Super Audio CD
|Media type||Optical disc|
|Read mechanism||650 nm laser|
|Developed by||Sony & Philips|
Super Audio CD (SACD) is a read-only optical disc for audio storage, introduced in 1999. It was developed jointly by Sony and Philips Electronics, and intended to be the successor to their Compact Disc format. While the SACD format can offer more channels (e.g. surround sound), and a longer playing time than CD, research published in 2007 found no significant difference in audio quality between SACD and standard CD.
The Super Audio CD format was originally introduced in 1999. Royal Philips Electronics and Crest Digital partnered in May 2002 to develop and install the first SACD hybrid disc production line in the USA, with a production capacity of 3 million discs per year. But SACD did not achieve the same explosive growth that Compact Discs enjoyed in the 1980s, and was not accepted by the mainstream market. By 2008, some considered the Super Audio CD a complete failure.
By October 2009, record companies had published more than 6,000 SACD releases, slightly more than half of which were European classical music. Jazz and popular music albums, mainly remastered previous releases, were the next two most numerous genres represented.
Many popular artists have released some or all of their back catalog on SACD. Pink Floyd's album The Dark Side of the Moon (1973) sold over 800,000 copies by June 2004 in its SACD Surround Sound edition. The Who's rock opera Tommy (1969), and Roxy Music's Avalon (1982), were released on SACD to take advantage of the format's multi-channel capability. All three albums were remixed in 5.1 surround, and released as hybrid SACDs with a stereo mix on the standard CD layer.
Between 2007 and 2008, Genesis re-released all their studio albums across three box sets. Each album in these sets contains the original album on SACD in both stereo and 5.1 mixes. The US & Canada versions do not use SACD but CD instead.
By August 2009 443 labels had released one or more SACDs. Instead of depending on major label support, some orchestras and artists have released SACDs on their own. For instance, the Chicago Symphony Orchestra started the Chicago Resound label to provide full support for high-resolution SACD hybrid discs, and the London Symphony Orchestra established their own 'LSO Live' label.
Many of the SACD discs that were released during the 2000s are now out of print and are available only on the used market. By the late 2000s, the major record companies were no longer regularly releasing discs in the format, with new releases confined to the smaller labels.
Playback hardware 
The Sony SCD-1 was a player, which was introduced concurrently with the SACD format in 1999, at a price of approximately US$5,000. It weighed over 26 kg (57 lb) and played two channel SACDs and Red Book CDs only.
The first two generations of Sony's PlayStation 3 (PS3) game console were capable of reading SACD discs. Starting with the third generation (introduced October 2007), SACD playback was removed. PS3 was capable of converting surround DSD to lossy 1.5 Mbit/s DTS for playback over S/PDIF using the 2.00 system software. The subsequent revision removed the feature.
Sound quality 
In the audiophile community, the sound from the SACD format is thought to be significantly better than older Red Book CD-DA format CD recordings. For example, one supplier claims that "The DSD process used for producing SACDs captures more of the nuances from a performance and reproduces them with a clarity and transparency not possible with CD.
Comparison with CD 
In September 2007 the Audio Engineering Society published the results of a year-long trial, in which a range of subjects including professional recording engineers were asked to discern the difference between SACD and compact disc audio (44.1 kHz/16 bit) under double blind test conditions. Out of 554 trials, there were 276 correct answers, a 49.8 % success rate corresponding almost exactly to the 50 % that would have been expected by chance guessing alone. The authors commented:
Now, it is very difficult to use negative results to prove the inaudibility of any given phenomenon or process. There is always the remote possibility that a different system or more finely attuned pair of ears would reveal a difference. But we have gathered enough data, using sufficiently varied and capable systems and listeners, to state that the burden of proof has now shifted. Further claims that careful 16/44.1 encoding audibly degrades high resolution signals must be supported by properly controlled double-blind tests.
Following criticism that the original published results of the study were not sufficiently detailed, the AES published a list of the audio equipment and recordings used during the tests.
Comparison with DVD-A 
Double-blind listening tests in 2004 between DSD and 24-bit, 176.4 kHz PCM recordings reported that among test subjects no significant differences could be heard. DSD advocates and equipment manufacturers continue to assert an improvement in sound quality above PCM 24-bit 176.4 kHz. Despite both formats' extended frequency responses, it has been shown people cannot distinguish audio with information above 21 kHz from audio without such high-frequency content.
SACD is capable of encoding recorded audio in either stereophonic sound or surround sound. Although SACD audio streams are encoded in a pulse-density modulation (PDM) scheme called Direct Stream Digital (DSD), a manufacturer may also write a Pulse-code modulation (PCM) "layer" compatible with conventional Compact Disc players.
SACD is a disc of identical physical dimensions to a standard compact disc; the density of the disc is the same as a DVD and it encodes audio using a process known as Direct Stream Digital. The SACD sampling rate is 2.8224 MHz and the resolution is one bit. A stereo SACD recording can stream data at an uncompressed rate of 5.6 Mbit/s, four times the rate for Red Book CD stereo audio. SACD recordings can have a wider frequency and dynamic range than conventional CDs.
|Format||16 bit PCM||1 bit DSD|
|Sampling frequency||44.1 kHz||2.8224 MHz|
|Dynamic range||90 dB (practical)
96 dB (theoretical)
|105 dB (practical)
120 dB (theoretical)
|Frequency range||20 Hz – 20 kHz||20 Hz – 50 kHz (practical)
2 Hz – 100 kHz (theoretical)
|Disc capacity||700 MB||4.7 GB|
|Discrete surround||Never implemented||Yes|
There are three types of SACDs:
- Hybrid: Hybrid SACDs are encoded with a 4.7 GB DSD layer (also known as the HD layer), as well as a PCM (Red Book) audio layer readable by most conventional Compact Disc players.
- Single-layer: A DVD-5 encoded with one 4.7 GB DSD layer. Single-layer SACDs are not backward-compatible with conventional CD players.
- Dual-layer: A DVD-9 encoded with two DSD layers, totaling 8.5 GB, and no PCM layer. Dual-layer SACDs can store nearly twice as much data as a single-layer SACD. Like single-layer SACDs, dual-layer discs are not backward-compatible with conventional CD players.
Almost all commercially released SACDs have included both stereo (dual-channel) and surround sound (multi-channel) mixes. A multi-channel mix need not be surround, however; some of the Living Stereo reissues (such as the RCA reissue of the 1957 Chicago Symphony Orchestra recording of Mussorgsky's Pictures at an Exhibition) use only the three front channels to reproduce the original three-track (3.0) stereo recordings. Nor is a surround mix obliged to use all six SACD channels (five full-range plus LFE). For example, the 2001 SACD release of Mike Oldfield's Tubular Bells (1973) remains in the quadraphonic 4.0 mix.
The CD layer exists primarily for backward compatibility, but is not required. If the CD layer is omitted, the SACD need not be limited to an 80-minute playing time. For stereo material, the space that would have been taken by the multi-channel program can be used to extend playing time to four hours or more. BIS has taken advantage of this to put all of Bach's organ music on five SACD-only disks.
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 is 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. Conversely, if a conventional CD is placed into an SACD player, the laser will read the disc as a CD since there is no high-resolution layer.
DSD is 1-bit, has a sampling rate of 2.8224 MHz, and makes use of noise shaping quantization techniques in order to push 1-bit quantization noise up to inaudible ultrasonic frequencies. This gives the format a greater dynamic range and wider frequency response than the CD. The SACD format 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 currently available players list an upper limit of 70–90 kHz, and practical limits reduce this to 50 kHz. Because of the nature of sigma-delta converters, one cannot make a direct technical comparison between DSD and PCM. DSD's frequency response can be as high as 100 kHz, but frequencies that high compete with high levels of ultrasonic quantization noise. With appropriate low-pass filtering, a frequency response of 50 kHz can be achieved along with a dynamic range of 120 dB. This is about the same resolution as PCM audio with a bit depth of 20 bits and a sampling frequency of 96 kHz. Thus, DSD looks inferior to a "standard" PCM 24bit/96 kHz even while using slightly more bandwidth than PCM (2.8224 Mbit/s vs 2.304 Mbit/s).
To reduce the space and bandwidth requirements of Direct Stream Digital (DSD), a lossless data compression method called Direct Stream Transfer (DST) is used. DST compression is compulsory for multi-channel regions and optional for stereo regions. This typically compresses by a factor of between two and three, allowing a disc to contain 80 minutes of both 2-channel and 5.1-channel sound.
Direct Stream Transfer compression was also standardized as an amendment to MPEG-4 Audio standard (ISO/IEC 14496-3:2001/Amd 6:2005 – Lossless coding of oversampled audio) in 2005. It contains the DSD and DST definitions as described in the Super Audio CD Specification. The MPEG-4 DST provides lossless coding of oversampled audio signals. Target applications of DST is archiving and storage of 1-bit oversampled audio signals and SA-CD. A reference implementation of MPEG-4 DST was published as ISO/IEC 14496-5:2001/Amd.10:2007 in 2007.
Copy protection 
SACD has several copy protection features at the physical level, which as of 2012[update] appeared to make the digital content of SACD discs difficult to copy. The content may be copyable without SACD quality by resorting to the analog hole, or ripping the conventional 700 MB layer on hybrid discs. Copy protection schemes 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, and SACDs can only be manufactured at the disc replication facilities in Shizuoka, Japan and Salzburg, Austria. 
See also 
- Audio format
- Audio storage
- Earlier attempts at higher fidelity that stayed within the CDDA standard: HDCD and XRCD.
- Later attempts at higher fidelity that stayed within the CDDA standard: Blu-spec CD, DSD-CD and SHM-CD.
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