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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 MP 2500 R the audio data read from the CD is „multiplied" to a higher
sampling rate (352,8 kHz) before it is converted back into analogueue 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 ten 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 MP 2500 R 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 MP 2500 R 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.
Oversampling 1 (Standard FIR Filter)
The long FIR filter is the standard oversampling process in digital technology,
offering extremely linear frequency response, very high damping, linear phase
characteristics and constant group delays. The disadvantage is the pre- and
post-echoes which are added to the signal. These „time range errors" tend to
affect the music signal's dynamics, precision and naturalness, and reduce
spatial orientation.
Frequency response and transient characteristics of the long FIR filter
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