[sdiy] Thoughts on a Polyphonic Synth

[Richard Wentk]

At 14:52 24/03/2006, Benjamín Velasco wrote:
>Hi list!!!
>
>After looking the amazing midi projects at 
><http://www.ucapps.de>www.ucapps.de i was wondering how difficult it would 
>to build for example a 8 note poly synth. With the MIDI to CV from this 
>page you could have 8 independent for each oscillator. Then you would need 
>perhaps 8 EG and 8 VCA for each note's volume? What other module you i 
>have to repeat? Maybe 8 EG for VCF??

Doing envelopes and LFOs in hardware is nuts. By the time you finish you'll 
have a card you can't afford to build and which won't work reliably for 
long enough to be useful.

It's much smarter to offload envelopes and LFOs to the processor, and let 
that do all the work. That also means you can include features like a 
key->env scaling, or key-> LFO rate scaling.

You will want one LFO per voice to get a rich vibrato. You'll also want 
velocity and AT control of modulation sources and destinations which is 
another reason to use software.

So basically each voice card is 2/3 VCOs, selector/volume hardware for the 
waveform mixing, routing for sync and FM, a noise source, a VCF and VCA. 
Control voltages are derived from the usual multiplexed bank of sample and 
holds driven from a DAC.

You'll need to multiplex the voltages onto a 12 or 16 bit data bus with 
addressing for each module/parameter.

Conceptually it's pretty simple. Building it is a little harder. ;-)

Richard