What is PD synthesis?


Phase distortion synthesis (also known as PD) is a form of synthesis originally developed by Casio company. They presented it for the first time in their CZ series of synthesizers. In the 1980`s Casio`s engineer Mark Fukuda invented this form of audio synthesis, which evolved from Cosmo Synth System. Pursuing this further, Cosmo Synth System was custom developed for Japanese synthesist-composer Isao Tomita who is regarded as one of the pioneers of electronic music. In order to make CZ line of synthesizers less expensive, Casio used digital synthesis without filter instead of analog based subtractive synthesis with a filter.


Phase Distortion application


The sound that came out from it was “ thinner “ comparing to the more expensive analog synthesizers. Phase distortion technique initially tried to simulate analog filters. It had in total eight different wave-forms. As well as the standard saw-tooth, square, and pulse wave-forms, it had a special double sine waveform, a half-sine waveform, and three wave-forms with simulated filter resonance: resonant saw-tooth, triangle, and trapezoidal wave-forms. 

Each digital oscillator could have one or two wave-forms. Unlike other synthesizers, where having multiple wave-forms cause those multiple wave-forms to mix together (parallel), the CZ synthesizers plays one waveform and then plays the other, and so on in alternation (series). This could cause the appearance of a sub-harmonic one octave below the nominal pitch of the sound, due to the period of the combined waveform taking twice as long as a single waveform would. It was possible to combine two non-resonant wave-forms together, and to combine a resonant waveform with a non-resonant waveform. However, it was not possible to combine two resonant wave-forms.


How does PD synthesis work?


Internally, phase distortion synthesis works by reading a sine wave table stored in memory. The most significant bits of an initial linear frequency counter is under envelope control transformed into a secondary phase angle signal. This signal reads the sine wave. Depending on the wiring, the transform will change with a possible wide variety of wave-shapes.

Phase distortion synthesis is a subset of the Phase modulation synthesis. in addition to this, PM synthesis is the usual method of Frequency modulation synthesis within hardware synthesizers. Those two forms of audio synthesis are similar but their implementation and the result is not equal. At phase modulation an oscillator modulator can have its own period. Phase distortion applies an angular modulator of straight line segments hard synchronized to the same period as its corresponding carrier. Additionally it modulates each cycle identically. 


Interactive phase distortion


After CZ series of synthesizers in Casio`s VZ-1 synthesizer a variation of PD synthesis implemented and named “ Interactive phase distortion “. This is an improved version of PD. It incorporated FM synthesis. Furthermore, it also had multiple oscillators which were combined in various configurable routing with possibility to modulate each other using phase modulation or ring modulation.  


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