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| SBM
Direct Conversion |
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Super
Audio CD |
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Super Bit
Mapping Direct Conversion
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As
seen in chapter 2 (Hybrid disc
structure), the upper
layer of a SACD will present a format compatible
with existing CD players, in other words a PCM
signal with a sampling frequency of 44.1kHz and a
resolution of 16 bits. However, there are many ways
to downconvert a signal from 1-bit/64Fs to
16-bit/1Fs. Indeed, many A-D converters can easily
perform this task. But a new process has been found
in order to retain the maximum possible signal
quality in the 16-bit words. This process, named
Super Bit Mapping Direct, allows to filter and noise
shape the DSD signal in a single stage. As a result,
interstage requantizing errors are here again
suppressed, ripple is eliminated, and aliasing is
minimised. This technology has been entirely
integrated in a one-stage FIR digital filter / noise
shaper, developed by Sony. The resulting
16-bit/44.1kHz signal is finally recorded on the
upper layer of the hybrid disc which consequently
fulfils the Red Book specifications for Compact Disc
and can be played in any existing CD player.
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As
explained above, a 20-to-24-bit precision can be
approached in 16-bit digital audio (SACD hybrid
discs) with Super Bit Mapping Direct. But this
technology can also be transposed to the standard
Compact Disc industry thanks to a dedicated
processor : the SBM Direct processor.
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Actually,
16-bit/44.1kHz downconversion is just one option for
the DSD bit stream, as the original 2.8224MHz
frequency has been carefully calculated to enable
high precision downconversion to all current PCM
sampling rates, thanks to simple integer
multiplications and divisions.
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Indeed,
the sampling frequency of DSD, 64x44.1kHz=
2.8224MHz, has been chosen to be a simple multiple
of most of the common sample rates in order to make
data conversion to existing audio formats as easy as
possible. As shown in the figure below, synchronised
sample frequency conversion is possible within a
reasonable processing clock rate. By using DSD
signal as the source material, jitter problems
related to asynchronous sampling frequency
conversion are completely eliminated !
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Synchronised sample
frequency conversion from DSD
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