[sdiy] polyphonic synthesis

[Paul Maddox]

Seb/Daniel,

> >     But, i understand that poly synths are far more complex.  Would i be
> > correct in saying that a poly synths needs  ( at least ) 1 VCO per note
of
> > polyphony?  I read that you would also need a VCF and a VCA for each
note,
> > but surely the synth could be simplified so as the individual oscillatos
wer
> > combined and then put through a single VCF and VCA?
>
> (I appologise for this blatently newbie post)
>
> I'm with ya.  I'd love to do a poly synth myself.  Unfortunately, my (lack
of)
> experience gets in the way.

Im currently building/designing a poly synth (its not easy)

Ok,  couple of things..
IDEALLY you would need seperate VCF/VCA for each...
think about it like this..

you hold down a low C, you filter sweeps (peeoooowwmmmm)... no you hit a
high note,
with ONE VCA/VCF for say 8 note poly, you get the filter sweep, but BOTH
notes, so its
almost like retriggering the bottom note..
If you're doing block chords its fine..

> Regardless, I still give it thought.  Here's what I think is needed for a
ten
> (hell, I gots ten fingers) voice polyphonic analog:

I would suggest 8 or 16note (micro's like number that are multiples of 8)

> A MIDI>CV converter with 10 output sets (at least CV and GATE), cascading
to an
> unused set with each note-on.  The closest I've found to this is the
midi2cv8
> kit from PAIA: http://www.paia.com/midi2cv.htm.  This kit only supports 4
> voices.  I suppose you could daisy chain 3 of these and mod the firmware
so
> only unplayed notes are kicked over to the next converter.

My Poly midi>CV supports 4 voices...
It would be easy enough to add more voices, 16,32,64 if you're a real
massochist.

> I really wouldn't want to skimp and pass the sum of all VCO's to a single
> filter and VCA set.  All modules would need to be built 10 times, with a
single
> control panel.  I believe this would be tough.  I need to learn how to
make one
> pot/switch control ten devices.  Relays come to mind for switches, but
pots...
> oh boy.

Its not too bad, design a  PCB, send it to PCB pool and order 10 :-)
dont use realys, noisy and high failure rate, use 4016's or FETs where you
can.
To control multiples you must make EVERYTHING CV controlled, this includes
things like
resonance pots (which are often attenuators, so you need an extra VCA)...
EG's also need to be voltage controlled, you also need to control the level
of the EG
(another VCA, unles syou do some clever stuff with the EG)..

> Patches could be passed through DIN jacks.

Hang on,
Ananlog synth, and then no knobs?
Link the cards via ribbon cable, makeing the odd numbers earth, this way
each CV has a ground either side of it.

> I can see tuning each voice to sound exactly the same becoming the biggest
> issue.  With all the talk on this list stressing the need to match
components,
> I'd imagine matching components across 10 identical modules would be a
> nightmare (understatement?).

yes, you would be in for a long session seeting it up..

>And what of thermal stability?  Even if matched
> well, the boards will still behave differently due to temperature
difference on
> each.

Why do you think peopl elike the warmth of old ananlog polys ?
the osc's are all slightly off..

> Basically, I'd like to do something like this before I die..  ( :

My advice (at the risk of being shot down for saying the 'D' word) would be
to use a digital oscillator.
These can be compact and easily made into polyphonic...
then you need only make seperate voice cards (VCF/VCA/EGs).
This gives you some REAL advantages (and now you see why analog polys died
quickly)
Everything can now be stored in memory (ok the all analog could too)
Everything is now ROCK SOLID as far as tuneing goes,
You only need to supply the voice boards with a dozen or so CVs
(A,D,S,R,Level, Cutoff, Q, Keyfollow, A,D,S,R, pan)
you can use one micro for the whole thing (midi/control panel/etc)
this means NO midi>CV convertors aswell...

If you want to take it even further, you could even do the EG's Digitally,
this is how the PPG does it
(Ok you knew Id get PPG in here somewhere), but many late80's poly synths
did it this way..

Paul Maddox
_______________________________________
Wavesynth home page;-
    Http://www.wavesynth.com
Modulus synthesizers home page;-
    Http://www.wavesynth.com/modulus