TTL CSYNC vs 75 OHM CSYNC?

[erik343]

Can somebody please explain the technical differences between TTL and 75 ohm composite sync?
I already know that the TTL voltages are five times higher than 75 ohm sync.

I am asking because I have a NES, SNES, and Atari 2600. I am going to eventually RGB (from Tim Worthington) mod all of these consoles, and some jumpers can be set to either TTL or 75 ohms. I am not sure which to set each to? I have a SNES cable with the 330 ohm resistor, and two other SNES cables with that resistor removed. I am using the Framemeister XRGB-mini as well hooked up to a 4K TV.

Thanks.

Erik W.

[Arthrimus]

If you are using a cable with no resistor on the Csync line then you need your mod to output 75ohm Csync for it to be safe. If you are using a cable with a resistor present on the Csync line you need your mod to output TTL Csync. Most console manufacturers like Nintendo and Sega chose to output TTL Csync from the AV port of the console and then add a resistor in the cable to bring it down to safe levels for consumer video equipment, therefore most scart cables made for these consoles include a resistor and expect to see TTL Csync.

I would personally recommend always going with TTL Csync for your mods, and using standard cables that include the resistor. That way you don't have cables floating around in your setup that would be unsafe to hook up to a stock system accidentally due to not having a resistor in line. There is no disadvantage to TTL Csync and a resistor that I am aware of from a video quality standpoint, so mods that output 75ohm Csync are kind of silly and unnecessarily create incompatible ecosystems that can be dangerous if interchanged.

[Guspaz]

Apart from the fact that the video source is supposed to output 75ohm to adhere to standards, and using a resistor to turn TTL into 75ohm is a hack.

I'd point out that if you use HD Retrovision component cables, you'll never have to worry about sync levels or even sync types again, but then I'd be shilling for products that I sell, and we can't have that

[Arthrimus]

Apart from the fact that the video source is supposed to output 75ohm to adhere to standards, and using a resistor to turn TTL into 75ohm is a hack.

That's really not the point. In an ideal world, Nintendo, Sega and others would have designed their consoles to natively output 75ohm Csync and none of their cables would have had resistors on the Csync line, but in reality that's not what they did, which means that mods that output 75ohm Csync natively need modified cables that have the resistor removed, thus making them potentially dangerous if they are later plugged into an unmodified system. So whether or not it ought to be this way, it is and the safest thing for the end user of these video mods to do is set them up to conform to the standard that the original console manufacturer used for their av port.

With all of that said, if you have an NESRGB mod installed that uses a custom Mini DIN AV port pinout instead of a console standard AV port, and you are going to be using custom cables that won't fit any stock consoles, then go ahead and use 75ohm Csync, because it won't endanger your setup if you can't accidentally plug it into a device that outputs TTL Csync. However, if you are using a Nintendo multi AV port for your mod, then it should be set up to output the same way a stock SNES would.

[erik343]

I found something weird: viewtopic.php?p=1144184#p1144184

It says that if you set J8 to use TTL (open), then if you have a resistor (like 330 uf) in the cable it will not work. Meaning, no matter what you set J8 (TTL or 75 ohms) to, then you cannot have a resistor in the cable for it to work? Not sure...

[matt]

If it were me, I'd just set it to 75 ohms. Of course that wouldn't be compatible with RGB cables designed for the SNES, but you can solve that problem by just having a dedicated cable for that system.

[naz]

What exactly happens if you mix the cables/resistors??? does everything explodes or do you just get something like oversaturation on the colors??

[maxtherabbit]

[makar1]

I'd point out that if you use HD Retrovision component cables, you'll never have to worry about sync levels or even sync types again, but then I'd be shilling for products that I sell, and we can't have that

Why would anyone with an RGB capable device like the Framemeister bother using HD Retrovision cables to transcode to YPbPr? They also require sync-on-composite which may or may not be present depending on the mod used.

If it were me, I'd just set it to 75 ohms. Of course that wouldn't be compatible with RGB cables designed for the SNES, but you can solve that problem by just having a dedicated cable for that system.

Doing this creates dangerous cables that will destroy the video scaler if used with a console that natively outputs TTL CSYNC. 75 ohm CSYNC should only be used to replace the composite video signal in order to reduce interference.

[Bassa-Bassa]

Is TTL/5V really that dangerous for TV sets and home devices (did anybody have a bad experience?)? I'm using a TTL CSYNC Saturn cable to a Keene SCART Commander and then to a Sony RGB CRT TV set from the mid-90s. Do you think that the SCART switcher may be suffering as well?

Is there something similar to a UMSA to attenuate the levels, but for RGBS? I don't have the means to add a resistor.

[Guspaz]

It won't fry the display sink immediately. It will do so slowly over time. IIRC it was pointed out in another thread that Micomsoft stopped listing TTL compatibility, and the speculation was that it was causing too many failures.

Why would anyone with an RGB capable device like the Framemeister bother using HD Retrovision cables to transcode to YPbPr? They also require sync-on-composite which may or may not be present depending on the mod used.

IIRC the Framemeister converts everything internally to YPbPr (well, YCbCr) and then does chroma subsampling. As do most capture cards. So if you're converting to component video either way, there's not really any downside to letting it be done in a high-quality specialized circuit.

[erik343]

Thanks for all the information and your opinions. I think either TTL or 75 ohm CSYNC is fine, and there are two sides to the story. I decided that my aim is just to have all of my consoles do 75 ohm CSYNC.

[makar1]

Thanks for all the information and your opinions. I think either TTL or 75 ohm CSYNC is fine, and there are two sides to the story. I decided that my aim is just to have all of my consoles do 75 ohm CSYNC.

75ohm level CSYNC is safe if it's placed on the composite video line of your console, removing the need for dangerous custom cables.