Phase Distortion Synthesis was pioneered by Casio in the 1980’s and is based on the idea of distorting the playback phase of a sine wave.
Phase distortion synthesizers like the Casio CZ-1000 are characterized by a very clean sound, similar in some cases to the Yamaha DX-7. The Casio’s 8-stage envelopes for Pitch, Wave shape, and Amplitude were superior to most synthesizers at the time. The main drawback of the Casio was a lack of velocity sensitive keys.
Of course with SynthEdit, you can use the Velocity output of the MIDI to CV module to improve the playability of the Casio sound. Vince Clark (of Erasure) used a rack full of Casio-CZ’s to achieve a “fat” sound. Now you can too!
How the Casio PD synthesizers worked.
Casio’s method of Phase Distortion used oscillators which generated modulator and carrier waveforms, which were synchronized to each other.
The modulators were various wave-shapes that could ‘distort’ the carrier’s sine wave into other shapes, to a degree derived from the Digitally Controlled Wave (DCW) envelope. In doing so, many harmonics were created in the output.
As the modulators were rich in harmonic content, they could create spectra more linearly, i.e. like traditional subtractive spectra, rather than emulating Yamaha’s phase modulation (PM/FM) synthesis.
Yamaha style PM does not require oscillator sync but is usually limited to sine waves, which meant that the output audio spectrum bears the non-linear structure of Bessel functions.
PD is a different type of PM – whose different modulation system caused significant difference in operation and sound between PD and PM. Thus the two methods are significantly different.
The phase transforms were assembled from piecewise linear functions under binary logic control and show the characteristic sharp knees (and for some transforms, even sudden jumps) as they move from minimum to maximum, where the frequency counter’s accumulator wraps around and starts over. The sharp knees are smoothed by the roundness of the modulated sine wave and not too noticeable in the resulting audio signal.
Recreating Phase Distortion synthesis with SynthEdit.
SynthEdit has a Phase Distortion Oscillator module that can be used to recreate that classic sound.
The Modulation Depth input controls the amount of distortion, with Zero Volts giving a pure sine wave and 10 Volts giving a fully distorted wave.
The example below shows the effect of progressively increasing the amount of phase distortion when Wave 1 and Wave 2 are both sine waves. Wave 1 is the main (carrier) oscillator and Wave 2 is the modulating oscillator for the Phase distortion.
There are 8 different wave-shapes to choose from, and you can select different combinations of the two waves-shapes.
The Phase Distortion Oscillator module is used slightly differently from the standard voltage controlled oscillator.
The wave-shapes available are: (with the addition of “none” for Wave 2)
The diagram below shows a typical Subtractive Synthesiser voice containing an oscillator, VCF and VCA sections. The Filter and Amplifier sections are each modulated with a envelope generator (ADSR) , one to control the Volume, and one to control the filter cut-off frequency.
Here is how you can achieve a similar result using the Phase Distortion Oscillator module, with one ADSR modulating the Phase Distortion level, and the second the Audio output level.
There is (usually) no VCF section, instead the ADSR that would have modulated a filter is connected to the Phase Distortion Oscillator’s Modulation Depth input plug. This gives a sound very somewhat similar to traditional subtractive synthesis, but without the need for a filter. Of course if you want to add a filter into the mix there’s nothing stopping you (Hybrid Synthesis).
A simple Phase Distortion Synth constructed in SynthEdit.
The basic synthesizer is quite a simple arrangement of a PD Oscillator, two ADSR modules and a VCA, but it can be made as complex as you like with more oscillators, (plus filters if you wish), and a noise source. As with most structures, you can add to them and make them as complex as your imagination and ingenuity will permit.
