it used the same FM synth chip that the original Atari ST did.
The Atari ST didn't have an FM synthesizer, it had a YM2413 which is just a 3-channel square wave/noise type deal... (a.k.a. PSG/SSG, same as the AY-8910). The Genesis uses the YM2612, with 6x 4-operator FM channels. Sega's custom chip also includes an SN76489 recycled from the SMS. The Z80 doesn't make any noise itself but most Genesis games use it to run music player routines or push samples out the DAC.
