January 1951 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
It's probably a safe
bet that most people, even at the dawn of color television, knew of the
competition which occurred for the adoption of three different methods of
implementation. Two of them - line-sequential by Color Television, Inc. (CTI),
and dot-sequential by Radio Corporation of America (RCA) - were fully electronic
while the third system by the Columbia Broadcast System (CBS) used a kludge of a
spinning color wheel placed in front of a black and white display. The CBS
field-sequential design used a synchronization component of the composite
transmitted signal to position the correct color screen (red, yellow, or blue)
in front of the screen as the electron gun scanned the CRT - analogous to how
World War I airplane machine guns were synchronized with the engine to fire
between propeller blades. Of course an out-of-synch scenario in the color wheel
was not as serious as with the machine gun. The worst that could happen with the
TV is a false color picture whereas with the machine gun your biplane instantly
became a glider. Although I poke some fun at the CBS solution, the fact is sheer
ingenuity was involved in all three. The ultimate selection of RCA's
dot-sequential system, which had not yet occurred when this article was
published, was based largely on its simpler solution to incorporating a
composite signal that was backward-compatible with the existing black and white
standard. What designers managed to pull off in a mere 6 MHz of bandwidth was
amazing as it included three color luminance and intensity signals, horizontal
and vertical synchronization on field edges, audio, and a guard band (see
color composite signal definition).
Color Television Systems
By Fred Shunaman
Of the three main systems of color television that have been battling for FCC
and public recognition, the tentatively approved CBS field-sequential system is
most prominent today. The FCC has stated, however, that the door is not irrevocably
closed against other systems, so interest remains strong in the runners-up. These
are the line-sequential system of Color Television Incorporated (CTI) and the dot-sequential
system developed by RCA.1
The pros and cons of these systems have been discussed with so much heat and
so little moderation that the radio-man is not quite sure of anyone of their technical
features. The public - at whom this barrage of facts and near-facts has been directed
- is hopelessly confused. The terms "compatible" and "incompatible" have been bandied
about to such an extent that many laymen believe that it would be possible to get
color pictures without modifying their present sets, if only a "compatible" system
of transmission were used. At the other extreme is a sizeable number who believe
that present sets will become useless as soon as color television starts.
Let us review some of the technical facts to help clear up the nonsense. We have
one system using relatively simple mechanical apparatus and two systems using more
complex electronic equipment to produce roughly similar results. All three systems
use standard black-and-white tubes with colored gelatine filters to insert the color
into the images.
RCA has, it is true, demonstrated a single tube which produces the three colors
with its own phosphors.2 This promises a color system without filters
and with only one instead of three kinescopes as used in the present RCA setup,
but whether a three-color tube can be mass-produced economically enough to be used
in home receivers remains to be seen. At least three types of three-color tubes
(RCA, Geer, and Du Mont) have been patented; none have yet been proven to be (or
not to be) practical.
Another abused catchword is "mechanical system." It is made even more puzzling
when CBS spokesmen remark in passing that their system could also work with electronic
color tubes. The fact is that the adaptability of any of the systems is a function
of the speed of switching from one color to another. Equipment that can be used
by the fastest-switching one can be used by the other two, but not vice versa! Colors
are switched more than ten million times a second in the RCA system, 15,750 times
in the CTI system, and only 144 times per second by the CBS method. Therefore either
CBS or CTI could transmit and receive with equipment suitable for the RCA method.
CBS could also use equipment of the type required by CTI's line sequences.
However, should CBS decide to rid itself of the stigma of a "mechanical system"
and go electronic, it would have to accept some of the disadvantages as well as
the advantages of the more complex systems. An excellent field-sequential system
could be built up with three cameras and three kinescopes, but it would be much
more costly than the color wheel. A field-sequential system could undoubtedly use
a three-color tube if such were available, but would be up against the same problems
of color crawl, etc. as is the dot-sequential system, and similar complex and expensive
methods would have to be used to solve them.
Color Illustration I - The CBS field-sequential system uses six
one-color fields to make up a complete color image.
With the color-wheel system now used by CBS (Color Illustration I), receiving
and transmitting equipment differ little from that used for black-and-white.2,3
Color is supplied by transparent discs divided into red, blue, and green segments
which rotate in front of camera and kinescope. The discs must be synchronized so
that each segment is in position while the corresponding color field is transmitted.
Thus, during a red field, a red filter ahead of the camera lens permits it to "see"
only the red light from the scene, and the blue and green are not photographed.
At the same instant, a red filter in front of the kinescope colors the partial image
for the viewer. The same thing happens during the blue and green frames, and the
eye receives the red, green, and blue primary images in such rapid succession that
it sees a picture in full color.
Instead of black-and-white's two interlaced fields per frame, with 30 complete
pictures per second, CBS pictures are composed of two interlaced color frames of
three fields each. There are 144 fields per second, with 24 complete pictures. It
was necessary to cut the number of lines from the standard 525 to 405 to transmit
the 144 fields within the regular 6-mc channel. This is the reason for Columbia's
Main advantages of the CBS system are its simplicity and low cost. Since the
only modifications required are the above-mentioned changes in the scanning frequency
and the addition of a color wheel, the CBS system requires no extensive or complex
new equipment. Transmitters and receivers for color-or for color and black-and-white
-can be constructed or modified at a fraction of the cost of adapting for either
of the other systems.
The chief disadvantage of CBS color is its incompatibility. Because of the different
line frequency, a standard receiver tuned to a CBS color broadcast will see nothing,
either in black-and-white or color. Another disadvantage is its lower definition,
either in black-and-white or color. Its 405 lines cannot reproduce fine detail as
well as systems using standard 525-line pictures. When used with a mechanical color
wheel, picture size is limited to about 12 inches.
Other disadvantages are flicker and fringing. Its sponsors claim that the high
field rate (144 per second) has fairly well eliminated flicker. Fringing - the breakup
of color at the edges of rapidly moving objects-is still something of a problem.
The CTI System
Color Illustration II - The CTI line-sequential system combines
all three primaries in each of its six fields.
The system demonstrated by CTI (Color Illustration II) is line sequential. Instead
of transmitting a whole field or frame in one of the primary colors, the color is
switched at the end of each line. Proponents of CTI's method claim that flicker
is reduced enough by line switching to permit the system to be compatible. However,
the 525 lines of the standard system introduce a problem. Since 525 is a multiple
of 3, the same line in each field would always be scanned in the same color. A system
had to be designed to skip lines regularly, so that all parts of the picture would
be scanned in three colors. By skipping, line 1 (for example) in the first field
may be scanned in red, in the third field in green, and in the fifth field in blue.
(Even-numbered lines would be scanned in the second, fourth, and sixth fields.)
CTI uses three lenses and three color filters ahead of its camera tube, so that
three images, identical except for color, are formed on the mosaic. Instead of being
speeded up as in the CBS system, the horizontal sweep is slowed down to one-third
standard, so that a single sweep will give three lines, one in each primary color.
Three cameras could of course be used. In that case a switching system would select
lines successively from each of them.
The CTI receiver may consist of three kinescopes, each with a color filter and
lens ahead of it. The lenses are so placed as to superimpose the three images on
a screen, where they appeal' as a full-color picture. It may also be a single tube,
with the three color rasters side by side on it, and the same optical mixing system.
CTI's great advantage is its compatibility. It uses the old 525-line interlaced
system. The disadvantages are complexity (as compared to CBS) and another peculiar
to a line-sequential system. This is line flicker or line crawl, in which the lines
seem to be crawling up or down the picture. It can be avoided to some extent by
the complex color interlace in which six fields are required for a single color
picture. The number of complete pictures is thereby decreased to ten per second,
which seems slow. Sponsors of the system say that the line-by-line color switch
prevents this from producing objectionable flicker.
RCA Dot-Sequential Color
Color Illustration III - RCA's dot sequential system, with four
fields per picture, has a complex interlace of dots.
Fig. 1 - RCA dot-sequential transmitter, showing mixing of high
Probably more has been said about the RCA (Color Illustration III) dot-sequential
system than both others combined. It is the most complex, the hardest to understand,
and offers the greatest possibilities for future development of any of the three
systems. Instead of breaking the color up into its primaries by fields and lines,
the RCA system breaks each line up into dots of primary color. Each color is scanned
or "sampled"4 3.6 million times per second, and a stream of colored dots
appear on the viewing screen. These combine to form a color picture much as do the
dots of a color plate used in printing books or magazines. The dots of color printing
do not fill the whole area, however, whereas those of RCA color television overlap
about 50%. The small size and rapid succession of dots reduces problems of flicker
and fringing to where they can be ignored.
Four fields are required for a picture. Two are the standard line interlace;
the other two trace over the same lines, but the color dots are displaced so that
a dot in field 3 is halfway between two dots of field 1 and one in field 4 halfway
between those of field 2. This, plus the 50% overlap, insures that all parts of
the scene are scanned in all three colors. There are 15 pictures a second, since
the standard 60-field system is used.
RCA's great advantage is compatibility, but it has another - that of greater
definition than its rivals. The high frequencies from each of its three color cameras
are mixed together, and the low frequencies are sent through the color sampler which
transmits the signals to produce color in the received picture. Fig. 1 shows how
this is done. Mixing the highs causes the fine detail of a scene to be reproduced
in each of the colors, no matter what its original color. Therefore large bodies
(which are reproduced by the low-frequency signals) are transmitted in color, while
points, edges, and outlines are actually in black and white.
Strange as it sounds, this actually works. If, for example, two adjacent sides
of a building appear in deep green, and the fine corner line that separates them
appears as black or white (depending on whether it is in sun or shadow) the eye
is satisfied. Indeed, there is reason to believe that the eye does not perceive
color in fine detail, and the mixed-highs principle may produce pictures closely
resembling what the eye sees in nature.
Disadvantages of the RCA system are the complexity and cost of the equipment
and its operation. Colors are switched more than 10 million times a second, instead
of 144 times as in the CBS system or the 15,750 times of the CTI system. The difficulty
of keeping the apparatus in perfect adjustment is enormously increased. Color drift
was one of the early problems of this system, and produced some interesting (but
to the engineers hair-raising) effects. Thus bananas on a plate might apparently
age, turning from yellow to brown as they were being carried to or from the center
of the picture.
This problem has been solved with a synchronizing system in which timing pulses
are transmitted to provide exact dot registry.
Many engineers point to these very problems, and the ones that still exist, as
one of the strong points in favor of RCA's system. This admittedly crude development
already produces images which some feel are equal to those of any system, and cannot
lag far behind by anyone's reckoning. Yet the system is new and at the beginning
of its development, whereas others are well in sight of the end of theirs. To say
that a system shows great room for improvement may not always be praise, but it
is a significant factor when planning for the future.
In typical RCA receiving equipment, three kinescopes are used, one for each of
the primary colors. The separate colors are mixed with the aid of dichroic mirrors,
which are transparent to two of the primaries and reflect the third. The viewer
sees a full-color picture on what appears to be the screen of the green tube, though
actually the red and blue components are reflected from the mirrors. As stated before,
a single three-color direct-viewing tube has been demonstrated, but is still in
the developmental stage.
Besides the three methods described, a number of other incipient color television
systems - not developed to the point of demonstration - have been proposed to the
FCC. None of them are likely to replace one of the present systems as the final
answer to color television, but the possibility cannot be excluded.
1 Television in Color. Fred Shunaman Radio-Electronics, January, 1950,
2 New Picture Tube for Color TV. Radio-Electronics, June, 1950, page 27.
3 Color Television. Harry W. Secor, Radio-Craft, Part I, June 1947, page
4 PPM - New Technique, Fred Shunaman Radio-Craft, February, 1946, page
314. Pulse Code Modulation, Fred Shunaman, February, 1948, page 28.
Posted December 29, 2020