Super Audio CD

Super Audio CD (SACD) is a read-only optical audio disc aimed at providing higher fidelity digital audio reproduction than the compact disc. Introduced in 1999, it was developed by Sony and Philips Electronics, the same companies that created the CD.


Overview

SACD uses a very different technology from CD and DVD-Audio to encode its audio data, a 1-bit delta-sigma modulation process known as Direct Stream Digital at the very high sampling rate of 2.8224 megahertz.

SACDs must always contain a 2-channel stereo mix and may optionally contain a surround mix (usually the 5.1 layout) as well. To be precise, the so-called surround mix does not have to be in the 5.1 format. The old quadraphonic 4.0 format will do as well, most noticeably on the 2001 SACD release of Mike Oldfield's Tubular Bells. The correct designation for the surround part of a SACD is "multi-channel", and usually has its own "Multi-Ch" logo on the back cover.


Multi Channel

With the use of Super Audio CD you can add various sound experiences. SACD offers 4 possilities of recording and listening to the music, using multiple channels, instead of the 2 channels of stereo.

3 Channels

In the 3 Channel setup we are using a left, right and middle speaker, the sound gets an "in the audience" effect. The rear speakers are not used in this case.

4 Channels

In a 4 channel setup we are using front-left, front-right, surround-left and surround-right speakers. This way the sound gets a more positional affect. You can hear a sound actually get "over" you.

5 Channels

In a 5 channel setup we add in addition to 4 channel sound, a center speaker that is also used adding more positional possibilities.

5:1 Channels

In the 5:1 setup we have in addition to the 5 channel sound a special subwoofer activated for more bass power.


DSD

SACD audio is stored in a format called Direct Stream Digital (DSD), very different from the conventional PCM used by the compact disc or conventional computer audio systems.

DSD is 1-bit, has a sampling rate of 2.8224 megahertz, and makes use of noise shaping quantization techniques in order to push 1-bit quantization noise up to ultrasonic frequencies. This gives the format a greater dynamic range and wider frequency response than the CD. Promotional materials about SACD supplied by Philips and Sony suggest that the system 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 players list an upper limit of 80-90 kHz.

The process of creating a DSD signal is conceptually not unlike taking a 1-bit sigma-delta analogue-to-digital (A/D) converter and removing the decimator which converts the 1-bit bitstream into multibit PCM. Instead, the 1-bit signal is recorded directly and in theory only requires a lowpass filter to reconstruct the original analogue waveform. In reality it is a little more complex, and the analogy is incomplete in that 1-bit sigma-delta converters are these days rather unusual, one reason being that a 1-bit signal cannot be dithered properly: most modern sigma-delta converters are multibit.

Because of the nature of sigma-delta converters, one cannot make a direct comparison of the dynamic range and the frequency response between DSD and PCM. An approximation is possible, though, and would place DSD in some aspects comparable to a PCM format that has a bit depth of 20 bits and a sampling frequency of 88 kHz, effectively making DSD a poor contender against the highest-resolution format common in PCM, i.e. 24-bit sampled at 192 kHz, which provides a (theoretical) additional 24dB of dynamic range and supports ultrasonic bandwidth approximately twice that claimed for DSD.

Because it has been extremely difficult to carry out DSP operations (for example performing EQ, balance, panning and other changes in the digital domain) in a 1-bit environment, and because of the prevalence of studio equipment such as Pro Tools, which is solely PCM-based, the vast majority of SACDs, especially where rock and contemporary forms which rely on multitrack techniques are concerned, are in fact mixed in PCM (or mixed analogue and recorded on PCM recorders) and then converted to DSD for SACD mastering.

To address some of these issues, a new studio format has been developed, usually referred to as "DSD-wide", which retains standard DSD's high sample rate but uses a multibit, rather than single-bit digital word length. This is essentially indistinguishable from PCM (it's sometimes disparagingly referred to as "PCM-narrow") but has the added benefit of making DSP operations in the studio a great deal more practical. The "DSD-wide" signal is down-converted to regular DSD for SACD mastering. As a result of this technique and other developments there are now a few digital audio workstations (DAWs) which operate, or can operate, in the DSD domain, notably Pyramix and some SADiE systems.

Note that high-resolution PCM (DVD-Audio, HD-DVD and Blu-Ray Disc) and DSD (SACD) may still differ in terms of fidelity at high-frequencies since DSD, thanks to its high sampling frequency, does not show the typical ringing effects of reconstruction filters used with PCM. On the other hand, DSD's dynamic range decreases quickly at frequencies over 20 kHz due to the use of strong noise shaping techniques which push the noise out of the audio band resulting in a rising noise floor above 20kHz. PCM's dynamic range, on the other hand, is the same at all frequencies. (Some high-end SACD players employ an optional low-pass filter set at 30 kHz for compatibility and safety reasons, suitable for situations where amplifiers or loudspeakers can't deliver an undistorted output if noise above 30 kHz is present in the signal.)

Although Sony and Philips claim 1-bit DSD processing is superior to PCM, almost all units present on the market (including Sony's top SACD players) convert DSD stream to 88 kHz PCM before outputting to D/A converters.


source & note
the most part is as is in wikipedia (in other words - copied)
other parts are from cdfreaks.com

this article was meant to be mailny about DSD modulation.

imho dvd-a is better than this. what's your opinion ?