Types of Sync
The term "sync" refers to the signal that helps the receiving device display the video image correctly. When you hear the term "sync" in retro gaming, it's generally referring to the fourth cable in RGB setups: The "s" in RGBs. This fourth signal coordinates the horizontal and vertical synchronization of the color and brightness information that's sent separately in the three RGB lines. When buying RGB cables, you'll often see choices of what signal is connected to the "sync" line. In the context of RGB SCART with video game consoles, Composite Video (cvbs) and Luma (from S-Video) both can be safely used as sync. Some consoles even generate their own dedicated sync signal, but there's often no benefit from using that.
Here's a visual example of what happens when a sync signal is disconnected. Since this is a static screen, the issue is mild, but if a game were being played, the signal would go crazy, or not be displayed at all.
Different Types of Sync
RGB (or RVB in France) is a shorthand for one of the following:
|RGsB||3||Sync on Green (SoG)||PlayStation 2|
|RGBS (Composite Sync)||4||RGB with Composite Sync (CSYNC) signal||Mega Drive / Genesis 1 & 2, SNES (NTSC), Neo Geo AES, Jaguar, Saturn, and PlayStation (with small mod)|
|RGBS (Sync on Luma)||4||RGB with Luma as the sync signal. A sync stripper can be used to extract CSYNC from the luma signal.||PlayStation, PlayStation 2, Dreamcast, N64, Xbox, SNES|
|RGBS (TTL Sync)||4||Same as RGBS (CSYNC) but voltage levels are 5x higher. For use with professional displays only - may fry consumer-grade equipment.||SNES (NTSC)|
|RGBcvS*||4||CSYNC over composite video (composite video Sync). Also known as "sync on composite". A sync stripper can be used to extract CSYNC from the composite video signal. If poorly-shielded cables are used, you'll end up with a lot of video interference and most commonly a "checkerboard" pattern.||Xbox, PlayStation, SNES (PAL), Saturn (PAL), and GameCube (PAL)|
|RGBHV||5||Independent sync (horizontal & vertical)||Dreamcast, Xbox 360|
You can build a circuit that strips all information from a video signal and outputs CSYNC. The most common scenario you'd want a "sync stripper" is for use with rare displays or processors that require them. The only common scenario that requires a sync stripper in an RGB SCART cable is if you're connecting to an Extron Crosspoint switch using sync on luma. You can use external adapters like the Sync Strike or you can make your own sync stripping circuits using a chip called the LM1881. If you're an expert who knows what they're doing, here's a guide to create your own sync stripper.