Can we discuss DC restoration circuits?

[old_tv_nut]

Originally Posted by miniman82

Cool,
See, this is where I get confused. If clamping to ground were sufficient, everyone would be doing it. However, my understanding is that the true black level is not at ground potential, but rather slightly above it.

http://en.wikipedia.org/wiki/File:Composite_Video.svg

In the above illustration, you can see that there is a blank level at .285v (the so-called 'blacker than black' level), and then slightly above that you have the 'true' black level at .339v (7.5 IRE). This is where studio monitors are calibrated to:
http://www.outside-hollywood.com/200...tudio-monitor/
So is it also the voltage you would ideally clamp to, or am I wrong?

I'm sure Cliff meant "blanking (back porch)" rather than ground. Fact is, there is no place in the signal where there is an NTSC black to clamp to. PAL solved this by making black equal to the back porch level. In an NTSC receiver, you must also clamp to back porch, and then adjust the black level down slightly to be correct. I repeat: it is not totally terrible to clamp to the sync tips, it may just require occasional adjustment when changing channels/sources. The back porch clamp is necessary in pro gear, since there will not be someone constantly adjusting for changes in incoming sync amplitude.

Regarding the clamping pulse width: the value in the IC is likely a compromise between burst gate and clamping pulse width. Ideally, the burst gate should be wide enough to include the whole burst, while the clamp pulse should be narrow enough to never include sync or active video. So, changing the value as Cliff suggests is a good idea.