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Re: oversampling

146.103.254.13

Posted by Werner on February 18, 2000 at 01:42:01

In Reply to: Re: oversampling posted by Mart on February 18, 2000 at 00:52:51:

Going back to theory: reconstruction is by definition an
analogue operation, for the simple reason that you want
to reconstruct when you need a clean analogue output (another
reason of course is that when still in the baseband-sampled
digital domain you can not reconstruct at all, as the
only frequency band your digital 'reality' knows is from
DC to fs/2, and the useful signal itself sits smack in
that band).

Oversampling expands the bandwidth of the digital realm
you're in, so that you can move a part of the reconstruction
job in.

Your remarks on feedback, opamps, and convergence are entirely
irrelevant since the theory of sampling simply does not
discuss implementation details. What if I chose to use
a non-feedback triode circuit with passive filters?
And anyway, there is no funny convergence in opamps, not
at these low frequencies...

>reconstruction circuit removes this artifact? I'm sure the digital circuit >can resolve this
>quicker w/o phasing in an infinite-order XO.

I'm not sure what you're talking about, but I feel confident
that neither are you :-) But OK, analogue filters always
marry a specific amplitude response to a specific phase
response (BTW, 'phasing' is not a verb belonging to the
dictionary of electronics!). In the digital domain you can
cheat bigtime, as you can control time: through the
use of memories and buffers you have access to the past
and to the future of the signal, which is neat. So in the
digital domain you can say, within some constraints, I want
THIS amplitude response, and I want THAT phase response
to go with it. However, this does not imply that all
digital audio filters do this, as many are just 1:1 translations
of their analogue cousins, but of higher order.

Now what you're referring to above, alluding to digital
reconstruction being better than analogue, is very simply
exactly what every oversampling CD-player since 1983 has been
doing. So there is nothing new here.

Again, the new 'upsampling' boxes are here because
1) manufacturers want to sell us more boxes
2) there is a limited need for them in the scope of DACs
that have been designed around an 88.2kHz or 96kHz
fs, which now are confronted with an installed
base of software sampled at 44.1kHz: you have to
translate between formats.

Upsamplers are no magic, no recipe for instant bliss. They
are a tool for a very limited job. And if something upsampled
turns out sounding a tad better, then that is just because
the new DAC you're going into may inherently sound a bit
better. But it might as well turn out the other way,
the upsampling process killing much of the original
information, especially if you move between non-integerly
related formats, like between 44.1k and 96k.

All else is marketing.

Listen, back in the early days of the dCS pro-grade upsampler,
they themselves stated that upsampled CD, played through the
Elgar DAC sounded a bit better, but that they were at a
loss as to why precisely. It was just a lucky coincidence.

This post is made possible by the generous support of people like you and our sponsors:

Kimber Kable

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