Col-R-Tel vs. Spectrac

Col-R-Tel weakness. We buy technology for its advantages. It makes life better. But here's a secret that engineers don't want to share: Every gadget has a flaw.

Flicker

Col-R-Tel's main weakness was flicker. The flicker resulted from Col-R-Tel's field-sequential reproduction process. And there's more: With bright images, the flicker became even worse than with average pictures. We still like Col-R-Tel. Sure, flicker's a weakness, but not an Achilles Heel. With a bit of engineering, maybe we could pare down the flicker.

Frame times of Col-R-Tel, Spectrac & NTSC. "Odd" & "even" refer to video field type. Spectrac halves vertical resolution.

The best way to reduce flicker is to increase the field rate. That is, if you scan video fields more often, the flicker smoothes out. Eventually, despite the sequence of still fields, humans perceive continuous motion. The CBS Color System used this method to reduce flicker. In Goldmark's CBS Color System, the field rate was 144 Hz. Compare that to the standard NTSC rate of only 60 Hz. Unfortunately, NTSC TV sets couldn't reproduce 144-Hz pictures. This incompatibility rapidly led to the demise of the CBS System.

• Reason 1: The famous Col-R-Tel circuit was a hot rod of sorts. It had the bare minimum of parts necessary for accurately reproducing colors. Engineers also designed the circuit for easy service. A technician could readily identify discrete parts. These parts were large enough for humans to maintain. The circuit also allowed modification.

• Reason 2: Col-R-Tel and Spectrac are parallel technologies. Certainly Col-R-Tel could take a page from Spectrac's instruction book and reproduce just two colors. The amount of flicker would fall dramatically.

The cost of this change? Two drawbacks come to mind. First, as in Spectrac, vertical resolution would drop by one half. The second drawback might be more noticeable: Two-color pictures offer a smaller range of colors (gamut) than do three-color pictures.

The Two-Color Gamut

Two vs. three-color gamut. The two-color gamut is a one-dimensional line between two complementary colors. Every color on the line is a combination of these two colors. The line can have height or width, but not both. In contrast, a three-color gamut is a two-dimensional plane with both height and width. This plane is really an array of lines just like the two-color gamut. Obviously this plane contains many more colors than just one line could hold. In theory, a three-color system can reproduce any color on the plane.

Are two colors enough? Despite three-color theory, the reduced, two-color gamut is sufficient for many pictures. Commercial, two-color movies offer proof. These movies were popular for decades. Both Technicolor^® and Cinecolor^® provided film stocks for two-color movies. See Cinecolor.

3-color TV can reproduce any color on plane RGB.

Edwin Land's Retinex color theory supports the idea that two colors are enough. Land felt that the retina and cerebral cortex (the “retinex”) work together to fill in the missing colors. Land's experiments reproduced full-color images from only red and white light. Today, NASA uses Land's methods to enhance photos.

Flesh tones. When you only have two colors, flesh tone accuracy is extremely important. Tests have proven that with correct flesh tones, the human eye forgives many color problems. For this reason, a TV must portray faces accurately, or at least naturalistically. The flesh of all races includes orange (or sepia). This fact is the basis for most “automatic tint control” (ATC) circuits. Such circuits generally reproduce colors in the yellow-to-magenta range as orange.