Super Audio 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 present there is some discussion as to whether 1-bit Sigma-delta modulation is suitable for high-quality audio Audio Engineering Society Convention Paper 5395.

At the optical level, SACDs are essentially DVD format, which is why inexpensive SACD-compatible DVD players are on the market.

While the frequency response of a normal CD is technically beyond the typical human's range of hearing, SACD offers an even wider response than CD does. For a consumer level format, it is probably much higher than necessary. However, for music mastering, and intermediate production, a wider frequency range can be advantageous.

SACD disks may contain a 2-channel stereo mix, a 5.1 surround sound mix, or both.

Hybrid SACDs are dual layer, with a CD layer (PCM digital audio/16 bits/44.1kHz) compatible with traditional CD players. Using noise shaped dithering, the hybrid disc's CD layer can approach up to 20-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. The disks can also come in Dual Layer (two high density DSD layers) and Single Layer (single, high density DSD layer) versions, both of which are only playable on SACD compatible hardware.

SACD also has copy prevention 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. SACD can't be played on a computer, nor can SACDs be created except by a licensed disc replication facility.

Currently, there appears to be a high resolution format war between Super Audio CD and DVD-Audio. Another promising challenger is the DualDisc format.

At present SACD format has only taken the interest of audiophiles, with virtually no mainstream acceptance. Most reviews in "high-end" hi-fi magazines hold the opinion that SACD is an improvement over CD, but does not meet the standards of a good Vinyl record (of course, that vinyl sounds better than even regular CD is contentious issue). On the average person's stereo it would be difficult to tell the difference between CD and SACD and as such the higher fidelity is not of interest to many people, especially given the inconvenience of not being able to make backup copies or transfer the music to a portable digital audio player. Some experiments have supported the idea that only a minority of people have good enough hearing (so called "golden ears") to hear the difference, even on very good equipment. If in the future hybrid discs were to become the main music format it is likely the SACD layer would not be played by most people, and as such the price would probably have to be roughly that of a regular CD. Several times mainstream consumers have chosen a cheap and convenient format with "good enough" sound quality over a state of the art format. For example, the rise of mp3 over CD audio.