[sdiy] Additive/Subtractive synthesis?

[Ishaan Dalal]

> > This seems the simplest on paper, since changing the Q/center freq of a
> > state-variable filter on the fly looks like a giant pain.
>
> Not if using OTAs.

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.
Ultimately, however, it is analog voltage controlled, so I might be able to
fix 'em somehow. My circuit can (in theory) actually produce 3 octaves, with
a one note per octave polyphony limitation (i.e., you can only play C in one
of the 3 octaves at once). 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.

> > On the other hand, subtractive would certainly be more compact, allowing
> > me to dispense with the individual oscillator for each note.

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 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. They also have a 4-quadrant quad analog multiplier
(MLT04), which I believe would serve just as well....what do you say?

> Additive synthesis is where each note is constructed from two or more
> simple (in)harmonic components (e.g. sinewaves) from separate oscillators.
> A recent example being the Kawai K5000 synthesizer (or its older cousin,
> the K5), which constructs each note from upto 128 digitally-generated
> sinewaves.

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

> Subtractive synthesis is where you start with a harmonically rich waveform
> (e.g. sawtooth) and filter out (subtract) some harmonic components to
> modify the sound.

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" :-)

> Anyway, terminology aside, it sounds like you have a fun project ahead.
> If I were you, I'd go for a simple one-oscillator-per-pnote synth to keep
> the circuitry simple, and so you don't need to bother with keyboard
> scanning or MIDI processing.

Precisely, a "piano" clone with a rudimentary ADSR and a somewhat variable
VCF is more than enough work for something due in two weeks time! :-)

Cheers, and thanks for the advice,
Ishaan