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Date Posted: 16:54:47 02/03/02 Sun
Author: Jain
Subject: Re: AGC sampling added to Rev7t1d2. For Potter and Phil
In reply to: Potter 's message, "Re: AGC sampling added to Rev7t1d2. For Potter and Phil" on 20:42:59 02/01/02 Fri

My provider was also kind...

Now my problem is this: I built the AGC fix circuit. Let's try to start it: right at the beginning there is no signal for the 3 chip to lock, and there are no signals on any port of the pic. The AGC hack circuit finds a certain voltage (might be zero or anything else) on the sampling cap and, normaly, aplies it on the AGC pin; sure that voltage is not the right one to make the IF IC to work properly, so there will never be any signal for the 3 chip to lock on.
This is not theory, it's for real and something has to be done for the system to work (fortunately, I didn't use the soft solution -- since I'm not ready with the Canton 4.2 version and I work with 3.0 -- but a hardware one and I have the posibility to make some arrangements).
Ideas? (I have one, but I want to test it first)
Jain

>Thanks Tom
>My lack of TV theory is starting to show
>I just used the Edison technique - try everything
>within reason until you're happy with the result - and
>was fortunate that the local provider was good enough
>to leave the VBI alone for me.
>
>
>>If the VBI is not raised then sampling during the last
>>50% VBI would work and give a good AGC voltage.
>>Unfortunately, many systems attenuate (or raise) the
>>VBI. This causes the AGC to have too much gain and if
>>it is sampled during this time, the overall gain will
>>cause the video signal, when recombined with a
>>regenerated sync, to be larger than 1vpp. This will
>>make the contrast and brightness between scrambled and
>>unscrambled channels to be different because the
>>regenerated sync always stays the same size.

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