Hi Rob, SACD means Super Audio Compact Disc.
The following is copied and pasted from two different sources. Some of the terms are technical, you might need a dictionary to understand it.
SACD, like DVD-Audio, is a new format for recorded music. The focus of this simple, yet elegant format is pure audio reproduction. Physically, it looks like an ordinary CD. Yet one Super Audio Compact Disc can contain three versions of the music program on two separate layers. All three versions of the program benefit from the use of the DSD (Direct Stream Digital) recording format: an encoding architecture that is significantly different from the LPCM (Linear Pulse Code Modulation) format used for digital recording over the past two decades. Digital audio, as found on the conventional CD and DVD formats, is LPCM.
As mentioned, a Super Audio CD is a two-layer "hybrid disc." A hybrid disc allows backward compatibility with conventional CD hardware. The layers break down as follows:
Layer One - a reflective, CD compatible layer, enabling playback of the stereo program on conventional CD players. It is our experience that a "Red Book" layer derived (down converted)from the DSD master, always results in a better sounding CD.
Layer Two - a high-density, semi-transmissive Super Audio layer. It contains the stereo and surround DSD programs. When played on a new Super Audio CD player, the full glory of this format is exposed. In both stereo and surround, consumers will experience frequency response from DC to over 100,000 Hz and a dynamic range greater than 120 dB - across the audio band.
The sound quality achieved through the use of DSD is exceptional.
Data Capacity
Although a Super Audio CD disc has the same physical specifications as a conventional CD, it is capable of storing approximately 7 times the amount of data (4.7GB). The conventional CD (reflective) layer can hold up to 74 minutes of 2-channel 16-bit PCM, 44kHz audio. The SACD (semi-transmissive) layer can hold up to 74 minutes of 2-channel and 6-channel 1-bit DSD 2.8224 MHz audio.
Direct Stream Digital Sound
The heart of SACD is the DSD recording format, and the desire to address the ever-present problem of multi-stage decimation and interpolation inherent in PCM systems.
DSD samples the analog signal and converts it to digital by 64X oversampling delta-sigma modulation (sampling frequency = 2.8224MHz). The result is a 1-bit digital representation of the audio signal. Rather than decimate the 1-bit signal into multibit PCM code, DSD records the 1-bit pulses directly.
On the playback side, the use of DSD eliminates the need for interpolation and steep anti-aliasing filters traditionally used to block alias frequencies.
In short, DSD facilitates the direct recording of a very accurate pulse which can be easily played back with excellent results.
6 Channel Surround Sound
As mentioned, the high-density layer can also contain a discrete DSD six channel recording of the music program. Each channel can be full bandwidth with 100kHz frequency response and a 120dB dynamic range. The result is a surround-sound image with remarkable resolution and transparency.
Hardware
To play the high-density layer, a Super Audio CD player is required. In addition to playing SACD discs, These players will also play your DVD-Video discs and audio CDs. You can continue to enjoy all of your favorite recordings in your existing digital disc collection.
Value Added Features
SACD has limited "value-added" capabilities. The musical performance can be accompanied by text and graphics.
Richard Carpenter has remained busy at Capitol records working on a new Carpenters compilation which is being mixed in 5.1 surround sound mix and will be released on SACD. 5.1 means that the music is mixed with five main channels: left, center, right, left surround, and right surround, plus a low-frequency effects (LFE) bass channel (called a ".1" channel because it covers only a fraction of the frequency range of the main channels). This new compilation will present the Carpenters like we've never heard them before. Along with the new CD, Richard is also working on a DVD. This is some great stuff to look forward to.