SNES SPC700 negative values for Main volume and Echo volume?

What exactly does a negative value mean in regards to the sound itself? As in technically, what does a negative value mean. The best I can describe it is that when you activate negative values, or mix negative and positive values you get a distinctly hollow sound. Distinctly SNES sound.

I am wondering what is technically happening to the instruments here, so that I can try to recreate the effect in a standard DAW with real instruments. Or some variation of it. I assume there is some form of phase cancellation happening, but I can't really tell what the hell I am hearing from an audio stand point. Depending on how extreme the sound is, it can flat out sound like the noise is coming from behind you.

From just a casual glance, it looks like it is more often used with the echo effect, and not so much with the main volume of a channel, but, with the C700 VST you can experiment with it in the extreme and get very wild hollow surround sounds.

I just wonder what the hell is going on under the hood. Anyone have any insight on this?

Negative volume just amplifies the signal with a negative multiplier symbol, the signal has inverted polarity. This is perfectly inaudible when used on both sides. However when volume is negative on one side and positive on the other, it create this strange (and sometimes annoying) effect. The sound can also cancel itself when mixed to a mono speaker, which is not a rare situation on old TVs.

Capcom uses this in the echo volume, for all their games, systematically.

By polarity are you just referring to an inversion of the phase for the output? Is that all it is?

Yes. Negative volume produces phase inversion. If you invert the right channel and not the left or vice versa, a matrix surround decoder (such as Dolby Pro Logic) will send it to the rear speakers.

Not only is a differential phase decoded by ProLogic as the surround channel, but it's also one of the signals that your wetware can't localize. (Hence why it sounds "hollow", "can't really tell what the hell I am hearing" and "sound like the noise is coming from behind you").

Your brain relies on the wavefronts arriving at your ears at subtly different times and in the same phase, so when you get a signal that can't correspond to a physical effect... you get the above.