Super Audio CD

SACD (Super Audio CD, 超级音频CD) is an optical disc audio format jointly developed by Sony and Philips.

The two companies announced a licensing program in 1998, and Sony launched the first players and discs in Japan in May 1999. SACD uses Direct Stream Digital (DSD) as the basic audio encoding for the high-density layer and supports both stereo and multichannel programs.

DSD records signals with 1-bit quantization and a sampling frequency of 2.8224 MHz—a frequency 64 times that of 44.1 kHz—and is often referred to as DSD64. It represents changes in the signal between adjacent samples via a high-speed bitstream and uses noise shaping to shift most quantization noise above the audible frequency range; the playback device requires a low-pass filter to suppress high-frequency noise. DSD and multi-bit LPCM use different digital representation methods, but both require complete analog-to-digital and digital-to-analog conversion, filtering, and production workflows.

SACD retains the 120-millimeter disc format, with the high-density layer using a data density similar to that of a DVD. Single-layer discs have only one high-density layer of approximately 4.7 GB, while dual-layer discs have approximately 8.5 GB. Hybrid SACDs combine a semi-reflective high-density layer with a standard CD layer compliant with the CD-DA specification on a single disc; traditional CD players focus on and read the CD layer, while SACD devices read the high-density layer. The two physical layers of a hybrid disc store independent programs. The CD layer must be 44.1 kHz, 16-bit stereo PCM, while the high-density layer is DSD; there is no compatibility process during playback that involves real-time downsampling from DSD to CD. The two layers may originate from the same master or may use different remastering, level, dynamic processing, or editing versions; therefore, identical track titles do not guarantee that the digital content is identical.

The high-density layer can be configured with both stereo and multichannel sections. Multichannel programs use up to six full-range channels, and producers may organize the content according to a 5.1 or other permitted speaker configuration; not all discs contain both stereo and multichannel versions. The player selects programs based on region directories; stereo and multichannel sections may have different mixes and track lengths. To control the data volume of multichannel DSD, SACD may use DST (Direct Stream Transfer) lossless compression. DST reduces the bitrate by leveraging statistical relationships between adjacent samples and channels, restoring the original DSD data upon decoding; its use depends on the program length, number of channels, and mastering choices. DST is unrelated to DTS and should not be confused with lossy encoding, which discards audio information.

SACD is a comprehensive specification covering physical media, file structures, program regions, and protection mechanisms; DSD is merely one of its audio encoding methods.

Files such as DSF and DFF can store DSD audio and be played back by computers or network players, but they lack the SACD disc directory, mix layer, and physical authentication data. Conversely, burning DSD files to a standard DVD or BD data disc will not result in a disc that complies with the SACD specification. The high-density layer of commercial SACDs contains content protection measures such as encryption, physical watermarks, and disc authentication. Even if a standard computer optical drive can read the CD layer, it typically cannot access the high-density layer as a conventional file system. A few specific optical drives or early video game consoles have read paths that can be exploited by specialized tools, but this is not part of the standard data access method provided by SACD for consumers.

Early playback devices mostly transmitted decoded audio via analog output, while later general-purpose players could use IEEE 1394, HDMI, or manufacturer-specific digital links. A device’s ability to read SACDs, internally decode DSD, transmit in native DSD, and output DSD after converting it to PCM are distinct capabilities. The fact that the receiving end displays PCM does not necessarily mean the disc uses PCM encoding; rather, it may indicate that the conversion occurred within the player itself.