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Date Posted: 18:10:40 08/27/01 Mon
Author: Tom
Subject: Re: Hey Sakman
In reply to: Rex 's message, "Re: Hey Sakman" on 17:25:50 08/27/01 Mon


>>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.

I'm not sure what you mean. The AGC doesn't change the amplitude of the video?

The transmitted rf is negative modulated A.M. video with the zero carrier point being the input level to the modulator that makes no carrier(somewhere around 3vdc).The lower the voltage, the stronger the rf signal. The demodulated signal's amplitude is directly related to the strength of the RF signal. A stronger RF signal gives a lower voltage out of the demodulator. In order to get a consistent 1vpp video out of the IF chip, the AGC circuit adjusts the gain of the RF. This changes the _amplitude_ of the demodulated video. If you change the amplitude of the video, of course you will change the DC level of any of it's components, sync level, black level,etc.

>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.

Potter's circuit taken from 5508W box does exactly that. It samples the AGC voltage at the filter pin and then applies that voltage right back to the filter pin for the rest of the frame.

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