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All about DVD Part 4: Perfect Pair
New sound formats: DVD-Audio and SACD
When Audio-CD format emerged, most music producers and music
lovers were absolutely sure that Audio CD provides the best
quality sound possible and nothing else will ever be required.
However, some audiophiles instantly refused to accept new Audio
CD and reverted back to old vinyl disks. At first no-one took
them seriously, however, as the time passed an increasing number
of listeners started to notice, especially when comparing CD
albums with the same records on vinyl, that CDs are not capable
to transmit full range of sound. Eventually it becomes clear
that there are scientific reasons behind this, namely dynamic
range and sampling rate.
Dynamic range is the difference between the loudest and
softest sounds in audio record. Sound data is recorded on CD in
16-bit PCM format and therefore its dynamical range does not
exceed 96 decibels. And the upper range for human ear is 120Db.
The difference is obvious. While the lack of dynamic range might
be hard to notice for the pop music, but for something like
symphony orchestra sound engineers often have a problem with too
soft or too loud instruments which are beyond dynamical range
and therefore lost in record.
Apart from dynamic range it also became clear that Audio CDs
have insufficient sampling rate. Digital record consists
of small pieces called samples, each of them contains
information about sound in particular point in time. The more
samples are per unit of time, the higher is the quality of
record. Basically, sample is a digital analogue of sound
oscillation and for that reason they are measured by using the
same unit, hertz (Hz). The maximum frequency of sound that
humans can hear is around 20 thousands hertz (20 kHz). To
reproduce sound accurately, the sound information have to be
digitized with sampling rate higher than a human can hear.
During Audio-CD development it was believed that doubling this
rate - up to 44.1 kHz would be sufficient. But for some records
part of sound was lost and turned into noise. Especially often
that happened for classical music and jazz, with high-frequency
instruments such as violin and flute being the most unlucky
ones. It is scientifically proven now that for the most accurate
sound the sampling rate has to be not lower than 64 kHz. This,
of course, is far beyond Audio CD capabilities.
To address these issues the DVD-Audio format was
developed. It took more time to approve than DVD-Video, as DVD-Forum had to justify
between two alternative technologies: one from Toshiba and the
other from Sony-Philips (they also developed Audio CD) alliance.
The problem was to decide, which format was better. Both of them
successfully solved the drawbacks of Audio CD.
Toshiba simply improved existing PCM standard and turned it into
24-bit one. The dynamic range grew to 144db and sampling rate
soared up to 192 kHz. These values exceed human ear capabilities
by a good margin. This format also had support for multi-channel
sound.
Sony and Philips took another route and
invented new single-bit audio format called DSD (Direct
Stream Digital). The idea was to read data from disk in
significantly smaller quantities (one bit at a time, compared to
16 bits for Audio CD and 20-24 bits for DVD-Audio) but
with insane sampling rate - 2.8224 MHz. Similar principle is
actually employed in the vinyl audio pickup. Due to this the
dynamical range for new format grew to 120db. Apart from that
multi-channel support was added along with compatibility with
Audio CD. The latter was achieved by adding second layer with
sound in Audio CD format. If such disk is inserted in generic
Audio-CD player, it will be played just like any Audio CD, with
all limitations of that format.
At the end, in 1999 Toshiba finally prevailed. This was due to
several reasons. First of all, DSD soundtrack is hard to
make even in studio environment. Second, when this technology
first appeared, there was no special equipment that could
support it. All existing devices were able to work with PCM
sound only, and DSD required conversion. This eventually
reduced the benefits of DSD to zero. Toshiba, however, selected DVD
as a medium for the new format, which made it possible to create
audio disks in this format even at home. The situation with
Sony-Philips creation was completely different. It had to be
recorded on special disks, which were similar in appearance and
sizes to normal DVD, but totally different otherwise. These
disks also required special equipment for printing and
recording. All of these were the reasons why Toshiba received
rights for the DVD-Audio brand-name.
But that didn't stop Sony and
Philips and they decided to
introduce their own format - regardless of DVD-Forum decision. The new
format was called Super Audio Compact Disc. As a result,
there was no single standard and neither SACD nor
DVD-Audio managed to even get close to ordinary Audio-CD
sales volumes.
The difference in sound between SACD and DVD-Audio
wasn't spotted even by most "advanced" audiophiles.
Let's examine each of the technologies in details:
DVD-Audio
Since DVD-Audio standard is based on ordinary DVD, such
disk can be double-sided or double-layered or a combination of
these two. Unlike SACD, a sound can be recorded on
DVD-Audio in different quality and in various formats,
stereo or multi-channel. Sampling rate for DVD-Audio can
vary from 44.1 kHz up to 192 kHz (however sound can only be
stereo at 192 kHz, as it is simply not enough space for
multi-channel sound of that quality). Generally,
DVD-Audio is able to deliver 1000 times more accurate
sound when compared with Audio CD!
To be able to fit multi-channel uncompressed sound (which
usually takes lots of space) developers have used special
lossless compression technology called MLP (Meridian
Lossless Packing), which in some ways is similar to PC
archiving packages such as Zip or RAR.
In order to be compatible with generic DVD players,
DVD-Audio disks can contain copy of the soundtrack
encoded in Dolby Digital and DTS on the other side of the disk.
Such disks are known as Hybrid DVD-Audio disc -
HDAD. Sometimes it is possible to encounter disks that
have only Dolby Digital\DTS encoded soundtracks, however,
such disks hardly have anything to do with DVD-Audio.
Apart from this, several sound-recording companies have
developed in 2004 a new format called DualDisc.
Essentially it is a double sided disc, one side of which is
single- or double layer DVD and the other side is plain CD. The
CD format used in such disk is slightly different form normal
CD. Standard thickness of CD or DVD disk is 1.2mm. Audio CD
layer occupies all that space, and DVD requires only half of it,
or 0.6mm. Therefore it is not possible to create a disk with
both proper DVD and CD layers as it would be 2mm thick and won't
fit in any drive. Due to this reason the developers decided to
cheat a little bit and reduced thickness of Audio CD layer to
0.9mm. As a result, it is possible to play DualDisc on most
players, but it is still too thick and doesn't fit in most cars
CD. In addition to this it can damage some Hi-End equipment, and
will also void the warranty. Another interesting effect of
making Audio CD layer thinner is the inability to record
anything to it via normal CD recording methods due to spherical
aberrations. This means that laser is not able to focus on data
pits and "see" only blurred image which cannot provide any
actual data. The problem was solved my making pits bigger,
however their amount reduced. As a result, the capacity of disc
has dropped and instead of standard 74 minutes such disks
contain only 60, and there are only 525 MB available instead of
650.
Most probably DualDisc technology won't last long. There
actually is a new technology to replace it -
Onedisc\DVDplus. German engineers managed to create
hybrid medium with standard thickness - 1.2mm. More importantly,
the capacity of CD side doesn't suffer; it still is able to
contain 650mb of data or 74 minutes of audio. The developers of
this standard, however, do not share their secrets, and how
exactly did they managed to overcome obvious problems is still a
mystery. Many recording studios have purchased license for this
technology and Onedisc will probably soon appear on shop
shelves.
Beside audio data, DVD-Audio disc is able to hold any
type of DVD-Video data. It can be video, menu, slides, etc (it
is also possible to put sound in DVD-Audio format on
DVD-Video disk). The standard also provides new technology for
storing text information, some sort of replacement for CD-Text.
Disk can contain titles, lyrics, information about performers,
etc. Text can be viewed on TV or audio player screen. Just as
DVD-Video, DVD-Audio can provide a selection of several
languages for text info.
DVD-Audio copy protection is also worth
mentioning. After famous DVD-Video hack manufacturers decided to
postpone DVD-Audio until better protection becomes
available. This better protection was CPPM (Content
Protection for Pre-recorded Media), which include several
different protection measures:
1. According to the new standard, DVD-Audio data is
encrypted. Unique set of keys can only be kept in legal player
device (that is, DVD-Audio player that comply with
CPPM requirements). In case if keys do not match the disk
(for example when DVD-Audio player is manufactured
without appropriate license), it will not be possible to decrypt
the data and DVD-Audio disk simply won't play.
Theoretically, it is possible to copy such keys by hacking the
device, but practically it is rather useless as if stolen keys
would be revealed, all later releases of DVD-Audio disks
simply won't support them. And most manufacturers perform
monthly revision of keys.
2. Contents of DVD-Audio are also bound to the physical
medium. This is achieved by recording "digital
watermarks" on the disk. This hidden information contains
disk details and the information about how many times this disk
can be copied. Watermarks are recorded with intervals of several
seconds and if the player discovers that watermarks doesn't
match medium it instantly stops playback.
Apart from this, if DVD-Audio player is equipped with
Firewire (iLink) digital out, it won't be possible to use
it for illegal copying too. All outgoing information is
encrypted and because of this it only will be possible to
connect the cable to an amplifier or receiver that supports
encryption. It will not be possible to connect player to the PC
and therefore won't be possible to copy digital output.
SACD disks come in three flavors - single layer,
double-layer and hybrid.
In case of SACD hybrid disks have two layers. Bottom
layer has Audio CD data and upper semi-transparent layer
contains SACD data. If such disk is inserted in normal CD
player, there will be no problems with playback - the player
simply won't notice the upper layer.
Single- and double-layered disks contain only SACD data,
also known as HD (high definition). Each HD layer takes
the same space as standard DVD layer - 4.7 GB. Sound can be
recorded in mono, stereo or multi-channel formats. Some
SACD player can read only mono or stereo disks, and it
will take a special device to read multi-channel disks. In most
cases, however, there are both multi-channel and stereo versions
present. Each type of soundtrack has its own space on the disk -
stereo data is recorded closer to the center of the disk,
multi-channel data comes next and additional data (such as
photos or lyrics) is recorded close to the edge. Each sound zone
contains at least 74 minutes of sound. There are, however,
purely multi-channel disks with several hours of audio on
them.
In ordered to reduce size of data, SACD developers had
used DST (Direct Stream Transfer) compression. This
compression is similar to MPL and resembles PC archiving
utility. It operates by compressing repeating sequences of bits
and can achieve significant reduction in size without any
losses.
Just as DVD-Audio does, SACD has complex set of
copy protection measures:
1. SACD disks are not compatible with computer DVD
drives, which makes illegal copying at home impossible.
2. Disk contents are inaccessible until special lead-in
SACD-Mark is read. This label is located in a hidden area
of the disk and contains information essential for the playback.
Only certified devices can read and decrypt it.
3. Disk contents are also protected by PSP-PDM (Pit Signal
Processing-Physical Disc Mark) watermarks. By changing pits
(tracks) width it is possible to write additional info on the
disk (for example, key from encrypted soundtrack). These
watermarks can be reproduced only on special licensed equipment.
Without them the playback will stop after only few seconds.
Besides, it is possible to create a visible picture on working
surface of the disk - for example, company logo, thus the
original disk can be recognized even with naked eye. To avoid
compatibility issues, the second layer in SACD is
protected as much as any normal Audio CD is.
For audio output mainly analog out is used, but encrypted
digital out via Firewire (iLink) is also possible, just
as in DVD-Audio. Coaxial and optics outputs are not supported at
all.
Summary
Most probably, both formats will still remain an expensive
technology for true music fans and audiophiles. Majority of
consumers already made their choice in favor of
medium-independent music format - MP3. So it is highly possible
that DVD-Audio and SACD will remain the top audio
quality available, as any higher quality doesn't have much sense
and will be too expensive to reach.
About the author:
Artem Chlegov is a editor and writer for DVDSoftwareGuide.com - a
DVD software review website.
Written by: Artem Chlegov
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