Quote:
Originally Posted by maxhifi
...Is the DC signal we are attempting to restore present at the output of the video detector?
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Yes. The DC component is broadcast. One of the consequences is that the RF power at sync tips or blanking or black level does not change due to variations in the scene content. Therefore (assuming the TV's AGC circuit is working on sync tips) the output of the video detector is similarly stable.
However, carrying this all the way to the CRT by using DC coupling all the way is not practical in tube sets (too much drift over time/temperature, plus changes every time you replace a tube). It is also not practical in solid state sets even if the transistors do not vary, due to component tolerances. Every stage would need the bias circuits replaced with adjustable potentiometers.
The only reasonable way to stabilize the video DC level is to do it close to the CRT, (or measure it at the CRT and use feedback to adjust it at an earlier point).
The ultimate solid state CRT TVs used feedback to control the CRT bias individually for red, green, and blue, thus achieving not only DC restoration but automatic CRT tracking. The video IC restored DC in the low-level luminance, then added a low brightness pulse at the top of the raster just after retrace (where you can't see it) to measure the CRT beam current
near black.
Why not measure zero current, exactly
at black, to set cutoff? Because a CRT has a power law current vs. voltage characteristic, so at zero current, the incremental current gain is also zero, which means there is no feedback signal.