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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 mechanism600 µm laser
Developed bySony & Philips
Usageaudio storage

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).

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 — with the typical sampling rate present on audio CDs currently being 44.1kHz at a resolution of 16 bit.

SACDs must always contain a 2-channel stereo mix and 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 remained in the old quadraphonic 4.0 mix. 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 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 removes 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 (source: http://www.sa-cd.net). 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, early Elton John, Derek and the Dominos' Layla, Aerosmith's Toys in the Attic, Meat Loaf's Bat Out Of Hell, 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 video game system 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

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 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.[1]

Many other vendors offer SACD playback capabilities in their product lines, although none has offered a portable, Walkman-style SACD player.

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 few A/V processors supporting DSD over HDMI 1.2 (such as the Marantz SR7001), but no players.

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

DSD

File:DSDlogo.gif

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.)

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.

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 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.

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 overssampling, 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. So improved quality may result by simply preventing 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 get 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. 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 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. [1]

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.

There seems to exist 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 spdif port can transfer. [2]

See also

References

  1. ^ "The Sony SCD-1 SACD Player". @udiophilia. Retrieved 2006-05-18.
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