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Watermarking |
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Super
Audio CD |
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Digital Watermarking
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Copyright piracy and
illegal duplication have always been a problem,
particularly to the major editing companies. Copy
management systems like ACMS or SCMS can
successfully defend copyright works against casual
duplication. However, they lack of efficiency to
fight large counterfeiting operations prepared to
mass produce illegal copies of digital medias.
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Copyright
data embedded within the audio signal would be an
excellent protection. However, many attempts in the
past have shown that such a signal was either too
easy to retrieve from the audio stream, or was
affecting the original audio signal, which
audiophile people would never accept from such a
high quality media. Indeed, introducing an
anti-piracy embedded signal should not bring the
audio quality back to a lower level.
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Recently,
a very clever solution has finally been found by
Philips and Sony to include copyright protection
information in the main audio stream, in such a
complex way that it can not be easily removed but
remains fully transparent to the original audio
data. It is actually a form of 'Digital
Watermarking' which stores the required copyright
data as a modulation of the width of the injection
moulded 'pits' on the disc substrate itself. Hence,
copyright data can not be replicated without the
glass mastering equipment used to make the original
disc stampers. It goes without saying that such an
equipment has been specifically designed and
carefully licenced.
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Furthermore,
the modulation of the pit's width can be
synchronised on consecutive turns of the disc in
order to form visible patterns on the disc itself.
Thus, faint text or graphics can appear on the
recorded side of the disc, hence the name of
'Watermarking'.
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A
new technology, called Pit Signal Processing (PSP),
has been developed in order to encode the copyright
protection information : the idea behind PSP is to
modulate the width of the pits on the
disc. However, an increase of the
laser beam
would affect both, the width and the length of the
mark, which would be a critical problem, since the
length is already used to modulate the audio main
stream.
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The
trick to modulate the width while keeping the
integrity of the audio main stream, consists in a
temporary feedback loop introduced between the PSP
unit and the quality-check system. As a result, the
recording process learns to control extremely
accurately the length of the marks cut into the
substrate, ensuring that the required width
modulation to encode copyright information does not
introduce unwanted length variations.
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PSP and Jitter
Reduction
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The increased precision
in the length of marks brought by the PSP technology
has also another great advantage, particularly for
the audiophile market. Indeed, thanks to the
Eight-to-Fourteen Modulation (EFM) used to encode
data onto a CD master (see channel
modulation), pits (and the spaces in-between) can
only vary in length in integer values between 3 and
11 units. However, although slight errors in pits
length would almost never result in a different
integer value (and would consequently not affect the
audio data), such variations may cause jitters, i.e.
variations in data timing. Such errors are due to
the normal tolerances of glass mastering and
injection moulding.
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The
resulting jitter affects the sound quality, as it
blurs the sound representation and increases the
high-frequency noise. Practically, jitter can make a
difference between different discs in a same batch,
or can be noticed while playing the same disc in
different players which would present more or less
susceptibility to jitter (here, emphasis has also to
be put on the quality of the laser mechanism which
will go from an extrem to the other while using
respectively low-cost or high-end products). The
very tight control on pit length provided by the PSP
system actually reduces pressing-induced jitter by
at least a factor two, and potentially much more. As
a result, audio quality, which is definitely
affected by the injection moulding process, can be
improved considerably.
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Beside
the visible Watermark on the playing surface of the
disc, PSP can also be used to store irremovable
information like the country of origin, mastering
house and pressing plant identification codes, glass
master matrix number, disc ISRC catalogue numbers,
etc.
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Digital
Watermarking based on the PSP system has already
proved its efficiency : if a disc with no or altered
copyright information is inserted in a SACD player,
play-back will start for a few seconds, and then the
disc is simply ejected from the player. It has
consequently been decided by law that all hybrid
players have to be equipped with suitable PSP
detection circuitry.
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Of
course, Pit Signal Processing is only applied to the
high-density layer of a SACD disc, so that the CD
layer remains fully compatible with the Red Book
specifications, and consequently can be played back
in any existing CD player.
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As
a conclusion, SACD seems to prove that a very
efficient copyright protection system can be
compatible with a high-end audio format, which is
not altered in any way by this system, and thus can
be used to reproduce the original music in its
finest details.
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