> phase is just the derivative of frequency, and the derivative is a linear operator (essentially a 6dB/oct EQ). FM (if strong enough to create negative freque

> phase is just the derivative of frequency, and the derivative is a linear operator (essentially a 6dB/oct EQ). FM (if strong enough to create negative frequencies) and PM are equivalent when you're modulating a signal with a sine wave. As soon as you chain modulators, they're no longer mathematically equivalent or capable of producing the same sounds, since frequency-modulating a signal by a complex wave is different from phase-modulating a signal by the same complex wave (which can produce sharp corners given a bounded input with sharp corners).