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Composite Sync in an analog RGB signal

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FusionITR

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For systems that output video vida RGBS (S being composite sync), is this signal suppose to be AC "TTL"? The reason why I ask is because probing a composite sync signal from my SNES shows about a 0 to -2.5Vpp sync signal (unloaded). I want to test the composite sync line on a few more old consoles to confirm this is accurate but until I do that I wanted to get some input here.

So this the C-Sync line suppose to be AC coupled too? If so that doesnt make any sense... why would you AC couple a TTL signal? And why is the peak amplitute only 2.5V? Also does the csync signal also have a 75 ohm output impedence like the RGB lines?

Also, does anybody know where I can get some spec sheets for analog RGB signals? If I had one or knew where to get one I'd be able to answer all these questions myself...

Thanks
 
Standard composite signal with sync is 1V or so into 75 ohms. Thus without the 75 ohm load, the level is likely twice that (or a little more than twice in your case).

It's not TTL. The analog video voltage standards were generated in the 1940's, long before TTL logic even existed. (Early TV sync generators consisted of an equipment rack full of tubes).

Component and composite video connections including sync are all 75 ohm.

The signal is likely AC coupled so that the receiver can ignore the DC level of the signal. It shifts the signal to the proper level inside the receiver.
 
Eh... you didn't read my post. I said composite-sync, not composite video. I know that composite video has a embeded sync signal along with the video burst but I'm talking about something totally different. There is a video standarded called RGBS, in which the sync signal is on a seperate wire. This is the standard all of the older video game consoles used and I think it's still used in a lot of applications that use low resolution video.
 
The confusion is in your using the term composite. Composite means the video and sync are on the same signal. There's no such thing as composite-sync. If it's sync by itself it's just video sync, not composite.
 
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.

**broken link removed**

Timing diagrams galore, plus it mentions 'CMOS level' for the sync, not TTL. Bizare no?

Blueteeth


Edit: **broken link removed**
**broken link removed**
 
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Wow I'm a complete idiot.... long story short I was the one who AC coupled the c-sync signal like 2 years ago when i was a total noob at electronics (hard to believe I came this far?). Problem is that the US SNES had a DC offset to their rgb lines and I use a japanese video converter (XRGB2) to feed into a VGA montor. This device expects AC coupled RGB inputs because the japanese snes is AC coupled. I stupidly added a AC coupling cap to the c-sync signal too, haha.

Anyhow I made this extremely complicated circuit to sync both DC and AC coupled sync and it was a complete waste of time =/
 
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