OPERATING INSTRUCTIONS FOR THE INT202 FIREWIRE INTERFACE
Page
10
Date: 03/10
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 ends up at 352.8kHz. 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 20kHz 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 specifiy. 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 chose 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.
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
nonethless 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
are correlated with the audio signal, hence the resulting
error is also correlated and thus there are distortions and
not just noise (noise would be uncorrelated). The
dithering technique now is used to de-correlate the error
from the signal. This can be achieved by adding a very
low level noise to the original 24 bit signal before
truncation. After truncation the signal does not show any
distortion components but a slightly increased noise
floor. This works like magic..... the distortion is replaced
by a small noise – much more pleasant.
