[sdiy] Digital accumulator VCO core?

cheater cheater cheater00social at gmail.com
Wed Feb 10 10:29:26 CET 2021

On Wed, Feb 10, 2021 at 9:41 AM Gordonjcp <gordonjcp at gjcp.net> wrote:
> On Wed, Feb 10, 2021 at 09:26:30AM +0100, cheater cheater wrote:
> > DCO: a digital expo controls a digital circuit. circuit sends a reset
> > signal to an analog accumulator, which is fed constant current. that
> > analog accumulator is then shaped in analog.
> > my idea: an analog expo controls an analog current. that current is
> > variable. the current feeds a digital accumulator. the accumulator has
> > a digital comparator.
> >
> > simplified:
> > DCO: digital control of analog accumulator
> > my idea: analog control of digital accumulator.
> That sounds like you're creating the worst of both worlds.  The analogue expo won't be in tune, ever.

Perfect! That's exactly what I want.

> When you say "digital accumulator" and "digital comparator", do you mean something like
>         phase = phase + theta
>         saw = phase
>         square = (phase > 50%)
> for theta being the "step" size between samples that you'd work out from sizeof(phase)*frequency/samplerate?
> Because that is going to alias like hell.

no, again, we're just replacing the capacitor with a digital circuit.
The ADC samples the current, the microcontroller adds the sample value
to an internal variable. that is /all/. No smart stuff.

> Given that the expo just turns a relatively sane 1V/oct control voltage into a V/Hz control voltage - or really μA/Hz control current - you're using a difficult-to-make analogue circuit to make your control input harder to cope with.

Perfect - it's beautiful

> If you look at how a practical DCO like the Juno 106 works, an expo lookup table in ROM drives a DAC, and another expo lookup table in ROM drives a digital counter, and you've got a squarewave off the counter that is used to reset the integrator which charges at a rate governed by the DAC.  In ROM you've got semitone steps and to get in between you use linear interpolation which gives just a few Hz error at the very top of the scale.

completely different idea to what I posted in the OP

> I'm not saying don't do it, I'm just saying that the analogue bit is a difficult bit that doesn't bring anything to the party.

It brings a lot of nice crappiness which I like

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