[sdiy] Additive/Subtractive synthesis?

[Neil Johnson]

Hi,

> I'm making a basic one-octave polyphonic analog synth for a university final
> project.

Cool!!

> My quandary is whether to use additive/subtractive synthesis.  For
> additive, I'd probably use a square or saw oscillator for each (one of
> eight) note, which would go through it's own op-amp bandpass and then
> mix into the final output.

That is not what the general definition of additive synthesis is.  What
you have described is all-note polyphony, where each note/key has its own
oscillator/filter/envelope circuit.  The upside is you can play as many
notes as you have keys.  The downside is you have _LOTS_ of circuitry,
which pushes up the cost.  And tuning is a bugger, as you'll have to tune
each note individually.  But, perhaps in your case since you are
restricted to one octave, that might not be such an issue.

Indeed, one can quite easily design a _very_ simple voice card (one osc,
simpler LP filter, simple AD envelopes).

> 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.

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

If you only had one oscillator, you would only be able to play one note at
a time.  Not polyphonic then, but monophonic.

> The problem is, how would I then make a VCF for each note with discrete
> op-amps?

But since you now only have one oscillator, you don't have "each note" to
worry about, so you can put more effort into making the sound more
complex, and interesting.

> [Note: The constraints do not allow me to use any of the CEM chips,
> though an SSM2164 VCA, or LM13700 OTA has been approved by the
> instructor].

Using SSM or CEM chips for a new design is bad, no matter what, since they
are no longer made, and remaining units should be kept for restoring old
synths.

But SSM2164's are cool... I happen to know a guy who sells them in small
quantities :-)

> Switched cap filters are an option, but again, they would require one
> oscillator per note, chalking up to the same as additive synth.

Again, this is _not_ additive synthesis.

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.

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.

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.

Cheers,
Neil

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