26 Australian
FEATURE
A
round fi fteen years have elapsed since
the Super Audio CD was introduced.
Sony and Philips had hoped that this
new 12cm optical disc format might
replace the CD. As we all know, both SACD and
DVD-Audio—the competing high resolution
audio disc format—never made any headway
outside the audiophile community.
But now, with increasingly fast Internet
speeds and increasingly cheap digital storage,
the high-resolution digital audio fi rst provided
on SACD and DVD-Audio is now available for
download. The SACD-equivalent downloads are
in the form of DSD—Direct Stream Digital, the
underlying digital format used for SACD. The
DVD-Audio-equivalent downloads are typically
in FLAC (Free Lossless Audio Codec) which
simply contains the granddaddy of digital audio
formats: PCM (Pulse Code Modulation).
But while the ground has shifted, the battle
continues. Since the start there have been those
convinced that SACD is in some fundamental
way ‘better sounding’ than PCM-based audio.
Today the battleground is DSD vs. PCM.
Many of the DSD claims are, I think, a
product of the marketing hype Sony and Philips
used when they launched SACD. These may
sound like harsh words, and they are, but let us
consider what the two companies managed to
achieve with that hype.
They managed to convince the audiophile
community that this digital format was some-
how more ‘analogue’ than Pulse Code Modula-
tion. But of course it isn’t. Digital is digital.
These are alternative ways of digitally capturing
a waveform, but in the end, each maxes out at a
particular level of accuracy determined in large
part by the bit-rate of the digital data.
Sony and Philips also made more specifi c
claims. In an insert in many SACDs, especially
in the early days, they said: ‘Where CD frequency
response extends to 20,000Hz, DSD technologycan theoretically reach 100,000Hz. Where CD has
dynamic range of 96dB, DSD recording can achieve
120dB across the entire audible range.’
Cool, huh?
But as we shall see, these claims are highly
misleading. In reality, it is a rare DSD recording
that has an effective frequency response much
beyond 30,000 Hertz. That isn’t because of the
recording. That’s because of the design of the Di-
rect Stream Digital process itself that was touted
as an improvement on PCM.
Indeed, it’s extremely hard to reconcile these
two statements:
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100,000 Hz’, and
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SACD recommendations’
The second of those is from the datasheet of
a popular PCM/DSD DAC chip, the Cirrus Logic
CS4398. Yes, even while Sony and Philips were
promoting a ‘100kHz bandwidth’ for SACD, its
formal specifi cation recommended that players
mMUFSUIFPVUQVUBUL)[What is DSD? How
does it differ from PCM?
PCM (Pulse Code Modulation) is simple to un-
derstand. At regular intervals the instantaneous
level of an analogue signal is measured. A record
is kept of those measurements. That record
can be used to reconstruct the original signal.
Because a discrete numbering system is used to
take the measurements, multiple generations of
digital copying ought not degrade the quality
of the record at all. The resolution of the system
is determined by two factors: how frequently
the measurements are taken, and the size of
the number-space upon which the measure-
ments are mapped. A CD measures samples
at 44,100Hz (samples per second) and uses a
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to be measured). DVD-Audio and Blu-ray permitup to 192,000 samples per second and a 24-bit
number space (allowing 16.7 million levels to be
measured).
DSD (Direct Stream Digital) does things
differently. Instead of mapping the instantane-
ous level of an analogue signal across a defi ned
number space, it represents the momentary level
of an analogue wave by the density of digital
‘1s’ in a stream of single bit digits. A series of ‘0s’
represents a low point in the waveform, a series
of ‘1s’ is a high point, while alternating ‘0s’ and
‘1s’ is the half-way point (that is, a zero voltage).
There’s more to DSD than this, but let’s pause
briefl y to note that this form of digital coding is
called ‘Pulse Density Modulation’, or PDM, and
is one of several alternatives to PCM.
To properly represent the analogue signal by
means of how tightly the ‘1s’ are packed, a lot
of numbers need to be fl ying, and this is indeed
the case with DSD. The form of it used on SACD,
and the great majority of extant DSD mate-
rial, has its bitstream proceeding at 2,822,400
bits per second per channel. By design, this is
exactly four times the bit-rate for a channel of
an audio CD. The use of a multiple of the CD
bit-rate allows a relatively clean downsampling
to CD standard for the release of recordings in
that format. It is also a multiple of the sampling
frequency used for CD: 64 times CD’s 44.1kHz.
This form of DSD has come to be known as
DSD64, or 2.8MHz DSD. (That is, 64 × 44,100 =
2,822,400—or 2.8MHz.)
In recent years a form of DSD with double
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ond—has appeared. This is known as DSD128
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available.)
With effi cient digital encoding, and in the
absence of lossless compression, the bit-rate
is proportional to the resolution of the digital
signal (that is, the accuracy with which it isDSD vs. PCM
Which is the Best?