[sdiy] How DCOs work

[Tom Wiltshire]

On 10 Oct 2016, at 15:52, Colin f <colin at colinfraser.com> wrote:

> 
> It misses the fundamental limitation of counter-based DCOs compared to VCOs,
> which is that frequency modulation of a VCO affects the shape of the ramp
> directly, and the period of the oscillator derives purely from the behaviour
> of the ramp.
> Frequency modulation of a DCO applies to the period only, as it can only be
> changed by re-loading the counter reset value.


Agree. Which is interesting because the DeepMind12 *does* implement audio-rate FM on the DCOs (or some of them, at least). I think it's probably only done with the LFO at the fastest rates, but given that you can't update a 100Hz bass waveform any quicker than 10 msec, I don't see really how they've managed it - if they really have. Until I get to play with one, I won't know for sure - it could be that the FM just breaks if you do it on lower frequencies.

Once upon a time, the fundamental limitation of DCOs would have been the discrete frequency steps, but with high frequency clocks and 32-bit counters, that's no longer an issue.

Still, it can't be impossible to update the charging CV and the counter value during a cycle these days. You'd have to work out what the required count should be given the new final count, but modern processors can do that quickly enough. So maybe the problem isn't insoluble.

Tom