Super Audio CD (SACD) is an optical disc format for audio storage introduced in 1999. It was developed jointly by Sony and Philips Electronics and intended to be the successor to the compact disc (CD) format. The SACD format allows multiple audio channels (i.e., surround sound or multichannel sound). It also provides a higher bit rate and longer playing time than a conventional CD. An SACD is designed to be played on an SACD player. A hybrid SACD contains a Compact Disc Digital Audio (CDDA) layer and can also be played on a standard CD player.
History The Super Audio CD format was introduced in 1999, and is defined by the Scarlet Book standard document. Philips and Crest Digital partnered in May 2002 to develop and install the first SACD hybrid disc production line in the United States, with a production capacity of up to three million discs per year. SACD did not achieve the level of growth that compact discs enjoyed in the 1980s, and was not accepted by the mainstream market. By 2007, SACD had failed to make a significant impact in the marketplace; consumers were increasingly downloading low-resolution music files over the internet rather than buying music on physical disc formats. A small and niche market for SACD has remained, serving the audiophile community.
Content Many popular artists have released some or all of their back catalog on SACD. Pink Floyd's album The Dark Side of the Moon (1973) sold over 800,000 copies by June 2004 in its SACD Surround Sound edition. The Who's rock opera Tommy (1969), and Roxy Music's Avalon (1982), were released on SACD to take advantage of the format's multi-channel capability. All three albums were remixed in 5.1 surround, and released as hybrid SACDs with a stereo mix on the standard CD layer. Some popular artists have released new recordings on SACD. Sales figures for Sting's Sacred Love (2003) album reached number one on SACD sales charts in four European countries in June 2004. Between 2007 and 2008, the rock band Genesis re-released all of their studio albums across three SACD box sets. Each album in these sets contains both new stereo and 5.1 mixes. The original stereo mixes were not included. The US and Canada versions do not use SACD but CD instead. By August 2009, 443 labels had released one or more SACDs. Many SACD discs that were released from 2000 to 2005 are now out of print and available only on the used market. By 2009, the major record companies were no longer regularly releasing discs in the format, with new releases confined to the smaller labels.
Technology
SACD discs have identical physical dimensions as standard compact discs. The areal density of the disc is the same as a DVD. There are three types of disc:
Hybrid: Hybrid SACDs have a 4.7 GB SACD layer (the HD layer), as well as a CD (Red Book) audio layer readable by most conventional compact disc players. Single-layer: A disc with one 4.7 GB SACD layer. Dual-layer: A disc with two SACD layers, totaling 8.5 GB, and no CD layer. Dual-layer SACDs can store nearly twice as much data as a single-layer SACD. Like most dual-layer DVDs, the data spiral for the first layer is encoded from the inside out, and the second layer is encoded starting from the point where the first layer ends and ending at the innermost part of the disc. Unlike hybrid discs, both single- and dual-layer SACDs are incompatible with conventional CD players and cannot be played on them. A stereo SACD recording has an uncompressed rate of 5.6 Mbit/s, four times the rate for Red Book CD stereo audio. Commercial releases commonly include both surround sound (five full-range plus LFE multi-channel) and stereo (dual-channel) mixes on the SACD layer. Some reissues retain the mixes of earlier multi-channel formats (examples include the 1973 quadraphonic mix of Mike Oldfield's Tubular Bells and the 1957 three-channel stereo recording by the Chicago Symphony Orchestra of Mussorgsky's Pictures at an Exhibition, reissued on SACD in 2001 and 2004 respectively).
Disc reading
Objective lenses in conventional CD players have a longer working distance, or focal length, than lenses designed for SACD players. In SACD-capable DVD, Blu-ray and Ultra HD Blu-ray players, the red DVD laser is used for reading SACDs. This means that when a hybrid SACD is placed into a conventional CD player, the infrared laser beam passes through the SACD layer and is reflected by the CD layer at the standard 1.2 mm distance, and the SACD layer is out of focus. When the same disc is placed into an SACD player, the red laser is reflected by the SACD layer (at 0.6 mm distance) before it can reach the CD layer. Conversely, if a conventional CD is placed into an SACD player, the laser will read the disc as a CD since there is no SACD layer.
Direct Stream Digital
SACD audio is stored in Direct Stream Digital (DSD) format using pulse-density modulation (PDM), where audio amplitude is determined by the varying proportion of 1s and 0s. This contrasts with compact disc and conventional computer audio systems using pulse-code modulation (PCM), where audio amplitude is determined by numbers encoded in the bit stream. Both modulations require neighboring samples to reconstruct the original waveform; the more neighboring samples, the lower the frequency that can be encoded. DSD is 1-bit, has a sampling rate of 2.8224 MHz, and makes use of noise shaping quantization techniques in order to push 1-bit quantization noise up to inaudible ultrasonic frequencies. This gives the format a greater dynamic range and wider frequency response than the CD. The SACD format is capable of delivering a dynamic range of 120 dB from 20 Hz to 20 kHz and an extended frequency response up to 100 kHz, although most available players list an upper limit of 70–90 kHz, and practical limits reduce this to 50 kHz. Because of the nature of sigma-delta converters, DSD and PCM cannot be directly compared. DSD's frequency response can be as high as 100 kHz, but frequencies that high compete with high levels of ultrasonic quantization noise. With appropriate low-pass filtering, a frequency response of 20 kHz can be achieved along with a dynamic range of nearly 120 dB, which is about the same dynamic range as PCM audio with a resolution of 20 bits.
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