[sdiy] 1-bit ADC for audio/audio delay

Mikael Kopimi Altemark maltemark at gmail.com
Wed Apr 13 08:39:41 CEST 2016


Isn't this the strategy used by Deltalab for their classic Effectron and
Super TimeLine delays? Their sound is a favorite of mine. Maybe there are
some interesting documents to find if you look for clues as to how they
implemented their solutions.

On Mon, Apr 11, 2016 at 11:59 PM, Tom Wiltshire <tom at electricdruid.net>
wrote:

> Hi All,
>
> I've been experimenting with the following circuit that I found online:
>
>         http://www.electricdruid.net/images/DeltaSigmaADC.png
>
> Original article is:
>
>
> http://electronicdesign.com/analog/low-cost-audio-delay-line-uses-1-bit-adc
>
> Everyone likes a simple audio delay, right? And you can't get much simpler
> for a digital delay than that, so I thought it'd be worth playing with.
> Note that the image in the original article has an error and takes the
> feedback from the wrong place, after the comparator instead of after the
> latch. My image is corrected.
>
> Now, the circuit works well enough, but it doesn't work as well as I'd
> expected. For now, I've got no shift register. I'm just doing the ADC, then
> feeding the bits out again so I can compare input and output signals for
> quality. I've done lots of experiments with PWM audio output, and I've got
> better results than I'm getting out of this. And this *should* work better
> than PWM, since the "PDM" output it produces should have less jitter than a
> PWM output would.
>
> In practice, it's quiet with no signal going in (what would be the worst
> case for PWM - the midpoint voltage) but when it gets a signal, there's a
> substantial amount of background white(ish) noise. Now, I understand that
> it's never going to be hi-fi (that's not the point) but I don't understand
> why I should get more noise with a signal than without.
>
> I've also been reading around Delta-Sigma convertors in general (of which
> this is a basic example) and I've noticed that this circuit arranges the
> typical components (comparator, integrator, differential amp) in a
> different order than many.
>
> Would swapping the passive RC filter for a active 2-pole filter improve
> things? Would a op-amp integrator stage help? (it'd be more linear at the
> extremes for starters..) These experiments are my probable next steps. And
> how would I go about converting such a Delta-Sigma modulator into a
> second-order modulator in the style of:
>
> http://www.beis.de/Elektronik/DeltaSigma/DeltaSigma.html
>
> (This is one of the examples that uses the same bits, but in a different
> order).
>
> Any pointers to further information appreciated, or clues as to what I
> might be doing wrong.
>
> Thanks,
> Tom
>
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-- 
Allt gott / Best wishes,
Mikael Altemark
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