The old Sega video connections are a poor choice from a technical point of view. Read below for more info than you probably wanted.darcagn wrote: I'm thinking for this mod I might do a 8-pin (full size) DIN connector with the same pinout as the Sega Genesis model 1, so that any off-the-shelf Genesis model 1 cables will work with it. Additionally, instead of mounting the connector to the back, I might just have a short female 8-pin DIN connector on a cable routed through the hole on the back of the console where the RF-out cable used to be, so it retains the same style as the original console configuration.
Thoughts?
Historical:
The first console to feature the Sega RGB port is the Master System. Most of the consoles connect the signals from the video encoder straight to the video port. The exception is the French consoles. They have no video encoder at all, the RGB signal from the graphics chip connects straight to the video port.
The Sega RGB cable (called the Adapteur R.V.B.) was included with all the French Master System consoles. There is no RF modulator or video encoder so RGB in all that is available. The cable contains a four channel video driver (for the RGB + sync lines) and a DC-DC converter to generate the SCART switching voltages.
The same arrangement was passed on to the Mega Drive. There was no special version for France without the video encoder. Now all models have one and the RGB signals connected straight from it to the AV port. The same cable is used as before. I presume this is all done in the the name of compatibility.
Now the Mega Drive II came out. I think this is a genuine mistake that the engineers forgot to put the resistor + capacitor on the board this time when they changed the connector. They also took the opportunity to make a dog's breakfast of the audio circuit. The new cable for the Mega Drive II in France (RGB still required as many didn't have PAL compatible TVs yet) was a horrible compromise where they put the forgotten components into the SCART end of the cable along with a DC-DC converter for the SCART switching. The DC-DC converter was noisy and most cables around are already modified to cut power to it. The capacitors were left out, presumably because they didn't fit.
Now for the technical bit:
The video encoder in the Master System and Mega Drive can drive the video line directly with the addition of a 75 ohm series resistor and coupling capacitor (usually 220u but the value is not critical). The resistor serves two purposes. First is to match the source impedance with the load so there are minimal reflections along the transmission line (cable), second it isolates the low impedance output of the video driver from the cable capacitance. The capacitance of a long cables can be enough to make the video encoder oscillate. This is a characteristic of all high speed voltage feedback amplifiers. Capacitive loads are not allowed. You can find warnings in the better video encoder datasheets about this.
To make a passive RGB cable for these consoles you need to put the resistors into the console end not the SCART end. Even if you don't care for good quality video, the video signal couples into the audio signal worse if the resistor is placed at the SCART end. The capacitor can at either end and it's only blocking DC. If you leave it out (like Sega did), it forces the video driver (in the video encoder chip) into class A mode (instead of class AB) and it makes the video encoder get very hot.
The reason I didn't choose this for my products should be obvious. It's got lots of history, technical flaws, and it's very expensive to make cables with components inside. Of course, I have to be able to supply cables which are up to a certain standard, not just ask to buy cables from somebody else which may or may not be junk.
The reason I didn't use the Neo Geo pinout is DIN connectors are a bit too big for my liking. The larger the hole the customer has to drill, the more likely it will end in a cracked shell. Also, (statistically) nobody has a Neo Geo.
I recommend separate cables for audio and video because it's the most cost effective way to get guaranteed good audio performance. The two cables can easily be bound together with heat shrink or cable ties. Pretty much every analog video connection that isn't SCART has separate connections for audio so it's hardly without precedent.
I'll finish by mentioning that these are only recommendations and you are quite welcome to use whatever connection you like.
I have learned a lot from this hobby and that's half of the fun to me (the other half, of course, playing games!). It definitely sounds like you should stay away from the Sega configuration for your products, and we for our installations. 
The orange S/PDIF-style phono jacks aren't staying, though, I just used them as placeholders for the drilling process until the right colors are shipped to me. The palette switch will be mounted on the right. I didn't have a step bit, and they seem to be quite expensive, so I used a very small dremel bit to create a hole and filed them out by hand.
