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Super Audio CD

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Super Audio CD
File:SACDlogo.png
Media typeoptical disc
Encodingdigital (DSD)
Capacityup to 7.95 GiB
Read mechanism650 nm laser
Developed bySony & Philips
Usageaudio storage

Super Audio CD (SACD) is a read-only optical audio disc format aimed at providing much higher fidelity digital audio reproduction than the compact disc. Introduced in 2000, it was developed by Sony and Philips Electronics, the same companies that created the CD. SACD is currently in a format war with DVD-Audio. Although neither side has made significant progress toward acquiring consumer acceptance, SACDs have an advantage over DVD-Audio in that most discs are hybrids compatible with existing CD players. Media players that can play many formats (including DVD-Audio and SACD) are available for under $100 [1].

Overview

Optical discs
General
Optical media types
Standards
See also

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 MHz. This is 64 times the sampling rate used in Compact Disc Digital Audio, which specifies 44.1 kHz at a resolution of 16 bit.

SACD authoring guidelines suggest that an SACD should always contain a 2-channel stereo mix[citation needed] though not all SACD have it (for example, in 2005 Sony Music Entertainment (Germany) GmbH released Charles Rosen’s performance of the Goldberg Variations as a hybrid SACD with 16-bit PCM and DSD 5.1 surround but no DSD stereo). They may optionally contain a surround mix — either 5.0 or 5.1 layout. Although the disc always stores all channels, the surround mix does not have to use them all, and some may be mute; for example the 2001 SACD release of Mike Oldfield's Tubular Bells remains in the quadraphonic 4.0 mix made in 1975, and the RCA reissue of the 1957 Chicago Symphony Orchestra recording of Mussorgsky's Pictures at an Exhibition marks the first time the original 3.0 (three track) recording is available in a consumer format. The correct designation for the surround part of a SACD is "multi-channel", and usually has either the label "SACD Surround" or 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 a "Red Book" layer compatible with most legacy 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 HD layer with no CD layer.
  • Dual-layer: Physically a DVD-9 DVD, a dual-layer SACD includes two HD layers totalling 8.5 GB, with no CD layer. This type is rarely used. It enables nearly twice as much data to be stored, but eliminates CD player compatibility.

Integration

As of April 2007, there have been over 4,500 SACD releases, a little over half of which appear to be classical. Jazz appears to have fewer releases, and popular music, mainly remastered previously released albums, appears to have even fewer releases than Jazz.[2][3]. Notable artists who have released some or all of their back catalog include: Peter Gabriel, Genesis (band), Bob Dylan, Elton John, The Moody Blues, The Rolling Stones and Nine Inch Nails. Pink Floyd's seminal album The Dark Side of the Moon (the 30th anniversary edition of 2003), The Who's seminal album Tommy (the 34th anniversary edition of 2003) and Roxy Music's Avalon (the 21st anniversary edition, 2003) were released on SACD to take advantage of the format's multi-channel capability. Both were remixed in 5.1 surround, and released as Hybrid SACDs, with a stereo mix on the standard CD layer.

Because most SACDs are now issued in a hybrid format only, such as the remastered Rolling Stones and Bob Dylan 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 may give the SACD format an advantage over DVD-Audio because the hybrid discs are designed to play on all standard Red Book CD players.

The format continues to attract a few major artists; however, the main interest continues to be classical and older remastered albums rather than new releases. SACD also continues to face sharp pressure from DVD-Audio and DualDisc releases and customer uncertainty over the competing formats. (This uncertainty may wane as players capable of both formats, such as the Denon DVD-1930CI, become affordable.)

Disc reading

File:Sacd model.jpg
How a hybrid Super Audio CD works

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 without difficulty 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, combined DVD/SACD players, in-car players[1], and Sony's PlayStation 3 game console.

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.[2]

Many other vendors offer SACD playback capabilities in their product lines, although none has offered a portable Walkman-style SACD player capable of playing the high definition layer of an SACD. Most portable CD players will play the conventional CD layer of a Hybrid SACD.

SACD players are not permitted to digitally output an unencrypted stream of DSD. Players initially supported only analog output; later some proprietary digital interfaces such as Denon Link permitted encrypted transmission of DSD. There are now two standard digital connection methods capable of carrying DSD in encrypted form: i.Link and HDMI (version 1.2 or later, standardised in August 2005).

i.Link is generally found on mid- to high-end equipment. HDMI is more common, being the standard digital connection method for high-definition video+audio, but as of December 2006, there are only a few A/V processors supporting DSD over HDMI 1.2 (such as the Marantz SR7001), but no players.

Some players, such as the PlayStation 3, don't output DSD over HDMI, but instead convert it to PCM.

DSD

File:DSDlogo.gif

SACD audio is stored in a format called Direct Stream Digital (DSD), which differs from the conventional PCM used by the compact disc or conventional computer audio systems.

DSD is 1-bit, has a sampling rate of 2.8224MHz, 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. Promotional materials about SACD supplied by Philips and Sony suggest that the system is capable of delivering a dynamic range of 120dB from 20Hz to 20kHz and an extended frequency response up to 100 kHz, although most currently available players list an upper limit of 80-90kHz.

The process of creating a DSD signal is conceptually similar to taking a 1-bit delta-sigma analog-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 bits provides a (theoretical) additional 24dB of dynamic range. Due to the effects of quantization noise, the usable bandwidth of the SACD format is approximately 100kHz, which is similar to 192kHz PCM.

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[citation needed]. 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 30kHz for compatibility and safety reasons, suitable for situations where amplifiers or loudspeakers can't deliver an undistorted output if noise above 30kHz is present in the signal.)

DST

To reduce the space and bandwidth requirements of DSD (2.8 Mbps per channel), a lossless compression scheme 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, typically allowing a disc to contain 80 minutes of both 2-channel and 5.1-channel sound.

PSP

File:PSPlogo.gif

SACD includes various copy protection measures of which the most prominent is Pit Signal Processing (PSP), a physical watermarking feature that contains a digital watermark modulated in the width of pits on the disc (data is stored in the pit length). The optical pickup must contain special circuitry to read the PSP watermark, which is then compared to information on the disc to make sure it's legitimate. Because the majority of DVD players and all DVD-ROM drives use an optical pickup that lacks this specialized watermark detection circuitry they cannot read the data on the SACD layer of a protected SACD disc. [4]

On hybrid SACD discs, PSP is only applied to the SACD layer – not to the CD layer.

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 [citation needed]. 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 20kHz, 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.

Conversely, the properties of DSD and the authoring process tend to discourage the kind of extreme compression and unpleasant-sounding hard digital clipping often found on PCM recordings. Unlike CD, which sets the 0dB level right at the theoretical PCM signal limit, and doesn't take into account oversampling, SACD sets the 0 dB level at 6 dB below the theoretical full-scale DSD signal, and prohibits peaks above +3 dB. DSD processing is less amenable to simple clipping to meet these limits, forcing more care to be taken during mastering. The extra headroom also eases the job of D-to-A converters in playback equipment, which often suffer overload distortion when fed the full-scale PCM common on heavily-compressed CDs.[3][4] Thus, improved quality may result by simply preventing the kinds of poor mastering often found on PCM, rather than any fundamental audible difference between DSD and PCM; PCM mastered several dB lower would also obtain the same benefit.

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, and allowing an 80kHz upper limit, when human hearing is less than 20 kHz may be considered useless. 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, appear to make SACD discs impossible to copy without resorting to the analog hole. 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. [5]

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, and it may be possible to get the raw DSD data from these links. 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.

There seems to be one solution for obtaining digital non-DRM output on SACD as well as DVD-A players. A Switzerland-based company is offering a modified output-board that taps into the digital datastream prior to D/A conversion as well as converting DSD to PCM that the S/PDIF port can transfer. [6]

See also

References

  1. ^ "Sony Announces Three Super Audio CD Car Stereo Players". HighFidelityReview.com. Retrieved 2007-01-18.
  2. ^ "The Sony SCD-1 SACD Player". @udiophilia. Retrieved 2006-05-18.
  3. ^ "Issues with 0dBFS+ Levels On Digital Audio Playback Systems". Audioholics. Retrieved 2007-05-10.
  4. ^ "Overload in Signal Conversion" (PDF). AES 23rd International Conference. Retrieved 2007-05-10.
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