[sdiy] Digital audio demo on Youtube

rsdio at sounds.wa.com rsdio at sounds.wa.com
Fri Feb 21 10:43:53 CET 2014


On Feb 18, 2014, at 11:58, Richie Burnett wrote:
> Sinc interpolation, or a digital filter that closely models it, is  
> the alternative. That's what most audio DACs from the last decade  
> do internally.

Actually, nearly all audio DACs from the last decade - and certainly  
all 24-bit audio DACs - are sigma-delta converters which literally  
cannot hold a constant value. So, it's not that they've solved the  
ZOH problem, but rather that they've created an ultrasonic  
oscillation problem in its place.

If you have an example of a recent audio DAC that isn't sigma-delta,  
then cite a part number. It would be interesting to take a look at  
its implementation and contrast it to the usual.

I've recently worked with a 125 MHz 14-bit binary-weighted current  
switching DAC, but although it can handle audio reasonably well it's  
not likely to be found in any audio products.

> There's nothing wrong with ZOH per say. It's just that it distorts  
> both the waveform and the frequency response significantly, and  
> then this really needs to be put right with a complicated analogue  
> filter that follows.

ZOH certainly affects the frequency response, but I wouldn't call  
that distortion. Of course, anything other than the original signal -  
whether noise, EQ, quantization, or other - is technically  
distortion, but I'm suggesting that it's significant to say that ZOH  
affects the frequency response without claiming that it distorts in  
addition to the response alteration. If I'm missing something, please  
elucidate.

> The point Monty was making is that there is nothing inherent in  
> digital audio that implies a ZOH should be part of the  
> reconstruction. In fact it isn't the best method and actually leads  
> to much confusion.

I personally think that it's irrelevant whether the DAC used ZOH, and  
therefore irrelevant that even the intermediate analog signals might  
be stair-stepped. The real point is that the final reconstructed  
signal has no stair stepping. How you get there shouldn't affect your  
perception of the final analog signal.

Brian Willoughby
Sound Consulting



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