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Old 11-06-2018, 04:05 PM
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Join Date: Jul 2004
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I don't think this is off-topic, although it's been discussed on VideoKarma before (and also has some relation to video color encoding, because, after all, they both affect the result).

The most often used technique (and used by Magnavox even before "TAC") was to spread the demodulation angles between R-Y and B-Y. This had the equivalent effect of reducing Q axis (green-purple) gain (color saturation). Early Maggies also had a switch that set the crt tracking to sepia tones (the tradename escapes me). Both of these changes moved colors on the orange side of the circle toward flesh tone. Sets that did this very strongly gave rise to "the tan cowboy on the brown horse riding into the orange sunset" look.

RCA had a unique hue correction system that actually worked on the phase of the reference subcarrier. It moved hues that were close to flesh hue even closer, without affecting the amplitude or phase of greens, blues, or purples.

Some Motorola tube sets with the single-tube chroma oscillator/demod had three color controls - the usual hue and saturation, plus a red/blue balance. This third control affected the color tracking towards either sepia or blue. It was actually needed for accurate tracking, as the demod outputs were DC coupled to the crt grids, but in the cheaper models, fixed resistors were used and the third control was eliminated. This single tube chroma circuit also had week greens to begin with, as they were taken off the cathode of the single demod tube and were too low in amplitude and not quite the right phase for correct color. So, that helped eliminate greenish flesh tones.

Trade names like "TAC" or "Instamatic" often arrived with solid state sets that had a single button to actuate the color "correction" along with AFC and preset hue and saturation controls, plus often some sort of automatic saturation adjustment, and sometimes a light sensor to adjust contrast and brightness according to room lighting.

RCA used an average chroma signal sensing adjustment. I hated it, but consumers seemed to prefer it, even though it could produce some over-saturated "calendar art" results if the customer preference was set too high, and on the other hand could have noticeable fading of color due to bright red clothing if the preference was set lower. Zenith used a peak chroma compression that prevented reds from blooming if the chroma in a transmission was too high. It could still result in noticeable errors such as oversaturated skin tones, especially if the preference controls were set wrong, but on the other hand did not have the saturation "breathing" with content change that the RCA sets could present.

Much of the need for "automatic color" went away gradually as the TV manufacturers and broadcasters cooperated on controlling variations in transmission, but analog cable systems introduced hue, saturation, and black level errors all over again, as they had many more proc amps to manage in their systems.
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