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Stream Digital |
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Super
Audio CD |
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Direct Stream
Digital (DSD)
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First,
let's investigate why there is a need for a new
digital audio format.
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Originally,
sound is analog, and it will remain so, except if we
change the morphology of our ears. For more than a
century, people have tried to capture sound and
store it on a medium, so that they can reproduce it
at will. First attempts in copying sound were very
natural : analog sound was stored in an analog way.
Actually, today's analog recording materials show
performances which could not be reached by standard
digital equipment. For instance, the best 30ips
half-inch analog recorders can capture frequencies
past 50kHz, and the residual noise of a good analog
console may be more than 120dB below its maximum
output level.
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But
these analog formats (vinyl or tape) are either too
big or too expensive to access these high-quality
specifications in domestic use, and they are quickly
subject to unavoidable and permanent deteriorations.
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Most
of these problems have been solved with the
introduction of digital technology in the audio
domain. Small size, friendly interface, and
especially no sound scratches (thanks to the error
correction algorithms of these new optical reading
systems). Compact Disc (CD) was presented as the
revolutionary audio format, with its 44.1kHz
sampling frequency, and its 16-bits quantization....until
now.
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Although
CD offers an efficient shield against disc
deterioration compared to analog devices, the audio
world was indeed still missing the high quality
performances of analog formats.
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As
a result, audio engineers have worked hard to define
a new digital format : to cover the frequency range
of the best analog tape machine, the frequency
response should be flat to near 100kHz and slowly
roll off at higher frequencies. Thus the sampling
frequency should set at over 500kHz. And to cover
the dynamic range of a good analog console, the
residual noise power up to 20kHz should be more than
120dB below maximum output level.
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Although
DVD-video has pop-up meanwhile with a 24-bit / 96kHz
resolution, it was noticed that more than 24 bits
were needed, even if the noise level was to be
around -120dB, as shown on the following drawing :
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Original
waveform of analog random noise
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-108dB
noise sampled at 44.1kHz and quantized to 24
bits
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| Same
noise sampled at 44.1kHz and quantized to 20
bits |
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Even
if the noise level is around -108dBFS, the second
drawing shows clearly that there is a waveform
degradation due to the 24 bits quantization when
compared to the analog signal (first drawing). As a
result, it is clear that more than 24 bit resolution
is needed to reproduce the slight nuances of an
original music track.
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The
2 companies (Philips and Sony) which did so much in
multibit PCM technology for digital audio have
consequently decided to move away from multibit in
order to reach an even 'higher' resolution.
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