OWNERS MANUAL FOR WEISS DAC202 D/A CONVERTER
Page
10
Date: 03/10
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
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.
I have given the example of a 24 bit recording which has
to be truncated to 16 bits. Where is the application in
High-End Hi-Fi audio? More and more signal processing
is implemented in the digital domain. Think of digital
equalizers, digital volume controls, upsamplers, digital
pre-amplifiers, decoders for encoded signals on DVD etc.
All those applications perform some mathematical
operations on the digital audio signal. This in turn causes
the wordlength of the signal to be increased. E.g. an
input signal to an upsampler may have a wordlength of
16 bits (off a CD), but the output signal of the upsampler
may have 24 bits or even more. This comes from the
fact that the mathematical operations employed in such
devices increase the word length. E.g. a multiplication of
two 2 digit numbers results in a four digit number. So
