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

[Jacob Watters]

I think the pt2399 works in a similar way. Try checking its datasheet to
point you in the right direction.

On Monday, 11 April 2016, Eric Brombaugh <ebrombaugh1 at cox.net> wrote:

> Tom,
>
> Looks like you're on the same path as Batz Goodfortune:
>
> http://www.all-electric.com/b&c.html
>
> I fooled around with that circuit many years ago - even updated it with a
> CPLD for the DRAM address generator.
>
> The main trouble is that it's only a 1st-order sigma-delta so it's not
> going to have super great performance. I suppose you could try building a
> higher-order sigma-delta, but those get pretty complicated and then you can
> start having stability issues. Performance of the 1st-order circuit will
> improve somewhat as you raise the oversampling ratio, but that's expensive
> since it requires longer shift registers for the same delay.
>
> You might have better luck using an SACD style codec instead of a
> home-made S/D. SACD codecs output the raw bitstream that hasn't been
> converted to PCM and incorporate higher-order converters.
>
> Eric
>
> On 04/11/2016 02:59 PM, Tom Wiltshire 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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