A variation on phase modulation, pioneered by Casio in the 1980s. Phase distortion is essentially a specific case of phase modulation, in which the modulation waveform is oscillator-sync'ed to the carrier. The carrier is always a sine wave, but the modulation waveforms consist of a number of harmonic-rich waveforms, such as sawtooth waves, which run at a somewhat higher frequency than the carrier. The oscillator sync technique has the advantage of minimizing the creation of inharmonic overtones, making it easier to comprehend for sound designers and performers who are accustomed to traditional subtractive synthesis methods. Mathematical techniques help eliminate the discontinuities in the waveform that is normally caused by oscillator sync, so that the results do not have the buzzy oscillator-sync sound (unless the performer wants them to). For Casio, the phase distortion technique also had the advantage of not infringing on Yamaha's Chowning FM patents.
Casio first introduced the phase distortion synthesis method on the Cosmo synthesizer prototype that it built for Isao Tomita in the early 1980s. Its first commercial product to use phase distortion was the CZ-101 synth, introduced in 1984.