[sdiy] Poly analog synths needing multiple signal paths?

[Olivier Gillet]

> I would think that you could have multiple oscillators, each getting its own CV, feed them into a mixer, then from there to filtering etc.

This can be done, this is called paraphony. There are a few
occurrences of synths like that - like the Poly-800, it's fun and
quirky, but the classic polys are not done like that, no way.

Let's say that you have all 8 oscillators sent into a mixer and a
single VCF/VCA, with a short decaying envelope with a low sustain
modulating the VCF and the VCA.

You play and hold a C. The envelope is triggered. And you hear a C
with a ptzong filter sweep, and then it sustains at a low level.
Now you play a G, one second later. Either:
a/ The envelope does not retrigger and you won't hear anything.
b/ The envelope retriggers and you hear the C+G with the filter sweep.
This is like the C has been retriggerd.

If you just want to play house stabs, not a big deal. If you do
anything more complex you have to deal with the fact that every new
note will retrigger whatever is still lingering/sustaining. If you
want notes to coexist in both a "decaying" and a "freshly hit" state,
you need one VCF/VCA per voice.

Another problem is filter key-tracking. If you play a low note and a
high note at the same time, you probably want the two of them to go
through filters with different cutoff values so that they have a
relatively homogenous timbre.

Olivier





On Tue, Oct 8, 2013 at 8:10 PM, Paul Anderson <wackyvorlon at me.com> wrote:
> I've read that polyphonic analog synths need a complete, separate signal path for each note of polyphony. I've never entirely understood why. I would think that you could have multiple oscillators, each getting its own CV, feed them into a mixer, then from there to filtering etc.  Any thoughts are greatly appreciated!
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