The YM2413 (OPLL) is a nine-voice 2-operator FM chip: each voice is a modulator warping a carrier's phase, shaped by a real ADSR envelope. It gave the MSX its FM-PAC cartridge and the built-in MSX-MUSIC of later machines. Fifteen instruments live in ROM, and there is one you build yourself, which makes it a good place to learn how FM synthesis actually works.
A five-note FM chord across the voices. Sound needs a click first — that is the browser's rule, not the chip's.
The whole chip. Click any block to jump to the page that takes it apart: the diagram is the site map.
After the PSG's three fixed squares and the SCC's drawn wavetables, the OPLL is the first MSX chip that properly synthesises: a modulator operator adds its output to a carrier's phase every sample, and that phase-warping is what turns a plain sine into a bell, a brass, a clav.
You mostly pick an instrument here (Violin, Guitar, Piano, Organ…) rather than design one, but slot 0 is eight bytes of your own, and any ROM patch can be copied into it and edited. That is where FM patch design is taught.
Set bit 5 of register 0E and channels 7-9 stop being
melodic: they become Bass, Snare, Tom, Cymbal and Hi-Hat, driven by a
shared noise generator. Six FM voices and five drums from the same
chip.
A machine with MSX-MUSIC still has its PSG, and often an SCC cartridge too. Together they are the MSX's sound family, and each has its own playground.
Seven pages along the signal path — one operator, its envelope, the second operator, the patch, the voices, rhythm, and the MSX. Each stands alone, and each ends with the same write-only register file, its own bytes lit.