What I want is simple: I want to modulate the volume (amplitude) of one channel, with the amplitude of another channel.
There's the vocoder effect which does something vaguely similar to what I'm looking... but it splits the audio in bands to (obviously) create the vocoder effect.
A sidechain compression/gating won't do it either.
So how can I do this?
If you have Max4Live (included in Live9 Suite), it should be easy enough to build a Live device that does the amplitude modulation exactly the way you want.
Otherwise, look for Ring Modulator and Amplitude Modulation VST plugins. The easy way to write a Ringmod plugin is to just do amplitude modulation. So the free plugins are likely to do what you want. Actually modeling an analog ring modulator takes more effort, but maybe that is the sound you are really looking for.
If you happen to have Absynth, I'm pretty sure you can accomplish AM or ring modulation on arbitrary incoming signals with that.
