[sdiy] VCDO using PIC16C771

[Johannes Öberg]

Am I missing something here; I always thought most modern VCO's were
this stable (32 steps / semitone ~ 1,5 cents) at least for the
duration of a song or so. So a regular autotune seems much more useful
if the precision isn't greater than this.

And the PIC ADC isn't very accurate or fast IIRC, and as it uses a
ramp converter it would also be affected by temperature.

Am I way off?

/Johannes

On 2/22/07, jean-pierre.desrochers at ville.quebec.qc.ca
<jean-pierre.desrochers at ville.quebec.qc.ca> wrote:
>
> I think the concept is not enough clarified here..
>
> Here is the trick:
> CV voltage 0-5VDC going to the 12 bits Pic ADC,
> The Pic reads the AD value and goes to a fixed serial EEPROM
> lookup table at that AD address to fetch a 16bits 'period value'.
> The XR2206 freq (square wave) is sent back to one of the PIC I/O pin.
> If the XR2206 freq is too low or too high according to the lookup data
> (at the specific CV input) the Pic send a PWM square wave
> filtered with a low pass to compensate the drift and bring
> back the CV/freq desired to the XR2206.
> Of course the low pass filter is fast enough to
> not create any portamento effect but enough eficient
> to have a nice DC feedback voltage.
> The serial EEPROM is a 4K X 16bits so it gives
> 32 steps between each semitones.
> 32steps X 12semitones X 8octaves = 3072 words (16bits)
>
> J-P Desrochers
> Quebec City
>