1 fundamentals of monochrome and colour tv system

1 fundamentals of monochrome and colour tv system
1
FUNDAMENTALS OF
MONOCHROME AND COLOUR TV
SYSTEM
Picture formation
A picture can be considered to contain a number of small elementary areas of light or
shade which are called PICTURE ELEMENTS. The elements thus contain the visual
image of the scene.
In the case of a TV camera the scene is focused on the photosensitive surface of
pick up device and a optical image is formed. The photoelectric properties of the pick up
device convert the optical image to a electric charge image depending on the light and
shade of the scene (picture elements). Now it is necessary to pick up this information
and transmit it. For this purpose scanning is employed. Electron beam scans the
charge image and produces optical image. The electron beam scans the image line by
line and field by field to provide signal variations in a successive order.
The scanning is both in horizontal and vertical direction simultaneously.
The horizontal scanning frequency is 15,625 Hertz.
The vertical scanning frequency is 50 Hz.
The frame is divided in two fields. Odd lines are scanned first and then the even lines.
The odd and even lines are interlaced. Since the frame is divided into 2 fields the flicker
reduces. The field rate is 50 Hertz. The frame rate is 25 Hertz (Field rate is the same as
power supply frequency).
Number of TV Lines per Frame
If the number of TV lines is high larger bandwidth of video and hence larger R.F.
channel width is required. If we go for larger RF channel width the number of channels
in the R.F. spectrum will be reduced. However, with more no. of TV lines on the screen
the clarity of the picture i.e. resolution improves. With lesser number of TV lines per
frame the clarity (quality) is poor.
A compromise between quality and conservation of r.f. spectrum led to the
selection of 625 lines in CCIR system B. Odd number is preferred for ease of sync pulse
generator (SPG) circuitary to enable interlace of fields.
The capability of the system to resolve maximum number of picture elements
along scanning lines determines the horizontal resolution. It means how many alternate
black and white elements can be there in a line. Let us also take another factor. It is
realistic to aim at equal vertical and horizontal resolution. We have seen earlier that the
vertical resolution is limited by the number of active lines. We have already seen that
the number of active lines are 575. so for getting the same resolution in both vertical
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and horizontal directions the number of alternate black and white elements on a line can
be 575 multiplied by Kell factor and aspect ratio. Therefore, the number of alternate
black and white dots on line can be 575 x 0.69 x 4/3 which is equal to 528.
It means there are 528 divided by 2 cyclic changes i.e. 264 cycles. These 264
cycles are there during 52 micro seconds. Hence the highest frequency is 5 MHz.
fh ig h e s t=
2 6 4× 1 06
52
=
5MHz
Therefore the horizontal resolution of the system is 5 MHz.
A similar calculation for 525 lines system limits the highest frequency to 4 MHz and
hence the horizontal resolution of same value.
In view of the above the horizontal bandwidth of signal in 625 lines system is 5 MHz.
Viewing Distance
Optimum viewing distance from TV set is about 4 to 8 times the height of the TV screen.
While viewing TV screen one has to ensure that no direct light falls on the TV screen.
Composite Video Signal (CVS)
Composite Video Signal is formed with Video, sync and blanking signals. The
level is standardized to 1.0 V peak to peak (0.7 volts of Video and 0.3 volts of sync
pulse). The composite video signal (CVS) has been shown in figure 1.
0.7V
1.5µ
Sec.
1.0V
0.3V
Front
Porch
4.7µ
Sec.
52µ
Sec.
Active Period
Back
Porch
Sync Tip
5.8µ
Sec.
12µ
Sec.
H Blanking
64µ
Sec.
H Period
Fig. 1 Composite Video Signal (CVS)
RF Transmission of Vision and Sound Signals
TV Transmission takes place in VHF Bands I and III and UHF Bands IV and V.
Picture is amplitude modulated and sound is frequency modulated on different carriers
separated by 5.5 MHz.
Also for video amplitude modulation negative modulation is employed because of the
following main advantages.
•
Pictures contain more information towards white than black and hence the
average power is lower resulting in energy saving. (Bright picture points
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correspond to a low carrier amplitude and sync pulse to maximum carrier
amplitude).
•
Interference such as car ignition interfering signals appear as black which is less
objectionable.
•
Picture information is in linear portion of modulation characteristic and hence
does not suffer compression. Any compression that may take place is confined
to sync pulse only.
•
The design of AGC circuit for TV Receiver is simpler.
AM produces double side bands. The information is the same in both side bands. It is
enough to transmit single side band only. Carrier also need not be transmitted in full and
a pilot carrier can help. However, suppressing the carrier and one complete side band
and transmitting a pilot carrier leads to costly TV sets. A compromise to save RF
channel capacity is to resort to vestigial side band system in which one side band in full,
carrier and a part of other side band are transmitted.
Part of L.S.B
Picture Carrier
-1.25 –0.75
0
Sound Carrier
U.S.B
5
5.5
Frequency MHz
Fig. 5 Theoretical representation of the side bands in VSB transmission.
3
THE PAL COLOUR TELEVISION SYSTEM
The Colour Television
It is possible to obtain any desired colour by mixing three primary colours i.e. Red, Blue
and green in a suitable proportion.
Additive Colour Mixing
The figure 10 shows the effect of projecting red, green, blue beams of light so that they
overlap on screen.
Y=
0.3 Red + 0.59 Green + 0.11 Blue
Fig. 10 Additive Colour Mixing
The Colour Television
It is possible to obtain any desired colour by mixing three primary colours i.e., red, blue
and green in suitable proportion. Thus it is only required to convert optical information of
these three colours to electrical signals and transmit it on different carriers to be
decoded by the receiver. This can then be converted back to the optical image at the
picture tube. The phosphors for all the three colours i.e. R, G and B are easily available
to the manufacturers of the picture tube. So the pick up from the cameras and output for
the picture tube should consists of three signals i.e. R, G and B. It is only in between the
camera and the picture tube of the receiver we need a system to transmit this
information.
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Colour television has the constraint of compatibility and reverse compatibility with the
monochrome television system which makes it slightly complicated. Compatibility
means that when colour TV signal is radiated the monochrome TV sets should also
display Black & White pictures. This is achieved by sending Y as monochrome
information along with the chroma signal. Y is obtained by mixing R,G & B as per the
well known equation :
Y
=
0.3 R + 0.59 G + 0.11 B
Reverse compatibility means that when Black & White TV signal is radiated the colour
TV sets should display the Black & White pictures.
If we transmit R, G, B, the reverse compatibility cannot be achieved. Let us see how :
If we transmit Y, R & B and derive G then :
Since
Y
=
0.3R + 0.59G + 0.11 B
G
=
1.7Y - 0.51 R - 0.19 B
In such a case what happens with a colour TV set when we transmit black and white
signal. R and B are zero, but G gun gets 1.7 Y. The net result is black & white pictures
on a colour TV screen appear as Green pictures. So reverse compatibility is not
achieved.
Colour Difference Signals
To achieve reverse compatibility, when we transmit Y, R-Y and B-Y instead of Y, R & B,
we do not take G-Y as this will always be much lower than R-Y and B-Y and hence will
needs more amplification and will cause more noise into the system. G-Y can be
derived electronically in the TV receiver.
In the previous paragraph we have seen
G
=
1.7 Y - 0.51 R - 0.19 B
So
G-Y =
-0.51 (R-Y) - 0.19 (B-Y)
Thus, colour difference signals fulfill the compatibility and reverse compatibility.
Because in this case the colour difference signals are zero if the original signal is
monochrome (i.e. R = B = G)
So if we take R - Y
R - Y = R - (0.3 R + 0.59 R + 0.11 R) = 0
Similarly B - Y = 0
As such colour difference signals are zero for white or any shade of gray whereas, Y
carries the entire Luminance information.
It is to be noted while R, G, B signals always have positive value R-Y, B-Y and
G-Y signals can either be positive or negative or even zero.
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The R-Y and B - Y chrominance signals may be recovered at the television
receiver by suitable synchronous demodulation. But sub-carrier is to be generated by a
local oscillator. This generated sub-carrier in the receiver must have same frequency as
that of transmitted sub-carrier and also the same phase. This is achieved by
transmitting 10 cycles of sub-carrier frequency on the back porch of H synchronizing
pulse. This 10 cycles sub-carrier signal is known as BURST or colour BURST.
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2
VIDEO CHAIN IN A TYPICAL
DOORDARSHAN STUDIO
STUDIO CENTRE
A Studio centre of Doordarshan has the following objectives:
1)
To originate programmes from studios either for live telecast or for recording on a
video tape.
2)
To knit various other sources of programs available at the production desk i.e.,
camera output from studios, feed from other kendras, outdoor, playback from pre
recorded tape, film based programs slides, video graphics and characters generator
etc. This knitting or live editing includes generation of special effects and desired
transitions between various sources.
3)
Processing/distribution of different sources to various destinations in technical
areas.
4)
Routing of mixed programme for recording/transmission via master switching
room and Micro Wave to the transmitter or any other desired destinations.
Activities in a television studio can be divided into three major areas such as :
1) Action area,
2) Production control room, and
3) Central apparatus room,
Action area
This place requires large space and ceiling as compared to any other technical area.
Action in this area includes staging, lighting, performance by artists, and arrangement to
pick up picture and sound. Hardware required for these activities in a studio (typical
size 20 x20x8.5 cubic meters) are:
1.
Very efficient air conditioning because of lot of heat dissipation by studio light and
presence of large number of persons including invited audience performing artists
and operational crew.
2.
Uniform and even flooring for smooth operation of camera dollies and boom
microphone etc.
3.
Acoustic treatment Keeping in mind that a television studio is a multi purpose
studio with lot of moving person and equipment during a production.
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4.
5.
Supporting facilities like properties, wardrobe, and makeup etc.
Effective communication facilities for the floor crew with the production control
area.
6.
Studio cameras (three to four) with one of the cameras fitted with teleprompter
system and pressure dolly.
7.
Luminaires and suspension system having grids or battens (hand/motorised
operation).
8. Pick up wall sockets for audio operations.
9. Tie lines box for video and audio lines from control room
10. Cyclorama and curtain tracks for blue and black curtain for chroma keying and
limbo lighting respectively.
11. Audio and video monitoring facilities.
12. Studio warning light and safety devices like fire alarm system and fire fighting
equipments etc.
13. Digital clock display.
Operational requirement from the technical crew may vary from programme to
programme. These requirements for lighting, audio pick up and special effects etc.
depends upon the programme requirement such as establishing a period, time, formal or
informal situation.
Production control area
Activities in this area are:1. Direction to the production crew by the producer of the programme.
2. Timing a production/telecast.
3. Editing of different sources available at the production desk.
4. Monitoring of output/off air signal.
Hardware provided in this area include:
1. Monitoring facilities for all the input and output sources(audio/video).
2. Remote control for video mixer, telecine and library store and special effect
(ADO) etc.
3. Communication facilities with technical areas and studio floor.
Vision mixing and switching
Unlike films, television media allows switching between different sources simultaneously
at the video switcher in Production control room operated by the Vision Mixer on the
direction of the program producer. The producer directs the cameramen for proper
shots on various cameras through intercom and the vision mixer (also called VM
engineer) switches shots from the selected camera/cameras with split second accuracy,
in close cooperation with the producer. The shots can be switched from one video
source to another video source, superimposed, cross faded, faded in or faded out
electronically with actual switching being done during the vertical intervals between the
picture frames. Electronics special effects are also used now days as a transition
between the two sources.
Vision Mixer (or Video Switcher)
Though the video switching is done by the VM at the remote panel, the electronics is
located in CAR. The vision mixer is typically a 10 x 6 or 20 x 10 cross bar switcher
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selecting anyone of the 10 or 20 input sources to 6 or 10 different output lines. The input
sources include: Camera 1, camera 2, camera 3, VTR1, VTR2, Telecine 1, Telecine 2,
Test signal etc. The vision mixer provides for the following operational facilities for
editing of TV programs:(i)
Take: Selection of any input source
or
Cut: switching clearly from one source to another.
(ii)
DISSOLVE: Fading out of one source of video and fading in another source of video.
(iii)
(iv)
SUPERPOSITION OF TWO SOURCES: Keyed caption when selected inlay is
superimposed on the background picture.
SPECIAL EFFECTS: A choice of a number of wipe patterns for split screen or wipe
effects.
The selected output can be monitored in the corresponding pre-view monitor. All the
picture sources are available on the monitors. The preview monitors can be used for
previewing the telecine, VTR; test signals etc. with any desired special effect, prior to its
actual switching.
The switcher also provides cue facilities to switch camera tally lights as an indication to
the cameraman whether his camera is on output of the switcher.
Present day PCR’s have:
•
•
•
•
•
24 input video special effects switchers
(CD 680 or CD 682-SP)
Character generators
Telecine/DLS remote controls
Adequate monitoring equipment
Character Generator(CG)
Character Generator provides titles and credit captions during production in Roman
script. It provides high resolution characters, different colours for colorizing characters,
background, edges etc. At present bilingual and trilingual C.G are also being used by
Doordarshan.
Character Generator is a microcomputer with Texts along instructions when typed in at
the keyboard is stored on a floppy or a Hard disk. Many pages of scripts can be stored
on the disk and recalled when needed, by typing the addresses for the stored pages, to
appear as one of the video sources.
Sync Pulse-Generator(SPG)
It is essential that all the video sources as input to the switcher are in synchronism i.e.,
start and end of each line or all the frames of video sources is concurrent. This
requirement is ensured by the sync pulse generator (SPG). SPG consists of highly
stable crystal oscillator. Various pulses of standard width and frequency are derived
from this crystal electronically which form clock for the generation of video signal. These
pulses are fed to all the video generating equipment to achieve this objective of
synchronism. Because of its importance, SPG is normally duplicated for change over in
case of failure.
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It provide the following outputs:
•
•
•
•
•
•
•
Line drive
Field drive
Mixed blanking
Mixed sync
colour subcarrier
A burst insertion pulse
PAL phase Indent pulses
Camera Control Unit (CCU)
The television cameras which include camera head with its optical focusing lens,
pan and tilt head, video signal pre-amplifier view finder and other associated electronic
circuitry are mounted on cameras trolleys and operate inside the studios. The output of
cameras is pre-amplified in the head and then connected to the camera control unit
(CCU) through long multi-core cable (35 to 40 cores), or triax cable.
All the camera control voltages are fed from the CCU to the camera head over the multicore camera cable. The view-finder signal is also sent over the camera cable to the
camera head view-finder for helping the cameraman in proper focusing, adjusting and
composing the shots.
The video signal so obtained is amplified, H.F. corrected, equalized for cable delays,
D.C. clamped, horizontal, and vertical blanking pulses are added to it. The peak white
level is also clipped to avoid overloading of the following stages and avoiding over
modulation in the transmitter. The composite sync signals are then added and these
video signals are fed to a distribution amplifier, which normally gives multiple outputs for
monitoring etc.
Light Control
The scene to be televised must be well illuminated to produce a clear and noise free picture. The
lighting should also give the depth, the correct contrast and artistic display of various shades
without multiple shadows.
The lighting arrangements in a TV studio have to be very elaborate. A large number of lights are
used to meet the needs of ‘key’, ‘fill’, and ‘back’ lights etc. Lights are classified as spot and soft
lights. These are suspended from motorized hoists and telescopes. The up and down movement
is remotely controlled. The switching on and off the lights at the required time and their dimming
is controlled from the light control panel inside a lighting control room using SCR dimmer controls.
These remotely control various lights are inside the studios.
Sound mixing and control
As a rule, in television, sound accompanies the picture. Several microphones are generally
required for production of complex television programs besides other audio sources also called
marred sound from telecine, VTR, and audio tape/disc replays. All these audio sources are
connected to the sound control console.
The sounds from different sources are controlled and mixed in accordance with the requirement
of the program. Split second accuracy is required for providing the correct audio source in
synchronisation with the picture thus requiring lot of skill from the engineer. Even the level of
sound sometimes is varied in accordance with the shot composition called prospective.
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Audio facilities
An audio mixing console, with a number of inputs, say about 32 inputs is provided in major studio.
This includes special facilities such as equalisation, PFL, phase reversal, echo send/receive and
digital reverberation units at some places Meltron console tape recorders and EMI 938 disc
reproducers are provided for playing back/creating audio effects as independent sources
(Unmarried) to the switcher.
Video Tape recorders
VTR room is provided at each studio center. It houses a few Broadcast standard Videocassette
recorders (VCRs). In these recorders, sound and video signals are recorded simultaneously on
the same tape.
Most of the TV centers have professional quality B-Format BCN-51 One inch VTRs. For
broadcast quality playback it is equipped with correction electronics i.e. a processor which
comprises velocity error compensation, drop-out compensation and time base correction. It also
comprises a digital variable motion unit enabling still reproduction, slow motion and visible search
operation.
New centers are being supplied with Sony U-matic high band VCRs along with ½” Sony
Betacam SP VCRs, DVC Pro. High bands VCRs are to be provided with digital time base
correctors where as Betacam has got built in DTBC with studio machines.
Post Production Suites
Modern videotape editing has revolutionised the production of television programs over the years.
The latest trend all over the world is to have more of fully equipped post production suites than
number of studios. Most of the present day shootings are done on locations using single camera.
The actual production is done in these suites. The job for a post production suites is:a.
To knit program available on various sources.
b.
While doing editing with multiple sources, it should be possible to have any kind of
transition.
c.
Adding/Mixing sound tracks.
d.
Voice over facilities.
e.
Creating special effects.
The concept of live editing on vision mixer is being replaced by “to do it at leisure” in post production suites.
1.
2.
3.
4.
5.
6.
7.
8.
9.
A well equipped post production suite will have:Five VTRs/VCRs, may be of different format remotely controlled by the editor.
Vision mixing with special effect and wipes etc. with control from a remote editor panel.
Ampex Digital Optics (ADO) for special effects.
Audio mixer with remote control from the editor remote panel.
Multi-track audio recorder with time code facilities and remote operation.
Character generator for titles.
Adequate monitoring facilities.
Supported by “Offline editing systems” to save time in post production suites.
One man operation.
Coverage of Outside events :
Outside broadcasts(or OBs) provide an important part of the television programs. Major
events like sports, important functions and performances are covered with an O.B. van which
contains all the essential production facilities.
Video Chain :
The block diagram on facing page connects all these sections and it can be observed that the
CAR is the nodal area. Now let us follow a CAM-I signal. CAM-I first goes to a Camera
electronics in CAR via a multi-core cable, the signal is then matched/adjusted for quality in CCU
and then like any other sources it goes to video switcher via PP (Patch Panel) and respective
VDAs(Video Distribution Amplifiers) and optional Hum compensator/Cable equilizers.
Output from the switcher goes to stabilizing amplifier via PP and VDAs. Output from the
stab. Is further distributed to various destinations.
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12
3
TV LIGHTING
1.
GENERAL PRINCIPLES
Lighting for television is very exciting and needs creative talent. There is always
a tremendous scope for doing experiments to achieve the required effect. Light is a kind
of electromagnetic radiation with a visible spectrum from red to violet i.e. wave length
from 700 nm to 380 nm respectively. However to effectively use the hardware and
software connected with lighting it is important to know more about this energy.
Light Source
Any light source has a Luminance intensity (I) which is measured in Candelas.
Candela is equivalent to an intensity released by standard one candle source of light.
Colour Temperature
One may wonder, how the light is associated with colour. Consider a black body
being heated, you may observe the change in colour radiated by this body as the
temperature is increased. The colour radiated by this body changes from red dish to
blue and then to white as the temperature is further increased. This is how the concept
of relating colour with temperature became popular. Colour temperature is measured in
degree Kelvin i.e. ( C + 273) . The table below gives idea about the kind of radiation
from different kinds of lamps in terms of colour temperature.
o
Standard candle
1930o K
Gas filled tungsten lamp2760o K
Projection bulb 3200o K
Flash-bulb
3800o K
HMI lamp
6500o K
Electronic flash tube
6000o K
Average day light
6500o K
Blue sky
12000 - 18000o K
Basic Three Point Lighting
Key light : This is the principal light source of illumination. It gives shape and
modeling by casting shadows. It is treated like "sun" in the sky and it should cast only
one shadow. Normally it is a hard source.
Fill Light : Controls the lighting contrast by filling in shadows. It can also provide catch
lights in the eyes. Normally it is a soft source.
Back light : Separates the body from the background, gives roundness to the subject
and reveals texture. Normally it is hard source.
Background Light : Separates the person from the background, reveals background
interest and shape. Normally it is a hard source.
In three point lighting the ratio of 3/2/1 (Back/Key/Fill) for mono and 3/2/2 for colour
provides good portrait lighting.
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4
TV CAMERA
Introduction
A TV Camera consists of three sections :
a)
A Camera lens and optics
:
b)
A transducer or pick up device
:
c)
Electronics
:
To form optical image on the face
plate of a pick up device.
To convert optical image into an
electrical signal.
To process output of a transducer to
get a CCVS signal.
CCD CAMERAS
Introduction
Any camera will need a device to convert optical image into an electrical signal. Now let us
consider a picture frame made of small picture element. For more sharpness or better resolution
we have to increase these elements. This picture frame can now be focused on to a structure of
so many CCD elements. Each CCD element will now convert the light information on it to a
charge signal. All we need now is to have an arrangement to collect this charge and convert it to
voltage. This is the basic principle on which CCD cameras are based.
Latest CCD Cameras
CCD were launched in 1983 for broadcasting with pixel count from a mere 2,50,000 which
increased to 20,00,000 in 1994 for HDTV application. Noise and aliasing has been reduced to
negligible level. CCD cameras now offers fully modulated video output at light level as low as 6.0
lumens. A typical specification for a studio camera now available in market are some thing like
2/3 inch, FIT, lens on chip CCD with 6,00,000 pixel, 850 lines H resolution, S/N more than 60 dB,
sensitivity F-8 (2000 lux) etc.
Fig. 5 Block Diagram of a typical Camera
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5
VIDEO TAPE FORMATS
Introduction
Format of Video tape recorder defines the arrangement of magnetic information of the
tape. It specifies :
•
•
•
•
The width of tape
Number of tracks for Video, Audio, Control, Time Code and Cue,
Width of tracks
Their electrical characteristics and orientation
All machines conforming to one format have similar parameters to enable compatibility
or interchange i.e. the tape recorded on one machine is faithfully reproduced on the
other. There are a number of formats in video tape recording and the number further
gets multiplied due to different TV standards prevailing in various countries e.g. PAL,
SECAM, NTSC and PAL-M.
Classification
• Composite Analog Formats (All reel/Spool type)
Quadruplex, 1” B format and 1” C format for professional Broadcast use.
• Heterodyne formats (Cassette)
U-matic LB, HB, SP; for semi-professional work.
• Component analog formats (Cassette)
Betacam, Betacam SP, M-II; for professional Broadcast use.
• Digital Composite/Component formats (Cassette)
D1, D2, D3, D5, Digital Betacam and DCT (Ampex); for professional
Broadcast use.
• Heterodyne domestic (Cassette)
VHS, Betamax, Video 8mm, S-VHS, Hi-8; for domestic and semi professional
use (S-VHS & Hi-8)
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Digital Composite/Component Formats :
These are ultimate in video recording as the information is recorded in digital form and
multi-generation dubbing is no longer a problem. The various digital formats in use are
as under :
a) D1 : It is the first digital standard and uses component system (i.e. CCIR 601
YUV, 4:2:2 format) using ¾” tape with writing speed of about 30 m/s and helical drum
running at 150 rps with segmented tracks. Digital coding is in 8-bit words with a raw
picture data rate of 216 M bits/sec. The 601 standard now provides the option for 10-bit
coding but is not implemented in D1 machines.
b) D2 : To reduce cost, D2 system was introduced by Ampex. D2 uses D1 cassette
of high coercivity ¾” metal tape with two pairs of heads scanning eight tracks per field.
D2 uses composite video signal for sampling, having 8 bit 4 times sub carrier frequency
sampling, with writing speed of about 28 m/s using helical drum running at 100 rps.
Data rate is around 150 Mb/s.
c) D3 : D3 was developed by NHK and Panasonic using composite system, ½”
metal particle VHS sized cassette thus saving cost. It records 8 bit digital video at a
sampling rate of 4 fsc (17.73 MHz) in 8 tracks per field. Data rate is similar to D2. It too
offers 4 digital audio 16-20 bit at 48 kHz and cue track with comprehensive slow motion.
Head drum rotates at 100 rps with a writing speed of 21.4 m/s. The signal recorded on
½ inch metal tape is more than twice the recording density of any other existing formats.
Because of its compact size it is suitable for camcorders. D3 cassette can record 4
hours of continuous recording. Multi-generation suffers in quality in comparison to
component machines.
d) D4 : 4 is perhaps an unlucky number in Japan as there is no D4.
e) D5 : Panasonic now has a new component system called D5 using ½” tapes. It
is successor to D3. It is digital component using same cassettes as D3 but running at
double speed. D5 can handle 4:3 or 16:9 aspect ratio with full restoration. For 16:9
sampling rate is 18 MHz with 8-bit coding.
f) Digital Betacam : Keeping in view the enormous success of Beta-SP, Sony
have announced a new digital version. Digital Betacam machine will record component
digital to the revised 10 bit CCIR 601 standard providing 2 hours running time large
cassette besides small cassettes of 40 minutes duration. Prototype machine has already
been displayed by SONY.
g) DCT Format by Ampex : It is a digital component format. DCT is an 8 bit
system, and uses ¾ inch tape drive and 2:1 bit rate reduction. Data rate is around half
of D1 component system at much lower cost than D1. It offers a record time of three
and half hours.
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6
HIGH POWER TV TRANSMITTER
Design
All the TV transmitters have the same basic design. They consist of an exciter followed
by power amplifiers which boost the exciter power to the required level.
Exciter
The exciter stage determines the quality of a transmitter. It contains pre-corrector units
both at base band as well as at IF stage, so that after passing through all subsequent
transmitter stages, an acceptable signal is available. Since the number and type of
amplifier stages, may differ according to the required output power, the characteristics of
the pre-correction circuits can be varied over a wide range.
Vision and Sound Signal Amplification
In HPTs the vision and sound carriers can be generated, modulated and amplified
separately and then combined in the diplexer at the transmitter output.
In LPTs, on the other hand, sound and vision are modulated separately but
amplified jointly. This is common vision and aural amplification.
A special group delay equalization circuit is needed in the first case because of
errors caused by TV diplexer. In the second case the intermodulation products are more
prominent and special filters for suppressing them is required.
As it is difficult to meet the intermodulation requirements particularly at higher
power ratings, separate amplification is used in HPTs though combined amplification
requires fewer amplifier stages.
IF Modulation
It has following advantages
• Ease of correcting distortions
• Ease in Vestigial side band shaping
• IF modulation is available easily and economically
Power Amplifier Stages
In BEL mark I & II transmitters three valve stages (BEL 450 CX, BEL 4500 CX and BEL
15000 CX) are used in vision transmitter chain and two valves (BEL 450 CX and BEL
4500 CX) in aural transmitter chain. In BEL mark III transmitter only two valve stages
(BEL 4500 CX and BEL 15000 CX) are used in vision transmitter chain. Aural
transmitter chain is fully solid state in Mark III transmitter.
Constant Impedance Notch Diplexer (CIND)
Vision and Aural transmitters outputs are combined in CIN diplexer. Combined power is
fed to main feeder lines through a T-transformer.
17
BEL 10 kW TV TRANSMITTER ( MARK – III )
A block diagram of BEL 10 kW TV Transmitter is shown in Fig. 10. It consists of :
a)
b)
c)
d)
e)
f)
g)
10 kW Transmitter MK-III.
Input Equipment Rack
Monitoring Equipment Rack
Control Console
Indoor Co-axial Equipment comprising of :
• U-link Rack with U-link panel A and B, T-Transformer and 10 kW Dummy
Load.
• Aural Harmonic Filter.
• CIN Diplexer
• Aural Notch Filter and Band Pass Filter.
Antenna system with junction box, feeder cables etc.
Power distribution equipment.
Fig. 10 Block Diagram of 10kW TV Transmitter (Mark-III)
SOLID STATE POWER AMPLIFIERS
Dual Driver
1)
Has got two identical sections. Each capable of delivering 10 W.
2)
Gets 28 V power supply through relay in 80 W AMP.
3)
Sample of output is available at front panel for RF monitoring.
4)
Provides A DC output corresponding to sync peak out put for vision monitoring
unit.
5)
Thermostat on heat sink is connected in series with thermostat or 80 W AMP and
provides thermal protection. (Operating temp. 70oC.)
18
Fig. 11 TX. Block Diagram (Mark-III)
Ref. Drg.No:-STI(T)745,(DC497)
Fig. 12 Aural PA Chain (Mark-III)
Fig. 13 Vision Chain of Exciter (Mark-III)
19
TRANSMITTER CONTROL SYSTEM
The transmitter control unit performs the task of transmitter interlocking and control.
Also it supports operation from control console. The XTR control unit (TCU) has two
independent system viz.
1.
Main control system. (MCS)
2.
Back-up Control System (BCS)
Functions performed by MCS (Main Control System)
XTR control
Interlocking
RF monitoring
Supporting operation from control console
Three second logic
Thermal protection for 1 kW and 10 kW vision PAs
Thermal protection for 130 Watt vision PA and Aural XTRa
Mimic diagram
Functions performed by BCS (Backup control system)
Transmitting control
Interlocking
System Description of Exciter :
Fig. 2 Block Diagram of TV Exciter (Mark-II)
Video Chain
The input video signal is fed to a video processor. In VHF transmitters LPF, Delay
equalizer and receiver pre-corrector precede the video processor.
Low Pass Filter : Limits incoming video signal to 5 MHz.
Delay Equalizer : Group delay introduced by LPF is corrected. It also pre-distorts the
video for compensating group delay errors introduced in the subsequent stages and
diplexer.
Receiver pre-corrector : Pre-distorts the signal providing partial compensation of GD
which occurs in domestic receivers.
Both the delay equaliser and receiver precorrector are combined in the delay equaliser
module in Mark III version.
20
DP/DG Corrector
This is also used in the exciter preceding LPF (mark III) for pre-correcting the differential
gain and differential phase errors occurring in the transmitter.
Video Processor
The block diagram of video processor is given in fig. 3.
Functions
• Amplification of Video signal
• Clamping at back porch of video signal.
Clamping gives constant peak power. Zero volt reference line is steady irrespective of
video signal pattern when clamping takes place otherwise the base line starts an
excursion about the zero reference depending on the video signal.
Fig. 3 Block Diagram of Video Processor (Mark-II)
Vision Modulator
The block diagram of Vision modulator is given in fig. 4 and schematic diagram is shown
in fig. 5
Functions
• Amplification of Vision IF at 38.9 MHz.
• Linear amplitude modulation of Vision IF by video from the video processor in a
balanced modulator.
IF Amplifier
IF is amplified to provide sufficient level to the modulator. It operates as an amplitude
limiter for maintaining constant output.
Modulator
A balanced modulator using two IS-1993 diodes is used in the modulator.
Band pass amplifier
Modulated signal is amplified to 10 mW in double tuned amplifier which provides a flat
response within 0.5 dB in 7 MHz band.
Fig. 4 Block Diagram of Vision Modulator (Mark-II)
21
Fig. 5 Schematic Diagram of Vision Modulator (Mark-II)
VSBF and Mixer :
The block diagram of VSBF and Mixer is given in fig. 6. It consists of following stages :
•
•
•
•
•
VSB filter
ALC amplifier
Mixer
Helical Filter
Mixer Amplfier
Fig. 6 Block Diagram of VSBF Mixer (Mark-II)
VSB Filter
Surface Acoustic wave (SAW) filter provide a very steep side band response with high
attenuation outside designated channel. It has a linear phase characteristic with a low
amplitude and group delay ripple. (Fig. 7.)
22
Fig. 7 Block Diagram of V.S.B.Filter (Mark-II)
ALC Amplifier
Automatic level control is provided to maintain the exciter output level constant.
Mixer : ANZAC type MD 108 is used as mixer.
Helical Filter : to attenuate harmonics by at least 30 dB (similar to the one in aural
section).
Mixer Amp. : Provides 34 dB gain. Output is + 15 dBm.
Local Oscillator
The block diagram of Local Oscillator is given in fig. 8. It supplies three equal outputs of
+ 8 dBm each at a frequency of fv + fvif. This unit has 3 sub units.
(1)
fc/4 oscillator : Generates frequency which is 1/4 of desired channel frequency.
Fine freq. control is done by VC1.
(2)
LO Mixer/Power divider : Here the above fc/4 frequency is multiplied by four to
obtain channel frequency of fc and then mixed with fvif. Power divider is also
incorporated to provide three isolated outputs of equal level.
Visual Transistorised Power Amplifier (VTRPA)
VSBF & Mixer output is amplified by VTRPA which is highly linear and also sufficient to
drive valve stages.
It is a 5 stage amplifier 2 N 3375 for the first three stages. 2 N 3632 for the four
stage and BLW 75 for the final stage. All stages are biased for class A operation.
In Mark II later versions only 3 stages are used.
1st Stage
(1)
CA 2870 B
Hybrid amplifier.
IInd Stage
(2)
CD 3400
2 W driver
IIIrd Stage
(3)
CD 3101
Output 10 W
The amplifier is air cooled by two AC fans fixed to the rear of the unit.
Fig. 8 Block Diagram of Local Oscillator (Mark-II)
23
AUDIO CHAIN
Aural Modulator
The aural modulator unit consists of audio amplifier, VCO, mixer and APC.
The block diagram of Aural modulator is given in fig. 9.
Fig. 9 Block Diagram of Aural Modulator (Mark-II)
Audio Amplifier
A balanced audio signal at + 10 dBm from studio is converted to unbalanced signal by
audio transformer T4. The output of this is taken through potentiometer to the input of
Hybrid Audio Amp BMC 1003. A 50 micro second pre-emphasis is also provided.
VCO
This is a varactor tuned oscillator. Its frequency can be varied by coil L4. Transistor TR17 forms the oscillator. VCO output is frequency modulated by the audio signal. Output
level is 0 dBm.
TV TRANSMITTER ANTENNA SYSTEM
TV Antenna System is that part of the Broadcasting Network which accepts RF Energy
from transmitter and launches electromagnetic waves in space. The polarization of the
radiation as adopted by Doordarshan is linear horizontal. The system is installed on a
supporting tower and consists of antenna panels, power dividers, baluns, branch feeder
cable, junction boxes and main feeder cables. Dipole antenna elements, in one or the
other form are common at VHF frequencies where as slot antennae are mostly used at
UHF frequencies. Omni directional radiation pattern is obtained by arranging the dipoles
in the form of turnstile (Fig.15) and exciting the same in quadrature phase. Desired gain
is obtained by stacking the dipoles in vertical plane. As a result of stacking, most of the
RF energy is directed in the horizontal plane. Radiation in vertical plane is minimized.
The installed antenna system should fulfil the following requirements :
a)
It should have required gain and provide desired field strength at the point of
reception.
b)
It should have desired horizontal radiation pattern and directivity for serving the
planned area of interest. The radiation pattern should be omni directional if the
location of the transmitting station is at the center of the service area and
directional one, if the location is otherwise.
c)
It should offer proper impedance to the main feeder cable and thereby to the
transmitter so that optimum RF energy is transferred into space. Impedance
24
mismatch results into reflection of power and formation of standing waves. The
standard RF impedance at VHF/UHF is 50 ohms.
Fig. 15 Turnstile Antenna and its Horizontal Pattern
Mixer
VIF signal from IF osc. and aural IF from VCO are injected at the base of TR1. The
mixer output is 5.5 MHz. This is processed, divided to produce a square pulse at 537
Hz. For phase comparison reference pulses are derived from TCXO oscillating at 1.1
MHz after suitable division. The phase difference develops error voltage if the freq
variation is present. This voltage is applied to VCO to correct frequency when PLL is
unlocked due to freq. shift.
AURAL MIXER
This is similar to vision mixer which translates AIF at 33.4 MHz to aural carrier
frequency. This unit consists of --(i)
Mixer
(ii)
Helical filter
(iii)
Mixer amplifier
Aural Transistor power Amplifier (ATRPA)
ATRPA raises the power of Aural carrier to 20 watts. There are four stages giving a 23
dB gain. The transistors used are 2N3856, 2N3375, 2N3632 and BLW 93. The unit is
cooled by two small fans fixed at the rear side of ATRPA.
Radiation Pattern and Gain
The horizontal and vertical radiation pattern are shown in fig. 19 and 20. The total gain
depends upon the type of the antenna panel and no. of stacks as given in table-1.
Fig. 19 Typical Horizontal radiation pattern
25
VESTIGIAL SIDE BAND TRANSMISSION
Another feature of present day TV Transmitters is vestigial side band transmission. If normal
amplitude modulation technique is used for picture transmission, the minimum transmission
channel bandwidth should be around 11 MHz taking into account the space for sound carrier and
a small guard band of around 0.25 MHz. Using such large transmission BW will limit the number
of channels in the spectrum allotted for TV transmission. To accommodate large number of
channels in the allotted spectrum, reduction in transmission BW was considered necessary. The
transmission BW could be reduced to around 5.75 MHz by using single side band (SSB) AM
technique, because in principle one side band of the double side band (DSB) AM could be
suppressed, since the two side bands have the same signal content.
It was not considered feasible to suppress one complete side band in the case of TV
signal as most of the energy is contained in lower frequencies and these frequencies contain the
most important information of the picture. If these frequencies are removed, it causes
objectionable phase distortion at these frequencies which will affect picture quality. Thus as a
compromise only a part of lower side band is suppressed while taking full advantage of the fact that:
i)
Visual disturbance due to phase errors are severe and unacceptable where large
picture areas are concerned (i.e. at LF) but
ii)
Phase errors become difficult to see on small details (i.e. in HF region) in the picture.
Thus low modulating frequencies must minimize phase distortion where as high
frequencies are tolerant of phase distortions as they are very difficult to see.
The radiated signal thus contains full upper side band together with carrier and the
vestige (remaining part) of the partially suppressed LSB. The lower side band contains
frequencies up to 0.75 MHz with a slope of 0.5 MHz so that the final cut off is at 1.25 MHz.
Standards
The characteristics of the TV signal is sections 1 and 2 refer to CCIR B/G standards. Various
other standards are given in Table 1.
Table 1
Frequency Range
Vision/sound carrier spacing channel width
Vision sound carrier spacing
5.5 MHz
Channel width
7 MHz (B) in VHF OR 8 MHz (G) in UHF
Sound Modulation
FM
FM deviation (maximum)
+ 50 kHz
26
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