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Date Posted: 13:19:33 03/09/02 Sat
Author: Tom
Subject: Potter, Why 64?
In reply to: Potter 's message, "Re: Rev7t1a" on 08:30:31 03/09/02 Sat

>I was able to detect normal video by counting the
>number of peds/frame preceding the burst IE if more
>than 64 (to rule out random noise/gremlins etc) then

I am curious as to why there are more than 64 PEDs for a normal signal but not for a scrambled signal. How big of a window are you looking for PEDs in? Are you looking from the start of the line until the burst? There is a PED at the end of the sync just before the burst on scrambled and unscrambled signals and a PED at the start of the HBI in scrambled signals only. Either one should give over 500 PEDs per frame. Please send me the code so I can see what you are doing, Thanks

>Also had much trouble with the very long ped at the
>start of the sync (caused by the excessively tall
>leading edge) and all was fixed by locking to the ned
>at the end of the sync as its duration was more
>suitable.
>
>>If your picture is shifted to the left then the
>>scrambled sync from your system is delayed or the
>>3chip is locking to the wrong edge. In a scrambled
>>signal, there is a PED at the start of the HBI (where
>>the signal becomes raised), a NED at the start of the
>>sync, a PED at the end of the sync, and a NED at the
>>end of the HBI. Rev7t1a locks to the NED at the start
>>of the sync. Some PAL versions lock to the PED at the
>>end of the sync. It is possible that the 3chip is
>>locking to the NED at the end of the HBI instead of
>>the NED at the start of the sync. If the lock check
>>in MakeVBI has been disabled or modified, then it can
>>lock on any repeating NED.
>>
>>The clamp signal from the PIC was used in old 3chips
>>to provide voltage to the clamp diodes during the
>>video, both inverted and normal. The normal signal
>>from the PIC is not on during inverted video.
>>
>>The 3chip has never detected normal video because
>>nobody thought it was important enough to incorporate
>>into the design and write code for it. It should be
>>fairly simple and I believe jpb did it but nobody
>>expressed any interest at the time.

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