Yeah, ADPCM is a form of audio file, it is then played by the Neo Geo’s sound chip (The YM2610) and through the systems DAC (YM3016) to get that Neo Geo sound quality. (DACs are super important, same as the types of audio file processing/compression, as I will mention much like old samplers). The chip is key, but not really the FM part for most games.
The Neo Geo used to rely more on the 4 voice FM capabilities at first, and still kind of did for the CD releases but due to it having the FM capabilities, plus SSG for sound effects and basic synth sounds, it’s 7 channels of sampling became more used (esp since the cartage had way more ROM space than any other console at the time) and allowed for a much wider variety of sound. Keep in mind, there are a lot of tricks used to make the samples sound good, for instance one thing I read was quite interesting
“A lot of games have several recordings of the same instrument playing different notes, rather than having only one sample and change the ADPCM-B playback frequency to vary its pitch”
Which is neat. But the Neo Geo is a lot like the SNES where it’s use of sampling was really what defined it’s sound more than the on board sound generation (unlike the genesis). Look at Earthbound for instance (my favorite game~) it has a dizzying array of samples used throughout mixed with little sound from the actual sound generation the SNES was capable of. Same thing goes for the Neo Geo, so just trying to replicate the FM part of the chip would only get you so far. I mean, the chip has a unique sound, but FM is FM, it can only do so much.
I’m not sure if perusing how to encode and playback samples is worth it. Keep in mind (I looked this up) the samples were “compressed as 4bit per sample and played back as 12bit” but, well, I guess what I’m trying to say is it’s like trying to emulate an old EMU sampler or something. But since very few people want the genuine Neo Geo sound there probably isn’t too much of a demand for software emulation of the hardware (unlike old EMU and Akai samplers).
Though MVSTracker exists, I read that it’s audio encoding is far from perfect though it does somewhat emulate the FM chip.
Hope this helps
(PS. I second old roland sound modules, they are amazing, esp the SC-88 Pro!)
sauce: https://wiki.neogeodev.org/index.php?title=YM2610