Phase Modulation (PM) is the shy brother of Frequency Modulation. The concept of FM is pretty simple: modulating the frequency of an oscillator (known as the ‘carrier’) according to another incoming signal (the ‘modulator’). The amplitude of the modulator is used to determine the carrier’s frequency.
The concept of Phase Modulation is a bit more complex, but its result is very similar to Frequency Modulation. The reason a lot of synthesizers that are known to use FM, actually use Phase Modulation, in order to avoid complications. Yamaha’s DX7 is an example of this. Here we will try to reveal some of concepts behind Phase Modulation.
Phase Modulation results in Frequency Modulation of the carrier, caused by the ballistic behavior of the modulator, Meaning, its speed and direction of movement. Confusing? Lets go for a drive.
Imagine a hypothetical car with a hypothetical speedometer that has negative speed measurement when you drive in reverse. If you drive forward and backwards at the same speed (lets say 10kmph) periodically you would be driving in a way a triangle signal is generated (see illustration). But if you look at your speedometer, you will notice it acts like a square wave toggling between 10kmph (when driving forward) and -10kmph (when driving backward). The shape of the car driving would be similar to the way pitch is being modulated in FM.
The shape of the speedometer movement would be similar to the way in which pitch is being modulated in PM. It is as if a speedometer is connected between the modulator and the carrier. So if we phase modulate a signal with a triangle oscillator we will get the similar result to frequency modulation by a square. On the same principle, modulating with a sine oscillator will result in a triangle like modulation.
PM has several advantages over FM. Its sound is a bit more subtle and easy on the ears, it is a lot easier to calculate, it enables self modulation without the need for feedback, and it enables modulation of any audio source and not just oscillators!
FleXor’s Phase Modulation effects are excellent for applying FM-esque effects on any audio source you wish, from sample oscillators, to guitars and drums. The phase modulation can also be used to create chorus and phaser effects when used with LFOs.