Phase distortion synthesis

Phase distortion (PD) synthesis is a synthesis method introduced in 1984 by Casio in its CZ range of synthesizers. In outline, it is similar to phase modulation synthesis as championed by Yamaha Music Corporation (under the name of frequency modulation), in the sense that both methods dynamically change the harmonic content of a carrier waveform by influence of another waveform (modulator) in the time domain. However, the application and results of the two methods are quite distinct.

Casio made five different synthesizers using their original concept of PD synthesis (with variations). The later VZ-1 and co's synthesis method Interactive phase distortion is much more similar to the aforementioned phase modulation, rather than a direct evolution of phase distortion; see below.

Generating harmonic content

Casio's implementation of PD used oscillators generated by modulator and carrier waveforms, synchronised to each other per-cycle. The modulators were various angular waves that could 'distort' the carrier's sine into other shapes, to a degree derived from the "DCW" envelope. In doing so, many harmonics were created in the output. As modulators were rich in harmonic content, they could create spectra more linear, i.e. more similar to traditional subtractive spectra, than Yamaha's phase modulation (PM/FM) synthesis. PM does not require oscillator sync but was for a long time limited to sine waves, which meant output spectra bore the non-linear hallmark of Bessel functions. PD is a different type of PM - whose very different modulators caused significant difference in operation and sound between PD and PM. Thus the two aren't directly equivalent.

Distortion synthesis

Distortion synthesis is a group of sound synthesis techniques which modify existing sounds to produce more complex sounds (or timbres), usually by using non-linear circuits or mathematics.

While some synthesis methods achieve sonic complexity by using many oscillators, distortion methods create a frequency spectrum which has many more components than oscillators.

Jean-Claude Risset was one notable pioneer in the adoption of distortion methods.

Some distortion techniques are: FM synthesis, waveshaping synthesis, and discrete summation formulas.

FM synthesis

Frequency modulation synthesis distorts the carrier frequency of an oscillator by modulating it with another signal. The distortion can be controlled by means of a modulation index.

The method known as phase distortion synthesis is similar to FM.

Waveshaping synthesis

Waveshaping synthesis changes an original waveform by responding to its amplitude in a non-linear fashion. It can generate a bandwidth-limited spectrum, and can be continuously controlled with an index.

Phase distortion

In signal processing, phase distortion or phase-frequency distortion is distortion, that is, change in the shape of the waveworm, that occurs when (a) a filter's phase response is not linear over the frequency range of interest, that is, the phase shift introduced by a circuit or device is not directly proportional to frequency, or (b) the zero-frequency intercept of the phase-frequency characteristic is not 0 or an integral multiple of 2π radians.

Audibility of phase distortion

Grossly changed phase relationships, without changing amplitudes, can be audible but the degree of audibility of the type of phase shifts expected from typical sound systems remains debated.

See also

References

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).