[sdiy] Additive/Subtractive synthesis?

[Neil Johnson]

Ishaan Dalal wrote:
> Actually, I was thinking of using the UAF42, and one of the application
> notes describes a way to digitally tune Q/ctr_freq, but uses 2 DACs.

Yes, but IIRC the DACs are in the signal path.  It'd be better to build a
simple OTA-based state-variable filter and generate the CVs from DACs.
Add enough filtering and you should be able to avoid zipper noise as the
DACs change.

> The way this is done is using a CMOS counter with divide by 2, 4, 8
> outputs, all three going to a 4:1 mux. Depending on the position of the
> key, the mux connects the relevant output to the VCA/F, effectively
> increasing the range to 3 octaves. Haven't worked out how to
> simultaneously tune the LP (or BP) filter.

Well, if your base signals are digital, whu not make a very simple
digital low-pass FIR filter with a shift register and some resistors?
Then you could modify the cut-off frequency by varying the clock
frequency.

> What I was thinking was, say, a master saw source at 220 Hz, going to
> the VCF of each key, which BP'd the appropriate Fourier multiple,
> probably amplifying it too, and each key sending this to the output
> mixer.

But that would only give you octaves, not the semitones (if I understand
you correctly...possibly not :-)  Also you'd end up with lots of high-Q
bandpass filters, and if they wandered off centre due to temperature
drift, etc, your notes would fade.

> > But SSM2164's are cool... I happen to know a guy who sells them in small
> > quantities :-)
>
> Actually, Analog Devices has SSM2164s in active production, at least the
> DIP/SOIC versions.

I know... been to my website yet?  Look in the "shop" section....

> If only I could use (and knew how to program :-) a DSP!

...You'd never get your project done in time!!!!

> Actually this is supposed to be a "Digital Logic Design" project, but I've
> managed to keep the instructor placated with stuff like PCM sampling
> (drums/cymbals and a 30-second sequencer). Wouldn't want to make it "too
> analog", at least I can claim the one oscillator/note as a "digital tone
> generator" :-)

Why not then implement a top-octave divider (ask Tim Ressel about this :-)
and generate all the tones in parallel.  There's quite a nice project to
keep you busy for a few weeks, especially if you investigate the spectral
purity and frequency accuracy of each tone.

Neil

--
Neil Johnson :: Computer Laboratory :: University of Cambridge ::
http://www.njohnson.co.uk          http://www.cl.cam.ac.uk/~nej22
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