[sdiy] Digital accumulator VCO core?

Brian Willoughby brianw at audiobanshee.com
Wed Feb 10 00:29:55 CET 2021


I'm not so sure it's "opposite" - more like a rather slight variation.

You're suggesting that the output of an ADC - a digital value - control a digital accumulator. That really sounds exactly like a DCO to me. Of course, there are many kinds of DCO.

I think you have a great idea for something to implement. I'm just suggesting that there's no need to pretend that a digital-controlled oscillator core is the opposite of a DCO. The internal ramp is 100% in control of the frequency in your design, and thus the pitch is digitally controlled. At least an analog integrator and comparator would have a totally analog period length, but your design completely avoids that part of the analog control because of your goal of eliminating the pitch error due to the fixed time required to discharge (or pre-charge) a physical capacitor in the analog domain.

Brian


On Feb 9, 2021, at 08:48, cheater cheater wrote:
> Yeah, a DCO is basically the exact opposite of what I'm proposing here.
> 
> On Tue, Feb 9, 2021 at 4:56 PM David Moylan via Synth-diy wrote:
>> 
>> Of course there's also the in-betweener Roland style DCO where the
>> digital side provides a voltage to control the charge rate of a saw core
>> and sync pulse to reset it.
>> 
>> On 2/9/21 10:32 AM, Richie Burnett wrote:
>>>> There was someone here some years ago who’d done a voltage-controlled
>>>> oscillator on a dsPIC. The cunning part of that was that the output
>>>> waveform was a triangle instead of a ramp. That massively reduces the
>>>> aliasing problems...
>>> 
>>> Neat!  I wonder if you could take that process all the way and have
>>> the NCO output a sinewave (to remove all aliasing.) Obviously you
>>> could easily use analogue electronics to derive a square wave from
>>> that with a comparator, but not sure how you could shape it into a
>>> triangle or sawtooth waveform!?
>>> 
>>> I guess it's all irrelevant really though as BLIT, BLEP, minBLEP,
>>> poly-BLEP etc, all work very well for directly generating the common
>>> analogue oscillator waveforms at 48kHz or 96kHz sample rates.
>>> 
>>> -Richie,
>>> 
>> 
>> --
>> David Moylan






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