[sdiy] Digital accumulator VCO core?

Brian Willoughby brianw at audiobanshee.com
Fri Feb 12 06:07:02 CET 2021

On Feb 10, 2021, at 00:26, 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.
> it's opposite to me because that's how i look at it :-) but maybe you
> view it differently

Sometimes, I find it more useful to think in terms of "discrete" versus "continuous" rather than simply trying to decide what is "digital" versus "analog."

A DCO has discrete frequencies because of the master crystal and counter dividers that are used to reset the ramp oscillator.

Your idea is an interesting one, and I think you should build it, but your "analog" parts are still discrete. One reason is that the "analog control" gets quantized by the ADC, and then you're stuck with discrete values. You can use 12-bit ADC for 4096 discrete steps, or you can use 16-bit ADC for 65536 discrete steps, but it's still a discrete control. Don't even try 24-bit ADC because those chips cannot hold a steady value due to the tricks used to "fake" 24-bit (I'm exaggerating a bit).

I would point out that the digital accumulator frequency is also discrete, but that's not really saying much since it's obviously digital. However, if you let a pure analog ramp VCO run through an ADC, then the frequency would not be discrete even though the waveform has been converted to digital.

Brian Willoughby

p.s. While you're experimenting with this digital accumulator (I can't wait to hear what results you get), I'm toying  around with ways to get continuous frequency control out of digital oscillators. I want to do this without using software BLEP and other tricks that consume a lot of processing power. That's because I want to get frequency out of the discrete realm and into the continuous realm. I don't know that it will *sound* better, but that doesn't stop me from wanting to try...

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