Some information from bouncy
"Some say that the instruments of the past themes in Sonic CD were created using the SNES or Genesis sound chip. Well, I can safely say that is not the case. What you've been hearing this entire time was compressed samples recorded off of synthesizers that were prevalent in the late 80s to early 90s. These said samples were then being played by the Sega CD's Ricoh RFC5C164 sound chip. Unlike the present, bad and good future themes that were Redbook Audio, the past tracks were like sequences to a degree. Now for the first time ever, you'll be able to hear how Tidal Tempest Past sounded like before all the compression. Please enjoy -- it was difficult to track down all the instrument sources. By the way, I DID add in a few extra channels just to give off the flare the present/good future/bad future JP tracks had. Because of the Ricoh sound chip's limitations, most past tracks ended up sounding empty."