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

Tom Wiltshire tom at electricdruid.net
Mon Apr 11 23:59:42 CEST 2016


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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