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PCM and DSD
MP 2500 R and DSD
MP 2500 R and PCM
Introduction
Two competing formats are available in the form of PCM and DSD, both of
which are used to store audio signals at very high resolution and quality. Each
of these formats has its own specific advantages. A vast amount has been
written about the relative merits of these two formats, and we have no intention
of participating in the dispute, much of which is less than objective in nature.
Instead we consider it our task to develop equipment which reproduces both
formats as effectively as possible, and exploits the strengths of each system to
the full.
Our many years of experience with both systems have clearly shown that PCM
and DSD cannot just be lumped together; it is essential to treat each format
separately, and take their specific requirements into account. This applies both
at the digital and analogue level.
For this reason the MP 2500 R employs two separate digital sections and two
D/A converter sections - each optimised for one format.
By its nature the DSD format involves a noise floor which rises above the range
of human hearing as frequency rises. Although this noise floor is not directly
audible, it does subject the treble units in the loudspeakers to a significant
load. It is also possible for the high-frequency noise to cause distortion in many
low-bandwidth amplifiers.
The lower the DSD sampling rate, the more severe the inherent noise, and it
cannot be disregarded, especially with the DSD64 format - as used on the
SACD. As the DSD sampling rate rises, the high-frequency noise becomes
increasingly insignificant, and with DSD256 it is virtually irrelevant. In the past it
has been standard practice to apply digital and analogue filtering processes in
an attempt to reduce DSD noise, but such solutions are never entirely without
side-effects on sound quality. For the MP 2500 R we have developed two
special techniques designed to eliminate the sonic disadvantages:
1.) The True-DSD technique, consisting of a direct digital signal path
without filtering and noise-shaping, plus our True 1-bit DSD D/A converter
2.) Analogue reconstruction filter with automatic bandwidth selection.
The PCM process makes extremely high-resolution sampling values available:
up to 32 bits. However, the sampling rate of PCM is significantly lower than that
of DSD, and the spacing in terms of time between the sampling values is
greater.
This means that it is extremely important with PCM to employ maximum
possible precision when converting the high resolution into analogue signals.
Here at our answer was to develop quadruple D/A converters which
provide a four-fold improvement in accuracy over conventional converters. A
further very important aspect of PCM reproduction is to reconstruct the curve of
the original analogue signal between the sampling points with great accuracy,
since these points are much more widely spaced in comparison with DSD. To
this end the MP 2500 R employs a polynomial interpolation process (Bezier-
Spline interpolation) developed in-house at , which in mathematical terms
delivers the smoothest curve for a given number of reference points (sampling
points). The output signal generated by Bezier interpolation exhibits a very
"natural" shape, devoid of the digital artefacts - such as pre- and post-
oscillation - which are usually produced by the standard oversampling process.
More detailed information on this can be found in the chapter "Technical
description, oversampling / up-sampling"
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