[sdiy] Controller design

[Thomas Burdick]

The reason for using VCOs instead of fixed-frequency oscillators is flexibility. The layout of the instrument is usually as follows: 10 buttons control 4 sets of reeds. Each set is tuned diatonically like a harmonica, with different notes on push and pull, and some overlap, as you guessed. The usual tuning is for two sets of reeds to be slightly detuned, another to be an octave down, and another an octave up; sometimes the same-ocatave pair are tuned a third off from one another, and there are other tunings as well. The bellows pressure not only controls volume, but also bends notes (and alters the color of the tone). Finally, because it's a diatonic tuning, you can play in two or three keys with a single instrument, but like a harmonica, you'll need one tuned to a different key to play anything else.
So, I'd like to reproduce the peculiarities of the traditional instrument (diatonic tuning, bent notes, bellows pressure affecting tone), but also to get more flexibility from the electronics than you can from metal reeds. If I use a linear CV, I can change keys by adding an offset voltage to the CV. And VCOs would let me adjust the amount of detuning between the two primary sets of reeds with a simple control, rather than retuning 20 different fixed oscillators.
I'm pretty happy with the VCO I have at the moment, which produces two triangle outputs offset by an octave. This means I can get four voices on four sets of reeds with 8 reasonable VCOs, which seems tenable. Especially compared to 80 fixed oscillators.
As for Gordon's question of why not use digital oscillators: that's a reasonable question. I'm comfortable with analog circuitry, and it's fun to work on. I'm less comfortable with microcontrollers, but I'll consider digitally generating the triangle core. That might make everything a little simpler.

> Date: Tue, 29 Dec 2015 13:02:01 -0600
> From: mskala at ansuz.sooke.bc.ca
> To: gordonjcp at gjcp.net
> Subject: Re: [sdiy] Controller design
> CC: synth-diy at dropmix.xs4all.nl
> 
> On Tue, 29 Dec 2015, Gordonjcp wrote:
> > Any good reason not to just generate the oscillators digitally?  VCOs suck.
> 
> Or, for that matter, why not have a separate fixed-frequency analog
> oscillator per note?  Tuning 20 fixed-frequency analog oscillators is
> probably easier than 4 VCOs, the control system would be much simpler, and
> it'd be unlimited polyphony.
> 
> -- 
> Matthew Skala
> mskala at ansuz.sooke.bc.ca                 People before principles.
> http://ansuz.sooke.bc.ca/
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