Super Audio CD
Super Audio CD (SACD) is a read-only optical audio disc format that can provide higher accuracy as well as surround sound compared to the audio CD format. Introduced in 1999, it was developed by Sony and Philips Electronics, the same companies that created the Compact Disc. SACD was 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.
SACD faces competition from several different formats. The CD continues to be the mainstream optical disk format for audio, mainly competing on price and the large installed base of compatible players. DVD-Audio was a format with somewhat similar features as SACD, but sales seems to have stagnated below the 2001-level. BD-Audio is a newer format that uses the Blu-ray Disc physical format for high-resolution, multichannel audio-only playback, although it remains to be seen what kind of support it will get from the content producers.
Although SACD and the traditional LP record both may have minuscule shares of the total market, they may be competing for the same customers within certain genres. Both are notably strong within classical, jazz and acoustic music, all of which has a notable appeal within the audiophile or high fidelity community. Some audiophiles think that the LP may outlive both CD and SACD in the long term.
With the media convergence foreseen, and popularity of non-physical media distribution such as iTunes, some think that physical media will lose market share to downloadable ones, even in the high fidelity market. Lossless flac media files with data rates equalling that of physical media are already available from niche record companies.