Hawes Mechanical Television Archive by James T. Hawes, AA9DT
Two-Color Television Systems, Part 2

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. For instance, flicker is one of Col-R-Tel's weaknesses. Flicker comes from Col-R-Tel's field-sequential reproduction process. And unfortunately, that's what makes Col-R-Tel great. And there's more: With brighter images, the flicker gets 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 can pare down the flicker.

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 uses this method to reduce flicker. In Goldmark's CBS Color System, the field rate is 144 Hz. Compare that to the standard NTSC rate of only 60 Hz. Unfortunately standard TV sets can't reproduce 144-Hz pictures. This incompatibility rapidly led to the demise of the CBS System.

Graphic 
          compares color frame times of Col-R-Tel, Spectrac and NTSC

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


Compatiblility. Varying the field rate isn't an option for Col-R-Tel. Col-R-Tel depends on capatibility with NTSC and must operate at NTSC's 60-fields-per second rate. In favor of economical color TV then, Col-R-Tel has a serious flicker problem. As we said, Col-R-Tel's fields match the NTSC rate. Yet a new color frame occurs only 10 times per second. (That figure considers interlace. If we ignore interlacing, Col-R-Tel makes 20 times per second, still flickery.)

The dim picture, another Col-R-Tel "weakness," actually offers some relief. With a dim picture, flicker is less noticeable than with a bright picture. In fact, the Col-R-Tel filters considerably dim the picture. So much so that you should watch Col-R-Tel in a darkened room. With a picture this dim, and a product this fun, maybe the flicker doesn't bother most viewers.

Spectrac & the Two-Color Answer

Spectrac poses a different answer to the flicker reduction vs. compatibility problem. Spectrac came on the scene in 1971. Spectrac can draw a field 300 percent faster than Col-R-Tel can. The secret is that Spectrac uses only two colors instead of three. Instead of the primaries red, blue and green, Spectrac substitutes red and cyan. This color swap accounts for a 150-percent increase. Spectrac doubles this increase by only coloring each line once. In effect, Spectrac skips every other line.

Why cyan and red? Cyan works because it's a mixture of blue and green. For best flesh tone reproduction, "Spectrac red" might be closer to red-orange than cherry red. By adding a small amount of green to the red, you achieve the desirable red-orange color. In color decoder terms, red passes through a phase-lead R/C or L/C network. Let's translate that for the viewer: Mr. Viewer, please twist the Hue Control. When you see the picture you want, you're done.

Flicker reduction. Could we reduce Col-R-Tel's flicker by cutting the number of primary colors? Yes, for two reasons...

  • Reason 1: The famous Col-R-Tel circuit is a hot rod of sorts. It has about the bare minimum of parts that we need to accurately reproduce colors. Engineers also designed the circuit for easy service. You can readily identify discrete parts, and they're large enough for humans to work on.

  • 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 drops by one half. The second drawback might actually be more noticeable: Two-color pictures offer a much narrower number 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 might be sufficient for most pictures. Commercial, two-color movies offer a practical proof. These movies were popular for decades. Both Technicolor® and Cinecolor® provided film stocks for two-color movies. See Cinecolor. 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 other 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. Some ATC circuits go further, suppressing cyan, blue or green during scenes with faces. Along similar lines, classic mechanical television (MTV) sets used neon orange to reproduce monochrome pictures. Due to the narrow-angle nature of MTV pictures, faces were the most common subject. With their neon tubes, even mechanical monochrome pictures tended to reproduce faces in lifelike color. The dominant orange is a mechanical television legacy that NTSC picked up as its I-vector. As we'll see, the NTSC hue "I" reproduces as an orange-red color.

Map of cyan and orange, two-color TV 
       gamut.
A two-color TV can reproduce colors along a line between the colors.

Map of phase angles for TV red, blue and green.
Three-color TV can reproduce any color on plane RGB.




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Page Directory

What is Two-Color TV?

Advantages & Disadvantages

Flicker Reduction

Col-R-Tel vs. Spectrac

Col-R-Tel flicker

Spectrac

2-color gamut

History

2-color NTSC

2-color Col-R-Tel

Assembly

Schematic

TV System Flicker Comparison

2.5-Color TV

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