WEISS Vesta Owner's Manual page 10

OWNERS MANUAL FOR WEISS VESTA FIREWIRE INTERFACE
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 from the CD and upsamples it to 88.2kHz or 176.4kHz
or even higher. The output of the upsampler is then fed to the D/A converters which in turn feeds
the reconstruction filter.
All modern audio D/A converter chips have such an upsampler (or oversampler) already built into
the chip. One particular chip, for instance, upsamples the signal by a factor of eight, i.e. 44.1kHz
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Page: Date: 10/08 /dw