[old_tv_nut]

All sets for a long time from this time onward had partial DC coupling in the luminance. This results in a partially floating black level depending on average picture luma. Consumer Reports complained about this for years and years. The manufacturers' reasoning was that it partially compensated for black level errors in broadcasts (and in analog cable systems, where consistency was much worse). It was finally resolved with solid state studio gear and solid state TV sets whose circuitry was stable enough to keep the proper black level once set. Consumer Reports was very pleased.

The CTC-5 also has partial DC coupling (not zero) in the color difference paths. Again, this counteracts circuit drift or changes when tubes are replaced, to some extent. It is also a technique for automatic color balance, called "integrate to gray," because many scenes are close to gray on average. This technique was even used in automatic color photo printing, where it affected the white balance, not the black balance as in the CTC-5. It is prone to "subject failure," that is, if a large part of the scene is a dominant color, the rest of the scene will be pushed in the complementary direction.

In color prints made on these early machines, I have seen pics of someone wearing a red sweater where the rest of the picture including their face was pushed toward cyan.

In the CTC-5, you can see the effect of this partial coupling if you play a DVD of a test screen that is all one color, like a complete red field for checking purity. The result is that the red is desaturated and foggy tomato-soup looking, even at a correct color level setting that shows small red areas in program material correctly. I have had thoughts about removing the caps that pass more AC than DC in the color difference channels, as the color difference outputs seem to have more than enough gain for dynamic scenes. The drawback might be that tracking would be more susceptible to tube differences and drift.

The CTC-7 and such later RCAs with X-Z demods have a DC restoration effect built into the color-difference amplifiers by drawing grid current when the negative horizontal blanking pulse is applied to the cathodes. But the luma channel is still only partially DC coupled.