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Stream Digital |
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Super
Audio CD |
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Direct Stream Digital (DSD) (cont.)
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As seen in the
previous section, it was decided to move away from
multibit systems in order to reach a higher
resolution with the new digital audio format. Indeed,
higher bit rates and sampling frequencies have
improved noticeably sound quality of PCM systems,
but these improvements are nowadays getting smaller
and smaller, mainly because of filtering.
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Steep
filters are usually required at the input of PCM
systems in order to block any signal above the
sampling frequency. For instance, a conventional
44.1kHz sampling system would require a brick wall
filter which passes 20kHz audio but rejects signals
above 22.05kHz ! Beside the difficulties to realise
such filters, requantization noise is also added by
the multi-stage or decimation (downsampling) digital
filters used in recording :
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Oversampling
has indeed often been used to overcome the problems
due to steep brickwall filters. However, multi-stage
interpolation (oversampling) digital filters are
then required, resulting in more quantization noise.
1-bit conversion technology (also known as Bitstream
or Delta-Sigma) is for instance quite common in PCM
systems nowadays : first, the original analog signal
is converted to a digital 1-bit representation,
thanks to a 64xFs modulation. Then, a decimation
filter provides a high-resolution signal (16, 18,
20, 24 bits) at the sampling frequency (Fs).
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Regarding
play-back, most of the current CD and DVD players
include a Delta-Sigma converter which makes the
opposite transformation, an interpolation filter
being necessary to first make the 64xFs signal.
Then, a delta-sigma modulator provides a 1-bit
signal, while a simple low-pass filter can finally
be used to get the analog signal.
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Conventional multibit
PCM
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During
the complete chain, an important amount of
requantization noise is added by the decimation (downsampling)
digital filters used in recording and by the
multi-stage interpolation (oversampling) digital
filters used in play-back.
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The
idea behind DSD is to simply remove these two
problematic filters (decimation and interpolation).
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Direct
Stream Digital
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As
in conventional PCM systems, the analog signal is
first converted to digital by 64x oversampling
delta-sigma modulation. But the 1-bit resulting
signal is directly recorded by the DSD system,
instead of using a decimation filter to get a
multi-bit signal with PCM recorders.
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