When you hear the term “sync” in retro gaming, it’s 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.
Overview Of Sync Choices
So, let’s step back for a moment and go over some of the more technical information behind sync. If any of this is confusing, just get an RGB SCART cable that syncs on composite video and don’t worry about it. If you’re curious, lets continue…
All analog video signals carry some type of sync signal; Composite video carries all of the video information (all of the colors and brightness combined), as well as the sync information to your TV. S-Video is similar to composite, but separates the signal into two parts: Chroma (color information) and Luma (brightness information and sync). Then, some console generate a sync signal all on it’s own, but that can be sent many different ways and if the RGB SCART cable isn’t built to spec, there’s a potential safety issue if using a console’s direct sync line.
To make things more confusing, it’s common to stumble across articles or forums posts where people use different names to describe sync (“regular sync”, “pure sync” “raw sync” , etc). This was extremely confusing to me when I first started learning about sync in game consoles and I want to make sure this page clarifies that! In the context of game consoles, there’s really only four types of sync any beginner or intermediate retro-gamer needs to know:
csync – This is just the composite sync info with nothing else on the line, outputted at a standard 75 ohm video signal of around 700mV or less.
ttl sync – This is essentially the same as csync, but voltages are over five times higher!!! Almost all standard equipment used requires csync and ttl can actually damage SCART equipment!
sync on luma – This is using S-Video’s luma pin as sync.
composite video (cvbs) as sync – This uses the composite video line as sync. The same as if you plugged a yellow composite video cable into the sync input of your TV/scaler.
I have a page with information on what components are required in each console’s RGB SCART cable, in order to output proper csync. It’s a good reference for experts who need custom cables, or people who want to check their existing cables. Once again, if any of this is confusing, just use RGB SCART cables that sync on cvbs: http://www.retrorgb.com/csync.html
When does choice of sync matter?
In the context of using a well-built cable into a modern scaler or RGB monitor, which sync signal you use probably won’t matter. Here’s the pro’s and con’s of each:
CVBS Pro’s: Safe to use and your cable could output both RGB and composite over SCART, if your device is compatible (like some TV’s and the RetroTINK 5x).
CVBS Con’s: If poorly-shielded cables are used, you’ll end up with a lot of video interference and most commonly a “checkerboard” pattern. Some devices like an Extron Crosspoint (and a few random RGB monitors) require csync and will not accept cvbs or luma.
Luma Pro’s: Safe to use, no chance of cvbs checkerboarding.
Luma Con’s: Some devices like an Extron Crosspoint (and a few random RGB monitors) require csync and will not accept cvbs or luma.
csync pro’s: If the cable is built correctly, it’s compatible with all RGBs devices and there’s no chance of cvbs checkerboarding.
csync con’s: Safety! If a cable is built wrong, you can send the wrong voltage from your console to SCART devices and potentially damage them. While this is generally the only con, it’s a serious one: Even if the voltage is safe, a cable built wrong can cause compatibility issues!
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. In this case, I’d strongly recommend using a gscartsw, as it can safely process sync from almost every console and you don’t need to worry about making your own.
The only scenario I’d ever suggest using a sync stripper in an RGB SCART cable is if you’re connecting to an Extron Crosspoint switch. In that case, I’d suggest putting the sync stripper in the SCART to BNC side of things and not the console’s cable. You can also use external adapters like the Sync Strike.
You can also make your own sync stripping circuits using a chip called the LM1881. I’ve created a detailed guide if anyone would like to try, however once again, there’s almost no reason to use one in most scenarios. If you’re an expert who knows what they’re doing, here’s a guide: http://www.retrorgb.com/syncstripper.html
Other Terms, Acronyms
You may come across terms similar to RGBs, such as “RGBHV” and “RGsB”. Just to clarify, here is a basic description of each:
– RGBs is simply Red, Green, Blue and sync. In RGBs, both the horizontal and vertical sync signals are combined into this one line.
– RGBHV is essentially the same as RGBs, however the horizontal and vertical sync signals are sent down their own individual lines, totaling 5 channels. “VGA” uses RGBHV.
– RGsB is “Sync on green”: In this scenario, the green cable also carries the horizontal and vertical sync signals, totaling only three cables. The only time you’ll run into RGsB in the classic gaming world is with the PlayStation 2 and it’s usually just better to use component video. More details in the PS2 section: https://www.retrorgb.com/playstation2.html
Hopefully this page covered all the basics you’d need to know about sync. I purposely over-simplified a few of the details in hopes that it would make this page easier to understand, but I’ll someday have a video with more detailed information about what sync does and how it interacts with different gaming equipment.