|Hawes Mechanical Television Archive
How Col-R-Tel Works
Color Pictures for the Black-and-White Budget
Col-R-Tel is a 1955, seven-tube color converter for monochrome TV receivers. The manufacturer is Color Converter Inc. of Columbia City, Indiana, near Fort Wayne. Thanks to a splendidly clever design, Col-R-Tel operation is nothing but reliable, simple and effective. Converter electronics recover the color signal and mix it with the monochrome signal. The resulting pictures still appear in black and white. Yet each video field now represents saturation values for one color of the picture. You view the TV through the spinning color wheel. Six wedges of colored plastic make up the wheel. As you watch TV, each wedge colors one video field. The wedges pass by so fast that you can't see them. Your eyes combine the rapidly changing fields into realistic color pictures.
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Block diagram. Above is a Col-R-Tel block diagram. The obsolete 1951 CBS system, though similar, is incompatible with Col-R-Tel. Col-R-tel owes much to its CBS ancestor: Both systems rely on the field-sequential color technique. Both also use a spinning color wheel. Both scan in the order red, blue and then green. Like the CBS system, Col-R-Tel marks yet another triumph of mechanical television technology. Yet unlike the CBS system, Col-R-Tel is compatible with NTSC, the US television system. Col-R-Tel achieves NTSC compatibility by being two converters in one...
On the block diagram, notice that the video signal splits into three paths. The split occurs at the CRT electron gun. Two paths diverge right at the gun. One is the original, monochrome output signal. The second path is the input signal to the Col-R-Tel chroma demodulator. The demodulator processes this signal and outputs a red, green, or blue color signal. At the demodulator input, a third path splits off. This third path is the input signal for the Col-R-Tel burst amplifier. The burst amplifier switches on only during horizontal blanking. For a very brief time, this amplifier accepts the station burst signal. After amplification and processing, this signal becomes the sync reference for the entire color adapter. By phase-shifting this reference, Col-R-Tel can detect separate red, green and blue chroma signals. Circuit operation details appear on succeeding pages.
Viewing experience. In the two demonstrations that I saw, Dave Johnson's Col-R-Tel adapter and DuMont TV maintained perfect sync. The picture was definitely color, and most pleasing. But picture colors were neither bright, nor saturated, nor finely detailed. The color gels really dimmed down the image. Dave compensated by cranking up the TV brightness and contrast. The shadows grew blocky and undetailed. But nobody seemed to mind. Then someone brought down the room lights. I didn't notice a lot of flicker. Yet on Col-R-Tel, motion occasionally caused banding artifacts. Similar artifacts appear in field-sequential color TV pictures from Apollo moon shots.
Overall, the images reminded me of my old RCA CTC-15 set with the color turned partway down. For the demonstration, Dave's program was a videotape. I suppose that the tape could have contributed some of the pastel color effect.
Despite my minor gripes, Dave's Col-R-Tel performed magnificently. The colors were accurate, and they certainly improved image depth. I doubt that anybody would have preferred monochrome. In partial compensation for the minor picture faults, the image had no convergence faults. I don't mean "no observable convergence faults." I mean absolutely no convergence faults, because field-sequential pictures simply avoid them: There's nothing to converge! All the pixels come from one gun.
Col-R-Tel installation requires you to connect several wires between the TV set and three converter boxes. You add the bulleted parts in the block diagram.
The System for builders. If you're a color wheel builder, investigate the Col-R-Tel system! Unlike the CBS system, Col-R-Tel is compatible with our NTSC TV system. While revolutionary in 1955, Col-R-Tel electronics are reasonably simple. Col-R-Tel contains very few tuned coils and transformers. Diodes, rather than triodes or pentodes, switch between color signals. Today, a builder might easily replace these diodes with solid-state devices. Col-R-Tel engineers eliminated color bandpass amplifiers. Field-sequential scanning reduces the number of output demodulator and driver stages from six to two. Many of the tubes are triodes, and most are dual or triple types. Unlike most color decoders, Col-R-Tel requires no delay line for the luminance signal.
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