ABC's of Color TV
August 1966 Radio-Electronics

August 1966 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.

In 1966, as color television was still in its early adoption phase, this Radio-Electronics magazine article demystified its core principles. Unlike black-and-white (B&W) TV, which only transmitted brightness signals, color TV had to encode hue and saturation while remaining compatible with existing B&W sets. A color camera used three tubes (red, blue, green) to capture light, while the receiver's CRT combined these primary colors additively - mixing 30% red, 59% green, and 11% blue produced white. Saturation (color intensity) was adjusted by blending pure hues with white light. The transmitter employed phase modulation at 3.58 MHz to embed color signals without disrupting the B&W signal. Only red and blue signals were transmitted; green was derived by subtracting them from the total video ("matrixing"). Receiver controls included COLOR (saturation) and HUE (tint), the latter calibrated using natural skin tones. This clever system ensured compatibility while bringing vibrant color to the nascent era of color broadcasting.

ABC's of Color TV

By Jack Darr

A lot of words have been written about color TV. Sad to say, most of them have been long and unfamiliar. With the help of some hairy mathematical formulas, they bred a huge litter of confusion among you men who have to work with color TV sets. Right? So, here is an article that covers the same basic principles, but in words of one syllable or less. Any math you find will be of the "my wife's checkbook" variety.

To service color TV sets, you have to know how they work: the basic principles. Actual circuitry is pretty simple, just as it is in black-and-white. Some of you more advanced men may think we're getting a little childish at times, but if you already know all about it, what are you doing reading this article, huh?

Where the Color Signal Comes From  

A black-and-white TV camera makes a picture by changing the light values of the scene into electrical signals. White is full output, black is no output. This is the video or brightness signal.

In color TV we need something else. Besides telling how bright an object is, we've got to tell what color it is. We still need the b-w signal, so that we can pick up the color signal on a b-w set. (This is "compatibility.") We have to have the color signals, too, and we have to put them in the same "space" (same band of frequencies) we once used only for b-w. So, we pull a sneaky: we make a b-w signal out of the color signals themselves!

A color camera has not one tube but three. With a system of special mirrors, each tube sees only one color: red, blue or green. At the receiver, the picture tube is a 3-in-1 type, almost like three tubes in one bottle. It can make red, green or blue pictures on the same screen with three independent electron guns.

Those are the two ends of our system. Now, let's see what we have to do to make not only color pictures, but black-and-white pictures too, using only red, green and blue light.

Colors of Different Kinds

First, let's talk about colors. Paint, ink and dye are subtractive colors. When white light - which is all colors - falls on a red flag, everything but the red is absorbed - subtracted. Only the red is reflected, so we see red. Red is a primary color in paint or ink or dye - one that we can't get by mixing any two other colors.

But we're dealing in light, and we have additive colors. The primaries are different. Mix blue and yellow paint, and you get green. Mix blue and yellow light, and you get white! In light, green is a primary color, and yellow is a mixture of red and green! The phosphors on a color picture tube make light in three primary colors: red, blue and green. Actually, we could use any other three primaries that when mixed together would produce white.

In color TV we can make any imaginable color just by using different proportions of our three primaries. You can actually make more colors than you can with the finest printing inks! The big problem here, of course, is to make a pure white using only colored light. (Black is pretty easy. All we have to do is turn everything off!)

After some little trouble, the engineers found out that they could make white if they used a mixture of 30% red, 50% green and 11 %. blue light. The odd percentages come out that way because of the response of the human eye to light of different colors. We see this odd-numbered mess as a nice pure white!

So, all we need to do is operate all three electron guns in the color picture tube together, keeping them in that 30-59-11 proportion, and vary their intensity as a group the same as we would the single gun of a b-w picture tube. Then we come out with a nice black-and-white picture. Now we can watch b-w programs on a color TV set, and the systems are compatible.

Making the Right Colors: Saturation

There's one more thing we have to do in color programs: not only reproduce a color (hue) like blue or orange or purple, but also reproduce how bright that color is. Red, for instance, can come in any of many shades from a deep, rich rose-red to a pale pink. The redness of a red, or the blueness of a blue, is called saturation. For pure red we just turn the red gun full on and turn the other two off. But if we need a pink we have to "add some white" to the red. This is like mixing paints.

Let's say that our color is a "half-saturated pink." All right, we've got 50% red and 50% white. Turn the red gun on to half of its maximum intensity. That's that 50% of our pink. Now, we need half white. Well, white is 30% of our 59% green and 11 % blue, as we said before. So, we divide these figures in half, and get 15% red, 29.5% green, 5.5% blue (which still makes white because the proportions are unchanged). We add that mix to the red signal already there, and we get a beautiful pink rose, sweater, nose or whatever the thing is.

This method works with all colors or combinations of colors. Our percentages always come out 100%; here, we have 50% pure red, plus (15 + 29.5 + 5.5 = 50) 50% white, which adds up to 100%, even in my wife's checkbook.

This is saturation. All it means is how much white there is in a particular color. The percentage of saturation is the ratio of pure color to white, and that's all there is to it.

In a color TV circuit, we have a color control. All this does is increase the "volume" of the color: full on, maximum color; half on, half "volume" on the color. Works just like a volume control does in the sound.

Adding the Color Signals

Our camera and transmitter output must be arranged so that no matter what kind of receiver we use, we get a good picture. So, we use the combined color signal as a b-w (video) output. The value of this signal, at any given instant, is the equivalent of a b-w signal. This is actually what you'd see if a color camera were looking at a scene that was all black-and-white or if a b-w camera were looking at this scene!

On a color program, the color signals will vary, of course, according to the color of the object the camera's seeing. However, the instantaneous total value of the camera's output still corresponds to the brightness of the object - in other words, is a b-w video signal. However, we have to add in the color information - the information that is the difference between the output of the red, the green and the blue camera tubes - and do it in such a way that it won't interfere with the regular b-w video signal.

We can't add it as another AM signal. That would be like pouring milk and water into the same pitcher and trying to pour milk out of one side and water out of the other. So, we change them to a form that we can mix and then separate later on. We use phase modulation instead of amplitude modulation.

At the transmitter, we use two balanced modulator circuits, plus a subcarrier oscillator at 3.579545 MHz (called 3.58 MHz from now on). We feed the red and blue color signals into these, having delayed one of them 90° in phase or, in effect, made it a fraction of a microsecond later than the other. The modulators convert the original color signals into (in effect) frequency-modulated signals. The subcarrier is cancelled out in each modulator. All we get out is the sidebands: the only information-carrying part of each signal. The carrier itself is not transmitted, to save postage. This sounds like a bad joke, but it's true. If we did transmit the unmodulated carrier, it would not only use up some of our transmitter power, but also create beat frequencies and other odd effects. We've got enough of those to contend with as it is.

But we'll need that subcarrier when we get to the receiver, for use as a reference. You can't say a signal is "90° lagging" unless you have a reference point! It's got to be 90° from something. So, we build a crystal-controlled 3.58-MHz oscillator into the receiver and lock it in phase with the one at the transmitter by sending along little samples of the 3.58 MHz that generated the non-suppressed sub-carrier. These are little shots, about 8 cycles each, and they're sent sitting on the back porch of each horizontal sync pulse. At the receiver, this burst is separated from the rest of the TV signal and fed into a phase-detector circuit that controls the receiver oscillator.

Now we have reinserted the 3.58-MHz signal, and can separate the phase-modulated signals. For example: If we had two marching bands, one in red uniforms and the other in blue, marching at the same speed, we could run them together so that every other man had a different color uniform. First a red bandsman, then a blue one, then another red one, and so on.

If we want to get them separated again, all we have to do is stand alongside and grab every other man and make him turn aside, as the band goes past. Which color bandsman we get depends on when we grab. This is a matter of timing, which is another way of saying phase. We can put a circuit in the receiver that will do the timing for us, comparing our "grabber" to the reference in the TV transmitter so we can separate the red and blue signals.

Now, we can - What? Someone asked, "What happened to the green? You're separating only red and blue! We've got to have some green, haven't we?" Yes, indeed. I thought you'd never ask! We want to save all the TV-signal space we can. So we send only the red and blue signals from the transmitter! However, we can get the value of the green signal back by an operation that is theoretically pretty ingenious, but actually pretty easy.

Our "whole signal" out of the camera is red, blue and green, right? If we take away the red and blue, we have green left. We can say that the whole signal equals 1. (Or, if you want to, the video.) In the receiver, we have our two signals, red and blue. So, we simply "subtract these from 1," and use the value we have left as green, which it is! This mixing and unmixing is a process you can call matrixing if you like long words, and it used to take about 9 tubes and a hatful of parts. Now, we do it with 3 little triodes and about 12 parts.

There is one more control besides the COLOR control on a color-TV receiver that affects the color. It is called HUE or TINT and about the best we can say is that it affects "the color of the color"!

When we get to the receiver, the color signals are in the form of phase-related signals. These are compared to a locked-in reference-oscillator signal, and the phase of each signal tells it what color to be. The amplitude tells it how much color to be (saturation) but it's the phase that determines what color (hue) it will show - red, green, purple, etc.

So, we add one more control - the HUE control. All it does is vary the phase of the reference oscillator signal just a wee bit. It doesn't take much; only a very few degrees of shift make the colors change a lot.

In all color sets, we use the color of a human face for our reference. In any other colored object, we don't really know whether it's a bright red or a pink, but we do know about what people ought to look like. So, to get the hue control set properly, we simply turn it" until the people look people-colored, and there we are.