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Date Posted: 21:51:17 08/25/01 Sat
Author: Tom
Subject: Re: Hey Sakman
In reply to: John 's message, "Hey Sakman" on 18:40:16 08/25/01 Sat

Here's some background on the DC restore problem. Maybe someone can come up with a better solution.

The AGC circuit in the IF chip looks for the lowest point in the video and assumes that it is the sync pulse. It adjusts the gain of the IF and RF so that the demodulated video sync is always at a set reference level, usually around .6v. With a suppressed sync video signal, the lowest part of the video is the darkest part of the video line, since the sync is raised above the video. It adjusts the gain so that the lowest part of the video is at about .6v, instead of the sync. Because the video brightness is constantly changing with the content of the picture, the AGC correction voltage and the gain of the IF circuit is constantly changing to try to correct it. replacing the cap at the IF chip's delay or filter pin will slow down the AGC's response and improve the picture but it doesn't completely fix it. Remember, the gain of the IF and RF amps is changing which means the amplitude of the demodulated signal is changing. This results in DC levels for the suppressed sync and video black that are constantly changing.

I haven't seen Digitalman's DC restore circuit, but I believe it samples either the sync or the video black and then uses that sample to dynamically adjust the clamp voltage to correct the DC level of the video. It doesn't correct the amplitude of the video as it changes. A better way may be to use the sample to correct the gain of the video buffer amp so that the black level or sync out of the amp is always at a reference level set by a pot. This can be done by biasing an op-amp type video amp at the zero-crossing point, sampling the sync(or black) at the output of the op-amp and comparing it to a reference level, and then feeding it back to the video amp as a gain correction voltage. The bias is necessary because as the gain goes down, the DC level must go up, and vise-versa.

Any other ideas?

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