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Date Posted: 08:30:31 03/09/02 Sat
Author: Potter
Subject: Re: Rev7t1a
In reply to: Tom 's message, "Re: Rev7t1a" on 20:55:33 03/08/02 Fri

May not make sense unless viewed with non-ttf font (eg Courier)
What my inverted video looks like
--
| |
| | burst (inverted)
| -v^v^v^-
| | |INVERTED VIDEO |
| | | |
--- sync | |

Unscrambled normal video

| |NORMAL VIDEO|
| | |
| sync | |
- -^v^v^v
| | burst (normal)
| |
--

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 the signal must not be scrambled so reset the 'LOCKED' flag and pass the signal untouched as if it was a bad signal. This was necessary as the 3chip was happily locking to normal video and therefore displaying it with an inverted burst. Note that the start of the front porch could be wrongly interpreted as the sync pulse in this signal depending on where the video was at the end of the previous line but lock should fail as the serration pulses in the Vsync would be displaced outside the sampling window.
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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