[sdiy] Casio CZ resonant waveforms / Windowed Sync

[Tom Wiltshire]

Hi All,

I've been messing with using the windowed sync technique that Casio used for 'resonant waveforms' on the CZ synthesizers:

http://www.electricdruid.net/images/cz/czresonantwaves.mov

(6.5Mb mov)

The technique is described on the CZ wiki page, but the Deep Synthesis site has much more detail:

http://en.wikipedia.org/wiki/Casio_CZ_synthesizers
http://homepage.mac.com/synth_seal/html/cz1.html

In short, it's a simple digital hard sync effect, but multiplied by a 'window waveform' to avoid the alias-causing discontinuity. In this case, the window is a simple downwards ramp, so that the waveform tapers to zero where the discontinuity would have appeared.

What shape window you use changes the sound considerably since you're doing a multiplication in the time domain(ring mod) which is equivalent to a convolution in the frequency domain. In English, roughly, the harmonics of the window shape get multiplied by the harmonics of the waveshape.

It's very simple to do, and gives a pretty good result. What you're hearing is 16-bit 48KHz, with an analog lowpass after it at about 1KHz or so (I repurposed some LFO hardware I'd designed). There's some distortion from my speakers being too loud for the computers mic, and the video is back-to-front, but neither of those bugs are the algorithm's fault! You could easily double the sample rate and give yourself another alias-free octave, and for the sine-based waveform, that would be enough.

Hope you like it.

Regards,
Tom