From Sega Retro
The Sample Music Playback System is a sound engine used in several games for the Sega Mega Drive/Genesis and the Sega 32X, including all major Sonic games except Sonic the Hedgehog Spinball, which uses GEMS instead. The engine's name has not been completely confirmed, as a large group of users have come up with serious doubts about it and no concrete proof has been found.
A few versions of this engine exist; most notably, both a 68K and a Z80 version were developed. It is possible to port songs between two games that use the SMPS engine; however, several games use a slightly different revision of the format, and the chance that the ported song will need extensive editing by hand (usually with a hex editor) is very high.
Many sceners have been working on the SMPS format, doing an extensive work of research. Most credit should be given to saxman, the man who started it all, but a lot of hackers continued his work up to our days, including Puto and Tweaker.
We now have a fairly complete knowledge of this format, and several tools are available to hackers who want to tweak the music in their hacks. In particular, the xm4smps tool created by Puto and Oerg866's xm3smps/oerg (official improved branch of nineko's original xm3smps) allow music to be imported into the SMPS format with ease, generally having very favorable results.
- A very comprehensive guide detailing the SMPS format and all its variants.