Phase distortion synthesis is a synthesis method introduced by Casio in the 1980's in its CZ range of synths, and similar to Frequency modulation synthesis. Basically it plays a sine wave, but by playing different sections of it at different speeds, it distorts it.

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The base sine wave, depicted here as a triangle wave for clarity (phase distortion can be done on a triangle wave too).

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The same, but with 50% phase distortion into a saw wave.

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With 100% phase distortion, the waveform has turned into a saw wave.

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      *          *
     *            *
    *              *
   *                *
  *                  *
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Here is with 50% phase distortion, but into a square wave this time.

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*               * 
*               *
*               *
*               *
*               *
*               *
*               ****************

100% phase distortion into a square wave.

Other phase distortion patterns that are supported on the CZ range are impulse, half-sine and double impulse. The CZ synths also generate windowed and synchronised sine waves.

Casio made five different synthesizers using this method of phase distortion synthesis.

The VZ-1 's synthesis method ("Interactive phase distortion") is probably closer to Frequency modulation synthesis than to phase distortion.

Internally, phase distortion synthesis works by modulating the period of each level of the base sine wave (instead of the frequency or phase as in Frequency modulation). Care must be taken, since if the average period of the whole waveform changes, the oscillator will go out of tune. Casio possibly circumvented this problem by using two oscillators, and keeping the phase distorted oscillator in tune by synchronising it to the other silent oscillator. Even with these precautions, tuning problems may occur, especially if the amount of phase distortion changes quickly.