In video transmission systems, synchronization or "sync" signals are the signals that establish the timing relationship between each location in the picture and the color information being transmitted at each moment. Sync pulses mark the start of each horizontal scan line and the start of each vertical refresh. Sync is required for the picture to be displayed correctly. There are different types of sync signaling used by different devices, and some devices support more than one kind of sync. Sync can be transmitted on the same wire as the video signal, or on separate wires. When you hear the term "sync" in retro gaming, it's generally referring to the fourth wire in RGB setups: the "S" in RGBS.
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:
| Type | Channels | Description | Console sources |
|---|---|---|---|
| RGB + sync on composite | 4 | Composite video (CVBS) as the source of sync. 0.7 V video and sync pulses of -0.3 V (1 Vp-p). [75 ohm termination] This is the format of SCART RGB and most RGB-modded TVs. Poorly shielded cables may cause interference (checkerboarding). | Xbox, PlayStation, PlayStation 2, SNES (PAL), Saturn (PAL), and GameCube (PAL) |
| RGBS (CSync) | 4 | Composite sync: horizontal and vertical sync on one sync line. Voltages are not standardized, and can be around 1 V or several volts. [75 ohm termination] Pulse polarity is negative (active low). Used by professional video equipment. Can often be fed to displays expecting RGB + sync on composite, and may solve interference problems. | Neo Geo AES, and PlayStation (with small mod) |
| RGBS (TTL CSync) | 4 | TTL composite sync: horizontal and vertical sync on one sync line using TTL signal levels (up to 5.5 V unloaded). Pulse polarity is typically negative (active low). Used by older computer equipment and video game consoles. Also accepted by most displays with an RGBHV input. Feeding TTL level sync to equipment expecting lower voltages requires an inline resistor in the cable, or else the receiving equipment may be damaged. | SNES (NTSC), Mega Drive / Genesis 1 & 2, Jaguar, Saturn (NTSC) |
| RGBHV | 5 | Horizontal and vertical sync on separate lines using TTL signal levels (up to 5.5 V unloaded). Sync pulses can be any polarity (active high or low). This is the standard for most computer equipment since VGA (VESA standard), and is supported by displays with a VGA connector or 5x BNC. | Dreamcast, Xbox 360 |
| RGsB (sync on green) | 3 | Sync pulses of -0.3 V on the green line. Less common and used by some old computer equipment. Accepted by some CRT computer monitors, including most Sony monitors. | PlayStation 2 and 3 at 480p and higher |
| RGB + sync on luma | 4 | Luma as the source of sync. Similar to sync on composite, but without a chroma signal. 1 Vp-p. [75 ohm termination] Can be fed to most displays expecting RGB + sync on composite, and may have less interference. | PlayStation, PlayStation 2, Dreamcast, N64, Xbox, SNES |
The word "composite" refers to different things. "Composite video" (CVBS) is a color video signal that combines luma and chroma on one wire. "Composite sync" (CSync) is a sync signal that combines horizontal and vertical sync on one wire.
Sync Strippers
A sync stripper or sync separator is a circuit that takes a composite video signal or luma signal as input and extracts the sync pulses to produce pure CSync. The most common scenario you'd want a sync stripper is for use with rare displays or processors that can't handle luma or composite video as the sync signal. 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.