to the rate of
change of flux. In case of video frequencies consisting of dc to 5 MHz, theoretically there would
be infinite number of octaves. Even if frequencies down to 25 Hz are required to be retained (dc
can be recovered by clamping), the band between 25 Hz to about 5 MHz would occupy nearly
17 octaves. This means that the average output on account of frequencies in the lowest octave
will be about 100 db below the output in the highest octave. It is very difficult to handle such a
large dynamic range where low frequencies run into noise levels. The octave problem could be
solved by translating video frequencies to higher frequencies by amplitude modulation. For
example, a carrier frequency of 10 MHz would provide sidebands in the range of 5 to 15 MHz.
This covers only two octaves. However, amplitude modulation is not used because of the following
reasons:
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(i) At the speeds used in video recording, it is difficult to keep good contact with the
head and this results in amplitude variations. Such amplitude variations are reproduced as
noise.
(ii) In order to avoid distortion it is necessary to use an ac bias having a frequency
atleast four times the carrier frequency. However, such a high frequency (4 × 10 MHz = 40
MHz) is not desirable on account of high frequency heating and other such losses.
Keeping in view the above drawbacks FM is used for video recording. A carrier frequency
of about 6 MHz is often employed. Its amplitude is kept quite large thus making use of any ac
bias unnecessary.
FM is insensitive to small amplitude variations. However, if present due to improper
contact between the tape and head, such variations can be removed by the use of amplitude
limiters. The dynamic range with FM recording is also quite small despite the fact that it has
a wider bandwidth. This is so because FM sideband frequencies which have significant
amplitudes occupy only a few octaves.
An additional advantage of FM is that it becomes possible to record and transmit even
the dc component of the video signal. Audio signals which accompany the scene are recorded
on the same tape by a separate head and played back by normal audio tape recording and
playback techniques.
Scanning Methods
The two methods commonly used for video recording are called transverse scanning and helical
scanning. In transverse scanning four quadruplex heads are used for recording and reproduction
while in helical scanning either one or two heads are employed.
Transverse (Quadruplex-head) Scanning. Transverse scanning, though expensive is
superior, and is therefore preferred in professional video tape recorders. Figure 10.6 (a) illustrates
Audio erase head
Video heads
Record head
Tape
Control track head
Vacuum
chamber
Cue
Cue record
erase
(a)
Capstan
head
Vacuum guide
Heads
Audio track
Video track
Guard band
Cue
Tape
Head drum
Support
(b)
Control track
(c)
Fig. 10.6. A transverse recording system (a) basic principle of transverse recording
(b) vacuum guide and head wheel (c) tape format.
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the unidirectional path of the tape past a rotating head wheel which has 4 record/reproduce
heads affixed to it at accurately spaced 90 degree intervals. Thus the heads rotate transversely
across the tape while it is pulled slowly from one spool to another. Each head comes in contact
with the tape as the previous one leaves it. The net result is a large relative motion between
the head gaps and tape surface.
In order to achieve good contact between the tape and head (see Fig. l0.6 (b)) the tape is
made to move in a curvature. The curvature is of the shape of head travel and this is given to
the tape by a vacuum pump arrangement to ensure that each head maintains a proper fixed
pressure contact with the oxide surface.
Because of tape movement and relative arc of the recording heads, the recorded track is
somewhat slanted (see Fig. 10.6(c)) in the direction of tape travel. At the top and bottom edges
of the tape, sync and audio signals are recorded respectively. A switching arrangement transfers
signal to the active head (the head which is in contact with the tape) at the appropriate moment.
A small guard band appears between any two slanted recording tracks on the tape. A full track
erase head is used before the tape goes to the drum that carries the recording heads.
Synchronization. While recording, the completion of one video track must correspond to
one field of picture scanning. Similarly during replay the video head must track it accurately
otherwise reproduced picture will get severely distorted. Therefore speed regulation of the
tape mechanism is very critical. The control signals recorded at the edges of the tape should be
synchronized and thus locked with head rotation. This is done by servo control methods. The
television signal to be recorded has its own pulse train but this cannot be used directly on the
tape since it needs drastic changes before use for synchronizing and control of drive speeds.
Servo Control System. A signal in the form of pulses is generated for each rotation of the
video head. This as shown in Fig. 10.7 is done either by a bulb and a photocell or by a small
magnet and coil combination. The drum has a small slit through which light from a bulb falls
Head motor shaft
Bulb
Magnet
Slot
Video
head
Video
head
Photocell
Head drum
Control signal
(a)
(b)
Fig. 10.7. Generation of control signal for video head speed
regulation (a) photocell method, (b) magnetic method.
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on a photocell once during each revolution of the drum. In the second method, a small magnet
fixed on the drum, induces a small voltage pulse as it passes over a fixed coil once during each
revolution. The drum pulses thus generated are compared with the incoming field sync pulses
obtained during recording. The comparator produces an error signal corresponding to the
difference in phase and frequency between the two signals. This is amplified and applied to
eddy current brakes provided on the head motor. The brakes adjust the motor speed to provide
necessary synchronization. On replay the drum pulses are compared with 50 Hz pulses derived
from supply mains. The error signal thus derived from the comparator is used for controlling
speed of the head motor. In addition to this the drum pulses are also used to ensure that the
head runs accurately at the centre of the recorded track. Maximum video output is the indication
of correct video head tracking.
The use of four recording heads together with switching arrangements, a vacuum system
for good tape contact with recording heads and elaborate synchronizing facilities contribute to
the high cost of this system. However, it is justified because professional VTRS which employ
quadruplex system of recording with a full bandwidth of 5 MHz providc such good quality
pictures that they look like a live programme.
Helical Scan Recording. Smaller and low-priced video tape recorders employ 0.5″ or 1″
tape and provide a bandwidth up to about 2.5 MHz. Such recorders normally use one head and
a relatively simple drive mechanism. In this system of recording the tape is wrapped around a
drum inside which the head rotates. The video head protrudes through a horizontal slit in the
drum to come in contact with the tape. Figure 10.8(a) shows the mechanical layout of this
method of recording. The video head rotates at 50 revolutions per second such that one field of
picture information is recorded in one revolution. As shown in the figure, the tape comes in
contact at the upper edge of the drum and leaves it at its bottom edge. Thus the recorded track
is in the form of a helix and this gives it the name of helical scanning. While a single video head
is used for both recording and playback, the tape passes before a full track erase head before it
goes around the drum. A typical one inch tape format is shown in Fig. 10.8(b).
Takeup
spool
Rotating
drum
Tape head
Rotating
drum
Video
head
Tape
Audio and control
track head
Tape
Capstan
spindle
(a)
Audio track
Guard band
Guard track
Video
track
(b)
Cue track
Control track
Fig. 10.8. Helical scanning system (a) single head wrap scanning (b) 1″ tape format.
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TELEVISION APPLICATIONS
It consists of an audio track and guard band at the top and audio cue, guard and control
tracks at the bottom. The video tracks across the tape are at an angle of 5° to the tape length
axis separated by 4 mil (0.004″) guard bands.
Quadruplex Head Recording and Playback Circuits. Figure 10.9 shows the basic block
schematic arrangements of recording and playback in a quadruplex head VTR. In record mode
the composite video signal of about 1 V P-P amplitude, as obtained from a camera set up, feeds
into a two stage wide-band video amplifier in the video tape recorder. The output from this
amplifier is fed to the FM modulator via a pre-emphasis network and driver.
Video
Video
amp
input
Pre-emphasis
circuit
Driver
amp
FM
modulator
Recording
head amp
Sync
separator
and shaper
Head drum
motor drive
Amp
Ultrasonic
biasing network
input
Four
video
heads
Head
drum
Servo
control cct
Audio
Slipring
brushes
Motor
Amp
Audio record
head
A.C. bias
oscillator
Fig. 10.9(a) Simplified block diagram of a quadruplex head video tape recording system.
Slipring
brushes
Video
heads
Switcher
amp
Equalizer
and
limiter ccts
FM
demodulator
Video
amplifier
AM
modulator
Head
drum
Motor
Head drum
motor drive
Motor
Capstan
motor drive
To
monitor
To TV
receiver
Servo
control ccts
Capstan
Speed control
signals
Amplifier
Audio head
Audio
output
Fig. 10.9(b) Simplified block diagram of a quadruplex head video tape system in the playback mode.
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The modulator output is amplified by the recording head amplifier and fed to the record
heads through slip-ring brushes. The accompanying audio signal is amplified, given an ac bias,
and then fed to the audio head. As shown in Fig. 10.9 (a) a sync separator circuit is also connected
across the output of the wide-band video amplifier. The sync pulses, after separation, are
suitably shaped and used to synchronize and control the head drum motor speed by servo
control techniques.
During playback (see Fig. 10.9 (b)) the video head outputs are collected through slipring brushes. These are then fed to an electronic switcher, which selects and amplifies the
signal from the head that is in contact with the tape. The selection takes place during horizontal
retrace blanking intervals so that the switching transients are not visible. The selected output
is fed to the FM demodulator through an equalizing network and several limiter stages. The
detected output is amplified by a four stage video amplifier before feeding it to a monitor or an
amplitude modulator. After modulation with the carrier frequency of any one of the band I
channels, the incoming programme can be viewed on a TV receiver.
The speed control signals are recovered from the corresponding tape tracks and processed
for driving the capstan motor and synchronizing the head drum motor drive. The audio signal,
as obtained from the audio playback head, is amplified and fed to the monitor or a modulator
for use in a TV receiver.
Video Disc Recorder. Optical video disc recording is a recent development. It uses a
laser beam in its pick up system. The video disc is 12″ (301.6 mm) in diameter and is made of
transparent plastic. Since its reverse surface is coated with aluminium to reflect the laser
beam, it has an appearance of a metallic disc. Sound and image signals are stored in tiny pits
located in a substrate l.l mm from the surface. There are close to 14 billion pits on one surface
of a disc. The reflection of the laser beam from the disc is intermittently interrupted according
to the distribution and width of these pits and the reflected laser beam is converted into electrical
signals. Since there is no friction from a stylus as in the case of conventional audio pickups, the
sound and image quality of the laser beam type video disc is almost permanent. The rotational
speed of the disc is so controlled that the relative beam velocity is constant from the outermost
edge to the innermost end. One revolution of each track forms one frame of picture and one
side of the disc can record up to 54000 frames. Both, one hour and half an hour duration dises
are now available. In the later type of video disc, sound is FM modulated and recorded in two
separate channels. Therefore, the two channels may be used for recording and reproduction of
bilingual programmes or high fidelity stereo music.
10.7 TELEVISION VIA SATELLITE
The conventional methods for extended coverage of TV by microwave space communication
and coaxial cable links are relatively expensive. Geostationary communication satellites
launched into synchronous orbits around the earth in recent years have enabled not only national
but also international television programmes to be relayed between a number of ground stations
around the world. Three artificial satellites placed in equatorial orbits at 120° from each other
cover practically the whole populated land area of the world.
High power, highly directive land based transmitters transmit wideband microwave
signals to the geostationary satellite above the transmitter. Each microwave channel has a
bandwidth of several tens of megahertz and can accommodate many TV signals and thousands
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of telephone channels or suitable combinations of these. The satellites usually powered by
solar batteries receive the transmission, demodulate and amplify it and remodulate it on a
different carrier before transmitting again. The transmitting antenna on the satellite, by the
use of a suitable reflector, can direct the radiated beam to a narrow region on the earth and
economize on power to provide a satisfactory service in the desired area. Higher power satellites
can provide large power flux densities so that smaller size antennas can be used for reception.
For national distribution the transmission is downwards from a wide angle antenna so that
the whole national area is ‘illuminated’ by the transmission if possible. For international
distribution the transmission is also towards the other one or two satellites (which are in line
of sight direction) from highly directive antennas. The demodulation-amplification-remodulation
transmission process is repeated in the second satellite. The final ‘down channel’ transmission
is received (in the same or a different country) by a large cross-section antenna and processed
in low noise receivers and finally reradiated from the regular TV transmitters.
There are a number of ‘INTELSAT’* satellites over the Atlantic, Pacific and Indian
Oceans operating as relay stations to some 40 ground stations around the world. The
international system of satellite communication caters to the continental 625/50 and the
American 525/60 systems. As television standards differ from country to country, the
transmitting station adopts the standards of the originating country. The ground station
converts the received signal with the help of digital international conversion equipment to the
local standards before relaying it.
Frequency modulation is used for both ‘up channel’ and ‘down channel’ transmission.
FM, though it needs a larger bandwidth, offers good immunity from interference and requires
less power in the satellite transmitter.
Frequency Allocation
The frequency bands recommended for satellite broadcasting are 620 to 790 MHz, 2.5 to
2.69 GHz, and 11.7 to 12.2 GHz on a shared basis with other fixed and mobile services. The
satellite antenna size and the RF power naturally depend upon the frequency of operation.
Space erectable antennas are used for the 620-790 MHz band, with the size limited to about 15
metres, while rigid antennas are used both for 2.5 and 20 GHz bands, the size being limited to
about 3 metres.
For the ground terminal, the maximum diameter of the antenna is restricted by the
allowable beamwidth and frequency. The cost and complexity of the receiver increases with
increase in frequency.
Extended Coverage of Television
Besides the use of satellites for international TV relaying, satellites can be used for distributing
national programmes over extended regions in large countries because of their ability to cover
large areas. For this, satellites can be used in three following ways.
(i) Rebroadcast System. In this system emission from a low power satellite is received
with the help of a high sensitivity medium size (about 9 m) antenna on a high sensitivity low
noise earth station. The received satellite programme is rerelayed over the high power terrestial
transmitter for reception on conventional TV receivers. This method is suitable for metropolitan
*INTELSAT stands for INternational TELecommunication SATellite-consortium.
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areas where a large number of TV receivers are in operation. This method also enables national
hook-up of television programmes on all distantly located television transmitters. For a large
country like India, to do so by microwave or coaxial cable links would be expensive.
(ii) Limited Rebroadcast System. In rural areas where clusters of villages and towns
exist, and the receiver density is moderate, low power transmitters can be used to cover the
limited area. This reduces the ground segment cost by eliminating the need for special frontend equipment, dish antennas and convertors for each receiver.
The SITE (Satellite Instructional Television Experiment) programme conducted by India
in cooperation with NASA of USA in 1975-76 was a limited rebroadcast system. A high power
satellite ATS-6 (Application Technology Satellite) positioned at a height of 36,000 km, in a
geostationary synchronous orbit with sub-satellite longitude of 33° East was used for beaming
TV programmes over most parts of the country. TV programmes from the earth station at
Ahmedabad were transmitted to ATS-6 at 6 GHz FM carrier with the help of a 14 m parabolic
dish antenna. The FM carrier had a bandwidth of 40 MHz. The ‘down transmission’ from the
satellite was done from a 80 W FM transmitter at 860 MHz. The transmitted signal consisted
of a video band of 5 MHz and two audio signals frequency modulated on two audio subcarriers
of 5.5 MHz and 6 MHz. This enabled transmission in two different languages and reception of
any one of these. A block diagram of this system is shown in Fig. 10.10.
ATS-6
Satellite
6 GHz
860
MHz
Mixer
BPF
3m dish
receiving
antenna
(b)
Local
osc
14m dish
Earth
station
(a)
RF
amp
Limtdiscrim
Amp
AF
amp
Front end
and converter
Mixer
860 MHz
IF
amp
BPF
IF
amp
Limtdiscrim
LPF
Video
amp
70 MHz
Local
osc
790 MHz
Antenna
feeder to
the indoor
receiver
fcP
Video
0
(c)
fcs2
fcs1
6
f(MHz)
Defl
ccts
5.5
Fig. 10.10. Direct reception system from a satellite. (a) ATS-6 Satellite (b) front end converter
and receiver block diagram (c) base band frequency spectrum.
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(iii) Direct Transmission. Direct reception of broadcast programme is the only possibility
in areas remote from terrestial broadcast stations. In this system the cost of reception is very
high even with high power satellite transmissions. This is so because of the need for a special
antenna to receive the signals and front-end convertor unit to modify the signals into
conventional broadcast standards. Advances in technology reducing the cost of low noise frontend for rcceivers may make direct individual reception feasible in the near future. Japan was
the first country to launch a medium scale satellite (‘YURI’) in April 1978 for experimental
purposes towards direct reception. It radiates two colour channels in the 12 GHz band. The
additional receiver equipment consists of 1 to 1.6 metres parabolic dish antenna and a frontend convertor to feed UHF-AM TV signals to the conventional receiver.
Cost of Satellite Communication
The cost of satellite communication would be very much lower than it is but for the limited life
of the satellite. The life is limited because a geosynchronous satellite using high gain antenna
requires close control of both its position and altitude in ‘Orbit’. The position and altitude
control rockets require fuel that has to be put in once for all before launch. Thus for a given
payload, the longer the life the heavier is the satellite and correspondingly expensive. All
communication satellites are therefore designed for a maximum operating life limited by its
positioning fuel capacity. This of course has an advantage too. Successive generations of
communication satellites can incorporate the latest developments in electronics and
communication technology, packing much more capacity into satellites of comparable size. It
is noteworthy that the cost per channel of one hop satellite communication has decreased over
the last decade by a factor of more than ten. We can therefore hope that with advances in
technology direct reception at reasonable costs will become a reality in a not too distant future.
10.8 TV GAMES
Television games is a relatively new application of digital electronics and IC technology to TV
products. The first of the solid-state games used Transistor-Transistor Logic (TTL). The earlier
set-ups provided logic for playing question answer games on the television screen. Later paddle
type games were developed which included generation of sounds to give a touch of reality to
the game being played. Then colour was added to the display and the challenge of contest
amongst players increased by programming the game electronics to adapt to the player’s skills.
Now with the development of microprocessors (µP) further sophistication has become possible,
where, for example in card games, players can compete against the computer and against each
other.
Though logic can be developed to play almost any game but most common and
commercially available games include tennis, soccer, squash and rifle shooting. The receiver
screen shows the game in progress and also displays its current score. As the game proceeds
the score display is updated properly. As an illustration, if the game being played is tennis, the
screen (see Fig. 10.11) will show the court lines, the net, the rackets and the ball. The movement
of the ball is fully shown as it is hit from one side to the other. In addition, individual scores are
displayed on both sides of the court. Suitable sounds are generated when any contact occurs
and these are reproduced on a loudspeaker in synchronism with the action. In some games like
football, players are also shown along with movements of the ball.
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TV screen
Court boundary
Ball
Net
Racket
4
2
Racket
Fig. 10.11. Tennis game display on a receiver screen.
Functional Organization of TV Games
Any television game consists of two parts-the game unit and TV receiver. The game unit contains
complete electronic circuitry necessary for generating signals, which when fed to the television
receiver display the current status of the game on the screen. Functionally a TV game unit
consists of three sub-units or blocks. These are: (i) player or user’s control unit, (ii) game cum
control circuit logic unit and (iii) RF oscillator and modulator section. Figure 10.12 shows
these blocks or units and also the manner in which the modulated game video signal is fed to
the receiver.
TV
receiver
Antenna
switch
TV game unit
Player(s)
or user(s)
controls
Game and
control
logic
RF osc
and
modulator
Fig. 10.12. Functional blocks of a TV game system.
The player control block contains various controls available to players for playing the
game. In addition, it has the associated circuitry for generating corresponding command signals
to initiate various actions.
The game and control logic section is the heart of any TV game. It produces video signals
necessary for displaying game characters and game field on the screen. The characters may be
simple paddles, bats, rackets or complex figures representing men, women and other objects
necessary for the game. The interface circuitry for both player and game-action control forms
part of the character and field video generation circuits. This section also provides logic circuits
for game-playing rules, score display and totalling during the game.
The control and logic section also inserts sound signals at appropriate points in the
game by pulse detection gating. 1n addition to all this it has a sync generation section which
develops both horizontal and vertical sync pulses needed to time the composite video signals
correctly.
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The composite video signal which contains full game and sync information feeds into
the VHF oscillator-modulator block. The oscillator is set for channels 3 or 4 and its output after
modulation provides input signal for the TV receiver. It is normally fed to the receiver input
terminals through a special antenna isolation switch. The output signal level from the oscillatormodulator unit is kept low to avoid any interference to other television sets operating in the
vicinity.
Development of TV Game Circuits
Earlier TV game units, built with TTL consisted of different general purpose chips
interconnected on a printed circuit board. These games being simple in nature needed a limited
number of printed circuit boards. However, with time, the number of games offered by the
same game unit increased and this made the units more complex. The corresponding complexity
of electronic circuits required to implement these games made the TTL hardware too bulky
and unmanageable. Accordingly manufacturers of TV games have now switched over to the
use of dedicated (special purpose) ICs and microprocessors for designing all types of complex
games and their logic circuits.
Dedicated ICs for TV Games
Single customer built LSI chips are now available which contain almost all the circuitry which
goes into the making of a game unit. Besides a modulator, a clock-generator and a regulated
power supply, such ICs need only a few discrete components like resistors and capacitors to
produce multi game units. TV game industry has made big strides during the past decade and
a large number of dedicated n-channel MOS chips, both for 625/50 scan (PAL-Colour) and 525/
60 scan (NTSC-Colour) systems are now available. These include a choice of ball and paddle
games with true game rules, realistic courts, and individual player identification. The battle
games offer all the thrilis and excitements of real battle scenes.
The organization of a TV game unit employing a specially built IC is illustrated in
Fig. 10.13. As shown there, all outputs (command signals) from the user’s panel and clock
generator chip feed into this IC. The various outputs from the IC are combined in a video
summer unit to form a composite video signal for feeding onto the modulator unit.
Clock
generator
User’s panel
and
command
circuits
Control
signals
Modulated audio
and video signals
Control logic
and circuits in
the game chip
(IC)
Video and
audio
summer
Video
and
audio outputs
RF osc
and
modulator
To
receiver
Composite video
signals
Regulated
power supply
Fig. 10.13. Block diagram of a TV game system employing a dedicated IC chip.
In order to illustrate various functions performed by such ICs, a 28 lead dual-in-line
dedicated IC package is shown in Fig. 10.14. It contains logic and controls for six selectable
games which can be played by one or two persons with vertical paddle motion. The games
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Pin No.
MONOCHROME AND COLOUR TELEVISION
Designation
Function
1
NC
No connection
2
VSS
Negative supply input, nominally 0 V (ground)
3
Audio output
Three different audio tones for hit, boundary,
reflection and score can be selected. The sound output
lasts for about 35 ms
4
VCC
Positive dc supply, + 7 V to + 9 V
5
Ball angles
(i) Input set to logic ‘1’ (open circuit) selects ± 20°
rebound angles
(ii) Input set to logic ‘0’ (Vss i.e. ground) selects ± 20°
and ± 40° rebound angles
6
Ball output
The ball video signal is output at this pin.
7
Ball speed
(i) Input set to logic ‘1’ selects a low speed of ball
motion
(ii) Input set to logic ‘0’ selects a higher speed of ball
motion
8.
Manual service
With input logic at ‘1’ the game stops after each
service. However, when logic is switched to ‘0’, the
circuitry changes to automatic service mode
9, 10
Player outputs
Right Player (RP) and Left Player (LP) video outputs
are available at these pins
11, 12
13
Right and Left Paddle/
As shown in the figure an R-C network connected
ball position (location)
to each of these pins enables vertical position control
of the paddle/ball through a 10 K potentiometer.
Bat size
(i) Logic 1—large paddle/bat size
(ii) Logic 0—small paddle/bat size
14, 15
NC
No connection
16
Sync output
Standard horizontal and vertical sync and blanking
pulses aie available at this pin.
17
Clock input
The output of the master clock (chip) is fed at this pin
(Frequency = 2 MHz)
18 to 23
Game Selection
To select a particular game the corresponding switch
(see figure) is set for logie ‘0’ i.e. connected to VSS.
Other game selection switches remain at logic ‘1’ i.e.
open circuit.
24
Score and field output
The score and field output video signals are available
at this pin.
25
Game Reset
The input switch is momentarily connected to Vss (logic
0) to reset the score counters and to start a new game.
Normally this pin connection stays at logic ‘1’.
26
Shot input
This input is driven by a positive pulse indirectly
obtained from the user’s panel to indicate a ‘shot’
27
Hit input
This pin is also driven by a positive pulse triggered
by the shot input if the target is hit.
28
NC
No connection
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which can be played are tennis, soccer, squash, practice (one man squash) and two rifle shooting
games. It features automatic on-screen score display from 0 to 15, sound generation for hit,
boundary and service, selectable paddle size, ball speed, two different rebound angles, automatic
or manual ball service and visually defined areas for all ball games. The video signal output is
suitable for black and white display on a standard domestic TV receiver. The functions of
various pin connections on this IC (see Fig. 10.14) are as follows.
1
10K
pot
14
Right
11
paddle/ball
location
25
15
28
NC
Clock input
Ball output
Game reset
19
Sync output
TV GAME
DEDICATED
INTEGRATED
CIRCUIT
20
Game
select
Audio output
Shot input
22
Hit input
23
Ball speed
5 Ball-angles ± 20° /± 20° and ± 40°
12
R
From clock
generator
6
Score and 24
field output
18
21
17
Left
paddle/ball
location
Manual service
Bat size
16
3
To summing
cct
500 Hz, 1 KHz,
and 2 KHz
26
27
7
8
13
Video
outputs
C
Vss 0V
2
RP
9
LP
10
Vcc+
4
To summing
cct
Fig. 10.14. Block diagram of a TV game dedicated IC. (Note: The pin
numbers and their locations are arbitrary).
TV Games with Colour Display
Initially, designers of TV games were hesitant to provide colour display because of the
complications of colour signal generation and modulation. However, now with the availabilityof
separate ICs for such purposes, addition of colour needs little more than adding one or two
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MONOCHROME AND COLOUR TELEVISION
integrated circuits to the schematic. National Semiconductor’s LMI 889 is one such IC which
accepts luminance (brightness), syne, chrominance (colour) and audio inputs and produces an
RF modulated composite video signal. This IC includes two RF oscillators which are tuned to
VHF low-band channels 3 and 4. Either output can be selected by applying a voltage to the
external R-L-C tank circuit. The sound oscillator is isolated from the rest of the IC, and can be
externally frequency modulated with a varactor diode or by switching a capacitor across the
tank circuit. The crystal controlled colour subcarrier oscillator feeds two chroma modulators
with quadrature signals for generating (B-Y) and (R-Y) colour-difference signals. Two RF
modulators then add video, chroma and sound to the selected carrier frequencies.
Microprocessor (µP) Controlled TV Games
Some of the available games have a µP as the basic control element and use plug in ROM
(Read Only Memory) cartridges to store game sets. Many side benefits are accrued by
incorporating a µP into a particular video game. Most obvious of these are the versatility of the
design, the multiplicity of games available and the ease with which a particular game design
can be modified.
A µP controlled game may include, as its functional components, a microprocessor,
Random Access Memory (RAM), Read Only Memory (ROM), Cassettes or other secondary
magnetic storage medium besides a key board or other player control blocks, a video interface,
a modulator and course a receiver as the display unit.
A block schematic of such a system is shown in Fig. 10.15. A brief description of the
various blocks follows:
(i) Display Unit. The television receiver is used as the display unit. The receiver handles the signal fed to it in the usual and displays appropriate patterns at desired positions on
its screen. The rate of display is made fast enough to maintain the illusion of continuity as is
the normal practice in television broadcasts. The display on the screen is organised by the
games system in a standard format which is usually 150 rows × 250 columns.
(ii) Player Controls/Keyboard and Interface. This unit provides a link between the system and players. The knobs on the keyboard are moved to initiate various actions. For instance in a tennis gams the rate and direction of displacement of the control knob will decide
how quickly and in which direction the bat will move for hitting the ball. The outputs from the
keyboard are analog in nature. The associated interface accepts the serial analog inputs and
converts them into parallel digital form for processing by the µP (microprocessor). Similarly
other command knobs generate appropriate analog signals which are necessary for a particular game.
(iii) Memory—RAM (Random Access Memory) and ROM (Read Only Memory). As shown
in the figure the memory consists of two blocks—RAMs and ROMs. The RAMs store information temporarily which continuously updated or changed on receipt of commands from the
players. The ROMs store fixed instructions for the µP for its processing the received data and
sending out command signals to associated units. The ROMs also contain instructions for
generating video signals for fixed patterns. For example in a football game, ROM stores the
dot pattern of a football. When the game is in progress, it feeds this data continuously to the µP
along with the information it receives about speed and direction of motion of the ball.
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TELEVISION APPLICATIONS
(iv) Row and Column Counters. The receiver screen is divided into 150 rows and 250
columns. This is done to determine the position of the object on the screen. The row and column counters are used to determine the row and column position of any object. To display a
particular object at a certain place on the screen, the counters are suitably set by the µP. The
counters in turn feed digital signals to the PVI (Programmable Video Interface) for generating
corresponding video signals. For example, to give the impression of a horizontal movement of
any object, the µP on receipt of such information from the memory, continuously changes the
column counter to indicate the next horizontal position. Accordingly the column counter continuously feeds digital command signals to the PVI for generation of varying video signals to
flash the object at its correct location on the screen.
Memory
Cassette
and
Interface
ROM
and
Interface
RAM
and
Interface
Row
co-ordinates
Microprocessor
(µP)
and Interface
Column
co-ordinates
Player controls
(Key board)
and Interface
Control
clock
generator
Column
counter
Row
counter
Indoor antenna
Command
signals
Composite
video
signals
Programmable
video
Interface
(PVI)
Sync
pulses
Audio
signals
Video
Modulator
(AM)
Audio
Modulator
(FM)
Combining
network
Selector
switch
TV
receiver
Sync
generator
Fig. 10.15. Simplified block schematic of a microprocessor controlled TV games setup.
(v) Programmable Video Interface (PVI). This unit generates video and audio signals on
receipt of digital commands from the µP both directly and indirectly. The µP continuously
feeds the PVI with dot pattern output of various objects and indicates their motion and location through Row and Column counters. The PVI also receives input from the clock generator
to produce audio signals to indicate various sound outputs at appropriate moment. The sync
pulses are also added here to form a composite video signal.
(vi) Sound Generator. In order to make playing of TV games more realistic, suitable
sounds are generated for various events Iike service, rebounds, shots, etc. The audio signals
are processed within the PVI. The µP is programmed to control a square wave generator (clock
generator) whose fundamental frequency, f0, is usually about 7800 Hz. For each different
occurrence a special bit (= 1) is set on. An 8-bit number (n) in the PVI determines the type of
sound to be generated like a rattle, collision, a shrill whistle etc. This as stated above is
determined by the µP and depends on the nature of game and the occurrence of different
sounds while the game is in progress. The PVI on receipt of a particular 8-bit number sends an
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MONOCHROME AND COLOUR TELEVISION
audio signal whose frequency is given by
f=
7800
f0
=
Hz
(n + 1) (n + 1)
The sound output can have a frequency as low as 30 Hz. The audio signal is either fed
directly to a separate loudspeaker or is reproduced on the receiver loudspeaker in the usual
way.
(vii) Microprocessor (µP). The heart of the TV games system is the microprocessor. It
processes the data fed to it and generates digital output which is used by the PVI to generate
proper video and audio signals continuously. The memory unit (RAMs and ROMs) inform the
µP how to process the data and send appropriate command signals. For example in a tennis
game, as the player moves the control knob in an effort to hit the ball it generates a particular
control signal. On receipt of this information via the memory unit, the µP decides whether row
and column locations of the racket and ball overlap or not (for a contact) at the same instance.
lf contact is made the direction of the ball is reversed. The ball’s dot structure remains the
same while its address location changes continuously to show the ball in motion. However, if
no contact is made the µP decides whether the point where the ball landed (X, Y co-ordinates)
are within the acceptable areas, that is, inside the court or outside it and accordingly gives
credit.
The output signal is also used to update the score on the screen. Similarly the µP decides
the nature of’ the sound to be produced and sends a command signal for its generation.
The above explanation is a simplified view of what actually goes on in the system. The
actual process involved is more complex and requires detailed and complicated programming.
(viii) Cassette Recorder and Interface. A large number of games can be stored in cassettes.
These are fed to the memory via a suitable interface. The cassette output is analog and the
interface converts it into digital form. This allows the RAMs to readily store the contents of the
required ‘game’ from the cassette player. When a different game is desired, the tape is advanced
or rewound till the desired game appears on the tape head. Information about one or two
games is permanently stored in the memory unit and thus these can be played without any
input from an external cassette receiver.
(ix) VHF Modulator. The video and audio outputs from the PVl together with sync pulses
can be fed directly to the receiver through a cable. However, in modern systems these signals
modulate VHF carriers of a particular channel (usually 2 or 3) as is normally done in a TV
transmitter. The modulated output is fed to the receiver input terminals via a coaxial cable. In
some designs the modulated signal is radiated through a small antenna. The receiver antenna
intercepts the radiated signal and processes it in the usual way to reproduce visual display on
the screen and sound in the loudspeaker.
References
1.
Riley, M.P. ‘Video Tape Recording’, Television August 75, Vol. 25 No. 10. Sept. 75, Vol 25, No. 11.
Oct. 75, Vol. 25, No. 12. Nov., 75 Vol. 27, No. 1.
2.
Wentworth, John W., ‘The Technology of Program Production and Recording’ Proc. 1RE, 50 (No. 5)
(May 1962) 830-836.
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TELEVISION APPLICATIONS
3.
Mahanvelu, A.S. and Rao B.S., ‘Basic consideration in the planning of Broadcasting Satellite
System’ J. 1nst. Electron, Telecom Engrs, New Delhi (No. 6) (June 1976), 347-355.
4.
Rao, B.S., Ramaiah and Swaminathan, V.L. ‘Television Receiver far Direct Reception from Broadcasting Satellite’, J. 1nst., Electron. Telecom Engrs, New Delhi (No. 8) (August 1973) 453-458.
5.
TV Games, WESCON Professional Program, 1976, Western Electronics Shown and Convention.
6.
Kam, L.I., Technical Aspects of Video Gamas, Signetics Corp. IEE Trans, on Consumer Electronics, CE-24, No. l, CE-23, No. 3.
Journal of Electronic Industry (JEI) Jan, March, June, July, Sept. and Oct. 1978.
Review Questions
1.
Draw the block diagram of a MATV system and explain how television signals are picked up
from several stations and distributed to various locations in an apartment building or hotel.
How is the impedance match maintained at different subscriber tap points ?
2.
Describe the main merits and applications of a CATV system. Draw a typical layout of this
system of signal distribution and label all the blocks. Why are amplifiers and equalizers required along trunk distribution lines ?
3.
How is a CCTV system different from regular TV broadcasts ? Enumerate various applications
of this system of television. Describe with suitable block diagrams various methods employed to
feed/transmit video signal to different monitors/receivers.
4.
Discuss special problems of video tape recording and explain haw these are overcome for recording video signals on a magnetic tape. What is a basic difference between transverse and helical
scan recording ? Explain with suitable diagrams the basic difference between these two methods
of video recording.
5.
Draw block diagrams illustrating record and playback modes of a quadruplex head VTR system.
Label all the blocks and explain sequence of operations both for recording and playback. How is
scanning and speed synchronization achieved in such a recording and reproduction system ?
6.
Describe briefly various systems which can be employed for distributing national television programmes by a satellite over extended regions in large countries like India. What is the function
of a special front-end used along with TV receivers for direct reception from a satellite ?
7.
Describe with a block diagram the functional organization of a TV game set-up and explain the
use of dedicated ICs for processing and control of analog signals generated at the user’s panels.
8.
Draw a schematic block diagram of a TV games set-up which employs a microprocessor for processing input data and generating digital outputs for the PVI.
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11
Video Detector
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11
Video Detector
From antenna to the input of video detector of a television receiver, it is all radio frequency
circuitry, and is similar to a superhetrodyne AM radio receiver. It is at the video detector that
picture signal is extracted from the modulated intermediate carrier (IF) frequency. This, after
detection is amplified and fed to the picture tube cathode or grid circuit for reproduction of the
picture.
The sound signal which is frequency modulated with the sound IF carrier frequency is
translated in the detector to another carrier frequency, which is the difference of the picture
IF and sound IF, i.e., 38.9 – 33.4 = 5.5 MHz. The intercarrier FM sound signal thus obtained is
separated at this stage or after the first video amplifier. It is then amplified and detected
before feeding to the audio section of the receiver.
11.1 VIDEO (PICTURE) SIGNAL DETECTION
The video detector is essentially a rectifier cum high frequency filter circuit to recover video
signal from the modulated carrier. Semiconductor diodes are used exclusively for detection
and need about 2 volts or more of IF signal for linear detection without distortion. The signal
to the detector is fed from the output of last IF amplifier stage. Either polarity of this signal
can be rectified by suitably connecting the diode, since both sides of the modulated envelope
have the same amplitude variations. This choice depends on the number of video amplifier
stages used and the manner in which the vidoe signal is injected in the picture tube circuit. It
should be noted, however, that polarity is not important in an audio system because the phase
of ac audio signal for the loudspeaker does not matter in reproduction of sound, but a polarity
inversion of video signal driving the picture tube would produce a negative picture.
The detector may use either series circuit or shunt circuit, the basic forms of which are
shown in Fig. 11.1. The series circuit arrangement is preferred because it is more suited for
impedance match between the last IF amplifier output and input of the video amplifier.
Diode
Video
IF input
C
C
R
Demodulated
video
output
IF
input
D
(a)
R
(b)
Fig. 11.1. Basic detector circuits (a) series (b) shunt.
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Video
output
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VIDEO DETECTOR
In most television receivers a positive going signal is obtained at the output of the detector
because this is the correct polarity for cathode injection in the picture tube after one stage of
video amplification. The choice of polarity is also influenced by the type of AGC used. In some
transistor receivers a negative going signal is developed, where a two stage video amplification
is employed for feeding the picture signal to the cathode of picture tube. A schematic
representation of the two types is illustrated in Fig. 11.2.
Black level
+ v0
+v
D
Black level
v0
0
Last IF
stage
t
0
t
Negative going signal
0
t
D
v0
–v
Negatively
modulated
video signal
Load and filter
network
– v0
Black level
Positive going signal
Fig. 11.2. Production of negative and positive going video signals
from a negatively modulated video signal.
11.2 BASIC VIDEO DETECTOR
The basic circuit of a video detector employing a diode is shown in Fig. 11.3 where a parallel
combination of C, a small capacitor and RL, a large resistance constitutes the load across which
vs
Cd
D
Last
IF
C
vs
t
0
–
+
RL
v0
v0
t
0
D.C. level
Video signal output
Fig. 11.3. A simple diode detector and filter circuit.
rectified output voltage v0 is developed. Note that the load is connected to anode of the diode to
develop a negative output voltage with respect to ground. The diode conducts during negative
half cycle of the input to charge ‘C’ up to a potential almost equal to the peak signal voltage vS.
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The difference is due to diode trop, since the forward resistance of the diode though small is
not zero. On the downward and positive half of each carrier cycle the diode becomes nonconducting, and during this interval some of the charge on capacitor ‘C’ decays through RL, to
be replenished at the next negative peak. The time constant of RLC network is kept large
compared with the time period of the applied IF signal. Then the circuit settles down to a
condition, in which short current pulses flow through the diode only during the tip of each IF
cycle to replenish the charge lost. The time constant of the network must, however, allow the
capacitor voltage to follow the comparatively slow variations of the envelope of the modulated
carrier. This condition can be shown to occur approximately when
1
= ωm ×
RL C
m
1 − m2
where ωm is the highest modulating frequency and m is the modulation index.With the highest
modulating frequency of 5 MHz and assuming an average modulation index of about 0.4, the
time constant (RLC) comes to nearly 0.08 µs. The period tC of the carrier (IF = 38.9 MHz)
frequency = 0.025 µs and that of the highest modulating frequency (5 MHz) = 0.2 µs. In practice
a time constant close to half the period of the highest modulating frequency is chosen for
effective and distortion free detection. This, in our case, is 0.1 µs and is nearly equal to the
calculated value of 0.08 µs. This time constant (0.1 µs) is seen to be much higher than the IF
time period and thus an output voltage that is very nearly equal to that of the envelope of the
AM wave is ensured. If the RLC time constant is made too small, the output waveform will
have a large IF ripple content which is not desirable. However, if the RLC product is kept too
large it will not affect the negative going half-cycle of the envelope waveform, but will cause
distortion of the positive going movements of the modulated envelope. This is known as positive
peak clipping.
Choice of RL and C
When the diode is conducting, it is obvious that some part of the output voltage is dropped
across Rd, the series diode forward resistance. The detector effeciency then depends on the
ratio of RL/Rd, where RL is the load resistance. Thus, for higher efficiency, a diode with a small
forward resistance must be chosen and RL should be kept as high as possible. As RL is increased,
C has to be reduced to maintain the correct time constant. The smaller the value of C the more
significant becomes Cd, the anode to cathode capacitance of the diode. A reference to Fig. 11.3
will show that C and Cd form a potential divider across the input circuit. During positive half
cycles of the applied carrier voltage, when the diode does not conduct, the entire signal voltage
splits across these two capacitors. This results in some unwanted positive half of the applied
voltage being developed across the load capacitor C. This reduces the net negative-going output
and hence the efficiency. Therefore in an effort to increase RL, C cannot be reduced too much
because then most of the unwanted positive going voltage would develop across RL and reduce
the net output voltage greatly.Since the voltage divides itself in inverse proportion to the
capacitances, and the larger voltage appears across the smaller of the two capacitors in series,
the ratio C/Cd should be as large as possible. If C is to be decreased to use a high value of RL,
then Cd must be very small.
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VIDEO DETECTOR
Choice of the Diode
For the time constant fixed at 0.1 µs, RL varying between 2.7 K and 5 K in parallel with C,
between 15 pF and 30 pF are commonly employed as load network for the detector in television
receivers. Since RL is only a few kilo ohm, the forward resistance of the diode must be as small
as possible. The diode capacitance Cd must also be very small because the value of C chosen for
the network is only around 20 pF. Some of the semiconductor diodes that meet the above
requirements are 0A79, IN69 and IN64, and are often used in video detector circuits.
It would be instructive to compare the values of RL and C chosen for the TV receiver
detector with that of a broadcast radio receiver. The corresponding values for a radio receiver
detector are : RL = 500 K and C = 100 pF (see Fig. 11.4) and are based on the same considerations
as explained for the television detector.
Filter circuit
Rf
D
vs
50 K
100
pF
C
IF = 455 KHz
fm = 5 KHz
100
pF
Cf
C = 100 pF
RL = 500 K
RL
500 K
v0
RL C = 50 s
Fig. 11.4. Detector and filter circuit of a radio receiver.
11.3 IF FILTER
The detector output voltage, v0 consists of three components—(i) the required video signal,
(ii) an IF ripple voltage superposed on the video waveform and (iii) a dc component of amplitude
almost equal to the average amplitude of the AM wave. The IF carrier component is removed
by passing the signal through an IF filter. The dc component, if not required, is blocked by
inserting a coupling capacitor in series with the signal path. However, the dc component forms
a useful source of AGC voltage and represents the average brightness of the scene.
A brief survey of the filter circuit used along with a radio receiver detector will be helpful
before looking into the special problems involved in detection of video signals. Such a detector
circuit with provision to filter out IF ripple frequency is shown in Fig. 11.4 Rf is chosen to be
very much greater than Xcf (reactance of Cf) at the intermediate frequency of 455 KHz. Most of
the IF voltage gets dropped across Rf and v0 is practically ripple free. Cf being small acts as an
open circuit for the audio signal. Rf and RL then form a potential divider for the desired audio
signal, and a part of this signal is also lost across Rf. However, since Rf << RL (nearly 1/10th)
the loss of audio signal is very small. The filter configuration shown is standard in radio receivers.
The RC filter circuit used in radio receivers is not practicable for video detectors because
of very low values of load resistance. A suitable value of Rf to attenuate IF would seriously
attenuate the video signal as well, because of small value of RL across which the video output
voltage develops. A series inductor is therefore used in place of Rf and the RC filter is thus
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MONOCHROME AND COLOUR TELEVISION
replaced by an LC filter (see Fig. 11.5). At the IF frequency (38.9 MHz), the inductor Lf has
much higher reactance than that of the shunt capacitor Cf but at the video frequencies the
D
Lf
100 H
Last
IF
vs
Cf
C
10 pF
RL
v0
Fig. 11.5. Basic video detector and fitter circuit.
reactance of Lf is much lower as compared to that of Cf. As labelled in Fig. 11.5, Lf = 100 µH and
Cf = 10 pF have been chosen to illustrate the filter action. At IF frequency the ratio of the
reactance of series inductor to that of shunt capacitor (XLf /XCf) is nearly equal to 60. This
means an attenuation of about 35 db for the ripple voltage and is considered adequate. However,
the attenuation of the video signal will be different for different frequency components of the
composite signal. The higher video frequencies suffer more attenuation than the lower ones.
In order to overcome this discrepancy the load is modified to include an inductor in series with
RL. The reactance of this compensating coil (LC) increases with frequency and thus the
magnitude of the complex load increases to counteract the additional drop across Lf, the series
inductor. A typical value of such a compensating coil is of the order of 200 µH. Such an
arrangement also takes care of the input capacitance of the following video amplifier which
would otherwise tend to attenuate high frequency components of the video signal. The filter
circuit thus modified, ensures low, and almost constant attenuation over the entire frequency
range. This is known as video bandwith compensation. The bandwidth must extend up to 6
MHz to include sound intercarrier frequency of 5.5 MHz and its FM sidebands besides the
video signal.
Figure 11.6, shows a practical video detector circuit. It employs a load resistance = 3.9 K
and a compensating coil = 250 µH. Its output is dc coupled to the video driver. The resistors R1
and R2 form a voltage divider across the 12 V dc supply to fix necessary forward bias at the
base of the emitter follower (driver). The capacitor C1 provides effective ac bypass across R2.
Lf
D
From
last video
IF
C2
To driver
of video
amplifier
Lc
250 µH
RL = 3.9 K
100Ω
C1
R2 15 K
+ 12 V
R1 = 4.7 K
+ 12 V
Fig. 11.6. A practical video detector circuit.
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VIDEO DETECTOR
Filter Circuit Modifications
A somewhat non-linear behaviour of the diode results in production of a series of harmonics
and beat frequencies at output of the detector. It is possible for these unwanted products to get
coupled to the tuner and go through a zero beat as the tuner is tuned (through the corresponding
RF range) to produce what are known as ‘tweets’. Such interference is usually confined to
channels that lie between 80 to about 180 MHz. To eliminate these interferences, the diode
and part of the filter circuitry are enclosed in the ‘can’ of last picture IF stage coupling
transformer for effective screening (see Fig. 11.6).
In some detector designs self-resonant chokes that are tuned to suppress specific troubling
frequencies are inserted in series with the signal path. Figure 11.7 shows such a circuit
configuration. Note that a major part of filter capacitors is provided by the stray and wiring
capacitances and the ‘wired in’ (physical) capacitors are much less in value and range from
5 pF to 10 pF. Both the series inductors Lfa and Lfb are made to resonate at the desired
frequencies by their self-capacitance and no physical capacitors are actually needed. In some
cases the filter capacitor Cf across the load is provided by the input capacitance of the video
amplifier stage so that no ‘wired in’ Cf appears in the circuit.
Ca
Cb
D
Last
IF
Lfa
vs Cs
C1
Lfb
C2
Cf
Lc
RL
Cin
v0
Fig. 11.7. A modified video detector circuit.
11.4 DC COMPONENT OF THE VIDEO SIGNAL
The video detector output includes a dc component which must be preserved for a true
representation of the transmitted picture. Therefore with dc coupling employed between the
detector and the video amplifier and video amplifier to the picture tube, all shades from white
to grey get correctly reproduced. However, in some receivers ac coupling is used between the
detector and video amplifier. The insertion of a coupling capacitor in series with the signal
path completely removes the dc component. The video signal waveform then settles down with
equal areas on either side of the zero voltage line. This is illustrated in the waveforms drawn
in Fig. 11.8. Note that this results in lesser contrast between two lines having different
brightness levels. Similarly an increase in the average brightness of the transmitted scene
results at the receiver in a depression of the black level, so that the reproduced range in
brightness is less than that of the original scene. Notwithstanding this disadvantage ac coupling
is sometimes used because of certain other merits. However, the dc level of the video signal
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MONOCHROME AND COLOUR TELEVISION
can be reinserted by what is known as ‘dc reinsertion’ technique before injecting it at the grid
or cathode of the picture tube. This is fully explained in Chapter 14.
v0
t
0
C
Peak white level
0
Grey line
Black level
White line
v0
Black level
Fig. 11.8 (a). Video detector output for two
different lines, one grey and the other white.
Note the black level is same for both the lines.
Fig. 11.8 (b). Effect of a.c. coupling. The
black level is now different and thus
the d.c. component is lost.
11.5 INTERCARRIER SOUND
In addition to recovering the composite video signal, rectifying action of the diode in the video
detector also results in frequency translation of the sound IF signal. The strong picture IF
carrier at 38.9 MHz acts as a local oscillator and heterodynes with the attenuated sound IF
carrier at 33.4 MHz to produce a difference frequency of 5.5 MHz. The resulting new IF together
with its FM sidebands is known as intercarrier sound signal. The video detector circuit is
modified (see Fig. 11.9) to extract the sound signal. As shown in the figure, a parallel tuned
circuit, commonly known as sound IF trap is inserted in the signal path. It is tuned somewhat
broadly with a centre frequency of 5.5 MHz. This offers a high impedance to the sound component
of the detected signal to remove it effectively from the video signal path. A tuned secondary
circuit delivers the intercarrier IF to sound section of the receiver. In some TV receivers the
intercarrier sound signal is allowed one stage of amplification in the video amplifier and then
separated through a trap circuit. It may be noted that in colour receivers two separate diode
circuits are used, one as a 5.5 MHz sound convertor and the other for video signal detection.
This done to reduce interference in colour pictures due to the beat note produced at the difference
frequency of sound carrier and colour subcarrier frequencies.
Compensating
network
To sound IF
D
4.7
pF
Lfa
Lfb
RL
4.7
pF
3.9 4.7 K
K
Video
amplifier
Sound trap circuit
Fig. 11.9. Video detector circuit with intercarrier sound trap.
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VIDEO DETECTOR
11.6 VIDEO DETECTOR REQUIREMENTS
It is now obvious that in practice the video detector diode load is more complex than a simple
shunt combination of resistance and capacitance because of the many functions it is required
to perform. The requirements are as follows:
(a) The detector load must provide a suitable impedance as seen through the diode, at
input of the detector to tune and damp secondary of the last IF coupling circuit
correctly.
(b) The detector load must remove from the output, the IF content in the signal as much
as possible. For this purpose the load usually includes one or two low-pass filter
sections.
(c) The detector load should have a trap circuit (a series rejector circuit) for separating
the intercarrier sound signal.
(d) The detector load must also include a provision to boost the higher video frequencies
to compensate for the loss due to input capacitance of the video amplifier.
It is obvious from these requirements that rigorous theoretical design of such a diode
detector is very complex and therefore, in practice the design is usully reached by empirical
methods based on filter circuit theory.
11.7 FUNCTIONS OF THE COMPOSITE VIDEO SIGNAL
Figure 11.10 illustrates various paths for the composite video signal as obtained from the
video detector. We can consider that the signal is coupled to several parallel branches for
different functions. Therefore, each circuit can be operated independently of the others. For
instance clipping the sync pulses in the sync-separator stage does not interfere with the video
amplifier supplying signal to the picture tube. Similarly, the AGC circuit rectifies the video
signal for developing AGC bias. With the same video signal the video amplifier provides complete
video signal to the picture tube for reconstruction of the televised scene. In some receiver
designs a cathode or emitter follower is used to isolate the video detector from these circuits.
In many receivers the video signal for the sync-separation circuit is tapped after one stage of
video amplification.
To video amplifier
From IF
amplifier
Video
detector and
filter circuit
To colour amplifiers
(in colour receivers)
For
picture tube
To AGC circuit
To sync separator
Fig. 11.10. Functions of the composite video signal.
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MONOCHROME AND COLOUR TELEVISION
Review Questions
1.
Draw the basic circuit of a video detector and explain design criteria for the choice of time constant
of the load circuit. What determines the polarity of the diode in the detector circuit ?
2.
Describe the factors that influence the choice of RL and C in the load circuit. Suggest suitable
values of RL and C for the 525 line system where IF = 45.75 MHz and fm = 4 MHz.
3.
Explain why is it necessary to employ and L-C instead of R-C filter to remove IF ripple from the
detected output. Give typical values of L and C and justify them.
4.
Why is the filter circuit generally modified to include self-resonant inductors in the signal path ?
What precautions are taken to prevent undesired harmonics from reaching the tuner ?
5.
Explain how compensation is provided in the detector load circuit to extend its bandwidth. Why
is it necessary to have a bandwidth of nearly 6 MHz ? Sketch the complete circuit and label all
components.
6.
How is the FM sound signal at 33.4 MHz translated to a new carrier frequency of 5.5 MHz and
separated from the composite video signal ?
7.
Give a practical video detector circuit imcorporating the following features: (i) effective IF filtering, (ii) suppression of harmonics, (iii) separation of the intercarrier sound signal, and (iv) frequency compensation. Give typical values of all the circuit components.
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Video Section Fundamentals
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12
Video Section Fundamentals
The amplitude of composite video signal at the output of video detector is not large enough to
drive the picture tube directly. Hence, further amplification is necessary, and this is provided
by the video amplifier. The manner in which video signal is applied to the picture tube (cathode
or control grid) decides the type of video section circuitry. The video signal on application to
the picture tube varies the intensity of its beam as it is swept across the screen by deflection
circuits. The gain control of the video amplifier constitutes the contrast control, whereas the
brightness control forms part of the picture tube circuit.
12.1 PICTURE REPRODUCTION
Figure 12.1 shows how the input video signal voltage for one line, impressed between grid and
cathode of the picture tube results in reproduction of picture elements for that line. It should
be noted that the results are the same when reversed video signal is applied between cathode
and control grid. In fact the video signal should so align itself that its black level drives the
grid voltage to cut-off. Any grid voltage more negative than that is called blacker than black
and this part of the video signal corresponds to sync-voltage amplitude. At the grid of picture
tube sync pulses really have no function, but these are used in the synchronizing section of the
receiver to time deflection circuits for vertical and horizontal scanning.
12.2 VIDEO AMPLIFIER REQUIREMENTS
In order to produce a suitable image on the screen of picture tube, the video amplifier must
meet the following requirements.
(i) Gain
The video signal must be strog enough to vary the intensity of the picture tube scanning beam
to produce a full range of bright and dark values on the screen. This is illustrated in Fig. 12.1,
where the signal amplitude is large enough to provide the desired contrast between white and
dark parts of the scene being televised. However, with a video signal having smaller peak-topeak variations, the brightness extends from dark, at cutoff bias, to some shade of grey, with
the result that there is less contrast between dark and light areas. Figure 12.2 illustrates the
light variations produced when the signal amplitude is reduced to about half as compared to
the signal amplitude required for full contrast. Any further reduction in the video signal
amplitude will result in a washed-out picture, and there will be little difference between dark
and light areas of the picture. A video signal amplitude of about 75 volts peak-to-peak is needed
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VIDEO SECTION FUNDAMENTALS
Illumination
White
Light transfer
characteristics of the
picture tube
Grey
shades
–100 –75
VGK
0
t
Bias
Blacker
than
black
t
Black
Horz
blanking
White
Black
Fig. 12.1. Light output with correct amplitude of the composite video signal.
Illumination
White
Various
o
shades
d
of grey
r
Grey
Dark grey
– 75
VGK
Black
0
Horz
blanking
Black
White
Fig. 12.2. Effect of insufficient video signal amplitude on brightness variations in the picture.
to obtain a picture with full contrast. Some colour picture tubes need higher signal amplitudes
of the order of 150 V peak-to-peak, for proper reproduction of the picture. With a detector
output betwen 2 to 4 volts in all tube receivers, a gain between 25 to 50, at the video amplifier
is considered adequate. A single pentode valve can develop this gain and most TV receivers
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MONOCHROME AND COLOUR TELEVISION
using tubes have one stage of video amplification between video detector and picture tube.
However, in transistor receivers where detector output seldom exceeds 2 volts, a two to four
stage video amplifier becomes necessary to fully modulate the beam of the picture tube.
(ii) Bandwidth
As explained in an earlier chapter higher frequencies are needed to reproduce horizontal
information of the picture. The lowest frequency for picture information in the horizontal
direction can be considered as 10 KHz when the camera beam scans all white and all black
lines alternately. Note that the active line period has been taken as 50 µs instead of the actual
period of 52 µs. Similarly when the beam scans half white and half black lines in succession,
the video frequency generated is 20 KMz. This is illustrated in Fig. 12.3 where different alternate
black and white widths have been closen to demonstrate the generation of high frequency
signal. Thus, for reproducing very minute details, a very high video frequency would be
necessary, but keeping in view the limitations of channel bandwidth, the upper limit has been
fixed at 5 MHz.
» 50 ms
Width of
o picture
e or » 50 ms
100 KHz = 5 ms »
200 KHz = 2.5 ms »
½ cycle
of
500 KHz = 1 ms »
Height
ei
of
the
e picture
p
» 18720
ms
8
= ½ cycle of
26.7Hz » 25 Hz
5 MHz = 0.1 ms »
(a) Horizontal information
(b) Vertical information
Fig. 12.3. Relationship between picture size and video frequencies.
The signal frequencies corresponding to picture information scanned in the vertical
direction are much lower compared with those for reproduction within a line. If the video
voltage it taken from top to bottom through all the horizontal lines in a field, the variation will
correspond to a half-cycle of a signal with a frequency of approximately 25 Hz. When the
brightness of the picture varies from frame to frame the resultant signal frequency is lower
than 25 Hz. However, this is considered as a change in dc level corresponding to a change in
the brightness of the scene; and this can become almost zero Hz (i.e., dc) when the average
brightness does not change over a long period of time. Ideally then, the video amplifier response
should be linear from dc to the highest modulation frequency of 5 MHz. This is possible only
when the video amplifier is direct coupled.
(iii) Fequency Distortion
The gain at high frequencies falls-off because of shunting effect of the device’s output
capacitance, stray capacitances and input capacitance of the picture tube. When ac coupling is
employed the gain decreases at low frequencies on account of increasing reactance of the coupling
capacitor. This inequality in gain at different frequency components of the signal is called
frequency distortion. Excessive frequency distortion cannot be tolerated because it changes
picture information. If high frequency content of the video signal is lost due to poor high
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VIDEO SECTION FUNDAMENTALS
frequency response the rapid changes between black and white for small adjacent picture
elements in the horizontal line cannot be reproduced. This results in loss of horizontal detail.
For example in the test pattern shown in Fig. 12.4, the individual black lines close to the
centre will loose their identity and instead appear as an illdefined black patch. Similarly small
details such as individual hairs of a person’s eyebrows do not appear clearly. However, in a
close up of the same face, where each area of the picture gets enlarged, the sharpness of each
detail improves because a relatively low frequency range is required for proper reproduction of
details. Frequencies from about 100 KHz down to 25 Hz represent the main parts of the picture
information, out of which frequencies from 100 KHz to about 10 KHz correspond to black-andwhite information of most details in the horizontal direction and frequencies from 10 KMz
down to 25 Hz represent changes of shading in the vertical direction. If the low frequency
response is poor, the picture as a whole is weak with poor contrast. The lettering, if any, is not
solid and the average brightness appears to be changing gradually from top to bottom of the
raster, instead of complete change of brightness in the actual scene.
Horz
resolution
Vertical
resolution
Fig. 12.4. Test chart for determining vertical and horizontal resolution.
(iv) Phase Distortion
Phase distortion is not important in audio amplifiers, because the ear does not detect changes
in relative phases of the various frequency components present in a given sound signal. However,
it is important in video amplifiers, since phase shift implies time shift, which in turn means
position shift in the reproduced visual image. The resultant shift in relative positions of the
various picture elements is detected by the eye as distortion. Therefore relative phases of all
the frequency components present in the video signal must be preserved. The time delay due
to phase shift is not harmfull if all frequency components have the same amount of delay. The
only effect of such uniform delay would be to shift the entire signal to a later time. No distortion
results because all components would be in their proper place in the video signal waveshape
and so also in the reproduced picture. Therefore, the phase angle delay should be directly
proportional to the frequency or all frequency components must have the same time delay.
This is illustrated in Fig. 12.5 (a) and (b). It should be noted that the signal inversion of exactly
180° in any one stage of the video amplifier does not mean phase distortion. There is no time
delay, but only a polarity reversal.
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MONOCHROME AND COLOUR TELEVISION
Phase
angle
delay °
Time
delay
(s)
Frequency
(a)
Frequency
(b)
Fig. 12.5. Phase response of the amplifier:
(a) Phase angles proportional to frequency.
(b) Corresponding time delay which is constant.
Figure 12.6 shows frequency response of an RC coupled amplifier where fL and fH are the
lower and upper 3 db down frequencies. The corresponding phase shift angles at these
frequencies are +45° and –45° relative to midband frequencies. Phase distortion is very important
+ 90°
Gain
(db)
Low
frequency
Mid
frequency
High
frequency
100%
+ 45°
Phase
– 70.7%
– 45°
Ha po
Half
power
er points
o ts
0
fL
(a)
f
0°
– 90°
fH
f
fH
fL
(b)
Fig. 12.6. (a) Frequency and (b) phase response of a practical amplifier.
at low video frequencies because here even a small phase delay is equivalent to a relatively
large time delay. As an illustration consider an amplifier designed to have fL = 2.5 Hz. The
phase shift at 2.5 Hz = 45° and that at 10 fL (25 Hz) it is nearly 6°. The relative time delay
6
10 6
×
≈ 660 µs, which in turn
360
25
would mean that the picture information due to the two frequency components (2.5 Hz and
between this frequency and midband frequencies would be
660 ~
− 10 lines. To correct this
64
discrepancy even if phase shift at 25 Hz is made = 1°, the corresponding time delay would be
120 µs and the picture information will get displaced by about 1.5 lines on the raster. The eyes
are very sensitive to time delay errors and see this as ‘smear’ on the picture. At very high video
frequencies the effects of phase distortion are not as evident on the screen because the time
delay at these frequencies is relatively small. For example, if fH, the upper corner frequency of
the amplifier is set at 5 MHz, the corresponding time delay with respect to midband frequencies
25 Hz) would get displaced with respect to each other by nearly
is only
45
1
1
~
×
−
th of a µs. The consequent displacement of the picture elements is
360 5 × 10 6 40
too small to be detected. Thus, a video amplifier with flat frequency response up to the highest
useful frequency, has negligible time delay distortion for very high video frequencies.
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VIDEO SECTION FUNDAMENTALS
(v) Amplitude Distortion of Nonlinear Distortion
If the operating point on the transfer characteristics of a device for a given load and signal
amplitude is not carefully chosen, amplitude distortion occurs where different amplitudes of
the signal receive different amplification. This can result in limiting and clipping of the signal
or in weak signal output. If sync pulse voltage gets compressed, synchronization may be lost,
because the video amplifier usually provides composite video signal for the sync separator.
Very often some gain has to be sacrificed to avoid amplitude distortion.
(vi) Manual Contrast Control
It should be possible to vary amplitude of the video signal for optimum setting of contrast
between white and black parts of the picture. Any control that varies the amount of ac video
signal will operate as a contrast adjustment. Therefore contrast varies when gain is varied in
either picture IF section or video amplifier. However, such a control is not possible in the IF
section because all receivers employ automatic gain control circuits to maintain almost a
constant voltage at the output of the video detector. Also, with intercarrier sound, any change
of gain in the IF section would affect the sound volume. Therefore, contrast control is provided
in the video amplifier, and this in effect, is the gain control of video amplifier.
12.3 VIDEO AMPLIFIERS
It is obvious from the preceding discussion that video amplifiers must meet several exacting
demands and this calls for careful and rigorous design considerations. The wide-band
requirement starting from almost dc to several MHz with minimum phase distortion is perhaps
the most stringent requirement. Both direct-coupled and RC coupled configurations are used
and each type has its own merits and demerits. Both types need high frequency compensation,
and this is met by shunt-peaking and series-peaking techniques. Though a dc amplifier does
not need any low frequency compensation, the RC coupled amplifier employs special low
frequency boost techniques to extend the bandwidth at the lower end of its response.
A basic RC coupled amplifier configuration, which applies to both tubes and transistors,
is shown in Fig. 12.7. It is designed to work under class ‘A’ operation. The gain of this amplifier
Cc
v0
0.1 mF
vin
Video
amplifier
Ct
18PF
RL
= 2.5 K
B+
B–
Rg =
250 K
Gain
(Contrast)
control
Fig. 12.7. Basic R.C. coupled amplifier
Note: The component values shown are of a typical video
amplifier employing a pentode.
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MONOCHROME AND COLOUR TELEVISION
falls off rapidly at high frequencies because of shunting effect of inter-electrode, input, and
stray capacitances in parallel with the load resistance RL. Video stages use low value of RL
compared with audio amplifiers because of large bandwidth requirements. Typical values are
2 KΩ to 8 KΩ in tube versions. Although gain is reduced, lowering RL extends the high frequency
response. The effect of Ct does not become important until its reactance is low enough to become
comparable with the resistance RL. Then, the shunt reactance Xct lowers the impedance ZL,
and this causes a fall in the gain at higher frequencies. The lower the RL, and smaller the
shunt capacitance Ct, better is the high frequency response. However, practical video amplifiers
generally use a relatively higher value of RL with peaking coils to boost the gain at higher
frequencies. A typical arrangement known as ‘shunt peaking’ is shown in Fig. 12.8. Here a
small inductor, L0 is connected in series with RL. This peaking coil resonates with Ct to boost
the gain at high frequencies (see Fig. 12.8 (b)), where the response of the uncompensated RC
coupled amplifier would normally drop off. The resistance of the coil is very small and so it
does not effect dc voltages and response at the middle frequencies. A peaking coil is effective
for frequencies above about 400 KHz.
vin
Cc
v0
Q
Ct
L0
RE
RL
0
v0
vfn
+
–
Bias
+
–
f
The response curve of Ct and L0
in parallel
0
v0
RB
Vcc
0
f
With shunt
peaking
Without shunt
peaking
(a)
f
(b)
Fig. 12.8 (a). Video amplifier employing shunt
coil (peaking) frequency compensation.
Fig. 12.8 (b). Frequency response of the amplifier.
Another arrangement to extend the high frequency response is known as ‘series peaking’
compensation (Fig. 12.9). In this circuit, LC is in series with the two main components of Ct. At
RD
Cc
LC
Q
Cin
Cout
v0
To picture
tube input
circuit
vin
0
v0
vfn
RL
+
–
RE
f
Effect of Lc
Rin
With series
peaking
+
0
(a) Video amplifier with series peaking
Without series
peaking
(b) Frequency response
Fig. 12.9. Video amplifier employing series peaking coil compensation.
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VIDEO SECTION FUNDAMENTALS
one side of LC is Cout of the video amplifier and on the other side is Cin of the next stage or input
capacitance of the picture tube. This arrangement reduces shunting capacitance across RL
which results in more gain, while LC resonates with Cin to provide a rise in voltage across Cin
at high frequencies. A series peaking coil usually has a shunt damping resistance such as RD,
the function of which is to prevent oscillations or ringing in the coil with abrupt changes in
signal.
The circuit of Fig. 12.10 combines shunt and series peaking which results in more gain,
extended high frequency response and improved transient behaviour.
Lc = 145 mH
Cc
v0
0.1 mF
5 PF
vfn
C0
Video
amplifier
RD = 10 K
L0
35 mH
Cln
12 PF
To picture tube
circuit
Rin
400 K
RL
4K
Contrast
control
R1
C2
.005 mF
Cf
10 mF
Rf
10 K
B+
B–
Fig. 12.10. Video amplifier employing both shunt and series peaking for high frequency
compensation and a special decoupling circuit to boost low frequency response.
Note that the component values shown in the circuit are for a tube amplifier.
The low frequency response is affected by increased reactance of the coupling and bypass
capacitors. This can be improved by using largest possible values of these capacitors. In adition,
the decoupling filter RfCf in the B+ supply line (Fig. 12.10) can be used to boost gain and reduce
phase shift distortion at very low frequencies. The capacitor Cf offers large reactance at low
frequencies and then Rf in series with RL becomes the effective load. The increased load results
in higher gain for low frequencies. This rise in gain compensates for the reduction in gain
caused by reactance of the coupling capacitor CC. Furthermore, phase shift caused by shunt
capacitor Cf is opposite to phase shift caused by series capacitor CC. As a result Cf tends to
correct phase distortion introduced by the coupling cpacitor. A small paper or ceramic capacitor
C2 is normally provided across Cf to bypass very high video frequencies because Cf, being a
large value electrolytic capacitor, has a non-negligible inductance and fails to provide a bypass
at these frequencies.
Gain Control
The change in gain of the video amplifier to provide contrast control can be effected in different
ways. A common method is to vary the cathode/emitter resistance R1. This resistance is left
unbypassed and its variation alters the negative feedback which in turn changes the gain of
the amplifier. The change in the value of R1 also varies the bias and the consequent shift in the
operating point can introduce amplitude distortion. To overcome this problem, in many video
amplifier designs, a potentiometer is provided at the output of the video amplifier to alters the
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MONOCHROME AND COLOUR TELEVISION
magnitude of the video signal applied to the picture tube. This method is the same as volume
control in an audio amplifier.
5.5 MHz Sound Trap
The video amplifiers usually have a trap circuit, tuned to the intercarrier sound frequency of
5.5 MHz to keep the sound signal out of the picture signal. If sound signal is not separated at
the video detector, the trap circuit can be modified to deliver the intercarrier sound signal to
the sound IF amplifier. This is illustrated in Fig. 12.11 (a) where L1 and C1 constitute the trap
circuit and winding L2 coupled to L1 serves as take-off point for the 5.5 MHz sound signal.
To sound
L2
v0
Cc
5.5 MHz
trap cct
Video
amplifier
Cc
v0
L1
C1
vin
Lc
Lc
IF amp
RL
L0
RD
vin
RD
Video
amplifier
L0
C
L
5.5 MHz
trap cct
B+
B+
–
(a)
RL
(b)
Fig. 12.11. Video amplifier circuits (a) with parallel resonant trap in series
with the output. The trap circuit also serves as the sound take-off circuit
(b) with series resonant 5.5 MHz trap in shunt with the load.
As shown in the figure, the trap circuit is in series with the output and when tuned to
resonance at 5.5 MHz, offers maximum impedance. Therefore, the sound signal is removed
because maximum voltage across the trap is developed at this frequency.
In Fig. 12.11 (b) another trap circuit arrangement is shown. Here L and C form a series
resonant circuit tuned to 5.5 MHz. This trap circuit is in shunt with the load, and at resonance,
provides practically a short circuit to the intercarrier sound signal. This prevents it from
appearing across the picture tube input.
If the 5.5 MHz sound signal together with its sidebands is not fully suppressed it causes
beat interference which results in diagonal lines on the picture having small wiggles. The
weave in the lines is the result of frequency variations in the FM sound signal. This effect is
also called ‘wormy’ picture. The interference disappears when there is no voice or music, leaving
just the straight lines corresponding to the 5.5 MHz carrier without modulation. The trap
circuits are tuned for minimum interference in the picture.
12.4 BASIC VIDEO AMPLIFIER OPERATION
Before attempting to discuss complete video section circuits it is desirable to recapitulate the
operation of a basic RC coupled amplifier, which after adding compensating elements serves
as the video amplifier. The circuit of such an amplifier employing a pentode is drawn in
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229
VIDEO SECTION FUNDAMENTALS
Fig. 12.12 (a). The voltage drop across RK provides grid bias and ensures class ‘A’ operation.
The load resistance RL provides output voltage between the plate and ground. While Rs is the
screen voltage dropping resistance. The bypass capacitor CS, connected between screen grid
and ground provides an effective short at all frequencies of interest. CK connected across RK is
a bypass for the ac component of the current to prevent degeneration of the input signal.
The input signal waveform and corresponding plate current and plate voltage waveforms
are shown in Figs. 12.12 (b) to (d). When the input signal swings to the extreme positive,
maximum current flows and the plate voltage swings to its minimum value. Again, when the
input signal attains its maximum negative value, the plate current becomes minimum and
plate voltage attains its maximum positive value. Thus, the output voltage is 180° out of phase
with respect to the input voltage. The dc component of the plate voltage is blocked by coupling
capacitor (CC) and the amplified ac component then appears across the output terminals of the
amplifier.
An RC coupled amplifier employing a transistor in the common emitter configuration
performs in the same way as its tube counterpart, but with a difference, that instead of voltage
it needs current drive for its operation. Normally potentiometer biasing is provided for setting
the operating point. The voltage and current polarities for an n-p-n transistor are the same as
with a vacuum tube. However, with a p-n-p transistor all signal polarities are negative with
respect to ground.
Cc
v0
P
Cc
vin
G1
0
RL
5K
25 K
Rg
CS
4 F
K
RK
t
RS
G2
250 v
P–P 4V
+ Vpp
VGK
–
(vin)
CK
(a) Amplifier circuit
(b) Input voltage
volts
iP
225
mA
40
185
150
P–P value = 31 mA
20
75
VP0
P–P value = 140 V
IP0 (average value)
t
0
t
0
(c) Plate current
(d) Plate voltage
Fig. 12.12. R.C. coupled amplifier.
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MONOCHROME AND COLOUR TELEVISION
12.5 COMPARISON OF VIDEO SIGNAL POLARITIES IN TUBE AND
TRANSISTOR CIRCUITS
As explained earlier a negative going signal is a must when the video signal is injected at the
cathode. Similarly positive going signal is required for feeding at the grid of the picture tube.
If opposite polarity is used the result will be a negative picture, in the same sense as a
photographic negative. Besides this, the polarity of video signal with respect to ground also
affects the biasing of the device employed in the video amplifier. These aspects are examined
in greater detail both for tube and transistor amplifiers using grid and cathode modulation of
the picture tube.
Grid Modulation of the Picture Tube
(i) Transistor Circuits. The circuit arrangement shown in Fig. 12.13 is of a direct coupled video
amplifier employing a n-p-n transistor to grid modulate the picture tube. The resistors R1 and
R2 together with RE, that is bypassed by CE, provide forward biasing at the base-emitter junction.
This biasing arrangement provides good stability of the operating point against device
replacement, temperature variations, dc supply changes and ageing of circuit components.
B+
Brightness control
1C
PIX
tube
R1
RL
B.C.
G1
vin
v0
K
C
n–p–n
B
vin
0
v0
E
R2
RE
Ico
t
vco
CE
Rx
0
0
t
Fig. 12.13. Direct coupled video amplifier circuit to grid modulate the picture tube.
Since a positive going signal is needed at the collector, a negative going video signal is
required to the base. The value of emitter resistance RE is chosen to set the operating point ‘Q’
for minimum collector current in the absence of input signal. In fact the base-emitter bias is so
set that when video signal is applied, the sync tips make the base most positive. Thus the
collector current becomes maximum for sync tip levels and the collector acquires minimum
positive potential. This is the correct polarity for obtaining minimum beam current of the
picture tube. Since the operating point of the transistor is set close to cut-off bias, the collector
voltage, in the absence of any video signal is highest, which makes the grid of the picture tube
least negative with respect to cathode. Therefore, with no video voltage drive, though the
transistor draws a minimum current, the picture tube beam current is maximum.
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VIDEO SECTION FUNDAMENTALS
Notice that the cathode of picture tube is returned to the brightness control which sets
the cathode at a voltage which is more positive with respect to ground than the grid. Brightness
control is achieved by varying net negative voltage between grid and cathode. The resistance
RX fixes the minimum limit of bias and prevents the possibility of a net positive voltage on the
grid.
If a p-n-p transistor is employed the base-emitter junction must be enough forward
biased, with no input signal, so that the positive going sync tips drive the bias backwards
towards the Ib = 0 point. Note that the input signal is all positive and the least positive level is
reached on peak-white and the transistor then passes maximum current. The maximum collector
current results in least negative collector voltage which in turn makes the picture tube grid
less negative with respect to its cathode and the beam current increases to reproduce the
peak-white values of the picture. It may also be noted that in the absence of any video signal
the transistor collector is at a minimum negative potential and the corresponding beam current
is large. Similarly a p-n-p transistor needs a dc source of opposite polarity than a n-p-n
configuration. This necessitates interchanging of the locations of orightness control and resistor
RX to make sure that the picture tube grid never attains a positive potential.
(ii) Vacuum Tube Circuit. In the case of a vacuum tube a positive supply voltage is
needed and a positive drive to the grid is necessary to increase the plate current. Therefore,
the working conditions of a vacuum tube video amplifier are exactly similar to those of a
n-p-n transistor. However, the potentials needed are much higher and the biasing technique is
somewhat different. Thus in a vacuum tube amplifier, with no input signal the tube draws a
minimum current, and the picture tube beam current is maximum.
Cathode Modulation of the Picture Tube
When the video signal is injected at the cathode of picture tube a negative going signal is
needed at the anode/collector of the video amplifier and this necessitates a positive going signal
at the grid/base of the amplifier.
(i) Tube Circuit. The necessary circuit details of the video section and the signal waveforms at the input and output of the video amplifier are illustrated in Fig. 12.14. The tube is
biased close to VGK = 0 point so that maximum current corresponds to peak-white and minimum plate current occurs on sync pulse tips. The anode voltage waveform is then negative
going as required for correct cathode modulation. The picture tube beam current is again high
with no signal at input of the amplifier.
(ii) Transistor Circuits. Fig. 12.15 shows a p-n-p configuration and associated waveforms. The transistor input circuit is back biased towards Ib = 0 µA so that on peak-whites the
base current and hence the collector current is very small. Minimum collector current results
in maximum negative voltage at the collector, so that once again the necessary negative going
collector signal is derived for the picture tube cathode. In the absence of any input video signal
the collector voltage is more negative and hence the signal beam current is maximum.
If a n-p-n transistor is employed in the video amplifier designed for cathode modulation,
the signal polarities both at the input and output terminals are exactly the same as shown in
Fig. 12.14 for a vacuum tube configuration. Therefore the picture tube beam current will be
high with no input signal to the amplifier.
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MONOCHROME AND COLOUR TELEVISION
B+
ip
PIX
tube
RL
RX
K
G1
v0
0
t
0
vin
v0
t
B.C.
Rg
RK
CK
t
0
– vin
Fig. 12.14. Direct coupled video amplifier circuit to cathode modulate the picture tube.
B–
0
PIX
tube
R1
t
RL
B.C.
K
v0
vin
G
– ic
p–n–p
0
t
0
R2
RE
CE
t
RX
– v0
– vin
Fig. 12.15. Direct coupled transistor (p-n-p) video amplifier circuit
to cathode modulate the picture tube.
These results may be summarized as follows:
(i) The sense of the video signal relative to black level seen either at the input or output terminals is the same in tube and transistor circuits.
(ii) When the tube or n-p-n transistor is called upon to deliver maximum plate/collector
current the p-n-p transistor has to pass minimum collector current and vice versa.
(iii) The picture tube beam current is maximum with no input video signal both for grid
modulation and cathode modulation when dc coupling is used.
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VIDEO SECTION FUNDAMENTALS
12.6 RELATIVE MERITS OF GRID AND CATHODE MODULATION OF THE
PICTURE TUBE
The main points to be considered in assessing relative merits of the two systems are as follows:
(i) Picture Tube Input Characteristics
This is a measure of the beam current change for a given change in the video signal drive
voltage. The advantage lies with cathode modulation. The beam current is determined by the
grid-cathode voltage and by the positive voltage on the first anode with respect to cathode.
With grid modulation, the only factor which determines the change in beam current results
from a given change in video signal voltage between the grid and the cathode. No other electrode
voltage is effected with the applied voltage. However, when cathode modulation is applied a
second factor influences the beam current. This is the voltage between the first anode and
cathode. Thus as the video signal input voltage moves, from black-level towards peak-white
the cathode moves more negative, not only to the control grid but also to the first anode. As
stated above the voltage on the first anode and cathode itself have a marked influence upon
the beam current of the picture tube. Thus the change in beam current brought about by
effective reduction in grid-to-cathode. negative bias, is further augmented due to increase in
positive voltage between the first anode and cathode. However, with grid modulation the voltage
between the first anode and cathode remains constant. If follows then, that for a given change
in the input signal voltage, the change in beam current is less with grid modulation than with
cathode modulation. For the same input, the beam current is about 20% greater on peak-white
with cathode modulation, or the video amplifier gain can be smaller for the same beam current.
(ii) Feed to the Sync Separator
Most commonly used sync separator circuits employ either a tube or a transistor, which is cutoff during the picture information part of the video signal but is driven into conduction by the
sync pulses. A negative going video signal is then needed at the grid of the tube or base of the
n-p-n transistor being used as a sync separator. With cathode modulation the right polarity is
available at the anode/collector of the tube/transistor. It is not so when grid modulation is
employed and thus cathode modulation has the added advantage over grid modulation of
automatically providing the right polarity of the video signal needed to drive the sync separator.
However, if a p-n-p transistor is used for sync separation, the grid modulation will provide
signal with the correct polarity. With transistor receivers employing a two stage video amplifier,
the question of feeding the sync separator is no longer important since both positive and negative
polarities of the video signal are always available.
(iii) Safety of Picture Tube in the Event of Video Amplifier Failure
With tube receivers or the ones employing an n-p-n transistor for video signal amplification
the advantage lies with cathode modulation if direct coupling is used. Should the emission of
the video amplifier tube fail or the n-p-n transistor stop conducting, the plate/collector current
drops to zero and the cathode of the picture tube attains a positive potential equal to B+ supply.
This immediately cuts-off the beam current and thus no damage is caused to the picture tube.
With grid modulation such a fault will make the grid highly positive causing excessive beam
current. The shorts between plate and cathode (collector and emitter) are rare and need not be
considered. When a p-n-p transistor is employed in the video amplifier the effects would be
opposite.
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MONOCHROME AND COLOUR TELEVISION
(iv) Cathode to Heater Voltage Stress
In tube receivers using series heater arrangement the picture tube is placed at or near the
ground end of the chain to give it. maximum protection against possibility of damage due to
short circuits across the heater line. Thus the heater is at negative end of the dc supply. With
grid modulation the cathode voltage of the picture tube is at a higher potential than when
cathode modulation is used. The stress is therefore more in the case of grid modulation. However,
in modern picture tubes the likelihood of cathode heater breakdown has been minimized. Also
with capacitive coupling the voltages get reduced, and therefore this point does not very much
influence the choice of modulation method.
In conclusion, it may be said, that the video amplifier is one of the most important
section of the receiver. It not only amplifies the video signal which extends from almost dc to a
very high video frequency of 5 MHz, it also acts as the source for feeding the video signal to the
sync separator and automatic gain control circuits. In most cases the sound signal is also
separated at this stage after amplification. It would then be desirable to take up a detailed
consideration of the video section design and circuitry. The next two chapters are devoted to
these aspects of video amplifiers.
Review Questions
1.
What are the essential requirements that a video amplifier must meet for faithful reproduction
of picture details ?
2.
How does phase distortion in the video signal affect the quality of the picture ? What causes
‘smear’ in the picture and how can this be minimized ?
3.
Draw the circuit diagram of an RC coupled amplifier employing an n-p-n transistor in common
emitter configuration and explain its operation as a voltage amplifier. Sketch its frequency
response and explain why the gain falls-off both at very high and low frequencies.
4.
Describe briefly with circuit diagrams the techniques employed to extend the bandwidth of an
RC coupled amplifier to accommodate full range of the video signal.
5.
Why are trap circuits provided in video amplifiers to attenuate frequency spectrum occupied by
the FM sound signal ? What is the undesired effect of sound signal on the reproduced picture ?
6.
Draw simple circuit diagram of a dc coupled video amplifier that feeds the grid of the picture
tube. Sketch suitable input and output voltage waveforms and justify that the chosen polarity of
the video signal will result in correct reproduction of the picture on the screen. Identify the
location of the brightness control in the circuit drawn by you.
7.
Discuss relative merits of cathode and grid modulation of the picture tube. Explain why cathode
modulation is considered superior to grid modulation.
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13
Video Amplifiers—Design Principles
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13
Video Amplifiers—Design Principles
The choice of basic amplifier that can be modified to meet the requirements of a video amplifier
is restricted to direct coupled and RC coupled configurations. The techniques employed to
achieve broad-band characteristics are same for tube and transistor amplifiers. However, their
mode of operation and impedance levels are quite different from each other. Therefore, while
explaining design fundamentals, video amplifiers employing tubes and transistors are
considered separately.
13.1 VACUUM TUBE AMPLIFIER
The basic circuit of a video amplifier employing RC coupling, together with its ac equivalent
circuits valid for medium, high and low frequency regions are shown in Fig. 13.1. The capacitor
C0 represents output capacitance of the tube, Cs stray shunt and wiring capacitance and Ci
input capacitance of the picture tube circuitry. All the three capacitances are effectively in
parallel, and when added constitute Ct = C0 + Cs + Ci. The total shunt cpacitance seldom
exceeds 20 pF and thus acts as an open circuit to frequencies in the low and midband ranges.
The coupling capacitor CC is chosen to be quite large to provide nearly a complete ac short,
even at very low frequencies.
Gain Expressions
It is an easy matter to deduce gain expressions for the three frequency regions from the
corresponding equivalent circuits of the basic amplifier configuration (Fig. 13.1). The results
are summarized below as a starting point for explaining wide-banding techniques.
(i) Gain at midband (Amid) (see Fig. 13.1 (b))
A(mid) = – gm R || ≈ – gm RL
...(13.1)
where R || = RL || Rg || rp ≈ RL, because load resistance in video amplifiers seldom exceeds
10 K-ohms. The minus sign signifies phase reversal of 180°.
(ii) Gain at high frequencies (see Fig. 13.1 (c))
A(HF) = – gmZt
where Zt = RL || X C .
t
Substituting for X C =
t
1
and simplifying yields
ωCt
| A(HF) | =
A(mid)
1 + (ωCt RL ) 2
∠ – θH
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237
VIDEO AMPLIFIERS—DESIGN PRINCIPLES
+
RS
CS
RL
B+
16 mF
CC
G2
G1
vin
Ct = C0 + CS + C1
CS
C0
Ct
Rg
v0
R|| = tp || RL || Rg = RL
K
vin = VG
1K
= vS
Fig. 13.1 (a). Basic R.C. coupled amplifier.
gmvg
rp
RL
»
v0
Rg
gmvg
v0
RL
Fig. 13.1. (b). A.c. equivalent circuit valid at medium frequencies.
rp
vg
CL
RL
Rg v0
CL
gmvg
tp
RL
Rg v0
gmvg
CL
RL v0
Fig. 13.1 (c). A.c. equivalent circuit valid at high frequencies.
CC
CC
gmvg
tp
RL
v0
Rg
»
RL
gmvg
Rg
v0
Fig. 13.1 (d). A.c. equivalent circuit valid at low frequencies.
Defining the upper 3 db frequency as
1
1
fH =
or ωH =
2πCt RL
Ct RL
| A(HF) | =
A( mid)
F ω IJ
1+ G
Hω K
2
∠ − tan −1
FG ω IJ
Hω K
...(13.2)
H
H
At ω = ωH, RL = X C and ∠θH = – 45° (relative to phase-shift at midband).
t
1
, Zt = 0.707 of its midband value and thus the gain at this
2πCt RL
frequency falls to become – 3 db with respect to the midband gain.
(iii) Gain at Low Frequencies (A(LF)) (see Fig. 13.1 (d)). Proceeding in the same way as for
the high frequency gain expression, and after a little manipulation, the gain in the low frequency
region can be expressed as
Note that at f = fH =
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MONOCHROME AND COLOUR TELEVISION
| A(LF) | =
A(mid)
F 1 I
1+ G
H ωC R JK
C
2
where
∠ + θL
rp RL
rp + RL
+ Rg ≈ Rg.
g
Defining the lower 3 db down frequency as
fL =
1
1
or ωL =
2πCC Rg
CC R g
the gain expression can be written as
| A(LF) | =
A(mid)
F ω IJ
1+ G
HωK
L
2
∠ + tan–1
FG ω IJ
HωK
L
...(13.3)
At ω = ωL, Rg = XCC and θL = + 45° (relative to phase shift at midband).
Note that at f = fL =
1
, Zt = 0.707 of its midband value, and the gain at this
2πCC R g
frequency again falls to –3 db with respect to midband gain. The frequencies fH and fL are
known as corner frequencies and the gain at these frequencies is 70.7% of the midband value.
Bandwidth
The bandwidth of an amplifier is defined as
BW = (fH – fL) ≈ fH =
1
2πCt RL
...(13.4)
In an RC coupled amplifier, even when RL is made as low as 0.5 K-ohms, fH seldom
exceeds 3 MHz. However, it is not possible to make the load resistance (RL), too small, because
the gain requirement (gain = gmRL) of the video amplifier is not fully met. Therefore, some
other means have to be devised to extend the frequency range upto 5 MHz without unduly
reducing RL.
13.2 HIGH FREQUENCY COMPENSATION
The bandwidth is normally extended by making the plate load complex in such a way that its
magnitude increases with increase in frequency. Thus, the compensation technique is aimed
at pushing up the upper –3 db frequency fH, which normally would occur at a relatively low
frequency due to the presence of shunt capacitance Ct. Negative feedback is also applied to
increase the bandwidth but this results in some loss of gain. The various HF compensation
techniques are as follows:
(a) Shunt Inductance Peaking
A small inductor of the order of 50 to 250 µH is added in series with the load resistor RL.
Though connected in series with RL, the coil is in fact a part of the shunt plate circuit. This is
llustrated in Fig. 13.2, where the compensated amplifier configuration together with its high
frequency equivalent circuit is drawn. As shown there, the effective circuit, in shunt with the
signal path, consists of Ct in parallel with series combination of RL and inductor Lpx. The
inductor increases the net plate circuit impedance at high frequency end of the frequency
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VIDEO AMPLIFIERS—DESIGN PRINCIPLES
range being handled, and thus partly compensates for the decreasing reactance of Ct. This
results in shifting the higher corner frequency fH to a still higher value to enhance bandwidth
of the amplifier.
B+
RL
RS
+
Lpx Shunt peaking coil
To picture tube
circuit
CS
v0
Ct
vg
RK
rp
+
Lpx
Ct
–
mvg
+
CK
XCK << RK
RL
(a)
(b)
Fig. 13.2. Video amplifier employing shunt peaking (a) Circuit (b) Equivalent circuit.
The combined impedance of RL, Lpx and Ct can be expressed as
L(X ) (X )
Z =M
MN ( R ) + ( X
L px
t
2
Ct
2
L
2
+ RL2 ( X Ct ) 2
L px
− X Ct ) 2
OP
PQ
1/ 2
...(13.5)
As stated earlier the gain at fH where XCt = RL, falls to 70.7% of the midband value.
Therefore, to extend the midband range, Zt should increase at this frequency to yield a gain
equal to the midband gain. This can be readily achieved by setting X L px =
RL
and XCt = RL (at
2
fH) in equation (13.5). This, on substitution, yields
LM FG R IJ × (R ) + (R )
H2K
Z =M
MM ( R ) + F R − R I
GH 2 JK
MN
L
t
L
3 db.
2
L
2
2
2 2
L
2
L
L
OP
PP
PP
Q
1/ 2
= RL
...(13.6)
Thus, the midband gain extends to fH, where, without compensation it was down by
Procedure for fixing RL and Lpx. It is necessary to first determine the total shunting
capacitance (Ct) and the highest frequency (f) up to which flat response is desired before fixing
the values of RL and Lpx. The highest frequency of interest in the 625 line system is 5 MHz. The
value of Ct can be estimated from the data of the tube chosen for the amplifier, and by measuring
stray capacitances if necessary.
With both f and Ct known, the values of RL and Lpx can be found as under.
RL =
1
2πfCt
F from f
GH
H
=
1
2πCt RL
I
JK
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...(13.7)
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MONOCHROME AND COLOUR TELEVISION
Since X Lpx is to have a value equal to RL/2,
∴
or
2πfLpx = 0.5RL =
Lpx =
On substituting RL2 =
we get
0.5
(from equation 13.7)
2πfCt
0.5
2 2
4 π f Ct
1
4 π 2 f 2 Ct 2
...(13.8)
in equation (13.8)
Lpx = 0.5 CtRL2
...(13.9)
This equation can be written in a general form as
Lpx = nCtRL2
...(13.10)
where ‘n’ can be made to have any value of < 1, in order to vary frequency response in the
region close to the new value of fH. It may be noted that the circuit will resonate if ‘n’ exceets
unity.
Effect of varying Lpx. If gain versus frequency plot of such an amplifier is drawn for
different values of ‘n’, it is revealing to note, that for values of ‘n’ greater than 0.5, the response
has a peak which becomes more pronounced as ‘n’ increases. Furthermore, increasing the
value of inductance (i.e., n) increases the amplitude of the hump and also steepens the rate at
which the gain falls off above fH. It is characteristic of compensating coils that while they lift
the response curve in the desired region, the subsequent fall-off of gain is more rapid than in
the uncompensated circuits. Too steep an edge in the response curve is not a desirable
characteristic, since it can give rise to a tendency to produce overshoot or transients. In fact no
single value of ‘n’ can give (i) constant gain throughout the pass-band, (ii) linear phase response,
and (iii) fast transient response without overshoot, all at the same time. It can be shown, that
for optimum frequency response, a value of n = 0.414, for least phase distortion; n = 0.322 and
for critical damping n = 0.25 is necessary.
In the practical development of a particular circuit, it is usual to start-off with an inductor
of value Lpx = 0.5 RL2Ct and then experiment with larger or smaller inductors until the desired
response is obtained. It may be noted that time delay due to phase shift at high frequencies is
very small and if linear phase characteristics are not obtainable while satisfying other
requirements, it will not cause any problems.
(b) Series Inductance Peaking
In this arrangement the compensating coil is inserted in series with CC, which means that the
inductor is in series with the signal path, rather than in shunt with it. Figure 13.3 shows this
circuit arrangement with its equivalent circuit. In practice, the coil is fixed very close to the
plate pin of the tube, and in this position it effectively separates the total shunt capacitance Ct,
into two parts, with C0 on the tube side and (Cs + Ci) on the other side of the coil. As seen in the
equivalent circuit, this arrangement takes the form of a low-pass filter. The value of Lpy is so
chosen, that the filter passes all frequencies within the required video band, but offers a rising
attenuation above the upper limit of this frequency hand.
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VIDEO AMPLIFIERS—DESIGN PRINCIPLES
B+
RS
RL
Lpy
To picture
tube circuit
CS
C 1 + C2 = C t
C1
vg
RK
rp
C2
–
CK
v0
Lpy
mvg
Ct
RL
C2
+
(a)
(b)
Fig. 13.3. Video amplifier with series compensation (a) Circuit, (b) Equivalent circuit.
Since the total shunt capacitance gets divided into two parts, it is possible to choose a
higher value of RL, because C0 is only across RL and not Ct as was the case in shunt compensation.
A 50% increase in RL becomes possible, i.e., Eqn. (13.7) can be modified to become
RL =
1.5
2πf H Ct
. ..(13.10)
This results in higher gain of the amplifier.
Choice of Lpy. It is obvious that the behaviour of the filter will be affected by the disposition
of the total shunt capacitance across the input (shown as C1) and across the output (shown as
C2) of the resultant filter configuration. A typical value of C2/C1 is 0.75. A useful basic design
formula for the inductor is given by
Lpy = nRL2Ct
...(13.11)
where n varies between 0.5 and 1. With a ratio of C2/C1 = p = 0.75, a value of n = 0.67 is
commonly used, since it gives optimum frequency response.
Though with series compensation more gain is possible and a better rise time performance
results, but the fall-off in gain just beyond and upper edge of the required band is much steeper.
This can cause excessive overshoot and even oscillations. This tendency towards ringing can
be reduced by connecting a resistance in parallel with Lpy. A typical practical value of such a
damping resistance is 5RL and varies between 15 and 20 K-ohms.
(c) Combined Shunt and Series Peaking Coils
Shunt and series inductance compensation can be combined to get a peformance slightly superior
to that of the series peaking circuit. The corresponding amplifier configuration with its
equivalent circuit is shown in Fig. 13.4. The following formulae may be used as a guide to
establish approximate values of Lpx and Lpy
Lpx = nxRL2Ct and Lpy = nyRL2Ct
...(13.12)
where nx and ny are the corresponding values of n and are dictated by the value of p, i.e.,
(C2/C1). As a starting point, approximate values can be determined with the help of the following
chart:
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MONOCHROME AND COLOUR TELEVISION
(i) Linear frequency response
(ii) Optimum phase response
(iii) Critical damping
p
nx
ny
0.6
0.14
0.58
0.72
0.1
0.46
0.8
0.063
0.39
B+
RL
RS
Lpx
Lpy
Lpy
CS
C1
vg
RK
C2
rp
v0
–
mvg
+
CK
Lpx
Ct
C2
v0
RL
(a)
(b)
Fig. 13.4. Video amplifier employing both shunt and series compensation
(a) Circuit (b) Equivalent circuit.
It may be noted that in any case it would be necessary to test experimentally with
various inductors to achieve the desired response. This is best done by using a visual display
system*. In addition, the transient response may be checked by feeding a square-wave signal
to the amplifier, and measuring the rise-time of the output waveform with a cathode-ray
oscilloscope.
Cathode Compensation. The basic principle of this method is to apply negative feedback
over the low and middle frequency regions, but arrange to remove it progressively in the HF
region. Because of negative feedback the overall gain gets reduced but it results in a considerable
increase in bandwidth. The simplest of the various possible circuit arrangements is shown in
Fig. 13.5 (a), where the value of Ck has been so chosen, that it completely bypasses Rk at high
video frequencies, but at medium and low frequencies, its reactance becomes comparable with
Rk. This results in negative feedback which increases as the frequency decreases. In fact at
medium and low frequencies, the reactance of Ct is large compared with RL, and that of Ck is
large compared with Rk, thus the amplifier effectively performs as one with a plate load of RL
and an unbypassed cathode resistance Rk. However, at higher frequencies when the shunting
effect of Ct on RL becomes appreciable, the reactance of Ck becomes comparable with Rk, and
this reduces feedback to improve gain and maintain the frequency response.
Condition for Maximum Flatness of Frequency Response. Gain of the above amplifier
that employs cathode degeneration can be expressed in the form
A(mid)
A(HF)
= 1 + jωCtRL + gmRk
1 + jωCt RL
1 + jωCk Rk
*Necessary details of visual display system are given in Chapter 28.
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...(13.13)
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VIDEO AMPLIFIERS—DESIGN PRINCIPLES
B+
B+
RL
RL
CC
CC
vin
vin
C1
Rg
RK
CK
C1
Rg
LK
RK
CK
(a)
(b)
Fig. 13.5. Widebanding by cathode compensation (a) Reactance of CK
comparable to RK at medium and low frequencies, (b) LK and CK
resonate at the upper edge of the video frequency band.
Differentiating Eqn. (13.13) with respect to ω and equating this equal to zero yields
RLCt = RkCk
...(13.14)
This is the condition that must be met for maximum flatness of the frequency response.
As already stated the improvement in bandwidth occurs at the expense of overall gain. However,
this sacrifice is worth it, because with negative feedback the amplifier gain becomes more
stable and in addition there is a considerable reduction of distortion in the output of the amplifier.
Another cathode compensation method is shown in Fig. 13.5 (b), where an inductor Lk in
series with Ck shunts the cathode resistor. The values of Lk and Ck are so chosen, that the
combination exhibits series resonance at the upper edge of the video frequency band. At
resonance the very low impedance of the series tuned circuit effectively short circuits the
feedback resistor and negative feedback is virtually reduced to zero. Typically, the ratio of
cathode to plate circuit time constants falls in the range of 0.5 to 2.0.
13.3 LOW FREQUENCY COMPENSATION
In video amplifiers that employ ac coupling a large coupling capacitor is normally used to
obtain a fairly low value of fL. No special low frequency compensation is thought necessary
because of the annoying effects of too good a low frequency response. This aspect is fully
explained in the next chapter.
Direct Coupled Video Amplifier
When direct coupling is used the question of low frequency compensation does not arise but
the problems of high frequency response are the same as with RC coupled amplifiers. Though
such amplifiers can amplify changes in dc level, they have other inherent problems of drift,
need for a highly regulated power supply and the high voltage dc source for adjustments of
voltages at the grid and cathode in the absense of ac coupling. All this adds to cost and therefore
in many cases partial dc coupling is preferred.
Selection of Tubes for Video Amplifiers
The ability to provide high gain and to handle signals up to 5 MHz are the primary considerations
in selection of tubes for use as video amplifiers. To achieve a high gain and large signal swings
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MONOCHROME AND COLOUR TELEVISION
without excessive distortion, pentodes and beam power tetrodes having high current and large
power dissipation ratings are preferred.
Figure of Merit
The figure of merit of a high frequency tube is defined as the product of gain and bandwidth.
This can be expressed as:
Gain × bandwidth = A(mid) × (fH – fL)
Substituting for A(mid) = gmRL and setting (fH – fL) ≈ fH =
Figure of Merit = gm × RL ×
1
we get,
2πCt RL
1
gm
=
2πCt RL
2πCt
...(13.15)
Obviously, larger the value of this expression, better is the tube for use as a videoamplifier. However, because of large power needs, and the consequent large electrode structure,
the output capacitance (C0) of such tubes cannot be made very small. This reduces the figure of
merit of such tubes (fH reduces) and it becomes necessary to provide HF compensation to
achieve the desired bandwidth. PCL84 is one such tube which has been specially designed for
use in TV receivers. The pentode section of this tube is used as a video amplifier whereas the
triode section is connected as a cathode follower for feeding video signal to AGC and sync
circuits.
Video Amplifier Circuit
Based on the design criteria developed in the earlier sections of this chapter, video amplifier
design employing tube PCL84 has been more or less standardized. A typical circuit is drawn in
Fig. 13.6 with component values labelled on it. The tube employs a load resistance to the order
of 4 K-ohms and with a steady plate current close to 18 mA, the amplifier delivers enough
peak-to-peak video signal to produce a full contrast picture.
+ 200 V
4K
From
video detector
RL
Lpy
Lp×1
To cathode of
picture tube
Lp×2
Ry
5.6 K
100 V
L.C.
22 K
To triode section
of PCL84
4.7 K
+ 200 V
0.7 V
47W
PCL84
+
CK
4 mF
4 KPF
0.1
mF
20 K pot
contrast
control
56 K
3 KPF
Fig. 13.6. Complete video amplifier circuit with gain control in the screen grid circuit.
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VIDEO AMPLIFIERS—DESIGN PRINCIPLES
245
The main design features of this video amplifier are summarized below :
(i) The plate circuit contains both shunt and series compensating coils, that are mutually coupled to provide adequate frequency broadbanding.
(ii) Cathode compensation is also provided by using a small cathode bypass capacitor.
(iii) The amplifier is designed for a full gain of about 30. The screen grid voltage is
varied to contol the gain and this serves as the contrast control.
(iv) The amplifier is dc coupled and has excellent low frequency response. In many designs
partial dc coupling is used for optimum results.
13.4 TRANSISTOR VIDEO AMPLIFIER
Gain requirement from both tube and transistor video amplifiers is usually the same, and
varies between 25 to 60, depending on the video signal amplitude available at the output of
video detector and the transfer characteristics of the picture tube. Transistor video amplifiers
are almost always direct coupled. This not only solves the gain and phase shift problems at low
frequencies, but also makes the use of large coupling capacitors unnecessary. Direct coupling
in transistor circuits does not present any serious problems so far as dc supply is concerned,
because the magnitudes of voltage needed are much less than in tube circuits. However, the
output transistor must have a VCC supply of the order of about 150 volts for delivering a video
signal of nearly 75 volts peak-to-peak to modulate the picture tube.
Transistors for Video Amplifiers
The output capacitance of transistors is comparable with that of tubes, but because transistors
operate at lower impedances, the frequency and phase response in the collector circuit of an
RC coupled transistor amplifier remains unaffected up to a higher frequency than in the tube
plate circuits.
The input capacitance of bipolar transistors is in general, much greater than that of
tubes, but its effect on the previous collector circuit, can be nullified by using interstage emitter
followers.
In a transistor amplifier the upper frequency limit is determined not by stray and shunt
capacitances, but by the reduction in current gain, as the cut-off frequency of the transistor is
approached. Low power transistors amplify up to very high frequencies and the problem is one
of limiting the bandwidth rather than extending it. However, this remark is not applicable to
power transistors. The high signal voltage, that the last video amplifier stage is expected to
deliver with restricted load resistance, needs large collector current operation. This in turn
needs a transistor of about 2 watt rating with a high breakdown voltage. When the above two
conditions are met, the desired gain at high frequencies cannot be easily achieved, because it
is difficult to make transistors with lesser collector to base capacitance and high junction
breakdown ratings. Therefore, it becomes necessary to use peaking coils in the collector circuits
of transistor video amplifiers to extend the high frequency range.
Amplifier Configuration
One high gain, high frequency transistor could by itself provide the required gain and bandwidth,
but input and output impedance requirements make a single stage transtor amplifier difficult
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MONOCHROME AND COLOUR TELEVISION
to design. Therefore, all video amplifier configurations are preceded by a driver stage, connected
as an emitter follower. The driver connects the video detector to the output stage and meets
the following requirements:
(i) It presents a high input impedance to allow the use of a high detector load (about 5
KΩ). This ensures higher detector efficiency and more output voltage.
(ii) It has low output impedance which facilitates matching to the input of the video
amplifier transistor.
(iii) Since the gain of the driver is less than one, it has a large bandwidth to ensure full
transmission of video and intercarrier sound signals.
(iv) The driver output is in phase with its input, and thus provides the correct polarity of
video signal for cathode modulation of the picture tube after one stage of amplification.
13.5 TRANSISTOR CIRCUIT ANALYSIS
Common emitter circuit arrangement is the best suited configuration as a video amplifier,
because of its moderate input and output impedances, high voltage and current gains besides
a large power output. Figure 13.7 shows hybrid-pi model of a transistor in the common emitter
mode. The equivalent circuit (model) has been simplified by reflecting, collector to base junction
capacitance (Ccb or Cµ), to the input loop and neglecting the high collector to base resistance
(rb′ c, or rµ).
rx
vbe
b
C
B
B
vb¢e
rp
Cp
Cm
rce
RL
gmvb¢e
E
Cin = Cp + Cm
= Cp + Cm (1 + gmRL)
Fig. 13.7. Hybrid-pi(π) model of a transistor in common emitter configuration.
In the equivalent circuit (See Fig. 13.7):
(i) rx is the base spreading resistance expressed as a lumped parameter. Its value varies between 50 to 150 ohms.
(ii) rπ is base to emitter junction resistance. Note that be common emitter input resistance in the ‘h’ parameter model, i.e., hie = rx + rπ.
(iii) gm (mA/V) is a constant of proportionality between collector current and base-emitter voltage. It varies with collector current and is governed by the relation
KT
I
gm = C , where VT =
(VT = 0.026 V at room temperature.)
V
VT
(iv) Cπ is the sum of diffusion capacitance and emitter to base junction capacitance.
(v) β is the short circuit current gain of the transistor at low frequencies. It varies with
collector current and falls-off rapidly at collector currents beyond 10 mA.
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VIDEO AMPLIFIERS—DESIGN PRINCIPLES
β = gm × rπ
...(13.16)
when β decreases, both gm and rπ are effected, and get reduced.
(vi) rce is collector to emitter resistance.
(vii) Cm (reflected Miller capacitance) = Cµ(1 + Av)
where Av is the voltage gain of the stage.
(viii) Cin (total input capacitance) = Cπ + Cµ(1 + Av)
...(13.17)
(ix) RL is external load resistance.
(x) v′be is the effective voltage between base and emitter.
fβ (Half Power Frequency)
The circuit of Fig. 13.7 takes the form shown in Fig. 13.8 (a) when RL is set equal to zero. Note
that rce disappears from the circuit once RL is made zero.
The input side of the circuit has a single time constant, consisting of rπ in parallel with
Cin. From this, 3 db down frequency
1
2πrπ Cin
fH = fβ =
RS
B
rx
B¢
vb¢e
b
vS
...(13.18)
rπ
ic = short
circuit current
Cin
gm vb¢e
Single time
constant
With RL = 0 Av = 0
Cin = Cin = Cπ + Cµ
Fig. 13.8 (a). Equivalent circuit for the calculation of short-circuit CE current gain.
A1
β
(–3db) 70.7%
20 db decade
Bandwidth
1.0
fB
fT
f(MHz)
Fig. 13.8 (b). Short-circuit CE current gain vs frequency.
The short circuit current gain, Ais, can be expressed as
Ais =
At f << fβ, Ais = β and for f >> fβ, Ais ≈
β
1+
f
fb
βfβ
f
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...(13.19)
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MONOCHROME AND COLOUR TELEVISION
When f is set equal to fβ, the short circuit current gain drops by 3 db (see Fig. 13.8 (b)).
This frequency fβ, at which the short circuit current gain becomes 70.7% of its maximum value
is the half-power or—3 db frequency. The frequency range up to fβ is then referred to as the
bandwidth of the circuit.
fT (Unity Current Gain Frequency)
fT is defined as the frequency at which Ais attains a magnitude equal to unity, that is:
1=
or
βfβ
fT
(from Eqn. 13.19)
fT = β fβ
...(13.20)
Substituting values of β and fβ from (13.16) and (13.18) in (13.20), we get
fT =
gm
2πCin
...(13.21)
Since at short circuit Av = 0, Eqn. (13.17) reduces to
Cin = Cπ + Cµ
fT =
∴
ωT =
or
gm
2π(Cπ + Cµ )
...(13.22)
g
gm
≈ m | since Cπ >> Cµ.
Cπ
(Cπ + Cµ )
...(13.23)
The above expression shows that fT is a function of the transistor parameters only.
Since fT controls the gain at high frequencies it is also known as ‘Figure of Merit’ of the transistor.
Most of the above parameters are listed in transistor manuals. However, some
parameters, if not given, can either be measured or calculated from the various relations
given above.
Voltage Gain of the Basic Amplifier
The circuit of the basic amplifier employing a BJT (transistor) in common emitter configuration
is shown in Fig. 13.9 (a). In its equivalent circuit (Fig. 13.9 (b)), biasing resistance RB has not
been included, because its shunting effect on input impedance of the transistor is negligible.
Similarly rce being very large, in comparison with RL has been neglected.
VCC
RB
RL
RS
v0
RS
CT
vS
vS
B
rx
B¢
vb¢e
b
a
rπ
C
Cπ × D
Rth
b
(a)
gm vb¢e
RL
v0
CT
E
(b)
Fig. 13.9. Basic transistor amplifier in CE configuration (a) Circuit (b) Equivalent circuit.
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VIDEO AMPLIFIERS—DESIGN PRINCIPLES
The Factor ‘D’
Before proceeding to find voltage gain, it will be useful to define a factor D, to which, the
corner frequency of the amplifier is related.
From Eqn. (13.17),
Cin = Cπ + Cµ (1 + Av)
Av at midband = gm RL
∴
Cin = Cπ + Cµ(1 + gmRL) ≈ Cπ + CµgmRL
Since
ωT =
∴
Cin = Cπ(1 + CµRL ωT)
gm
(from Eqn. 13.23)
Cπ
...(13.24)
= Cπ × D
D = (1 + CµRL ωT)
where
...(13.25)
Voltage Gain
From the equivalent circuit of the amplifier (Fig. 13.9 (b)), the expression for voltage gain at
any frequency (ω) can be expressed as
− βRL
Av =
(rπ + rx + RS )[1 + jωDCπ (rπ || RS + rx )]
If follows from this expression that
| A(mid) | =
βRL
(rπ + rx + RS )
....(13.26)
Gain Bandwidth Product
The corner frequency occurs when the reactance of Cin (= Cπ × D) equals Rth, where Rth is the
Thevenin’s equivalent of the circuit (Fig. 13.9 (b)) to the left of points a and b.
Rth =
rπ ( RS + rx )
rπ + RS + rx
...(13.27)
where RS is the source resistance.
On equating X Cin = Rth and some manipulation we get :
fH =
βfβ
B
1
where ωβ =
and the other factor
rπ Cπ
B=
or ωH =
βω β
B
rx + rπ + RS
RS + rx
...(13.28)
fH (= bandwidth) can also be expressed as
f
B
× T (since fT = fβ × β)
β
D
B ωT
×
ωH =
D
β
fH =
or
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...(13.29)
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MONOCHROME AND COLOUR TELEVISION
Finally, Gain × Bandwidth = A(mid) × ωH
=
βRL
B ωT
×
×
(from Eqs. (13.26) and (13.29))
rπ + rx + RS
β
D
Substituting for B from Eqn. (13.28)
G×B.W=
ωT
RL
×
D
RS + rx
...(13.30)
13.6 GUIDELINES FOR BROAD-BANDING
Equation (13.30) serves as a guideline for explaining the means to extend high frequency
response of the amplifier with or without sacrificing midband gain. This is explained by
considering separately all the constituents of the Gain-Bandwidth expression.
(a) The gain-bandwidth product increases with decrease of source resistance RS. Thus a
reasonable first step while designing a video amplifier is to choose the lowest possible
value of RS. This requirement is readily met, since the driver stage is an emitter
follower, and its output resistance can be made very low without any appreciable loss
in gain.
(b) ωT (fT = βfβ) does not stay constant and drops-off both at very low and high values of
emitter current. However, there is a range of IE (emitter current) over which it stays
high and substantially constant. Therefore, it is advantageous to fix the transistor
operation, in this region, as far as possible.
(c) Once the transistor and its operating point (IE or IC) have been chosen, rx gets fixed
and cannot be varied.
(d) If Cµ, that forms part of factor D is decreased, the bandwidth will increase with no
corresponding loss in gain. However, the value of Cµ is dictated by the VCC supply
chosen or the maximum collector voltage rating of the transistor. Therefore, Cµ is
more or less fixed and cannot be changed for extending high frequency region of the
amplifier.
(e) The last variable is RL, that can be changed to control bandwidth. But, this too cannot
be varied much because of large peak-to-peak output voltage required to modulate
the picture tube, and the maximum permissible dissipation of the transistor.
Output Circuit Corner Frequency
Input capacitance of the picture tube, together with transistor output and wiring capacitances
easily add up to about 15 pF and very much limit the value of RL. In fact the net output
capacitance (CT) of the video amplifier in parallel with RL provides another corner frequency
which turns out to be lower than the input circuit corner frequency. This, then controls the
bandwidth of the amplifier. As stated above, decreasing RL will push up the output corner
frequency, but the load resistance cannot be made too small because it will increase the power
dissipation of the device.
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VIDEO AMPLIFIERS—DESIGN PRINCIPLES
13.7 FREQUENCY COMPENSATION
It is clear from the above discussion that a high fT and high power dissipation transistor is
necessary for video amplifiers. These two requirements, though necessary, are mutually
contradictory to a large extent.
In the past these requirements were met by cascoding, where the power dissipation was
shared equally by the two transistors employed in such a configuration.
With advances in technology, transistors with high power dissipation and reasonably
high fT have now become available. However, the configuration still requires some high
frequency compensation and is normally provided by a shunt or peaking coil in the collector
circuit.
This and other relevant details are explained by a design example.
Video Amplifier Design Data
Output voltage
Bandwidth
Detector output voltage
Voltage gain
Configuration
Coupling
VCC supply
Transistor
75 V (p-p)
5 MHz
2 V (p-p)
40
Common Emitter
Direct
150 V
BF 178
Transistor parameters at IC = 15 mA are : rx = 50 ohm, rπ = 200 ohms, Cπ = 100 pF, β = 20,
fT = 120 MHz, Cµ at 150 V = 1.25 pF, max collector dissipation = 1.7 watts, minimum collector
emitter breakdown voltage = 145 V.
Choice of RL and Operating Point
To avoid non-linear distortion due to saturation and cut-off, the best course for such a large
output is to draw several load lines on the characteristics of the chosen transistor and calculate
distortion for each load by the usual three or five point analysis. This would help to decide the
optimum value of RL. Note that too small and too large a value of load resistance is not acceptable
for reasons already explained.
Such an exercise on the characteristics of BF178 led to the following results:
IC ≈ IE = 15 mA
RL = 4.9 K
VCE (min) inclusive of drop across RE = 30 V
(RE is the emitter resistance)
This leaves 120 V to accmmodate the output signal with enough margin for the blanking
excursion.
For class ‘A’ operation the following relations are valid:
Pmax =
2
VCC
4( RL + RE )
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...(13.31)
252
MONOCHROME AND COLOUR TELEVISION
IE =
VCC
2( RL + RE )
...(13.32)
Choosing RE = 100 ohms and allowing a 10% limit in VCC variations, i.e., VCC max = 165 V,
Pmax from eqn. (13.31) = 1.36 watts and IC ≈ IE from eqn. (13.32) = 15 mA.
The calculated value of IC checks with that found graphically. Similarly the calculated
value of Pmax is within the max. permissible dissipation. However, the transistor would need a
suitable heat sink and this is always provided.
Amplifier Circuit
The circuit of the amplifier is drawn in Fig. 13.10 (a) and its equivalent circuit valid at high
frequencies is shown in Fig. 13.10 (b). Besides other circuit elements the two corner frequencies
(break points) are labelled as f, 3 db (in) and f, 3 db (out) in the equivalent circuit.
+ VCC
Lpx
RL
R1
K
PIX
Tube
BF 178
RS
RE
100W
CE
300 PF
(a)
Driver circuit
representation
B
B¢
rx
RS
vS
CT
R2
vS
vbe
Rb
rp vb¢e
Lpx
Cin
CT
gm vb¢e
f–3 db(in)
(b)
RL
f–3 db(out)
Fig. 13.10. Transistor video amplifier (a) Circuit (b) Equivalent circuit valid at high frequencies.
Voltage Gain
Equation (13.26) can be modified to include the effects of biasing network and the inadequately
bypassed RE. Putting Rb = R1 || R2 (biasing network resistors) and adding (1 + β)RE ≈ βRE
(reflected emitter resistance) to rπ, the new voltage gain can be calculated. With the given
design values even if Rb is taken as 7 K it can be neglected in comparison with the other
shunting resistances. With this assumption the new voltage gain
βRL
| Av(mid) | =
...(13.33)
Rs + rx + rπ + βRE
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VIDEO AMPLIFIERS—DESIGN PRINCIPLES
Assuming
Rs = 150 Ω, | Av(mid) | ≈ 40
This nearly checks with the midband gain computed from the approximate relation
valid with feedback:
| Av(mid) | ≈
RL 4.9 K
=
= 49
100
RE
Input Circuit Corner Frequency f3 db (in)
f3 db (in) =
where
1
2πCin Rth
...(13.34)
Cin = Cπ + Cµ(1 + Av) = 100 + 1.25(1 + 40) = 150 pF
Equation (13.27) can be modified to include βRE. This takes the form
Rth = rπ || (Rs + rx + βRE) =
On substituting numerical values, Rth =
∴
f3 db (in) =
rπ ( Rs + rx + βRE )
rπ + Rs + rx + βRE
...(13.35)
200(150 + 50 + 2000)
≈ 180 Ω
200 + 150 + 50 + 2000
10 12
1
=
≈ 5.8 MHz
2π × 150 × 180
2πCin Rth
This result shows that the input circuit does not need any compensation and would
safely transmit up to the highest modulating frequency of 5 MHz.
Output Circuit Corner Frequency (f3 db (out))
The total collector network capacitance in a well laid out receiver would by typically as follows:
Picture tube cathode and leads
= 7 pF
Heat dissipator
= 3 pF
BF 178 output capacitance
= 3 pF
Total capacitance CT
= 13 pF
∴ f3 db (out) (uncompensated) =
1
1
=
≈ 2.6 MHz.
2π × 4.9 K × 13 pF
2πRL CT
Frequency Compensation
The above result shows that the output circuit has a lower corner frequency and hence would
determine the extent of compensation needed to push this corner frequency to about 5 MHz.
The design criteria for calculating the values of peaking coils are the same as used in
tube circuits. For shunt compensation:
From equation 13.9 we have
Lpx = 0.5 CTRL2
On substituting RL = 4.9 kΩ and CT = 13 pF
Lpx = 0.5 × 13 × 10–12 × (4.9 × 103)2 ≈ 158 × 10–6 H = 158 µH
An inductor of this value will form part of the collector load to provide necessary
compensation.
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MONOCHROME AND COLOUR TELEVISION
Broad-Banding by Negative Feedback
In addition to shunt compensation the emitter resistance RE is bypassed by a very small (300
pF) capacitor CE (see Fig. 13.10 (a)) to provide negative feedback at medium and low video
frequencies. This not only improves the bandwidth but also ensures stable operation of the
amplifier. The value of CE is determined by the consideration that emitter and collector network
time constants should be approximately equal.
13.8 VIDEO DRIVER
The output resistance of the driver together with the input resistance and capacitance of the
video transistor form a network whose time constant should be compatible with the required
bandwidth. Assuming 3 db point at 5 MHz and with Cin = 180 pF
R(out) ≤
10 12
≈ 150 ohms
2π × 5 × 10 6 × 180
Therefore, ideally, ignoring Rin, the output resistance of the driver stage should not
exceed this value. This checks with the value of RS used while determining midband gain and
input corner frequency.
However, in most practical designs. a value of 500 ohms can be used, because of increase
in bandwith available by partially decoupling the emitter resistance of the output transistor.
The emitter follower will have a gain nearly equal to 0.9. This will feed about 1.8 V video
signal to the output transistor which in turn will deliver ≈ 75 (p-p) (gain ≈ 40) for the picture
tube.
Video Detector Loading
A high frequency transistor like BF 184 having β = 75 and fT = 300 MHz, if employed as an
emitter follower with RE = 470 ohms will have Rin = 75 × 470 = 35 KΩ, and Cin = 5 pF. This is
acceptable to a video detector circuit having a 3.9 KΩ load resistance.
Video Driver Biasing
The bias in the driver stage must be carefully set to permit maximum collector voltage swing.
With a high input signal, the output will clip if the transistor cuts off or if VCE reaches zero.
The result is loss of detail in dark grey or white parts of the picture and a buzzing tone in the
sound output. For this reason the upper biasing resistance is often a pre-set variable resistor.
The video detector diode is invariably direct coupled to the driver and thus held at the same
steady bias voltage as the base of the emitter follower. The biasing network is often designed
to provide a small forward bias on the diode to reduce distortion on small input signals.
13.9 CONTRAST CONTROL METHODS
Contrast control is a manual control for setting level of the video signal fed to control grid or
cathode of the picture tube. Its setting determines the ratio of light to dark in the picture.
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VIDEO AMPLIFIERS—DESIGN PRINCIPLES
Contrast Control in Vacuum Tube Video Amplifiers
(a) Cathode Network Control. In this method negative feedback is applied in one form or the
other, taking into account the biasing requirements of the tube. Figure 13.11 (a) shows one
such method, where the adjustment of the contrast control does not change the operating
point of the amplifier. This maintains a constant black level of the video signal.
(b) Plate Network Control. In this arrangement (Fig. 13.11 (b)) the contrast control
potentiometer regulates the magnitude of the video signal to the picture tube. The stray
capacitance of the potentiometer and its connecting leads can reduce high frequency response
of the amplifier. To minimize shunt capacitance the control is usually mounted close to the
video amplifier with the shaft mechanically coupled to the front panel of the receiver. In addition,
the capacitors shown along with the potentiometer, provide frequency compensation to maintain
the same frequency response, at different settings of the contrast control.
CC
To plate
circuit
B+
Video
input
B+
Video
input
Rg
Rg
RL
RK1
RK
Contrast
control C
1
LPX
CK
C1
C2
B+
B–
C3
R2
Contrast
control
To picture
tube circuit
R3
RK2
(b)
(a)
Fig. 13.11. Contrast control circuits in vacuum tube video amplifiers
(a) Control network in cathode circuit, (b) Control network in plate circuit.
Contrast Control in Transistor Circuits
(a) Base Network control. A contrast control technique that maintains a constant black level is
shown in Fig. 13.12 (a). If the values of R3 and R4 are chosen to give a voltage that is equal to
the black level of the video signal at the emitter of Q1, the black level of the signal fed to Q2 will
remain constant over the contrast control range. Because of bandwidth requirements, R2 should
not be higher than 1 K-ohm. The parallel value of R3, R4 should be about one quarter of the
value of R2 thus giving a contrast range of about 5 : 1.
(b) Emitter Network Control. Fig. 13.12 (b) shows one type of emitter network contrast
control. It is a degeneration control. When the arm of potentiometer R4 is at ground, its resistance
is unbypassed causing maximum feedback. This results in small video output. Any variation of
the arm towards the emitter reduces feedback to deliver more output. This provides the desired
contrast control. C1R1, and C2R2 are video peaking networks which cause higher gain at low
contrast settings for high frequencies, making the picture sharper.
(c) Collector Network Control. As shown in Fig. 13.12 (c) the 25-K frequency-compensated
potentiometer operates like a volume control. Setting of contrast control determines the peakto-peak amplitude of the video signal taken from the collector of the video amplifier and
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MONOCHROME AND COLOUR TELEVISION
coupled to the cathode of picture tube. Because of the high impedance level of the collector
network, stray capacitances place severe restrictions on the circuit layout. The compensation
should be so provided that for any setting of the contrast control almost equal time constants
are obtained in the two arms of the connecting network.
+ VCC2
Lpx
+ VCC1
RL1
RL
R3
Q1
From video
detector
Q2
a¢
a
To AGC
circuit
Rb2
To picture
tube circuit
R2
R4
R1 Contrast
control
RE
CE
(a)
+ VCC
Lpx
RL
R3
PB
From driver
output
To picture
tube circuit
Contrast control
C3
C1
R4
R1
C2
R2
(b)
+ VCC
Lpx
R1
Contrast
control
RL
Video
input
C1 C C
25 K
R2
RE
CE
R4
R3
B+
(c)
To cathode of
picture tube
Brightness
control
Fig. 13.12. Contrast control circuits in transistor video amplifiers
(a) Base network control (b) Emitter network control (c) Collector network control.
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VIDEO AMPLIFIERS—DESIGN PRINCIPLES
13.10 SCREEN SIZE AND VIDEO AMPLIFIER BANDIWDTH
It may not be immediately apparent but the size of the screen on which the picture is reproduced
will also govern how much finer details the picture should possess. On a small scrren the
number of active lines that carry video information are very close to each other. This disables
the eye to distinguish fine details, unless the viewer comes very close to the screen. The reason
stems from the fact, that unless the two adjacent objects subtend an angle of one minute or
more at the observer’s eye, they cannot be seen as distinct units. With small screens the distance
necessary for the eye to resolve details is so short that the viewer normally never comes that
close to the screen.
Manufacturers of small screen TV receivers take advantage of this fact and design video
amplifiers with a bandwidth that is much less than 5MHz. This reduction, in the highest
modulating frequency to be reproduced, also makes full bandwidth in the RF and IF tuned
amplifiers unnecessary. All this results in considerable reduction in the overall cost of the
receiver which is a big factor in a highly competitive consumer goods industry.
Review Questions
1.
Draw small signal equivalent circuit of an R.C. coupled amplifier employing a vacuum tube and
derive expressions to show that its bandwidth ≈
1
, where RL is the load resistance and Ct
2πRLCt
the total shunting capacitance.
2.
Describe briefly the methods normally employed to extend the bandwidth of an R.C. coupled
amplifier to meet video signal requirements. Show that in order to extend midband to the higher
corner (– 3 db) frequency, the required value of shunt peaking coil inductance, Lpx = nCtRL2,
where n < 1.
3.
What are the relative merits of shunt and series compensation techniques employed to extend
the bandwidth of an amplifier. Illustrate your answer by drawing equivalent circuits of amplifiers
employing the two types of compensation.
4.
Describe different methods of low frequency compensation. What are the special problems of
direct coupled amplifiers ?
5.
Draw equivalent circuit of a grounded emitter amplifier and derive expressions for voltage gain
and gain-bandwidth product.
6.
Design a video amplifier to meet the following requirements :
Output voltage = 60 V (p-p), bandwidth = 4 MHz, detector output voltage 1.2 V (p-p), voltage gain
= 50, configuration—common emitter, coupling—direct, VCC supply = 100 V, Transistor-BF 177.
Its approximate parameters are :
rx = 50 ohms, rπ = 200 Ω, cπ = 100 pF, β = 25, fT = 120 MHz, Cπ at 100 V = 1 pF, maximum collector
dissipation = 0.8 watt. Take total collector network cpacitance = 16 pF. Assume any other data if
necessary.
7.
Describe with suitable circuit diagrams different methods of contrast control used in both tube
and transistor video amplifiers. Mention relative merits of each type.
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14
Video Amplifier Circuits
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14
Video Amplifier Circuits
The essential requirements of video section circuitry and general wideband techniques were
explained in the previous two chapters. The merits of cathode over grid modulation of the
picture tube were brought out in an earlier chapter. This is now the most preferred method of
feeding video signal to the picture tube unless there are strong reasons in favour of grid
modulation. However, there are several methods of coupling the video amplifier to the picture
tube. This, together with other relevant circuit details, is discussed in this chapter.
Various Coupling Methods. Though it cannot be denied that for near perfect reproduction
of the transmitted picture, dc link has to be maintained between the video detector and picture
tube, but dc coupling has its own problems which when fully taken care of add to the cost of the
receiver. Therefore in many video amplifier designs full dc coupling is dispensed with yet
maintaining optimum reception which is subjectively acceptable. The various possible coupling
arrangements between the video amplifier and picture tube may be classified as:
(a) DC coupling
(b) Partial dc coupling
(c) AC coupling with dc restoration
(d) AC coupling
Though the circuit details differ from chassis to chassis, typical circuits of each type are
examined to identify merits and demerits of the various types of coupling.
14.1 DIRECT COUPLED VIDEO AMPLIFIER
A commonly used video amplifier employing PCL 84 (pentode-triode) is shown in Fig. 14.1.
The video signal is directly coupled from video detector to cathode of the picture tube. The
main features of this circuit are as follows:
(i) Frequency Compensation
The plate circuit contains both shunt and series peaking coils to provide enhanced high frequency
respouse. Additional broadbanding is achieved by using a small (0.003 µF) cathode bypass
(Ck) capacitor. The network L1R1 in the grid circuit of the tube provides frequency compensation
to offset its input capacitance.
(ii) Contrast Control
Gain of the amplifier is controlled by varying dc voltage (potentiometer R6) at screen grid of
the pentode. This becomes the contrast control. The need for two decoupling capacitors at the
screen grid arises from the fact that electrolytic capacitors have a small self-inductance which
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260
261
VIDEO AMPLIFIER CIRCUITS
is sufficient to introduce considerable reactance at high frequencies. Therefore, to provide
adequate decoupling at high frequencies the 4 µF electrolytic capacitor (C1) is shunted by a
small 0.005 µF capacitor (C2) as a high frequency bypass.
+
RL
200V
4K
L4
L2
R10
5.6K
R2
56K
L3
80V P–P
SPG
+
R3
From
video
detector
200V
K
PCL 84
Contrast
control
22K
L1
R1
4.7K
Rk
47
C1
4mF
Ck
.003mF
C2
.005
mF
To sync
separator R4
1K
+150V
R6
PCL 84
(Triode
section)
22K
R5 To AGC
circuit
20K
Pot
500K
Pot
Brightness
control
R8
R9
C3
0.1mF
VDR
G1
G3
C4 G2
.001
mF
470K
LHT
+500V
2.2M
R7
Horz
Vertical
blanking blanking
(50 Hz) (15625 Hz)
Fig. 14.1. Direct coupled video amplifier.
(iii) Sync and AGC Take-off Points
The triode section of PCL 84 is connected as a cathode follower. The resistors R4 and R5 form a
potential divider at the cathode of the triode to feed necessary video voltage to the sync separator
circuit. AGC circuit is fed directly from output of the cathode follower. The use of cathode
follower avoids any loading effects from sync separator and AGC circuits and thus fully isolates
the video amplifier from these circuits. In the absence of such a provision some additional
capacitance appears across the output of the amplifier and tends to lower its high frequency
response.
(iv) Flyback Suppression Pulses
Field and line flyback suppression pulses are injected at the control grid and first anode of the
picture tube through isolating networks. These negative going pulses are of sufficient amplitude
to cut-off the beam current during flyback intervals.
(v) Brightness Control and Switch-off Spot
Brightness control is achieved by varying positive voltage at the grid (G1) of the picture tube
with potentiometer R8 that is connected in series with a VDR (voltage dependent resistance)
across B+ supply. The VDR has a special function to perform. When the receiver is switched-off
the time-base circuits stop immediately and the picture tube spot assumes central position on
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262
MONOCHROME AND COLOUR TELEVISION
the screen. The cathode and grid potentials rapidly becomes equal to chassis potential since
the B+ voltage disappears. However, the picture tube’s cathode remains hot for some time and
keeps emitting electrons. At the same time the EHT capacitance formed by the aquadag coating
of the tube does not immediately discharge since the resistance of the EHT circuit is very high.
For a few moments, therefore, beam current continues to flow with zero grid-cathode bias and
no deflection fields. A bright spot known as ‘switch-off spot’ appears on the screen centre,
which can in due course burn a small portion of the phosphor coating on the screen. Suppression
of the switch-off spot is brought about in this circuit by the VDR which forms the lower arm of
the brightness control potential divider. When the receiver is switched off and the B+ voltage
disappears, the resistance of the VDR immediately becomes very high. This allows the charge
on the associated capacitor C3 to remain for a short time so that the grid is momentarily
positive with respect to cathode. The result of this is that a high beam current passes for a
brief instant and this discharges the EHT smoothing capacitor. This happens as the normal B+
voltage is decreasing and before the raster finally collapses. Thus the heavy beam current is
spread over the screen face and not concentrated on a central spot.
14.2 PROBLEMS OF DC COUPLING
Direct coupling, though very desirable has the following stringent requirements, which, when
provided for add very much to the cost of the receiver.
(a) Regulated EHT Supply
Regulation of normal type of EHT systems used in most television receivers is not good. Full
contrast range to be handled by the picture tube with dc coupling puts a heavy demand on the
high voltage supply. On signal levels that correspond to peak whites, excessive beam current
flows and this results in a drop of voltage. In turn this tends to an instantaneous increase in
deflection sensitivity so that the picture expands (blooms) as the voltage falls. The increase in
deflection sensitivity is due to the fact that as EHT voltage falls, forward velocity of the electrons,
that constitute beam current, decreases. The electrons then spend a longer time under the
deflection field, and are deflected more by a given field than they would normally be. Therefore,
either a well regulated EHT system should be provided, or the natural range of brightness
levels, which occur in the original transmitted picture, should be artificially restricted at the
receiver. The latter, that is, partial loss of dc component of the video signal is lesser of the two
evils and involves a commercial compromise. This is explained in another section of this chapter.
However, as stated earlier, in terms of absolute picture fidelity, both the complete retention of
dc component and a well regulated EHT system are necessary.
(b) Beam Current Limiting
In a dc coupled video stage, for cathode injection, the picture tube can be driven to a high
brightness level if the input signal is removed. This increase in beam current is expensive both
in high voltage supply source and the life of the picture tube.
A simple circuit which limits the mean beam current to a pre-determined value (without
limiting the contrast range) is shown in Fig. 14.2 (a). With this circuit arrangement, dc coupling
is maintained only on low key scenes, where it is most important. In this circuit,
V3 = (Id + Ib)R1 = IdR1 + IbR1,
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VIDEO AMPLIFIER CIRCUITS
The diode will remain conducting and clamp the picture tube cathode to collector of the
amplifying transistor, so long as IbR1 < V2.
+ VCC
Lpy
RL
VCo
RB1
L0
vin
RB1
vin
D + V3 K
Lpx
+V2
Id
RB2
Ib
G1
RB2
C1
RE
Brightness
control
R1
Fig. 14.2 (a). Beam current limiting.
RE
RL
+ Vcc
Ry
C1
0.1mF
R1
R2
Picture
tube
Vertical
blanking
Fig. 14.2 (b). Partial dc coupling.
At the threshold of limiting,
Ib × R1 = V2 = V3
Therefore
Id = 0
Beyond this threshold V2 < V3 and the capacitor C1 charges to V3 – V2, that is, the
picture tube drive is now ac coupled. The picture tube, therefore, receives an additional backbias proportional to the excess mean drive.
It may be noted that it is not possible to establish precise limiting threshold because the
mean value of V2 varies with the picture content. However, by a suitable choice of the value of
R1, excess of picture tube EHT current and the consequent blooming (or breathing) of the
picture are prevented. In video circuits, that employ partial dc coupling, beam current is
automatically reduced, making use of a diode unnecessary.
(c) Other Direct Coupling Problems
Besides the need for a regulated EHT supply and beam current limiting, the complexity in
direct coupled amplifiers arises on account of the following :
(i) It is necessary to have a stable (regulated) B+ source to avoid any drift in the output
of the amplifier.
(ii) The reflections from any passing aeroplanes result in a steep rise and fall of input
signal at the antenna of the receiver. This, despite an efficient fast acting AGC, causes
a momentary flutter of the reproduced picture. It occurs because of very good low
frequency response of the amplifier that extends down to zero Hertz.
(iii) There is a possibility of heater to cathode insulation breakdown during picture highlights because of high dc voltage, equal to the plate voltage that appears on the
cathode of the picture tube.
(iv) For cathode injection, if the detector output is directly coupled to the video amplifier,
the latter must be biased to conduct heavily when no signal is present. This is expensive
both in B+ current and life of the device.
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MONOCHROME AND COLOUR TELEVISION
14.3 PARTIAL DC COUPLING
As mentioned earlier the solution to direct coupling problems, as a compromise, lies in
attenuating the dc voltage before applying it to picture tube and providing a low frequency
bypass to reduce some of its annoying effects. This arrangement is a common feature in most
television receivers and is known as ‘partial dc coupling’. The relevant portion of a transistor
video amplifier is shown in Fig. 14.2 (b). In this circuit
(i) C1, R1 and R2 constitute dc attenuator and low frequency filter circuit. The long
time constant circuit R1, C1 in series with video signal path to the picture tube cathode makes
the aeroplane flutter effect less annoying. The capacitor C1 (0.1 µF) fails to bypass very low
frequencies, with the result, that R1 provides series attenuation in the signal path to offending
low frequency pulsations. As obvious, low frequency components of the video signal get attenuated by a factor R2/(R1 + R2), and this is what gives the circuit the name ‘partial dc coupling’.
(ii) The values of R1 and R2 are chosen so as to considerably reduce the dc voltage that
reaches the cathode of picture tube. This not only attenuates the low frequency content of the
FG
H
R
IJ
K
2
video signal but the consequent reduction in the working dc voltge VCO × R + R at the cath1
2
ode reduces the magnitude of dc voltages required at the accelerating and focusing anode of
the picture tube. This in turn results in a saving in the cost of power supply circuit.
(iii) The reduction in the cathode voltage reduces any possibility of heater cathode breakdown of the picture tube.
(iv) Since the dc component is partly removed by potential divider action of R1 and R2,
the difference in the mean level brightness from scene to scene is reduced. This restricts the
overall range to be handled by the tube and hence limits the maximum demand that is made
on the EHT system.
Video Amplifier Circuit
A transistorised video amplifier circuit with emitter follower drive and partial dc coupling is
shown in Fig. 14.3. The salient features of this circuit are :
(i) Signal from the video detector is dc coupled to the base of Q1. This transistor combines the functions of an emitter follower and CE amplifier. The high input impedance of
emitter follower minimizes loading of the video detector. The sync circuit is fed from the collector of this transistor, where as signal for the sound section and AGC circuit is taken from the
output of the emitter follower.
(ii) The output from the emitter follower is dc coupled to the base of Q2. This is a 5 W
power transistor, with a heat-sink mounted on the case. The collector supply is 140 V, to
provide enough voltage swing for the 80 V P-P video signal output.
(iii) In the output circuit of Q2, contrast control forms part of the collector load. The
video output signal is coupled by the 0.22 µF (C2) capacitor to the cathode of picture tube. The
partial dc coupling is provided by the 1 M (R2) resistor connected at the collector of Q2.
(iv) The parallel combination of L1 and C1 is tuned to resonate at 5.5 MHz to provide
maximum negative feedback to the sound signal. This prevents appearance of sound signal at
the output of video amplifier.
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VIDEO AMPLIFIER CIRCUITS
+ 20V
1.8K
To sync
separator
2.4V
1.8K
L2
0.22mF
Contrast 5K
control Pot
0.22mF
1.2V
+ 140V
To sound
IF amp
14V
Q1
Driver
15PF
90V
1M
L1
47K
To AGC
circuit
+ 12V
68W
20K
C1
0.002mF
5.5 MHz
trap
Brightness
control
100K Pot
10K
470W
Video detector
circuit
R2
Q2
output
1.8V
80V P–P
C2
47K
Neon
lamp
R3
15K
C3
EHT
.047mF
+ 140V
.0022mF
Vertical
blanking
Fig. 14.3. Video amplifier employing partial dc coupling.
(v) The neon bulb in the grid circuit provides protection of a spark-gap since the neon
bulb ionizes and shorts to ground with excessive voltage. The ‘spark gaps’ are employed to
protect external receiver circuitry from ‘flash overs’ within the tube. The accumulation of charge
at the various electrodes of the picture tube results in the appearance of high voltates at the
electrodes, which if not discharged to ground, will do so through sections of the receiver circuitry and cause damage.
(vi) Note that dc voltages at the base and emitter of the two transistors have been suitably set to give desired forward bias.
(vii) Vertical retrace blanking pulses are fed at the grid of the picture tube through C3,
and the grid-return to ground is provided by R3.
(viii) Brightness control. The adjustement of average brightness of the reproduced scene
is carried out by varying the bias potential between cathode and control grid of the picture
tube. In the circuit being considered a 100 KΩ potentiometer is provided to adjust dc voltage at
the cathode. This bias sets correct operating point for the tube and in conjunction with the
video blanking pulses cuts-off the electron beam at appropriate moments.
The setting of grid bias depends upon the strength of signal being received. A signal of
small amplitude, say from a distant station, requires more fixed negative bias on the grid than
a strong signal. The dependency of picture tube grid bias on the strength of the arriving signal
is illustrated in Fig. 14.4. For a weak signal, the bias must be advanced to the point where
combination of the relatively negative blanking voltage plus the tube bias drives the tube into
cut-off. However, with a strong signal the negative grid bias must be reduced, otherwise some
of the picture details are lost. Since the bias of the picture tube may required an adjustment
for different stations, or under certain conditions from the same station, the brightness control
is provided at the front panel of the receiver.
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MONOCHROME AND COLOUR TELEVISION
Brightness
Picture tube
transfer characteristics
Average
brightness
Black
VGK
–90
–60
–30
DC bias
0
Weak signal
Strong
signal
Reduced bias
Fig. 14.4. Optimum setting of contrast control for different
amplitudes of the video signal.
The effects of brightness and contrast controls described earlier overlap to some extent.
If setting of the contrast control is increased so that the video signal becomes stronger, then
the brightness control must be adjusted to meet the new condition, so that no retrace lines are
visible and the picture does not look milky or washed out. Too small a value of the negative
grid bias allows average illumination of the scene to increase thus making part of the retrace
visible. In addition, the picture assumes a washed out appearance. Too low a setting of the
brightness control, which results in a high negative bias on the picture tube grid, will cause
some of the darker portions of the image to be eliminated. Besides this overall illumination of
the scenes will also decrease. To correct this latter condition, either the brightness control can
be adjusted or the contrast control setting can be advanced until correct illumination is obtained.
If the brightness control is varied over a wide range the focus of the picture tube may be
affected. However, in the normal range of brightness setting made by the viewer, changes in
focus do not present any problem.
It is now apparent that despite the fact that video signal, as received from any television
station, contains all the information about the background shadings of the scene being televised,
an optimum setting of both contrast control and brightness control by the viewer is a must to
achieve desired results. Many viewers do not get the best out of their receivers because of
incorrect settings of these controls. However, to ensure that retrace lines are not seen on the
screen due to incorrect setting of either contrast or brightness control, all television receivers
provide blanking pulses on the grid electrode of the picture tube.
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VIDEO AMPLIFIER CIRCUITS
14.4 CONSEQUENCES OF AC COUPLING
As already explained, dc coupling though desirable, adds to the cost of receiver. Partial dc
coupling does not reduce fully the circuit complexity and other side effects of dc coupling. This
suggests the use of ac coupling from video detector to picture tube. However, before doing so, it
would be desirable to review the consequences of ac coupling. Figure 14.5 (a) shows video
signals for two lines taken at different moments from a television broadcast. One signal
represents a line from a predominently white picture while the other belongs to a black
background. As they come out of the video detector, their sync pulses are aligned to the same
level. When amplified by a dc coupled amplifier, they get inverted but retain their common
blanking level (Fig. 14.5 (b)). At the picture tube, with a suitable fixed bias, black levels of the
two signals automatically align themselves along the beam current cut-off point. This happens
because of different dc contents in the two signals. Thus dc components of video signals enable
scenes with different background shadings to be correctly reproduced on the raster without
having to change the setting of brightness control.
t
0
–vin
(a)
Predominantly
white picture
Predominantly
black picture
v0
t
0
(b)
Fig. 14.5. Video amplifier signal waveforms for two different pictures
(a) Input voltage (b) Output voltage.
Now consider that these signals are passed through a capacitor as would be the case if
ac coupling were employed. This is illustrated in Fig. 14.6 by an equivalent circuit and associated
waveshapes. In the equivalent circuit (see Fig. 14.6 (a)) dc component of each signal has been
represented by a battery and the ac component by a generator. The combined signal feeds into
an RC circuit. On application of any composite signal the coupling capacitor will charge to a
value equal to the battery voltage. However, the ac video content will cause the capacitor to
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MONOCHROME AND COLOUR TELEVISION
charge and discharge as the applied voltage exceeds and falls around the dc voltage across the
capacitor. Thus, while the dc component is blocked by the capacitor, the current which
continuously flows through the load resistance develops an ac voltage drop across it. The
resulting output waveshapes and their locations along with corresponding input waveshapes
are shown in Figs. 14.6 (b) and (c). Note that while the output waveshapes are almost identical
to their input counterparts, their sync and blanking levels no longer align with each other.
Each signal has set itself around the zero axis as a consequence of ac coupling. This leads to
the following undesired effects.
(i) Visible Retrace Lines
The retrace lines become visible because the blanking pulses do not remain at a constant level
and may not have enough amplitude to cause retrace blanking. Most modern receivers,
irrespective of the coupling employed, incorporate special vertical and horizontal retrace
blanking circuits as a means of preventing retrace lines from becoming visible.
+
vc
–
C
Video
signal’s
Input
ac component
voltages
and
dc component
R
v0
Output
p
voltages
ag
+
–
(a)
vin
v0
vin
v0
+
+
+
+
t
0
0
t
DC component
–
t
0
0
t
DC component
–
–
(b)
–
(c)
Fig. 14.6. Effect of RC coupling on the dc component of video signals
(a) equivalent circuit of video signal and RC network (b) a predominantly
white scene, and (c) a predominantly dark scene.
(ii) Possible Loss of Sync
This occurs due to loss of dc component. The sync pulse amplitudes now vary with picture
content and this can lead to inadequate amplitude of sync pulses at the output of sync separator.
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VIDEO AMPLIFIER CIRCUITS
Therefore, if synchronization is to remain stable, the dc level must be restored (sync pulses
must line up) before the video signal is fed to the sync separator circuitry.
(iii) Loss of Average Brightness of the Scene
This means that bright and dark scenes may not be easily distinguishable. With loss of dc
component the average brightness information is lost. Thus signals from different brightness
backgrounds will lose this identity and reproduce pictures with a background of some grey
shade.
(iv) Poor Colour Reproduction
In colour television a change in the luminance (brightness) signal will cause a change in the
brightness of a colour. Therefore loss of dc component of the video signal will result in poor
colour reproduction.
14.5 DC REINSERTION
As explained earlier, relative relationship of ac signal to the blanking and sync pulses remains
same with or without the dc component. Furthermore, brighter the line, greater is the separation
between the picture information variations and the associated pulses. As the scene becomes
darker, the two components move closer to each other. It is from these relationships that a
variable bias can be developed to return the pulses to the same level which existed before the
signal was applied to the RC network.
DC Restoration with a Diode
A simple transistor-diode clamp circuit for lining up sync pulses is shown in Fig. 14.7 (a). The
VCC supply is set for a quiescent voltage of 15 V. In the absence of any input signal the coupling
capacitor ‘C’ charges to 15 V and so the voltage across the parallel combination of resistor R
and diode D will be zero. Assume that on application of a video signal, the collector voltage
swings by 8 V peak to peak. The corresponding variations in collector to emitter voltage are
illustrated in Fig. 14.7 (b). The positive increase in collector voltage is coupled through C to the
anode of diode D, turning it on. Since a forward biased diode may be considered to be a short,
it effectively ties (clamps) the output circuit to ground (zero level). In effect, each positive sync
pulse tip will be clamped to zero level, thereby lining them up and restoring the dc level of the
video signal.
+ VCC
RL
+ 15V
+
vin
VCE
C
VCE
C
v0
–
R
–
4V
+
D
+ v0
19V
0
15V
–4
Clamp level
VCO
11V
0
(a)
t
DC level
–8
t
(b)
(c)
Fig. 14.7. Diode dc restorer (a) circuit (b) collector voltage (c) output voltage.
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MONOCHROME AND COLOUR TELEVISION
In the case under consideration the diode will cause the coupling capacitor to charge to
a peak value of 19 V. However, during negative excursion of the collector voltage the capacitor
fails to discharge appreciably, because the diode is now reverse biased and the value of R has
been chosen to be quite large. The average reverse bias across the diode is – 4 V, being the
difference between the quiescent collector voltage and peak value across the capacitor. Note
that as the input video signal varies in amplitude a corresponding video signal voltage appears
across the resistor R and it varies in amplitude from 0 to – 8 V (peak to peak). This, as shown
in Fig. 14.7 (c), is the composite video signal clamped to zero.
Similarly as and when the average brightness of the scene varies the capacitor C charges
to another peak value thereby keeping the sync tips clamped to zero level.
Reversing the diode in the restorer circuit will result in negative peak of the input
signal being clamped to zero. This would mean that the dc output voltage of the circuit will be
positive. The video signal can also be clamped to any other off-set voltage by placing a dc
voltage of suitable polarity in series with the clamping diode.
Limitations of Diode Clamping
It was assumed while explaining the mechanism of dc restoration that charge across the coupling
capacitor C does not change during negative swings of the collector voltage. However, it is not
so because of the finite value of RC. The voltage across C does change somewhat when the
condenser tends to discharge through the resistor R. Another aspect that merits attention is
the fact that whenever average brightness of the picture changes suddenly the dc restorer is
not capable of instant response because of inherent delay in the charge and discharge of the
capacitor. Some receivers employ special dc restoration techniques but cost prohibits their use
in average priced sets.
14.6 AC COUPLING WITH DC REINSERTION
Figure 14.8 is the circuit of a video amplifier employing ac coupling with dc restoration. The
coupling capacitor C1 offers negligible reactance to the high frequency content of the video
signal and it gets coupled to the picture tube grid directly. With no input signal, C2 charges to
the steady dc potential existing at ‘X’ through R4, R6 and R7. With arrival of video signal the
potential at ‘X’ falls during negative swing of the collector voltage. This causes C2 to discharge
through R4, R3, VCC source, R7 and diode D1. However during positive voltage swing at the
collector, C2 fails to regain its charge because during this interval the diode is reverse biased
and R6 has been chosen to be too large. Thus the reduced voltage across C2 is maintained at
this level during intervals between sync tips because of the relatively large value of C2 and
associated resistors. The resultant difference of potential between ‘X’ and VC2, that effectively
appears between ‘Y’ and ground (see Fig. 14.8) gets applied to the grid via isolating resistance
R5. This amounts to restoring dc component of the video signal, which otherwise is blocked by
the coupling capacitor. When the average brightness of the scene increases the video sync tips
move further away from the picture signal content and the point ‘X’ then attains a new less
positive potential during sync tip intervals. This further lowers the potential across the capacitor
C2. The enhanced difference in potential between the point ‘X’ and the new value of VC2 gets
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VIDEO AMPLIFIER CIRCUITS
applied to the grid of the tube. This, being positive, reduces the net negative voltage between
the grid and cathode and the scene then moves to a brighter area on the picture tube
characteristics. Any decrease in average brightness of the scene being televised will have the
opposite effect and net grid bias will become more negative to reduce background illumination
of the picture on the raster. Thus the diode with the associated components serves to restore
the dc content of the picture signal and the difference in potential between ‘X’ and ‘Y’ serves as
a variable dc bias to change the average brightness of the scene.
v0
Last
video
amp
R1
C1
t
0
v0
L2
L1
vin
C2
X
R3
R2
+
–
R4
–
+
VC2
R5
To grid
of picture tube
Y
R6
D1
VCC
R7
Fig. 14.8. Practical dc restorer circuit.
14.7 THE AC COUPLING
Cost is a strong determining factor in the design of commercial television receivers. If it is
possible to reduce the cost of a set without impairing the picture quality too much, then a
sacrifice in quality for cost is justifiable and is made in some receiver designs. Therefore many
receivers use only ac coupling. In other words the dc component is removed from the signal
and never reinserted.
The AC Coupled Video Amplifier
Figure 14.9 shows an ac coupled video amplifier. The coupling capacitor (0.22 µF) and resistance
of the brightness control network constitute the ac coupling network. The contrast control is
located in the emitter circuit of the first video amplifier. It is also ac coupled to the output
video amplifier. The amplifier employs the usual broadbanding techniques. It has a sound trap
(resonant) circuit in the emitter lead of the output transistor. An interesting feature of this
circuit is the provision of a spot-killer switch. This swith opens when the receiver is switched
off. Its operation removes dc voltage at the cathode of picture tube reducing grid-cathode voltage
to zero. The residual beam current increases and quickly discharges the EHT smoothing
capacitor thereby reducing intensity of the switch-off spot.
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MONOCHROME AND COLOUR TELEVISION
+
24 V
100
0 V P–P
RL
From
video
detector
150 mH
+ 24 V
0.22 mF
1K
Q1
33 K
10 mF
Contrast
control
CC
15 K
Q2
300 W
Pot
SPG
8.2 K
+
400 V
2.2 M
100 K
2.2 K
27 W
2000 PF
.001
mF
180 K
Tuned
to
5.5 MHz
1000 PF
Spot
killer switch
500 mH
10 W
+
150 V
–
EHT
470 K
20
mF
250 K Pot
Brightness
control
+ 550 V
100 W
33 K
Vertical
blanking
Horz
blanking
Fig. 14.9. AC coupled video amplifier circuit.
14.8 VIDEO PREAMPLIFIER IN AN IC CHIP
The use of integrated circuits between video detector and video output amplifier is very common
in all solid state TV receivers. TBA 890 is one such dedicated IC which performs the following
functions. Figure 14.10 shows circuit connection at various pins of this IC.
To sound IF
5.5 MHz trap
+ 130 V
39 PF
From
video
detector
(Pin 8 of TCA 540
if used)
3.5 K
2.2 K
10
PF
4.7 K
NC
15 nF 50 mF
15
1
2
14
13
100
W
0.2 mF
+
16
AFC flyback
pulses
Vertical sync pulses +
(To vertical osc)
12
11
10
9
3
16 V
0.1 mF
Delayed AGC
to tuner
+
1.5
K
56
W
4
5
+
10 nF 16 mF
6
7
8
100
W
180
W
125
mF
150
W
100 mH
Contrast
control
100 mH
TBA 890 IC
100 W
+
+ 16 V
680 W
1K
1 nF
1K Pot 56 W
33 nF
220
W
AGC to
100 K Pot 1st IF
47 K
33 nF AFC –
AFC
output
flyback
(To Horz osc)
pulses
47
mF
1.5 K K
2N
3501
0.22 mF
82 W
1.5 K 390 W
OA79
470 K
+
33 K
G1
G2
22 K
130 V
Brightness
control
100 K
pot
68 nF
56 K
AGC level
Fig. 14.10. Video output amplifier driven by the IC TBAS90.
(i) First Video Amplifier
The output from video detector feeds at pin 9 and this is also the sound take-off point. The
video preamplifier employs atleast two stages of differential amplifiers and is preceded by a
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VIDEO AMPLIFIER CIRCUITS
driver stage to provide impedance matching. The voltage gain from this stage is about 70 db.
Input signal from the detector is clamped at 3 V and the video output is obtained at pin 11 of
the IC. The output drives video output transistor 2N 3501 through a contrast control network
as shown in the figure. The VCC supply to the IC is a stabilized + 16 V derived/from the low
voltage (LV) rectifier and filter network.
(ii) Sync Separator
The sync separator receives input from the video preamplifier and is suitably biased to deliver
clean sync pulses. The circuit also employs a noise suppression circuit. Integrated vertical
sync pulses are fed to the vertical oscillator through a capacitor from pin number 14 on the IC.
The amplitude of the vertical sync output is around 11 V.
(iii) AFC Circuit
The horizontal AFC circuit employs a single ended discriminator. It derives sync pulse input
from the sync separator and flyback pulses of opposite polarity from the horizontal output
transformer at pins 4 and 10. The vertical blanking pulses are also added at pin 10 from the
vertical output transformer. The AFC control voltage is available at pin 2 and is fed to the
input of horizontal oscillator through an anti-hunt filter circuit. AFC output voltage ranges
from 2 to 10 volts.
(iv) AGC Circuit
The IC includes a keyed AGC circuit and receives flyback pulses through pin 10 along with the
AFC circuit. The AGC output voltage varies from 1 to 12 V and is fed to the IF section from pin
7 as shown in the figure. Delayed AGC voltage for the tuner is available at pin 6 and its
amplitude varies from 0.3 to 12 V.
Review Questions
1.
Enumerate the various coupling methods employed between video detector and picture tube.
Why does dc coupling add to the cost of the receiver ?
2.
Describe the main features of the dc coupled video amplifier shown in Fig. 14.1.
What is a ‘switch-off ’ spot ? Explain how its undesirable effect on the screen is minimized by
using a VDR in the brightness control circuit.
3.
Explain briefly the essential requirements, which must be met, while providing dc coupling in
the video section of the receiver. Describe with a suitable circuit diagram how a diode can be
used to limit beam current of the picture tube to a safe upper limit.
4.
What do you understand by partial dc coupling ? Explain with a circuit diagram how some of the
annoying features of dc coupling are almost eliminated by partial dc coupling. Justify that it is a
reasonable compromise between cost and quality.
5.
Describe the consequences of ac coupling. Show with a suitable circuit diagram 1and illustrations how dc component of the video signal is restored back by diode clamping in an otherwise ac
coupled video amplifier.
6.
Explain how the removal of dc component from the video signal affects the overall contrast range
and necessitates repeated adjustments of the brightness control.
7.
State the arguments that are advanced in favour of employing ac coupling in the video section of
a TV receiver. Draw the circuit diagram of a typical coupled video amplifier and explain its main
features.
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15
Automatic Gain Control and
Noise Cancelling Circuits
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15
Automatic Gain Control and
Noise Cancelling Circuits
Automatic gain control (AGC) circuit varies the gain of a receiver according to the strength of
signal picked up by the antenna. The idea is the same as automatic volume control (AVC) in
radio receivers. Useful signal strength at the receiver input terminals may vary from 50 µV to
0.1 V or more, depending on the channel being received and distance between the receiver and
transmitter. The AGC bias is a dc voltage proportional to the input signal strength. It is obtained
by rectifying the video signal as available after the video detector. The AGC bias is used to
control the gain of RF and IF stages in the receiver to keep the output at the video detector
almost constant despite changes in the input signal to the tuner.
15.1 ADVANTAGES OF AGC
The advantages of AGC are:
(a) Intensity and contrast of the picture, once set with manual controls, remain almost
constant despite changes in the input signal strength, since the AGC circuit reduces
gain of the receiver with increase in input signal strength.
(b) Contrast in the reproduced picture does not change much when the receiver is switched
from one station to another.
(c) Amplitude and cross modulation distortion on strong signals is avoided due to reduction
in gain.
(d) AGC also permits increase in gain for weak signals. This is achieved by delaying the
application of AGC to the RF amplifier until the signal strength exceeds 150 µV or so.
Therefore the signal to noise ratio remains large even for distant stations. This reduces
snow effect in the reproduced picture.
(e) Flutter in the picture due to passing aeroplanes and other fading effects is reduced.
(f) Sound signal, being a part of the composite video signal, is also controlled by AGC
and thus stays constant at the set level.
(g) Separation of sync pulses becomes easy since a constant amplitude video signal
becomes available for the sync separator.
AGC does not change the gain in a strictly linear fashion with change in signal strength,
but overall control is quite good. For example, with an antenna signal of 200 µV the combined
RF and IF section gain will be 10,000 to deliver 2 V of video signal at the detector output,
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AUTOMATIC GAIN CONTROL AND NOISE CANCELLING CIRCUITS
whereas with an input of 2000 µV, the gain instead of falling to 1000 to deliver the same
output, might attain a value of 1500 to deliver 3 V at the video detector.
Basic AGC Circuit
The circuit of Fig. 15.1 illustrates how AGC bias is developed and fed to RF and IF amplifiers.
The video signal on rectification develops a unidirectional voltage across RL. This voltage must
be filtered since a steady dc voltage is needed for bias. R1 and C1, with a time constant of about
0.2 seconds, constitute the AGC filter. A smaller time constant, will fail to remove low frequency
variations in the rectified signal, whereas, too large a time constant will not allow the AGC
bias to change fast enough when the receiver is tuned to stations having different signal
strengths. In addition, a large time constant will fail to suppress flutter in the picture which
occurs on account of unequal signal picked up by the antenna after reflection from the wings of
an aeroplane flying nearby. With tubes, a typical AGC filter has 0.1 µF for C1 and 2 M for R1.
IF
amplifier
RF amp
Video signal
input
AGC
rectifier
Decoupling
network
R2
Rectified
output
C2
R1
C3
R3
AGC bias
line
C1
RL
AGC
filter
Fig. 15.1. Basic AGC circuit.
For transistors, typical values are 20 kΩ for R1 and 10 µF for C1. The filtered output
voltage across C1 is the source of AGC bias to be distributed by the AGC line. Each stage
controlled by AGC has a return path to the AGC line for bias, and thus the voltage on the AGC
line varies the bias of the controlled stages.
15.2 GAIN CONTROL OF VT AND FET AMPLIFIERS
The gain of a vacuum tube or FET amplifier can be determined from the equation AV = gmZL,
where gm is the transconductance of the device and ZL the impedance of the load. The impedance
is determined by the components used in the tuned circuit and does not lend itself to simple
manipulation. However, gm of both tubes and field-effect transistors can be controlled by varying
their bias. Hence all AGC systems vary the bias of RF and IF stages of the receiver to control
their gain. The variation of gm with control grid voltage for a vacuum tube is illustrated in
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MONOCHROME AND COLOUR TELEVISION
Fig. 15.2(a). The transconductance is smaller near cut-off but increases as the bias decreases
towards zero. It also decreases if operation of the tube is brought close to saturation. The
region near cut-off bias is used for AGC operation. The self-bias is chosen to fix the operating
point for high gain. The AGC voltage which is negative gets added to it and shifts the operating
point to change the gain. The resulting change in gain compensates for variations in the input
signal thereby maintaining almost constant signal amplitude at the output of video detector.
Figure 15.2 (b) shows how the control grid is returned to the AGC line for negative bias. In
tube circuits a negative bias varying between – 2V and – 20V is developed by the AGC circuit
depending on the strength of incoming signal. In order to obtain high gain and minimum cross
modulation effects, pentodes with remote cut-off characteristics are preferred in video IF
amplifier circuits.
IF amp
gm
IF
transformer
vin
100 W
Cut-off
bias
5K
VGK
–30
–20
–10
0
470 PF
AGC bias
–2 V to –20 V
(a)
0.001 mF
(b)
Fig. 15.2. AGC action in tube circuits (a) plot of gm vS grid bias,
(b) method of returning AGC bias to the control grid.
15.3 GAIN CONTROL OF TRANSISTOR AMPLIFIERS
The transistor is a current controlled device. Therefore, in transistor amplifiers it is desirable
to consider power gain instead of voltage gain for exploring means of their gain control by AGC
techniques. The power gain (G) of a transistor amplifier may be determined by the equation,
G≈
β 2 RL
Rin
where β and Rin are the short circuit current gain and input resistance of the transistor
respectively. Here again it is convenient to vary one of the parameters of the transistor in
order to control overall amplification of the receiver. The magnitude of β depends on the
operating point of a transistor which is established by the base to emitter (VBE) forward bias.
Shifting the operating point, both towards collector current cut-off and collector current
saturation causes a decrease in β which in turn reduces the power gain. Figure 15.3 shows the
effect of change in VBE on collector current and power gain of an amplifier employing a silicon
transistor. A number of conclusions may be drawn from the curves shown in the figure. First,
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AUTOMATIC GAIN CONTROL AND NOISE CANCELLING CIRCUITS
the amount of change in VBE which is necessary to shift the operating point of the transistor
from cut-off to saturation is small, only 0.4 to 0.5 V. This is much smaller as compared to about
30 V in a vacuum tube. Second, at some optimum value of forward bias (0.7 V in this case)
power gain of the amplifier is maximum and does not change much for small variations in the
bias voltage. However, the gain decreases as the bias is either increased (shifting the operating
point towards saturation) or decreased (moving it towards cut-off).
IC
Saturation
Cut-off
0
0.4
0.7
G
0.4
Towards
cut-off
VBE
Forward
AGC
Reverse
AGC
0
1.0 V
0.7
VBE
1.0 V
Towards
saturation
Fig. 15.3. Variation of collector current (IC) and power gain (G) of a transistor
amplifier as its base to emitter voltage (VBE) is varied.
15.4 TYPES OF AGC
If the operating point is shifted towards saturation for controlling the amplifier gain, it is
called forward AGC. An AGC system which operates by shifting the operating point towards
cut-off is referred to as reverse AGC.
In many TV receiver designs, either forward or reverse AGC is exclusively employed for
affecting gain control. However, in some receivers both forward and reverse AGC are
simultaneously employed in different parts of the RF and IF amplifier chain. It may be noted
that receivers which use either reverse or forward AGC do not operate the amplifiers at peak
gain but fix the no-signal operating point at such a value that the stage gain may be increased
or decreased without having to move to the other side of the power gain peak.
Reverse AGC
The power gain curve in Fig. 15.3 is not symmetrical, that is, the reverse AGC region of the
curve falls off more rapidly than does the forward AGC region. This means that reverse AGC
will require a smaller change in voltage for full gain control than will forward AGC. However,
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MONOCHROME AND COLOUR TELEVISION
operation in this region, which is close to cut-off makes the receiver more susceptible to overload
and cross modulation distortion on strong signals.
The circuit of Fig. 15.4 (a) is of a single stage transistor (n-p-n) IF amplifier employing
reverse AGC. The voltage divider formed by R1 and R2 provides a suitable fixed forward bias
from the VCC supply. The resistor R3 and capacitor C1 constitute the AGC decoupling network.
To 2nd
IF amp
+
From
mixer
–
R4
C1
+
C2
+ VCC
0.005 F
R1
VCC
R2
1K
R3
– AGC bias
Fig. 15.4 (a). Tuned amplifier with reverse AGC.
Forward AGC
Forward AGC is often preferred for controlling gain of video IF amplifiers because it is more
linear in its control action. Besides a change in β, the input resistance (Rin) of the transistor
also decreases with increase in forward bias. This results in a power mismatch between the
tuned IF transformer and the transistor, thereby providing an additional control on power
gain.
To 2nd
IF amp
+
From
mixer
–
C2
R4
C1
+
AC
ground
R1
VCC
R2
R3
C3
R5(large
resistance)
+ VCC
+ AGC bias
Fig. 15.4 (b). Tuned amplifier with forward AGC.
A single stage tuned IF amplifier employing forward AGC is shown in Fig. 15.4 (b). In
this case, for any increase in signal strength the base to emitter forward bias must increase to
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281
shift the operating point of the transistor towards saturation. Similarly a decrease in signal
strength would require a decrease in forward bias. To achieve this, the AGC system must
deliver a positive going voltage to the base of amplifier. If a p-n-p transistor is used the forward
AGC system would develop a negative going voltage proportional to the signal strength. As
shown in the figure, amplifiers employing forward AGC often use a large resistor (R5) in series
with the collector circuit. When AGC voltage varies to increase collector current, effective VCE
decreases, thereby allowing the transistor to approach saturation quickly for faster AGC action.
AGC is applied to the tuner and 1st and 2nd IF stages but not to the third or last IF
stage because amplitude of the input signal to the third IF amplifier is quite large and any
shift in the chosen optimum operating point by the application of AGC would result in amplitude
distortion. Another reason for not applying AGC bias to the last IF stage is the fact that the
AGC control is proportional to stage gain and this is more suited to RF and first two IF stages
because the RF signal amplitude here is quite small and these stages can be designed for more
gain without any appreciable distortion.
15.5 VARIOUS AGC SYSTEMS
An ‘average’ or simple AGC system, as used in radio receivers, is not suited for control of gain
in TV receivers. The average value of any video signal depends on brightness of the scene
besides signal strength and so is not a true representation of the RF signal picked up at the
antenna. For example, a dark scene would develop more AGC bias as compared to a white one,
the signal strength remaining the same. This, if used to control the gain of the receiver, would
tend to make dark scenes more dark and white ones more bright.
With the present system of transmission, the carrier is always brought to the same level
when synchronizing pulses are inserted irrespective of the average level of the video signal.
The amplitude of the sync level would change only if the signal strength changes. The sync
amplitude level, then can serve as the true reference level of the strength of the picked up
signal. The system based on sampling the sync tip levels is known as ‘Peak’ AGC system.
Either the modulated picture IF carrier signal or the detected video signal can be rectified by
the AGC stage to supply AGC bias voltage. In most receivers, however, the AGC circuit uses
signal from the video amplifier because the higher signal level allows better control by AGC
bias. It is necessary to provide dc coupling between the video amplifier and AGC system in
order to keep the pedestals of the video signal aligned. The peak rectifier output then will be a
true measure of the signal picked up by the antenna.
Peak AGC System
A typical peak detector circuit is shown in Fig. 15.5, where a separate diode is used to rectify
the signal which is fed to it through capacitor C1 from the output of the last IF amplifier.
During positive half cycles of the modulated video signal, diode D1 conducts and the capacitor
C1 charges to peak value of the input signal with the polarity marked across the capacitor.
During periods other than sync pulse intervals the diode is reverse biased and no current flows
through it. However, the capacitor tends to discharge through secondary winding of the IF
transformer and R1. Time constant of the discharge path is 270 µs and this being much greater
than the line period of 64 µs, the capacitor discharges only partially and regains charge
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MONOCHROME AND COLOUR TELEVISION
corresponding to the sync tip (peak) amplitude on each successive sync pulse. Thus the current
that flows through R1 is proportional to the peak value of the modulated video signal and the
voltage drop across it becomes the source of AGC bias.
D2 Video detector
circuit
Lp
Ls
Last IF
stage
270 PF
+ –
R2
Discharge path
of C1
AGC
voltage
C2
D1
C1
560 K
–
–
0.22
mF R1
+
+
1M
Fig. 15.5. Peak AGC system.
Negative voltage drop across R1 is filtered by R2 and C2 to remove 15625 Hz ripple of the
horizontal sync pulses. The output AGC voltage thus obtained is fed to IF and RF stages as an
AGC control voltage.
Drawbacks of non-keyed AGC. The peak AGC system which is also called the non-keyed
AGC system suffers from the following drawbacks, though it measures the same signal
strength.
(a) The AGC voltage developed across the peak rectifier load tends to increase during
vertical sync pulse periods because the video signal amplitude remains almost at the
peak value every time the vertical sync pulses occur. This results in a 50 Hz ripple
over the negative AGC voltage and reduces gain of the receiver during these intervals.
The reduced gain results in weak vertical sync pulse which in turn can put the vertical
deflection oscillator out of synchronism causing rolling of the picture. To overcome
this drawback a large time constant filter would be desirable to filter out the 50 Hz
ripple from the AGC bias. But with too large a time constant the AGC voltage fails to
respond to fast changes like aeroplane flutter and quick change of stations.
(b) In fringe areas noise pulses develop an additional AGC voltage which tends to reduce
the overall gain. This effect is more pronounced for dark scenes. The net effect is that
S/N ratio further deteriorates and this results in a lot of snow on the picture.
(c) Even when the input signal strength is quite low, a small AGC voltage gets developed
and this reduces the gain of the receiver, when actually, maximum possible gain is
desired for a satisfactory picture and sound output.
To overcome these drawbacks special AGC circuits known as ‘keyed’ or ‘gated’ AGC
circuits have been developed and are used in almost all present day television receivers. The
problem of reduction of gain with weak input signals is resolved by using ‘delayed’ AGC action.
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AUTOMATIC GAIN CONTROL AND NOISE CANCELLING CIRCUITS
Keyed AGC System
In this system, the AGC rectifier is allowed to conduct only during horizontal sync pulse periods,
with the help of flyback pulses derived from the output of the horizontal deflection circuit of
the receiver. Video signal is also coupled to AGC rectifier to produce AGC voltage proportional
to signal strength. However, AGC tube or transistor is generally biased to cut-off so that it
conducts only for the short time the keying pulse is applied. This ensures that the rectifier
conducts only when the blanking and sync pulses are on. As shown in Fig. 15.6 (a) the flyback
pulses are generated during the retrace period of horizontal sweep circuit. Thus the time of
flyback pulses corresponds to the time of sync and blanking, assuming that the picture is in
full synchronization. The gating or AND function means that both inputs must be ‘on’ at the
same time to produce AGC output.
On
Flyback pulses
from H.O.T.
Off
Signal from
video amplifier
Fig. 15.6 (a). Keying pulses at horizontal sync rate for AGC circuit.
+
Delayed
RF AGC
0.22 mF
70 V
R1
8.2 M
C3 R2
2.2 M
400 V P–P
Keying pulses
R5
A
0.001 mF
+
–
Horizontal
output transformer
(H.O.T.)
C1
AGC winding
R3
IF AGC
– 2 V to – 20 V
0.01 mF
C2
R4
B
390 K
180 K
+ 115 V
Discharge path
–
of C1¢
230 K
+
– 25 V
+
56 K
R6
From
video amp
80 V P–P
50 K
R7
330 K
140 V
AGC control
Fig. 15.6 (b). Typical triode keyed AGC circuit.
The basic circuit of a keyed AGC system employing a triode is shown in Fig. 15.6 (b).
Video signal is directly coupled from video amplifier to the grid of AGC tube. Because of dc
coupling the grid is at + 115 volts and so the cathode is maintained at + 140 volts to develop a
grid bias voltage equal to – 25 V (115 – 140). Without any video signal on the grid the tube
stays beyond cut-off. No dc potential is applied to the plate of the triode, but instead, positive
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MONOCHROME AND COLOUR TELEVISION
going flyback pulses derived from the horizontal deflection output transformer are fed through
C1, which drive the plate positive during the sync pulse periods. The video signal of large
amplitude drives the grid positive and clamps it at almost zero potential when the signal rises
towards blanking level, so that the tube can conduct as the plate is pulsed positive at that
time. When plate current flows, it charges C1 with the plate side negative. The path for the
charging current includes cathode to plate circuit in the tube, C1 and AGC winding on the
horizontal output transformer (H.O.T.). Between pulses when the tube does not conduct, C1
partially discharges through the path marked on the diagram to charge C2, the AGC filter
capacitor. The time constant of the filter circuit is much larger than 64 µs and therefore a
relatively steady dc voltage is developed to serve as the AGC source. Since the tube conducts
only during horizontal sync pulse periods, even when the vertical sync pulse train arrives,
AGC voltage developed across the points A and B represents true signal strength and has no
tendency to vary during vertical sync periods. The potentiometer R7 is adjusted for optimum
grid bias. The function of R6 is to isolate the AGC tube form the video amplifier which supplies
video signal. The chain of resistors (R1 through R4) is for delayed AGC action which is explained
in a subsequent section of this chapter.
Transistor Keyed AGC
A basic keyed AGC circuit designed to develop a positive AGC voltage is shown in Fig. 15.7.
The p-n-p transistor Q1 is biased to cut-off under no signal conditions. Negative going retrace
pulses are applied to the collector circuit. The base-emitter junction gets forward biased when
the video signal at the base approaches its minimum value. This corresponds to sync pulse
periods and it is then that the collector is pulsed ‘on’ by the flyback pulses. The keying pulses
are fed to the collector of the keyer transistor via diode D1. When the transistor conducts,
current flow through R1, R2, Q1, D1, winding on H.O.T. and C1 thereby charging it with the
polarity marked on it. The voltage developed across C1 is the AGC output voltage. Diode D1
prevents discharge of C1 during the time between keying pulses. In the absence of D the charge
on C1 will forward bias the collector to base junction of Q1 and allow the capacitor to discharge
resulting in loss of AGC voltage. However, the diode has the correct polarity to couple negative
flyback pulses from AGC winding to the collector of the transistor. The capacitor C1 and resistors
R3 and R4 form a filter to develop a steady dc voltage for controlling overall gain of the receiver.
Flyback pulses
AGC
voltage
+ 1.5 V to + 4 V
R4
25 V P–P
R3
5.6 K
6.8 K 10 mF
Winding on D
1
4.7 K
+ H.O.T.
Q1
C1
–
+ 10 V 3.2 V + 2.8 V
to 3.7 V
to 3.3 V
4K
3K
Pot R
2
AGC level
370 W
R1
Fig. 15.7. Typical transistor keyed AGC circuit.
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From
video amp
2 V P–P
AUTOMATIC GAIN CONTROL AND NOISE CANCELLING CIRCUITS
285
It may be noted that in comparison to vacuum tube circuits, the peak-to-peak amplitude
of the keying pulses and video signal is much smaller. Typical transistor circuit values are 25
V peak-to-peak for flyback pulses and 2, V p-p for the video signal. For an n-p-n transistor the
polarity of both the inputs is opposite to that needed for a p-n-p transitor.
15.6 MERITS OF KEYED AGC SYSTEM
(a) A long time constant to filter out 50 Hz ripple is no longer necessary because conduction
takes place only during the horizontal retrace periods and no undue build up of
voltage occurs during vertical sync intervals. The relatively short time constant filter,
used to remove 15625 Hz ripple, enables the AGC bias to respond to flutter and fast
change of stations, thereby ensuring a steady picture and sound output.
(b) AGC voltage developed is a true representation of the peak of fixed sync level and
thus corresponds to the actual incoming signal strength.
(c) Noise effects are minimized because conduction is restricted to a small fraction of the
total line period.
15.7 DELAYED AGC
The picture produced on the raster should be as noise free as possible. This is achieved by lownoise circuits. However, despite careful circuit design, each stage in the receiver contributes
some noise. The cumulative effect of this, if not checked, would be a noisy picture. Noise effect
can be overcome by high amplification of the incoming RF signal.
The amplified signal will then swamp out effects of stage noise as it is processed by the
receiver. It is, therefore, necessary to operate the RF amplifier at maximum gain, particularly
for weak RF signals. In the circuits discussed so far, the AGC voltage is fed not only to the first
and second IF amplifiers, but also to the RF amplifier. Thus the negative AGC bias would
reduce the gain of RF amplifier even for low-level RF signals. This undesirable effect is overcome
by delaying the AGC voltage to the RF amplifier for weak RF signals. The technique used
which is a delay in voltage, not in time, is called delayed AGC.
Delayed AGC Circuit
In Fig. 15.6 (b) delay action is achieved through the voltage divider R1, R2, R3 and R4 tied
between B + supply and ground. This places the RF AGC take-off point (grid of the RF amplifier)
at approximately + 70 V to ground. Since the cathode of the RF amplifier tube returns to
ground, the grid to cathode circuit acts like a diode and is forced into conduction. Since a
conducting diode may be considered to be a short, the AGC take-off point is clamped to
approximately zero volt because the cathode of the amplifier returns to ground. The bias clamp
will continue to conduct until the input signal becomes sufficiently strong to provide a negative
AGC voltage which is large enough to overcome the forward bias applied to the bias clamp
diode and turn it off. This will restore normal AGC action to the RF amplifier for input signal
amplitudes higher than a predetermined level. It can be set by varying the potentiometer R7.
Another method of obtaining delayed AGC is to use a separate diode for clamping action.
This method is explained later while discussing a typical tube AGC circuit.
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MONOCHROME AND COLOUR TELEVISION
15.8 NOISE CANCELLATION
The need for minimizing noise set-up in AGC and sync separator circuits has given rise to
several methods of noise cancellation. Three commonly used methods are described. In
Fig. 15.8 (a) diode D2 is used as a switch which opens in the presence of noise preventing it
from reaching the video amplifier. The necessary forward bias enabling it to pass noise free
video signals is set by the potentiometer R3. When a strong noise signal arrives the diode gets
reverse biased thereby stopping noise pulses from reaching the video amplifier. Since the video
signal to both AGC keyer and sync separator is obtained from the output of video amplifier,
noise pulses are prevented from reaching these circuits.
Noise
pulse
To sync
separator
Bias
D1
L1
0
0
Video
amp
D2
Video detector
L2
Noise
gate
From last
video
IF amp
R1
+
R2
To picture
tube circuit
R4
To keyed AGC
circuit
C1
R3
Noise gate
level
Fig. 15.8 (a). Diode noise gate circuit.
In the noise cancellation circuit of Fig. 15.8(b) the signal inputs to the control grid (G1)
suppressor grid (G3) and plate (P) of a pentode are applied in such a way that the tube conducts
only if all the three inputs are present at the same time. This arrangement is often referred to
as a coincidence gate or an AND gate. Noise cancellation is achieved by setting the control grid
bias in such a way that it goes to cut-off when a strong noise pulse arrives.
Special tubes with sharp cut-off characteristics have been developed for use in TV
receivers and 6HA7 is one such tube—a twin pentode, where one section is for AGC and the
other for sync separation. As shown in Fig. 15.8 (b) the cathode, control grid and screen grid
are common to both the sections. The control grid serves as the noise gate for both the circuits.
However, there are two suppressor grids and two plates to separate sync output from one
pentode and AGC voltage from the other.
Another noise cancellation circuit is shown in Fig. 15.8 (c), where noise is eliminated by
using a separate noise gate. It separates noise from the composite video signal, amplifies it
and then adds it to the inverted composite video signal. The noise gate is a grounded base
amplifier, normally set to cut-off. Any incoming noise pulse of sufficient amplitude counteracts
the fixed reverse bias and sets the amplifier into conduction. Thus the noise pulse is amplified
without inversion of its polarity. The gain of the noise gate amplifier is set equal to that of the
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AUTOMATIC GAIN CONTROL AND NOISE CANCELLING CIRCUITS
video amplifier. Since the two noise pulses are equal in amplitude but opposite in polarity,
they cancel on addition. Thus the AGC and sync separator circuits remain immune to incoming
noise pulses. This system is frequently used in solidstate receivers.
Plate +
voltage
Keying
pulses
+
AGC
bias circuit
VG3(1)
+ 95 V
From plate
of video amp
VG1
From grid of
video amp
C2
+
–
RL
C1
Ip1
P1
G3(1)
250 V
Sync
output
70 V
P–P
N
G3(2) + 110 V
80 V
P–P
From video
amp
+ 170 V
0
0
VG2
K
+
Noise pulse N
(N)
VG3(1) +
P2
G1
700 V
P–P
t
0
+ 170 V
G2
Keying
pulses
t
N
3V
P–P
N
135 V
VG1 –
+
I
Noise
gate
N
Cut-off
p1
Plate current
+ 135 V
t
0
Fig. 15.8 (b). Keyed AGC and sync separator circuits with a common noise gate.
Noise pulse
No noise pulse
Videop
amp
Keyed
AGC
AGC
output
Keying pulses
Noise gate
(Grounded
base amp)
Noise pulse
only
Fig. 15.8 (c). Noise cancellation by a separate noise gate amplifier.
15.9 TYPICAL AGC CIRCUITS
As already explained there is a considerable difference between the methods of controlling
gain of vacuum tube and transistor amplifiers. Transistor keyed AGC circuits are complex
than their tube counterparts. In some circuits three or more transistors are used to develop
the required AGC voltage. There is no standard approach to such circuits and they may vary
from chassis to chassis. Some typical AGC circuits employing tubes and transistors are discussed.
Keyed AGC with a Twin-Triode
The keyed AGC circuit shown in Fig. 15.9 employs a twin-triode for generating AGC voltage.
One section (V1) of the tube is used for developing a dc voltage proportional to the peak value
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MONOCHROME AND COLOUR TELEVISION
of the input video signal. The second triode (V2) is connected for keyed AGC action. The video
signal is dc coupled to the grid of V1 through a frequency compensating network. It is connected
as a cathode follower with R3 as its load resistance. The time constant of R3 in parallel with C3
is chosen to be quite large as compared to the line period of 64 µs. Therefore, the voltage which
develops across this network is a dc voltage proportional to the peak value of sync pulses. This
is direct coupled to the grid of V2 which conducts during flyback pulse intervals to charge the
capacitor C2. In between sync pulses C2 discharges to develop a negative voltage at point A
with respect to ground. The potentiometer R5 is varied to adjust optimum bias voltage for V2 to
permit sufficient conduction for developing suitable AGC voltage with a known input signal
strength. Note that any strong noise pulse will make G2 positive with respect to K2 causing
clamping action. Thus noise pulses are prevented from developing any AGC voltage. The
resistors R7 and R8 form a potential divider and the voltage which develops across the filter
circuit R8–C5 is fed to the IF section of the receiver.
Flyback pulses
From video amp
400 V P–P
50 V P–P
B+
+
R1
56 K
C1
82 PF
–
P1
C2
A
250 V
Delayed
AGC
V2
R4
G1
K1
G2
R5
AGC
level
C3
12 K
R7
100 K
D1
K2
Cin
.01 mF
4.7 M
B
R9
P2
V1
R2
100 K
R10
R3
470 K
C4
1 mF
R6
C6
10 K
Pot
15 K
2 mF
IF AGC
R8
330 K
C5
2 mF
Fig. 15.9. Keyed AGC with delay circuit for RF amplifier.
The values of resistors R7 through R10 are so chosen that at low RF signal levels D1
remains forward biased from B + supply. Thus point B of the AGC circuit is clamped to ground
and no AGC voltage gets applied to the RF amplifier. Any change in the negative potential at
A affects the potential at point B through the isolating resistor R9. When the input signal
strength increases, say when another strong channel is selected, point A becomes more negative
with the result that at a predetermined voltage level the potential at B changes to become
negative. This reverse biases diode D1 removing its clamping action. Thus full AGC voltage
becomes available on the RF amplifier AGC bias line. This then amounts to a delay in applying
AGC to the tuner section of the receiver.
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AUTOMATIC GAIN CONTROL AND NOISE CANCELLING CIRCUITS
AGC Circuit for Solid-State Receivers
A typical circuit is shown in Fig. 15.10 where transistor Q1 is keyed to develop AGC bias and
Q2 serves as the AGC amplifier. The video signal is dc coupled at the base of Q1 and its emitter
is fed with a suitable dc voltage to reverse bias the emitter-base junction in the absence of any
video signal. The flyback pulses are fed at the collector of this transistor through the diode.
The pulsed current completes its path back to the winding on the H.O.T. through C1 which
then charges with positive polarity towards ground. R5-C1 acts as the AGC filter and this
voltage is dc coupled to the base of Q2. The forward-bias on the emitter-base junction of this
transistor varies with the voltage across C1 and thus controls the collector current through
load resistance R10. A strong input signal at the antenna will develop more negative voltage
across C1, thereby, reducing the forward bais on Q2. This in turn will decrease its collector
current making the collector more positive. The receiver employs forward AGC control on the
RF and IF amplifiers and the voltage developed at the collector is then of the right polarity to
decrease gain of these stages. R7-C3 constitute an additional filter circuit for the RF bias line.
R8 is the isolating resistance and R9-C4 acts as filter for IF bias line. Another transistor can be
added to this circuit to achieve delay action for the RF amplifier.
Flyback pulses
+ 27 V
+ VCC
8V
P–P
From
video amp
8V
P–P
Driver
+
R11
R10
47 K
Keyer
Q1 (BC 147)
33 K
200 V R1
1K
047 mF
Winding on
H.O.T.
R2
C2
R3
2.5 mF
330
–
C
+ 1
R4
1.5 K
Q2 Amplifier
(BC 147)
R8
R
6
R5
120
4.7 K
3.3 K
K Pot
R7
22 K
680
R9 C
4
RF amp bias
2 to 5 V
+
C 25mF
– 3
+
25mF
–
IF amp bias
4 to 12 V
Fig. 15.10. Keyed automatic gain control circuit with AGC amplifier.
Improved AGC Circuit
An improved AGC circuit is shown in Fig. 15.11. The bias for the two IF amplifiers is developed
by Q2, the AGC ‘keyer’. For the RF amplifier, bias is supplied by Q3, the AGC driver. It will be
seen that there are different modes of AGC operation, depending on the level of incoming
signal. On very low signals, no AGC bias is developed, and the controlled IF and RF stages are
fixed biased. On medium-level signals the AGC keyer turns on and biases the two IF amplifiers.
As the signal level increases further the AGC driver is also turned on, biasing the RF amplifier,
while the bias on the controlled IF amplifiers continues to increase. A point is reached beyond
which the bias of the IF amplifiers is clamped. When the incoming signal level becomes very
high, both RF bias and IF bias again increase to reduce the gain of the controlled amplifiers.
The circuit operates in the following manner. The negative going (sync) composite video
signal from the video detector is fed to the video driver Q1 (p-n-p) which is connected as an
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MONOCHROME AND COLOUR TELEVISION
emitter follower and develops at its emitter a negative-going composite video signal,
superimposed on a + 3.8 V dc level. This is dc coupled to the base of Q2 through R15, an isolating
resistor. The AGC keyer Q2 is normally at cut-off. This is so because its emitter-base junction
is reverse biased and no dc voltage is applied at its collector. The transistor Q3 is also normally
gated to cut-off. Therefore, with no or on very low-level RF signals, both Q2 and Q3 remain in
cut-off.
Q4 1st video
IF
R14
R11
C5
R20
Q5
C8
91
2nd video
IF
R6
C7
C6
470
R7
4.2 V
+ 3.5 V
Negative keying
pulses
From
video detector
3V
Video
driver
Q1
3.8 V
R18
D1
1.5 V
L1
C4
50 mF
R17
R19
R16
R3
R4
+ 12 V
R10
D4
1.5 K
Delay
Adj
Winding on
H.O.T.
AGC
Q2 keyer
150 W
R15
5.6 K
R13
R12
1.5 K
+ VCC
12 V
D2
82 W
4.5 V
D3
R2
560
+ 3.5 V
100
R1
680
+ VCC
12 V
4V
+
–
0.5 V
VDR1
C1
4 mF
R8
AGC
Q3 driver
4.7 K
+ 1.5 V
Delayed AGC
to tuner
R9
C3
Decoupling
network
R5
680
C2
3 mF
Fig. 15.11. Improved AGC circuit. Note that the voltages shown are with no signal applied.
The series dc voltage divider consisting of R1 through R5 develops about + 3.5 V at the
junction of R3 and R4 from the 12 V dc supply. This potential is coupled via diode D2 to the base
of transistor Q5 in the second IF amplifier. Bias for the base of Q4 (first video IF amplifier) is
derived from the junction of R6 and R7 located in the emitter lead of Q5. The operating points
of both Q4 and Q5 are chosen for forward AGC control.
From the same VCC source another circuit is completed through D3, VDR1 and R8, thereby
providing about 4 V across C1 and 0.5 V at the base of Q3. The voltage drop across R5, the
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AUTOMATIC GAIN CONTROL AND NOISE CANCELLING CIRCUITS
291
emitter resistance of Q3 is about 1.5 V on account of current which flows through it from the
VCC source. This is enough to keep Q3 in cut-off. The voltage drop across R5 (1.5 V) is fed as bias
voltage to the RF amplifier through the decoupling network R9-C3. Thus with very low input
signal levels the IF and RF amplifiers operate at a fixed bias chosen to provide maximum gain.
When the incoming RF signal level increases sufficiently the negative sync pulse
amplitude at the emitter of Q1 overcomes the reverse-bias on the emitter-base junction of Q2.
As a result, the negative-going horizontal keying pulses injected in the collector circuit of Q2
from the H.O.T. cause collector current flow during sync pulse intervals. This results in a
higher voltage across C1 and thus D2 is reverse biased. The increased potential on C1 is now
coupled through R10 and R11 to the base of Q5. The enhanced forward bias on Q5 while increasing
its collector current takes it to a region of reduced power gain. Similarly the increased voltage
drop across R7 is enough to shift the operating point of Q4 to reduce its gain. Thus the gain of
both, 1st and 2nd video IF amplifiers is reduced. Note that the increased positive AGC voltage
appearing across C1 is prevented from reaching the emitter of Q2 by diode D3 and will therefore
not effect the current flow in this transistor. Similarly diode D1 which couples keying pulses to
the collector of Q2 prevents application of positive AGC voltage in its collector circuit. For the
signal levels just considered the AGC driver transistor Q3 remains cut-off and no additional
AGC bias for the RF amplifier is developed.
When the incoming RF signal level increases sufficiently, the large positive AGC voltage
developed across C1 decreases the resistance of the voltage dependent resistance VDR1, with
the result that a higher dc voltage is applied to the base of Q3. This turns on Q3 and the
resultant increased positive voltage across its emitter resistance (R5) furnishes additional RF
AGC bias. Potentiometer R12 sets the collector voltage level of Q3 thus determining the condition
for conduction of this transistor. R12 therefore acts as the tuner AGC delay control.
With still higher signal levels, IF AGC voltage will increase unitl it becomes 0.5 V more
positive than the voltage across R12. D4 then conducts clamping the IF AGC voltage at that
level. Any added increase in the received RF signal will now cause the RF amplifier AGC
voltage to increase rapidly. Finally on very strong signals, when the tuner AGC voltage exceed
7 V, Q3 conducts heavily causing the voltage drop across R5 to rise rapidly. The additional
voltage drop across R5 raises the potential at point A to such a level the diode D2 again gets
forward biased. Thus the IF AGC will again increase causing further reduction of gain in the
IF section.
15.10 AGC ADJUSTMENTS
Some receivers do not have any AGC adjustments. Other receivers have as many as three
adjustments that affect the operation of the AGC system. In case a noise cancellation control
forms part of the AGC circuit, it should be first advanced till the noise cancellation circuit
becomes inoperative. This will ensure that no sync inversion occurs while other adjustments
are made. After all other AGC adjustments have been made the noise threshold control should
be advanced to the point of most stable sync.
Many receivers have a tuner AGC delay control. This control should be adjusted on
medium-strength signals. As the tuner delay is increased the signal will become noise free.
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MONOCHROME AND COLOUR TELEVISION
With too much delay, strong signals will cause overloading of amplifiers. Indications of overload
are buzz in the sound and bending of the picture.
The third type of control is an AGC level control, sometimes called the ‘threshold control’.
This adjustment sets the voltage to which the sync tips must rise to give AGC action. The
effect of this control is to adjust the detector level. In the absence of service notes the output
from the detector must be estimated. This normally lies between 2 to 5 volts. However,
manufacturers’ instructions, if available, should be followed for all adjustments.
AGC Circuit in an IC Chip
Weak signal sections of the receiver which commonly use integrated circuits include video IF,
video detector, AGC, AFC, video preamplifier and sound strip. The various functions performed
by one such IC (TBA 890) were discussed in the previous chapter. The BEL CA3068 is another
IC which has complete video IF subsystem and tuner AGC for monochrome and colour receivers.
This integrated circuit consists of nearly 39 transistors, 10 diodes, 67 resistors and 18 capacitors.
Figure 15.12 is a simplified block diagram of CA3068. Note that the tuned circuit filters and
decoupling resistors and capacitors are connected externally at the corresponding pins of the
integrated package.
Tuned
filter
9
13
AFT
drive
14
From 6
tuner
1st
IF amp
2nd and
3rd video
IF amp
AGC
to tuner 7
AGC
delay
Keyed
AGC
circuit
Video
detector
Video
amplifier
19
Video
output
3
Keying
pulses
AGC
noise
gate
Fig. 15.12. Simplified block diagram of the AGC section in IC CA3068 (BEL).
The IF output from the tuner is fed at pin 6 to the first video IF amplifier. Horizontal
flyback pulses are needed for AGC action. The dynamic range of the IF AGC is 55 db. Delayed
AGC for the tuner is available at pin number 7, with a AGC voltage variation from 2.2 V to 4.5
V. Besides noise immunity circuits, the IC employs a zener diode as an RF bias clamp to
prevent application of excessive AGC bias. The chip also has a provision for ‘service switch’
which can be used to isolate the AGC stage for fault finding.
Review Questions
1.
What are the advantages of using AGC in television receivers ?
2.
Describe the basic principle of automatic gain control and show how it is applied to tube and
transistor amplifiers.
3.
What is meant by ‘forward AGC’ and ‘reverse AGC’ in transistor tuned amplifier circuits ? Why
is forward bias control preferred to reverse bias method of gain control ?
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AUTOMATIC GAIN CONTROL AND NOISE CANCELLING CIRCUITS
293
4.
What is the basic principle of peak AGC system ? Explain how the control voltage is developed
and applied to IF and RF amplifier stages of the receiver.
5.
What is delayed AGC and how is it developed ? Why is delayed AGC applied only to the RF
amplifier and somtimes to the first IF amplifier of the receiver ? Why is AGC not applied to the
last IF amplifier ?
6.
Describe with a simple circuit the basic principle of a ‘keyed AGC’ system. How does it overcome
the shortcomings of a ‘non keyed’ (peak AGC) control system ?
7.
A keyed AGC system employing tubes is shown in Fig. 15.9. Explain how the AGC voltage is
developed and applied to RF section of the receiver. What is the function of diode D1 ?
8.
Draw circuit diagram of a keyed AGC system employing transistors and having a noise gate.
Explain how the AGC voltage is developed and amplified.
9.
Explain briefly various types of noise gates used to suppress noise pulses in the video signal
before it is applied to the AGC and sync separator circuits.
10. The circuit of an improved keyed AGC system is shown in Fig. 15.11. It is designed to operate in
different modes depending on the level of incoming RF signal. Explain step by step how the
control voltage is developed and applied to RF and IF sections of the receiver.
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16
Sync Separation Circuits
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16
Sync Separation Circuits
The synchronising pulses generally called ‘sync’ are part of the composite video signal as the
top 25 percent of the signal amplitude. The sync pulses include horizontal, vertical and
equalizing pulses. There are separated from the video signal by the sync separator. The clipped
line (horizontal) and field (vertical) pulses are processed by appropriate line-pulse and fieldpulse circuitry. The sync output thus obtained is fed to the horizontal and vertical deflection
oscillators to time the scanning frequencies. As a result, picture information is in correct position
on the raster. The sequence of operations is illustrated in Fig. 16.1 by a block schematic diagram.
From video detector
Video
amp
Composite
video signal
To cathode of
picture tube
H
Sync separator
and noise
cancellation
circuit
V
Integrator
Vertical deflection
oscillator and
sweep generator
(50 Hz)
Sync
pulses
only
Differentiator
Horz
AFC
Horz deflection
oscillator and
sweep generator
(15625 Hz)
DC control voltage
Vertical
deflection
amplifier
Horz
deflection
amplifier
50 Hz
To vertical
deflection
coils
15625 Hz
To horizontal
deflection
coils
Fig. 16.1. Block diagram of the sync separator and deflection circuits in a television receiver.
16.1 SYNC SEPARATOR—BASIC PRINCIPLE
The problem of taking off the sync pulses from the video waveform is a comparatively simple
one, since the action consists of merely biasing the device used in the circuit, in such a way,
that only the top portions of the video signal cause current flow in the device. This is readily
achieved by self-biasing the tube or transistor used in the circuit.
Two basic circuits, one employing a tube and the other a transistor are shown in Fig. 16.2
to illustrate this method of sync separation. Self-biasing or automatic bias means that the dc
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296
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SYNC SEPARATION CIRCUITS
bias voltage is produced by the ac signal itself. The requirements are to charge the input
capacitor by rectifying the input signal while it approaches its maximum value and have an
RC time-constant long enough to maintain the bias on the capacitor between peaks of the ac
input signal. The video signal is normally obtained from the video amplifier and coupled to the
input of the sync-separator circuit. In Fig. 16.2 (a) video signal is fed to the grid of the triode
with sync pulses as the most positive part of the waveform. In the quiescent state there is no
+
+
VPP
RL
Video
input
+
RL
v0
C1
–
ip
C2
–
2 mF
50 K
R1
Fig. 16.2 (a). Basic sync separator circuit
employing a triode.
Dynamic
characteristics
+
Video
input
0.1 mF
1M
VCC
R2
Fig. 16.2 (b). Basic sync separator circuit
employing a transistor.
iC
Collector current
Saturation pulses
O
Forward bias
Dynamic
characteristics
Plate current
pulses
Cut-off
Reverse bias
GK
v0
t
O
Sync pulse tips
which cause
grid current
flow.
Se -b as
Self-bias
t
Horz sync
pulses
Input
composite
video signal
t
Effective
bias line
Slicing level
Fig. 16.2 (c). Illustration of tube circuit operation. Fig. 16.2 (d). Illustration of transistor circuit operation.
bias on the tube. On the arrival of the signal, first few pulses drive a heavy grid current and
the capacitor C1 quickly charges up with the grid side negative. Between peaks of the input
signal, C1 discharges slightly through R1. The R1, C1 time constant is made large enough to
keep C1 charged to about 90 percent of the peak positive input. The effect is to develop an
automatic negative bias so that the operating point sweeps back form VGK = 0 to a point well
beyond cut-off. After a few pulses the tube settles down into a steady bias such that it is
completely cut-off except during the positive going sync pulses. The tube then operates under
‘class-C’ condition i.e., biased well beyond cut-off. This is illustrated in Fig. 16.2 (c). To maintain
a steady bias, the sync tip levels of the video signal make the grid slightly positive every cycle
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MONOCHROME AND COLOUR TELEVISION
to cause a small grid current flow, which quickly replenishes the charge lost by the capacitor
through the grid leak resistance R1. The average negative bias developed across the capacitor
C1 then controls the plate current during sync pulse periods, which in turn gives rise to
corresponding negative-going voltage pulses at the plate. Between pulses, the plate voltage
rises to + VPP since there is no voltage drop across the plate load resistor.
The corresponding transistor circuit is shown in Fig. 16.2 (b) where C2, R2 coupling
provides self-bias between the base and emitter of the transistor. The capacitor charges because
of the base current that flows during sync amplitude levels of the composite video signal. The
circuit operation is illustrated in Fig. 16.2 (d) where the negative voltage developed across C2
reverse biases the emitter-base junction in such a way that only positive sync voltage drives
the transistor into conduction to produce sync output in the collector circuit. The sync amplitude
varies between VCC and collector voltage corresponding to the maximum collector current.
The bias voltage values in transistor circuits are less than in tube circuits because the
base-emitter junction requires only a fraction of a volt as forward bias, to produce collector
current output. The RC time constant is the same (about 0.1 second) as in tube circuits but
with large C and small R because of the lower input resistance of a transistor.
16.2 SYNC SEPARATOR EMPLOYING A PENTODE
The triodes have relatively large interelectrode capacitances and therefore their performance
in sync-separator circuits is inferior to that of pentodes. Accordingly a pentode is preferred to
a triode in such circuits. A typical sync separator employing a pentode is shown in Fig. 16.3.
B+
80 V P–P
150 K
56 K
270 K
2000 PF
R3
From video 8.2 K
amplifier
C1
0.1 mF
C2
C5
R4
C3
– 40 V
75 PF
C4
To vertical
oscillator
3900 PF
R2
270 K
R1
1M
0.1 mF
15 K
To horizontal
AFC
Integrating circuit
Fig. 16.3. Vacuum tube (pentode) sync separator circuit. Note that the
grid bias voltage with a normal input signal is about – 40V.
The components and DC source voltage are so chosen that both plate and screen-grid voltages
are low. This limits the dynamic region of the tube to a narrow range, with a cut-off bias of the
order of about 5 volts. With the input voltage on, the plate voltage attains a minimum value
during sync tip intervals. The actual minimum plate voltage obtained, and hence the sync
peak-to-peak amplitude, depends on the value of plate load resistance and peak plate current.
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SYNC SEPARATION CIRCUITS
It should be noted (see Fig. 16.2 (c)) that the effect of driving the tube into grid current on each
sync pulse tip is to clamp the sync pulse tops to zero (ground) potential, so that the sync tips
align at the same level despite wide variations in the inter-pulse periods. This in effect amounts
to restoration of dc component of the video signal, which is lost due to the presence of series
coupling capacitor C1.
One may ask as to why the video signal is not dc coupled from video amplifier to syncseparator. The reason is that changes in dc conditions, both because of variations of the average
brightness of the scene and signal amplitude, when stations are changed, are bound to take
place over a period of time and would make it very difficult to arrange for a constantly efficient
sync separator action. The ac coupling, with its own automatic and flexible dc restoration
function, provides the signal in the form, where it becomes easy to slice-off sync pulses of equal
amplitude. The resistance R3 provides isolation between the sync separator and video amplifier
circuits. Because of saturation occurring at the sync tip level of the video signal, any noise
resting on the sync tips does not get reproduced in the output.
16.3 TRANSISTOR SYNC SEPARATOR
A sync separator employing an n-p-n transistor is shown in Fig. 16.4. The capacitor C1 tends to
charge up to the peak input signal voltage less the base-emitter forward voltage drop. There is
a marked difference in the signal voltage amplitude necessary to drive tube and transistor
sync separators, since a transistor requires only about 0.6 V or so at the base to produce collector
current output. An input video signal of the order of 5 to 10 V is all that is necessary to give rise
to output sync pulses whose amplitude approaches about 50 volts peak-to-peak.
50 V P–P
10 V P–P
4700 PF
0.22 F
From collector
of the video
amplifier
R3
C1
C4
47 K
R3
C2
– 2.7 V
R2 3.3 K
R1
+
820 K
15 K
22 K
330
PF
C3
To vertical
oscillator
330 PF
+ 130 V
24 V
To
horz AFC
Integrating circuit
Fig. 16.4. Transistor sync separator circuit. Note that the reverse
bias voltage with a normal input signal is about – 2.7V.
As shown in Fig. 16.4 a transistor sync separator often employs a fairly high value of
collector load resistance with the result that the transistor bottoms (saturates) at a very low
value of base (input) current. Full amplitude sync pulses are thus developed for a wide range
of input signal strength levels. In general, a higher value of load resistance reduces the peak
drive current needed to bottom the transistor but leads to broadening of the output pulses.
This is due to charge storage effect, where the charge carriers stored in the base region take
longer to dispel, if the collector load resistance is increased. This necessitates the use of
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MONOCHROME AND COLOUR TELEVISION
transistors having fast switching capability which implies that the transistor must have a
high upper cut-off frequency. In sync separators which employ switching transistors, a small
forward bias is sometimes given to ensure that tips of the pulses drive the transistor well into
the bottoming condition to produce good clean output pulses.
The factors which must be kept in view while designing a transistor sync separator can
be summarized as follows:
(i) To achieve a reasonable voltage gain the β of the transistor should be large.
(ii) A transistor with a small output leakage current must be chosen because the leakage
current lowers the collector voltage and thus reduces net amplitude of the sync output.
(iii) To ensure a steep front edge of sync pulses a high frequency transistor is desirable.
(iv) Since the transistor is off most of the time a low power transistor can be employed.
16.4 NOISE IN SYNC PULSES
Noise pulses are produced by ignition interference from automobiles, arcing brushes in motors,
and by atmospheric noise. The noise is either radiated to the receiver or coupled through the
Noise pulse
Video
input
Excessive
grid/base current
Chassis potential
Clipping level
0V
Average bias
Grid
or base
v0
t
0
Noise pulse
Duration of blocking
Fig. 16.5 (a). Effect of a strong noise pulse on sync output.
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SYNC SEPARATION CIRCUITS
200 PF
Video
input
C2
R2
270 K
0.1 mF
C1
Sync
separator
R1
Sync
output
1M
Fig. 16.5 (b). Double time-constant bias circuit at the input of a sync separator.
power line. Especially with weak signals the noise can act as false synchronizing pulses.
Furthermore, when noise pulses have much higher amplitude than the sync voltage, large
grid/base current flows which charges the coupling capacitor to a much higher voltage than is
normal and this results in a noise set-up. Because of the long time constant of self-biasing
network, the sync separator is held much beyond cut-off for a period which depends on the
amplitude and width of the noise pulse and the time constant of the input circuit. As shown in
Fig. 16.5 (a) the sync separator gets blocked and there is weak or no sync output till the bias
returns to its normal value. During strong noise periods, the picture does not hold still until
synchronization is restored again. Thus in order to reduce the effect of noise, sync circuits
generally employ one or more of the following techniques :
(i) Double Time-constant for Signal Bias
The time constant of the grid/base leak-bias circuit, at the input of sync separator, must be
long enough, to maintain bias from line to line and through the time of vertical sync pulses in
order to maintain a constant clipping level. As stated earlier, a time constant of the order of 0.1
second is adequate for this purpose. But such large values would result in long blocking on
strong noise pulses. In a similar way too long a time-constant will tend to increase the negative
bias then its normal value during vertical sync intervals when the composite video signal
voltage stays close to its peak value. This results in shortening of horizontal signal pulses soon
after the vertical pulse train during each field. However, if the time constant is made too short
to overcome the above drawbacks, this may not maintain bias between sync pulses, specially
during the vertical sync pulse time. The result may be inadequate sync separation during and
immediately after the vertical sync pulses. Therefore the problem of reducing the effect of high
frequency noise pulses without changing the time constant of the average bias network is
solved by providing a double time constant circuit at the input of the sync separator. The
circuit configuration is shown in Fig. 16.5 (b). It may be noted that the two sync separator
circuits described earlier (Fig. 16.3 and Fig. 16.4) also have double time constant circuits at
their inputs. The network R1,C1 provides the normal grid/base leak-bias, with a time constant
of 0.1 second for the sync signal. The small capacitance C2 (200 pF) and resistance R2 (270 KΩ)
provide a short time-constant. The double time constant thus provided enables the negative
bias to change quickly to reduce the effect of noise pulses in the input to the sync separator.
The capacitor C2 being small can quickly charge when noise pulses produce grid/base current
thus increasing the bias for noise. The change in voltage across C1 and C2 is inversely
proportional to their capacitance values. Therefore a noise pulse will charge C2 to a voltage
500 times more than that across C1. Since R2C2 time constant is 50 µs, C2 can discharge through
R2 between sync pulses. Thus the bias is maintained at the normal value for sync pulses.
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MONOCHROME AND COLOUR TELEVISION
Another advantage which accrues by the addition of a short time-constant circuit is that
it acts like a frequency response compensator and maintains the sharp rise time of input sync
pulses.
(ii) Sync Clipper after Sync Separator
The sync output limited by saturation does not result in sharp sync pulses. The clipping level
is also not uniform because of shift in self-bias caused by sharp noise pulses. Many sync-separator
circuits have a sync clipper stage where sync output of the separator is clipped and amplified.
The purpose is to provide sharp sync-pulses with equal and high amplitude, free from noise
pulses and without any camera signal. Clipping in successive stages allows the top and bottom
of sync pulses to remain sharp and prevents noise pulses from having higher amplitude than
the sync.
(iii) Use of Noise Cancellation Circuits
The use of a double time constant circuit as a noise suppressor is only suitable for non-recurring
noise. If noise pulses are periodic, the small charge on C1 (see Fig. 16.5 (b)) contributed by each
noise pulse will be cumulative and noise set-up will still occur. Therefore, in many sync separator
circuits some form of noise suppression switch is provided. Several such noise cancellation
circuits which were described in the previous chapter along with AGC circuits are also used in
sync separator circuits.
16.5 TYPICAL TUBE SYNC SEPARATOR CIRCUIT
The separator circuit shown in Fig. 16.6 employs a pentode as a sync-separator followed by a
triode which provides gain and ensures sharp edged sync output. The video signal is coupled to
B+
220 K
R5
R4
4.7 KPF
82 K
8.2 K
R8
C3
470 PF
0.01 mF
From video C
1
amplifier
2.2 M
circuit
R7
R3
V1
R2
120 K
8.2 K
110 V
C5
1.5 KPF
To AFC
circuit
V2
470 K
–1V
C2 – 6 V
R1
L1
To vertical
sweep oscillator
C4
2.5 mF
R6
6.8 K
Fig. 16.6. A typical vacuum tube sync separator circuit.
the separator through a cathode follower from the video amplifier. A double time-constant
circuit is provided at the input to reduce the effect of noise pulses on clipping level. The triode
also operates on self-bias developed through network R3, C3, which couples the pentode to the
triode. Since the amplitude of the sync output at the plate of the pentode is almost steady, the
time-constant R3, C3 has been chosen to be quite small (2.5 ms). This is adequate, both for a
steady auto-bias generation and suppression of occasional excessive noise pulse amplitudes.
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SYNC SEPARATION CIRCUITS
The load of the triode consists of a small inductor L1 and an 8.2 KΩ resistor in series.
Sync pulses to the vertical oscillator are fed from plate of the triode whereas the input of the
horizontal automatic frequency contol (AFC) circuit is connected to the junction of L1 and
resistance R7. The sharp peaked pulses that develop across the inductor when triode conducts
are of the desired shape and amplitude for feeding to the AFC circuit. The coil L1 is shunted by
a resistance R8 to suppress onset of self oscillations when shock excited by sharp plate current
pulses.
In some AFC circuits, discussed in the next chapter, a balanced sync pulse output is
necessary. This can be easily obtained by inserting a suitable resistor in the cathode lead of the
triode. The two outputs, one at the plate and the other at the cathode of the triode are then
180° out of phase with respect to each other.
16.6 TRANSISTOR NOISE GATE SYNC SEPARATOR
A transistorized sync-separator employing a noise gate is shown in Fig. 16.7. The noise gate
transistor Q2 is in series with the emitter of the sync separator Q1. The tansistor Q2 is so
biased that it normally stays in saturation. The sync separator then operates normally and its
emitter current completes its path through Q2, which has only about 0.6 V between its collector
and emitter when saturated. Any sharp noise pulse in the video signal gets coupled to the base
of Q2 through diode D2 and this cuts-off the noise gate transistor Q2. In the absence of any
collector current through Q2, the emitter of Q1 rises to + 20 volts and this blocks the transistor
Q1. Hence no sync separation occurs during the noise pulse interval. It may be noted that
normal amplitude of the negative video signal fed at the input of Q2 is not sufficient to turn if
off. However, if a noise spike with a negative amplitude beyond the sync tip appears, it turns
the noise gate off. However, noise pulses which are not longer than sync tip will not cause the
noise gate to turn off.
+
470 K
N
340 V
180 K
100 K
4.7 K
From
video
amplifier
circuit
Vertical sync
output
Q1
0.056 mF
0.068 mF 0.0068 mF
+
D1
20 V
0.015 mF
39 K
0.0056 mF
27 K
120 K
N
D2
Q2
From video
detector
output
27 K
0.05 mF
Noise gate
transistor
Fig. 16.7. Transistor sync separator with a noise gate.
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Horz sync
output
304
MONOCHROME AND COLOUR TELEVISION
16.7 IMPROVED NOISE GATE SYNC SEPARATOR
The circuit of Fig. 16.8 is another example of a typical sync clipper cum noise invertor. Q1, an
n-p-n transistor, acts as the sync separator. In the absence of any signal, the base-to-emitter
voltage is zero and the transistor is off. When a positive-going signal appears at its input,the
transistor is turned on. As shown along the circuit a 4 V (p-p) positive composite video signal is
applied at the sync input terminals through the isolating resistor R6. This turns Q1 on and
base current flows drawing the vertical sync pulses through C1, D1, emitter-base junction and
D2. The resulting current charges the capacitor C1 with the polarity marked across it. Since
the sync pulse is the highest component of the composite video signal, the voltage developed
across C1 at the end of the vertical sync pulse (which is 160 µs wide) is approximately 4 V.
When the vertical sync pulse passes, the base of Q1 is held negative with respect to the emitter
and it is cut off again. Capacitor C1 tends to discharge through R1 during the interval of 18.84
ms between vertical sync pulses but the time-constant C1R1 is so large (220 ms) that C1 can
discharge only about 8 percent of the voltage across it. When the next vertical sync pulse
arrives, its peak is approximately 8 percent more positive than the change on C1. Thus Q1 is
again turned on during the sync pulse interval and this part of the input signal gets amplified
to develop a 20 V p-p negative going sync pulse at the collector.
22 V
+ VCC
4V P–P
input
D3
5.6 K
1K
0.0056 mF
R6
C2
R9
15 K
D4
0.47 mF
+
C5
R8
R5
5.6 K
C6
3K
To horz
AFC circuit
D1
Q1
1M
Q2
R7
5.6 K
R1
0.1 mF
R3
C3
C1
0.22 mF
220 mF
100 K
R2
D2
0.01 mF
To vertical
oscillator
+
C4
R4
6.8 M
Q1 – Sync separator
Q2 – Noise inverter
Fig. 16.8. Improved transistor noise gate sync separator.
After the vertical pulse interval, the horizontal sync pulses which are 4.7 µs wide, arrive.
Because the charge on C1 is still high to keep Q1 in cut-off, the horizontal sync pulses are
provided another path through C2 to turn it on. Base current again flows during the horizontal
sync pulse interval, charging capacitor C2. In the 59 µs interval between consecutive horizontal
sync pulses, C2 discharges through R2 (R2C2 = 560 µs) by about 12 percent only and thus keeps
Q1 in cut-off state. When the next horizontal sync pulse arrives, it is sufficiently positive to
turn Q1 ‘on’ again. The negative horizontal sync pulses developed at the collector of Q1 are
again of about 20 V peak-to-peak. Thus there are two separate input paths for the vertical and
horizontal sync pulses, ensuring that both are amplified by Q1, while eliminating the video
information between sync pulses.
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The fact that the positive horizontal sync pulse is coupled directly to the base of Q1, back
biases diode D1 during the horizontal sync pulse interval. This prevents the base current of the
transistor from discharging C1. Diode D2 prevents transistor conduction in the reverse direction
when the negativegoing signal adds to the charge on the base input capacitors to exceed the
base-emitter voltage rating.
Noise Invertor. As shown in Fig. 16.8 the noise invertor transistor Q2 together with its
associated circuitry is in parallel with the input to the sync separator. This offsets the effects
of high amplitude noise pulses in the following manner. The diode D4 rectifies the normal
incoming positive going composite video signal and acts as a peak detector on account of the
long time constant of R4C4(R4C4 = 3.2 sec). The resulting dc voltage across C4 is equal to peak
of the sync pulses. This positive voltage gets applied to the cathode of D3 through resistors R8
and R7. In turn this reverse biases D3 under normal signal conditions and no signal appears
through D3 at the base of Q2. It stays in cut-off and does not affect the normal operation of the
sync separator. However, when a sudden noise spike appears as shown in the figure, voltage
across C4 cannot change quickly, and the diode D3 will couple the spike through C5 to the base
of Q2. The transistor Q1 then conducts heavily and effectively shorts its collector circuit to
ground. This results in shorting of the entire input signal to the sync separator, including
video information and sync pulses, during time of the noise spike. As soon as the noise pulse
disappears, however, Q2 cuts off again and the next video signal and sync pulses will be
uneffected. Capacitor C6 prevents any dc short across Q2.
16.8 SYNC AMPLIFIER
An amplifier is not always used before a sync separator. It will depend on the type of transistor
used as a separator and the polarity of the sync pulses at the output of the video detector. If
the video detector output is taken from its anode and its amplitude is sufficiently high, it can
be fed directly to a p-n-p type sync separator. This eliminates the need for a sync amplifier
before the separator. If an n-p-n type transistor is employed, as in Fig. 16.8, it is necessary to
reverse the polarity of a negative-going video signal at the output of the detector before it can
be coupled to the separator. In that case the output of a common emitter video amplifier may
be used as input to the sync separator.
IC Sync Circuit
In some receivers sync-separator function is performed in an integrated circuit. This IC is a
part of the sync-AGC module that combines noise inversion, sync separation and AGC.
Noise cancellation and amplification are first performed in the IC. The resultant noise
free video signal emerges at a particular pin of the IC and is applied to a time-constant network
that is kept outside the IC. This enables the use of different time-constant network to suit a
particular receiver design. The output from the network is fed back to the IC where sync
separation takes place. Over 20 volts of separated sync is available, both positive and negative
going at different pins. The IC also receives gating pulses from the horizontal output circuit for
AGC operation.
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Review Questions
1.
Draw the basic circuit diagram of a sync separator employing a triode with grid leak bias. Comment on choice of the time-constant of the biasing circuit. Why is a pentode preferred to a triode
in tube sync-separator circuits ?
2.
Draw the circuit of a sync separator employing a p-n-p transistor. Show input and output waveforms. Why are high-frequency-transistors with small output leakage current employed in sync
circuits ?
3.
What are the undesirable effects of high amplitude noise pulses in sync-separator circuits ?
Explain how the use of a double time-constant biasing circuit overcomes the effect of noise in
input signal to a sync separator. Why is a sync clipper often employed after the sync separator ?
4.
Explain the operation of sync separator shown in Fig. 16.6. What is the use of inductor in plate
circuit of the triode ?
5.
Draw the circuit diagram of a typical transistor noise gate sync separator and explain its operation.
6.
The circuit of an improved noise gate sync separator is shown in Fig. 16.8. Describe its operation
and in particular explain how the noise invertor transistor Q2. cancels the effect of noise pulses.
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Sync Processing and AFC Circuits
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Sync Processing and AFC Circuits
The receiver has two separate scanning circuits, one to deflect the electron beam, of the picture
tube, in the vertical direction and the other in the horizontal direction. Each scanning circuit
consists of a waveform generator, i.e. oscillator and a power output stage. The synchronizing
pulses obtained from the sync-pulse separator are used to control the vertical and horizontal
deflection oscillators, so that picture tube is scanned in synchronism with the original picture
source at the transmitter. The horizontal sync pulses hold the line structure of the picture
together by locking in the frequency of the horizontal oscillator; and the vertical sync pulses
hold the picture frames locked-in vertically by triggering the vertical oscillator. The equalizing
pulses help the vertical synchronization to be the same in even and odd fields for good interlacing.
17.1 SYNC WAVEFORM SEPARATION
This means separating the vertical and horizontal sync pulses. It is the difference in the pulse
time duration of the horizontal (line) and vertical (field) sync pulses which makes it possible to
separate them. The horizontal sync pulse with a width of 4.7 µs and repeated at 15625 Hz
represents a high frequency signal, whereas the vertical sync pulse with a total width of 160 µs
which repeats 50 times in a second, is relatively a very low frequency signal. Therefore, the
vertical and horizontal sync pulses can be separated from each other by RC filters. A low-pass
filter, connected across the incoming sync pulse train, will develop appropriate trigger pulses
O
Q1 – Emitter follower
Q2 – Sync separator
H
–
2K
From
video amplifier
Q1
t
33 V sync pulses
– VCC
VCC
5 F
H EE
0.047 F
100 K
EE
To vertical
osc
vo1
R1
1K
X
Y
C1
LPF
= 50 to 60 S
Q2
2K
Clipping
level
vo2
C2
HPF
= 0.5 S
To AFC circuit
R2
Spiked horz sync pulses
Fig. 17.1. Separation of vertical and horizontal sync pulses.
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309
for synchronizing the vertical oscillator. Similarly, a high-pass filter will deliver sharp
differentiated pulses for the horizontal oscillator from the same pulse train. This, as shown in
Fig. 17.1, is done simultaneously by feeding output from the sync pulse separator to the lowpass and high-pass filter configurations, connected in parallel. The resistor R1 and capacitor
C1 constitute the low-pass filter, also known as integrating circuit. The high-pass filter, also
known as differentiating circuit, consists of C2 and R2 and has a very small time constant.
The integrated output across C1 that builds up 50 times during one second is used for
triggering the vertical oscillator. However, the spiked (differentiated) output that develops
across R2 is fed to the automatic frequency control (AFC) circuit, the output of which is employed
for holding the horizontal oscillator at the correct frequency. The use of the AFC circuit ensures
correct synchronization even in the presence of noise pulses. The serrated vertical sync pulses
also develop only a spiked output across R2, because the time constant of the circuit R2C2 is
relatively too small to produce any appreciable voltage across R2.
The sync separator shown in Fig. 17.1 is preceded by an emitter follower to isolate it
from the video amplifier. The small amount of forward bias at the base of Q2 ensures good
bottoming and thus clean output sync pulses are fed to the filter circuits. The output waveshapes
from the integrating and differentiating circuits are shown alongside corresponding filter
configurations.
17.2 VERTICAL SYNC SEPARATION
The time constant of the low-pass filter circuit (see Fig. 17.1) is chosen to be much larger than
the width of each serrated vertical pulse. This value is not very critical and a time constant
R1C1 of about ten times the serrated pulse width is adequate. When the combined sync waveform,
beginning with horizontal and equalizing pulses appears at the input of such a circuit, the
capacitor C1 charges along an exponential curve governed by the time constant of the filter
circuit. Since this time constant is very large compared with the duration of the horizontal and
equalizing pulses, the voltage output across C1 during these intervals of the input wave is a
very small fraction of the ultimate value. This is shown in the output waveform of the integrator.
When the trailing edges of the horizontal or equalizing pulses appear, the small charge stored
on the capacitor discharges along an exponential curve governed by the same time constant.
The overall result is a very small toothed voltage across the capacitor, which lasts for a time
comparable to the width of each horizontal or equalizing pulse, and thus the amplitude of the
filter output voltage is negligible during the horizontal and equalizing sync periods.
However, when the vertical sync pulse (serrated) arrives, cumulative charging of the
capacitor occurs, because the duration of each serrated vertical sync pulse is long compared
with the gaps between serrations. Consequently,the charge accumulated from the first input
serration (29.7 µs) has little opportunity to discharge during the following notch (2.3 µs). The
next broad pulse adds to the charge already built-up across the capacitor which again is only
partially discharged during the next gap. The five broad pulses which constitute the vertical
sync pulse thus cause a gradual increase in voltage across the capacitor, with small spikes
superimposed on it. At the output of the low-pass filter, therefore, the voltage rise appears to
be almost smooth corresponding to the vertical pulse. This pulse amplitude is substantially
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MONOCHROME AND COLOUR TELEVISION
greater than the small spikes caused by the horizontal and equalizing pulses. As soon as the
vertical sync pulse has passed the integrated output pulse decays almost to zero during the
post-equalizing pulse period, and stays at this level during the horizontal pulse train that
follows.
The purpose and effectiveness of the equalizing pulses is apparent from the plot shown
in the figure. The inclusion of equalizing pulses, before and after the field pulses, ensures
identical integrated output despite insertion of field waveform in the middle of one line for one
field, and at the end of the line on alternate fields.
In many vertical sync control circuits the integrated field pulse waveform is clipped by
a reverse biased diode, along the line XY as shown in Fig. 17.1,and thus the voltage changes
across C1 due to line pulses are not seen by the field oscillator circuit at all.
Cascaded Integrator Sections
A very large time constant for the integrating circuit removes horizontal sync pulses, and also
reduces the vertical sync amplitude across the integrating capacitor. This rising edge of the
sync pulse is then not sharp and this can lead to incorrect triggering of the oscillator.However,
when the R1C1 time constant is chosen to be relatively small the horizontal sync pulses cannot
be filtered out and serrations in the vertical pulse produce notches in the integrated output.
Thus though the vertical sync pulse rise is quite sharp, the output voltage attains the same
amplitude at different charging excursions. This can also lead to wrong synchronization and
so the notches must be filtered out. The resulting outputs with large and small time constant
are illustrated in Fig. 17.2 (a).
Input pulse
Serration
v
Insufficient
integration
(RC too short)
Excessive
integration
(RC too large)
0
t
Fig. 17.2 (a). Effect of time-constant on vertical sync output.
R1
R2
10 K
vi
0.005 F
10 K
C1
0.005 F
C2
v0
Fig. 17.2 (b). Two-section integrator for vertical sync.
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R1
22 K
vi
2.2 nF
C1
R2
R3
39 K
100 K
2.2 nF
C2
4.7 nF
C3
v0
Fig. 17.2 (c). Three-section integrator for vertical sync.
To overcome the above described discrepancy, most receivers employ a two-section
integrating circuit with each RC section having a time constant of about 50 µs. Such a circuit
is illustrated in Fig. 17.2 (b). The operation of the circuit can be considered as though the R1C1
section provides integrated voltage across C1 that is applied to the next integrating section
R2C2. The overall time constant for both sections together is large enough to filter out horizontal
sync pulses while the shorter time constant of each section allows the integrated voltage to
rise more sharply because each integration is performed with a time constant of 50 µs. In some
designs even a three section integrator is provided and such a configuration is illustrated in
Fig. 17.2 (c).
17.3 HORIZONTAL SYNC SEPARATION
The high pass filter circuit and the differentiated output are shown in Fig. 17.1 along with the
sync separator circuit. The time constant of this circuit (R2C2) is kept much smaller (normally
1/10th) than the width of the horizontal pulse. A time constant between 0.5 µs to 1 µs is often
employed.
The physical action of the differentiator is as follows. When a leading edge of the incoming
pulse train is applied to the C2,R2 circuit, the initial voltage across the capacitor C2 is zero, and
so full amplitude of the leading edge appears across the resistor R2 and the output wave follows
almost exactly the shape of the input leading edge. When the flat top of the input rectangular
wave is reached, on further charging of the capacitor occurs, and the circuit discharges along
an exponential curve governed by the timeconstant of the circuit. Since this time-constant is
very short compared to the duration of input pulse, the discharge completes itself before the
trailing edge of the pulse arrives.
When the trailing edge of the input pulse occurs, it produces another pulsed output (see
Fig. 17.1) of opposite polarity to that of the first pulse. Since the trailing edge component
extends in opposite direction to the leading edge output, it has no effect on the triggering of the
horizontal oscillator.
The equalizing and vertical sync pulses (notched) produce two leading edge components
when they occur during each line scanning interval of 64 µs. The extra leading-edge pulses
occur when the horizontal oscillator is insensitive to sync pulses and hence have no effect on
its frequency. As was explained in Chapter 3, the leading edge of each horizontal sync pulse,
alternate equalizing pulses, and alternate serrations of the vertical sync pulse are correctly
timed to initiate the horizontal retrace periods. Moreover as explained above, the differentiating
circuit is insensitive to the flat top portions of the rectangular waves, that is, the amplitude of
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the differentiated output is independent of the duration of the input pulse. There is, therefore,
no particular response to the vertical sync pulses, except at their edges and they as such have
no effect on the triggering of the horizontal oscillator.
17.4 AUTOMATIC FREQUENCY CONTROL
The direct use of the incoming sync pulses to control vertical and horizontal sweep oscillators,
though simple and most economical is normally unsuitable, because of their susceptibility to
noise disturbance arising from electrical apparatus and equipment operating in the vicinity of
the television receiver. The noise pulses extending in the same direction as the desired sync
pulses cause greatest damage when they arrive during interval between the sync pulses. Noise
pulses which are most troublesome, possess high amplitude, are of very short duration and
tend to trigger the oscillator prior to its proper time.
When the vertical oscillator is so triggered the picture moves vertically, either upwards
or downwards, until proper sync pulses in the signal again assume control. If the horizontal
oscillator is incorrectly triggered a series of lines in a narrow band will be jumbled up, giving
the appearance of streaking or tearing across the picture. The horizontal sweep system of the
receiver is effected more by noise pulses than the vertical system.
To understand this fully, it is necessary to examine the nature of interfering voltages.
The energy of the noise pulses is distributed over a wide range of frequencies. For a peak to
occur, the phase relationship amongst various frequencies must be such as to permit their
addition to form a high amplitude pulse. This condition, however, usually exists only for a
brief interval which explains the small width of these pulses.
The high frequency noise pulses, when they reach the vertical sync separator, i.e. lowpass filter, get suppressed along with the line sync and equalizing pulses because of large time
constant of the circuit. The presence of this low-pass filter (the integrating network) is mainly
responsible for greater immunity to noise pulses enjoyed by the vertical system. This explains
why no special circuit is used between the sync separator and the vertical oscillator. However,
when a wide noise pulse is received, it contains enough energy to cause off-time firing of the
vertical oscillator, but the annoyance caused to the viewer on account of occasional rolling of
the picture is seldom great.
On the other hand, circuit leading to the horizontal oscillator, being a high-pass filter,
passes noise pulses readily along with the narrow horizontal sync pulses. This results in serious
interference with the normal functioning of the horizontal sweep oscillator which, in turn,
results in frequent ‘tearing’ of the picture. In order to ensure that the horizontal oscillator
operates at the correct frequency, and is basically immune to noise pulses, all horizontal
deflection oscillators are controlled by some form of a circuit known as the automatic frequency
control circuit (AFC circuit).
The AFC circuit receives sync pulses and output from the horizontal oscillator
simultaneously and compares them regarding their phase and frequency. The discriminator in
the AFC circuit develops a slowly varying voltage, the magnitude of which depends on deviation
of frequency of the horizontal oscillator from its correct frequency. In case the oscillator frequency
is equal to the incoming horizontal sync frequency, i.e. 15625 Hz, no output voltage is developed.
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The AFC circuit output is filtered by a low-pass filter to obtain an almost dc voltage, which
then controls the frequency of the horizontal sweep oscillator. Thus the use of AFC circuit and
a low-pass filter at its output, eliminates the effect of sharp noise pulses. The use of this indirect
method of frequency control results in excellent horizontal oscillator stability and immunity
from noise interference.
AFC Operation
The block schematic arrangement of a frequently used AFC circuit for the horizontal deflection
oscillator is illustrated in Fig. 17.3. Horizontal sync voltage and a fraction of the horizontal
deflection voltage, normally taken from the horizontal output circuit, and suitably processed
to form horizontal flyback pulses, are coupled in the sync discriminator. The discriminator
consists of two diodes and associated circuitry. It detects the difference in frequency and develops
a dc output voltage proportional to the difference in frequency between the two input voltages.
The dc control voltage indicates whether the oscillator is ‘on’ or ‘off ’ the sync frequency. The
greater the difference between the correct sync frequency and the oscillator frequency, larger
is the dc control voltage. This dc control voltage is fed to a large time constant filter, the output
of which is used to control the oscillator frequency. The shunt by-pass capacitor of this lowpass filter eliminates the effect of noise pulses. A large time constant filter could not be used
directly, in the horizontal system ; because while suppressing noise pulses it would have
prevented the desired horizontal sync pulses from reaching the horizontal sweep oscillator.
This explains the need and use of the AFC circuit.
Sync input
DC control voltage
Sync
discriminator
R1
LPF
Horz
oscillator
Horz output
amplifier
C1
To deflection
circuit
R2
C2
Integrator
Flyback pulses
Fig. 17.3. Block diagram of the horizontal AFC system.
The automatic frequency control circuit is generally called flywheel sync, sync lock,
stabilized sync or horizontal AFC, based on the technique employed to develop the control
voltage. Most present day receivers use either a push-pull or a single-ended phase discriminator
AFC circuit for sensing any error in the horizontal oscillator frequency.
17.5 AFC CIRCUIT EMPLOYING PUSH-PULL DISCRIMINATOR
A typical circuit arrangement employing push-pull phase discriminator is shown in Fig. 17.4.
The sync pulses of equal amplitude but of opposite polarity are obtained from the phase splitter
circuit and coupled to the diodes D1 and D2, through capacitors C1 and C2 respectively. R1 and
R2 are of equal value and act as load resistors to the two diodes. The diodes are so connected
that the application of sync pulses of opposite polarity forward biases them simultaneously.
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MONOCHROME AND COLOUR TELEVISION
The horizontal deflection waveform, phase reversed and coming effectively from a voltage
source is applied at the point marked ‘A’ in the circuit diagram. This is actually obtained from
a winding (L1) on the horizontal output transformer in the form of flyback pulses and then
processed by the integrating network R6, C3.
+ 10 V
B+
100 Ω
R3
+
From
sync separator
0.033 µF
10 µF
Phase
splitter
C4
E1
10V P–P
C1
–
5.6 K
–
1 K R1
+
100 Ω
+
–
C3
9
R8
15 K
0.1 µF
R10
C5
Flyback
pulses
2.2 K
A
D2
E2 8V P–P
–
D1
0.033 µF 1 K R2
+
– +
V2
C2
R7 680 Ω R
R5
R4
VB
+
0 –
V1
0.3 µF C6
R6
L1
330 Ω
Winding
on H.O.T.
DC control voltage
to horz oscillator
3.3 µF
Fig. 17.4. Push-pull AFC circuit.
It is convenient to study the functioning of this circuit under three different conditions:
(a) when sawtooth feedback voltage is only present, (b) when only sync pulses are present and
(c) when both sawtooth and sync pulses are simultaneously present.
(a) When only sawtooth voltage is applied (no sync pulses) D1 conducts during the negative
and D2 during the positive half of the sawtooth cycle, charging C1 and C2 with polarities shown.
When voltage peaks have passed C1 discharges via R3, B+, B–, (ground) R8, R7 and R1. Similarly
C2 discharges through R2, R7, R8, B– and R4. This raises the lower end of R2 (marked V2) above
ground potential and the upper end of R1 (marked V1) below ground potential. Equilibrium is
reached when | V1 | = | V2 | =
1
2
E2 (peak-to-peak) of the applied sawtooth voltage. Thus VB
equals zero and the upper end of R7 stays at ground potential. When put in another way this
means that the two equal and opposite currents flowing through R8 develop a net zero voltage
across it. In steady state, small pulses of make-up current flow at the positive and negative
peaks of the sawtooth through D2,C2 and C1,D1 respectively.
(b) When only sync signal is present, the positive going sync pulses are coupled from the
collector of the sync splitter through C1 to the anode of D1. At the same time negative going
sync pulses are coupled via C2 to the cathode of D2. As a result of these pulses, current flows
along the path C1, D1, D2, C2, R4, B–, B+, and R3 thereby charging the capacitors C1 and C2 to
approximately peak value of the sync pulses with polarities shown across them. During the
time between sync pulses, the capacitor C1 discharges through R3, B+, B–, R8, R7 and R1. At the
same time the capacitor C2 discharges via R2, R7, R8 and R4. Two equal but opposite voltages,
caused by equal capacitor discharge currents develop across R8. These voltages cancel each
other leaving a net zero voltage across R7 and R8. Thus VB equals zero and this point continues
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SYNC PROCESSING AND AFC CIRCUITS
to be at ground potential. Note that V1 and V2 are of such polarity that both the diodes are
reverse biased during the time between sync pulses.
(c) Now if sync and sawtooth voltages are applied simultaneously, as they would be
under normal conditions, three cases are possible—(i) pulses in synchronism, i.e. oscillator
frequency is correct, (ii) oscillator frequency is more than the sync frequency, and (iii) oscillator
frequency is less than the sync frequency.
(i) For this case as shown in Fig. 17.5 (a), sync pulses and sawtooth voltage arrive
symmetrically, i.e. the sync pulses occur when the sawtooth is passing through its zero point
during retrace. As a result the circuit behaves as though each signal were applied to the phase
detector independent of each other. Thus the sync pulses deposit equal charges on C1 and C2,
causing smaller but identical make-up currents to be drawn from the sawtooth source. The
magnitudes of V1 and V2 remain equal (VB = 0) and no control voltage is developed across R8.
Therefore, the frequency of the horizontal deflection oscillator remains unchanged.
(ii) When the oscillator frequency is high (see Fig. 17.5 (b)) the sync pulses arrive late in
a relative sense, i.e. when the sawtooth is already in its positive half cycle. Thus the sawtooth
forward biases D2 and reverse biases D1 with the result that D2 conducts harder than D1. Thus
more charge is added to C2 and less to C1. During the discharge periods of these capacitors
unequal currents flow through R8 and a positive error voltage is developed across it. The
discharge currents also cause V2 to increase and V1 to decrease from their quiescent values
making VB positive. The low-pass network R9, C5 filters the control voltage and feeds a dc error
voltage to the horizontal oscillator. This forces the oscillator frequency to return to its normal
value.
Sync pulses
Sync pulses
Sync frequency
= 15625 Hz
E1
VAK(D1) = VAK(D2)
0
(a) f osc = f sync, dc control voltage = 0
(VAK D2)
VAK(D1) < VAK(D2)
0
(VAK D1)
(b) f osc > f sync, dc control voltage is positive
Sync pulses
Retrace Trace
0
VAK(D1) > VAK(D2)
(c) f osc < f sync, dc control voltage is negative
Anode to cathode voltage across diode D1
Anode to cathode voltage across diode D2
Fig. 17.5. Timing relationships between reference sync pulses and sawtooth voltage obtained
from output of the horizontal deflection oscillator circuit. (a) correct oscillator frequency
(b) oscillator frequency higher than sync frequency (c) oscillator frequency lower than sync frequency.
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(iii) As shown in Fig. 17.5 (c) when the oscillator frequency is low, sync pulses occur
during the time the sawtooth is in its negative half cycle forward biasing D1 more than D2.
Analogous arguments establish that for this case a negative control voltage is developed across
R8. This, after filtering is applied to the horizontal oscillator causing its frequency to increase
to its normal value.
Thus the sync discriminator continuously measures the frequency difference between
the sawtooth and sync pulses to produce a dc correction voltage that locks the horizontal
oscillator at the synchronizing frequency. In Fig. 17.4, R10 and C6 constitute an anti-hunt,
circuit whose function is described in a later section of the chapter.
17.6 SINGLE ENDED AFC CIRCUIT
The single ended or Gruen phase detector AFC circuit does not require push-pull sync input.
Instead, as shown in Fig. 17.6 (a), negative going sync pulses of about 30V P-P are coupled
through C1 from the sync separator to the common cathode ends of the two diodes D1 and D2.
The phase detector diodes are also fed flyback pulses obtained from a winding (L1) on the
horizontal output transformer. The pulses are passed through an integrating circuit (R5, C4)
and converted into a sawtooth voltage of about 20V P-P before being fed at point ‘X’ in the
circuit.
+
0
–
Feedback sawtooth
C3
1000 PF
B+
20 V P–P
R5
C4
30 V
From
sync
separator
27 K
180 K
R3
0.001 F
Q1 2.2 K
C1
B–
T1
30 V
T3
T2
Winding
on H.O.T.
DC
AFC voltage
R6
X
R4
L1
C5
R1 V1
D1
C2 68 pF
100 K
C6
820 PF
R7
0.033
F C
7
R2 V2
55 K
0.1 F
D2
180 K
Fig. 17.6 (a). Single ended (GRUEN) discriminator for horizontal AFC.
Circuit Operation
Assume that only sync pulses are applied to the phase detector. During the time T1 (see sync
pulse waveform) the sync separator is cut-off and its collector voltage is equal to B+ voltage. As
a result the coupling capacitor C1 is fully charged. When the negative sync pulse (T2) arrives,
i.e. Q1 saturates, the two diodes are simultaneously forward biased. Note that the diodes are
effectively in parallel for the sync input due to the large (820 pF) capacitor C5 to ground from
the anode of D1. This enables C1 to discharge through two different paths. One path for discharge
is through R3, Q1, B– and D2 while the other is completed via R3, Q1, B–, C4, C3 and D1. In this
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process C3 is charged with positive polarity towards C4. Because of very short time constant
during the discharge period, C1 discharges almost completely.
During the following long period (T3) between sync pulses both diodes are turned off
and C1 charges through two independent paths. One charging path is through B+, R4, R3, C1,
and R2. The second path is completed via B+, R4, R3, C1, R1, C3 and C4. The capacitors C3 and C4
being practically short at the sync frequency, two equal and opposite voltage drops (V1 and V2)
develop across R1 and R2 respectively producing a net zero voltage at point X (across the two
diodes) with respect to ground. In addition C3 discharges and attains its earlier status. The
voltage drops V1 and V2 provided reverse bias across D1 and D2 respectively and thus permit
conduction only during peak values of the applied signals.
It may be noted that a small (62 pF) capacitor C2 is connected across D2. This provides
frequency compensation and ensures that the sawtooth voltage-drops, across D1 and D2 are
identical in magnitude and waveshape. However, despite this precaution to correct mismatch
between the components that constitute the phase detector, in practice a small voltage of the
order of a few tenths of a volt does exist between point X and ground.
When only sawtooth voltage is applied at X, practically the entire voltage appears across
D1 and D2 in series opposition and forces them into conduction alternately. When the sawtooth
is positive to ground, D1 is turned on and the current which flows through it and R2 charges C3.
During the negative half cycle, D1 is turned off, but D2 conducts through R1, C3 and C4 thereby
discharging C3. Once again voltage drops V1 and V2 are equal and opposite and zero net voltage
is developed at point ‘X’.
When sync pulses and sawtooth voltage are simultaneously applied, as is necessary for
normal operation of any AFC circuit, three possible conditions can occur. These are illustrated
in Fig. 17.6 (b) with sync waveform ‘A’ as the reference.
A
0
B
0
C
0
D
0
Reference horizontal sync
Trace
Retrace
f osc = f sync
Control voltage is zero
f osc > f sync
Control voltage is positive
f osc < f sync
Control voltage is negative
Fig. 17.6 (b). Three possible conditions of horizontal sweep relative
to sync pulse in a single ended AFC circuit.
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First (see waveform B), the sync pulses may occur when sweep (sawtooth) voltage is
passing through zero. It is as if the sync pulses were applied without the sawtooth and vice
versa. Consequently, no dc error voltage is produced. In a practical circuit, however, as stated
earlier, a small voltage appears across the two diodes.
Second, the frequency of the oscillator may be higher than the sync pulse frequency.
When this happens (see waveform C in Fig. 17.6 (b)) horizontal sync will occur during negative
half cycle of the feedback sawtooth voltage. In this event D2 conducts harder than D1, leaving
a net potential across C3 with positive on its plate that is tied to the anode of D1. Subsequently
when C3 discharges via D1 and R2, V2 becomes greater than V1. Thus a net positive voltage is
developed across R1 and R2. This control voltage at X when fed after filtering to the horizontal
oscillator forces its frequency to decrease.
In the third case the frequency of the oscillator may be lower than the sync pulse rate.
Under this condition (waveform D) sync pulses will occur during positive half cycle of the
feedback sweep voltage with the result that D1 will conduct harder than D2. Consequently C3
will now attain a net negative charge on its plate tied to D1. On discharging through D2,R1, the
capacitor C3 now develops a higher voltage across R1 than R2. Thus the net voltage ( | V1 | –
| V2 |) across point ‘X’ and ground will be negative. Such a control voltage forces the oscillator
to increase its frequency returning it to its normal operation.
17.7 PHASE DISCRIMINATOR (AFC) WITH PUSH-PULL SAWTOOTH
A sync discriminator in which flyback pulses of opposite polarity are coupled to the two diodes
is illustrated in Fig. 17.7 (a). The flyback pulses of large amplitude are obtained from the two
ends of a centre tapped winding, wound on the horizontal output transformer. These pulses
are coupled at the points, marked A and B on the circuit diagram, where they are integrated
by two RC networks (RA, CA and RB, CB) to develop sawtooth outputs of opposite polarity. The
amplitude of the sawtooth voltage around its centre-zero axis is of the order of 20 volts.
3.3 KPF
A
CA
RA
390 K
DC control
voltage
39 K
D1
Winding
on horizontal
output
transformer
+
RB
B
400 V
R1
C1
100 K
C2
D2
R2
1.5 KPF
R3
100 K
20 V
39 K
CB
3.3 KPF
Fig. 17.7 (a). Phase discriminator circuit with push-pull sawtooth.
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SYNC PROCESSING AND AFC CIRCUITS
Reference
horz sync
Oscillator ‘on’
frequency
Oscillator fast,
D1 conducts
more than D2
Oscillator slow,
D1 conducts
less than D2
Diode D1
Diode D2
Fig. 17.7 (b). Timing relationships between the reference sawtooth and sync pulses.
The anodes of diodes D1 and D2 are tied together and positive going sync pulses from the
sync separator and invertor circuit are applied at this junction. The dc control voltage that
develops across the diode load resistors R1 and R2 depends on deviation of the horizontal
oscillator frequency from that of the incoming sync pulses. The waveforms in Fig. 17.7 (b) show
how the net voltage across each diode varies in accordance with variations of the oscillator
frequency. As explained in the previous (single ended) circuit it is the charging and discharging
action of the sync coupling capacitor C1, through two different paths that results in the
development of a net zero positive or negative control voltage. The control voltage is coupled to
the grid of a reactance tube, normally the triode portion of a pentode-triode tube. Network
C2,R3 is provided to compensate for any imbalance in the ac voltage applied to the two diodes.
The pentode section together with its triode acts as a sinusoidal oscillator cum waveshaper.
The reactance tube acts as a variable capacitor/inductor across the tank circuit of the oscillator
to keep the frequency of oscillations at 15625 Hz.
It may be noted that in any sync discriminator circuit it is possible by reversing the
polarity of the sawtooth voltage, which is fed to the discriminator, to obtain control voltage of
opposite polarity for the conditions of a fast or a slow oscillator. The dc control voltage, as
obtained from the various discriminator circuits, varies between ± 2 and ± 6 volts.
17.8 DC CONTROL VOLTAGE
The time constant of the RC filter provided at the output of the discriminator, determines how
fast the dc control voltage can change its amplitude to correct the oscillator frequency. A time
constant much larger than 64 µs is needed for the shunt capacitor to bypass horizontal sync
and sawtooth components in the control circuit while filtering out noise pulses. However, a
large time constant may not permit the control voltage to change within a fraction of a second
when sync is temporarily lost while changing channels. Also, if the time-constant is too large,
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the dc control voltage may be effected by the vertical sync pulses, causing bend at the top of the
picture. A typical value of the AFC filter time-constant is about 0.005 second, i.e., a period
nearly equal to 75 horizontal lines.
Hunting in AFC Circuits
The filtering circuit that follows the diode section of the discriminator controls the performance
of the AFC circuit. Too large a time constant makes the control sluggish, while insufficient
damping, on account of too small a time constant, causes the oscillator to ‘hunt’ returning to
the correct frequency. Excessive hunting in the AFC circuit appears as ‘weaving’ or ‘geartooth’ on the picture.
Oscillator hunting due to
difference in timing
between error voltage and
oscillator frequency
15626 Hz
15625 Hz
0
Time
Error voltage without
antihunt circuit
Error voltage
+
–
15624 Hz
15626 Hz
Horizontal
oscillator
frequency
Oscillator frequency
hunting
+
0
15625 Hz
Error voltage with
antihunt circuit
–
Error voltage
Horizontal oscillator frequency
The manner in which the oscillator frequency deviates from the correct value on account
of hunting is illustrated in Fig. 17.8 (a). In order to prevent this a double section filter is often
used for the dc control voltage. In this network a shown in Fig. 17.8 (b), the R1C1 time constant
of 0.005 sec is large enough to filter out noise, horizontal sync and flyback pulse effects. The
Fig. 17.8 (a). Horizontal oscillator hunting and its correction by the antihunt circuit.
From AFC
circuit
R1
To horizontal
oscillator
1M
0.005 F
C1
R2
C2
56 K
0.1 F
Antihunt
circuit
Fig. 17.8 (b). AFC filter circuit with antihunt network.
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SYNC PROCESSING AND AFC CIRCUITS
321
second section, R2 and C2 in series, is known as the ‘anti-hunt network’. The relatively low
resistance of R2 serves as a damping resistance across C1 making the output voltage more
resistive and less capacitive, thereby reducing time delay (see Fig. 17.8 (a)) in the change of
control voltage.
Review Questions
1.
Draw basic low-pass (integrating) and high-pass (differentiating) filter configurations, which
are employed to separate vertical and horizontal sync information. Comment on the choice of
time constants of these circuits. Sketch accurately, output voltage waveforms of the filter circuits,
when fed with a pulse train separated from the incoming composite video signal.
2.
Why is a cascaded network preferred for developing vertical sync pulses ? Why is it not necessary
to employ an AFC circuit for developing control voltage for the vertical deflection oscillator ?
3.
Draw a basic (block schematic) AFC circuit and explain how the control voltage is developed.
Explain fully how the effect of noise pulses is leiminated.
4.
Draw a typical push-pull sync discriminator (AFC) circuit and explain with the help of neatly
draw waveforms, how a control voltage, proportionate to the deviation of horizontal oscillator
frequency is developed.
5.
Why is a single-ended AFC discriminator preferred to the push-pull circuit ? Draw its circuit
diagram and explain with the help of necessary waveforms, how the control voltage develops,
when the oscillator frequency is (i) correct, (ii) fast and (iii) slow.
6.
Why is an anti-hunt circuit used while filtering the error voltage obtained from any AFC discriminator ? Draw its circuit configuration and explain how hunting is suppressed.
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18
Deflection Oscillators
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18
Deflection Oscillators
In order to produce a picture on the screen of a TV receiver that is in synchronism with the one
scanned at the transmitting end, it is necessary to first produce a synchronized raster. The
video signal that is fed to the picture tube then automatically generates a copy of the transmitted
picture on the raster.
While actual movement of the electron beam in a picture tube is controlled by magnetic
fields produced by the vertical and horizontal deflection coils, proper vertical and horizontal
driving voltages must first be produced by synchronized oscillators and associated waveshaping
circuits. As illustrated in Fig. 18.1, for vertical deflection the frequency is 50 Hz, while for
horizontal deflection it is 15625 Hz. The driving waveforms thus generated are applied to
power amplifiers which provide sufficient current to the deflecting coils to produce a full raster
on the screen of picture tube.
Free running relaxation type of oscillators are preferred as deflection voltage sources
because these are most suited for generating the desired output waveform and can be easily
locked into synchronism with the incoming sync pulses.
Vertical
sync input
DC control
voltage
Vertical
oscillator
50 Hz
Wave
shaping
circuit
Horizontal
oscillator
15625 Hz
Wave
shaping
circuit
To vertical deflection
amplifier
To horizontal deflection
amplifier
Fig. 18.1. Deflection oscillators and waveshaping.
The oscillators commonly used in both vertical and horizontal deflection sections of the
receiver are:
(i) Blocking oscillator,
(ii) Multivibrator,
(iii) Complementary pair relaxation oscillator,
(iv) Overdriven sine-wave oscillator.
324
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DEFLECTION OSCILLATORS
It may be noted that complementary pair circuits are possible only with transistors
while all other types may employ tubes or transistors.
As explained earlier, both vertical and horizontal deflection oscillators must lock with
corresponding incoming sync pulses directly or indirectly to produce a stable television picture.
18.1 DEFLECTION CURRENT WAVEFORMS
Figure 18.2 illustrates the required nature of current in deflection coils. As shown there it has
a linear rise in amplitude which will deflect the beam at uniform speed without squeezing or
spreading the picture information. At the end of ramp the current amplitude drops sharply for
a fast retrace or flyback. Zero amplitude on the sawtooth waveform corresponds to the beam at
centre of the screen. The peak-to-peak amplitude of the sawtooth wave determines the amount
of deflection from the centre. The electron beam is at extreme left (or right) of the raster when
the horizontal deflecting sawtooth wave has its positive (or negative) peak. Similarly the beam
is at top and bottom for peak amplitudes of the vertical deflection sawtooth wave. The sawtooth
waveforms can be positive or negative going, depending on the direction of windings on the
yoke for deflecting the beam from left to right and top to bottom. In both cases (Fig. 18.2) the
trace includes linear rise from start at point 1 to the end at point 2, which is the start of retrace
finishing at point 3 for a complete sawtooth cycle.
+I
2
Trace
Retrace
0
I
t
3
1
O ccycle
One
e
(a)
+I
1
Trace
3
I(P–P)
–
0
t
2
I
Retrace
(b)
Fig. 18.2. Deflection current waveforms (a) for positive
going trace (b) for negative going trace.
Driving Voltage Waveform
The current which flows into the horizontal and vertical deflecting coils must have a sawtooth
waveform to obtain linear deflection of the beam during trace periods. However, because of
inductive nature of the deflecting coils, a modified sawtooth voltage must be applied across the
coils to achieve a sawtooth current through them. To understand this fully, consider the
equivalent circuit of a deflecting coil (Fig. 18.3) consisting of a resistance R in series with a
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MONOCHROME AND COLOUR TELEVISION
pure inductance L, where R includes the effect of driving source (internal) resistance. The
voltage drops across R and L for a sawtooth current, when added together would give the
voltage waveform that must be applied across the coil. The voltage drop across R (Fig. 18.3 (a))
has the same sawtooth waveform as that of the current that flows through it. However, the
FG
H
voltage across L depends on the rate of change of current vL = L
di
dt
IJ
K
and the magnitude of
inductance. A faster change in iL, produces more self induced voltage vL. Furthermore, for a
constant rate of change in iL, the value of vL is constant. As a result, vL in Fig. 18.3 (b) is at a
i
1
vR
R
0
t
Coil current
v
vL
Deflection
coil
L
vR
0
t
(a)
vL
0
t
(b)
v
t
0
Trapezoid
(c)
Fig. 18.3. Current and voltage waveshapes in a deflection coil
(a) voltage drop across the resistive component of coil impedance
(b) voltage drop across the inductive component of coil impedance
(c) resultant voltage ‘v’ (VR + vL) across input terminals of the
coil for a sawtooth current in the winding.
relatively low level during trace time, but because of fast drop in iL during the retrace period,
a sharp voltage peak or spike appears across the coil. The polarity of the flyback pulse is
opposite to the trace voltage, because iL is then decreasing instead of increasing. Therefore, a
sawtooth current in L produces a rectangular voltage. This means, that to produce a sawtooth
current in an inductor, a rectangular voltage should be applied across it. When the voltage
drops across R and L are added together, the result (see Fig. 18.3 (c)) is a trapezoidal waveform.
Thus to produce a sawtooth current in a circuit having R and L in series, which in the case
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DEFLECTION OSCILLATORS
under consideration represents a deflection coil, a trapezoidal voltage must be applied across
it. Note that for a negative going sawtooth current, the resulting trapezoid will naturally have
an inverted polarity as illustrated in Fig. 18.4.
i
0
t
Trace
Retrace
v
0
t
Trace
Retrace
Fig. 18.4. Inverted polarity of i and v.
As explained above, for linear deflection, a trapezoidal voltage wave is necessary across
the vertical deflecting coils. However, the resulting voltage waveform for the horizontal yoke
will look closer to a rectangular waveshape, because voltage across the inductor overrides
significantly the voltage across the resistance on account of higher rate of rise and fall of coil
current.
Effect of Driving Source Impedance on Waveshapes
In deflection circuits employing vacuum tubes, the magnitude of R is quite large because of
high plate resistance of the tube. Therefore, voltage waveshape across the vertical deflection
coils and that needed to drive the vertical output stage is essentially trapezoidal. However, in
a horizontal output circuit employing a tube, the waveshape will be close to rectangular because
of very high scanning frequency.
When transistors are employed in vertical and horizontal deflection circuits, the driving
impedance is very low and equivalent yoke circuits appear to be mainly inductive. This needs
an almost rectangular voltage waveshape across the yoke. To produce such a voltage waveshape,
the driving voltage necessary for horizontal and vertical scanning circuits would then be nearly
rectangular. Thus the driving voltage waveforms to be generated by the deflection oscillator
circuits would vary depending on deflection frequency, device employed and deflection coil
impedance.
18.2 GENERATION OF DRIVING VOLTAGE WAVEFORMS
Sawtooth voltage is usually obtained as the voltage output across a capacitor that is charged
slowly employing a large time constant to generate the trace period and then quickly discharged
through a short time constant circuit to obtain the retrace period. The initial exponential rise
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MONOCHROME AND COLOUR TELEVISION
of voltage across the capacitor is linear, and thus alternate charging and discharging of the
capacitor at the rate of deflection frequency results in a sawtooth output voltage across it. This
is illustrated in Fig. 18.5(a) where CS is allowed to charge through a large resistance R1 from
a dc (B+) source. The charging time is controlled by switch ‘S’ which is kept open during the
trace period at the deflection frequency. At the end of trace, switch ‘S’ is closed for a time equal
to the retrace period, and the capacitor discharges quickly through a small resistance. R2.
Actually, switch ‘S’ represents a vacuum tube or a transistor that can be switched ‘on’ or
‘off ’ at the desired rate. When the tube or transistor is in cut-off state, it corresponds to ‘off ’
position of the switch. In the ‘on’ state during which the active device is allowed to go into
saturation, the tube or transistor conducts heavily allowing the capacitor to discharge through
its very low internal resistance which corresponds to resistance R2 shown in series with the
switch. In this application the tube or transistor is called a ‘discharge device’ and capacitor CS
is often referred to as ‘sawtooth capacitor’ or ‘sweep capacitor’.
As mentioned earlier the trace voltage should rise linearly. For this, only linear part of
the exponential volt-time characteristic is used. To achieve this, the time constant (RC) of the
circuit should at least be thrice the trace period. Same result can also be achieved by employing
a higher B+ voltage. The waveshapes shown in Fig. 18.5(b) illustrate the effect of charging
time constant (RC) and source voltage B+ on linearity and magnitude of the sawtooth voltage.
Trapezoidal Voltage Generation
As explained earlier it is often necessary to modify the sawtooth voltage to some form of a
trapezoidal voltage before feeding it to the output stages for obtaining linear deflection. Figure
18.6 shows a basic circuit for generating such a voltage. It is the same circuit discussed earlier
but employs a transistor as discharge switch and has a small resistance RP (peaking resistance)
in series with the sawtooth capacitor CS. The transistor which is biased to cut-off by battery
VBB is driven into saturation by the incoming, large but narrow positive going pulses. It thus
acts as a discharge switch to produce a fast retrace. During long intervals in-between positive
pulses, CS charges towards B+ through the large resistance R1 to provide trace voltage. Since
the value of RP is small as compared to R1, voltage developed across it is quite small while CS
charges. However, on arrival of a positive pulse, Q1 goes into full conduction thus providing a
very low resistance path (small RC) for the capacitor to discharge. The high discharge current
which also flows through RP develops a large negative voltage pulse across it. This is illustrated
by the waveform drawn along RP in Fig. 18.6. As shown by another waveform, the spiked
voltage across RP adds to the sawtooth voltage across CS to produce a trapezoidal voltage v0
between point ‘A’ and ground. Note that exact charge and discharge periods must be in
accordance with the synchronized vertical and horizontal scanning rates. This function is
assigned to the vertical and horizontal oscillators.
R1
S
DC
C
supply
p
Charging
path
+
CS
Discharge
path
v0
R2
Fig. 18.5 (a). Basic circuit for generating a sawtooth voltage.
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DEFLECTION OSCILLATORS
v0
High B+
Same time
constant
Low B+
Linear
trace
Retrace
Nonlinear
trace
0
Long time
constant circuit
Short time
constant circuit
Time
Fig. 18.5 (b). Effect of charging time constant (RC) and source voltage
(B+) on linearity and magnitude of v0.
B+
R1
v0
CS
Q1
Vin
A
RB
VBB
–
VC
S
VR
P
RP
+
V0
Fig. 18.6. Generation of trapezoidal sweep voltage.
18.3 BLOCKING OSCILLATOR AND SWEEP CIRCUITS
This oscillator may be thought of as a tuned-plate (or collector) configuration that is disigned
to produce an extreme case of intermittent oscillations. The feedback transformer is polarized
to produce such a large amount of feedback that the cumulative action is almost instantaneous.
In circuits employing tubes, the grid current that flows on account of regeneration develops
such a large negative self-bias that the tube is immediately driven to much beyond cut-off.
This prevents the circuit from generating continuous sinusoidal oscillations, at the natural
resonant frequency of the feedback transformer, depending on its inductance and stray
capacitance. Thus only a single short pulse of large amplitude is generated. The cycle is repeated
when the self-bias returns to the conduction region. The number of times per second the oscillator
produces pulse and then blocks itself, is the pulse repetition rate or oscillator frequency.
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MONOCHROME AND COLOUR TELEVISION
Similarly in a transistor blocking oscillator, feedback action switches the transistor
between saturation and cut-off at a rate that is controlled by choice of time constant in the
input circuit of the oscillator. This repetition rate or oscillator frequency is chosen to be 50 Hz
and 15625 Hz for the vertical and horizontal deflections respectively.
Vacuum Tube Blocking Oscillator and Sweep Generator
The switch S in Fig. 18.5 (a) can be replaced by a blocking oscillator to control the charge and
discharge periods of the capacitor. Such a circuit is illustrated in Fig. 18.7(a) where tube V1
acts both as a blocking oscillator and discharge tube. The grid voltage waveform is illustrated
in Fig. 18.7 (b). When the oscillator is cut-off by blocking action, CS charges through the series
resistance of R1 and R2 towards B+. During oscillator pulses the tube conducts heavily and its
plate resistance falls to a very low value. This provides a very low time constant path, for the
capacitor CS to discharge, with discharge current in the same direction as the normal plate
current during oscillator conduction. Thus the blocking oscillator behaves like a switch where
the ‘on’ and ‘off’ periods are automatically controlled by the frequency of blocking oscillator.
The ‘on’ and ‘off’ periods are set equal to the retrace and trace periods respectively.
C2
LP
V1
T1
C1
LS
R1
2M
+
CS
–
To vertical
output stage
0.05 mF
0.01 mF
C3
R3
R4
Sync
input
2M
1M
R2
4M
Hold
control
Height (size)
control
VC
S
B+
40
0V
350 V
0
t
(a)
Tube conducts
+
t
0
Cut-off bias
Grid voltage
–10 V
Discharge path
t = C1 (R3 + R4)
–120 V
(b)
Fig. 18.7. A vacuum tube blocking oscillator (a) circuit (b) grid voltage waveform.
Frequency Control
Frequency of the oscillator can be adjusted by varying resistance R4 which is part of resistance
in the grid-leak bias circuit. A lower time constant and smaller value of R4 will allow a faster
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DEFLECTION OSCILLATORS
discharge for a higher frequency. Increasing the time constant C1(R3 + R4) results in a lower
oscillator frequency. The oscillator frequency control is adjusted to the point where sync voltage
can lock the oscillator at the sync frequency to make the picture hold still. For this reason the
frequency adjustment resistor R4 is generally called the hold control. The range of frequency
control for the vertical oscillator is usually from 40 to 60. For the horizontal oscillator, which
employs an automatic frequency control, the horizontal hold adjustment is usually provided in
the AFC circuit.
Height Control
The capacitor CS is allowed to charge through R1 and R2 towards B+ for a fixed interval equal
to the trace period. During this period tube V1 (Fig. 18.7(a)) remains is cut-off. Decreasing the
resistance R2 reduces time constant. Hence CS charges at a faster rate and attains a higher
voltage at the end of trace period. Thus a reduction in the value of R2 increases amplitude of
the sawtooth voltage which, after amplification, causes a larger current through the deflecting
coils to increase size of the picture. Similarly, increasing resistance R2 results in reducing size
of the picture. The fixed resistance R1 in series with potentiometer R2 limits the range of
variation for easier adjustment. This method of size control is generally used in vertical deflection
circuits and is known as height control.
Synchronizing the Blocking Oscillator
A blocking oscillator in its free running state is not very stable and its frequency changes with
variations in electrode voltages. This, however, can be easily controlled and kept constant by
an external sync signal. The frequency can be synchronized either by sync pulses that trigger
the oscillator into conduction at the sync frequency or by changing the grid bias with a dc
control voltage. The vertical sweep oscillator is usually locked with pulses obtained by
integrating the vertical sync pulses, whereas the horizontal oscillator frequency is synchronized
with the dc control voltage produced by the horizontal AFC circuit.
Synchronized operation
Free
running
unni
t
Grid voltage
0
Cut-off
bias
Sync pulses
10 V
Sync
0
S
t
Fig. 18.8 (a). Blocking oscillator synchronization with positive trigger pulses.
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MONOCHROME AND COLOUR TELEVISION
+
t
0
Drive voltage
Cut-off bias
Controlled
frequency
10
0V
Control
voltage
Free running
Fig. 18.8 (b). Effect of positive dc control voltage on frequency.
The waveforms of Fig. 18.8 (a) illustrate how a vacuum tube blocking oscillator can be
synchronized by small positive pulses injected in the grid circuit of the oscillator. The sync
voltage is applied in series with the grid winding of the transformer through a capacitor as
shown in Fig. 18.7 (a). The positive sync pulses arrive at the time marked ‘sync’ when the
declining grid voltage is close to cut-off and cancels part of the grid bias voltage produced by
the oscillator. A small sync voltage is sufficient to drive the grid voltage momentarily above
the cut-off voltage. This initiates plate current flow and then the oscillator goes through a
complete cycle. The next positive sync pulse arrives at a similar point, in the following cycle,
forcing the oscillator to begin the next cycle. As a result the sync pulses force the oscillator to
operate at the sync frequency.
Free Running Frequency of the Oscillator
Operating the oscillator at the same frequency as the synchronizing pulses does not provide
good triggering, because the oscillator frequency can drift, above the sync frequency, resulting
in no synchronization. This is because the sync pulse will have no controlling influence when
the tube or transistor has already been switched into conduction by its own bias. For best
synchronization, the free-running oscillator frequency, is adjusted slightly lower than the forced
or sync frequency so that the time between sync pulses is shorter than the time between pulses
of the free running oscillator. Then each synchronizing pulse occurs just before an oscillator
pulse and forces the tube or transistor into conduction thereby triggering every cycle, to hold
the oscillator locked at the sync frequency. If the free-running frequency is too low, the sync
pulses will arrive early and fail to raise the bias to a level that can cause conduction because
the grid bias will still be at a large negative value.
This also explains why equalizing pulses or any noise pulses which occur at the middle
of the cycle fail to trigger the oscillator. False triggering due to noise pulses that occur close to
the sync pulses can be reduced by returning the grid to a positive voltage instead of the chassis
ground.
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DEFLECTION OSCILLATORS
DC Control Voltage
A dc control voltage (see Fig. 18.8 (b)) can also be added to the existing bias voltage on the grid
of the blocking oscillator tube or base of the oscillator transistor, to control its frequency. As a
result of this addition, less time is needed for the bias voltage to reach its conduction level to
start the next cycle. This method is often used to synchronize the horizontal oscillator frequency.
Transistor Blocking Oscillator Sweep Generator
In a blocking oscillator which employs a transistor, the method employed to turn the transistor
‘on’ and ‘off’ for generating sawtooth output is different than that employed with vacuum
tubes. This frequently involves the use of sawtooth wave generated by the transistor itself,
rather than by a grid-leak bias action as used in tube circuits.
Figure 18.9 is the circuit of a typical vertical blocking oscillator-sawtooth generator,
where sweep capacitor CS is in the emitter lead of the transistor. The combination of voltage
divider resistors R1 and R2 and potentiometer R3 that are connected across the VCC supply,
provide necessary forward bias to the transistor. When VCC supply is switched on, the rising
collector current through the primary of feedback transformer T1 generates a voltage across
its secondary, which drives the base more positive relative to emitter. This has two effects. The
first is an increase in collector current, which in turn increases positive feedback at the base.
This action is regenerative until saturation is quickly reached. The second effect is, that as the
increasing base current drives the transistor to saturation, the capacitor C2 discharges making
the conduction period still shorter. During the brief period when Q1 conducts heavily, the
capacitor CS charges quickly (from the emitter current) to produce retrace period of the sawtooth
wave. As Q1 saturates, the magnetic field about the transformer T1 stops expanding. The positive
voltage that was induced at the base of Q1 then disappears. As a result, the combined effect of
near-zero voltage at the base, because of potential drop across R2 and a positive voltage on the
+
+
R3
– C
1
R7
D1
R1
R8
LP
T1
LS
Vertical
hold
R2
C2
+
18 V
VCC
D2
Vertical
sync input
Q1
R4
R5
R6
Height
control
CS
To driver
input
C3
5 V (P–P)
Drive voltage at the base of Q1
2.8 V (P–P)
vcs (sweep voltage)
Fig. 18.9. A transistor vertical blocking oscillator.
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MONOCHROME AND COLOUR TELEVISION
emitter due to charge on CS, reverse biases the base-emitter junction and the transistor
immediately goes to cut-off. This sudden change in collector current and the consequent collapse
of magnetic flux in the primary of T1 induces a large reverse voltage in the secondary winding
which aids in keeping the transistor in cut-off state. This voltage at the base could damage the
transistor but for the protective action of diode D1. The diode acts as a short across the secondary
(base) winding of the transformer during the brief back-emf period, protecting the transistor.
After Q1 is cut-off, base capacitor C2 starts charging towards positive voltage at the
junction of R1 and R2. It reaches this positive voltage level very quickly and then levels off.
This is indicated by the base voltage waveform drawn along the circuit diagram. As soon as Q1
cuts off and emitter current ceases, CS starts discharging through R5 producing trace portion
of the sawtooth waveform. When the capacitor has discharged to the point where emitter
voltage no longer keeps Q1 cut-off, it turns on and the entire cycle is repeated.
The frequency of the oscillator is controlled primarily by the time constant R5, CS. Note
that during discharge of capacitor CS, Q1 is cut-off and network R5, CS is isolated from rest to
the circuitry. This provides excellent frequency stability.
Any change in the setting of potentiometer R3 alters forward biasing and thus controls
the instant at which Q1 breaks into conduction. This changes frequency of the oscillator and so
potentiometer R3 acts as ‘hold control’. Similarly potentiometer R6 which controls the magnitude
of sawtooth voltage that is fed to the driver is called ‘size’ or ‘height control’. In transistor
circuits the driver, which is an emitter follower, is necessary to isolate the oscillator from low
input impedance of the corresponding output stage.
Synchronization
Sync pulses received as part of the composite video signal from the transmitting station to
which the receiver is tuned are applied after due processing through diode D2 to the tertiary
winding of the feedback transformer. While D2 prevents vertical oscillator waveforms from
being fed back to the sync circuit, the tertiary (third winding) on T1 provides isolation between
the oscillator and sync circuit.
The positive going sync pulses drive the base more positive and turn on Q1 a little earlier
that it would have normally under free running condition. The sync pulses thus lock the vertical
oscillator to the transmitter field frequency to prevent any rolling of the picture.
Another transistor blocking oscillator circuit is shown in Fig. 18.10. In this circuit
sawtooth network is in the collector circuit and the sawtooth capacitor CS charges during trace
period and discharged during retrace interval. The setting of potentiometer R2 controls initiation
of conduction of the transistor and thus acts as ‘hold control’ over a narrow range of the oscillator
frequency. The output voltage across CS is of the order of 1 volt and is enough to operate the
following driver stage.
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DEFLECTION OSCILLATORS
+
18 V
330 W
D1
R2
Hold
control
To input of
the vertical
stage driver
R1
+
CS
100 mF
T1
Q1
R3
5 mF
+
VCS
C1
D2
Sync input
0
t
Fig. 18.10. Vertical deflection blocking oscillator.
18.4 MULTIVIBRATOR DEFLECTION OSCILLATORS
A multivibrator is another type of relaxation oscillator which employs two amplifier stages,
where the output of one is coupled to the input of the other. This results in overall positive
feedback and the circuit operates such that when one stage conducts, it forces the other to cutoff. Soon the stage that cuts off returns to conduction to force the first stage to cut-off. This
sequence repeats to generate square or rectangular output with a frequency that is controlled
by the coupling networks between the two amplifier stages. As in the case of a blocking oscillator
the multivibrator is used as a controlled switch to charge a capacitor through a resistance to
generate the required sawtooth wave output. The amplifiers may employ tubes or transistors
as active devices.
Multivibrators may be classified as bistable, monostable and astable. A bistable
multivibrator has two stable stages and needs two external trigger signals to complete one
cycle of oscillation. The monostable type has one stable stage and completes one cycle of output
with only one external pluses. However, and astable multivibrator is a free running type and
does not need any external trigger pulse for its normal operation. It is this type of multivibrator
that is employed as a deflection oscillator and its frequency is synchronized with the horizontal
AFC voltage or vertical sync pulses. Multivibrators can also be classified on the basis of coupling
between stages. The two types that are used in TV receivers are plate (or collector) coupled
and cathode (or emitter) coupled.
Transistor Free Running Multivibrator
The circuit configuration shown in Fig. 18.11 (a) is of a free running collector coupled
multivibrator where two common emitter amplifiers are cross coupled to provide positive
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MONOCHROME AND COLOUR TELEVISION
feedback. Note that the base resistances are returned to VCC supply to ensure precise operation
of the multivibrator. The circuit operation can be easily explained if the sequence of operations
is followed from the instant when one transistor just conducts and the other goes to cut-off.
Figure 18.11 (b) illustrates the collector and base voltage waveforms for one complete
cycle. At the instant marked t0, transistor Q2 just conducts to saturation and transistor Q1
returns to cut-off. As this happens, the rising voltage at the collector of Q1 charges the capacitor
C2 to VCC, since, VBE(sat)) ≈ 0. The charging current of C2 flows through the base of Q2 to complete
its circuit. RB2 is selected to provide enough current from VCC to the base of Q2, to keep it in
saturation, even when the charging current of C2 becomes zero and C2 charges to VCC.
+ VCC
VCE1
VCC
RL1
RB1
RB2
C2
+
R1
R2 C1
+
vCE1
E
0
t t
VBE1 + 0 1
C3
vCE2
E
vBE2
E
t2
t
t
0
(Q1)
Q2
vBE1
E
On
(Q1)
RL2
v0
Q1
Off
–VCC
vCE2
VCC
Sync input
(Q2)
(a)
0
vBE2
tA
tB
On
Off
t0 t1
t2
t
(Q2)
–VCC
Off
(b)
RB1
Q1
C1
On
–
+
vC1
+VCC
+
VCC
Q2
0
t
t1
vC1 (vC2)
–VCC
(c)
(d)
Fig. 18.11. Free running transistor multivibrator (a) circuit (b) collector and base
voltage waveforms (c) relevant circuit with Q1 ‘off ’ and Q2 ‘on’ (d) charging
curve of capacitor C1 from VCC towards VCC.
In a similar manner C1 would have got charged to VCC in the previous cycle when Q1
was in saturation. Actually at t = t0 the capacitor C1 which was previously charged to VCC gets
earthed with its positively charged plate towards ground, the moment Q2 goes into full
conduction. As shown in Fig. 18.11(c), C1 is then in parallel with emitter-base junction of the
‘off’ transistor Q1. This puts a reverse bias on Q1 equal to – VCC at t = t0, which is well beyond
cut-off bias of the transistor.
The capacitor C1 now starts charging from – VCC towards + VCC as shown in Fig. 18.11 (d).
At t = t1 the negative voltage across C1 reduces to zero and permits base current flow in the
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DEFLECTION OSCILLATORS
transistor Q1. This action is regenerative and Q1 instantly goes to saturation which in turn
cuts-off Q2. The time constant RB1C1 controls the off period (t1 – t0) of Q1 and can be set equal
to the retrace period of the required sawtooth wave.
The moment Q1 goes into saturation the positively charged plate of C2 is effectively
grounded through Q1. It then begins to charge towards + VCC at a rate determined by the time
constant RB2C2. Once again when VC2 = 0, the second transition takes place to complete one
cycle of operation. This cycle then repeats and permits the circuit to function as a free running
multivibrator. The time constant RB2C2 can be made equal to the trace period. As shown in the
circuit, RB1 consists of R1 and R2 where R2 is a potentiometer to adjust the retrace period and
thereby controls the frequency of the multivibrator.
The output voltage at either of the collectors is a rectangular wave with an amplitude
= VCC – VCE(sat).
Multivibrator Frequency
As shown in Fig. 18.11 (b) the total period
1
= tA i.e. (t – t0) + tB i.e. (t2 – t1).
f
A reference to Fig. 18.11 (c) will show that VC1 takes a time equal to tA to return to zero
from – VCC while charging toward + VCC . Then at the instant t1, νc1 = 0 and we can write.
T=
0 = VCC – (VCC + VCC) exp (– tA/RB1C1)
This expression when solved yields tA = 0.69 RB1C1.
Similarly it can be shown that tB = 0.69 RB2C2.
∴
T = tA + tB = 0.69 RB1C1 + 0.69 RB2C2.
Synchronization
The synchronizing pulses may be positive or negative and may be applied to the emitter, base
or collector of one or both the transistors. The frequency of switching action of the astable
multivibrator is kept lower than that of the synchronizing pulses, to force the transistor to
switch states slightly before the free-running transition time. The sync pulses are applied at
the base of the controlling transistor. The ‘on-off’ periods of the multivibrator are used to
generate sawtoothed output across a capacitor as explained in the previous sections.
Cathode/Emitter Coupled Multivibrator
In this type of multivibrator only one RC coupling is provided between the two amplifiers.
Positive feedback for regenerative action is obtained through a common resistance in the
cathode/emitter leads of the two amplifiers. The oscillator action is explained by considering a
cathode coupled multivibrator. As shown in Fig. 18.12 (a) the coupling from tube V1 to V2 is
through an RC network while from V2 to V1 it is through the common cathode resistance RK.
When V1 conducts the reduced plate voltage is coupled to the grid of V2 via C2 thereby cutting
it off. Thus the cut-off period of V2 depends on the time constant C2 (100 KΩ + R2) while C2
discharges. However, when V2 goes into conduction, the cut-off period of V1 depends on the
time constant for charge of C2. The charge path is through the low resistance of grid-cathode of
V2 (when grid current flows), RK and RL1 while V1 is off. As a result C2 charges fast to provide
a small cut-off period for V1. The cathode coupled multivibrator therefore automatically produces
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MONOCHROME AND COLOUR TELEVISION
an unsymmetrical output as V2 must remain in cut-off for a much longer period than V1. Thus
V2 provides the trace period and V1 the retrace period.
Sawtooth Generation
The circuit of Fig. 18.12 (a) can be modified as shown by dotted chain lines to generate a
sawtooth output for feeding into the horizontal output deflection amplifier. The sawtooth
capacitor is connected in the plate circuit of V2, which stays in cut-off for a long time and
conducts for a short time. This stage then acts as a discharge tube to generate a sawtooth
output as explained earlier. Figure 18.12 (b) illustrates how the sawtooth voltage output
corresponds to cut-off and conduction periods of V2. The variable resistance R2 in the grid
circuit of V2 serves as the frequency (hold) control. The grid resistance R1 for V1 does not
control the oscillator frequency because its grid voltage is controlled by the voltage drop across.
Rk. However, this grid is best suited for frequency control because of its isolation from the
oscillator voltages.
Frequency Control
The horizontal sweep oscillators are normally synchronized by the negative dc control voltage
obtained from the AFC circuit. In the cathode coupled multivibrator (Fig. 18.12 (a)), negative
dc control voltage is applied at the grid of V1. The added negative grid voltage reduces plate
current of V1 when it conducts. This results in a smaller drop in its plate voltage and less
negative drive at the grid of V2. Then less time is needed for C2 to discharge down to cut-off.
This reduction in the cut-off period results in an increased multivibrator frequency. Variations
in the AFC negative voltage thus apply necessary correction in the cut-off period of V2 to keep
the oscillator synchronized with the horizontal sync pulses.
B+
Ip1
Ip2
RL1
V1
R1
To horz
deflection amplifier
v0
680 PF
CS
330 PF
1M
CC
V2
C2
Sync
input
RL2
470 PF
v0
100 K
Conducting
100 K
Frequency
control
+
1K
R2
Cut off
Trace
RK
(a)
(b)
t
O e cy
One
cycle
le
Fig. 18.12. Cathode coupled multivibrator (a) circuit (b) waveforms.
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Retrace
339
DEFLECTION OSCILLATORS
+
vg2
Free running
cycle
Locked
cycle
0
t
Cut-off
bias
–V
+
Sync
pulses
0
1
2
3
4
5
6
t
Positive sync pulses as they appear on the grid of V2
Fig. 18.13. Grid voltage waveform of a multivibrator synchronized by sync pulses.
The oscillator is pulled in to the sync frequency at time t3.
Multivibrator Synchronization
As stated earlier, while discussing sync processing circuits, either positive or negative sync
polarity can be used with multivibrators. A Positive pulse, applied to the grid of a tube in cutoff, can cause switching action if the pulse is large enough to raise the grid voltage above cutoff. A negative sync pulse at the grid of V1 when it is in conduction, results in a more stable
operation and is generally used. In fact, the negative pulse applied to the gird of V1 gets amplified
and inverted to appear as a large positive pulse at the grid of V2.
It is highly improbable that the first sync pulse which arrives will succeed in
synchronizing the oscillator. The waveforms shown in Fig. 18.13 illustrate how the oscillator is
gradually pulled into synchronism. It would be pertinent to mention here that for a blocking
oscillator only a positive sync pulse can cause synchronization.
Multivibrator Stabilization
As the grid voltage approaches its cut-of value, it becomes increasingly sensitive to noise pulses
which might have become part of the signal. A strong such pulse, arriving slightly before the
synchronizing pulse can readily trigger the oscillator prematurely and cause rolling or tearing
of the picture.
To ensure stability of operation, resonant circuits are employed in some sweep circuits.
A cathode coupled multivibrator of the type shown in Fig. 18.12 (a) is redrawn in Fig. 18.14 (a),
with a resonant stabilizing circuit placed in the plate circuit of tube V1. the frequency of the
resonant circuit is adjusted to 15625 Hz. The tuned circuit is shock excited by periodic switching
of the tube from an ‘on’ to an ‘off ’ condition. As a result the sinusoidal output of the resonant
circuit modifies the voltage waveforms at the plate of V1 and grid of V2. This is illustrated in
Fig. 18.14 (b), where of particular importance is the grid waveform of triode V2. It may be noted
that the grid voltage now approaches cut-off very sharply and only a very strong noise pulse
will be able to trigger the second triode prematurely.
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MONOCHROME AND COLOUR TELEVISION
B+
L1
Resonant circuit
tuned to
15625 Hz
RL2
C
1
100 K
V2
RL1
Sync
input
Cut-off
250 V
C2
CC
V1
CS
C3
47 PF
(i) Plate voltage waveshape
of V1 without resonant
circuit
(iv) Grid voltage of V2
v0
with no resonant
circuit
(ii) Voltage across
resonant circuit
Rg
Cut-off
(i + ii) = (iii)
RK
(a)
(iii) Plate voltage of V1 (v) Grid voltage waveform
of V2 with resonant
with resonant circuit
circuit
(b)
Fig. 18.14. Cathode coupled multivibrator with a stabilizing resonant circuit
(a) circuit (b) waveforms.
18.5 COMPLEMENTARY-SYMMETRY RELAXATION OSCILLATOR
A complementary-symmetry relaxation oscillator, designed to drive the vertical deflection output
circuit, is illustrated in Fig. 18.15 (a). Transistors Q1(p-n-p) and Q2(n-p-n) which are directly
coupled, constitute the oscillator pair, while Q3 is the waveshaping transistor. Resistors R1
and R2 form a potential divider across VCC supply through the decoupling network R3, C6, to
provide positive voltage both at the base of Q1 and collector of Q2. The voltage at the emitter of
Q1 is developed by capacitor C1, when it charges towards VCC (+ 20 V), through resistance R4
and potentiometer R5 connected in series.
At the instant dc supply is switched on to the circuit, both the transistors are at cut-off,
because the base of Q1 is biased positively and its emitter is at zero potential. However, capacitor
C1 starts charging at once driving emitter of Q1 positive. When the rising voltage across C1
offsets the positive voltage at the base of Q1, the transistor turns on. This makes the base of Q2
positive which also goes into conduction. The collector current of Q2 flows through R1 and the
resulting drop across it, lowers the potential at the base of Q1, thus making it more negative
with respect to its emitter. This results in increased current through Q1 and the regenerative
feedback action that follows soon saturates Q1. When Q1 is on, its emitter current starts
discharging C1. As soon as the emitter voltage drops sufficiently to remove forward bias on Q1
it is driven out of conduction. This in turn cuts off Q2, thereby completing the regenerative
cycle. The capacitor C1 starts charging again towards VCC to repeat the sequence of events
explained above.
In the absence of any sync input, Q1 and Q2 repeat the on-off cycle at a rate determined
by the time constant C1(R4 + R5). This time constant determines frequency of the oscillator.
Potentiometer R5 is the ‘hold control’ and can be varied to change the frequency. A negative
going vertical sync pulse applied at the base of Q1, through the integrating networks R9, C3
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DEFLECTION OSCILLATORS
and R10,C4, brings the transistor out of cut-off before it normally would. This synchronizes the
oscillator to the applied sync pulses.
The voltage waveform (i) at the emitter of Q1 (see Fig. 18.15 (b)) is a sawtooth wave
developed by the charging and discharging of C1. Waveform (ii) is the sharp positive pulse
developed at the emitter of Q2 when both the transistors are turned on.
+
20 V
VCC
R3
+ VCC From horz
output circuit
R5
C6
R8
R1
R4
R9
Sync
input
C3
R10
C4
Q1
R2
i
i
+
R7
C1
C5
Q2
To vertical
output
amplifier
iii
ii
Q3
+
C2
ii
iii
R6
Fig. 18.15 (a). Complementary symmetry relaxation oscillator.
Fig. 18.15 (b). Wave shapes
at the emitters of Q1, Q2
and collector of Q3.
Wave Shaping
The wave shaping transistor Q3 is normally biased to cut-off. It is triggered ‘on’ by the positive
pulse developed at the emitter of Q2 and directly coupled to its base. Capacitor C2 connected
across the collector and emitter of Q3 charges during the ‘off ’ period of Q3 and discharges when
the transistor turns on for a short interval of time. The resulting sawtooth voltage across C2
(waveshape(iii)) is coupled through C5 to the vertical output stage of the receiver.
The rate at which C2 charges during the ‘off ’ interval of Q3 is determined by resistors R7
and R8. R7 is a potentiometer that can be varied to control the amplitude of the vertical sweep
and through this the height of the picture.
The voltage towards which C2 charges is derived from a rectifier in the horizontal output
circuit. Any change, in the horizontal deflection output voltage, will also alter the dc voltage
fed to this circuit and affect the amplitude of the sawtooth voltage. Thus, if horizontal size of
the picture changes, vertical size also changes proportionately, preserving the ratio of height
to width of the picture.
18.6 SINE-WAVE DEFLECTION OSCILLATORS
High frequency sine-wave oscillators are more stable in their operation as compared to
corresponding relaxation oscillators. Since the horizontal sweep frequency is quite high such
oscillators are often used in horizontal deflection circuits. For such an application the oscillator
is overdriven so that the tube or transistor acts like a switch and allows a sawtooth forming
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MONOCHROME AND COLOUR TELEVISION
capacitor to charge and discharge. The frequency of the oscillator is controlled by a reactance
tube or transistor placed between the AFC circuit and oscillator.
Reactance Tube Sweep Generator
A typical circuit of a vacuum tube horizontal deflection oscillator cum sweep generator is shown
in Fig. 18.16. It employs a tuned grid configuration where mutual coupling between coils L1
and L2 can be varied to change the oscillator frequency. The desired frequency is 15625 Hz.
The screen grid of pentode V2 acts as plate for oscillator action. This isolates the oscillator
circuit from large plate voltage variations which occur on account of switching action of the
tube.
B
400 V
39 K
50 V
3 KPF
4V
D1
1.5 KPF
From sync
separator circuit
D2
100 K
68 PF
100 K
1M
470
KPF
390 K
1.5
KPF
39 K
V1
3 KPF
C1
R2 125 V
V2
39 K
1K
L1
3 KPF
L2
33 K
R3
680 W
0.22 mF
130 V
4.7 K
175 V
1.7 V
200 V
150 W
4 mF
145 V
1
KPF
150
K
100 KPF
v0
CC
C1
470 PF
R1
56 K
40 V
Fig. 18.16. Reactance controlled horizontal sweep generator.
The oscillator uses self-bias and develops about – 15 V at the control grid. For a pentode,
this voltage is enough for class C operation. The oscillator is overdriven so that plate current
flows in short-duration pulses only during extreme positive peaks of the grid voltage, The
capacitor C1 charges towards B+ to develop trace period when the tube is off. The retrace is
formed when the tube conducts for a short duration. The high (39 K) plate load resistance R2
ensures a large (130 V P-P) sweep voltage.
Frequency Control
The reactance tube V1 shunts the oscillator tank circuit and appears as a capacitive reactance.
As explained in Chapter 7, Such a behaviour can be simulated by a capacitive feedback between
plate and control grid of the tube. The equivalent capacitance across the output terminals of
the tube is proportional to mutual conductance (Ceq = gm × RC) of the triode. In the circuit
under consideration, the bias of the reactance tube is the combined result of AFC derived grid
voltage and voltage drop across R3, the cathode resistor. And drift in the oscillator frequency
results in a change in the dc control voltage. This in turn shifts the operating point to change
gm of the reactance tube. The consequent change in the simulated equivalent capacitance Ceq,
forces the oscillator to correct its frequency. The continuous feedback action results in a very
stable operation of the oscillator.
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DEFLECTION OSCILLATORS
Reactance Transistor Sweep Generator
Figure 18.17 shows a transistor sine-wave oscillator whose frequency is controlled by a reactance
transistor. In this circuit a Hartley oscillator is used. It performs the same functions as its
vacuum tube counterpart discussed earlier. The output of the sine-wave transistor oscillator is
usually fed to the horizontal output circuit through a driver to provide isolation between the
output stage and oscillator circuit.
Horz osc
From
AFC
Reactance
transistor
C2
0.027
F
15 K
Q1
5 F
0.01 F
1K
R2
C1
360
3.3 K
+
– 1V
0.0068 F
Q2
19 V
470
To horz
output
amplifier
330
Hold
control
Bias
voltage
22 V
Fig. 18.17. Transistor sine-wave horizontal deflection oscillator and its associated reactance transistor.
As shown in the circuit diagram a reactance transistor (Q1) is used between the AFC
output and horizontal oscillator. It acts like an inductor instead of a capacitor, In order to
simulate such a behaviour the phase shift network C2, R2 couples some of the oscillator’s voltage
into the emitter of the reactance transistor and at the same time shifts its phase by 90°. Since
the feedback is returned to the emitter, transistor Q1 can be considered to be a common base
amplifier. As such, the emitter voltage and collector current are 180° out of phase. Thus the
transistor current lags behind the collector voltage by 90°. Therefore, the oscillator ‘sees’ the
reactance transistor as if an inductor has been connected across its tank circuit.
Any change in the oscillator frequency is sensed by the AFC circuit which couples a
proportionate dc error voltage into the base of the reactance transistor. This shifts the operating
point of the transistor in the same way as AGC bias does to control gain of tuned amplifiers.
Any decrease in oscillator frequency results in a positive dc control voltage thereby
increasing forward bias of the reactance transistor. The resulting increase in collector current
shifts the operating point where the transistor gain is lower. In turn, this acts to decrease the
effective reactance at the output terminals of the transistor. This causes the total inductance
shunting the oscillator tuned circuit to decrease. As a result, frequency of the oscillator is
increased to return to its correct value. The reverse would occur if the bias were to decrease for
any increase in the oscillator frequency.
Review Questions
1.
Sketch and label the current waveforms that must flow in the deflection yoke coils to produce a
full rester. Explain the basic principle of generating such waveforms.
2.
Why is a trapezoidal voltage waveform necessary to drive the vertical deflection coils ? What is
the effect of source impedance and frequency on the shape of the driving voltage waveform ? How
is the basic sawtooth voltage modified to obtain the desired driving voltage waveform ?
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MONOCHROME AND COLOUR TELEVISION
3.
Draw the circuit diagram of a blocking oscillator-cum-waveshaper which employs a single triode
and discuss its operation. Explain the operation of ‘hold control’ and ‘height control’ as provided
in the circuit drawn by you.
4.
A blocking oscillator that employs a transistor is shown in Fig. 18.9. Explain how the circuit
operates to develop a sawtooth voltage. In particular, explain the operation of hold and height
controls. Why are diodes D1 and D2 provided in the feedback transformer circuit ?
5.
Explain with suitable waveforms how the frequency of the blocking oscillator is controlled with
the help of sync information. Why is the free running frequency of the oscillator kept somewhat
lower than the desired frequency ? What will happen if the uncontrolled frequency is higher than
the correct frequency ?
6.
Draw the basic circuit of a multivibrator employing transistors. How are the feedback networks
designed to obtain correct trace and retrace periods ? Explain how the sync pulses control the
frequency of such an oscillator.
7.
The circuit of a complementary-symmetry relaxation oscillator is given in Fig. 18.15 (a). Describe
briefly how the circuit operates to generate sawtooth output voltage. Discuss with suitable waveforms how the sync pulses keep the oscillator in synchronism with corresponding oscillator at
the transmitter.
8.
Why is a sine-wave oscillator preferred in horizontal deflection circuits ? Explain with a circuit
diagram how a reactance tube/transistor connected between the AFC circuit and oscillator operates
to maintain a constant frequency.
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