CZ like Phase Distortion Synthesis

The CZ synthesizers of the 80s used digital oscillators which implemented a unique form of waveform synthesis (called « Phase distortion »).

The phase distortion (« PD ») technique consists of reading a sine wave at different speeds during its period, hence distorting the original wave. Depending on the way the reading speed is changed, different « target waves » are obtained (such as saw, square or pulse like waveforms). By increasing the amount of speed changes, the wave goes from sine (when the amount is 0) to the target wave (saw, square, etc.). The change in timbre resembles the effect of a low pass filter applied to the target wave. Another method consisting in putting sine waves inside a « window » is used with some target waveforms, to mimic the effect of a resonant filter.

This technique is purely digital and so the « color » is somewhat different from the « analog subtractive » method.

You can learn more in this Wikipedia article