Upsampling: Threat or Menace?
There's a problem with writing under a byline. People know
how to reach you when they think you're full of it. And one of the things I love about my
readers is how personally they take it when they think I'm wrong.
Which I frequently am. I'm not an E.E., so a lot of the
time when I report on a new technology or an unexpected consequence of an old one, I have
to rely on what I do know and understand. Whenever a manufacturer makes a claim for
a product that I do not know for a fact to be true, I try to indicate that the claim is
just that, a claim. And I try not speculate about why something happens when I don't
understand it -- I simply report what I have heard.
One area where I was beginning to feel uncomfortable was
the whole upsampling/oversampling business. So far I've reviewed the Bel Canto and
Perpetual Technologies "upsampling" DACs -- and I heard improvements using them
and reported on those perceived improvements. And in at least one conversation with me, a
Musical Fidelity executive represented the A3CD player as capable of upsampling to 96/24.
But something didn't seem quite right. I'm not saying that anyone was deliberately
misleading me, but there seemed to be a lot of foot-shuffling and throat-clearing whenever
I asked anyone about exactly how it all worked.
I remember being at Madrigal one day and asking if the 30.6
"upsampled." Looks of consternation all around -- what exactly did I mean? I
explained upsampling as it had been explained to me and received blank looks of
incomprehension from some of the finest digital engineers on the planet. "But every
DAC does that!" one of them blurted.
So I wasn't exactly unprepared when I called Ayre's
Research Director, Charles Hansen, last week to confirm some facts about his K-1x
preamplifier and, instead, received an earful. "I'm concerned about the things that
have been written about upsampling," Charlie said. "There's a lot of
misinformation being spread around and you've got a great opportunity to clear the air on
this issue."
Okay, Charlie. What's your point?
"Upsampling almost always makes a difference, and it
can make an improvement -- but it's not at all for the reasons most people are saying it
is. Upsamplers come in two varieties -- built-in and external. Any external upsampler is
going to affect jitter, for better or for worse.
"Every CD player ever built, except the very first
Sony -- the CDP-101 -- had a digital filter in it, and they all oversample. Upsampling and
oversampling are the very same thing and anybody who tries to tell you differently is
misstating the case. What 'upsamplers' do is basically put two digital filters in a row --
one brings it from 44.1kHz to 88.2kHz or whatever it does, and then the other filter
brings it up some more.
How's that, Charlie?
"The composite filter has more computational
horsepower and will perform better than a single filter, according to traditional
measurements. You can get the same improvement by using a better (more powerful) digital
filter in the first place or by rolling your own. But there's no magic in it.
"And there's nothing unusual about putting two digital
filters in a row -- virtually every digital filter is a cascade of 2x stages, because it
costs less than accomplishing the entire filtering process in one go."
"An additional digital filter will have two effects:
First, there's usually a reduction in out-of-band noise components. While this is
generally true, its impact will depend on how effective the original digital filter is,
and how sensitive the rest of the playback chain is to out-of-band noise.
"And also, the overall response of the digital filter
will be altered. The first stage of a digital filter has virtually all of the effect on
both the in-band audio frequency response and on the transition-band, which will affect
the impulse response. The subsequent stages only affect the stop-band response
(out-of-band noise).
"When you add an 'upsampler,' it then becomes the new
first stage of the composite digital filter and now determines the overall audible
characteristics. The first stage of the existing digital filter now becomes the second
stage, and so forth.
"Depending on the 'upsampler,' and depending on the
original digital filter, the new first stage will have either more or fewer taps. So,
adding an 'upsampler' will almost certainly change the characteristics of the first
stage (because it is very unlikely to exactly duplicate the first stage of the existing
digital filter)."
Is this good?
"A traditional digital engineer would assert that more
taps are a good thing, as they reduce out-of-band noise, as well as allowing for a sharper
transition from the passband to the stop-band. However, the experienced audio engineer
might well be skeptical about adding additional taps, as the time (impulse) response is
degraded. The fact is, we don't really know what a good digital filter is. For an
extreme viewpoint on this topic, you can refer to www.sakurasystems.com/reviews/a2.kusunoki.html,
where the author advances the viewpoint that any digital filter is a bad
idea."
So, there is controversy over what filters do?
"It's perfectly legitimate to say that, if you add
this box to your setup, there's an improvement in sound. If you've got a setup with an
off-the-shelf digital filter and you add a better digital filter to it, you'll get an
improvement in sound.
"The best place to read about this right now, since we
haven't written anything about it, is on the Madrigal website (www.madrigal.com/UPCONVERSION.htm).
It's technical and it's long, but everything in it is complete and accurate and pretty
much covers the full scope of the issue."
Do you feel that companies are misleading the public on
the subject?
"Well, not exactly. I think some companies are being
pretty coy about it. But the public really wants something like this. It's like trying to
sell a seven-year-old on Santa Claus -- it doesn't take much selling. There's an
incredible urge to believe in free toys. Well, audiophiles have thousands of CDs and they
have thousands of LPs and the last thing they want to be told is they need to buy a new
format to get better sound out of their favorite performances. Their urge to believe in a
magic converter is incredibly powerful.
"And there really are benefits from improving digital
filters. Please don't misunderstand me, I'm not saying you should avoid any product that
'upsamples,' I'm just saying you shouldn't buy into any claims about the benefits of
'upsampling.' The public thinks that 'upsampling' is a new technology that offers new
benefits that are not available any other way. They have been (mis)led into thinking that
they can turn their Red Book CDs into 96/24-quality discs. And that's simply untrue."
So now you know what I know. Do I believe Charlie and
disbelieve the manufacturers of "upsampling converters?" Not entirely. What
Charlie says makes a lot of sense to me -- as does Madrigal's posting on the subject. But
Charlie and Madrigal also manufacture digital products -- products in competition for the
same market -- so it's conceivable they're grinding their own axes.
But Charlie's message of "believe what you hear but
doubt the explanations you're being given" is good advice for any audiophile. It's
the only way to cut through the claims and counter-claims -- at least the only way to do
so without going crazy or becoming a cynic. (But that's the subject of another column.)
I'm glad he took the time to explain his views to me (us!)
and I'd be glad to publish any responses from any of the companies producing upsampling
converters, especially if they can shed further light on this controversial subject.
...Wes Phillips
wes@onhifi.com
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