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Date Posted: 05:18:48 02/01/02 Fri
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 07:36:26 01/31/02 Thu

The thing is that IF IC samples the bottom of the Hsync to determine the level of AGC and the burst porch for brightnes (the sandcastle pulse, that has other functions too). With that idea in mind, as long as there are Hsync pulses (during VBI) the DC level on AGC filter pin must be constant, no matter what's the content of the (video part of) line, black, white or any intermediate... When Hsync pulses are missing (active part of the frame) IF IC is sampling, but, Hsync missing, it finds something else and the DC level at the AGC pin will vary (acording to the average af the video content of the line). That's why it's somehow strange your findings that it is convenient to sample during a black line. The voltage level at the AGC filter pin should be the same during VBI and keeping that level during the rest of the frame will assure the right amplitude of the video signal, burst included. But, as I said , you have more experience that I have and probably the best aproach is trial and error (if the IF IC is acting in a different manner--hard to belive, but not impossible-- results will be different).
I hope all this mostly theoretical discutions won't bother too much people expecting something more practical; what we are trying is to clarify things to do the best with the simplest means.
Thanks
Jain

>I found it essential to avoid sampling any Hsync
>pulses as in my case they are normal in the blanking
>interval (first 23 lines in each frame) which would
>introduce an error into the average sampled voltage
>(ie lowers it slightly). If the colour burst is
>centred on the black level it could be included in the
>sample as it's average will be the same. I tried
>sampling on more than 1 line but it was less
>successful. I borrowed some experience from
>Macrovision remover circuits which also need to sample
>an accurate black level and use it to replace the
>troublesome line(s).
>
>
>>You have much more experience that I have, related to
>>this 3chip. Alow me to say that sampling for AGC would
>>be good if it is done durring 90% of the VBI (AGC
>>level must be the same for that period of time). It's
>>important to mention that the 47 uF cap on AGC pin
>>must be removed (the original one should be in place).
>>If Potter is thinking to use the AGC hack pulse for
>>seting the blancking level, then it is justified to
>>have it short, and in the place mentioned. Otherwise
>>that pulse could be as long as 1 mSec or so.
>>Jain
>>
>>>I chose a line that is always black on all channels
>>>(16 for me) and sampled from the end of the burst to
>>>near the end of the line (~50uSecs). This is the
>line
>>>after the one carrying the data packet. I'm also
>>>considering the DigitalMan DC restoration as this
>>>could be driven by the same sampling pulse and
>>>eliminate the clamping level adjustment for even
>>>better all-round results.
>>>
>>>>Folks,
>>>>
>>>>I did a very quick hack to my Rev7t1d2 code for SA
>>and
>>>>added AGC sampling support for Potter's circuit. It
>>>>uses RA3 (audio privacy on Canton 4.2, available on
>>>>the 3 pin header), and samples for about 800 nSec.
>>>>during line 23. I haven't properly timed it to the
>>>>front porch or anything like that, I just threw it
>>in.
>>>> 800 nSec. may not be long enough, and since I
>>haven't
>>>>built Potter's circuit to test it may not work at
>>all,
>>>>but let me know how it goes and I'll fix it when I
>>>>have some time.
>>>>
>>>>I sent the code to Jain and Sakman, I don't have
>>>>Potter's email address. Once it's working the way
>we
>>>>want it to, I'll zip it up like the Rev7t1d2 code
>and
>>>>Tom can post it if he likes.
>>>>
>>>>- Phil

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