Phase distortion synthesis

Casio's implementation of PD used oscillators generated by modulator and carrier waveforms, synchronised to each other per-cycle.

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.

As well as being more capable of generating traditional linear spectra, the CZ synthesizers can also emulate resonant filter sweeps.

The window function can take various shapes, including sawtooth and triangle, thus determining the 'basal' spectrum upon which the resonant effect is superimposed.

Casio's own later engine named Interactive Phase Distortion (iPD), which featured in their VZ synths (VZ-1, VZ-10M, and VZ-8M; the first two also rebadged by Hohner as the HS-2 resp.

Additionally the VZ-10M oscillators offer 8 different waveforms, allowing more complex sound generation than the DX7 which uses only sine waves.

Figure 19 from the USPTO CZ-series patent application depicting how to eliminate the sudden jumps in the variable resonance circuitry (here showing the second harmonic coming into view.)