What is all about sync and composite signals?

[Fix_Metal]

Hi everybody.
What I mean is: every old console I've seen (at least for what concearns to Sega) had both composite and sync.
When hooking up for RGB, every console uses composite signal @ #20, so that if RGB is not available in that particoular scart input (TV side) it will output composite.
So what's this sync signal? And what's its purpose/frequency?

[KillingBeans]

RGB can't be displayed without a matching sync signal.

Pin 20 on the SCART input has two functions. When the voltage applied to pin 16 is less than 0,4v the TV will use pin 20 as the video feed (decodes both R, G, B, H-sync and V-sync from pin 20) . But when the voltage is 1-3v  the TV discards the R, G and B from pin 20, but still uses the extracted sync signals as timing for the R, G and B signals it gets from the SCART input.

When using SCART as an RGB input, the raw sync signal from the console can also be applied to pin 20 instead of composite video. In theory that'll give you a slightly more crisp picture.

Just beware that some consoles have unbuffered sync on the AV output. MegaDrive is one of them. It needs a tiny mod to make the sync usable.

[Fix_Metal]

RGB can't be displayed without a matching sync signal.

Pin 20 on the SCART input has two functions. When the voltage applied to pin 16 is less than 0,4v the TV will use pin 20 as the video feed (decodes both R, G, B, H-sync and V-sync from pin 20) . But when the voltage is 1-3v  the TV discards the R, G and B from pin 20, but still uses the extracted sync signals as timing for the R, G and B signals it gets from the SCART input.

When using SCART as an RGB input, the raw sync signal from the console can also be applied to pin 20 instead of composite video. In theory that'll give you a slightly more crisp picture.

Just beware that some consoles have unbuffered sync on the AV output. MegaDrive is one of them. It needs a tiny mod to make the sync usable.

Thanks, this makes things clear. Well if it's unbuffered, a buffer will do the job, but I don't see any crap with just composite in #20.
Just for further inspection, the sync line is all about v-sync (50/60Hz? sin wave?) or is more complex than that?

[KillingBeans]

Well if it's unbuffered, a buffer will do the job

The MegaDrive actually has a built-in buffer in the video encoder. I't just not utilized. All you need to do is re-route the signal.

Just for further inspection, the sync line is all about v-sync (50/60Hz? sin wave?) or is more complex than that?

It's Compostite Sync. Combined H-sync and V-sync. Digital pulses.
It controls the scan lines, the frames and the blanking.

[Fix_Metal]

It's Compostite Sync. Combined H-sync and V-sync. Digital pulses.
It controls the scan lines, the frames and the blanking.

Thx