It is a six channel FM chip, with the PSG components gutted out of it (though I guess things that would've gone to the PSG are still there, showing its OPN DNA). The OPN2 didn't have an independent sampler channel, but it could sample....at a price: Channel six could either function as an FM channel, or as a fairly basic DAC sampler (much better at sampling than NES, but not as good as these other chips with dedicated ADPCM, or the SNES, or heck, even the PC Engine). So, what that means is without other methods of producing percussion, the Genesis, though technically a six channel FM chip, in effect, only could realistically use five FM musical voices at once (except, of course, when there was no percussion in the track, then you could have six concurrent non-percussive FM instruments).
In the case of the Genesis, it DID have PSG...but not via the YM2612. Again, the YM2149 was gutted out of the YM2612, though the "PSG-shaped-hole" in the 2612 is still apparent for those with eyes to see. Instead, it carried over the Texas Instruments SN76489 PSG that was in the 8-bit 3rd ten Sega Mastersystem. This was principally for backwards compatibility with SMS games, but did work in concert with the OPN2 giving the system a total of 10 channels...when used. Many games use the PSG...and many games don't.
The OPN2, had an interesting bug in it called "the ladder effect", where the negative half of the waveform was ever so slightly delayed. This had an impact on sound that was generally viewed as unfavorable, so there were filters built in to the system to combat this. Also, as a cost saving move, the OPN2 did not use an "adder" to mix down the individual sound channels for stereo output (like the others), but instead, used a method called a "Time Duplex Multipler". TDM is where instead of adding the channels together to mix them down for output, what it did was only output one channel at once, but toggled between those channels so fast that the human ear can't detect it, and perceives that it's hearing all six channels at once - even when in reality, it is actually only hearing one channel at any given time. This TDM technology was (is?) also used in telecommunications applications.
There was a slightly modified version of the OPN2, the OPN2c (YM3438), but I can't recall the particulars of how it's different (other than it's a CMOS version), and don't feel like looking it up. I don't think it was used in any consoles, though it might've been used in the arcade. The OPN2 was not used in the arcade as best as I'm aware, with the lone exception of "Thunder Force AC", which is just a slightly-modded-for-arcade-play version of Thunder Force III, and as best as I can tell, is running entirely on Sega Genesis hardware, only in an arcade form factor.
There are several soundtracks I could share that use both FM and PSG (with the resulting sound being very similar to the OPNA (2608), only with less robust percussion and samples). The PSG in the Genesis IS inferior to the YM2419 built into the YM2608, but in the context of just supplementing a louder, stronger FM, the differences are so incredibly negligible, that it's usually quite difficult to discern.
To give you the purest taste of the YM2612, I felt my main soundtrack example should be from a game that does NOT use PSG or even DAC samples. This soundtrack I'm about to share, namely, Castlevania Bloodlines, is 100% FM cover to cover. In this application, the YM2612 will sound similar a YM2151 (OPM), but with fewer channels, ladder effect, etc. It will only come very close to measuring up...without quite making it. Still, considering this chip at least has the ability to sample, while the OPM doesn't, there's still at least a case to be made for YM2612 in a 2612 vs 2151 contest.
Afterwards, I will share at least one example of all the other Genesis modes (FM+DAC+PSG, FM+DAC-PSG, and FM-DAC+PSG) as well as sharing something from the FM Towns Marty that uses the chip.
Meanwhile, enjoy Castlevania! :-)
Now, for the opposite extreme, here are a couple soundtracks WITH PSG AND DAC. In this mode, the Genesis can sample, and has a total of 10 channels.
Streets of Rage 2 - composed by the legendary Yuzo Koshiro [SALUTE!]
Here's another to use FM + DAC + PSG (and one of my favorite soundtracks on the system) - Sword of Vermilion
And another - Ristar
And here's another - same thing: Had always assumed the percussion was at least partially sampled, but it's actually all FM. In fact, this whole thing is FM, there's no PSG either. This is the only partially completed soundtrack to the unreleased Genesis version of Time Trax - composed by Tim and/or Geoff Follin (in both NTSC 60Hz tempo, and PAL 50Hz - which was authorial intent). This one's AMAZING through and through!
Most of time when you're dealing with FM drums, it's really obvious - like this soundtrack (this one is also all FM, no DAC or PSG) - Burning Force
Or the Dick Tracy Soundtrack (which is FM and a splash of PSG, without DAC)
Or this track from Arnold Palmer Golf (which is also FM and PSG, without DAC)
Okay, now let's have a few that DO feature DAC, but DO NOT feature PSG. Let's start with Mortal Kombat (by Matt Furniss, a contemporary to the Follins, and they're all British).
Or how's about another Matt Furniss rendition: Wolfchild!
Lastly for DAC, but no PSG, and almost lastly for this blog post, here's the Puggsy soundtrack - also by Matt Furniss.
And lastly for Matt Furniss, and the Genesis - here's a track that uses no DAC or PSG (or even percussion of any kind), but is just this amazingly rich, harmonics saturated, haunting FM organ sound - Options screen from Bram Stoker's Dracula. This may be my favorite FM sound made on the Genesis. It's so big, and rich, and ambient...and smooth! Paradoxically, it's both frightening, and soothing all at once. I also like how you can still hear "hints of sine" in it. I wish someone would make a YouTube video of this extended for like three hours. I'd just sit and listen to it all. This particular video is playing this back on an emulator. On real hardware there aren't all the pops...but there is a fuzzy buzz. While I prefer real hardware sound to emulation overall, I'll take the pops in the emulation over the fuzzy buzzy of the real chip in this instance.
Okay. So no more Genesis, but a focus on the YM2612 would be incomplete without looking at at least one thing from the FM Towns/FM Towns Marty - the only other system the chip appeared in. Let's wrap up here tonight with Genocide!
Many FM Towns games used CD-Audio, and bypassed the sound chips completely. Others took advantage of off-board MIDI sound systems, also bypassing the built-in chips. This soundtrack is the only full soundtrack I've been able to find to use the system stock chips.
There are two chips at work here, a Ricoh RF56C68 ADPCM chip (VERY similar to the Ricoh RF5C124 in the Sega CD) - and then, of course, our YM2612. Actually, though it's not the same hardware at all, the pairing actually results in a very similar sound to the Neo Geo AES/MVS. VERY SIMILAR!
The Ricoh ADPCM is an absolute beast of a chip though, so it's no surprise it's in the driver's seat, with the OPN2 just kinda playing support. Still, you can clearly hear it almost the whole time, and it's the EXACT SAME hardware pushing FM as we've been hearing in the Genesis.
Here you go:
Ah heck, one more FM Towns - also a Genocide game: Genocide 2! Then we're done, I promise!