Super Audio CD
(SACD) is a read-only optical audio disc aimed at
providing much higher fidelity digital audio reproduction than the compact
disc. Introduced in 1999, it was developed by Sony and Philips
Electronics, the same companies that created the CD. There is currently a
format war with DVD-Audio, with neither side making any significant
inroads to consumer acceptance. Media players that can play many formats
(including DVD-Audio and SACD) are available for under $100, so both
formats are likely to co-exist (as do DVD-R and DVD+R).
SACD uses a
very different technology from CD and DVD-Audio to encode its audio data,
a 1-bit delta-sigma modulation process known as Direct Stream Digital at
the very high sampling rate of 2.8224 MegaHertz — with the typical
sampling rate present on audio CDs currently being
44.1kHz.
SACDs must always contain a
2-channel stereo mix and may optionally contain a surround mix (usually
the 5.1 layout) as well. To be precise, the so-called surround mix does
not have to be in the 5.1 format. The old quadraphonic 4.0 format will do
as well, most noticeably on the 2001 SACD release of Mike Oldfield's
Tubular Bells. The correct designation for the surround part of a SACD is
'multi-channel', and usually has its own 'Multi-Ch' logo on the back
cover.
There are three types of
SACDs :
Hybrid :
The most popular of the three types, hybrid discs include an audio CD 'Red
Book' layer compatible with Compact Disc players, dubbed the 'CD layer,'
and a 4.7 GB SACD layer, dubbed the 'HD layer.'
Single
layer : Physically a DVD-5 DVD, a single layer SACD includes
a 4.7 GB SACD layer with no CD layer (i.e. one HD layer
only).
Dual-layer : Physically a DVD-9 DVD,
a dual-layer SACD includes two SACD layers with no CD layer (i.e. two HD
layers). This type is rarely used. It enables twice as much data to be
stored, but disables CD player compatibility.
Integration
Currently the most interest in
Super Audio CD discs has been from audiophiles and the recording industry
which helps explain why most of the new releases in this format are
classical, jazz and audiophile discs, although the biggest sellers are The
Dark Side Of The Moon and Brothers in Arms. It remains an interesting
question whether adoption of the new formats will be driven from the top
down or bottom up. So far the trend has been from the top down with many
labels adopting SACD for their new releases and re-releases, but mostly in
the areas of classical music and jazz (the traditional hi-fi
standards).
As of September 2006, there have
been over 4,000 SACD releases, about 40% of which are classical music.
However, some more popular albums, from Keane, Snow Patrol and Incubus'
Crow Left of the Murder, have been released in SACD format while many more
popular older albums have been re-released in SACD format including most
of Peter Gabriel's catalogue, most of Bob Dylan's catalogue, Pink Floyd's
seminal album The Dark Side of the Moon (the 30th anniversary edition of
2003; there are also plans to release Wish You Were Here) and Roxy Music's
Avalon (the 21st anniversary edition, 2003). The two latter albums were
released on SACD to take advantage of the format's multi-channel
capability. Both were remixed in 5.1 surround (leaving the original stereo
mix intact), and released as Hybrid SACD's as an incentive for record
buyers to switch from CD to SACD.
Genesis have announced that
their back catalogue will be remastered and released on SACD in 2007. The
format continues to attract major new artists.
Because most SACD's are issued
in a hybrid format only, such as the remastered Rolling Stones albums
released in 2002, many music buyers are building an SACD collection even
if they have no SACD playback equipment and are not specifically inclined
to purchase SACDs. This is believed to give the SACD format an advantage
over DVD-Audio because of the hybrid discs' compatibility with
conventional CD players.
Also, since most SACDs are
playable on any CD player and computer, they can also be played in cars,
whereas most people do not have a DVD player or Playstation 2/3 in their
car, which would be required for DVD Audio. To play DVD-Audio with a
computer DVD-ROM drive, you also need a high resolution sound card that
supports DVD-A.
Most surround sound/AV receivers
can do some processing on multi-channel audio in order to improve the
speaker matching and account for the room acoustics. Better quality
surround receivers do this in a DSD bitstream for digital signal
processing at high fidelity.
However, many more buyers are
choosing lower fidelity and convenience, in the form of MP3s and similar
lossy compressed formats, than are upgrading to get higher fidelity with
SACD. This is no doubt because most people listen to music outside their
house on portable devices, and sacrifice some degree of fidelity for
portability. Another reason is that people want to download music, which
requires download times or file sizes well below what SACD streams would
demand.
Disc
reading
Objective lenses in conventional
CD players have a longer working distance, or focal length, than lenses
designed for SACD players. This means that when a hybrid SACD is placed
into a conventional CD player, the laser beam passes the high-resolution
layer and gets reflected by the conventional layer at the standard 1.2 mm
distance, and the high-density layer is out of focus. When the disc is
placed into an SACD player, the laser is reflected by the high-resolution
layer (at 600 µm distance) before it can reach the conventional layer. To
the same point, if a conventional CD is placed into an SACD player, the
laser will read the disc with no problem since there is no high-resolution
layer.
Playback
hardware
Hybrid Super Audio CDs (which
include both a Stereo CD and a Super Audio CD layer) can be played back on
CD players. To hear the Super Audio CD Stereo and on many discs the Super
Audio CD Multichannel layer require the use of a Super Audio CD
player.
As would be expected, Sony and
Philips, as designers of the CD and SACD formats, have the most players on
the market in many guises such as standalone players, DVD Home Audio kit
and game consoles.
The Sony SCD-1 is a well-known
player which was introduced at time the SACD format was introduced to the
public. It weighs well over 26 kg (57 lbs) and is often 'modded' by its
owners to 'improve' the sound. The SCD-1 was introduced before
multi-channel SACDs existed and only plays two channel SACDs or red-book
CDs. It is no longer sold. When introduced in 1999, it sold for
$5,000. Many other
vendors offer SACD playback capabilities in their product lines, although
none has offered a portable, Walkman-style SACD player.
There is currently no way to
pass an unencrypted stream of DSD out of an SACD player. There are
currently a small number of players that offer a Firewire DSD output but
the content is encrypted. HDMI 1.2, standardised in 2005, can also carry
encrypted DSD, but as of October 2006, there are very few A/V processors
that support this (such as the Marantz SR7001), and no SACD
players.
The Sony PlayStation 3,
currently expected by November 2006, has been announced to include SACD
support.
DSD
SACD audio is stored in a format
called Direct Stream Digital (DSD), very different from the conventional
PCM used by the compact disc or conventional computer audio
systems.
DSD is 1-bit, has a sampling
rate of 2.8224 megahertz, and makes use of noise shaping quantization
techniques in order to push 1-bit quantization noise up to ultrasonic
frequencies. This gives the format a greater dynamic range and wider
frequency response than the CD. Promotional materials about SACD supplied
by Philips and Sony suggest that the system is capable of delivering a
dynamic range of 120 dB from 20 Hz to 20 kHz and an extended frequency
response up to 100 kHz, although most players list an upper limit of 80-90
kHz.
The process of creating a DSD
signal is conceptually not unlike taking a 1-bit sigma-delta
analogue-to-digital (A/D) converter and removing the decimator which
converts the 1-bit bitstream into multibit PCM. Instead, the 1-bit signal
is recorded directly and in theory only requires a lowpass filter to
reconstruct the original analogue waveform. In reality it is a little more
complex, and the analogy is incomplete in that 1-bit sigma-delta
converters are these days rather unusual, one reason being that a 1-bit
signal cannot be dithered properly: most modern sigma-delta converters are
multibit.
Because of the nature of
sigma-delta converters, one cannot make a direct comparison between DSD
and PCM. An approximation is possible, though, and would place DSD in some
aspects comparable to a PCM format that has a bit depth of 20 bits and a
sampling frequency of 192kHz. PCM sampled at 24 bit/192kHz provides a
(theoretical) additional 24dB of dynamic range but does not provide
anywhere near the same sampling rate of 2.8MHz as with DSD
recording.
Because it has been extremely
difficult to carry out DSP operations (for example performing EQ, balance,
panning and other changes in the digital domain) in a 1-bit environment,
and because of the prevalence of studio equipment such as Pro Tools, which
is solely PCM-based, the vast majority of SACDs, especially where rock and
contemporary forms which rely on multitrack techniques are concerned, are
in fact mixed in PCM (or mixed analogue and recorded on PCM recorders) and
then converted to DSD for SACD mastering.
To address some of these issues,
a new studio format has been developed, usually referred to as 'DSD-wide',
which retains standard DSD's high sample rate but uses an 8-bit, rather
than single-bit digital word length, but still relies heavily on the noise
shaping principle. It becomes almost the same as PCM (it's sometimes
disparagingly referred to as 'PCM-narrow') but has the added benefit of
making DSP operations in the studio a great deal more practical. The main
difference is that 'DSD-wide' still retains 2.8224MHz (64Fs) sampling
frequency while the highest frequency in which PCM is being edited is
352.8kHz (8Fs). The 'DSD-wide' signal is down-converted to regular DSD for
SACD mastering. As a result of this technique and other developments there
are now a few digital audio workstations (DAWs) which operate, or can
operate, in the DSD domain, notably Pyramix and some SADiE
systems.
Note that high-resolution PCM
(DVD-Audio, HD-DVD and Blu-Ray Disc) and DSD (SACD) may still differ in
terms of fidelity at high-frequencies since DSD, thanks to its high
sampling frequency, does not show the typical ringing effects of
reconstruction filters used with PCM. On the other hand, DSD's dynamic
range decreases quickly at frequencies over 20 kHz due to the use of
strong noise shaping techniques which push the noise out of the audio band
resulting in a rising noise floor just above 20kHz. PCM's dynamic range,
on the other hand, is the same at all frequencies. (Some high-end SACD
players employ an optional low-pass filter set at 30 kHz for compatibility
and safety reasons, suitable for situations where amplifiers or
loudspeakers can't deliver an undistorted output if noise above 30 kHz is
present in the signal.)
Comparing SACD and
CD
Many people feel that even a
moderately good system should reveal a significant difference between SACD
and either CD or DVD-Audio. The late film composer Jerry Goldsmith, for
example, fiercely backed SACD and several albums of his film scores and
compositions are available as Hybrid Multichannel SACDs.
Few home audio systems can
accurately reproduce sounds above 20 kHz, and most recording chains are
designed around this limit. Modern pop music is typically compressed to a
small percentage of the maximum available dynamic range, and thus would
not benefit from the extended dynamic range available in SACD. In
comparison, acoustic performances of jazz, folk, classical and alternative
music can definitely benefit from the lack of amplitude compression that
an extended dynamic range affords.
Increasingly, home audio
playback systems are home cinema multichannel and this single feature may
prove to be the most important when considering the differences between
Compact Discs and the newer distribution formats. CDs are stereo and both
SACD and DVD are multichannel-capable. In addition, SACDs can be authored
to be both forward and backward compatible with existing CD
players.
It has been argued that SACD and
DVD-Audio are merely attempts to add copy-protection features rather than
representing actual improvements in recording and listening technology.
However, in the hands of a competent engineer and producer, these formats
provide additional capabilities and features that can create a more
engaging and compelling listening experience. Record label owner David
Chesky of Chesky Records has said that 'we can give you a much better
ride' with Super Audio CD discs and has set August 2007 as the date when
all of his company's music releases will be issued only on Hybrid Super
Audio CD discs, compatible with both CD and SACD
players.
Copy
protection
SACD has several copy prevention
features at the physical level which, for the moment, appears to make SACD
discs impossible to copy. These include physical pit modulation and 80 bit
encryption of the audio data, with a key encoded on a special area of the
disk that is only readable by a licensed SACD device. The HD layer of an
SACD disc cannot be played back on computer CD/DVD drives, nor can SACDs
be created except by the licensed disc replication facilities in Shizuoka
and Salzburg.
It is possible to capture the
DSD digital audio signal after the decryption stage right before the
digital to analog converters of an SACD player, but since there is
practically no way for the public to make their own SACD discs this does
not pose a major threat.
A number of new SACD players
have encrypted IEEE 1394 (also called FireWire or i. Link) digital outputs
carrying DSD data, it may be possible to get the raw DSD data from the
link. The protection mechanism used is Digital Transmission Content
Protection (DTCP), which can be used in 'Copy Once' or 'Copy Never' modes.
It is unlikely, however, that the SACD license agreement rules permit
anything but the 'Copy Never' mode to be used.
Wikipedia information about
Super Audio CD. This article is licensed under the GNU Free Documentation License . It uses material
from the Wikipedia article 'Super Audio CD'
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