[sdiy] Generating a large number of CV outputs

Sun Dec 10 20:36:32 CET 2023

The carry-bit NCO technique is also equivalent to a 1-bit first order sigma
delta DAC.  Probably doesn't make it much easier to understand, but it does
make the design conclusions from sigma delta usable.  Ex. noise shaping
curves, required Over Sampling Rates vs ENOB, reconstruction filtering
(see: https://en.wikipedia.org/wiki/Delta-sigma_modulation).

To help myself visualize this, I plotted the output bit patterns for each
input level.  Both modulation algorithms (1-bit carry NCO and SD-1)
produced the same chart.

DC level is y-axis, time is x-axis.
[image: image.png]
Repeated 4 times to help imagine the oversampling:
[image: image.png]

For comparison basic PWM looks like:
[image: image.png]

By looking across a horizontal line, you can easily see the carry-bit
approach has a vast reduction in low frequency content compared to PWM.
That is, the digital signal 'noise' is being pushed into higher
frequencies.  However, there's still plenty of repeating pattern in the
NCO/SD-1

On Fri, Dec 8, 2023 at 1:43 PM Tom Wiltshire <tom at electricdruid.net> wrote:

> Honestly, it's not. It's just that damn simple. The version I actually
> wrote was all hardware module set-up specific to the device, and basically
> nothing else, so wouldn't tell you anything. Turning it all into software
> loses a lot of the benefit, so doesn't make much sense, except to explain
> it.
>
> Play with some NCOs a bit, and then get them to make a single pulse when
> they wrap. And then - Bingo! You're there!
>
> If you thought something was missing, what exactly? Ask a more specific
> question and perhaps I can help fill in a blank. I well remember how weird
> this stuff seemed when I first met it, so I think I understand the position
> you're in now.
>
>
> > On 8 Dec 2023, at 21:18, cheater cheater <cheater00social at gmail.com>
> wrote:
> >
> > I don't get it. Most of the algorithm is missing.
> >
> > On Fri, Dec 8, 2023 at 10:07 PM Tom Wiltshire <tom at electricdruid.net>
> wrote:
> >>
> >>
> >>
> >> On 8 Dec 2023, at 20:30, cheater cheater <cheater00social at gmail.com>
> wrote:
> >>
> >> On Fri, Dec 8, 2023 at 5:06 PM Tom Wiltshire <tom at electricdruid.net>
> wrote:
> >>
> >>
> >>
> >>
> >> On 8 Dec 2023, at 14:41, Matthew Skala via Synth-diy <
> synth-diy at synth-diy.org> wrote:
> >>
> >> If PDM means PWM with bit-reversal before the comparison (such as Richie
> >> describes), then it does indeed lock you into a lower sampling rate, and
> >> that's one reason I skipped describing *that* technique.  But PWM with
> >> bit-reversal seems not to be what you mean when you say PDM.
> >>
> >>
> >> That's not what I meant when I said PDM, certainly.
> >>
> >> The way I generated it is using an NCO. The NCO generates a single-shot
> output pulse everytime the phase accumulator wraps.
> >>
> >> Now consider what happens with a simple 8-bit NCO. If our frequency
> increment is 2, for example, we get a single output pulse every 128 clocks,
> or 2 pulses per 256 clocks. Notice that they will be nicely spaced apart,
> not next to each other like PWM. The output frequency would be (clock
> frequency / 128) in this situation.
> >> If the increment is 8, we get a output pulse every 32 clocks, 8 pulses
> per 256 clocks, and again, they're nicely spaced out. The output frequency
> is now up to (clock /32) so there's been a big improvement, just by getting
> away from those extreme values a little bit.
> >> As the increment climbs, the accumulator wraps more and more often. At
> freq=128, every other clock is an output and we reach our maximum output
> frequency of (clock/2). As the increment goes above half, we start staying
> high for more than a single pulse, and the waveform effectively turns the
> other way up and we get a mirror image of the effect we've seen from 0-128.
> >>
> >> HTH,
> >> Tom
> >>
> >>
> >> I've read this a few times but I'm struggling to understand what's
> >> going on. Can someone type out an algorithm or something like that?
> >> Would appreciate it a lot. Thanks.
> >>
> >>
> >> Ok, it'd look something like this:
> >>
> >> phase: 16-bit variable (for example)
> >> freq: 16-bit variable (same as phase)
> >>
> >> So we do:
> >>
> >> // Increment phase accumulator
> >> phase = phase + freq
> >>
> >> // Did phase wraparound?
> >> If (phase>65535) { // There are probably better ways to do this test,
> if it's even required.
> >> // Ok, phase wrapped
> >> phase = phase % 65536
> >> // output a pulse a single clock in length
> >> <this is implementation dependent!>
> >> }
> >>
> >> All you need to do is run this code at a fast clock rate and you're
> off. Of course, the better way is if you can hand some of this overhead to
> hardware, and some modern uPs include NCOs as a peripheral and can be set
> up for this "single shot pulse output" mode, which means that the while
> thing boils down to simply updating the "freq" frequency increment variable
> with your current output. It's dead simple.
> >>
> >>
> >> There's a page about NCOs in general on my website:
> >>
> >> https://electricdruid.net/direct-digital-synthesis/
> >>
> >> HTH,
> >> Tom
> >>
> >>
>
>
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