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Date Posted: 08:25:23 02/14/02 Thu
Author: Potter
Subject: Re: DC restore: question for Potter
In reply to: Jain 's message, "DC restore: question for Potter" on 12:13:32 02/13/02 Wed

I know what you mean and I tried it with & without some impedance (up to 5k) in the output with no noticeable change - the original setup just set a voltage ~2.5V via a pot across the power supply as the reference for the clamping diodes which tie the lowest part of the signal (the sync tips for normal video) to this reference minus the forward drop across the diode and hopefully leaves the black level very close to the reference voltage. Germanium diodes would be better as their Vf is closer to the size of a good sync than silicon. But with inverted/suppressed sync you'd expect it to be messed up because the actual video is being lowered by the clamping action especially if the video is inverted where the lowest level is shifting from line to line (looks OK though). I took the inverted output to be negative feedback which stabilises the whole thing and I'm only taking the sample on a line with normal sync.
Something to think about when there's a quiet moment.
>Hi Potter
>Since i solved (with your help) the AGC problem, now
>I'm bugging you with this one (I don't know if
>Digitalman is available): why, in the dc restore
>circuit, the second op-amp is in inverting mode (the
>lower the sampled black level, the higher the clamping
>level -- and therefore the higher the brightness of
>the picture on the screen)? It doesn't sound normal
>but maybe I'm missing something...
>On the other hand the output of the op-amp provides a
>low impedence path for the signal durring the active
>part of the line (for the part of the signal below the
>threshold of the diode openning voltage);
>fortunatelly, the video-content is usualy with litle
>components below average. Nevertheless, I think it
>would be better to have a 1-10k resistor in series
>with the output of the op-amp.
>Your opinion?
>Jain

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