Articles and whitepapers

5/5/2003

What is Super Audio CD (SACD)?

By Sony

Super Audio CD is a new standard for pure audio which offers playback quality that is far richer than conventional CD sound. Exponents maintain that it allows you to feel the artistry of the performer, the ambience of the hall, and even the colour of the music, like never before.

Super Audio CD (SACD) represents a revolution in the way digital data is recorded and played back, by converting sound signals using 1-bit Direct Stream Digital technology. Its exceptional sound quality exceeds the reproduction capabilities of most existing hi-fi equipment, and so requires new audio products with higher specifications.

The SACD format is capable of storing stereo recordings, as well as multichannel recordings and additional data. SACD players are capable of playing traditional CDs, and special hybrid CD/SACD discs can be played in traditional CD players.

Hybrid SACD logos - these play on traditional CD players

SACD logos - these only play on SACD players

Improved audio qualtiy

Developed by Sony and Philips in the early 1980s, the CD system converted music signals into digital signals for recording and playback. CD uses a PCM (Pulse Code Modulation) recording format, in which the analogue music signal is converted into digital data by sampling the signal 44,100 times a second (44.1kHz), and converting each sample into a 16-bit binary number. The digital data is stored on the CD, and when played back, is converted back to analogue signals that we can hear via loudspeakers or headphones.

When the CD format was being developed, PCM represented the most advanced technology available for recording. However, although the playback frequency of 44.1kHz with 16-bit quantisation was designed to match the audible range of human hearing (i.e. a frequency response of over 20kHz and a dynamic range of up to 96dB), it represented just one small part of all the sounds that exist in nature.

With the continuous progress being made in digital technology, it became possible to develop a new format with even higher sound fidelity than CD. Sony and Philips once again joined forces - this time resulting in the creation of Super Audio CD - but rather than just offering improvements in the sampling frequency and quantisation, SACD represents a whole new approach to digital recording by converting sound signals using 1-bit Direct Stream Digital (DSD) technology.

Direct Stream Digital

The DSD process uses a 2.8224MHz sampling rate to record a stream of 1-bit pulses that are capable of capturing a full range of naturally occurring sounds, with unparalleled audio resolution.

Put simply, the DSD format expresses music signals as a concentration of pulses. Although the DSD sound signals expressed are digital data (i.e. concentrations of 1s interspersed with 0s, and vice versa), their pattern is extremely similar to the actual sound wave you would hear. So with very little processing, it is possible to retrieve the original analogue signals with amazing accuracy, from this digital data.

Compared to PCM, the digital data processed with DSD follows the shape of the analogue wave much more closely. DSD's simplified mechanism for recording and playback results in a frequency response of over 100kHz and a dynamic range of over 120dB across the audible frequency range. The end result is music reproduction that is remarkably pure and faithful to the original.

Furthermore, the DSD format can accommodate more than four times the information of the current CD/PCM format, prompting the development of large-capacity discs for SACD use. With this extra capacity, a standard SACD can accommodate a two-channel stereo recording on the same disc as a multichannel six-track recording, as well as extra data such as still images, song text and other information.

CD compatibility

Three disc variations are available for the SACD format. The single-layer and dual-layer SACD discs contain either one or two high density (HD) layers respectively. A further version is a hybrid disc, which consists of one HD layer and one CD layer. There is forwards and backwards compatibility between SACD and CD because the CD layer of the hybrid disc can be read by conventional CD players, and SACD players can playback standard CDs.

Three SACD variations

If the CD layer was produced using SBM Direct (Super Bit Mapping Direct) technology, the high-quality DSD format data is accurately down-converted for even higher-quality playback from this CD layer.

Security

To combat the problem of music piracy, SACD uses five complimentary means of visible and invisible security. Each provides a different obstacle to unauthorised copying of the disc's contents, and if one line of defence is broken, the others continue to operate, providing the strongest protection against piracy.

Multichannel Audio

One of the most significant advances in Super Audio CD's listening experience is multichannel sound. SACD Multichannel enfolds the listener in a new sensory experience as sound from multiple speakers recreates the original concert venue in its true character and spatial dimensions.

Each of the world's great concert halls has its own, distinct sonic signature caused by sound reflected from the walls and ceiling that gradually decays, lending a particular depth to instruments and voices. SACD Multichannel records all components of sound, both direct and reflected, through multiple channels that can be recreated through the same number of loudspeakers. This allows instruments to keep their position and dimension, making the sonic fingerprint and space of the hall real parts of the listening experience.

A typical multichannel SACD playback system includes a multichannel-capable player with six channels of independent amplification and six independent speaker outputs. Five speakers typically surround the listener in circular fashion, in accordance with ITU (International Telecommunications Union) recommendations, and the remaining channel is used for low frequency enhancement (subwoofer).

More choice on one disc

The development of Direct Stream Transfer (DST) lossless coding makes it possible to store both stereo and multichannel DSD content on a single High Density layer. Separate data areas are provided for stereo and multichannel content, giving artists the flexibility to provide two different mixes of the same content on a single layer of the disc.

Faithful rendition of concert hall ambiance is just one of the many possibilities of SACD multichannel sound. The discrete surround channels, which are totally separate from the main front channels, can also be used to record any content the artist or producer desires, whether it be sound effects, choruses located in the rear, or other innovative recording techniques. Moreover, archived three-channel and four-channel recordings can be faithfully re-released in the SACD format, while the future may bring any number of new multichannel recording techniques yet to be invented.

A range of speaker configurations are possible to suit different domestic settings and produce the sound experience that you desire.

www.superaudio-cd.com