Chapter Four: Synthesis
6. Principles of Audio-rate Frequency Modulation
This article explains the phenomenon of audio-rate frequency modulation of sound, which was explored and used compositionally by John Chowning of Stanford University around 1970. His discoveries eventually lead to the design and release of the Yamaha DX-7 family of instruments, one of the most successful synthesizers of all time.
Sub-audio-rate frequency modulation
If the output of an oscillator is applied to control the frequency of another oscillator, frequency modulation (FM) will result. The oscillator providing the control source is referred to as the modulator, the oscillator providing the signal is referred to as the carrier. If the modulating oscillator is tuned below audio-rate (or approximately 20 Hz), sub-audio frequency modulation or vibrato will result. As the modulating waveform rises (increases in amplitude), so too will the frequency of the carrier; as it falls, so too will the frequency of the carrier. The rate of the vibrato is determined by the modulator's frequency, the depth of the vibrato (or how far above and below its center frequency the carrier will be pushed) is determined by the modulator's amplitude, and the shape of the vibrato is determined by the modulator's waveform. It is important at the outset of the discussion to realize that the modulator is not part of the signal path--it is never heard directly, only its effect on the carrier frequency.
A schematic of a basic frequency modulation patch using the same symbols from Module 4 is illustrated below: