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Technically, any time you connect a modulation source such as an envelope or LFO (Low Frequency Oscillator) to the pitch control voltage input of an oscillator, the result is frequency modulation. Connect an LFO, and the result is vibrato.

However, frequency modulation more often refers to a synthesis technique where the pitch of an oscillator is varied (modulated) very quickly – at audio rates – by another oscillator. The result is a complex side of harmonics that may either be nicely in tune our clangorous and “out of tune” with the fundamental pitch of the main oscillator. This technique was pioneered by John Chowning at Stanford University and popularized in the 80s by the Yamaha DX-7 digital synthesizer.

This technique is a mainstay of “West Coast” synthesis. Several manufacturers sell complex oscillator modules that contain two oscillators already connected to do FM synthesis. As this technique creates a lot of harmonics, it’s common to start with relatively simple waveforms such as sine waves, although that’s not required.

Other things to look for in an oscillator to frequency modulate include a linear (rather than normal 1v/octave exponential) control voltage input, and the ability to go “through zero” (where an oscillator can go down to zero cycles per second and then back up again in pitch with an inverted waveform, in essence running backwards) – both of these more closely replicate the typical Chowning style of FM, although they are not requirements for experimenting with frequency modulation.

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