Technical Description Digital Filters / Oversampling - T+A DAC 8 Betriebsanleitung

Technical description
Digital filters / Oversampling
Oversampling The audio data on for example CDs is stored at a sampling rate of 44.1 -
i. e. for each second of music 44.100 sampled values are available for
each channel. In the DAC 8 DSD the audio data read from the CD is
„multiplied" to a higher sampling rate (352,8 kHz) before it is converted
back into analogue music signals. This process delivers a very much
better, more finely graduated signal to the converter, which can then be
converted with correspondingly higher precision. The raised sampling rate
is a calculating process for which there are many different mathematical
methods. In almost all digital audio devices which exploit the advantages
of increased digital sampling rate a process known as a FIR filter is
employed for this purpose. At  we have been carrying out research
for more than twenty years, aimed at improving the oversampling
process, because the standard FIR method has one drawback to set
against its indisputable advantages: it adds small pre- and post-echoes to
the music signals. At  we have developed mathematical processes
(known as Bezier polynomial interpolators) which do not share this
disadvantage. For this reason they should sound better and more natural
than the usual standard process. Since the calculating procedure
employed by us is considerably more complex than the standard method,
the DAC 8 DSD features a high-performance digital signal processor
(DSP) which carries out the over-sampling process with immense
precision (56 bit) using special algorithms developed by .
The freely programmable DSP which we use is capable of carrying out
the oversampling process using any method of calculation. For this
reason we have implemented a slightly modified Bezier process (filters 3)
in the DAC 8 DSD in addition to the pure Bezier process (filter 4),
together with two variants of the standard process (filter 1 and filter 2). For
more information on the different processes please refer to the next
section. You can switch between the various algorithms, then decide for
yourself which of the filters gives the results you prefer.
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