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Homebrew YPrPb/NTSC transcoder sync questions?

Hello all... not sure if this is on-topic, but I figured someone might
know something about it.

My new roomate has an HDTV-ready set with component in, and I like the
idea of using a real, honest-to-goodness modeline out of the VGA port to drive
the TV. I've been scouring the 'net for some good plans to build a transcoder
(VGA->Component video), but figured I'd ask here too, in case anyone knows.

I'm looking to build a "Swiss army knife" transcoder. Takes VGA in,
loops VGA back out, and provides component out (YPbPr), but also provides
composite and S-Video out. The second circuit (composite/svideo) is easy, since
it's all done in the AD724 chip. The first I've got issues with:

Does anyone know why all the plans for transcoders on the 'net have H/V
sync generators (one-shot timers, delayed logic gates, etc)? Why can't it just
use the sync pulse from a custom modeline off the VGA port? Also, does anyone
know where to find the specs for sync pulse length for HDTV modelines?

Thanks for any advice:
-Cory


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* The prime directive of Linux: *
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