Quote:
Originally Posted by miniman82
My CRT tests new...
As I understand it, during bright scenes any black (or otherwise dark) objects will become lighter. It's really obvious if you're watching something in letterbox format, because the black boxes at the top and bottom of the screen will get 'lighter' as opposed to staying black like they should. Really ought to be called black level restoration instead of DC restoration, that would make more practical sense.
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Ok. let's clear up some things;
1) lack of DC restoration means on a bright picture, the blacks get too black (your black bars in letterbox are black and dark objects go too black and lose detail) (also, colors get over-saturated); on a dark picture, the opposite happens - your black bars in letterbox turn gray (also, colors get washed out).
2) this problem is in the luminance circuit only - you do not have to worry about the color difference drive on the CRT grids. Chroma may appear to be AC coupled when you look at the circuit, but it is not. There is a DC restorer action in the outputs so that the color balance does not drift depending on scene content. (Imagine if having a large red object in the scene caused the color to shift over-all toward cyan - this would be the color equivalent of the lack of luminance DC coupling, and would not be acceptable.)
3) different sets have different amounts of DC coupling depending on designer's choice
4) the amount of DC coupling can vary in some designs depending on the settings of the contrast and brightness controls
5) Adding DC restoration is a worthwhile experiment to improve picture quality