[sdiy] How DCOs work

[rsdio at audiobanshee.com]

What you're missing is that the integrator slope would change to the new frequency instantly, but not the reset (until the next period). There would be at least one cycle where the slope was too high or too low, resulting in loss of amplitude (when modulating to a higher frequency) or clipping (when modulating to a lower frequency). If the modulation is constantly changing, as it would be with FM, the slope would never be correct for the period. As you noted, that would really only be a loss of amplitude when modulating to higher frequencies, but it could be a large loss of amplitude if the modulation depth is large. But modulation can also lower the pitch of the DCO and the slope would exceed the expected range, clipping the audio circuits.

Brian


On Oct 11, 2016, at 6:24 PM, 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?
> 
> From: Synth-diy On Behalf Of Adam Inglis
> Sent: Tuesday, October 11, 2016 5:15 PM
> Subject: Re: [sdiy] How DCOs work
> 
> On 11 Oct 2016, at 12:52 AM, Colin f <colin at colinfraser.com> wrote:
>> straight ramp for every cycle, where a VCO would have a convex or concave
>> ramp shape dependent on the direction of pitch modulation.
> 
> This is really interesting. I’d never thought about it like that - so frequency modulation distorts the waveform in a VCO but not in a DCO. 
> What is the audible difference?