tri-Ace IMA ADPCM? by soneek at 8:16 PM EDT on April 6, 2017
It looks like the BGM format tri-Ace uses in Heaven x Inferno and Star Ocean anamnesis (.aac extension) might be IMA ADPCM. Below are my findings, which might not be 100% accurate.
The raw data start offset is at 0x200, and in blocks of size 0x40. It looks like every block starts with 4 uncompressed pcm16 samples (8 bytes), then the remaining 0x38 bytes are IMA. Rewriting the data without those first 8 bytes in each block, and parsing as IMA with GENH doesn't work perfectly, but I can identify songs.
It may be some MS IMA variation. The data looks normal and the header matches, but there is something funny going on. Normally the 2nd header short per ch is the step_index, from 0 to 88. But here it's often 0x7F and goes +0x300. I tried some mods but nothing playable. Maybe some basic encryption? Some more small (mono) files may help, but that's all I can do so I'll let the big boys figure out the rest. You can try MS-IMA in GENH to get some Sakuraba(?) guitars along with hellish noise.
This AAC format is also used in their PS3 and X360 games (inside bigfiles), interesting it does use .aac (could be renamed to ".aace" for vgmstream?). I think bxaimc was going to add support but they modify it in every single game, those crazy funsters.
I wish I had mono samples. But yes, those battle themes are Sakuraba's work. The 1st battle theme is supposed to sound like this. Hoping someone is able to figure out tri-Ace's stupid variation on IMA.
Thanks, I tried doing some more stuff and sounds kinda listenable without the hellish noises, but still not too great. Some kind of QuickTime IMA? I'll persevere.
Finally figured out, it was Yamaha ADPCM (AICA-like), somehow. Decoding is slightly noisy but eh. I added it for the lulz, too bad(?) they switched the music to Ogg in later versions, oh well.