[sdiy] How DCOs work

[Tom Wiltshire]

On 12 Oct 2016, at 05:16, Andrew Simper <andy at cytomic.com> wrote:

> On 12 October 2016 at 09:24, David G Dixon <dixon at mail.ubc.ca> wrote:
>> 
>> It seems to me (and, bear in mind that I know nothing about DCOs) that one could use a standard VCO circuit to control the current to an integrator, and use a digital circuit for the reset.  In this way, the only "tracking error" would be a slight loss of amplitude at high frequencies, and FM would still distort the slope.
>> 
>> What am I missing?
>> 
> 
> You could actually use this to generate some cool "sync" type sounds
> with a "single" oscillator, apply a voltage to the pitch of the VCO
> for FM and then it will re-sync at the correct pitch :)

There was(is) a mono synth that actually did this - Fredrik Carlqvist's Minod Vorga:

http://www.minod.com/products/minodvorga/vorga.html

There may be others, I don't know. If I was building a DCO synth, I'd certainly include it.

It's very simple to do, and I once experimented with the idea on the breadboard. You can set the ramp core up with a comparator that resets the integrator like normal, and also feed in a reset pulse form the counter. Setting the control current correctly gives you the normal ramp wave, reset by the counter reset pulse. Pushing the control current higher forces additional resets from the comparator and gives you sync sounds.

Tom