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When Compact Disc was brought out by Philips and Sony in 1982, it was believed to be the perfect medium. Twenty years later, Compact Disc is still in full development. Many experienced and knowledgeable people, such as recording engineers, and leaders of the audio industry claimed that the digital medium was not that perfect. They quickly realised that the 16/44.1 resolution that conventional (“Red Book”) Compact Disc had on offer was not enough to match or even improve upon the musical qualities the LP had to offer. ( for an extensive description of the shortcomings of CD, see the CD-page).Although most of the engineers knew that the resolution of CD had to be enhanced, the technology simply wasn’t available. By incremental progress they succeeded in getting a 24 bit/96kHz resolution onto a conventional CD, but only if the CD was recorded in 24/96 ánd if the digital-analog converter in the CD-player was compatible with that kind of resolution. That way, conventional CD came very close to offering a sound that sounded was almost ‘analog’. It seems a strange paradox that the digital revolution of CD, and the emission of LP was followed by a process of further development in which the engineers tried to make the CD sound as good as the medium it had made disappear. This is of course because of the fact that the consumers adapted to the new medium so quickly due to its practical advantages, and not its ultimate sound quality. The CD may have offered digital encoding, but compared with the often-abused LP, it was CD's random access, portability, indestructibility, and lack of surface noise that helped it establish a beachhead.
It was the appearance of the Digital Versatile Disc (DVD) that made it possible for the engineers to come up with a succesor of CD, that could meet the consumers demands of practicality, ánd offer truly superior sound quality. They could now record in a resolution of 24 bits/192 kHz, resulting in a sound finally surpassing that of LP, and match the ease of handling of CD. For these reasons of sound quality they called it Super Audio CD. It was originally co-developed by Sony and Philips, but it took only a few months for other companies to buy their participation and develop SACD-players of their own. The recording technology however is still monopolised by Sony, and is called Direct Stream Digital.
A key provision of SACD is the potential for a dual-layer "hybrid"
disc. A hybrid disc has two information layers; the bottom layer is identical
to that of a CD, and contains 16-bit/44.1kHz digital audio (the so-called
"Red Book" layer). The second "high-density" layer
uses much smaller information-carrying pits to store more data. This high-density
layer is semi-transmissive, meaning it reflects light from the SACD playback
laser, but is transparent to a CD player’s laser.
A hybrid SACD can present CD-quality sound to any CD player, and also
deliver high-resolution digital audio when played on an SACD machine.
SACD’s backward compatibility with CD has several important implications.
The first is that you can play a hybrid SACD in your car or portable CD
player, then move the disc inside for SACD playback in your listening
room. Second, there’s no dual inventory for record companies and
retailers; they carry a single disc for both CD and SACD customers. Third,
CD consumers will buy into the SACD format without making a conscious
decision to stop buying music in one format and start buying it in a new,
incompatible format. After several years of buying what they think are
CDs, the average consumer will theoretically have amassed a library of
SACD titles.
The hybrid disc is only one of three possible disc options. Record companies
could also choose to release music only in high-resolution form, with
a single high-density layer and no backward-compatible CD layer. Or the
third option, a disc with two high-density layers for about 150 minutes
of high-resolution playing time.
The high-density layer contains not only a two-channel stereo version
of the music, but can also store a six-channel mix . Artists and record
companies will decide whether to release two-channel or multi-channel
discs. SACD thus works equally well for two-channel and surround-sound
listeners.
Packing eight channels (a two-channel plus a six-channel mix) of high-resolution
digital audio in the same space that contained two channels of relatively
low-resolution CD-quality digital audio is made possible by three techniques.
First, the information-carrying pits in the high-density layer are about
half the size of conventional CD pits. Second, the distance between tracks
is about half that of CD. Third, the digital audio data are compressed
with a lossless coding system that more efficiently encodes that data.
For example, rather than store a stream of eight consecutive zeros, the
code 8X0 may be recorded instead. This is a crude oversimplification of
a very complex technique. What’s important, however, is that this
data compression system is perfectly lossless – that is, the same
ones and zeros that go in are the same ones and zeros recovered from the
disc. Consequently, there’s no reduction in sound quality. SACD’s
lossless compression is contrasted with lossy compression systems such
as Dolby Digital that discard musical information and degrade fidelity.
Direct Stream Digital (DSD) Encoding
The compact disc and most professional digital audio systems rely on Pulse
Code Modulation (PCM) to convert analog audio into digital information.
In PCM encoding, the audio signal is sampled at precise intervals. At
each sample point, a binary number is assigned to represent the audio
signal’s amplitude at the time the sample was taken. Assigning this
number is called "quantization." Sampling is akin to taking
snapshots of the audio waveform; quantization is "weighing"
the audio signal contained in the snap-shot. A movie camera is a form
of sampler; the images aren’t continuous, but are taken at discrete
time intervals.
Sony and Philips believed that PCM encoding had been so improved that
any future advancement would be minimal. Consequently, they devised a
radically different method of encoding digital audio called Direct Stream
Digital (DSD). In DSD encoding, the analog audio waveform is sampled at
2.8224 million times per second. Each sample, however, generates only
one bit of information. The musical information is encoded in the width
of the 1-bit-high pulses.
The data rate from DSD is exactly four times that of 16-bit/44.1kHz digital
audio. (CD produces an audio datastream of 705,600 bits per second per
channel.) This four-fold increase in the number of bits representing the
music allows DSD to encode more musical information over a wider audio
band-width compared with CD.
Using a single bit to encode the musical signal’s amplitude results
in a very high noise floor. But because DSD’s 2.8224MHz sampling
frequency produces a theoretical bandwidth of 1.4MHz (according to the
Nyquist Theorem, the audio bandwidth is half the sampling frequency),
that noise floor is spread over this very wide frequency range. This noise
floor can also be shifted from one frequency range to another, a technique
called noise shaping. In DSD, noise in the audio band is shifted to a
higher frequency where it becomes inaudible (Figure 3). Note that the
total noise power in the audio signal doesn’t change; noise shaping
merely shifts its frequency away from the audioband.
This technique of quantizing the audio signal with one bit is far from
new; the earliest digital audio systems used this so-called "delta-sigma
modulation." One advantage of delta-sigma modulation is that it is
a much simpler process than PCM encoding and decoding. DSD requires no
decimation and interpolation filtering as does PCM, processes that introduce
noise and errors into the signal. In fact, the DSD bitstream looks remarkably
like the analog signal it represents . In theory, this DSD bitstream could
be converted to analog with a low-pass filter made from a single capacitor.
DSD provides a dynamic range of 120dB and an audio bandwidth of more than
100kHz. Note that the 120dB dynamic range is available up to 20kHz; above
that frequency the noise shaping technique described earlier reduces dynamic
range. Nonetheless, DSD can encode information well above 20kHz, a first
for a consumer digital audio system. DSD can also encode a 10kHz square
wave, something 44.1kHz PCM cannot come close to achieving. Some designers
consider this ability a fundamental prerequisite for high sound quality.
A Direct Stream Digital signal can be converted to a 16-bit/44.1kHz PCM
signal for storage on an SACD’s CD-compatible layer. This processing,
which Sony calls Super Bit Mapping Direct, preserves some qualities of
the high-resolution DSD bitstream. Record companies can thus issue hybrid
CDs from a single DSD master. This has the benefit of improving the CD
layer’s sound quality.
Al these technical advantages would of course be useless if they would not result in a thouroughly enhanced sound quality offered by SACD. As with CD, several recording engineers, leaders of the industry and influentials in the matter will be quoted on their opinion on the new medium.