Primarily to ensure backwards compatibility with the 8-bit Mastersystem, the Sega Genesis, in addition to the six-channel YM2612 FM chip that it brought to the party, still contained the Texas Instruments SN76489 PSG (3 channel + 1 noise channel). The Genesis was able to tap into the PSG for its music, for a total of 10 channels, and often did. You can tell that the PSG is being used by that tell-tale sing-songy square wave tone.....or can you?
As it turns out, the FM chip, while it can't make an actual square wave (sine wave only), it can do an amazing job of "faking" a square wave sound with a 2-op, 2:1 ratio voice (resulting in two full sine curves in the positive, followed by two full sine curves in the negative, ultimately making a very "square-like" pattern). The result is a VERY convincing "square" tone.
Today, I'll share with you a few tracks that exclusively use the "FM fake" for square tones, followed, for contrast, by a few that exclusively use real PSG square waves. We'll also talk about why a composer would want to waste complex FM channels for basic PSG-like tones, when there's a real PSG "right down the hall" that they could use instead (turns out there actually are a few contexts where there is good reason for doing this)
We'll start out with perhaps the most incredible example of FM fake square, the continue theme from TMNT Tournament Fighters. This track sounds like it's made up almost entirely of PSG, when in fact, there is ABSOLUTELY NO PSG in it at all. There isn't even any DAC sampling on the FM 6 channel. Nope, this is 100% FM:
One reason for using an FM fake is if you want "stereo square" since the actual PSG chip is mono, the TMNT track is a great example of stereo square.
Another reason for wanting to use the fake is for deep pitches (since the SN76489 cannot pitch very deep at all.) For an example of the FM fake being used for this purpose, here's a track from Contra: Hard Corps.
There's not a lot of "square" going on here. But at the 0:44 mark, you have this big square that starts really deep, and slowly slides up to a high pitch, only to turn around and slide back down to the low one (much lower than the actual PSG can go). That square is fake. It's FM (another hint is how it has to cut out once or twice to accommodate some other FM operation):
PSG is much fainter in the mix (especially on real hardware, where it's much softer than in emulation.) Even on tracks with PSG square leads, the lead sounds kinda soft (as we'll hear next in a few tracks with real PSG lead). So, if you wanted a square sound that's in your face, it's FM for you.
Here's an example of dominant FM "square" (though the square tones are a little less perfect on this one) - The Pit - Mortal Kombat:
Here's another example of fake squares cutting to the front (though these squares are a little more obviously fake than the others.) However, while the "squares" are fake, the sine wave chords (the cheery, happy "doot doot doot doot" sounds) are REAL sine waves - made by the FM chip itself. This is "Woody Land 1 and 2" from Mr. Nutz:
But the trade off is they're just slightly less "sing songy" than the real thing, as you will hear.
I have three tracks for you now that use prominent PSG square (the real thing). You will notice the square tone is a little more sing-song, but you will also notice it is much softer, even in emulated tracks (though I will endeavor to find examples running on real hardware)
The first is "In the Bar" (Stage 1-2) from Streets of Rage 2 (real hardware, I believe):
This is all well and good, but to really get an idea of how close the FM fake can sound to the real PSG (and yet, how different it sounds) perhaps we should hear the two in the same context, so I have for you both the Arcade CPS1 version - YM2151(OPM) FM /MSM6258 PCM (with FM fake square lead), and the Genesis version - emulated (with real PSG lead) of Chun Li's theme from Street Fighter II.
Here's the Arcade version:
And here's the Genesis version:
Now, I've made a lot of noise about how PSG is fainter on real hardware than it is in emulation. Why this is, I honestly don't know. I would speculate that there was some filter in real hardware that was not emulated in, but that's just bald conjecture on my part. If anyone out there can either confirm my hypothesis, or provide us the real reason instead, I'm all ears.
But I wanted to demonstrate for you the difference in sound (most obvious in the PSG levels). I'll be sharing two videos of "King Turtle" from Jewelmaster, the first one emulated, and the second one on real hardware.
The PSG in this track is understated, even in emulation (and almost imperceptible on real hardware). It's the simple droning notes that appear during the "chorus" (starting at 0:04). Even in emulation, they're not dominant, but they're still very obvious.
Here's the emulated version:
Emulated version first:
And real hardware:
Now, I'm almost done, but there's one more curveball on this FM fake square vs PSG real square thing, and that's this: What if they're using BOTH?
This first track uses both, but uses them differently. The melody voice is an FM fake square, while the PSG is performing a very faint three-part harmony on the melody voice (I LOVE this track, by the way.)
Federal Patrol Office - Star Cruiser (JP Mega Drive only - real hardware):
...they have PSG and FM square doing double melody. The same notes...at the same time (no delay effect). PSG does add harmony off and on, but the melody voice is BOTH PSG AND FM AT THE SAME TIME!
I don't know why they'd do that. The two voices are similar enough that it would't change the tone much, especially considering the FM would be louder and all but completely drown out the PSG voice anyway. So I'm kinda stumped here. I suppose it does create a little bit of a phasy character since the overtones of both sounds are slightly different, and slightly clash in a pleasing, intriguing way, so maybe that's what they were going for?
But whatever the reason that's what all three of these tracks do. They're all from Sonic 2. We'll start with Mystic Cave Zone (2 player):
Mystic Cave Zone (1 player):
And, Oil Ocean Zone. I you listen REALLY closely at 0:07-0:13, buried deep in the mix is a mid-range plain sine wave (like we heard in three part chords on the Mr. Nutz track - only this is an individual sine). At that moment, it's also only the PSG making square since the FM channel that was making the square took a break for the sine wave and another voice.
Finally, here's Whirlwind from Shinobi III. I think this might be emulated, but if so, it's a really good emulator. This does the same as the Sonic games. There's an FM/PSG double melody, with multiple PSG channels all teaming up to make the PSG more luscious and rich sounding. I presume once again that the addition of the FM voice is to help bring the lead to the forefront a little better.
Okay! So that's real PSG square and FM fake "square" on the Sega Genesis, and while we were at it, it's also emulation vs real hardware as it relates to PSG. Hopefully 19 tracks (including this last one) is enough to paint a fairly full picture.
Let me know if I missed anything, or if I got anything wrong. Otherwise, I hope that opened your eyes, and ears to this little interesting dynamic in the Genesis' sound. I'll leave you with a great chip tune called "Pixel Protector" made by Jredd (aka Trevin Hugues), which uses real PSG for the melody, and an FM fake square for the bass):
Jredd - Pixel Protector - Soundcloud