Methods of generating television test patterns.

Methods of generating television test patterns.
Calhoun: The NPS Institutional Archive
DSpace Repository
Theses and Dissertations
Thesis and Dissertation Collection
1950
Methods of generating television test patterns.
Hancotte, John Joseph
Monterey, Calif. : Naval Postgraduate School
http://hdl.handle.net/10945/14436
Downloaded from NPS Archive: Calhoun
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METHODS OF GENERATING
TELEVISION TEST PATTERNS
J. J. Hancotte,
Jro
IvETHODS OF GENERATING
TELEVISION TEST PATTERNS
by
Joseph Hancotte, Jr»
Lieutenant, United States Navy
J"©!!!!
Submitted in partial fulfillment
of the requirements
for the degree of
MASTER OF SCIENCE
IN
ENGINEERING ELECTRONICS
United States Naval Postgraduate School
Annapolis, Maryland
1950
This work is accepted as fulfilling
the thesis requirements for the degree of
IdASTSR OF SCIENCE
in
ENGINEERING ELECTRONICS
from the
United States Naval Postgraduate School.
*
PREFACE
During the winter term of the third year of the postgraduate Electronics course I was stationed at the General
Electric Company plant in Syracuse, New York, developing a
monoscope camera.
During this time I became interested in
different methods of test pattern generation.
I am indebted to Mr. o. H. V7iggin,
engineer, for as-
sistance and suggestions while I was with the General Electric Company, and to the engineering personnel of the Radio
Corporation of America for their help in obtaining information,
I am
also indebted to Professor P. E. Cooper of the
Postgraduate School for advice and assistance in preparing
this thesis.
ii
TABLE OF CONTENTS
Page No,
INTRODUCTION
^^
Chapter I
TEST PATTERNS
2
1.
Functions
2o
RMA.
3,
RCA Indian head test pattern
4o
Crosshatch test pattern
5«
Equipment tests
2
test pattern
2
.
7
,
.
.
•
10
15
Chapter II
I4ETH0DS OF GENERATING TEST PATTERNS
1.
Monoscope camera
2.
Flying-spot scanner
3»
Iconoscope and image orthicon
4«
Grating generator
19
19
.
.
.,...•
cameras .....
25
35
37
Chapter III
SV/EEP CIRCUITS, POVfER SUPPLIES, AND VIDEO AlIPLIFIERS
OF PATTERN GENERATORS
o
.
U2
1.
Monoscope camera
1+2
2.
Flying-spot scanner
56
3.
Image orthicon and iconoscope cameras
67
Chapter IV
COMPARISON OF
IIETIiODS
OF PATTERN GENERATION
iil
69
LIST OF ILLUSTRATIONS
Figure
1,
Page
Radio Manufacturers* Association resolution
chart
,
3
...,,»
S
2»
R.C.A.
3,
R.C.A, personalized test pattern with cross-
Indian head test pattern
hatch pattern superimposed
/»-•
«
Special test pattern to check linearity, with
Crosshatch pattern superimposed
.......
5»
Defocusing action of magnetic focusing
6,
Internal construction and operating voltages
.
.
•
of the 2F21 monoscope tube
?•
.
o
2?
Average characteristics of the
5V/P15
flying-
Schematic diagram of R.C.A. type V/A-3A grat3S
Block diagram of R.C.A, type TK-IA monoscope
43
camera
13.
Schematic diagram of R.C.A. type TK-lA mono44
scope camera
14 •
30
32
ing generator
12.
*
Block diagram of flying-spot video-signal
generator
11.
23
Spectral-energy emission characteristic of
spot cathode-ray tube
10.
21
Internal construction and operating voltages
phosphor P15
9.
14
23
of the 5V^15 flying-spot cathode-ray tube
8o
12
Grid circuit and grid signal waveforms of
vertical sweep feedback amplifier
iv
47
LIST OF ILLUSTRATIONS
(Cont.)
Page No.
Figure
15.
Effect on video signal of losing dc component.
51
16.
Idealized keyed clamp circuit
51
17.
Method of adding synchronizing pulses to
«
R.C.A. type TK-IA monoscope camera
,....,
55
«
59
18.
An equalizing network
19.
Schematic diagram of an equalizing amplifier
and video mixer
2O0
.
.
6I
•
Average grid-drive characteristics of type
I6AP4 kinescope, and a
tj'-pical
amplitude re-
sponse of gamma-correction amplifier
21,
65
Schematic diagram of a gamma-correction amplifier
66
INTRODUCTION
One of the greatest requirements of the television
industry is a means of insuring that set standards for
broadcasting and manufacturing be upheld.
In order to ac-
complish this purpose a quick and easy method of testing
for quality is most desirable.
It is difficult to deter-
mine the cause of imperfect reproduction of scanned objects
if the televised image is moving rapidly and is not fam-
iliar to the viewer.
Accordingly, several forms of static
test charts have been devised for use in testing the reso-
lution and geometrical form of the generated image.
Of equal importance is the necessity for a fairly simple and reliable means of converting test pattern informa-
tion into a video signal of excellent quality.
A device
that accomplishes this is termed a pattern generator.
(1)
CHAPTER I
TEST PATTERNS
1.
Functions.
Television test patterns are designed to provide checks
on resolution, linearity, phase shift, ringing, quality of
interlacing, focus, aspect ratio, picture size, shading,
cathode-ray tube spot characteristics, and optical systems
of projection receivers.
Resolution is a measure of the frequency response of a
systems
It is measured in "lines'*,
indicating the maximum
number of equally spaced black and white lines that could be
accomodated in the vertical height of the pattern, each line
being distinguishable.
Ringing refers to undesirable oscil-
lations in sweep circuits or in video amplifiers due to poor
transient response.
The quality of interlacing is a measure
of how well the horizontal lines of one field fit between the
lines of the next field.
Aspect ratio is the ratio of picture
width to picture height and is set at four to three in this
country.
Shading refers to the corrective measures necessary
to obtain a uniform picture from a uniform distribution of
light.
It is desirable to reproduce gray shades as accurately
as possible.
2.
RJ.1A
test pattern.
The television test pattern or resolution chart illustrated in Figure 1 was designed by the Radio Manufacturers*
Association to standardize television resolution measurements.
(2)
To provide maximum utility, various branches of the tele-
vision engineering field were canvassed and suggestions obtained for its preparation.
In using this or any other test
pattern, resolution should be read only after equipment has
been adjusted to have a minimum of distortion.
Scanning,
shading (if the system employs shading), low frequency phase
shift, and focus, should be adjusted before reading resolu-
tion,
3ize, linearity, and aspect ratio are included in the
scanning adjustment.
Whatever the type of pattern generator
used, the exact total area of the pattern, whose boundaries
are indicated by arrow heads, should be utilized.
Vertical
sweep linearity is checked by comparing the spacing of the
short horizontal bars at both top and bottom of the picture
with that of the bars midway between.
Similarly, horizontal
sweep linearity is checked by comparing the spacing of the
vertical bars in the square at each side of the picture with
the spacing of the bars in the center square.
Aspect ratio
is checked by measuring the lengths of the gray scales in
the central circle.
If the horizontal and vertical scanning
is linear and the horizontal and vertical scales are equal
in length then the aspect ratio is correct*
If the pattern generator uses shading, check it by exam-
ining the monitor to see if the background is an even gray,
or use a waveform monitor and note if the average picture
signal axis is parallel to the black level both at line and
field frequencies.
To further aid in obtaining correct sha-
ding, adjust it until the gray scale reading is a maxim.um
(4)
and the same for all four scales*
If black streaking follows either of the two horizontal
black bars at the top or bottom of the large circle, it is
an indication of low-frequency phase shift.
The presence of
bright vertical lines closely following the black bars indicates high-frequency phase shift*
Cathode-ray beam focus adjustments are made for a maximum resolution reading, first of the horizontal scanning
and then of the vertical.
Due to beam characteristics a
maximum adjustment for one may not be the maximum adjustment
for the other,
A compromise adjustment should then be made.
Resolution is read by taking the maximum numerical readings on the wedges at which the separate lines can be resolved*
Horizontal resolution is read on the vertical wedges and vertical resolution on the horizontal wedges.
Resolution in the
central portion of the picture will usually be greater than
that measured by the wedges in the four corner circles.
This
may be due to failure in achieving optimum results for one or
more of the previously mentioned adjustments or due to inherent cathode-ray tube distortion.
All bars for checking sweep linearity are spaced for
200 lines resolution.
The resolution circles in the center
and in the four corners are used to test spot ellipticity on
cathode-ray tubes.
The resolution of the circles in the
corners (150) was made less than the resolution in the center
(300) because of added deflection defocusing in these areas.
The two sections of single line widths, 50-100-150-200-
(5)
250-300, and 350-400-450-500-550-600 provide an accurate
means of checking for ringing.
The multiple lines in the
wedges could prove confusing if used for this check.
These
sections also test the ability of the system to reproduce
isolated details.
One of the wedges is calibrated in megacycles as well
as in lines.
The following development shows how the con-
version is made from lines to megacycles.
If there are N
number of equal width black and white lines that can be accomodated in the vertical height of the pattern, then there
will be
^
lines that can be accomodated in the horizontal
3
width of the Dattern or
^ lines
of each color.
Assuming
3
wave will represent black to white variations, and
a sine
that the active time of horizontal trace is O.84H = 53.3
microseconds, then the frequency corresponding to
is
fj^
=
~
X ~-rr =
N lines
0.0125 X 10^' cycles per second =
0.0125Nmegacycles,
The four diagonal lines are used to check the quality
of interlacing.
Pairing of the interlaced lines is indica-
ted by jagged lines.
This is not effective if there is no
interlacing whatsoever.
The four crosses, one on each edge, are used for align-
ment of the optical systems of projection receivers.
The
four corner circles are so positioned that they should be
visible on receivers whose picture corners are masked.
The gray scales vary approximately logarithmically
from maximum white brightness to about l/30th of that value.
(6)
These shaded areas indicate nonlinear amplitude distortion
in the system.
The gray background of the chart provides
a satisfactory balance with the whites so that a studio
system set up by the use of this chart will televise an average scene without the need for additional adjustments.
3,
R.C.A, Indian head test pattern.
The test pattern illustrated in Figure 2 has been one
of the most popular in the industry.
monoscope tube.
It is used in the 2F21
Television stations often have their iden-
tification letters added to it.
The large circle has a rad-
ius three quarters of the pattern width so that the standard
aspect ratio is maintained.
The circle also shows the geo-
metrical symmetry of the scanning motion and reveals nonlinearity in the vertical or horizontal scanning directions.
The four circles in the corners have the same purpose and
are situated in the four regions of the pattern where geo-
metrical distortions, as well as defocusing of the scanning
beam, are most likely to occur.
The whole pattern is cros-
sed by a grid of fine lines which reveal any orthogonal dis-
tortion at any part of the image.
Five sets of resolution wedges are included.
The main
wedges within the central circle are calibrated by the numbers 20, 30, 45, and 35 v/hich stand, respectively, for 200,
300, 450, and 350 line resolution.
Open spaces in the cen-
termost line of each v/edge indicate the position of the calibration within each wedge.
The central concentric circles
have 300 line resolution, indicated by the number 30.
(7)
The
four sets of resolution wedges at the corners have similar
calibration markings.
The
tv;o
oblique wedges within the center circle are
tonal values, having different degrees of shading.
By tak-
ing the innermost or black section of each wedge as 100 per
cent, the degrees of shading of the other sections, reading
outward, are 75, 50, and 25 per cent, respectively.
The horizontal black bars below the central circle have
lengths that are logarithmically related.
The length of
each bar is 71 per cent of the length of the line above it.
As mentioned in discussing the RI^ chart, streaking following any of the bars is an indication of low-frequency phase
shift.
The two sections of single line widths arranged in two
vertical columns on either side of the central circle have
the same purpose as do those in the
RIvIA.
chart.
The numbers
indicate the width in lines of the nearest rectangle.
The Indian head is useful for judging over-all performance, especially contrast and average brightness which are
most easily judged on a pictorial subject.
The ends of the
diagonal lines merk the edges of a pattern having half the
width of the over-all pattern.
The lines are checks on the
interlace.
Test patterns containing wedges are sources of square
waves at controllable frequency.
If no scanning is used in
the vertical direction, and the beam current is reduced to
prevent possible burning of the signal plate by electron bom-
(9)
bardiaent, the horizontal scanning motion will pass in a
single line over one of the vertical resolution wedges, and
in so doing produce a rectangular signal v/ave.
The horizon-
tal trace is positioned by varying the amount of direct cur-
rent through the vertical deflection coils.
The frequency
of the square waves produced by the horizontal scanning can
be adjusted from roughly 50 kilocj'-cles to
5
megacycles de-
pending on the positioning of the horizontal trace and on
the horizontal scanning amplitude and frequency.
The ver-
tical sweep may be used in a similar way to produce frequencies rajiging from 300 cycles per second to 10 kilocycles,
/f.
Crosshatch test pattern.
In order to make certain that deflection systems of
pattern generators and cameras are linear, a bar or cross-
hatch generator is utilized.
The generator may be used in
conjunction with a monitor to test the sweep linearity of
pattern generators or of iconoscope or image orthicon cameras.
The simple check of observing a test pattern, trans-
mitted by the generator under test, on a monitor that has
been previously adjusted for linearity is not entirely satisfactory.
Any error in adjusting the monitor will be dup-
licated in adjusting the generator deflection.
It is only
necessary for the beams in the pattern generator and the monitor tube to travel at the same velocity across a picture to
give proper distribution in the reproduced picture.
This ve-
locity is not necessarily uniform.
In order to determine when constant scanning velocity
(10)
has been achieved in the device that generates the pattern
it is necessary to make a comparison between space intervals
marked upon the pattern and intervals of time.
To accomplish
this, two pictures are superimposed on a standard picture
monitor.
One picture is the generated pattern, and the other
picture is a Crosshatch time pattern, synchronized by the sweep
synchronizing pulses.
Using the time pattern as a standard,
various sections of the test or space pattern can be measured
and compared.
For example, if the Indian head test pattern
is used, the diameters of each of the four corner circles
should measure the same and should equal one-quarter of the
diameter of the large, central circle, all m.easured in intervals of the Crosshatch time pattern.
The results will be
dependent upon the linearity of the pattern generator scanning, but will be independent of the linearity of the monitor
scanning.
Figure
3
illustrates the appearance of a person-
alized test pattern and Crosshatch pattern on a monitor.
The
unequal spacing of the vertical bars of the Crosshatch pattern indicate that the horizontal sweeps of the pattern gen-
erator and monitor are equally nonlinear.
For still more accurate testing of a camera or pattern
generator a special test pattern may be devised.
Let us as-
sume that the bar generator controls are set so that the
Crosshatch lines on the monitor are about the width of one
scanning line.
If every seventh scanning line of the raster
is blanked, and the vertical blanking period is 7.5 per cent,
then the number of visible horizontal lines of the Crosshatch
(11)
pattern will be
^^
lines will be seen.
x 0.925 = 69.4, or 69 full horizontal
If the vertical lines of the time pat-
tern mark intervals of one one-hundredth of the horizontal
scanning cycle, and the horizontal blanking period is l6
per cent, then the number of visible vertical lines of the
Crosshatch pattern will be 100 x 0.84 = 84.
The raster being
4 units wide and 3 units high (aspect ratio of 4 to 3), then
each horizontal interval will measure 4/84 or 1/21 unit, and
each vertical interval will measure 3/69 or 1/23 unit.
Now a test pattern must be designed consisting of a
system of black diamond-shaped dots on a white field as in
Figure 4.
The dots are spaced so as to bear a definite re-
lationship to the Crosshatch pattern.
If each tenth verti-
cal and horizontal line of the Crosshatch pattern is to be
represented by a dot then there will be 9 dots in each horizontal row across the chart and 7 dots in the vertical rows.
Since the test chart will also have an aspect ratio of 4 to
3
then the separation of the horizontal dots will be 10/21
unit with a space of 2/21 unit at either end of the row.
The
separation of the vertical dots v;ill be 10/23 unit with a
space of 4^/23 or 9/46 unit at either end of the columno
The
units, of course, will depend upon the scale to which the
test pattern is constructed,
V/hen the test
pattern is fully scanned by the pattern
generator, the pattern generator sweeps v/ill be exceedingly
linear when each successive dot, vertically and horizontally,
corresponds to each successive tenth line of the Crosshatch
pattern as illustrated in Figure 4»
(13)
The pattern generator
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centering controls may be adjusted to bring about the
coincidence.
The scanning of the monitor may be overdriven to mag-
nify the picture for close inspection of a portion of it.
The centering controls of the monitor may be adjusted to
permit examination of a portion of the picture at a time.
A Crosshatch generator may be designed to present a pattern consisting of any number of vertical and horizontal
lines.
The horizontal bars are formed by broad blanking
pulses, whereas the vertical lines are formed by narrow pulses.
In most Crosshatch generators the bars are about 10
per cent of the space between bars.
The bars are of the or-
der of from one to two lines in width if the system has good
high-frequency fidelity.
Due to the sharpness of the ver-
tical lines in the Crosshatch pattern, a poor high-frequency
characteristic in the monitor amplifier or beam defocusing
of the kinescope can be observed.
The pattern provides a good check on uniformity of focus
if the deflection yoke has no defects.
Stray magnetic fields
in the vicinity of the kinescope will show up as curvature
of the scanning lines.
This is often caused in the yoke by
assymetrical capacitances of the coil sections*
5.
Equipment tests.
Test patterns are very helpful in the television manu-
facturing industry,
A manufacturer of television equipment
is concerned with how well his product will perform when re-
producing video signals.
Short-cut tests have been tried.
(15)
but give poor correlation with the results obtained when an
actual picture is reproduced.
However, the
qualitj?-
of the
latter is difficult to reduce to a quantitative basis unless
the test pattern or picture has a specific character which
can be accurately converted into a reliable video signal*
The test pattern is adnirably suited for this purpose*
Simple tests insure the quality of the video signal used
for rating a cathode-ray tube or kinescope.
If the scanning
on the test pattern is reduced and that on the kinescope is
maintained at normal, an enlargement of the scanned portion
of the signal plate will be seen on the kinescope.
This en-
largement removes the possible limitation of kinescope resolution.
Also, the reduced scanning lowers the frequency
band of the video signal so that the video aiaplifier does not
limit the resolution.
Under these conditions, the focus of
the scanning beam of the pattern generator can be accurately
set to give high resolution, from 500 to 600 lines.
If the
finest detail in the pattern can be resolved, it is evidence
that the scanning spot in the pattern generator is smaller
than the finest detail to be transmitted, and, therefore, that
the pattern generator spot size is not limiting resolution.
After the focus of the pattern generator is set for maximum resolution, the resolution of the pattern generator's
video amplifier can be checked.
This is done by making the
pattern generator's scanning normal size and increasing the
scanning on the kinescope.
The latter is necessary to remove
the possible limitation of kinescope resolution.
(16)
The resolu-
tion of the amplifier is easily checked on the test pattern
by noting the resolution of the upright "V's",
The overall
resolution of the pattern generator should be appreciably
more than the resolution to which the kinescope is to be
rated.
V/hen the
scanning of the kinescope is reduced to
normal, the limits of the kinescope resolution can be de-
termined and reliable test data obtained.
If more detail is
visible in the enlarged pattern than in the normal pattern,
then the kinescope spot size is limiting resolution.
Since the resolution in all parts of the scanning pattern on a kinescope may not be uniform, a test pattern is
available for checking all parts of the pattern under similar
conditions.
The pattern is divided up into several sections,
each one of which carries
"Vs" corresponding
to resolutions
Also, tones between black and white are
of 150 to 450 lines.
included to give a check on the modulation characteristic of
the kinescope.
With such a pattern, the kinescope can be
rated under different bias conditions with various amounts
of video signal input.
An illuminometer can be used to check
the light output for a definite signal.
In the development and production testing of receivers
a standard source of
tages.
high-quality signal has numerous advan-
The previously mentioned test patterns serve as good
"yardsticks" for measuring receiver characteristics.
By
modulating a small transmitter with the pattern generator
output a very useful test signal can be obtained for readily
checking receivers.
A test similar to that mentioned for
(17)
I
I
kinescopes will indicate whether or not the receiver amplifiers are limiting resolution.
If on enlarging the receiver
scanning amplitude no improvement in detail results, then
the amplifiers are limiting the resolution*
V/hen a
television system is installed, numerous tests
must be made to adjust the various circuits.
materially aid such testing.
Test patterns
For instance, any extraneous
signals entering the grid circuit of an iconoscope or image
orthicon can be detected by substituting a monoscope tube
in the circuit to generate a test pattern.
in a monoscope tube should be constant.
The beam current
Since the video
signal from the monoscope is directly proportional to the
beam current, any variation in beam current is revealed as
a modulation of the video signal.
Therefore, any unwanted
signal or hum in the circuits Is revealed,
When the shading signals which are sometimes added to
the iconoscope or image orthicon video signal are removed,
the video-amplifier can be checked for pick-up and frequency
response by using an externally generated test pattern for a
video signal.
Such tests help to separate confusing factors
which often combine to give poor over-all operation©
(18)
CHAPTER II
I-IETHODS
1.
OF GENERATING TEST PATTERNS
Monoscope camera.
In this chapter various methods of test pattern genera-
tion will be described,
Kmphasis will be placed on the gen-
eration of the low level video signal, and the methods of
amplification and correction will be discussed in the following chapter.
Probably the most widely used method of test pattern
generation is the monoscope camera,
A monoscope is
a type of
tube designed to produce a video signal of a test pattern or
a picture that is perm^anently enclosed in the tube.
Although
not suitable for developing a signal which represents action,
excellent fidelity can be obtained for a still picture which
contains half-tones or consists only of lines.
The monoscope tube to be described
type, the RCA 2F21.
is
the most recent
It is similar in appearance to an ordi-
nary cathode-ray tube.
It consists of an electron gun, a
signal plate, and a collector enclosed in a highly evacuated
envelope.
The electron beam is scanned over the signal plate
by an electromagnetic deflection system.
taken directly from the signal plate.
The video output is
Electrostatic focusing
is used,
V/hen the
target of the electron beam is a flat surface
such as in the monoscope tube, electrostatic focusing is su-
perior to magnetic focusing.
liagnetic focusing utilizes the
principle that an electron entering a magnetic lens system
(19)
will be deflected if the electron possesses a component of
velocity that is radial with respect to the axis, as shown
in Figure 5(a)
for paths "a",
'*b'%
and "d".
The electron
will spiral, and for a correct value of magnetic field
strength, will intersect the axis at a point P on the target.
This action is independent of the radial component of
velocity of the electron.
If the target is a plane, then for
correct focusing at the target center there will be a defo-
cusing of the spot at other points on the target as the beam
is deflected.
At point Q all paths will be longer.
As a re-
sult, an end view of the paths, as illustrated in Figure 5(b)
would show that an electron traveling with a radial component of velocity would have described path oe"f if path oe"e
represents its path when the beam is not deflected.
Thus
electrons that do not travel the main path "c" in the electron beam will result in a defocusing action when magnetic
focusing is used.
There is a slight defocusing action in
the picture corners when electrostatic focusing is used
caused by the mutual repulsion of the electrons in the beam.
This action, however, is also present in magnetic focusing.
The electron gun which supplies the scanning beam is of
high quality in order to obtain a superior video signal.
The
electron beam must be very small when it strikes the signal
plate to obtain good resolution.
The beam current is reas-
onably high in order to make the output video current as large
as possible.
The beam size is the more important factor, how-
ever, and limits the beam current.
The final anode of the
gun operates at 1000 volts, and a beam which focuses to a
(20)
Source of
Figure 5(a)-Focusin(T synten urging localized na.pnetlc field
oe"G represents end view of undeflected path "a" to point
i,
oe'e represents end Triev.; of undeflected r»ath "d" to point p'
oe-^f represents end vior; (relative to
deflected path "c") of
psth ^'\" deflected toward Doint :.
oe E represents end vie^;. (relative to
deflected peth "c") o^^
path "d" deflected towards point Q.
f:
Figure
5("b)-:5nd
(21)
vie^i;-
enlarf^ed
spot, the width of which is about l/500th of the pattern
height, can be obtained for currents of several microamperes.
Figure 6 shows the internal construction and operating voltages of the 2F21,
The
2
5/16" X
3
I/16" signal plate is located at the end
of the tube remote from the electron gun.
aluminum foil 0.004" thick and carbon.
It is made from
The surface of the
aluminum has a natural coating of alumj.num oxide which has a
reasonably high secondary-emission ratio v/hile the carbon has
a relatively low ratio.
As the plate is scanned the differ-
ence in the magnitude of secondary-emission currents deter-
mines the amount of video current.
Since this difference is
greater than unity, more video current is developed than if
only the primary current of the beam were utilized.
Aluminum foil developed for advertising and packing purposes as well as special inks developed for printing on metal
foils make satisfactory materials for signal plates.
Thus
the advantages and flexibility of commercial printing processes can be utilized.
The desired pattern or picture is prin-
ted on aluminum foil v/ith a black-foil ink.
Before sealing
the signal plate in the tube it is fired in hydrogen.
This
removes the volatile matter from the ink and leaves it prac-
tically pure carbon.
In order to have the output of the mon-
oscope correspond to outputs from iconoscopes and image orthicons the picture on the signal plate has blacks and whites
reversed, since in the outputs of these tubes black is positive.
Although the aluminum oxide is white in appearance, it
(22)
i
^>J^
TO
I*
±
D*'
—caw.
t=::c,
+ ^ro T6
*2io
/.
PigTjre 5-Internr-l construction and operating voltages
of the P??! no no scope tube
fc.3v.
-ffi/
+ 200V
(TO F^U5I»V6
COWTR&L)
+^0000
V.
Figure 7- Internal construction and operating voltsf^es
of the 5.VP15 flying-spot cathode-ray tube
(23)
I
I
I
has a higher secondary-emission ratio than carbon and, therefore, produces a signal which corresponds to black.
The following proceedure is used to print the signal
plates.
Photo-engravings are made of the subject matter.
The black-and-white material is treated as a line-cut, but
the half-tone material must be broken into a number of dots
of various sizes depending on the half-tone value.
This is
done when the photo-engraving is made by photographing the
material through a suitable screen,
A screen is used which
will break the picture into more elements than are used in
the television scanning system.
Thus this technique of ob-
taining half tones does not limit the resolution of the television system, and the half-tone effect is reproduced, just
as in a newspaper photograph.
The picture is made up of num-
erous dots of various sizes.
The secondary emission current from the signal plate is
collected by a conductive aquadag coating on the bulb wall.
This coating is operated at a potential positive with respect to the signal plate.
The secondary electrons are fed
through a side connection to R^ and
C^^,
See Figure 6,
Here
the current divides, some going back to the cathode through
the high-voltage power supply, the rest going to ground and
up through the grid resistor of the first video amplifier
back to the signal plate.
The latter electrons form the video
signal across the grid resistor.
The peak to peak value of the pattern-electrode signal-
current is approximately 0.5 microamperes.
(24)
The beam current
is about 30 per cent greater.
Usual practice is to operate
the monoscope with the pattern electrode at ground poten-
tial to avoid undesirable pick-up of extraneous signals, in-
cluding hum, at the first video amplifier grid.
Adjustment
of output signal level is made by means of the control of
A
the first grid voltage which regulates the beam current.
transformer with a high-voltage heater winding must be used
for supplying the heater power to this tube since the cathode
is operated about 1000 volts below ground potential.
Resolution capability of the monoscope is about 50O lines,
but it is possible to get higher resolution from some tubes
with low orders of beam current and signal output.
Because
there are not any half-tones in the video signals from a
monoscope except those that are created by the limitation of
resolving power of the beam, the signal is rich in the higher-order harmonics which make up the corners of a square wave.
This type of signal is exceptionally good for shov/ing the
transient response of video amplifiers,
2.
J'lying-spot scanner.
Another type of test pattern generator, much more recent than the monoscope, is the flying-spot scanner.
Early
experimenters with cathode-ray systems found an easy way of
televising fixed pictures by placing photographic negatives
against the fluorescent screen of a tube whose beam
in a conventional manner,
V7as
swept
A phototube placed in front pro-
duced a series of video pulses that could be applied to a
second cathode-ray tube circuit.
(25)
Dispersion of the light
I
i
spot through the glass introduced one of the main difficul-
ties toward attaining good reproduced detail, however, so
some sort of lens system was often introduced between the
screen of the transmitting tube and the film*
These early experiments had the advantage of simplicity,
but at scanning speeds suited to televising live scenes the
delay characteristics of the phosphorescent screen caused a
smearing effect with accompanying loss of detail.
A solution to this difficulty has been provided by the
RCA
5V/P15 tube,
a five inch cathode-ray tube intended pri-
marily for use as the scanner in a flying-spot video-signal
generator.
It has the advantage of permitting a change of
picture or test pattern at v/ill, and of reproducing the picture with the halftone fidelity of photographic film..
The type 15 phosphor with a metallized back has a spec-
tral-emission characteristic with peaks in the blue-green
and near-ultraviolet regions as shown in Figure 8.
The ul-
traviolet radiation has a persistence characteristic v;hich
is appreciably shorter than that of the visible region.
Thus,
by utilizing only the ultraviolet radiation, it is possible
to minimize blurring or trailing in the reproduced picture^
The metallized back effectively doubles the radiant energy
of the flying spot compared with the energy obtainable from
an unmetallized screen.
Magnetic deflection and electrostatic focusing are used
with the
5V/P15
to obtain essentially uniform focus over the
useful screen area.
Optically, the tube face has a quality
(26)
SJe dtaM^-'^riy i-^pW<|j^^
(27)
'e|f"!p^eip^air
^
I
and flatness which will not limit the perforraance of a high
quality objective lens needed to provide maximom signal
resolution*
There is an external conductive coating on the neck of
the tube v/hich, when grounded, prevents corona between yoke
and neck.
Corona would damage the yoke insulation and cause
breakdov/n in the glass of the neck.
The neck also has an in-
ternal coating which is at the high potential of anode 2,
The resistance of these coatings is high enough that damping
of the yoke deflection energy is negligible.
The capacitance
between the two coatings is in the range of lOOuuf to
500jiji-f
.
It serves as a filter capacitance for the high-voltage power-
supply unit.
An external moisture-repellent insulating coating on the
bulb cone miniiriizes sparking over the glass bulb under con-
ditions of high humidity,
V/hen
humidity is high, a continu-
ous film of moisture has a tendency to form on untreated
glass.
If a high-voltage gradient is present, this film may
permit sparking to take place over the glass surface.
The
tube should be protected as much as possible from contamination such as finger prints and dust which might absorb moisture and provide electrical leakage paths that increase in
conductivity v/ith high humidity.
The dust also reduces the
am.ount of radiation through the bulb face.
The internal construction and operating voltages of the
5VrP15 are
shown in Figure 7.
Anode
2
should be operated be-
tween 15,000 and 27,000 volts, brilliance and definition de-
(28)
creasing with decreasing voltage.
of the tube with anode
2
The ultraviolet output
at 20,000 volts is shown in Figure
9(a), and with anode 2 at 27,000 volts in Figure 9(b).
The
size of the arbitrary units is the same in both figures.
Also shov/n is the effect of variation in grid 1 voltage on
anode 1 current and on anode 2 current.
Soft x-rays are produced when anode
approximately 20,000 volts.
2
voltage is above
Experiments conducted at the
RCA laboratories have shown that at most they are very weak.
These rays can, however, constitute a health hazard unless
the tube is adequately shielded.
Relatively simple shielding
should prove adequate.
Grid 2 is incorporated in the 5^^15 to prevent interaction between the fields produced by grid 1 and anode 1.
It
may also be used to compensate for the variation in the grid
1 voltage for cutoff in individual tubes.
By adjusting the
voltage applied to grid 2, v;ith due consideration to its maximum rated value of 350 volts, it is possible to fix the grid
1 bias at a desired value, and obtain almost the same anode-
current characteristics for individual tubes having different
cutoff voltages.
Adjusting grid 1 cutoff in this way not on-
ly makes grid drive more uniform, but also reduces variations
in anode 1 current.
Since grid
2 at
most drav/s only negli-
gible leakage current, its voltage may be obtained from a po-
tentiometer inserted in the anode 1 voltage divider mentioned
in the next paragraph.
Focusing is controlled by adjustment of the ratio of
anode 1 voltage to anode 2 voltage.
(29)
This ratio is ordinarily
i
(30)
1
adjusted by variation of anode
1
voltage.
Anode
1
voltage
can be obtained from a potentiometer in a voltage divider
across the high-voltage supply.
Because the ultraviolet
efficiency of phosphor P15 increases somewhat faster than
the beam-current density, the signal output rises as the
flying-spot is brought to focus.
It is, therefore, desir-
able to provide uniformity of focus over the entire scanned
raster in order to obtain optimum signal output as well as
to obtain good resolution.
Resolution of better than 700 lines at the center of the
reproduced picture can be produced by the 5V/P15,
To obtain
such resolution in the horizontal direction, it is necessary
to use a video amplifier having a band-width of about 9 meg-
acycles.
By the use of the control grid in the 5WP15, a modulation pattern can be superposed on that from the transparency,
A great many unusual artistic effects and double-image effects
are possible by this addition.
Studio directors, after a few
experiments using simple equipment, can devise
manj'-
spectac-
ular background effects v/ithout preparing extensive artv;ork.
The blue-green radiation of the 5^'^15 decays hyperbol-
ically to about 30 per cent of its initial value in 1.5 micThe ultraviolet radiation has an equivalent ex-
roseconds.
ponential decay with a time constant less than 0,05 m.icroseconds.
The frequency response of the ultraviolet radiation
is substantially constant for a range of 3 megacycles and
then decreases exponentially toward zero at approximately 100
megacycles.
(31)
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(32)
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A flying-spot video-signal generator consists essentially
of (l) a flying-spot cathode-ray tube with associated power
supplies, deflection yoke, and scanning circuits to provide
a small, rapidly moving source of radiant energy;
(2)
an
optical system arranged to project the raster on the subject
to be scanned;
(3)
a multiplier phototube with associated
power supply to intercept the radiation transmitted or re-
flected by the subject and convert it into video signals; and
(4)
a video amplifier.
The subject may be a slide transpar-
ency, motion picture film, or an opaque object.
A block diagram of such a system arranged for use with
a slide transparency is shown in Figure 10.
Several of the
details of this diagram will be covered in the next chapter.
For best results, the enlarger type objective lens should be
designed for low magnification and, preferably, be corrected
to handle ultraviolet radiation.
The diameter of the object-
ive lens should be adequate to cover the slide to be scanned.
For use with 35 millimeter slides the Kodak Enlarging Ektar
f:4.5 lens with focal length of 100 millimeters, or equivalent, is suitable.
Satisfactory filters for absorbing the visible and passing the ultraviolet radiation of the screen are any of the
following:
Eastman V/ratten Nos. 18A, 34, and 35, as well as
the Corning Nos. 9863 and 5970.
The choice of filter for a
particular generator design is affected by a compromise between the permissible loss of signal output through absorption by the filter on the one hand, and the ©.mount of trailing which can be tolerated, or the extent of equalization
(33)
I
i
needed, on the other hand.
Trailing results from the lag in buildup and decay of
output from the screen.
As the flying-spot moves across a
boundary from a light to a dark area of the subject being
scanned, the persistence of energy output from the screen
results in continued input to the phototube from the light
area during the time the dark area is being scanned.
Thus,
the light area trails into the dark area in the reproduced
picture.
Similarly, as the flying-spot moves from a dark area to
a light area,
the lag in buildup of the screen output causes
the dark area to trail over into the light area.
As a result of these effects, the reproduced picture has
an appearance similar to that produced by a signal deficient
in high frequencies.
Therefore, it is necessary to enhance
the high-frequency response of the video amplifier by intro-
ducing equalizing netv/orks of the resistance-capacitance type
with suitable time constants.
Sufficient equalization should
be provided to give the desired square-wave response.
The decay characteristics of most standard phosphors are
such as to require considerable equalization provided by net-
works with different time constants in several stages of the
video amplifier.
Their relatively long decay generally re-
sults in appreciable reduction of the useful signal-to-noise
ratio.
Compared with standard phosphors the persistence of the
PI5 screen is comparatively short so that less equalization
is needed.
If the P15 is used without an ultraviolet filter.
(34)
less equalization is required than for other standard phos-
phors, but a complex network is nevertheless needed because
the decay characteristic is not a simple exponential curve,
but a curve of a complex function.
V/hen used
with a filter
to pass only the ultraviolet radiation, the P15 has a per-
sistence so extremely short that the small amount of equalization needed can be supplied by a single network.
As a
result, circuits and adjustments are simplified, and substan-
tially the same signal-to-noise ratio is obtained, in spite
of filter absorption, as with the arrangement using the total
radiation from the phosphor,
3.
Iconoscope and image orthicon cameras.
Another method of generating test pattern or still picture information is to focus it on the mosaic of an iconoscope or the photocathode of an image orthicon pickup tube*
The monoscope camera has been used extensively for the gen-
eration of test patterns for several years.
The flying-spot
scanner is such a recent development, however, that it is
not yet in general use.
Thus, in many instances the desired
picture is placed on a stand or suspended from a wall, and
a standard television camera is used to televise it, the cor-
rect lens for the distance involved being used to focus the
picture on the mosaic or photocathode of the camera tube.
Considerable caution must be exercised, particularly with an
image orthicon, since a still picture that is focused for any
length of time on the photocathode will tend to "burn" an
image of the picture on it»
Considerable time will thereafter
be required to allow this image to fade away*
(35)
There are two other methods of focusing still pictures
on a camera tube.
A recent development by the Radio Corpo-
ration of America is called the Video Announcer,
It is a
device designed to facilitate caiaera alignment in television
field operation particularly during adverse weather conditions.
It also provides a means of presenting static pic-
tures.
The Video Announcer being small and light can be quickly
and easily installed on most television field cameras, using
the 50 millimeter lens normally supplied with the camera.
It
uses single frame exposures of 35 millimeter film strip, with
adjustments provided for proper alignment, framing, and illumination of the film»
The equipment can accomodate a film of approximately 70
frames*
RMA and local station test patterns are generally
included at the ends of each strip along with a few trans-
parent frames which can be used to wipe off any images burned
on the image orthicon tube of the camera.
is done
Camera alignment
with the RMA test pattern.
The other method is used with television film cameras.
Iconoscopes are usually used in these cameras since they are
capable of a better signal-to-noise ratio than are image orthicons.
Iconoscopes give poor performance under insuffi-
cient lighting conditions, but that difficulty is not serious
when televising film.
Most film cameras may be used with a
motion picture projector or a slide projector.
The slide
projector presents a convenient arrangement for quick changing
of still pictures.
It can be used in the studio in much the
(36)
i
same way that the Video Announcer is used in the field.
4»
Grating generator.
In order to generate the Crosshatch pattern which is
used to check the linearity of pattern generators a special
type of pattern generator is used.
As previously mentioned
the output of this generator as viewed on a monitor is a series of horizontal and vertical bars equally spaced in time.
As a typical example the RCA type WA-3A grating generator will
be described.
A schematic diagram of this unit is shown in
Figure 11.
Signal inputs from a sync generator are negative horizontal driving pulses of 15,750 c.p.s,, negative vertical
driving pulses of 60 c.p.s,, and negative mixed blanking pulses.
There is an internal power supply using a full wave
rectifier regulated to produce 250 volts plate supply.
To form the vertical bars, negative horizontal driving
pulses are inverted and amplified by the first half of V-1
and differentiated and clipped by the second half.
The output
of V-1 synchronizes a free running cathode coupled multivi-
brator composed of V-2 and V-3 of such a configuration that
V-3 conducts for only a relatively short period of time, and
hence the output taken from the plate of V-3 consists of narrow negative pulses.
The "VERTICAL BAR Ri^GE" switch, S-2,
is a coarse control of the multivibrator frequency,
and the
"VERTICAL BAR FREQUENCY" potentiometer, R-19, is for fine
adjustment of the multivibrator frequency.
The "WIDTH" con-
trol, R-22, varies the screen potential of V-3, determining
(37)
Ikmv
-|p-|rV^
TMROUSHi^UT
v-9
(SSJ7
ilUlirL
'-^"i
09J7
T-
Jilil
(38)
IJUlJL
^MMMMS
5TASBS HOUIZONTAL
V-IO f'" SMAP? ftuAINf
- _ UNCHANOBe.
11
>I80-K
Figure 9 - Schema fi c
WA-3A Grating Generator
how heavily the tube conducts.
It affects the multivibrator
frequency and hence necessitates readjustment of the "VERTICAL
BAR FREQUENCY" control.
The "VflDTH" control provides adjust-
ment of bar width down to 10 per cent of the space between
A "VERTICAL BARS" on-off switch, 3-1, provides plate
bars.
supply voltage for the multivibrator.
The output of the multivibrator is amplified and inverted in V-4, whence it appears on the grid of the first half
of a mixer tube V-8 as positive pulses of vertical bar fre-
quency.
To form the horizontal bers, negative vertical driving
pulses are inverted and amplified by the first half of V-5
and differentiated and clipped by the second half.
The output of V-5 synchronizes a free running cathode
coupled multivibrator composed of the two halves of V-6.
The
"HORIZONTAL BAR RANGE" switch, S-3, is a coarse control of
the multivibrator frequency, and the "HORIZONTAL FREQUENCY"
potentiometer,
R-8/«.,
brator frequency.
is for fine adjustment of the multivi-
A "HORIZO^ITAL
BilRS" on-off switch, 3-4,
provides plate supply voltage for the multivibrator.
The output of V-6 synchronizes a one-shot cathode coupled
multivibrator composed of the two halves of V-7 whose purpose
is to provide a width control for the horizontal bars.
The
"V/IDTH" control, R-63, determines the duration of conduction
of the output half of V-7 and provides adjustment of bar
width down to 10 per cent of the space between bars.
The output of V-7 feeds one half of V-8 which is a mixer
for the horizontal and vertical bar pulses, the two halves of
(39)
the tube having a common load.
The output of V-8 is fed to V-9 where the mixed signals
are held to equal levels by the limiting action of driving
the tube to cutoff.
In the plate circuit there is a peaking
coil, L-1, to boost the high frequency response.
The "CON-
TRAST CONTROL" potentiometer, R-40, varies the plate supply
and hence the amplitude of the output signal.
The output of V-9 appears inverted across the load of
mixer, V-10.
The input to V-15 is mixed negative blanking.
The output of this tube can be taken from either plate or
cathode through switch, S-5, to provide either polarity
whence it is fed to mixer, V-11.
with mixer, V-10,
This tube has a common load
The bar signals and blanking signals ap-
pear across this load and are fed to V-12 where they are held
to equal levels by the limiting action of driving the tube
to cutoff,
V-12 drives the output tube, V-13,
The "OUTPUT BLANXING
LEVEL" potentiometer, R-109, varies the screen voltage of
V-I3, and thus determines the output amplitude.
A positive
blanking output can be taken from the cathode of V-I3.
There
is coupling from the plate of this tube to the output tube,
V-14o
A negative blanking output can be taken from the cath-
ode of this tube through the "OUTPUT POLARITY" switch, S-7.
For a signal of negative polarity the "POLARITY" switch
should be set to the
should be set to the
"—
'*
**
—"
position, and the "BLAl^ING" switch
position.
For a signal of positive
polarity the "POLARITY" switch should be set to the
'*+"
sition, and the "BLANKING" switch should be set to the
{/,0)
po-
"—
**
position.
In order to produce white bars the "POLi\RITY**
switch should be set to the "+" position and the "BLANKIMG"
switch should be set to the
"-«-"
position.
V7hen the cross-
hatch pattern is to be superimposed on a video test pattern,
the 'BLANKING^ switch should be off.
To use a Crosshatch generator for setting the linearity
of pattern generators the two outputs are injected into a
receiver or monitor video amplifier.
The monitor's linearity-
controls are then adjusted for maximum equality of bar spacing,
and then the pattern generator's linearity controls are ad-
justed to make equal space intervals of the test pattern
occupy equal time intervals of the Crosshatch pattern.
(a)
CHAPTER
SVffiEP
III
CIRCUITS, POVrER SUPPLIES, mi) VIDEO Ai^PLIFIERS
OF PATTERN GE1O31AT0RS
1.
Mono scope camera.
In order to make the description of pattern generators
complete, a discussion of pov/er supplies, the generation of
sweeps, and video amplification must also be included.
They
are common to all systems of test pattern generation excluding the Crosshatch generator.
A complete monoscope camera
will be described, following which items peculiar to other
systems of pattern generation will be discussed.
A block diagram of the RCA monoscope camera type TK-lA
is shov7n in Figure 12.
a
Standard RMA pulses are obtained from
synchronizing generator to drive the vertical and horizon-
tal deflection circuits which supply the sweep currents for
the monoscope deflection yoke.
The synchronizing generator
also supplies mixed blanking pulses which are added to the
video signal in the video amplifier.
An external power
source of 280 volts at 200 milliamperes, d.c, is required
to supply all tubes.
The accelerating voltage for the mon-
oscope beam is obtained from the internal high voltage supply.
Figure 13 is a schematic diagram of the TK-IA monoscope
camera.
High voltage for the monoscope tube is obtained from
a self-contained power supply.
The power transformer, T-8,
supplies all filament voltages, and the high voltage for the
monoscope tube.
A IB3GT/8OI6 tube is used as a half -wave
rectifier to obtain the high d.c. voltage necessary for op-
(42)
P«IVING
PULSei
FROM
GCN CRAT OR
VCRTICAL.
wLsr
VPRTICAU
O
ORiJirs
HIGH
VOUTASE
Suppu»
OUTPUT
VIOEO
4-
8UANKIM6
VIDEO
AMPLiriER
MONOSCOPe TJBE
HOfliZONTAL
"MTb
PULSE
"In—ninr
HORIZONTAL
DCfLECnON
IT
ClRCJiri
COMPOSITE-
VtRTfCAU • HORiZ.
o
BLANKING
AN\Pl.l^(eR
in_r
Fif:ure l^'-^lock diagram of ilOA type TK-lA nonoscope camera
(43)
NOTK:AH IWSISTAMCg VALOCS
4 CArACITANCE VALUES
IN
IN
QHh^l
4WATT
MlCltQ-FAR*05
UWIXJS rHOICATED QTHERWlSt..
CW
TUM
CuacKWISt
Hty&
STAKPCD OH
WITH TURK
^Ertpg /txf/y
NOTE:-
REMOTE FOCUS CONTROL 25 MEG.
REMOTE CAIN CONTROL lOO.OOCi^SW
BOTH VARIABLE RE5ISTOR5 TO GROUND
ROTATION.
MrER TO COftRESPOHPtMa
r^**.-^-^.^
CC^HiHATlON
ngfT
TVMa
,
15
Figure
W—Schematic Diagram of Monoscope Cam
m
(44)
eration of the monoscope tube.
The positive side of the high
voltage supply is grounded so that the signal plate of the
monoscope tube may be at ground potential, and the output of
-1250 volts is connected directly to the cathode of the monoscope tube.
Since one side of the monoscope filament is tied
to the cathode, a separate filament v/inding of high voltage
rating is required on the power transformer.
The control
grid is tapped at a point about 30 volts more negative than
the cathode voltage.
The setting of the potentiometer, R-60,
from which the grid derives its voltage determines the amount
of monoscope beam current.
a
The potentiometer is shunted by
991 voltage regulator tube which maintains the grid cathode
voltage and hence the beara current constant,
ITocusing is
controlled by the voltage on anode 1 which is obtained from
a tapped bleeder resistance, R-38, across the output of the
high voltage supply.
There is also provision for remote fo-
cusing by a high resistance shunt between anode 1 and pin
number 6 of the connector jack, J-1,
A
2,5 megohm variable
resistor connected between this point and ground serves as
a remote focus control.
Filtering of the rectifier voltage is accomplished by
the 1500 henry choke, T-9, and the 1 microfarad capacitors,
C-20 and C-21, in the negative side of the circuit.
Effect-
ive filtering is required to prevent modulation of the gen-
erated video signal.
The power supply is voltage regulated
by one triode section, V-llA, of a 6SL7.
All of the return
current for the power supply flows through the load resistor,
R-61, of the triode.
The voltage across R-6I, in series with
(45)
that across capacitor C-20, represents the total voltage of
the power supply.
The function of tube, V-llA, is to vary
the voltage across R-6I so as to offset any variation in
total voltage.
If,
for example, the total output becomes
more negative, this change is transmitted through C-21 decreasing the current through V-llA and raising the plate
voltage of the tube.
This increase across R-6I tends to
offset the negative rise across C-21.
To obtain vertical deflection negative vertical driving
pulses are applied to one grid of V-14.
These are amplified,
inverted, and applied to the grid of the other half of "V-14,
a sawtooth generator.
Between pulses C-24 charges towards
the plate supply voltage, and is discharged during the pulses
through the low impedance of the tube.
The time constant of
charge of 0-21+ is relatively large, and since only a small
part of this rise is utilized, the voltage sawtooth applied
to one grid of V-13 is quite linear.
The vertical sweep amp-
litude is varied by means of the HEIGHT" control, R-69,
which changes the slope of the sawtooth*
After two stages of amplification in V-13 the positive
going savrtooth is applied to the grids of the dual triode,
V-12, both halves of which are connected in parallel.
This
tube drives the vertical deflection coil through the transformer, T-10.
One-half of V-11 is used to provide negative
feedback, its plate being connected in parallel with the plate
of the input section of V-13»
Input voltage for this half
of V-11 is obtained from R-79 which is in series with the
(46)
?i^re
14(p;)-nrid circuit of vertical s^reep feedback
amplifier in monoscope camera
ei
figure
14(13)
-Grid signal waveforms of vertical sweep
feedback amplifier in monoscope camera
(47)
vertical deflection coils.
A sawtooth voltage is developed
across R-79 by the deflection current.
R-81 is the
"'^/ERTICAL
The potentiometer
LINEARITY" control.
To illustrate how
it functions consider the circuit shown in Figure 14(a),
where the input is a voltage increasing linearly v/ith time.
Let C represent C-27 and R represent R-81 and R-128 in the
grid circuit of the feedback amplifier.
To obtain the output voltage, e^, across R:
at
Letting
Cj
Then:
Ei = c£CatJ
=»
eo-^''[Q
= -ft
=
a(RC-RCe"'^)=a.RC6-e-^c)
The output voltage or the signal on the grid of the feedback
amplifier can be varied by varying RC between the limits of
37,500 and 162,500 microseconds,
A plot of the two extremes
of grid signal is shown in Figure 14(b) for the duration of
one field.
Thus, essentially, the circuit provides variable
attenuation of low frequencies with respect to high frequencies back at the vertical saw tooth amplifier to compensate
for the greater attenuation of high frequencies through the
amplifier and driver stages*
To obtain horizontal deflection negative horizontal
driving pulses are applied to one grid of V-18.
These are
amplified, inverted, and applied to the grid of the other
half of V-18, a trapezoid generator.
Between pulses C-32
charges toward the plate supply voltage, and is discharged
(48)
during the pulses through the low impedance of the tube
and R-126,
of the
The horizontal sweep amplitude is varied by means
'n'/IDTK**
control, R-92, which changes the slope of the
rising portion of the trapezoid*
The trapezoid is applied through the driver tube, V-17,
end transformer, T-llj to the horizontal deflection coils.
V/hen
the retrace interval commences, the horizontal deflection
coil charges up its shunt capacitance, which then discharges
back through the deflection coil and commences to charge the
shunt capacitance in the opposite direction.
At this point
another trapezoidal voltage is applied across T-11.
er tube, ¥-16,
is cut on,
The damp-
and the oscillation in the deflec-
tion coil and its shunt capacitance is damped through V-l6.
The signal on the grid of the damper tube is a partially dif-
ferentiated sawtooth whose shape can be varied by adjustment
of "LINEARITY CCNTROL*», R-128, to control the rate of damping
current through the tube.
CONTROL'*, R-102,
the tube.
The bias is varied by "LINEARITY
to vary the amount of damping current through
The current through the horizontal deflection coils
actually reverses in direction during the sweep.
A fraction of the voltage across the horizontal deflection coils is taken from the junction of R-119 and R-120 to
blank the monoscope grid during retrace time.
"CENTERING**
controls are provided in both sweep circuits which determine
the amount of direct current through the deflection coils.
The video amplifier consists of seven stages of signal
amplification and also includes provisions for clamping and
the addition of mixed blanking to the signal which can be
(49)
clipped at the desired level.
The amplifiers utilize combined
shunt and series compensation giving a response that is fairly uniform from about 30 c.p.s. to 8 megacycles.
The overall
gain of the video amplifier is about 1000.
The signal plate of the raonoscope is connected to the
grid of the first amplifier tube, V-1, through a compensating
network composed of coil, T-1, and resistors, R-3 and R-4«
A similar network is included in the plate circuit of each
additional video amplifier tube.
Overall gain is controlled
by potentiometer, R-7, v/hich varies the screen voltage of V-1.
There is also provision for remote gain control in the form of
a shunt resistance, R-105,
ber
from the screen of V-1 to the num-
pin of the chassis connector, J-l.
5
A 0.1 megohm vari-
able resistance connected between this point and ground serves
as the remote gain control,
A clamping circuit follows the second stage of video
amplification.
The purpose of clamping is to establish a set
or fixed voltage which will represent the black level for the
signal.
The level must be the same for all lines delivered
by the pattern generator.
This is accomplished by bringing
the signal to the same potential during each blanking period.
Thus all values of light must then be represented by various
signal levels, extending in only one direction from the black
level.
Figure 15(a) shows a signal which, for the first few
lines, is all black and consists of only the sync pulses
which extend beyond the black level.
At time
t^^
the signal
goes to all white and remains there except as it is inter-
(50)
'Vli
N;
BLACK
le:\/£l
11
MilliilliilUlIlM
111
mmm
Pigure 15(o)-:)C component present
Figure lL(b)-DC component lost
C
/
Figure 16-Idealized keyed ol?3mp circuit
(51)
rupted by the horizontal and vertical blanking pulses.
If
the signal were passed through a capacitor and the dc compo-
nent lost, it would appear as in Figure 15(b).
The white
has slipped to dark gray, and the black is far beyond the
true black level.
Besides establishing the black level, clampers perform
another function*
Since the amplifiers preceding the clamp
attenuate the low frequencies by coupling with small capacitors, troubles from hum and low frequency noise are minimized.
The dc restoration will insure that each field contains the
same amount of light or average voltage.
This amounts to the
reinsertion of low frequencies above 30 cycles per second.
Figure I6 shows an idealized equivalent of a keyed dc
restorer.
The key is closed for a small portion of the blank-
ing interval.
to ground.
C,
V^hen the
key is closed the output voltage goes
A charging or discharging current flows through
limited only by R,
C is
small enough so that before the
key is opened, it becomes completely charged, and current
through it has dropped to practically zero.
C
now possesses
a charge representing the difference between the signal volt-
age and ground.
After the key is opened the charge cannot
change since no path exists for current to flow.
The signal
is transmitted through C as if it were infinite in size.
When
the keying interval again returns, the signal may be at an in-
correct level, but the key, when closed, will force the output to the correct level, and the charge will be changed to
agree with the new difference between the input voltage and
the correct output voltage.
If the level, however, needed
no changing no current would flow into or out of C.
(52)
The
keyed circuit thus restores the dc component by holding the
black level at a fixed voltage during the blanking interval
which may be considered the dc axis.
The signal extends
always in one direction from this axis.
The actual clamp circuit of the monoscope camera as
shown in the schematic diagram, Figure 13, consists of
a
twin
diode, V-9, both halves of which are driven into conduction
by push-pull pulses, one set of pulses being horizontal dri-
ving pulses, and the other set being inverted horizontal
driving pulses.
The latter are obtained from across part of
the load of the horizontal pulse amplifier in the horizontal
deflection circuit.
The two sets of pulses are essentially
equal in magnitude.
The two halves of V-9, are driven through
capacitors C-2 and C-34,
The time constants (C-2) (R-113-4-R-
112) and (C-3i+) (R-lll) are long compared to the pulse dura-
tion, and therefore these capacitors do not charge up appre-
ciably,
w'hen
both halves of V-9 are driven into conduction
the plate side of R-113 and the cathode side of R-111 rise
and fall by the same amount,
C-8 charges or discharges quick-
ly due to a small time constant, and the grid of the following amplifier, V-6, is brought to ground potential.
A negative bias voltage of about 1/2 volt for the two
preceeding stages is taken from the junction of R-112 and
R-113,
Following the clamper there are two more stages of
amplification before mixed blanking is added to the video
signal.
In the grid circuit of the first of these two am-
plifiers, V-6, there is a
wsv/itch,
(53)
S-2, which makes it pos-
sible to remove the partially amplified output of the mon-
oscope tube, and insert a test signal, such as a square wave,
for checking the response of the remainder of the video
system*
The blanking pulses are combined with the video signal
in the plate circuit of V-2<,
Blanking pulses from the sync
generator are amplified and inverted by V-7,
The pulses are
then com.bined with the video signal across the load resistor,
R-32, common to V-2 and V-7,
The blanking pulses extend in
the positive direction at this point as does the black level
of the video signal.
They are added to the signal at the po-
sitions corresponding to monoscope blanking.
After another stage of amplification in V-3, the signal
is fed to the grid of a clipper tube, V-4,
The signal has
black in the negative direction and is clipped to the desired
level by adjustment of the "BRIGHTl^JESS" control, R-55.
This
control is a potentiometer in the high voltage power supply
bleeder netv^ork.
on the grid of
cut off.
It is used to adjust the negative voltage
V-Zj.,
and hence determines where the tube will
If the negative bias is set high, then the image
will become darker because the negative blanking voltage will
drive the tube into cut-off rather quickly.
This effectively
decreases the voltage variation between bright and dark and
hence brings the brighter portions of the image closer to
the black level.
It is the blanking level which determines
the black or reference level.
The output of V-4 is coupled to the parallel grids of
V-.8
and V-10,
These two output tubes are provided so that
(54)
I.)
>
rl'
::i
Q
uj
CO
o
.®
^+
f,;
_j
_
t/i
ir,
C
.
n-t
!>
•.
<
o
b
r
U
•
-«
I
(55)
«
the picture signal may be viewed on a monitor while the output is being fed to the equipment being checked or to the
modulator of a transmitter
It is necessary to use standard synchronizing pulses in
conjunction with the output of the RCA TK-IA monoscope camera
in order to synchronize the sweeps of the receiver or moni-
tor on which the pattern appears.
If the output of the mono-
scope is to be transmitted then the synchronizing information
must be added to the video signal.
Some manufacturers cause
the synchronizing information to be added in the monoscope
camera itself.
Synchronizing information could be added to the output
of the RCA TK-IA monoscope camera by the addition of i6SN7
tube as illustrated in Figure 17.
Negative synchronizing
pulses are amplified and inverted by the sync amplifier.
Through use of a common load with the clipper stage synchro-
nizing information is added to the video information.
The
potentiometer in the cathode circuit of the sync amplifier
is a "SYNC GAIN" control,
2,
Flying-spot scanner,
A flying-spot scanner uses a similar system of power
supplies, sweep circuits, and video amplifiers.
RCA is plan-
ning to demonstrate a flying-spot scanner in the near future.
Although not completed at this writing, enough information is
available to discuss how a flying-spot signal generator differs from a monoscope camera.
sweep circuits can be used.
(56)
Similar power supplies and
The d.c. power supplies for the
5WP15 tube should consist of 20 kllovolts for recoirmiended
anode
2
supply, and a negative supply of about 100 volts,
depending on equipment design, for grid 1,
1 is
Voltage for anode
obtained by tapping from a bleeder resistance across
the 20 kllovolts supply.
The high voltage for the multiplier
phototube can also be obtained from a tap on the bleeder resistance.
The only fundamental differences are in the video
amplifier of the flying-spot scanner.
In describing the 5Y^15 flying-spot scanner it was men-
tioned that an equalizing network would be necessary to compensate for the short delay characteristics of the P15 phosphor*
This equalizing network would be complex if the entire
radiation of the tube were used.
By making use of only the
ultraviolet radiation, however, the equalization can be supplied by only one network.
V/hen a
transition from black to white is scanned, the
spot in time has an exponential shape and a changing position*
Since the buildup of the spot intensity is practically instantaneous, v;hen the spot moves from behind a mask into an opening, at first only the light from the spot being hit by elec-
trons strikes the phototube.
An instant later, as the spot
moves farther into the opening, the light from the spot being
hit by the electrons has the light from the spots which had
been hit a short time before added to it, since they are still
emitting some light*
The light input to the phototube is
therefore proportional to the integral of the light-decay
characteristic.
Since the decay curve is an exponential func-
(57)
tion, its integral is also exponential,
as the scanning
spot moves from white to black, the light falling on the
phototube decreases exponentially.
That is, both the rise
and fall signals follow the same law.
Figure 18(a) shows how a signal would appear as a result
of scanning a v/hite bar on a black background without equal-
ization.
The voltage is zero up until time
t-j_.
At
t^^
v^hen
the beam reaches the white bar the voltage v/aveform becomes
ECl-e
••<'«^^
where E is the ultimate value for white,
t
is
measured in microseconds, and 0,05 microseconds is the time
constant of decay for the ultraviolet radiation from the FI5
phosphor.
At time t2, when the beam again reaches the black,
the voltage wave form becomes
Cft" o»^
,
or the complete
expression for the voltage is:
e(t)- E[(i-t-lii^)u(t-t,)~
(i~e-^) u.(t-U)]
If now an equalizing network is made up as shown in
Figure 18(c), where RC equals the time constant of decay of
the ultraviolet radiation « 0.05 microseconds, and r is made
considerably smaller than R, it will be shown that the output
^•ft)
is very nearly a squarewave when the voltage
figure 17(a) is the input.
Using Laplace transforms:
(58)
^(t) of
e(t)
t.
Vii
-
ejt)
i(t)
H
t
eo(t)
I
?i{^ure ir(c)-.i;qus.li^lr:g net^jorl
(59)
-.
£r
{[,-e-^ft-t^][u(t-t.)]-[|-e-«ft-t^^[uft-t.)]}
where
S' 7]^
If r is made considerably smaller than R then eo{t)
a square wave.
For instance, if
C^lO^^i
fUa^
I hen
S-d
is nearly
500
i-
R
—
5000
(5-(5o)(jo^o)(/flxw-'^)
=
5000 a
.
r^SOOn
/U/^*
.
0. OS* sec.
"
_
//
o.6 $-/« s
A plot of this voltage is dravm in Figure 18(b) to the
same time scale as for e{t).
It can be seen that at the
expense of an attenuation of 11 to 1, that the
(60)
t5.me
constant
1
(
4-5
o
(61)
of the response to a square ivave of the flying-spot scanner
has been reduced by the
sairie
factor.,
Figure 19 is a schematic diagram of an equalizing amplifier using the above method of equalization.
The ultraviolet
light from the flying-spot tube is focused on the cathode of
a 931-A multiplier phototube.
The cathode is connected to a
regulated supply of -725 volts, and each successive dinode
is approximately 75 volts more positive.
The anode is approx-
imately 50 volts more positive than dinode 9*
The output of
the multiplier phototube feeds the equalizing amplifier
through a compensating network.
Equalization is accomplished
by the method of coupling to the following amplifier.
There
are "GAIN" and "REIvIOTE GAIN" controls in the equalizing amp-
lifier identical to those in the RCA TK-IA monoscope camera.
Also shov/n in Figure 19 is a 12AT7 video mixer.
This
circuit will be included in the RCA flying-spot scanner.
By
focusing the light output of a flying-spot tube on the edge
of a mirror that is formed from two mutually perpendicular
surfaces, the light output of the flying-spot tube can be
split, half of it going to one phototube and half to another,
Vi'ith
such an arrangement a studio can fade or switch from one
slide to another.
Shown in the diagram is one of the photo-
tube multipliers with its associated amplifiers.
Referring
to this as the "left unit" there are also provisions for in-
serting a "right unit".
By means of the potentiometers, R-1
and R-2, on a coromon shaft a fade can be made from one unit
to the other, or one picture can be superimposed on another.
The left half of the mixer tube passes the video signal from
(62)
L
the left unit and the right half passes the picture from
the right unit.
If it is desired to change from one picture
to another v/ithout fading then switch, S-1,
is switched from
the »'FADE" position to the ''INSTAITTAJ^JEOUS" position.
By use
of switch, 3-2, the signal from either unit can be selected.
In the
" INSTANTANEOUS"
position the left half of the mixer
tube is used for both units.
At the output of the mixer
there is a sv/itch, 3-3, for selecting either polarity de-
pending on whether the picture is a positive or a negative.
The circuits shown in Figure 19 can be substituted for
the monoscope tube and its first two amplifiers, V-l and V-5,
in the RCA TK-IA monoscope camera.
The output of the mixer
will then feed into the clamper tube, V-9.
There will be an additional type of amplifier in the
RCA flying-spot scanner that does not generally appear in
monoscopes although it well could.
In order to obtain max-
imum fidelity of shade reproduction, the light output of a
television receiver should be directly proportional to the
light input to the photosensitive device.
"gamma" should be unity.
That is, the
In the flying-spot pickup, the
voltage output is directly proportional to the light input.
Since the beam current of a flying-spot tube is unmodulated,
there is no opportunity for the Introduction of amplitude
distortion in the output of the tube.
Nor is there ampli-
tude distortion in the output of the multiplier phototube,
since it is characteristically a linear device.
Therefore
if linear amplifiers are used, the output voltage of the
flying-spot video-signal generator will have voltages pro-
(63)
•
portional to light input
A kinescope, however,
is not linear,
since it requires
more volts to give the same change in light output at low
light levels than at high light levels.
Figure 20(a) shows
the light output plotted against signal input for a typical
kinescope, the I6AP4,
If a nonlinear amplifier is provided
in the flying-spot video-signal generator with the recipro-
cal of the kinescope characteristic, then the relation be-
tween input and output light will be linear.
Such an ampli-
fier is called a gamma-correction amplifier.
Figure 21 is an example of a gamma-correction amplifier.
It is being used by RCA in their flying-spot scanner.
The
input to the two grids of the 12AU7 from a clipper stage is
a
video signal with black positive.
The various crystal di-
odes, D-1, D-2, D-3, and H-U are biased to different voltages
by their associated potentiometers.
For small signals (white
level) the only conduction path to the grid of the following
amplifier is through the IIK resistor R-1,
For a slightly
larger signal let us assume the bias on D-1 is such that it
will conduct.
An additional current path is thus provided to
the grid of the following stage.
As the input signal gets
larger or approaches the black level more current paths are
opened and the signal on the grid of the following stage has
been amplified more strongly.
The cutoff biases of the crys-
tal diodes are independently controlled so that the gamma-
correction characteristic may be set as desired.
A typical
plot of amplification for this stage is shown in Figure 20(b),
(6/.)
(65)
-«
.-1
O
I
c^^
n
o
>
)
I
^1
(66)
It is important that the signal-to-noise ratio in the black
region be very good, as it is in the flying-spot scanner, in
order for this type of amplification to be practicable since
the signal voltages corresponding to the dark regions are
amplified more than the voltages corresponding to the white
regions.
This amplifier could be inserted in the video amplifier
of the TK-lA monoscope camera between V-4 and the
tubes.
tv;o
output
It can be used with either the monoscope or the flying-
spot scanner.
Due to the poorer signal-to-noise outputs of
image orthicons and iconoscopes it has not been used in con-
function with them to any great extent*
3«
Image orthicon and iconoscope cameras.
Image orthicon and iconoscope cameras are very similar
to monoscope cameras and flying-spot signal generators, dif-
fering mostly only in the method of pick-up.
Due to the pe-
culiarities of the iconoscope tube, an iconoscope camera requires modifications to its sweep circuits and a type of com-
pensation in the video signal known as shading.
Because the mosaic of an iconoscope is scanned on the
same side that the light enters, the scanning beam is tilted
at a 30 degree angle.
This necessitates "keystone" or ver-
tical sweep modulation of the horizontal sweep and distortion
of the vertical sweep in order to get a resultant sweep of
the mosaic that is linear.
Also, due to uneveness in the
secondary emitting properties of the mosaic surface, and due
to uneveness in the attracting fields adjacent to this sur-
(67)
•
face, secondary electrons fall on the mosaic in an uneven
shower.
A variation in charge distribution results over
the mosaic which gives rise to uneven picture signal compo-
nents knovm as "dark spot".
Shading signals must be inserted
in the video amplifier and adjusted on a trial-and-error
basis to give a uniform picture output when a uniform dis-
tribution of light falls on the mosaic
(68)
»
CHAPTER IV
COMPARISON OF METHODS OF PATTERN GENERATION
The monoscope camera and the flying-spot scanner are
capable of producing test patterns of the highest quality^
Both methods have an excellent signal-to-noise ratio, and
both are able to produce resolutions of about 500 lines
Image orthicon cameras are inferior in that they have a lower signal-to-noise ratio while iconoscope cameras have poorer
resolution.
In addition neither tube is as capable of repro-
ducing half-tones as faithfully as a monoscope or flying-spot
tube#
The monoscope camera provides the simplest method of
pattern generation.
and few adjustments.
Once set up it requires little upkeep
The same video signal can be obtained
from day to day, and the quality is not effected by such
variables as poor optical focus, dark spot, and amplifier
noise
In addition it is reasonably small and light and can
be moved about.
It is the ideal equipment for factory or
studio testing.
The flying-spot scanner can be equally
adapted to testing, but is more complicated and expensive
than a monoscope camera.
The optical system of the flying-
spot scanner requires that it be kept free from vibration
and shock, and that the focus of the lens system be carefully
adjusted.
There are several factors that limit the use of image
orthicons and iconoscopes for testing.
the item of expense.
Primarily there is
The life of an image orthicon varies
(69)
from 200 to 500 hours, and a tube costs about ll^OO.
The
life of an iconoscope varies from 500 to 2000 hours and
a tube costs about ^500,
Monoscopes, however, have a life
A flying-spot
of about 10,000 hours and cost about $100.
pick up tube has the short life of an image orthicon, but
only costs about $70 to replace.
There is also the objection that a still picture focused on the photocathode of an image orthicon for just a
few minutes will burn in an image of the picture.
This ob-
jection entirely eliminates the image orthicon for lengthy
testing.
An iconoscope is capable of generating a fairly
high quality picture, but it requires very careful alignment
and compensation.
The flying-spot scanner is the most versatile of all
the pattern generators in presenting still pictures.
The
monoscope camera is necessarily confined almost exclusively
to the generation of a test pattern since it would require
a separate tube for every different picture.
Slides can be
changed at will in the flying-spot scanner so that the unit
may be used to air a station ^s test pattern when no programs
are being televised, and then used during the program.s to
televise any still pictures such as advertisements.
Mention
was made that the flying-spot scanner could be used v/ith
opaque pictures and moving picture film.
The use of opaques,
however, requires excellent technique and is not yet coinmer-
cially feasable.
A special projector is required for moving
picture film, and to date the flying-spot scanner has not
been used for this purpose.
(70)
Ultimately, it seems quite likely that the image orthicon
camera v/ill be used exclusively for televising action, in the
studio and outside, the flying-spot scanner will be used for
televising motion pictures and stills, and the moncscope will
be confined to factory testing and possibly generation of a
television station*
s
test pattern.
(71)
•
BIBLIOGRAPHY
!•
Bauer, J. A, Television receiver production test equipment. Communications, 27:8-11, September 1947.
2o
Bedford, A.V. Figure of merit for television performance, R.C.A. Review.
3:36-44, July 1938.
3«
Bertero, E.P, Video announcer.
10:304-309, June 1949.
4»
Burnett, C.E, The monoscope,
April 1938.
5.
Coimnittee on Television Transmitters.
chart 1946,
RIviA
R.C.A, Review,
R,C.A, Review. 2:414-420,
B}Ak resolution
Bulletin ED-2502-A, September 1947
6.
Duke, V.J, A method and equipment for checking television
scanning linearity. R.C.A, Review, 6:190-201,
October 1941.
7.
Klver, M,S, Television production line testing-part II,
Radio-electronic engineering, 12:6-9, January 1949.
8.
Klver, M.3, Television production line testing-part III,
Radio-electronic engineering. 12:16-21, February 1949.
9.
Radio Corporation of America. Grating generator type
V/A-3A instructions,
Camden, New Jersey, Engineering
Products Department.
10.
Radio Corporation of America, Monoscope camera Type TK-IA
instructions. Camden, New Jersey, Engineering Products
Department, 1948.
11.
Radio Corporation of America, 5""^15 tentative data.
Harrison, New Jersey, Tube Department, 1948.
12.
Szlklal, G.C.; Ballard, R.C.; and Schroeder, A.C. An
experimental simultaneous color-television systempart II. Proc. I.R.E. 35:862-870, September 1947.
13.
Terman, F,E,
1947.
14«
^li'endt,
15.
Zeluff, V. Tubes at work.
June 1948.
Radio engineering.
New York, McGraw-Hill,
K,R, Television dc component.
9:85-111, March 1948.
(72)
R,C,A, Review,
Electronics. 21:124-126,
DUB
DA!I!S
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i
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13118™
Hancotte
Methods of generating
television test
patterns.
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