[sdiy] Digital accumulator VCO core?

Gordonjcp gordonjcp at gjcp.net
Wed Feb 10 09:43:24 CET 2021

On Tue, Feb 09, 2021 at 11:06:55AM -0700, Eric Brombaugh wrote:
> I did something like this back in early 2008 using a little Cypress PSoC.
> There's a full write-up of it here:
> http://ebrombaugh.studionebula.com/synth/dvco/index.html
> The basic concept was then converted to dsPIC and became the basis of
> several digital VCOs for Modcan and Synthesis Technology, then was further
> ported to STM32 for a few more designs.
> The key notion is doing the expo conversion in software using bit shifts and
> lookup tables. With this approach it's possible to have excellent tracking
> across 10 octaves with only a 12-bit ADC. Pitch resolution with 12-bits is
> about 3 cents which is slightly better than the commonly agreed human
> sensitivity of about 5 cents. A better ADC and higher resolution expo table
> would allow even better resolution.

I found that with the 10-bit ADC on the atmega328 I could get 6 cents steps which is noticeable if it "jitters" between steps or if you sweep incredibly slowly.  If you drive it with a sequencer then even the slowest practicable portamento that actually allows you to arrive at your desired note before the end of the beat doesn't show it up.  Hell, even with just semitone steps a 303 sequencer driving it makes it slide without really audible stepping.


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