[sdiy] VCO autotune for poly analog modular

[Jeff Farr]

I can't add much in technical terms here, but I've been thinking about
similar things myself lately for a XR2206 based sweep generator.  I
was thinking about using a MCU to control the frequency input to the
chip and the lookup table idea is how I figured an autotune feature is
best implemented.  However, I don't know the best way for the MCU to
measure the actual pitch of the oscillator.  The simplest way is
probably to feed the squarewave out from the osc to the MCU and count
the peaks, but IIRC I read something about using a reference
oscillator/phase cancellation and some sort of RMS measurement as
well.. sounds complicated..

On 4/3/07, mrmike <mrmike at clickbang.com> wrote:
> Yes, I'm insane.
> I'm trying to build a polyphonic analog modular based on five ASM-1s.
> Think Obie 4 voice, rather than Prophet-5, as a design style.  I would,
> however, like to be able to set an accurate single voice across all five
> modules. Basically, all five panels could have their own controls, but a
> switch on panel one will disable the front panel controls  on panels 2-5
> and slave them to the controls on panel one.  In this mode,  an autotune
> function would be useful.  Has anyone ever implemented one of these in a
> DIY polysynth?
>
> I was thinking of having the tune function compare VCO freq at a given
> CV voltage at several points across the 0-5V range for each VCO, then
> create a lookup table with a compensation map for each VCO.  As each CV
> for each voice comes in, its corresponding spot in the table is found
> and the tuning offset voltage is added to the initial CV and then sent
> to the VCO. Is this a dopey idea?
>
> Thoughts welcome!
>
> mrmike
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