The S-SMP (perhaps: Sony - Sound & Music Processor?) is the audio CPU used by the Super Nintendo Entertainment System. It consists of two integrated units, the SPC700 and a digital signal processor (DSP), as well as 64 kB of SRAM which is shared between the two units, and 64 bytes of ROM for booting. The SPC700 is an 8-bit CPU similar to the MOS6502 processor which runs audio software. The DSP is an 8-channel 16-bit processor. It mixes channels, handles stereo panning, adds echo, and can envelope samples. The chip was designed by Ken Kutaragi of Sony for Nintendo.
Unlike the Nintendo Entertainment System which used a 4-channel PSG, or the Genesis which used FM Synthesis, the S-SMP uses fully digital audio samples like a module. Some sound engines used instrument samples given to them by Sony themselves. One of these instruments was the infamous slap bass patch from the Korg M1. More times than not though, composers, sound designers and programmers just used instruments from keyboards they had at the time. Sometimes the instruments were picked out by the composers and other times they were picked out by the programmers since they designed the sound engines and software.
The chip functions by booting up when the SNES is turned on, then, at various points in the game, driver software, audio samples, and music data is loaded from the game cartridge into the S-SMP's RAM. When the SNES CPU encounters code to play a song or sound effect, it relays the message to the SPC700 which handles it. The DSP takes the audio samples, mixes them according to the music, and sends the result to the autio out jack on the SNES.
Since the audio processing unit of the SNES is almost entirely self-contained, a simple memory dump is usually all that is necessary to extract an SNES game's music, and these dumps exist as the SPC format. Some games access the S-SMP in more complicated ways, altering the RAM as the song plays, so these games cannot have their audio ripped, and instead rely on the SNSF format.
When SNES ports of NES games were made, the extra channels were usually used to add an echo or detune effect.
- en.wikipedia.org/wiki/Nintendo_S-SMP - Wikipedia.