[sdiy] Digital accumulator VCO core?

Daniel Roberts danmroberts at gmail.com
Tue Feb 9 23:21:02 CET 2021


Along these lines, I'm curious if it's possible to digitally generate a
triangle wave at half the needed frequency via a DAC, but then double the
frequency in hardware via a full-wave rectification followed by DC-removal
frequency doubling circuit? Via chaining of frequency doublers,
could someone get away with generating digital signals at a relatively low
sample rate with such a scheme?

Dan

On Tue, Feb 9, 2021 at 4:36 AM Tom Wiltshire <tom at electricdruid.net> wrote:

> One thing I don’t understand about this approach is “Why?”. What’s the
> *advantage* if you’re going to try and keep all the parts that create
> errors?
>
> 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, and since triangle-core oscillators are common, there’s
> a lot of wave shaper designs you can tack on the end. But the exponential
> conversion was done inside the chip for obvious reasons. The ADC sample
> rate was fast enough for it to do convincing FM.
>
> The only synths I know of that runs oscillators at anything like 12MHz is
> the Novation Peak/Summit. The ‘Oxford’ oscillators in those instruments use
> an FPGA and a clever DAC technique to run at very high speeds and avoid the
> need for anti-aliasing measures. I’ve never seen it done anywhere else. The
> development of decent virtual analogs follows the development of good
> algorithms like BLEPs that enable you to produce good quality band limited
> ramps and  pulses *at normal sample rates*. The point of those algorithms
> is they work fine at 48KHz.
>
> Tom
>
> ==================
>        Electric Druid
> Synth & Stompbox DIY
> ==================
>
>
>
> > On 9 Feb 2021, at 07:46, cheater cheater <cheater00social at gmail.com>
> wrote:
> >
> > There's a couple reasons to measure current. First of all, this is
> > core only - so the exp etc happen in analog still. And because of
> > that, those electronics will act differently when fed into low
> > impedance and high impedance. Hence measuring current. The issue with
> > many virtual analog synths is that they think an oscillator is just a
> > shaped ramp, essentially just a very simplified accumulator capacitor,
> > and so they don't model all the electronics that happen around that
> > capacitor. That's a problem and if you want to replace /just/ the
> > accumulator with digital electronics then you should definitely also
> > include all the other stuff around the oscillator, since that is also
> > going to have its own character.
> >
> > When switching, what you actually want to do is to continue
> > accumulating in an aliased manner, but when you're outputting, you
> > instead limit the slew rate to something outside the audio range. This
> > way you can avoid aliasing. You can play back a band limited sample of
> > a switch, using a technique like MinBLEP etc. But at 12 MHz, who
> > cares, right? You just bandlimit the thing somewhere, and you're done
> > with it. (note: you might think you probably don't want to run the
> > synth engine at 12 MHz, but that's what most virtual analogs run at,
> > 192 kHz at 64x oversampling)
> >
> >
> > On Tue, Feb 9, 2021 at 6:03 AM Brian Willoughby <brianw at audiobanshee.com>
> wrote:
> >>
> >> I don't follow the need for a current stage.
> >>
> >> I have designed with DAC chips that output current, but I've never seen
> an ADC that directly measures current. It's worth noticing that CV is a
> voltage, and feeding a current into a shunt resistor will convert that
> current back into a voltage. You might as well just feed the CV directly to
> the ADC of your choice. In other words, skip the current stage altogether.
> >>
> >> You could even skip the ADC and just feed a digital value in via MIDI
> or some other digital communications channel, although I'm only suggesting
> that as an option in addition to a CV input.
> >>
> >> If you still want the bad tempco effect, then you can use another ADC
> channel to read that separately, and offset or scale the CV value
> proportional to the "temperature."
> >>
> >> You probably want to select a DSP instead of the general purpose CPU so
> the code can keep up with accumulating and generating internal samples at
> 12 MHz. Your accumulator would then be much larger than the 16-bit input,
> and thus the code wouldn't run out of resolution even at 12 MHz.
> >>
> >> What you describe is reminding me of the Roland GR-300 voice, although
> that is discrete logic rather than code accumulating the values. In fact,
> your idea could be implemented in discrete hardware, especially if the ADC
> has parallel data outputs.
> >>
> >> I didn't know what you meant by solid state switches until I realized
> that  you're talking about the reset at the end of a ramp core period. It's
> true that a DSP could accumulate like an op-amp and capacitor without the
> limitation of finite discharge time. One drawback, though, is that an
> instantaneous change in the accumulator value would create aliasing of
> infinite harmonics that would be folded back into the ramp wave - but using
> a wavetable would already have plenty of digital artifacts so perhaps this
> isn't a concern.
> >>
> >> Brian Willoughby
> >>
> >>
> >> On Feb 8, 2021, at 19:43, cheater cheater wrote:
> >>> Hi all,
> >>> I was wondering if anyone ever experimented with replacing the
> >>> accumulator core with a digital setup. Essentially you would have
> >>> control voltage turned into current being shunted to ground, and you'd
> >>> sample this current with the ADC, accumulating it in software. A
> >>> 16-bit, 12 MS/s ADC like the Cirrus Logic WM8196SCDS/V costs only ~$2
> >>> - $3 depending on how many you need, and my guess is that this would
> >>> be more than enough bit depth and sampling rate to preclude numerical
> >>> error. I think this would be able to do FM at audio rate. So does
> >>> anyone do this sort of thing?
> >>>
> >>> The nice thing is you don't need to use solid state switches any more,
> >>> your switchover is immediate, so no tracking error due to that. Also
> >>> perfect for complex wave shapes, you could have a VCO like that easily
> >>> output a waveform from a wavetable while still being fully capable of
> >>> FM.
> >>>
> >>> You might want to use a shunt resistor with bad tempco ... just to
> >>> keep things feeling "analog" ...
> >>
> >
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