OPERATING INSTRUCTIONS FOR THE INT202 FIREWIRE INTERFACE
Page
9
Date: 03/10
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 ringing due to the
high steepness. In the early days of digital audio these
side effects have been recognized as beeing one of the
main culprits for digital audio to sound bad.
So engineers looked for ways to enhance those filters.
They can’t be eliminated because we are talking laws of
physics here. But what if we run the whole thing at
higher sampling rates? Like 96kHz or so? With 96kHz we
can allow frequencies up to 48kHz, so the reconstruction
filter can have a transition band between 20kHz and
48kHz, a very much relaxed frequency response indeed.
So let’s run the whole at 96kHz or even higher! Well –
the CD stays at 44.1kHz. So in order to have that analog
lowpass filter (the reconstruction filter) to run at a
relaxed frequency response we have to change the
sampling frequency before the D/A process. Here is
where the Upsampler comes in. It takes the 44.1kHz
