I’ve been looking for the pinout OSSC uses for the Dsub 15. I mean, I get that it uses the same pinout as VGA for VGA sources, but what about RGB? YPbPr? Audio?
VGA is RGB; RGBHV, specifically. For RGBS, same as RGBHV, but composite sync goes on the horizontal sync line. RGsB is the same as well, but with no dedicated sync line. For YPbPr, sames as RGsB, matched to cable color: Y to G, Pb to B, Pr to R.
You do need to keep in mind that AV3 has limited low pass filter capabilities, and that the wiki recommends connecting older consoles (which I believe means all of the consoles you mentioned) to the other inputs.
I have an OSSC1.5 with the audio upgrade board so I know mine isn’t wired for audio through the 15 pin Dsub, but are the newer ones?
No; VGA cables do not carry audio. The OSSC 1.6 has a 3.5mm TRS jack for audio input for AV3, located next to the DE-15 port.
I don’t have one, but I believe the OSSC 1.5 with the expansion audio board can only accept one audio input at a time, either via SCART on AV1 or via the 3.5mm jack next to the DVI port; meaning all of your video inputs will have to share this one audio input.
Luckily, the HD Retrovision cables were engineered to be used together with a few Y-cables, so they can share AV2 and connect NES RGB modded AV Famicom, SNES, N64, GameCube (with mods), Master System, Genesis 1/Sega CD, Genesis 2/32X, Saturn, Neo Geo, and TurboGrafx-16 (with Super SD System 3), and more. I can kill a lot of birds with two stones.
You can also kill either the OSSC or your consoles if you turn two of them on at the same time (accidentally or otherwise) when they’re connected with simple Y-cables, as this may cause voltage to spike on the video and/or audio lines and exceed specification. (Just because HD Retrovision says you can do it doesn’t mean you should.)
I know you don’t want to use a switch, but, if you need to have two cables share AV2, I would not recommend anything less; use an automatic switch if you want it to be hands-off.