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Super Audio CD

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Super Audio CD

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Super Audio CD (SACD) is a read-only optical audio disc aimed at providing 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.

Overview

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.

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). This type was often used by Sony Music Entertainment.
  • 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.

Integration

Indeed there is a format war between Super Audio CD and DVD-Audio. Another challenger is the DualDisc format. At present, these advanced formats have mainly taken the interest of audiophiles, with relatively little mass market acceptance.

As of May 2005, there have been approximately 3,000 SACD releases, about 40% of which are classical music (source: http://www.sa-cd.net). However, some more popular albums have been made as SACDs, including most of Peter Gabriel's catalogue, most of Bob Dylan's catalogue, Pink Floyd's seminal album Dark Side of the Moon (the 30th anniversary edition of 2003) 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, but not on the competing Multi-Channel compliant DVD-Audio as an incentive for record buyers to switch from CD to SACD. The competing DVD-Audio had its own incentive albums not released on SACD for the same purpose, including Queen's The Game and A Night at The Opera.

Because some discs 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 don't especially care about SACDs. This is bound to give the SACD format an advantage over DVD-Audio now that SACD playback equipment, in the form of "Universal" players, is inexpensive and readily available. At the same time, buyers who are looking for SACDs find hybrid discs attractive if they also play music on conventional CD players.

One issue in favour of DVD-Audio is the current lack of algorithms and hardware for dealing directly with DSD or Direct-Stream Digital audio (see below), the audio encoding method used by SACDs. 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. However, currently this cannot be done on DSD audio without first converting it to PCM audio like that used in DVD-Audio. Better quality surround receivers can convert the DSD bitstream to 192 kHz LPCM (linear Pulse-code modulation) 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 or DVD-Audio. This is no doubt because most people listen to music outside their house and couldn't hear the difference on their portable playback equipment anyway. Another reason is that people want to download music, which requires download times or file sizes well below what SACD or DVD-Audio streams would demand.

The Sony PlayStation 3, currently expected by November 2006, will include SACD support.

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 regular 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 m 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.

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.

Because of the nature of sigma-delta converters, one cannot make a direct comparison of the dynamic range and the frequency response between DSD and PCM. An approximation is possible, though, and would place DSD in some aspects comparable to a PCM format that has a bitdepth of 20 bits and a sampling frequency of 88 kHz, effectively making DSD a contender against the highest-resolution format among DVD-Audio PCM types, i.e. 24-bit sampled at 192 kHz.

Note that the two formats may still differ in terms of fidelity at high-frequency sounds since DSD, thanks to its high sampling frequency, does not show the typical ringing effects of reconstruction filters used with PCM. On the other side, DSD's dynamic range decreases quickly at frequencies over 20 kHz due to the use of strong noise shaping techniques, while PCM's dynamic range 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 harmonics above 30 kHz are present in the signal.)

Although Sony and Philips claim 1-bit DSD processing is superior to PCM, almost all units present on the market (including Sony's top SACD players) convert DSD stream to 88 kHz PCM before outputing to DA converters.

Comparison of SACD, DVD-Audio, and CD

There is no evidence that human beings are sensitive to audio frequencies above 20 kHz, and most people over the age of 35 are unable to hear sounds above 1516 kHz at 72 dB. There is consensus among some hi-fi experts that the ability of an audio system to reproduce sounds above 20 kHz is not needed strictly for reproduction of the amplitude aspect of musical content.

Within the limits of typical human hearing capabilities, the common digital audio formats, including CD, SACD and DVD-Audio, appear to be equivalent for typical listeners using low quality equipment. While some independent double-blind tests have failed to show distinguishable differences between SACD/DVD-Audio and the same audio stream downsampled to CD-Audio's sample rate and dynamic range, many people (usually the weakest link in objective quality assessment) do insist that a simple A/B comparison on even a moderately good system should reveal a stunning difference between SACD and either CD or DVD-Audio. The late film composer Jerry Goldsmith, for example, fiercely backed SACD.

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 or DVD-Audio. In comparison, acoustic performances of jazz, folk, classical and alternative music can definitely benefit from the lack of amplitude compression that extended dynamic range afford.

Increasingly, home audio playback systems are multichannel and this single feature is 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 while DVD-Audio titles can be authored to be both forward and backward compatible with existing DVD-Video 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, the SACD and DVD-Audio formats provide several additional features that can create an engaging and compelling listening experience.

Copy protection

SACD has several copy prevention features at the physical level which, for the moment, appears to make this format nearly impossible to perfectly 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. SACD can't be played on a computer, nor can SACDs be created except by a licensed disc replication facility. Copying the music may still be done via an analogue stage (for example, line-out of the SACD player to the line-in of a CD recorder), but doing so is imperfect since the conversion to and from analogue is lossy.

There has been speculation it may be possible to capture the digital signal after the decryption stage but before the digital to analog conversion stage of an SACD player, which would make it possible to create a bit perfect copy of the SACD. Because most of existing SACD players convert DSD directly to 88 kHz PCM, there exist add-on boards for several players on the market that allow to capture SACD music (only stereo though) through S/PDIF port. Also, since 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.

See also

External links


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Music Sound, v. 2.0, by MultiMedia

This guide is licensed under the GNU Free Documentation License. It uses material from the Wikipedia.


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