I heard in numerous places that Yamaha DX7 used Phase Modulation in spite of being advertised as an "FM synthesizer".
An opensource synthesizer called ZynAddSubFX has both PM and FM modulation modes available.
Both produce very similar results for sine wave modulators (hence no big deal with the DX7), but very different results for triangle wave ones.
FM produces a single gliding tone when modulating a sine with a triangle - you can see the triangle pattern on a spectrogram when the modulator is slow enough.
PM produces two tones of different pitch, and alternates between them. Spectrogram view looks like a square wave.
FM seems to read the absolute level of modulating waveform for modifying pitch of the carrier wave.
PM on the other hand seems to read the "slope" or the angle of the modulating waveform. A perfect analog square or pulse wave should produce no audible change at all, as it only contains neutral angles (when viewed from "above").
Native Instruments FM8 and Image-Line Sytrus both seem to perform what is called "PM" in ZynAddSubFX (even though FM8 has "FM" in it's name).
What is going on? What is the difference between Phase Modulation and Frequency Modulation? How do they relate?