alt Hacker NewsI think the article is sorta wrong about the sound setup, the Mega Drive I does have a sound input on the expansion port, and mixes it into its sound output. Otherwise stuff like RF cables wouldn't get Mega CD based audio (and you can do silly stuff like a Mega Drive II, which doesn't have a headphone port, with a Mega CD I).
I was going to say the patch cable setup was just a passive aid to take the Mega Drive I's minijack stereo output (the big DIN AV connector on the rear only does mono) to a more serious two RCA jack setup. But looking at a schematic to check, it does do more stuff, and apparently connecting the patch cable will reroute stuff so the sound mixing is done on the Mega CD side, not the Mega Drive side (early Mega Drive revisions are somewhat infamous for showing that Sega hadn’t quite mastered the dark arts of analogue sound circuitry).
(I would double check some of this, but my Mega Drive / Mega CD setup isn't to hand, and the CD drive is broken anyway, although I understand the JP/PAL piano tune on the logo screen is all from the Mega CD side?).
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So the problem with the Model 1 that the mixing cable is trying to solve is that the expansion port on that model has audio input pins, but no audio output pins. Now strictly speaking, this is sufficient if you just want to use the MD1's headphone jack (stereo) or A/V out (mono), but in addition to the analog audio circuitry not being amazing (though honestly the MD1 is much better than the MD2 here in general) it also imposes a fairly aggressive low-pass filter. This is rather undesirable for 44.1 kHz CDDA. As you note, the mixing cable allows the RCA outputs to be mixed on the CD hardware side which works around this issue.
On the MD2, they added audio output pins to the expansion port in addition to the existing audio input pins. This allows them to achieve the same effect with no mixing cable.
The Sega CD manual says that the RCA jacks are for connecting the Sega CD to a stereo system or stereo TV. I guess they found that doing the sound mixing on the Sega CD made the output cleaner which would have been useful for people wanting to use it as a CD player instead of just for games. The mixing cable is necessary to use the RCA jacks on the Genesis 1 because the system does not have a sound OUTPUT on the expansion connector, only a sound INPUT. The Genesis 2 repurposed a couple of previously unused expansion pins as sound outputs, which let the model 2 Sega CD do the audio mixing without needing a patch cable.
> I think the article is sorta wrong about the sound setup, the Mega Drive I does have a sound input on the expansion port, and mixes it into its sound output.
The Megadrive has a stereo sound input on the cartridge port too, for a total of 3 stereo sound sources (the YM2612, the MegaCD, the cartridge) plus one mono source (the SN76489 PSG).
As far as I know no original release ever used the cartridge input, but nowadays it is supported by many flashcarts to emulate the MegaCD or to provide CD quality audio tracks (MSU-MD and MD+ romhacks), and it has been used by some recent games.