Composite video vs composite sync and Demystifying RGB video (2024)

Figure 1. CSYNC over composite video format is transmitted over four wires to SCART pins 15, 11, 7 and 20.

Figure 2. Composite sync (CSYNC) format transmits only the horizontal and vertical sync information to SCART pin 20.

Advantages of composite sync

A composite sync RGB SCART cable can reduce a video effect often referred to as jail bars, which are faint on-screen horizontal lines displayed throughout game play and are more noticeable on patches of solid colours, and when using a modern flat screen television in-conjunction with a sync over composite video RGB SCART. This is due to the amount of channels occupying one wire and in the case of composite video this totals five (red, green, blue, VSYNC & HSYNC) as seen in figure 1, so cross-over interference is a common problem. This effect is legendary on the Sega Mega Drive / Genesis model 1 & model 2 when using a “sync over composite video” RGB SCART cable, however not exclusively reserved to those Sega consoles. Luckily both Mega Drive / Genesis consoles output composite sync as well as composite video, therefore, you just need an RGB SCART wired to support composite sync, and bingo! No more jail bars. Another problem often experienced with sync over composite video RGB SCART cables are sync issues when connecting consoles to video up scalers like the Framemeister XRGB mini. Problems includes screen drop-offs resulting in a blank screen during game play and also another effect called dot crawl. These two problems can occur with the Super Nintendo, NTSC version, but can be cured using a composite sync RGB SCART cable, YAY!

Composite sync extracted from composite video? What is this black magic?

Unfortunately the most commonly used format in the world of retro gaming after-market cables is the sync over composite video RGB SCART cable, including those cheap arse cables of eBay (oops). Consoles which can output composite sync include the Sega Mega Drive / Genesis 1 & 2, Super Nintendo (NTSC), Neo Geo AES, Atari Jaguar, Sega Saturn (JAPANESE model) etc. For consoles which only output sync over composite video, there is a solution. The composite video aspect of the video signal can be removed using a special circuit called a sync separator, sync cleaner or sync stripper as seen in figure 3. This removes the composite video signal and leaves a clean composite sync signal. A sync separator circuit can be inserted directly into a SCART plug and the voltage needed to power the circuit is drawn directly from the console and works without any problems. For consoles which don't output composite sync for example the Original Xbox,Sony Playstation, Super Nintendo PAL, Sega Saturn PALand Nintendo Gamecube Pal we supply these cables with a built in sync separator circuit called an LM1881 which is a surface mount chip attached to our custom printed circuit SCART board, as seen below. Or retrofit your existing RGB SCARTs with our Sync Blaster PCB, which incorporates an LM1881 sync separator board.

Summary

The RGB format requires a synchronising signal called composite sync (CSYNC) which is composition of Horizontal and vertical signals, so hence the name "composite" sync, as both signals are mixed together down the same signal wire. CSYNC resolves drop out issues on the xrgb-mini and also removes on-screen anomalies like checker board effects on the SNES and jail bars on the Mega Drive/Genesis. In an ideal world all SCART cables would use composite sync, however, this is not the case as I will explain below.

A lot of games consoles manufactures decided to mix the composite sync (CSYNC) signal over another signal called composite video (CVBS) resulting in three signals down one wire, namely horizontal sync + vertical sync + CVBS. I like to call this situation "sync over composite video". Some consoles do output CSYNC when using the correctly wired SCART cable, for example the following consoles can output CSYNC including the Atari Jaguar, SNES (NTSC), Super Famicom, Mega Drive etc. For consoles which don't output CSYNC you can add a sync cleaner in the loop to achieve CSYNC.

Ultimately the best format to use is composite sync (CSYNC) as this only consists of two channels of information transmitted to SCART pin 20, namely HSYNC and VSYNC. And the less channels occupying one signal wire means less interference.

For more information on sync see My Life in Gaming RGB 103 video below which goes into depth on the subject.