I just had a pretty good idea. An inverted sawtooth is just a sine wave with additional sine waves being out of phase by a certain amount (essentially it's supposed to be subtracted). I'm not sure how good an emulator VGM Music Maker is, but it has an option to turn each operator on individually at a certain time, causing the phase shift I want. Trying it out seems to work, I can create a pretty good inverted sawtooth with algorithm 7. With a custom driver, I could make it even better.

You could invert just one of the stereo channels, I think.

Doing that is Useless in your case, you will end up inverting the whole chip output

If you still want to do that, depending on what hardware you are on, if it is a Sega Genesis you can use the PSG to output an inverted Saw wave and it will phase with the Feedback operator...

Yep, I actually tried that exact thing. Inverting a wave seems like something the hardware would have built-in, which is why I asked here first. Either way, it'll be relatively easy to construct some hardware to do that outside the chip. Thanks, though.

AKAIK you can do an inverse saw (ramp) in some way, by using the SSG envelope (4) Inverted loop, when the envelope goes short you end up with a ramp, assuming AR is 31... the problem is that it will make another tune than your main note and the volume will also be quieter (especially on real hardware)... it is up to you to try to find a setting that can make an acceptable phasing I doubt that you ends with something useful but you can have one note with for example:
ALGO 7  OP1 Feedback 7 and lower volume and OP4 with SSG (4) inverted loop.
You will hear what you search for when you reach the point that the key you play is almost in tune with the period of the SSG loop..

BUUUT... you can do exactly the same phasing thing with the PSG and this will be far more easy an controllable

I don't think there is. There's definitely no straight forward way to do it.

Sawtooth + same sawtooth inverted and phase shifted = rectangle wave. Detune the inverted wave by a little, and you get a phasing duty cycle.

What's the difference?

After messing around a bit, I found using just operator 1 with feedback of 4, I can create a very good sawtooth wave. Is it possible in some way to invert that wave before playback? Right now, the wave starts at max volume, and then goes to zero before rising sharply again. I want it to start at zero, and go to max volume.