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Some thoughts on filters

> A digital filter is just that, a filter, the interpolation comes
> about by virtue of that filtering action. The lost information is
> not retrieved, in fact, as outlined above, further information must
> be lost in the filtering process.

Hi Peter, thanks for responding. I read your posting with interest
and I don't think we actually disagree on much of the technical
background. I have snipped sections of your posting that I do not
have time to address, but I have ead these sections and may well
respond later.

You are absolutely correct that the A/D conversion stage is where
the information is lost and nothing can get it back. Where we
disagree is on the best strategy to reconstruct the information
that did survive the A/D process form the digital data.

The reconstruction low-pass filter is not an adjunct to this
operation, it is an essential part of the operation - see my
article on the subject at http://www.stereophile.com/reference/25 .
The original low-pass filtered waveform presented to the A/D
converter will be constructed _exactly_ by a perfect low-pass
filter. However, there are no perfect filters. The sin x/x filter
I referred to in my article depends on there being both
pre- and post echo for its operation. However, pre-echo is about
an unnatural an effect as can exist and while a continual waveform
will be reconstructed correctly, the pre-echo might well be heard
as a degradation with a discontinuous waveform, ie, transients.
This is what I believe you dislike about the sound of filters like
this.

One solution is to use a low-pass filter that is better-behaved in
the time domain, the so-called "spline" function types made popular
by Wadia then Pioneer, and currently offered as a option by many
manufacturers. These filters are more "leaky" when it comes to
content outside the CD's passband and suffer from in-band rolloff,
which is perhaps why they are not as popular as the first sort.

Another solution is to use an analog-domain filter, which is what
early CD players did. Problems are that good filters are very
expensive, the unit-unit variability is unacceptable, and the
difficulty of controlling the inevitable ringing.

Whichever filter the designer chooses to use is a matter of trading
tradeoffs. There is no right or wrong. However, if you choose not to
use a low-pass filter _at all_, then you will no longer be
reconstructing the original waveform. You are instead passing along
to the downstream components a series of pulses, or more correctly
pulses that have been integrated into steps by whatever bandwidth
limitation exists in the player's output stage. This waveform
includes the original but also includes all the ultrasonic images of
that original.

As this signal passes through the low-pass functions of the
preamplifer and power amplifier circuitry, and ultimately the
loudspeaker, a semblance of the original waveform will emerge. But,
that semblance will be arbitrary, and totally dependent on the
owner's downstream components. This is what I referring to as
"poor engineering," the leaving to chance of what is a fundamental
part of the D/A conversion process.



> On the issue of worse sound, may I say first of all that since you
> have never heard one of our DACs under what you would call
> controlled circumstances, the comments about the sound cannot be
> taken for more than conjecture and coming from a leading figure in
> the audio press it hardly improves industry credibility.

Note that I am not stating that such a DAC cannot sound good. One of
my own writers, Peter van Willenswaard, has gone on record in the
pages of Stereophile as stating that your original non-filtered DAC
sounded superb. I am merely pointing out that whether or not such a
DAC sounds good will be dependent on chance.

In addition it may well sound worse, if the downstream components are
upset by the higher-than-usual level of ultrasonic energy from the
unfiltered images. I agree with you that if such components _are_
upset, they are examples of bad engineering. But such components
exist and the audiophile has no way of knowing if his preamp and
power amp will react to the Audionote DAC badly or not. And as I
said, he may even prefer the sound, even if it is demonstrably
incorrect.

> I can easily help you correct this, by getting you any Audio Note
> DAC of your choice, you may still not like it after hearing it, but
> that is OK too, all I ask is that you let at least one other
> Stereophile reviewer listen to it as well.

No deals. If you wish to send me an Audionote DAC then I will review
it for the magazine as with any other component I am sent.



> It behoves people in your position to keep an open mind at all
> times.

I don't disagree. But all of us are prisoners of our experiences
and education, you as much as me Peter. In the case of your DAC,
whether or not it sounds good - -and as I said, one of my own
writers has stated that it does do so -- by itself it doesn't output
a waveform that is the same as that that went into the microphones
and A/D converter.

John Atkinson
Editor, Stereophile



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