FM and PM synthesis - what is the difference and why?

Question

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.

The difference?

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?

Answer 1

frequency modulation (FM) is the manipulation of the Hz of a sound or "Operator". Operators generate sound waves at a certain speed (Hz) which dictates the pitch of that specific sound, for example, bass sounds usually have a range of around 20-250Hz whereas leads and synths have ranges between 500-15000Hz. By using Operators the sound is able to shift and modulate in regards to the various speeds of each sound that is generated resulting in a tone or sound being played at different wave lengths and frequencies. In theory, FM synthesis is effective for creating incredibly complex sounds based off of the rapidly changing frequencies of each operator, it is complex but it is easier to make intense and diverse sounds using FM rather than Subtractive Synthesis.

Phase Modulation is another way of saying FM (Which was coined by Yamaha in the early 1980s). If you want to get the science of phase modulation however it is effectively a method used to cancel out certain frequencies by playing 2 or more sounds at the same frequency range for a very short period of time, creating a shifting sound. By making these tones hit at a certain time it causes the sound to cancel out or PHASE and result in the very noticeable gliding sound that can be heard for example when a plane takes off.

Phase Modulation isn't really a type of synthesis as it is pretty much another way of saying Frequency Modulation. Both of these techniques do very similar things, however, Phase Modulation is used more in the generation of Operator tones and Phasing effects.

Hope this clears everything up for you

Answer 2

They are the same in terms of output. The difference in terminology likely comes from the fact that digital oscillators are implemented by incrementing a variable representing phase into a lookup table. The step of each increment is calculated from the intended frequency: phase += freq*modulation_index*table_size/sample_Rate with "freq" being the modulation signal plus the carrier frequency. The other terms can be treated as some constant, but that multiplication has to take place at some point. Since the thing being modulated in a digital oscillator, sometimes called a phase accumulator, is phase pointer, that term might have made more sense, but really it's the phase increment, and change in phase is frequency.

Answer 3

Late to the party, but here is a graph showing the difference.

They do sound quite different, i.e.: DX7 (pm) & Synergy (FM)