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When video signal is passed through a capacitor, the DC component is lost. The result is a black level that varies with the amplitude of the signal. That's why on TVs with zero DC restoration (as are the vast majority of 50s black and white sets) there is no such thing as a fade to black because black content appears gray and very dark content appears very washed out. This was an acceptable condition because content with normal lighting looked normal. But with the contrast adjusted for scenes of ordinary brightness, extraordinarily bright content (think J.G. Wentworth ads!) will result in the detail of darker objects being lost as black goes blacker. Just like any other circuit, there are conditions under which it can fail to work properly, but the question is how "properly" it worked in the first place. I have sets with DC restoration that ranges from excellent to poor. I have succeeded in restoring the restoration in sets that have a well-designed circuit that stopped working, but I've never been able to improve it in sets where it barely seems to make a difference.
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