[sdiy] How DCOs work

[Jacob Watters]

Thanks for sharing the video. A lot of it was just review, but there was
some new bits of info too.

Some if you might be interested in an alternative that I tested -
http://www.jacobwatters.com/blog/a-different-kind-of-dco/

It is a way of possibly getting the best of both worlds. I haven't tested
it with the Arduino yet, but it is still on my list of things to do.


Jacob Watters

On Wednesday, 12 October 2016, Mattias Rickardsson <mr at analogue.org> wrote:

> On 10 October 2016 at 16:52, Colin f <colin at colinfraser.com <javascript:;>>
> wrote:
> >> From:  Tom Wiltshire
> >
> >> In the video, he argues that the ramp core part is
> >> merely another wave shaper, since it would stop without the counter, and
> >> that would stop without the master clock, so there's only really one
> oscillator
> >> in the synth - the master clock Which is true in a way, but kind of
> means
> >> there's no such thing as a "DCO", since you've just defined them away.
> >> Instead, you've got an oscillator followed by digitally-controlled
> analog wave
> >> shapers.
> >
> > Another way to look at it is that the counters are oscillators.
> > Instead of charging a capacitor with voltage, they charge a 16-bit
> register
> > with numeric increments.
> > Instead of a voltage comparator resetting the timing cap, a numerical
> > comparator resets the counter register.
> > To say that the DCOs are not oscillators because they would stop
> oscillating
> > if the master clock was turned off is like saying VCOs are not
> oscillators
> > because they wouldn't run without the power supply.
> > In reality you have to consider the circuit as a whole.
> >
> >> But I'm nitpicking. It's a considered and knowledgable discussion of the
> >> technology, which is a rare thing with DCOs!!
>
> I agree! Thanks for an interesting video giving an interesting
> discussion and we realize that it's not that easy to define things in
> the DCO realm. If we really need to define them...? :-)
>
> After some thoughts I think I'd define a DCO in a somewhat
> non-technical way, but in a way that corresponds to our view of what a
> VCO is and what we mean by the very word "oscillator" in synthesizers:
>
> Most of us would probably agree that a typical "VCO" comprises 1) an
> oscillator core and 2) waveshapers. Right? We don't usually nitpick
> about the fact that the core is the true oscillator and that the
> waveshapers are just something else that needs to be fed with a signal
> from an oscillator. The "oscillator" in a synthesizer is the thing
> that makes the "sound" that is later filtered and modified and in the
> end sent to the outputs - this is a sloppy definition that somehow
> works even for us technical boffins.
>
> So I'd define "DCO" in a similar fashion as the thing that makes the
> sound in the synth. Hence a "DCO" is 1) the ramp core part and 2)
> waveshapers and 3) the timing pulse mechanism that resets the ramp
> core. To say that the rampcore is just another waveshaper because it
> doesn't oscillate by itself might be true on a technical level and is
> interestingly thought-provoking, but it's a quite useless definition
> since the process of "oscillating by itself" is quite uninteresting in
> a DCO. The clock pulse feeding the ramp core - or the single master
> clock feeding the dividers, for that matter - is not a little
> fascinating contraption that oscillates by itself, it's just a clock.
> Wiggling back and forth is what clocks do, and that doesn't make them
> sole candidates for being called the synth's oscillator. Nobody would
> argue that the oscillator in a soft synth is the computer's CPU
> crystal. :-)
>
> > Give me VCOs any day :)
>
> I've learned to live with DCOs as well, but it took quite some time. ;-)
>
> On 10 October 2016 at 17:03, Tom Wiltshire <tom at electricdruid.net
> <javascript:;>> wrote:
> > On 10 Oct 2016, at 15:52, Colin f <colin at colinfraser.com <javascript:;>>
> 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.
>
> Do we really need to assume that the DCO counter is modulated at all?
> How about FM-ing just the charging CV to the ramp core, and let it
> reset at its frequency like normal?
>
> /mr
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