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Date Posted: 06:39:09 02/04/02 Mon
Author: Potter
Subject: Re: AGC sampling added to Rev7t1d2. For Potter and Phil
In reply to: Jain 's message, "Re: AGC sampling added to Rev7t1d2. For Potter and Phil" on 16:54:47 02/03/02 Sun

I didn't invent the AGC fix - its just what I found existing in the Jerrold box. My code only takes a sample if the current line is inverted and the the decoder is locked onto the signal so on power-up or changing from a non-scrambled channel you'd expect that it's impossible to get a signal to work with but that's not the case. The output of the sample & hold only biasses and stabilises the voltage on the filter pin, it doesn't provide all of it so there is a very workable signal there even if the S&H is totally dead or not even existing. This is why a capacitor achieves acceptable results, all it does is extended the time constant with the penalty of not being able to respond to rapid signal level changes ( say with a microwave signal in a somewhat fringe area coming through trees in a storm )


>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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