September 1957 Radio & TV News
Wax nostalgic about and learn from the history of early
electronics. See articles from
Radio & Television News, published 1919-1959. All copyrights hereby
The main purpose for
bothering to reprint articles like this one on analog color TV theory is to
reveal the complexity and ingenuity that went into cramming a lot of information
into a relatively (at the time) small bandwidth.
Signals within signals and signals riding on top of and below other signals
was the name of the game, and pulling it off successfully required many well-designed
and well-aligned circuits. Anyone old enough to remember watching a show
on analog television can appreciate the difference between a high quality set
with self-adjusting capability and a cheap set that required constant fiddling
with the tiny, fluted knobs on the back. I, by the way, always had (and still
have) the cheap sets. A bad picture on today's digital displays consists of
screwy color tones or a few missing pixels, but at least you can stand to watch
your movie or ball game. If an analog set started acting up, the picture could
creep to the top or bottom of the screen, the horizontal and/or vertical scan
synchronizations could scramble the picture into an indiscernible mess, multipath
combined with a poor receiver could cause ghost images, along with many other
annoying phenomena. Proof of improvement is that instances of having a foot
put through a TV screen nowadays is vastly more likely due to a poor performance
on the part of a sports team than to a crappy picture.
Practical Color TV for the Technician
By Ken Kleidon
National Color TV Manager Hycon Electronics
Part 2. What service practitioners should know about the components of the
color video signal.
There are four areas of information, as
stated in the preceding article, with which the service technician must become
familiar if he is to service color receivers successfully. These areas cover
all aspects of the transmitted color signal, the special color circuits used
in the receiver, the new type of picture tube used at the receiving end, and
the special service techniques and procedures required. This article will be
primarily concerned with the signal.
Because of the compatibility requirement, a monochrome receiver must be capable
of receiving a color transmission and of reproducing directly from it a picture
in black-and-white without modifications or additions to that receiver. To facilitate
this requirement, the same transmission standards imposed on monochrome signals
apply equally to color signals. The latter must contain, at least, all the information
provided by a black-and-white broadcast and the same specifications must apply,
including the 6-mc. bandwidth for the channel, placement of the sound carrier
at 4.5 mc. above the picture carrier, and so on.
When the transmitted monochrome signal is analyzed from the standpoint of
the service technician, it is found to consist of three component signals -
one relating to video information, another to sound information, and a third
to synchronizing information. A color transmission must carry each of these,
but it also includes separate, additional information relating to color. Since
this added intelligence must be contained within the same 6-mc. bandwidth that
is allotted to the monochrome transmission, this color-signal content has been
devised in such a way that it will not interact or interfere with the monochrome
signal and that it will not affect operation of the circuits in a receiver designed
for black-and-white reception only.
As a result of this seemingly odd relationship between these separate but
related monochrome and color signals, the manner in which a color TV picture
is processed and reproduced in the receiver is quite distinctive. First the
monochrome signals are processed by circuits similar to those in conventional
monochrome receivers to produce a black-and-white picture. Then the color signals
are separately processed by additional circuits especially designed to respond
to them. The resultant color-producing information is then superimposed over
the monochrome picture to produce an image in color.
Fig. 1. Chrominance signal (broken line) squeezes into channel bandwidth.
Fig. 2. Color burst (broken line) is added to horizontal pulse's back
Fig. 3. Expansion of block in Fig. 4 labeled "color circuits."
This is one system in popular use, but others exist.
That this manner of producing the end result is indeed used can be demonstrated
in a practical way without going into technical details, if a properly adjusted
color receiver is tuned to a color TV broadcast. If the color (or chroma) control
is rotated to its minimum position, a black-and-white picture results. This
is what has happened: turning down the chroma control has had the effect of
discontinuing operation of the special color-processing circuits, or at least
of preventing their output signals from reaching the picture tube. The separate
monochrome circuits continue to operate, however, and a black-and-white picture
A practical analysis of the transmitted color signal reveals that it includes
five components. Three of these - video, sound, and sync - are identical to
those found in monochrome transmissions. The other two are incorporated to permit
the addition of color. Since the sound, signal is virtually a separate transmission
on a separate, although related, frequency and since it is not affected by the
fact that we are dealing with either a monochrome or color broadcast, we can
put it aside. The video (or brightness, or luminance) information, which provides
variations in light or dark, is interwoven with the sync signal in standard
monochrome practice. The purpose of the latter signal is simply to make sure
that the variations in light occur in the right places on the screen of the
In dealing with color information, we have a somewhat similar situation:
the chrominance signal, one of the two new components in the transmission, carries
variations in color; while the color-burst or color-synchronizing information,
the second of the two added signals, helps the receiver establish and separate
the colors from the chrominance information provided, and makes certain that
the right colors are being fed to the picture tube at the right time and in
the right places.
With the help of Fig. 1, we can see how the chrominance signal is squeezed
into the limited bandwidth available Actually it co-exists with already present
video information occurring at the same frequencies. Everything that appears
in solid line pertains to the signals with which we are already familiar in
the case of monochrome transmissions. A color subcarrier at 3.579545 mc., usually
referred to as 3.58 mc. for convenience, is shown in broken line. The extent
of its modulation sidebands are also shown in broken line.
Actually, in order to describe a full range of color variations electronically,
we need two signals, not one. If both of these can be varied over a wide range,
and the final color produced is the result of the combination of these two,
then we have an almost infinite range of possible combinations. This gives us
a wide potential for representing different hues (red, green, blue, etc.) and
different degrees of color intensity, or saturation.
Since the limited bandwidth available for any channel makes it difficult
enough to squeeze in even one additional carrier (at 3.58 mc.) , both of the
signals required for chrominance information are ingeniously modulated onto
this single carrier in such a way that they do not interfere with each other.
It is as though two subcarriers at exactly 3.58 mc. were used. One, however,
although it is at exactly the same frequency, is 90 degrees out-of-phase with
the first. Hence, these two are said to be in quadrature. In this way, if we
can adjust circuits in the receiver to be sensitive to the difference in phase
between these two signals, we can have the effect of separate signals in the
Since these chrominance signals are added in the form of amplitude modulation
and since the 3.58-mc. frequency at which they occur falls within the 4-mc.
bandwidth within which black-and-white video information also occurs, we have
an additional problem. Because the receiver's video detector is designed to
respond to amplitude modulation at this frequency, the color-carrying 3.58-mc.
signal will show up as a rather fine-grained beat interference, marring the
monochrome picture. To avoid this, the subcarrier that has been so carefully
devised to provide us with desired additional information is filtered out and
discarded at the transmitter! Its effect is not lost however: its modulation
sidebands continue to be transmitted; and provision is made for reinserting
the carrier in the receiver itself, safely away from the monochrome circuitry,
so that it may once again be presented effectively with its sidebands. In a
conventional black-and-white set, of course, no such reinsertion is made.
The second new element added to the transmitted signal for use by color-receiver
circuits is shown in Fig. 2. In solid line, we see the familiar horizontal
blanking and synchronizing pulse, with video (luminance) signal visible just
to either side of it. Inserted on the back porch of this pulse are 8 cycles
of sine-wave signal at exactly 3.58 mc., as indicated by the broken lines. Although
this color-burst signal, as it is known, has no noticeable effect on the operation
of the sync and deflection circuits, it is picked up by certain added circuits
in the color set that make important use of it.
Fig. 4. In this block diagram of a color TV receiver, the five basic
components of the signal transmitted (identified in text) are shown in the
various paths they follow through various receiver circuits. Except for the
color block, note basic similarity to monochrome circuitry.
It is principally used to synchronize a subcarrier reference oscillator built
into color sets, tuned to 3.58 mc., in a manner that may be compared to that
in which the 15,750-cps pulse is used to synchronize the horizontal oscillator
in all TV receivers. In this way, the transmitter tightly controls the receiver's
reference oscillator in phase as well as frequency. Thus the reference oscillator
provides a reliable substitute for the sub carrier that has been filtered out
at the transmitter and permits establishing the accurate phase relationship
that is necessary to distinguish between the two quadrature signals that make
up the chrominance information.
At this point we would do well to summarize our knowledge of the signal.
The monochrome transmission has three separate components, relating to video,
sound, and sync. Two more are added, for a total of five, to make up the complete
compatible color signal. One of these, the chrominance signal, can be regarded
as the color video signal. The other, the color burst, is another sync solely
for use by the special color circuits. It is used to synchronize a 3.58-mc.
reference oscillator in much the same way as the horizontal sync pulse is used
to control the horizontal oscillator.
If we follow the course of these signals inside of a color receiver, we note
that all five of them - the video (V), the sound (S), the sync or deflection
(D), the chrominance (C), and the color burst (B) - enter the antenna and proceed
through the tuner and i.f. amplifier stages together, as shown in Fig. 4.
From this portion of the set, the 4.5-mc. sound i.f. carrier may be separated
and sent directly on to the conventional sound circuits.
The remaining signals go to the video circuits (detector and video amplifier).
The sync or deflection signal is taken off for feeding to the sync circuits,
which operate the horizontal and vertical oscillators. In addition, sync pulses
are generally used to operate the keyed-a.g.c. circuits found in color sets.
Video information is amplified and supplied to the picture tube. The color-burst
and chrominance signals are applied to and processed by the color circuits.
The resulting color video information is applied to the picture tube, where
it is added to the existing monochrome image.
The same system for processing color intelligence is not used in all receivers.
However, as a starting point, the block marked "color circuits" in Fig. 4
has been separately expanded in Fig. 3 to correspond to one of the popularly
used color systems.
Since the color burst occurs during horizontal sync-pulse time, many circuits
in the color-processing section take the pulse, in one form or another. It is
applied, for various purposes, to the color killer, the burst keyer, and the
bandpass amplifier. Also applied to the latter section are the chrominance signal
and the color burst. After amplification, the burst is separated by the keyer,
applied to the burst amplifier, and then fed to the 3.58-mc. color-reference
sub carrier oscillator. Here it performs its important function of synchronizing
The chrominance signal, after leaving the bandpass amplifier, is passed on
to the two color-signal demodulators. In this receiver, they are the B-Y and
G-Y demodulators. Y stands for the black-and-white (or luminance or brightness)
component. B, G, and R stand for the three primary colors, blue, green, and
red, used in color television, from which all other colors and color combinations
are made. B-Y, then, would stand for all blue signal information minus the information
concerning its brightness, or the amount of black or white with which it is
mixed. (The latter, of course, is inserted separately by the monochrome that
is supplied and which is then "painted over" with the appropriate colors.) Similarly,
G-Y and R-Y stand for the green-only and red-only information.
After the B-Y and G-Y (or blue and green) information has been removed from
the total chrominance information found in the transmitted signal, it is possible
to develop the R-Y signal from what remains without resort to a separate demodulator.
These three color-difference signals, as they are called, are subsequently applied
to the three guns in the picture tube.
Much detailed information concerning the exact nature of the color signals
has been left out deliberately. It is hoped that enough information has been
covered, however, to give a broad understanding of what these signals are and
to assist in understanding receiver function with respect to them.
(To be continued)
Color and Monochrome (B&W) Television Articles
Posted September 23, 2014