[sdiy] Formant filters, yay!

Tom Wiltshire tom at electricdruid.net
Thu Mar 16 13:47:11 CET 2017


On 13 Mar 2017, at 23:36, "Richie Burnett" <rburnett at richieburnett.co.uk> wrote:

>> I've done similar stuff with sine or triangles oscs (although you can try squares and ramps for fun, obviously). The key part is the "windowing"...
> 
> Tom, I remember you making a demo video of this running on a dsPIC a long time ago.  Weren't you trying to implement the "resonant low-pass" waveform from a casio CZ synth at the time?
> 
> From what I recall it's possible to mimic other filter types like high-pass and band-pass too using this technique, by messing about with the phase where the sinusoidal oscillation starts from when it is sync'ed by the lower frequency windowing function.

Indeed I was! I'd been reading the CZ patents and whatever else I could find, and started experimenting with these kind of "windowed sync" waveform experiments. And yes, you can get HP and BP sounds too, by changing the window IIRC.

The original thread is in the archive:

	http://synth-diy.org/pipermail/synth-diy/2011-July/033213.html

and the video I posted is still online, amazingly:

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

Incidentally, whether you sync the windowed waveform or not is one of the differences between the several flavours of this type of thing. Some places say you should, other places seem not to bother.

Since then I've also done a much more specifically vocal version, again on dsPIC, using five sets of windowed sine waves to generate a full vowel spectrum. The nice thing is that you can add together five sine oscillators, then window the sum, and you get five tuneable formants and one basic pitch.
It sounded vocal, ish, but I wasn't that impressed. It made a series of AEIOU-type sounds when you twiddled it, but seemed like a lot of work for that result. I've had pretty similar sounds out of a couple of resonant filters.

Tom





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