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Date Posted: 17:25:50 08/27/01 Mon
Author: Rex
Subject: Re: Hey Sakman
In reply to: Tom 's message, "Re: Hey Sakman" on 21:51:17 08/25/01 Sat

>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

...

>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.

The first few lines of the video signal above the screen actually has a valid sync pulse. This is the reason why (at least on my TV) the first 10% of the display is brighter than the rest when the AGC cap was not installed.

Can you not do a circuit that detects the lowest point of
the video signal during the first few lines and then hold it
as a reference ?

>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.

This does not really change the amplitude of the signal.
It just shift the DC offset of the video signal.

>Any other ideas?

It might be more useful if you use that feedback path to the AGC pin and use the first few video line as your reference.

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