auto-tune, was Re: [sdiy] Design solutions for analog polyphony and/or banks ofVCOs ?

[john mahoney]

----- Original Message ----- 
From: "jbv"

> Last but not least, tuning a bank of VCOs is crucial.
> Is there any auto-tune schemo / procedure available somewhere ?
> I recall reading somewhere that some poly from the 80's (Seqential I
guess)
> featured an auto-tune circuitry...

JB,
The Oberheim OB-X has auto-tune, and I assume the OB-Xa and OB-8 do, as
well. So you could study those schematics.

Aside from that, here are some of my thoughts. This is mere brainstorming on
my part.

Auto-tune requires:
-- a reference oscillator (crystal-controlled DCO?)
-- a reference pitch CV that *should* drive the VCO to the ref. freq.
-- a way to select a certain waveform of one VCO at a time
-- a way to compare the selected VCO to the reference osc
-- a means to adjust a trimpot for each oscillator
-- muting of the audio output (set the VCAs to zero output, for example)

For the tuning trimpots, I would use digital trimpots:
http://www.maxim-ic.com/DigitalPotentiometers.cfm
An increment/decrement interface would be the simplest, although the Maxim
digipots with inc/dec are available only in audio taper. I think that linear
taper is preferable, but the inc/dec is easier to deal with. Some of their
pots have non-volatile memory, but this probably isn't needed if you will be
running the auto-tune proc each time the synth is powered up.

The tuning procedure for each VCO would be something like this:
-- select the VCO
-- reset VCO's tuning trimpot to minimum
-- drive the VCO with the reference CV
-- compare VCO to reference
-- if VCO freq is less than the reference ** , increment the trimpot
-- loop back to the comparison until pitch is good
-- if the trimpot has reached maximum, the VCO is out of spec (optional:
disable that voice)

** I have no idea how this is done. Maybe with a PLL?

I hope this helps!
--
john