Yep, 'composite sync' is where both Hsync and Vsync are on the same line. I've used a few little circuits to merge the two to TTL, from VGA to RGB video used in many (very) cheap colour displays. Namely PSone LCD's and car TFT panels.
I believe they are indeed meant to be TTL compatable, and as such, shouldn't be AC coupled. That siad, it doesn't mean to say it should be a healthy 0-5V, true TTL takes +2V to be the threshold for a 1. So your 2.5V *should* be fine.
Component video sync - Wikipedia, the free encyclopedia
Although because you are dealing with games consoles, it could very well be a propietary thing, steering away form conventional standards, as those consoles were never designed to be interfaced with non-nintendo equipment.
I wouldn't have thought nintendo would bother using component video, just good 'ol composite video, with seperate mono audio...modulated onto a carrier. I can't see the SNES having any advantage to having seperate sync and colour lines.
The only real info I can give you is from a display I have which takes analogue RGB and composite sync.
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Timing diagrams galore, plus it mentions 'CMOS level' for the sync, not TTL. Bizare no?
Blueteeth
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