The first chip in the OPN line, the YM2203 (called "OPN") only had three FM channels, and no sampling capabilities, but it did have an entire YM2149 PSG built in. YM2149 is essentially identical to a General Instruments AY-3-8910, which had three pulse wave channels (almost exclusively using plain square waves), and one noise channel. Though this is a series about FM, and this is an "FM chip", it really is just as much a "PSG" chip, and that's the side of the house that will actually get more talk time in this post:
The YM2149/AY-3-8910 chip was very comparable to the NES' 2AO3 in terms of its pulse width options. NES had four modes, with a total of three timbres (since two of them sound identical to each other, and are thus redundant), 12.5%, 25%, 50% (square), and 75% (redundant to 25%). The AY/YM had 25%, 50%, and 85%, though again, the 50% plain square was almost exclusively used. Both had very good noise channels. And both could use periodic noise musically (though I don't think NES ever did this, except for the metallic voices in Quick Man (Mega Man 2)). I believe AY/YM used this mode more often. Both AY/YM and 2A03 could pitch themselves comparably deep.
The biggest difference between the two is a) NES is two pulse, one triangle, and the AY/YM is three pulse, and b) the NES, albeit a very, very crappy one, and not frequently used, had a dedicated PCM sampler channel, and the AY/YM did not. If it was going to sample, it had to do it by tricking the periodic noise to do it, which sounded even worse than the NES's crappy sampler, and tied up a channel, where all four of NES's "normal" channels would still be free during sampling. One advantage to the AY/YM, though, is that its square waves had this great haunting quality to them that the crisper NES lacked.
Anyway, that's the AY/YM vs NES in a nutshell, and the basics of the YM2203 (OPN) FM chip as well. I know we spent more time talking PSG than FM in this post, but as I said above, it really is about an equal component in this very "FM/PSG-duet" sounding chip, and the FM side, between what was already said in the OPM post, and the remainder of this series will be fleshed out quite nicely.
OPN appeared in several arcade games, and also in the NEC PC 88/98. Our examples are the Y's I and II soundtracks for the PC88, composed by the legend - Yuzo Koshiro, the Sorcerian PC88 Soundtrack and a standalone track from Advanced Power Dolls 2 for the PC98.
The first three are more typical examples of the OPN's sound, though the Advanced Power Dolls 2 track shows that the OPN is actually capable of quite a bit more advanced sounding things - in fact, I had originally thought I was listening to the OPNA, because I thought the drums were sampled - nope...FM. Also I thought there might've been more than three FM channels happening at once, but again....nope. You really can do some pretty amazing things on the OPN!
Advanced Power Dolls 2 - PC98 - Believe in Motivation (composed by Hiroto Saitoh)