WEISS INT203, Руководство пользователя

Страница: 13 / 34

7
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 96 kHz or so? With 96 kHz we can allow frequencies
up to 48 kHz, so the reconstruction filter can have a transition band between
20 kHz and 48 kHz, a very much relaxed frequency response indeed. So let’s run
the whole at 96 kHz or even higher! Well — the CD stays at 44.1 kHz. 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 44.1 kHz
from the CD and upsamples it to 88.2 kHz or 176.4 kHz 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. 44.1 kHz ends up at
352.8 kHz. Such a high sampling frequency relaxes the job of the reconstruction
filter very much; it can be built with a simple 3
rd
order filter.
So, how come that upsamplers are such a big thing in High-End Hi-Fi circles?
The problem with the upsamplers is that they are filters again, digital ones,
but still filters. So in essence the problem of the analog reconstruction filter
has been transferred to the digital domain into the upsampler filters. The big
advantage when doing it in the digital domain is that it can be done with a
linear phase response, which means that there are no strange phase shifts near
20 kHz and the ringing can also be controlled to some extent. Digital filters in
turn have other problems and of course have quite a few degrees of freedom for
the designer to specify. This means that the quality of digital filters can vary
at least as much as the quality of analog filters can. So for a High-End Hi-Fi
designer it is a question whether the oversampling filter built into the D/A chips
lives up to his/her expectations. If not, he/she can choose to design his/her own
upsampler and bypass part of or the whole oversampler in the D/A chip. This
gives the High-End Hi-Fi designer yet another degree of freedom to optimize the
sonic quality of the product.
1.4 Dithering
You have probably not heard the term dithering in conjunction with audio.
Actually it is a term widely used in the professional audio realm but not so much
in the High-End Hi-Fi market.
What is dithering? Suppose a digital recording has been made with a 24 bit
A/D converter and a 24 bit recorder. Now this recording should be transferred
to a CD, which has just 16 bits per sample, as you know. What to do with
those 8 bits, which are too many? The simplest way is to cut them off, truncate
them. This, unfortunately, generates harmonic distortions at low levels, but
which nonetheless cause the audio to sound harsh and unpleasant. The harmonic
distortion is generated because the eight bits, which are cut off from the 24 bits,