[martinmunk]

I've had the "pleasure" of trying to transmit data / timecodes through active speakers with a lot of signal processing.

I don't imagine any QAM-like stuff working. FSK does, but the transitions get a little wonky, and the different frequencies will often have different phase shifts. ASK modulation should work, but haven't gone there yet.

But its interesting. I know a thing or two (literary) about encoding in the radio spectrum, but none of the encodings I've found deals with systems that are so un-lineary.

[brian-armstrong]

In my experience, the non-linearity is not the issue. It's the multipath that gets you with these things. Your signal echoes off of everything. I was able to get QAM working with Quiet[1] just fine but only at a few centimeters. FSK is much more reasonable. If you're having issues with intersymbol interference it's probably still multipath getting you. I'd say slow the symbol rate down to avoid it. I've had very good success with FSK with Quiet.

1: https://quiet.github.io/quiet-js