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encoded). So it seems pretty obvious that
DSD64 is significantly better than the
PCM system used on the CD.
That a newer digital system should
be better than a particular form of PCM
specified in the late 1970s and early
1980s should not be surprising. But is it
better than PCM in general? That’s the
real question.
Let’s use those 2,822,400 bits per
second per channel, and see what they’d
yield if we used PCM at that rate. At
2,822,400bps a PCM system with 24-bits
of resolution could have a sampling
frequency of 117,600 Hertz, yielding a
useful top end of 55,000 Hertz. With
20 bits of resolution the Fs would be
141,120 Hertz, giving close to a 70,000
Hertz bandwidth.
As you can see, there is no reason in
principle why a similarly-specified PCM
system shouldn’t give startlingly good
results... especially in the light of studies
which suggest that in double-blind trials
the sound of DSD and CD-standard
PCM is indistinguishable.*
But What About 100kHz?
You can read the Sony/Philips claim
about SACD as suggesting that DSD
can carry up to 100kHz in bandwidth,
and at the same time offer a noise floor
of 120dB, which would indeed be an
impressive feat since PCM couldn’t do
that in the same bandwidth (unless com-
pressed, like DVD-Audio).
But there are tricks that you can pull.
A raw implementation of most digital
encoding systems results in a low level
of quantisation noise that is evenly
spread across the frequency spectrum.
Such a thing is rarely permitted these
days. Possibly audible quantisation
distortion is eliminated by including
an extremely low level of random noise
(called ‘dither’). But this is random with
a purpose: it is shaped so that most of
the noise is in the top octave of the bandwidth,
where the human ear is relatively insensitive,
leaving the critical mid-bands with much lower
levels of noise. This is the heart of Sony’s Super
Bit Map (SBM) digital processing, but there are
plenty of other variations, such as the one that’s
used in the Sony/Philips DSD process. To make
sure it gets that –120dB noise result ‘across the
entire audible range’, as it puts it, it shapes the
noise so that the great majority is up at the top
end of the frequency spectrum.
Indeed, there is so much noise up there that
in the real world, any claims of a 100kHz band-
width—in any usable sense—are ludicrous. The
reason is straight-forward: at some frequency
which is usually between 23,000 and 30,000
Hertz, the level of the signal falls below the level
of the noise. As frequency increases, the level of
the signal continues to fall, but the level of the
noise continues to rise.
This is best seen in pictures. Let’s start with
something ideal. I created a clean non-dithered
1kHz sine wave in 192kHz, 24-bit PCM format
at a level of –12dBFS (peak). Then I converted it
to DSD64 and DSD128 and then, for analysis,
back again to 192kHz, 24-bit PCM. Why this
last step? There are very few analysis and editing
tools that work on native DSD. Some facilities
which use DSD convert to DXD—24-bit PCM at352.4kHz—for mixing. Do be aware that
there are some parameter choices avail-
able for DSD encoding and decoding, so
different encoders may yield different re-
sults... but only slightly different. In the
main, these results are representative.
So here’s a spectrum showing the
resulting signal: Graph 1. The blue trace
is the original PCM, Mustard is DSD64,
Green is DSD128. There’s the noise
inherent in DSD, shifted off into the
ultrasonic band. In the case of DSD64
it doesn’t kick in until above 20kHz.
DSD128 pushes this out to 40kHz. The
rapid drop-off in the DSD64 noise above
about 40kHz is the output ultrasonic
filtering (the software does this for PCM
conversion, but usually it’s built into
the DAC).
Let’s look at these test signals a dif-
ferent way. Let us zoom in at the top of
our 1kHz sine waves: Graph 2.
Same colours. All were at the same
level. I have just shifted them apart a
litte for visual clarity. The slightly larger
dots are samples. Notice how the PCM
at the top is smooth, the DSD64 is
somewhat bumpy, and the DSD128 is
wobbling up and down from sample
to sample. Let’s not be confused. The
bumps and the wobbles aren’t audible.
They are simply the ultrasonic noise
inherent in DSD. I present this here
to demonstrate one thing only: when
it comes to clinical accuracy, PCM is
superior to DSD.
But, you might object, all that’s test
signals. How about real music? Good
point. I played a snippet of DSD64
music from an audiophile DSD label—
recorded in 2015, direct to DSD—and
captured the output from the DAC.
I chose a section with acoustic guitar
recorded quite loudly to ensure plenty
of high frequency content. The result is
shown in Graph 3.
As you can clearly see, at 28.5kHz
the actual music signal sinks below the level of
the DSD noise, which continues to rise to around
40kHz, where it starts to be defeated by the DAC’s
low-pass filter.
Now let’s pause and consider all this for a
moment. It is not in the least surprising that
the inherent resolution of 192kHz at 24-bits is
better than DSD64. Remember DSD64’s bitrate
is around 2.8 megabits per second. The per-
channel bitrate for 24/192 is around 4.6Mbps.
But clearly 24/192 PCM is also more inherently
accurate than DSD128, even though the latter
has an even-higher bitrate: 5.6Mbps.
Yes, it is claimed that DSD can capture tran-
sients more effectively than similar bitrate PCM.DSD vs. PCM — Which is the Best?
Graph 1. 1kHz sine noise spectrum. PCM (blue) vs. DSD64
(mustard) vs. DSD128 (green).Graph 3. Audiophile DSD64 Acoustic RecordingGraph 2. 1kHz waveform comparison. PCM (blue) vs. DSD64
(mustard) vs. DSD128 (green). (Traces offset for visual clarity)