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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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The
delta-sigma analog-to-digital converter uses a
negative feedback loop to accumulate the audio
waveform. If the input waveform, accumulated over
one sampling period, is greater than the value
accumulated in the negative feedback loop during
previous samples, the converter outputs a digital
'1'. If the integrated value of the input waveform
is lower, a digital '0' is output.
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The DSD
analog-to-digital converter
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With
such a Pulse Density Modulation (PDM) process, the
instantaneous amplitude of the analog waveform is
represented by the density of pulses. Thus, a full
positive waveform will result in all 1s, while a
full negative waveform will be all 0s. A flat (zero)
signal will be represented by alternating 1s and 0s.
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DSD
pulse train
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The
DSD pulse train 'looks' remarkably like the analog
waveform it represents. The pulse point up where the
analog waveform approaches full positive and down
where the analog wave approaches full negative.
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Like
for PCM, such a PDM signal is highly resistant to
distortion, noise, wow and flutter of recording
media and transmission channels. But unlike PCM, DSD
signals looks quite analog, and digital-to-analog
conversion can be as simple as a low-pass filter.
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However,
5th order noise shaping filter are required in
practice to match the ultra-high signal-to-noise
ratio of DSD : this shifts the noise up in
frequency, out of the audio band.
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The
specifications of the resulting signal are as good
as those of the best analog formats, which have
however problems and constraints DSD does not know.
DSD signals can indeed present a frequency response
from DC to 100kHz, plus a dynamic range greater than
120dB, across the audio band.
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