MSU-1 is a theoretical expansion chip used to play CD-quality audio on the SNES, addressing up to 4GB of data (or unlimited? Can't remember nor can I find an accurate source). It was developed for emulator use for BSX/SNESCD (the latter of which was a cancelled project from Nintendo) hacks and (as such) was never officially supported on the SNES. Additionally only SNES9X and BSNES support emulation with SD2SNES being the only hardware option.

I think it's safe to say that while official, commercial expansions of the era could be interesting for ROM export (such as S-DD1, a compression chip), this oversteps the boundaries of what should be allowed for spc as a format.

Curious to hear your thoughts.

the format description does say "music for the SPC700 soundchip" which doesn't include expansions so i think we are basically covered here, it could be mentioned in restrictions on the lyceum i guess

definitely it has no place in the format, you can play CD quality audio with it haha. if i ever find the time i want to incorporate it into the one track on my album that had live drums layered on it, so it all technically plays in a rom

yeah! just remember that it's 1 channel of stereo audio and you should be good for that application

I think the other expansions are fine, though. The SPC700 soundchip doesn't really work entirely on its own, it's a co-processor after all. For that reason I think that the other expansion being used to augment playback in some way but not replace it is fine; really the only chip that can use the cartridge pins like that (to my knowledge) is MSU-1. I could see SA-1 being used to eliminate lag on high stress ROM exports or something. That's just my opinion though...

no one's been ballsy enough yet to use it as a second allgear in this way

probably due to lack of available tools to do this