Super audio CD (SACD) is a new audio recording format aimed at providing higher fidelity audio reproduction than the compact disc. It was developed by Sony and Philips, the same companies that created the CD.
Although the same physical size as the compact disc, SACD uses a different kind of digital audio signal, Direct Stream Digital (DSD). DSD is 1-bit and has a sampling rate of 2,822,400 Hertz. This gives the format a greater dynamic range and wider frequency response than the CD. At the optical level, SACDs are essentially DVD format, which is why inexpensive SACD compatible DVD players are on the market.
All (?) SACD disks contain a 2 channel stereo mix. Many also contain a 5.1 surround sound mix.
Hybrid SACD's are dual layer, with a CD layer (16 bits/44.1kHz) compatible with traditional CD players. using a process called Super Bit Mapping Direct, the hybrid disc's CD layer can approach 20-to-24-bit precision . This means if the format takes off retailers will be able to stock one title that is compatible with the installed base of CD players as well as new SACD players.
SACD also has copy protection features at the physical level, which for the moment appear to make this format nearly impossible to pirate. These include 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.
Currently, there appears to be a high resolution format war between Super Audio CD and DVD-Audio.