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.

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. A further reduction: The primary colors are 180° apart (not in quadrature). For this reason, only one chroma demodulator would be necessary.

• 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.

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 was an add-on color wheel and chroma decoder for U.S. monochrome TVs. With Col-R-Tel, a monochrome TV could decode standard color telecasts and display them in full color. Col-R-Tel's main flaw was that it could only display one color at a time. The viewer noticed 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 replaced the color wheel. Compared to the wheel, the belt was smaller, ran more quietly, and drew less current.