Hawes Mechanical Television Archive by James T. Hawes, AA9DT
How Col-R-Tel Works, Part 2

Col-R-Tel Demodulator Section

Block diagram of Col-R-Tel demodulator section.

The CRT. In the Col-R-Tel system, the signal starts at the CRT. During installation, we splice a yellow wire into the CRT cathode (or sometimes grid) circuit. This wire is actually a three-way Y-cord. It carries the luminance signal down a split path. As Col-R-Tel luminance encounters the yellow wire, it arrives at three destinations. Here they are...

  • The CRT (same as before)

  • The Col-R-Tel chroma demodulator

  • The Col-R-Tel burst amplifier
During horizontal blanking, the CRT and color phase selector turn off. The chroma demodulator receives no subcarrier signal, because all the selector diodes shut off. Meanwhile, the burst amplifier snaps on. When blanking ends, the CRT and color phase selector reawaken. The burst amplifier naps.

The chroma demodulator is a synchronous detector circuit. A standard, AM envelope detector won't work, because TV stations suppress the chroma carrier. In the chroma demodulator, one input accepts the chroma signal. The other input accepts one of three subcarriers. The chroma signal comes from the TV station. The three subcarriers come from the adapter color phase shifter. Each carrier is a continuous wave, 3.58-MHz signal. The three signals differ only in phase. Each phase can demodulate only one of these chroma signals:

  • The red minus luminance or R-Y signal ("V")

  • The blue minus luminance or B-Y signal ("U")

  • The green minus luminance or G-Y signal
The adapter's disc commutator selects which color to demodulate. The chroma demodulator receives the proper subcarrier signal and detects one chroma signal. The demodulator rejects the other two chroma signals. It also rejects the color burst.

The color difference amplifier sends the proper color signal back to the CRT. By proper, I mean the signal that corresponds to the color wedge before the CRT. At the CRT, the chroma signal mixes with the luminance signal.

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