Two-Color TV, part 1

Hawes Mechanical Television Archive

by James T. Hawes, AA9DT

Two-Color TV, Part 1

What is Two-Color TV?

Complementary colors. We're familiar with three-color TV. After all, it's been available since 1951 or so. (At least in the US.) Two-color TV is almost the same thing, only it uses two primary colors. Yes, that's possible! But how could we build full-color images with just two primary colors? Simple. All we need is a TV set that displays two complementary colors. These colors become our primaries, replacing red, blue and green.

$Art: fraction '2/3' on field of question marks.$

The only requirement is that some combination of the two colors produces white. Preferably, white should emerge when the two colors mix in near-equal quantities. In typical applications, the two colors should produce natural-looking flesh tones. For example, red-orange and cyan are most desirable two colors. Red-orange makes realistick flesh tones, while cyan makes acceptable (if imperfect) skies and grass.

Advantages & Disadvantages

What is the disadvantage of two-color TV? The gamut or number of possible colors is lower than with three colors. Then why would we want to use two colors? For several reasons...

Simplicity. Two colors reduce circuit complexity by 30 percent.
Bandwidth. With only two colors, we can fit a TV signal into less space. For this reason, Even three-color television systems such as NTSC and PAL only transmit two colors. (These systems further reduce bandwidth by matrixing two colors onto one signal.)
Low noise. The narrower bandwidth means that we have a lower-noise signal. This signal travels farther before it disappears into the background static.
Research. Viewers mentally fill in missing colors. Research by Edwin Land of Polaroid fame proves this phenomenon. More later.
A special advantage for field-sequential displays: Dramatically less flicker than with three-color TVs.

Wizard of Oz
shots off two-color TV by Cliff Benham: Dorothy, Scarecrow

Cliff Benham's famous, 2-color TV with Dorothy from The Wizard of Oz. Mouse over for Dorothy & Scarecrow.

Flicker Reduction

For field-sequential TV. Of all these advantages, flicker reduction is the most important. Why? Because flicker is the most irritating disadvantage of field sequential TV. Field-sequential TV is the “CBS method” that DLP projectors and 3D games use. In history, Col-R-Tel converters (not the CBS System) were the most prominent field-sequential technology. Cameras that operated on the Col-R-Tel standard journeyed to the moon with Apollo astronauts. (See Moon Col-R-Tel.)

For those who haven't read our Col-R-Tel pages: Col-R-Tel is an add-on color wheel and chroma decoder for U.S. monochrome TVs. With Col-R-Tel, a monochrome TV can decode standard color telecasts and display them in full color. Col-R-Tel's main flaw is that it can only display one color at a time. The viewer notices this effect as flicker. As we'll see, Spectrac, a later converter, solved the flicker problem. In fact, Spectrac proved that two colors could reduce flicker while depicting natural-looking people and scenes. Meanwhile, Spectrac also introduced a novel, two-level scanning belt that replaces the color wheel. Compared to the wheel, the belt is smaller, runs more quietly and draws less current.