Super Audio CD
Super Audio CD (SACD) is a read-only optical disc format for audio storage, introduced in 1999. It was developed jointly by Sony and Philips Electronics, and intended to be the successor to the Compact Disc (CD) format.
|Media type||Optical disc|
|Capacity||4.38 GiB / 4.7 GB|
|Read mechanism||650 nm laser|
|Developed by||Sony & Philips|
The SACD format offers more audio channels (e.g. surround sound), a higher bit rate, and longer playing time than a conventional CD. The SACD is designed to be played on an SACD player; however, hybrid SACDs contain a track that may also be played on a standard CD player.
The Super Audio CD format was introduced in 1999. Royal Philips and Crest Digital partnered in May 2002 to develop and install the first SACD hybrid disc production line in the United States, with a production capacity of three million discs per year. SACD did not achieve the same level of growth that compact discs enjoyed in the 1980s, and was not accepted by the mainstream market.
The first broadcast of a SACD Disc on BBC Radio was in March 2000. The programme was an interview between Eric Kingdon (Sony), Colin Mackenzie (Hi-Fi Corner) and Kit Frazer (BBC Radio Scotland). The disc played was Mariah Carey 'Mariah'.
By 2007, SACD had failed to make a significant impact in the marketplace; consumers were increasingly downloading low resolution music files over the internet rather than buying music on physical disc formats. A double blind study published by the Audio Engineering Society in 2007 failed to demonstrate the ability to discern a sound quality difference between CD and SACD in different listener populations. A small market for SACD has remained, serving the audiophile community.
By October 2009, record companies had published more than 6,000 SACD releases, slightly more than half of which were 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, the rock band Genesis re-released all of their studio albums across three box sets. Each album in these sets contains the album on SACD in both new stereo and 5.1 mixes. The original stereo mixes were not included. 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 from 2000-2005 are now out of print and are available only on the used market. By 2009, the major record companies were no longer regularly releasing discs in the format, with new releases confined to the smaller labels.
SACD is a disc of identical physical dimensions as a standard compact disc; the density of the disc is the same as a DVD. There are three types of disc:
- 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.
- 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.
Unlike hybrid discs, both single- and dual-layer SACD's are incompatible with conventional CD players and cannot be played on them.
|CD layer (optional)||SACD layer|
|Disc capacity||700 MB||4.7 GB|
|Audio encoding||16-bit pulse-code modulation||1-bit Direct Stream Digital|
|Sampling frequency||44.1 kHz||2.8224 MHz|
|Audio channels||2 (stereo)||Up to 6 (discrete surround)|
|Playback time if stereo||80 minutes||110 minutes|
Commercial releases commonly included both surround sound (five full-range plus LFE multi-channel) and stereo (dual-channel) mixes on the SACD layer. Some reissues however, retained the mixes of earlier multi-channel formats (examples include the 1973 quadraphonic mix of Mike Oldfield's Tubular Bells and the 1957 three-channel stereo recording by the Chicago Symphony Orchestra of Mussorgsky's Pictures at an Exhibition, reissued on SACD in 2001 and 2004 respectively).
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 0.6 mm 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.
Direct Stream Digital
SACD audio is stored in a format called Direct Stream Digital (DSD), which differs from the conventional pulse-code modulation (PCM) used by the compact disc or conventional computer audio systems. In PCM, the audio amplitude at any one moment is determined by the most recent single number encoded in the bit stream (16 bits encode a single number in CDs), while in DSD, the amplitude is a function of the entire past sequence of bits that have been read up to that moment.
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 20 kHz can be achieved along with a dynamic range of nearly 120 dB, which is about the same dynamic range as PCM audio with a resolution of 20 bits.
Direct Stream Transfer
To reduce the space and bandwidth requirements of 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.
SACD has several copy protection features at the physical level, which made the digital content of SACD discs difficult to copy until the jailbreak of the PlayStation 3. 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 disc 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 and Salzburg. However, PlayStation 3 with a SACD drive and appropriate firmware can use specialized software to extract a DSD copy of the HD stream.
Sound quality parameters achievable by the Red Book CD-DA and SACD formats compared with the limits of human hearing are as follows:
|Dynamic range||90 dB,
120 dB (with shaped dither)
|105 dB||120 dB|
|Frequency range||20 Hz – 20 kHz||20 Hz – 50 kHz||20 Hz – 20 kHz (young person);|
upper limit 8 to 15 kHz (middle-aged adult)
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 a compact disc audio (44.1 kHz/16 bit) conversion of the same source material 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. When the level of the signal was elevated by 14 dB or more, the test subjects were able to detect the higher noise floor of the CD quality loop easily. 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.
Comparison with DVD-A
Contradictory results have been found when comparing DSD and high-resolution PCM formats. 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. A 2003 study found that despite both formats' extended frequency responses, people could not distinguish audio with information above 21 kHz from audio without such high-frequency content. In a 2014 study, however, Marui et al. found that under double-blind conditions, listeners were able to distinguish between PCM (192 kHz/24 bits) and DSD (2.8 MHz) or DSD (5.6MHz) recording formats, preferring the qualitative features of DSD, but could not discriminate between the two DSD formats.
The Sony SCD-1 player was introduced concurrently with the SACD format in 1999, at a price of approximately US$5,000. It weighed over 26 kilograms (57 lb) and played two-channel SACDs and Red Book CDs only. Electronics manufacturers, including Onkyo, Denon, Marantz, Pioneer and Yamaha offer or offered SACD players. Sony has made in-car SACD players.
The first two generations of Sony's PlayStation 3 game console were capable of reading SACD discs. Starting with the third generation (introduced October 2007), SACD playback was removed. All PlayStation 3 models however will play DSD Disc format. PlayStation 3 was capable of converting multi-channel 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.
Unofficial playback of SACD disc images on a PC is possible through freeware audio player foobar2000 for Windows using an open source plug-in extension called SACDDecoder. Mac OS X music software Audirvana also supports playback of SACD disc images.
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Such a format—Super Audio CD (SACD)—has been out there since September 1999. And now, it's dying.
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