Super Audio CD - Technology

Technology

SACD is capable of encoding recorded audio in either stereophonic sound or surround sound. Although SACD audio streams are encoded in a pulse-density modulation (PDM) scheme called Direct Stream Digital (DSD), a manufacturer may also write a Pulse-code modulation (PCM) "layer" compatible with conventional Compact Disc players.

SACD is a disc of identical physical dimensions to a standard compact disc; the density of the disc is the same as a DVD and it encodes audio using a process known as Direct Stream Digital. The SACD sampling rate is 2.8224 MHz and the resolution is one bit. A stereo SACD recording can stream data at an uncompressed rate of 5.6 Mbit/s, four times the rate for Red Book CD stereo audio. SACD recordings can have a wider frequency and dynamic range than conventional CDs.

There are three types of SACDs:

• Hybrid: Hybrid SACDs are encoded with a 4.7 GB DSD layer (also known as the HD layer), as well as a PCM (Red Book) audio layer readable by most conventional Compact Disc players.
• Single-layer: A DVD-5 encoded with one 4.7 GB DSD layer. Single-layer SACDs are not backward-compatible with conventional CD players.
• Dual-layer: A DVD-9 encoded with two DSD layers, totaling 8.5 GB, and no PCM layer. Dual-layer SACDs can store nearly twice as much data as a single-layer SACD. Like single-layer SACDs, dual-layer discs are not backward-compatible with conventional CD players.

Almost all commercially released SACDs have included both stereo (dual-channel) and surround sound (multi-channel) mixes. A multi-channel mix need not be surround, however; some of the Living Stereo reissues (such as the RCA reissue of the 1957 Chicago Symphony Orchestra recording of Mussorgsky's Pictures at an Exhibition) use only the three front channels to reproduce the original three-track (3.0) stereo recordings. Nor is a surround mix obliged to use all six SACD channels (five full-range plus LFE). For example, the 2001 SACD release of Mike Oldfield's Tubular Bells remains in the quadraphonic 4.0 mix.

The CD layer exists primarily for backward compatibility, but is not required. If the CD layer is omitted, the SACD need not be limited to an 80-minute playing time. For stereo material, the space that would have been taken by the multi-channel program can be used to extend playing time to four hours or more. BIS has taken advantage of this to put all of Bach's organ music on five SACD-only disks.