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 acknowledged.
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
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.
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
Fig. 1. Chrominance signal (broken line) squeezes into
Fig. 2. Color burst (broken line) is added to horizontal
pulse's back porch.
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 results.
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 receiver.
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 set.
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 that oscillator.
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)
Posted September 23, 2014