[sdiy] poly?

[Rainer Buchty]

> hello everyone, I feel a little dumb asking this, but I never really
> understood how to make a synth polyphonic. what are some different
> methods?

I'd say there are some "fake" methods and only one real method.

To go polyphonic, you replicate the entire voice. So if you consider a
voice consisting of 3 oscs, 2 EGs, 2 LFOs, 4 VCAs and a VCF, then you need
to have a set of these modules for every voice and end up with an insanely
huge (and expensive, when done in hardware) system. That's the reason why
the polyphony of analog synths is usually somewhere between 6 and 16
voices (Korg produced cost-reduced but fully polyphonic synths some time
ago, see JH's excellent pages on his PS3200 clone for more information)
and with a rather limited modulation matrix compared to modular systems;
you will also find at least one microprocessor in polyphonic synths which
performs some of the modulation (LFOs, EGs, routing) in software to reduce
hardware complexity.

Also, you would need a special keyboard circuitry which assigns the
pressed keys to one of the sets. Something which you want to accomplish
using a microprocessor/controller and not with discrete parts.

Now for the fake solutions:

The cheapest way to turn a monophonic synth into some sort of polyphonic
machine is the arpeggiator. Works nicely with chords, but is rather
unusable for true polyphonic play.

With the Poly800 (IIRC) Korg came up with a similar idea: Instead of
replicating the entire voice, only the oscillator was replicated. So you
were able to hit a chord but VCF/VCA were shared. Also not really suitable
for polyphonic play.

HTH,
	Rainer