delta conversion

Super Audio CD

Super Audio CD (SACD) is a read-only optical audio disc format that can provide higher fidelity digital audio reproduction than the Red Book audio CD. Introduced in 1999, it was developed by Sony and Philips Electronics, the same companies that created the Compact Disc. SACD is in a format war with DVD-Audio, but neither format has managed to replace regular audio CDs.


SACD is a disc of identical physical dimensions to the compact disc but it 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. Because the resolution of SACD is 16 times smaller than CDDA, the bitrate for a given channel is only 4 times as large.

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." It is not uncommon for hybrid discs to carry the "Compact Disc Digital Audio" logo to show that the disc is CDDA-compliant.
  • 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.

SACD authoring guidelines suggest that an SACD should always contain a 2-channel stereo mix 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 an SACD is "multi-channel", and usually has either the label "SACD Surround" or its own "Multi-Ch" logo on the back cover.


As of February 2008, there have been over 5,000 SACD releases, slightly more than 50% of which appear to be classical. Jazz and popular music albums, mainly remastered previous releases, are the next two most common genres released to date. Notable popular artists who have released some or all of their back catalog include Alice In Chains, Aerosmith, Björk, Black Sabbath, Peter Gabriel, Genesis, Depeche Mode, Bob Dylan, Elton John, The Moody Blues, The Rolling Stones, Eleanor McEvoy, Nine Inch Nails, Santana and David Bowie. 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. All three were remixed in 5.1 surround, and released as Hybrid SACDs with a stereo mix on the standard CD layer.

Because most SACDs are issued in a hybrid format only, such as the remastered Rolling Stones and Bob Dylan albums released in 2002, music collectors may build and enjoy an SACD collection even if their only disc player is not designed to read the SACD's higher fidelity DSD encoding. The ability to play SACD hybrid discs on all standard Red Book CD or DVD video players is considered a significant advantage of the SACD format over DVD-Audio. Today, there are many affordable "universal" multiformat players which play not only SACD but also competing formats DVD-Audio or DualDisc.

Despite relatively wide consumer availability to SACD players, however, high definition audio formats continue to attract few major record labels, which in the mid-2000s increased marketing of low-cost compressed audio formats. The main interest continues to be classical and older remastered albums rather than new releases. The record label Mobile Fidelity focuses on this type of remastering.

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 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, requires a Super Audio CD player.

As would be expected, Sony and Philips--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, and Sony's PlayStation 3 game console. (As of the 2.00 upgrade, PS3 is capable of SACD 5.1 playback via an optical cable. It achieves this by converting the audio to a 1.5 Mbit/s DTS format. The immediate revision after this removed the feature.)

The Sony SCD-1 is a well-known player which was introduced concurrently with the SACD format in 1999 for a price of approximately US$5,000.. It weighs well over 26 kg (57 lb) and is often modified by its owners to improve the sound. The SCD-1, no longer produced, was introduced before multi-channel SACDs existed and only plays two channel SACDs or red-book CDs.

Many other electronics manufacturers, including mid-level vendors Denon/Marantz, Pioneer, Yamaha offer SACD playback capabilities throughout their product lines. None, however, has offered a portable 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).

The older i.Link interface is generally found on older mid- to high-end equipment and some current top of the line units from the Japanese manufacturers. HDMI is more common, being the standard digital connection method for high-definition video+audio. Most new mid-level and higher 2007 model year and later A/V processors support the HDMI 1.2 specification's DSD over HDMI feature. Most boutique manufacturers still do not support DSD. Some HDMI 1.1 spec DVD players convert DSD to LPCM and then pass it to an HDMI 1.1 spec or later processor. Lower end processors usually convert the DSD to LPCM, higher end ones usually convert it to LPCM for bass management or DSP but can also process it natively at the expense of DSP and bass management. Some new DVD players from Oppo Digital, Pioneer, Onkyo, etc. now support HDMI 1.2 or 1.3 and will pass DSD over HDMI as well as LPCM. Be aware that some players, for instance, Onkyo DV-SP504, will not support DSD or LPCM over HDMI without downscaling it to 48kHz. SACD or DVD-A will be played through anlaog outputs instead. The older i.Link interface has been dropped from all but high end A/V processors and DVD players.

Some players, such as the PlayStation 3 (not the 40GB version), do not output DSD over HDMI, but instead convert it to PCM.


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 2.8224 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. 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 currently available players list an upper limit of 80–90 kHz.

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 analog 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 192 kHz. PCM sampled at 24 bits provides a (theoretical) additional 24 dB of dynamic range. Due to the effects of quantization noise, the usable bandwidth of the SACD format is approximately 100 kHz, which is similar to 192 kHz 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 rock and contemporary music which relies on multitrack techniques — are in fact mixed in PCM (or mixed analog 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 2.8224 MHz (64Fs) sampling frequency while the highest frequency in which PCM is being edited is 352.8 kHz (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, owing to its high sampling frequency, does not show the ringing effects that PCM shows with certain types of signals when sharp reconstruction filters are employed, but instead it shows constant high levels of noise at the same frequencies this ringing would show in 192 kHz 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 20 kHz. 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.)


To reduce the space and bandwidth requirements of DSD (2.8 Mbit/s per channel), 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.

Pit Signal Processing

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.

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. 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.

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.

Few home audio systems can accurately reproduce sounds above 20 kHz, and most recording chains are designed around this limit. Modern popular music is often compressed to a small percentage of the maximum available dynamic range, and thus would not significantly benefit from the extended dynamic range available in SACD. However, electronic and organ music offer a wide natural dynamic range, and audiophiles 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 0 dB 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 DACs in playback equipment, which often suffer overload distortion when fed the full-scale PCM common on heavily-compressed CDs. Thus, improved quality may result from simply preventing the kinds of poor mastering often found on PCM, rather than from any fundamental audible difference between DSD and PCM. PCM mastered several dB lower would also obtain the same benefit.

A 2007 article published in the Journal of the Audio Engineering Society reported the results of a study, which concluded that listeners could not hear the difference between a high-resolution two-channel recording and a CD-quality downsampling of the same recording except when "unpleasantly (often unbearably) loud. The article concluded that many high-resolution releases sounded better than their CD counterparts, but attributes this to mastering differences.

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, or ripping of the conventional 700MB layer on hybrid discs. 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.

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 no practical way for the public to make their own SACDs, this does not pose a major threat.

The failure of the format to gain any significant market share further reduces the incentive, whether commercial (e.g. Slysoft's payware AnyDVD for DVDs and Hi-Def discs) or curiosity to create copying tools (e.g. "DVD" Jon's DeCSS for DVDs). The quick defeat of protection methods used on HD DVD and Blu-ray Disc, when they accounted for only 4% of digital movie sales in 2007, shows that even a marginal market share can create demand for technologies that enable consumers to bypass the protection.

A number of new SACD players have encrypted IEEE 1394 (also called FireWire or i.Link) or HDMI 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.

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