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.
Complementary requirements. In television, not just any two complementary colors will
suffice. There are rules: First, a nearly equal combination of the two colors must produce white.
Second, the two colors must produce natural-looking flesh tones. (That leaves out yellow and
blue.) Red-orange and teal are the most desirable pair. Mix exactly half of each and you get
white. Red-orange makes realistic flesh tones, while teal makes acceptable (if imperfect)
skies and grass. (Teal is a blue-green hue that is slightly more blue than cyan.) A reliable
second choice for the two colors is red and cyan. But the formula for white requires unequal
quantities, 30 percent red to 70 percent cyan.
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
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
Cliff Benham's famous, 2-color TV with Dorothy from The Wizard of Oz.
Mouse over for the Scarecrow.
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.
Benham 2-color TV displays witch from The Wizard of Oz. Mouse over for