Simulating A Resonant Filter
Figure 19 from the 1985 CZ-series patent shows how to emulate the variable resonance found in analogue voltage-controlled filters:
- (a) The base frequency counter, wrapping around every period.
- (b) The resonance frequency counter at a slightly higher frequency, being reset (or "synced") when the base counter wraps around.
- (c) The resonance frequency counter used as a sine wave readout. Note the nasty sudden jump at the reset!
- (d) The inverted base frequency counter.
- (e) Multiplying c by d. The sudden jump in c is now leveled out.
To summarize in other terms, the filter implementation is a form of digital hard sync, but adds a triangular descending volume envelope running within the sync period. Because the volume always ends at zero, the discontinuities that would cause aliasing in native digital hard sync are avoided. Filter sweep effects are generated in the same way that hard sync effects are generated: by modulating the frequency of the resonance waveform, the timbre changes, adding and subtracting harmonics as desired.
Casio made five different synthesizers using this method of phase distortion synthesis.
The VZ-1's synthesis method ("Interactive phase distortion") includes Frequency modulation synthesis as well as an improved version of phase distortion.
Read more about this topic: Phase Distortion Synthesis
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