Wells JASON Owner's Manual page 9

OWNERS MANUAL FOR WEISS JASON CD TRANSPORT
A physical law states that in order to represent any given analog signal in the digital domain, one
has to sample that signal with at least twice the frequency of the highest frequency contained in
the analog signal. If this law is violated so called aliasing components are generated which are
perceived as a very nasty kind of distortion. So if one defines the audio band of interest to lie
between 0 and 20 kHz, then the minimum sampling frequency for such signals must be 40kHz.
For practical reasons explained below, the sampling frequency of 44.1kHz was chosen for the CD.
A sampling frequency of 44.1kHz allows to represent signals up to 22.05kHz. The designer of the
system has to take care that any frequencies above 22.05kHz are sufficiently suppressed before
sampling at 44.1kHz. This suppression is done with the help of a low pass filter which cuts off the
frequencies above 22.05kHz. In practice such a filter has a limited steepness, i.e. if it suppresses
frequencies above 22.05kHz it also suppresses frequencies between 20kHz and 22.05kHz to some
extent. So in order to have a filter which sufficiently suppresses frequencies above 22.05kHz one
has to allow it to have a so called transition band between 20kHz and 22.05kHz where it gradually
builds up its suppression.
Note that so far we have talked about the so called anti-aliasing filter which filters the audio signal
ahead of the A/D conversion process. For the D/A conversion, which is of more interest to the
High-End Hi-Fi enthusiast, essentially the same filter is required. This is because after the D/A
conversion we have a time discrete analog signal, i.e. a signal which looks like steps, having the
rate of the sampling frequency.
Such a signal contains not only the original audio signal between 0 and 20kHz but also replicas of
the same signal symmetrical around multiples of the sampling frequency. This may sound
complicated, but the essence is that there are now signals above 22.05kHz. These signals come
from the sampling process. There are now frequencies above 22.05kHz which have to be
suppressed, so that they do not cause any intermodulation distortion in the amplifier and speakers,
do not burn tweeters or do not make the dog go mad.
Again, a low pass filter, which is called a „reconstruction filter", is here to suppress those
frequencies. The same applies to the reconstruction filter as to the anti-aliasing filter: Pass-band
up to 20kHz, transisition-band between 20kHz and 22.05kHz, stop-band above 22.05kHz. You may
think that such a filter is rather "steep", e.g. frequencies between 0 and 20kHz go through
unaffected and frequencies above 22.05kHz are suppressed to maybe 1/100'000th of their initial
value. You are right, such a filter is very steep and as such has some nasty side effects.
For instance it does strange things to the phase near the cutoff frequency (20kHz) or it shows
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Page: Date: 10/04 /dw