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Date Posted: 05:28:32 02/13/02 Wed
Author: Jain
Subject: Re: AGC sampling added to Rev7t1d2
In reply to: Potter 's message, "Re: AGC sampling added to Rev7t1d2" on 00:32:35 02/13/02 Wed

I studdied the solution for dc restoration and I'm not sure is doing the right thing. The point where from the circuit is sampling the dc level doesn't sounds right. It means you have to sample the real black level of the signal, not the black level set in 3 chip (let's note that the input at the sampling circuit is the same --determined by the ratio of the two resistors in the base of the first transistor in the output amplifier-- during the sampling period; there is no other switch open in the same time, provided the code was not modified to open the normal one). Also, the black level will always depend on the amplitude of the synchro pulse, witch is set by 3 chip at the same level, no matter the real level of the signal comming from IF IC (that's why I proposed rethinking the idea of shifting the real blanking portion of the signal instead of inserting an artificial synchro, but it seems everybody is hapy with the actual results...). I am still thinking which is the right way to do it.
Jain

Jain

>I tried the DC restore today on the 50MHz SX board and
>it seems to work fine using my AGC control pulse.
>Couldn't get a better result than with the 5k pot
>though which may just mean that it was already as good
>as I could get it.
>I tried with LM358 as specified and a TL072 and the
>result was the same.

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