19
Appendix A
Some information on the
DSD format — properties,
caveats, uses.
The DSD (Direct Stream Digital) Audio format is used with the well-known
SACD disc. The SACD has some form of copy protection implemented, thus an
SACD is not easily rip-able into a computer file.
A.1
Quantizing
DSD is a digital signal which uses a 1 bit quantization of the audio signal at a
sampling rate of 64 times the sampling rate of a CD (64
×
44100 Hz = 2
.
8224 MHz).
The CD format uses a 16 bit quantization at a 44.1 kHz sampling rate, thus the
data rate of a DSD signal is four times the data rate of a CD, which gives the
SACD the potential to represent the audio signal more precisely. The noise floor
of a signal at 44.1 kHz/16 bit is about 16
×
6 dB = 96 dB below full scale. With
the DSD signal the noise floor is 1
×
6 dB below full scale, so obviously there have
to be taken some measures to get down the noise floor, else a DSD recording
would be very noisy. Here the high sampling rate of DSD comes in handy. I.e.
with special filtering techniques (noise-shaping) it is possible to move the noise
in the audio band (0–20 kHz) out to higher frequencies above 20 kHz. Thus the
noise floor in the audio band can be brought down to very low levels (well below
the 16 bit noise floor of a CD) at the cost that the noise is at a fairly high level
at frequencies above 20 kHz.
A.2
Filtering
This high level/high frequency noise has to be suppressed before the signal
hits the amplifier and speakers, in order to avoid amplifier misbehaving and/or
speaker blow-up. A low pass filter is used to suppress the noise. In a native
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