NGFX SoundBuilder

More RE - this time Blastar's NGFX SoundBuilder Z80 ROM.

I wanted to learn more about how the Neo Geo sound works in general, and how the SoundBuilder Z80 ROM works in particular. Together with the Neo Geo Development Wiki, the YM2610 (incomplete) datasheet and some sample driver code, I was able to decypher most of it today.

Pretty neat how Blastar has designed his driver to cater for most situations. It provides 5 channels of ADPCM allocated in a round-robin fashion for short samples, a 6th channel that can play background sounds (optionally looped) for longer samples or voice/music, and a 7th independent channel (again, optionally looped) that can also play background sounds. On the NGCD, the 7th channel is replaced - seemlessly - with CDDA tracks.

The way it works is that the driver code itself doesn't need to change, just the data table(s) at the end of the ROM. One for the (optional) eye-catcher, and one for the game sounds. After you insert the samples into the GUI and allocate channels, it produces a Z80 ROM image and accompanying PCM data ROM ready to go for either cart or CD systems.

Now that I understand how it all works internally, hopefully I can get the sound working in Xevious without any glitches sooner rather than later.