Hawes Mechanical Television Archive by James T. Hawes, AA9DT
How Col-R-Tel Works

Performance Comparison

Illustration of people jumping through hoops

How does Col-R-Tel compare with standard color processors? The short answer is that Col-R-Tel is a very narrowband demodulator. Narrowband isn't necessarily bad. After all, Col-R-Tel achieves its goal of bang for the buck. Col-R-Tel's circuit might be as good as some X and Z decoders. Like Col-R-Tel, X and Z decoders are narrowband circuits.

A little color TV theory. Here's the long answer: The I and Q chroma subcarriers from the TV station are 90 degrees apart. The B-Y (U) and R-Y (V) chroma signals are 33 degrees off the I and Q axes. B-Y and R-Y are the chroma signals that eventually produce blue and red. TV vector diagrams indicate that these two chroma signals are also 90 degrees apart.

Simplified circuits can demodulate red, green and blue on almost any axis. After all, the NTSC color wheel has 360 degrees. Still, axes that deviate from I and Q must sacrifice bandwidth. That's why we call some demodulators narrowband decoders. A very common narrowband decoder is the R-Y and B-Y (or U and V) circuit. An X and Z demodulator is even narrower yet. The X and Z demodulator uses two axes that are only 62 degrees apart. This narrow separation reduces the circuit cost. Most viewers think that this circuit produces acceptable color. Yet the narrower angle between axes also increases crosstalk between chroma signals. Crosstalk muddies displayed colors. Undoubtedly, Col-R-Tel shares the crosstalk problem.

No matrix. At an X and Z decoder output, a resistive matrix improves color fidelity. The matrix converts the X signal to an R-Y signal. The matrix also converts Z to B-Y. The matrix operates by adding a sample of Z to X, and vice versa. Each sample shifts the signal phase enough to color correct the image. Because Col-R-Tel only decodes one color at a time, matrix color correction is beyond Col-R-Tel.

Decoding Green. The station doesn't transmit the third chroma signal, G-Y. This is the chroma signal that eventually produces green. Most TV sets recover G-Y by adding samples of the B-Y and R-Y signals. Because Col-R-Tel only processes one color at a time, it can't add two signals. Instead, like some conventional sets, Col-R-Tel generates a third subcarrier to recover G-Y. On vector diagrams, the G-Y vector appears at about 7:00. In vector terminology, that's -106 or so from the B-Y signal. Without the advantage of a resistive matrix, Col-R-Tel creates green by phase shifting negative R-Y. Negative R-Y (roughly greenish cyan) resides on the 270-degree axis. That's 6 o'clock. Col-R-Tel's 64-degree shift should produce a yellowish green. With the hue control, the viewer can adjust this color. Not bad.

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