Since I'm a guitar player who uses pitch to MIDI in order to trigger synthesizer modules and uses guitar into HRM based modeling synthesizers, I would have to say that the guitar itself is the most expressive modulator out there.
The Guitar does FM (note selection) modulation, velocity dynamic modulation (how hard the note is picked), pitch modulation (string bends) and envelope generator (note duration control) all initiated through the guitar playing technique itself.
Also in the case of HRM Tech or the old Roland gr 300 analog synth or routing the input of a guitar signal through a synthesizer module filter, the guitar string serves as the oscillator.
So I guess I'm saying that a human is the most important modulation source. Wait what's the question again?
