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

[Harry Bissell Jr]

> 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)
> 

> 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?


In the Prophet V... the scale and offset "pots" are
never touched.  Four (five?) voltages are output from
the DAC... and then a lookup table is created in RAM
for the "offset" needed to make the VCO conform to the
correct response.  Values in between these points are
interpolated linearly.

You need not use a PLL for frequency determination...
just use a counter in the micro to total the cycles
over a known gate time... (frequency counter). The
crystal
reference from the micro should be good enough.

No micro ? now THAT would be tough.

(BTW the P5s tuning is only accurate through the
digital
(DAC) driven interface... but it is in practice pretty
close to 1V/oct for external signals)

H^) harry