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1.5.3 Filtering
If a digital signal is converted to analogue the analogue signal contains not only the original signal, but as
well it’s mirror image which lies in the frequency domain beyond one half of the sampling frequency. This
mirror image (aliasing) can cause unwanted interference with the original signal and thus must be filtered out
before passing the signal to the amplifier.
If the original sampling rate of 44,1 kHz is used the filter slope must be positioned somewhat above 20 kHz
and has to be very sharp in order to let the audio signal pass and to eliminate the aliasing components. Such
filters cause a large phase deviation at the end of the pass band and have often also amplitude deviations.
This leads to a harsh reproduction of music and can also affect the localization of solo instruments and
voices.
Upsampling to higher rates makes it possible to set the filter frequency far out of the audio signal range. For
example at 192 kHz sampling rate the filter must take effect at 96 kHz. In this frequency region no music
signal is present. Thus the filter can theoretically not affect musical reproduction.
1.5.4 Digital- / analogue conversion
C8 and C9 use highly precise 24-bit differential converters to reproduce the analogue signal out of the digital
data. Two converters on the same chip are used to output balanced signals. These signals are fed into a
differential amplifier. The difference between the signals is twice the audio signal (because one of the signals
is inverted) and the difference of the inaccuracies of the converters. As the two converters are on the same
chip, their inaccuracy is nearly the same and thus also nearly eliminated by the differential amplifier.
The second advantage of this differential technique is that the (very low) individual noise coming from the
converters is reduced by 3 dB.
The result is a clearly audible advantage in dynamic of the music signal and an audibly improved
reproduction of the finest details.
1.6 FM-Tuner
The tuner can be adapted to different reception situations. You can set bandwidth, and sensitivity values in
order to achieve optimal sound quality from aerial antenna as well as from cable. With it’s high sensitivity the
tuner can also work with a simple indoor antenna.
The stereo decoder offers high channel separation as well as very low noise.
The RDS section (audio data system, not available in all countries) is processor controlled and shows You
station names and texts with additional information about the program You are listening to.
The station memory allows You to store up to 63 stations. It stores not only their frequency, but also the
individual setting of sensitivity, bandwidth and mode (mono/stereo).
