Technical data

OWNERS MANUAL FOR WEISS DAC202 D/A CONVERTER
Page 7 Date: 03/10
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 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 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.
Reconstruction Filters
Reconstruction filters have been mentioned in the "Upsampling,
Oversampling and Sampling Rate Conversion in General"
paragraph above. If you have read that paragraph you know
what the purpose of the reconstruction filter is. The main point
about this analog filter is that its frequency response should be
as smooth and flat as possible in order to have a virtually linear
phase response. The DAC202 employs a 3
rd
order filter for that
purpose.
Analog Output Stages
The DAC202 employs separate output stages for the main
output and the headphone output. Both stages use state of the
art operational amplifiers with high slew rate. A topology with a
very low output impedance has been chosen. This assures that
the performance of the DAC202 and the subsequent amplifier
combination is not compromised by the cables between the two
or by the input impedance characteristics of the amplifier.
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, 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