Note: Descriptions are shown in the official language in which they were submitted.
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PREVENTION OF CD-AUDIO PIRACY USING SUB-CODE CHANNELS
CROSS-REFERENCE TO RELATED APPLICATIONS.
This application corresponds to a continuation-in-part of U.S. Patent No.
6,988,206 filed April 14, 2000, which is a continuation-in-part of U.S. Patent
No.
6,425,098 filed August 9, 1999, which is a continuation-in-part of U.S. Patent
No.
6,256,738 filed October 20, 1998. All of these patents are assigned to the
assignee of
the present patent application.
FIELD OF THE INVENTION
The present invention relates generally to protection of intellectual property
rights, and specifically to prevention of unauthorized copying of recorded
audio,
computer-readable media.
BACKGROUND OF THE INVENTION
Compact disk (CD) recordings are a preferred means for distributing both
digital audio recordings and computer software. CD audio recordings are almost
universally made in accordance with standard 908 of the International
Electrotechnical
Commission (IEC), entitled, "Compact Disc Digital Audio System" (Geneva,
Switzerland, 1987). The 908 standard, also known as the Red Book, defines how
audio
data are to be encoded and recorded on the disk, enabling the data to be
played back
using standard -digital decoding chips. Different standards apply to other
types of data,
such as computer software, which may be recorded on CD, as well. For example,
the
Blue Book standard specifies multi-session recordings, that combine audio and
other
data on a single disk.
Original music CDs are produced by stamping plastic blanks with a master
stamp piece, which is produced using costly, specialized equipment.
Alternatively, a
recordable CD (CDR) may be created by authorized plants according to customer
request. Because of the high cost of CD recording equipment, CD recordings
were
considered, until recently, to be relatively secure against unauthorized,
copying. This
situation has changed recently, as inexpensive CD recording devices and
read/write
media have become available to consumers. Thus, it is now easy, using a
conventional
personal computer with a read/write CD drive, or with one read drive and one
write
drive, to make exact, digital copies of all types of CDs. The CD content is
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read out by the computer, byte by byte, from one CD, typically a legitimately-
purchased
original CD, and then written to a blank CD so as to make a pirate copy.
Similar copying
methods may be used with other media, such as Digital Video Disks (DVD).
Financial losses
to the recording and software industries due to such pirate copying are
estimated to be in the
billions of dollars.
Another common method for pirate copying is via the digital audio output
provided by
many CD players, such as the S/PDIF (Sony/Philips Digital Interconnect Format)
output. This
output format is specified by IEC standard 958, entitled "Digital Audio
Interface" (Geneva,
Switzerland, 1993), which is incorporated herein by reference. The S/PDIF
output can be used
legitimately to connect the CD player to high-quality converters, speakers and
other audio
components. On the other hand, since it reproduces the audio content of the CD
bit by bit, the
S/PDIF output is also easy for pirates to use in making illegal copies. In
addition to the audio
content, the S/PDIF data includes a sub-code, known as the serial copying
management system
(SCMS) sub-code, that indicates whether the source of the data is an original
recording or a
copy. According to SCMS, commercially-available recording equipment is not
supposed to
allow users to record audio from a S/PDIF stream that is marked as a copy.
This protection,
however, is easily circumvented by pirate copiers.
SUMMARY OF THE INVENTION
It is an object of some aspects of the present invention to provide improved
methods
and apparatus for preventing unauthorized copying of recorded, computer-
readable media.
It is a further object of some aspects of the present invention to provide
methods and
apparatus for producing compact disks that are resistant to unauthorized
copying.
It is still a further object of some aspects of the present invention to
provide methods
and apparatus for preventing unauthorized digital copying of audio disks.
In preferred embodiments of the present invention, signals recorded on an
original
storage medium are protected against unauthorized copying by changes made in a
sub-code
channel, preferably the Q-channel. For this purpose, a data-writing unit
comprises an encoding
block, which receives audio signals and encodes the signals in accordance with
standard 908
of the International Electrotechnical Commission (IEC). A modification-
insertion block of the
data-writing unit alters the encoded signals, by introducing a modification in
a sub-code Q
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channel of the encoded signals. A recording block of the data-writing unit
subsequently
records the altered signals on the storage medium, which typically comprises a
compact disk
(CD).
Preferably, the altered signals are erroneous according to standard 908, or
according to
implementations of the standard used in CD-ROM drives, but nevertheless do not
prevent
playback of the audio signals from the original medium following recording.
Thus, the
original CD is able to perform in a manner substantially unaffected by the
alteration. By
contrast, the alteration causes a substantially-unrecoverable error when an
attempt is made to
create an unauthorized copy of the CD. The error manifests itself in that
either the
unauthorized copy cannot be produced at all, or the copy is corrupted in such
a way as to
render it substantially unusable. Preferably, authorized software residing in
the data-writing
unit or in other hardware is enabled to duplicate the CD, such that authorized
copies also can
be played back, unaffected by the alteration.
In some preferred embodiments of the present invention, the modification-
insertion
block inserts the modification in the sub-code Q channel by specifying a
modified absolute or
relative track time of the audio signals recorded on the CD. CD copying
algorithms known in
the art rely on non-corrupted track times, and are therefore unable to make
unauthorized copies
of CDs which are prepared using these embodiments of the present invention.
Optionally, the
original track time is stored on the original CD in reserved bits of the sub-
code channels that
are defined by standard 908. Authorized copying software retrieves the
original track time
from the reserved bits, and is thereby enabled to make fully-functioning
copies of the original
CD.
Alternatively or additionally, the modification is introduced by adding one or
more
additional sessions on the disk, with a lead-in that point to tracks in the
program area of the
first session and includes special modes to block an unauthorized computer
from reading the
session.
Further alternatively or additionally, the modification-insertion block
inserts the
modification in the sub-code Q channel by specifying a modified index number
of a track of
the audio signals recorded on the CD. CD copying algorithms known in the art
rely on the
non-corrupted index number of the track, and are therefore unable to make
unauthorized
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copies of CDs which are prepared using these embodiments of the present
invention.
Optionally, the original index number of a track is stored on the original CD
in reserved
bits of the sub-code channels that are defined by standard 908. Authorized
copying
software retrieves the original index number of the track from the reserved
bits or from a
second session on the disk, and is thereby enabled to make fully-functioning
copies of the
original CD.
Preferably, in order to allow an authorized personal computer (PC) to play the
contents of the protected disk, the original contents are protected by digital
rights
management (DRM) technology, typically by compressing (using any suitable
format
known in the art, such as MP3 or VQF) and encrypting the contents. These
encrypted data
are stored in a manner that is hidden to conventional CD driver software, for
example in
reserved sub-code channel bits, in the pause area or lead-but area, or in a
second session
on the CD. The data can be decrypted and played back only by using authorized
software,
which is preferably stored in the second session.
Further preferably, the modifications introduced in the CD are such as to
cause
changes in a S/PDIF output or other digital output generated by a CD player in
which the
protected CD is played. The changes in the digital output do not interfere
with audio
playback of the CD recording by compatible equipment, such as audio speakers
with a
digital/analog converter. The changes do, however, prevent unauthorized
copying of the
CD content using the digital output, for example, as a digital input to a
recorder.
Optionally, in addition to the modifications introduced in the sub-code
channel,
further modifications are intentionally introduced in the audio data as an
added
impediment to unauthorized copying. The modifications are introduced in such a
way that
ancillary data on the disk, such as error detection codes, which are provided
by the 908
standard (or other applicable standard), enable a CD player either to correct
or conceal the
modifications during playback or to ignore them altogether. When an
unauthorized copy
is made of the medium, however, the ancillary data are ineffective in
overcoming the
intentional modifications in the original medium, with the result that faults
occur in the
copy that are substantially unrecoverable. Methods for introducing such
modifications are
described in U.S. Patents 6,256,738 and 6,425,098.
There is therefore provided, in accordance with a preferred embodiment of the
present
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invention, a method for protecting data recorded on an original storage medium
against
copying using an unauthorized data readout device, the method, including:
receiving data encoded in accordance with Standard 908 of the International
Electrotechnical Commission;
altering a portion of the encoded data to introduce a modification in a sub-
code Q
channel of the data, such that the altered data are inconsistent with an
implementation of
Standard 908 used in the data readout device, but do not prevent playback of
the data
following recording thereof; and
recording the data, including the altered data, on the original storage
medium, whereby
an application which plays back the recorded data runs in a manner
substantially unaffected by
the alteration of the data, but a substantially unrecoverable error occurs in
unauthorized
copying of the original storage medium.
Preferably, the medium includes a compact disk, and the data include digital
audio
data.
In a preferred embodiment, altering the data includes modifying an absolute
time in the
data to be recorded on the medium. Preferably, a correct absolute time is
stored in reserved
sub-code bits on the medium. Most preferably, modifying the absolute time
includes altering
absolute times recorded in a plurality of consecutive blocks of a selected
track.
In another preferred embodiment, altering the data includes duplicating one or
more
blocks of the data to a new location on the storage medium, without changing
the absolute
time recorded in the sub-code Q channel of the one or more blocks.
In still another preferred embodiment, altering the data includes modifying a
point
index number in a track of the data to be recorded on the medium. Preferably,
the method
includes storing a correct index number in reserved sub-code bits on the
medium.
In yet another preferred embodiment, altering the data includes generating a
multiple-session recording, including one or more audio and data sessions,
wherein the
modification in the sub-code Q channel is introduced in one or more of the
data and audio
sessions. Preferably, generating the multiple-session recording includes
creating a lead-in to
one of the sessions that points to one or more tracks located in a first audio
session on the
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medium. Alternatively or additionally, generating the multiple-session
recording includes
creating a lead-in to one of the sessions that includes a larger number of
tracks than is
specified by the standard. Further alternatively or additionally, generating
the multiple-session
recording includes creating a lead-in to one of the sessions that indicates
that the recording
medium is shorter than it is in actuality. Still further alternatively or
additionally, generating
the multiple-session recording includes inserting a modified absolute time at
a start of one of
the sessions. Yet further alternatively or additionally, generating the
multiple-session
recording includes creating multiple sessions at least two of which are
identified as audio
sessions.
In a preferred embodiment, recording the data includes compressing and storing
the
data in a data session on the medium, for use in making an authorized copy of
the medium.
In a further preferred embodiment, the encoded data include encoded audio
data, and
the method includes altering a portion of the encoded audio data such that the
altered data are
inconsistent with the implementation of Standard 908 used in the data readout
device, wherein
recording the data on the medium includes recording ancillary data which are
used by a
processor in the application to operate upon the altered portion of the
encoded audio data such
that the application plays back the data in a manner substantially unaffected
by the alteration of
the encoded audio data, but which ancillary data are ineffective in correcting
the altered
portion of the encoded audio data upon copying of the data, so that the
alteration causes a
further substantially unrecoverable error in the unauthorized copying of the
original medium.
Preferably, the ancillary data include error detection codes.
There is further provided, in accordance with a preferred embodiment of the
present
invention, a method for protecting data recorded on an original storage medium
against
unauthorized copying, including:
receiving data encoded in accordance with Standard 908 of the International
Electrotechnical Commission;
altering a portion of the encoded data to introduce a modification in the
data, so that a
digital output of the altered data will be inconsistent with an implementation
of a digital output
standard, such as Standard 958 of the International Electrotechnical
Commission, but the
modification will not prevent playback of the data following recording
thereof, and
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recording the data, including the altered data, on the original storage
medium, whereby
an application which plays back the recorded data runs in a manner
substantially unaffected by
the modification in the data, but a substantially unrecoverable error occurs
in unauthorized
copying of the original storage medium based on the digital output.
Preferably, altering the portion of the encoded data includes altering one or
more
control bits in the encoded data, which control bits are reproduced in the
digital output.
Additionally or alternatively, altering the portion of the encoded data
includes manipulating
bits of audio data within a frame of the encoded data.
There is also provided, in accordance with a preferred embodiment of the
present
invention, apparatus for protecting data recorded on an original data storage
medium against
copying using an unauthorized data readout device, including:
an encoder, adapted to receive a stream of data for recording on the medium
and
encodes the data in accordance with Standard 908 of the International
Electrotechnical
Commission;
a sub-code generator, operative to alter a portion of the encoded data by
introducing a
modification in a sub-code Q channel of the data, such that the altered data
are inconsistent
with an implementation of Standard 908 used in the data readout device, but do
not prevent
playback of the data following recording thereof; and
an eight-to-fourteen bit (EFM) modulator, coupled to modulate the altered data
for
recording on the original storage medium, whereby an application which plays
back the
recorded data runs in a manner substantially unaffected by the alteration of
the data, but a
substantially unrecoverable error occurs in unauthorized copying of the
original storage
medium.
In a preferred embodiment, the apparatus includes a data session generator,
operative to
provide a data session to the EFM for recording on the original storage
medium, so that the
data recorded on the medium include multiple-session data, and the defect in
the sub-code Q
channel is introduced in at least one of the multiple sessions.
There is additionally provided, in accordance with a preferred embodiment of
the
present invention, a data storage medium that is resistant to copying using an
unauthorized
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readout device, on which medium data encoded in accordance with Standard 908
of the
International Electrotechnical Commission are stored, a portion of which
encoded data is
altered by introducing a modification in a sub-code Q channel of the data,
such that the altered
data are identified as inconsistent with an implementation of Standard 908
used in the data
readout device, but do not prevent playback of the data following recording
thereof, whereby
an application which plays back the recorded data runs in a manner
substantially unaffected by
the alteration of the data, but a substantially unrecoverable error occurs in
unauthorized
copying of the original storage medium.
The present invention will be more fully understood from the following
detailed
description of the preferred embodiments thereof, taken together with the
drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic, sectional view of a compact disk (CD), in accordance
with
recording practice known in the art;
Fig. 2 is a schematic block diagram that illustrates apparatus for producing a
copy-resistant CD, in accordance with a preferred embodiment of the present
invention;
Fig. 3 is a schematic illustration of details of a data structure written to a
CD, in
accordance with recording practice known in the art; and
Fig. 4 is a block diagram that schematically illustrates duplication of data
on a CD for
the purpose of preventing unauthorized copying, in accordance with a preferred
embodiment
of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Reference is now made to Fig. 1, which is a schematic sectional view of an
audio
compact disk (CD) 20, showing the overall layout of the disk, in accordance
with recording
practice known in the art. An information area 22 includes a lead-in (LI) area
24, a program
area (PA) 26 and a lead-out (LO) area 28. Information area 22 progresses
outwards in a
widening spiral, and contains all of the recorded contents of CD 20. Lead-in
area 24 and
lead-out area 28 both contain P and Q channels, which include timing and track
information
used during playback of the CD. Program area 26 consists of up to 99 tracks of
audio
information (e.g., music), as well as accompanying sub-code information.
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Preferred embodiments of the present invention provide a number of different,
complementary techniques for modifying the data in CD 20 in order to frustrate
pirate copiers.
A number of these embodiments are described in detail hereinbelow. Generally
speaking, the
modifications fall into the following categories:
= General disk layout modifications - for example, recording several sessions
on an
audio CD. The second session can be used to mislead personal computers (PCs)
that
are commonly used in pirate copying, typically by adding conflicting pointers
in the
lead-in of the second session. Preferably, the last session on the CD is a
data session
that contains the music in compressed form. Most preferably, the music is
encrypted
so as to enable playback directly from the disk and/or encoded using digital
wrapper
technology (also known as digital rights management - DRM) to enable download
and
playback through secure software from the disk of an authorized user.
= Modifications of the audio data, by insertion of noise, preferably as
described in the
above-mentioned U.S. 6,425,098,
= Modifications of the subcode information, particularly in the Q channel.
These
modifications may include, for example, changing the absolute or relative
time, index
pointers, table of contents (TOC) information (listing too many or too few
tracks, or
misleading pointers between multiple sessions), control, byte modifications,
pregap
length manipulation. Other such modifications will be apparent to those
skilled in the
art.
= Low-level modifications to the structure of the bits recorded on the CD.
Such
modifications are described in detail in the above-mentioned U.S. Patents Nos.
6,256,738 and 6,425,098,.- They typically involve recording sequences of bits
on
the CD that cause hardware errors in some CD-ROM drives used in PCs, or
prevent
them from interpolating over erroneous audio data. Alternatively or
additionally, bit
sequences may be recorded on the CD that do not appear in the conventional
eight-to-fourteen bit (EFM) lookup table specified by the 908 standard.
These modifications may be used in CD 20 in different combinations, depending
on
application needs.
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Fig. 2 is a schematic block diagram that illustrates apparatus 40 for
producing a
copy-resistant CD, in accordance with a preferred embodiment of the present
invention. The
original sound, or analog audio, is converted to digital format suitable for
encoding on a disk,
such that the information can subsequently be played back (decoded) by a
standard player of
audio CD 20. In a first stage of encoding, a digital bit stream is generated
which represents the
continuous analog sound signal. The analog signal is converted into discrete
digital samples
by an analog-to-digital (AID) converter 30.
To enable detection and correction of errors during playback, parity bits are
added to
the bit stream. An error-correction encoder 32 employs the commonly used Cross-
Interleaved
Reed-Solomon Code (CIRC) to reorder the digital data. A sub-code generator 36
adds a byte
of sub-code for every thirty-two bytes of error-corrected audio. The 908
standard prescribes
the contents of the sub-code channels to be added at this stage. Sub-code
generator 36,
however, alters the standard sub-codes in such a manner that ordinary playback
of the CD is
substantially unaffected, but unauthorized copying is prevented. In
particular, the alterations do
not compromise the integrity of the bit stream and do not generate CIRC errors
on the disk.
Methods of such alteration are described hereinbelow.
Apparatus 40 is also shown to include an optional data session generator 37,
for use in
producing multiple-session disks that include both audio and data sessions. As
described
hereinbelow, the data session is configured so as to prevent an unauthorized
computer from
reading the CD. An audio compressor 35 is optionally provided to compress the
audio input
for inclusion in such a multiple-session disk in an encoded form.
It is known in the art to map a standard sixteen-bit word of sound data into
two
fourteen-bit "optical" symbols, and to include in the recording additional
information, such as
playback control and display functions. An eight-to-fourteen modulator (EFM)
34 combines
the output of error-correction encoder 32 with the output of sub-code
generator 36 and data
session generator 37. Each bit of encoded data is preferably physically
inscribed onto a master
for producing CD 20, using a laser beam recorder (LBR) 38 to create a series
of microscopic
pits on the master. Alternatively, a dedicated burner 39 is used to produce CD
20 directly on a
recordable CD (CDR).
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Fig. 3 is a schematic illustration of details of the data structure received
by EFM
modulator 34, in accordance with the 908 standard. A block 58 of data for
recording on a CD
contains 98 frames, each frame made up of 32 bytes of data, or symbols,
received from
encoder 32 (or from the data session generator 37) and one eight-bit "sub-
code" word. The
eight bits in each sub-code word are respectively called P, Q, R, S, T, U, V,
and W.
The Q bits from 98 consecutive frames are collectively called a sub-code Q
channel 64.
The sub-code Q channel contains program and timing information, including
synchronization
bits SO and Si, 4 control bits, 4 address bits, 72 data bits, and a 16-bit
cyclic redundancy check
code (CRCC). In lead-in area 24, the 72 data bits include relative time (R-
Time, or track time)
data 74 and track position, while in program area 26, relative time and
absolute time (A-Time,
or disk time) data are stored. The relative time for each block indicates the
length of time from
the beginning of a current track to the beginning of the block. The absolute
time indicates the
length of time from the beginning of program area 26 to the beginning of the
block. Time is
measured in units of minutes, seconds and frames (different from the 98 data
frames in a
block), with 75 time frames in a second. The Q channel also includes a track
number (TNO)
and in-track indices, also referred to as point indices. In lead-in area 24,
the Q channel carries a
table of contents (TOC).
Table I below schematically illustrates alteration of absolute time values of
sub-code Q
channel 64, in accordance with a preferred embodiment of the present
invention. In typical
encoding of CD 20 according to standard 908, absolute time starts at the
beginning of program
area 26 and increases linearly to the end of lead-out area 28. Copying
software known in the
art uses the absolute time as a pointer, in order to ensure smooth
reproduction of the original
audio and synchronicity of timing between recorded frames and played-back
frames.
Preferably embodied, a modification is introduced into the absolute time
values of the
sub-code Q channel, as shown in the "altered" column in the table, thereby
increasing the
speed at which time appears to pass in this track.
TABLE I
Original time Altered time
Min Sec Frame Min Sec Frame
0 (start 0 0 0 0 0
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track)
0 0 1 0 0 2
0 0 2 0 0 4
0 0 3 0 0 6
0 0 4 0 0 7
0 0 5 0 0 8
... ... ... ... ... ...
0 0 71 0 0 74
0 0 72 0 1 0
(end 30 42 5 36 20
track)
5 (pause) 30 43 5 30 43
5 30 44 5 30 44
So as not to adversely affect stereo play-back of the audio, the absolute time
for
original CD 20 is preferably increased in several consecutive blocks out of
every 150
consecutive blocks, and returned to normal in a pause area between tracks, as
shown in the
5 table above. A standard CD player will be unaffected by these intentional
distortions of the
absolute time. An unauthorized copy of CD 20, however, will contain
discontinuities in the
music, for example, where absolute time jumps from frame 2 to frame 4 to frame
6, in the
table above. The resulting signal on the CD copy will have a series of
constant-amplitude,
audible, distorted and generally unmelodious sounds.
Optionally, the original, correct absolute time information is stored in the
reserved bits
of the sub-code R-W channels (Fig. 3), which are currently unassigned
according to standard
908. Authorized software is enabled to make copies of original CD 20 by
accessing the data
stored in the R-W channels.
Alternatively, rather than introducing modifications in the absolute time,
modifications
that produce audible effects in unauthorized copies are introduced in sub-code
Q channel 64 by
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changing point index and/or mode values. Like the absolute track time, the
point index
provides a series of "checkpoints" within a track. A modified series of point
indices will cause
mute areas in an unauthorized copy of CD 20, but will not affect the playback
of the original
CD. Optionally, as described above, the correct indices are stored in the R-W
channels, and
authorized software is enabled to copy the original CD by retrieving the
indices from these
channels.
Preferably, modifications introduced in the CD are such as to cause changes in
a digital
output, such as a S/PDIF output, generated by a CD player in which the
protected CD is
played. The digital output data stream comprises a sequence of 32-bit words,
known as
sub-frames, each carrying between 16 and 24 bits of audio data, along with
preamble and
control bits. One of the control bits in each sub-frame is a sub-code bit,
similar to the sub-code
bits in each frame of data on the CD. The sub-code bits in a block of 192
frames (typically
384 sub-frames) in the S/PDIF data stream carry most of the contents of the
sub-code Q
channel being played on the CD, including control bytes, the track number and
the time.
Preferably, the modifications that are introduced in the Q channel on the disk
are such as to be
carried over to the S/PDIF data. Although these modifications do not interfere
with audio
playback of the CD recording by speakers and other compatible equipment, they
do impede
unauthorized copying of the CD content using the digital output.
As specified by the above-mentioned IEC 958 standard, the S/PDIF signal
contains
four control bytes copied from the CD. Therefore, in an alternative embodiment
of the present
invention, these control bytes are used to frustrate illegal copying of the
audio contents. For
this purpose, in recording the control bytes on the CD, in accordance with the
Red Book
standard, the track mode is set to "data" for a number of blocks, typically
between one and ten
blocks, in a repetitive cycle. The CRCC is then recalculated so that this
control byte is valid
from the standpoint of data accuracy. When a recording is made from the S/PDIF
output of the
CD player, however, this control byte setting will typically cause mutes or
clicks to appear in
the resulting audio stream.
In a further alternative embodiment, the bits in the S/PDIF envelope are
shifted or
otherwise manipulated in order to confound unauthorized recording of the CD
contents. When
the 32 bits in a S/PDIF sub-frame are fed to a digital/analog converter for
audio playback, the
converter takes all 32 bits and plays back the audio irrespective of the
shift. A digital recorder,
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however, will seek the audio data in a given subset of the bits of the sub-
frame, and the control
data in another subset, disjoint from the audio data, where these data are
supposed to be
according to the standard. Therefore, manipulating the bit order can block
recorders while still
enabling audio playback.
Fig. 4 is a block diagram that schematically illustrates another method for
preventing
unauthorized copying of a CD, using data session generator 37, in accordance
with a preferred
embodiment of the present invention. Whereas an audio CD has only a single
session 90,
apparatus 40 with session generator 37 can produce CDs having multiple
sessions, including,
for example, one or more audio sessions 100 and at least one data session 102.
Audio CD
players read only the first session on a multiple-session CD, in this case
session 100 and ignore
the rest. DVD players read and are "aware" of data session 102, as well, but
still play back
audio session 100. When the CD is inserted into a CD-ROM drive, however, the
computer
can access any of the sessions and therefore will be confused by the
modifications embedded
in the second or subsequent sessions. The present embodiment therefore
operates by
introducing conflicting data into the sub-code Q channel of data session 102
or of additional
audio sessions, subsequent to the first audio session. Most preferably, there
are at least two
audio sessions out of a total of three or more sessions on the disk.
Preferably, modified data are introduced into audio session 100, as well. The
modifications are introduced in such a way that ancillary data on the disk,
such as Cross
Interleave Reed Solomon Code (CIRC) error detection codes, which are provided
by the 908
standard (or other applicable standard), enable a CD player either to correct
or conceal the
modifications during playback or to ignore them altogether. When an
unauthorized copy is
made of the medium, however, the ancillary data are ineffective in overcoming
the intentional
modifications in the original medium, with the result that faults occur in the
copy that are
substantially unrecoverable. Methods for introducing such modifications are
described in the
above-mentioned U.S. Patents 6,256,738 and 6,425,098.
Despite the modifications in the sub-code channels and/or audio data, an audio
CD
player will reproduce the audio signals on the disk without difficulty. A
computer CD drive,
however, will be confused by either the multiple sessions on the CD or the
modified data in
audio session 100 (or by both) and will not be able to load or play the disk,
let alone copy it.
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Modifications in the added audio and/or data sessions are preferably
introduced by one
or more of the following mechanisms:
= Generating a lead-in that points to tracks located in the program area of
the first
session.
= Generating a lead-in with a larger number of tracks than is allowed by the
908
standard.
= Modifying the lead-in to simulate a shorter CD.
= Modifying the absolute time at the beginning of the session.
Following these methods, Table II below describes changes made in the sub-code
Q
channel of a multiple-session CD, in order to prevent computer copying of the
audio content
without interfering with audio playback. The first session on the CD is an
audio session, based
on the Red Book standard, while the second and subsequent sessions may be
audio or data
sessions, based on the Blue Book, as described above.
TABLE II
Session Standard Modification
1 Lead-in (2'/2-3 Add pointers BO and CO to the second and third
min) sessions, per Blue Book format.
Pause length 2 Use different pregap length, per Blue Book format,
sec preferably set to value between 1 and 3 sec.
Accurate Start A-Time at non-zero value, e.g., 10 sec, to mask
A-Time beginning of disk for computer CD drive.
2 Audio track Audio track length 4 sec (minimum time per Red
Book).
3 Lead-in (1 TOC points to 99 non-existent data tracks at time
min) 00:02.
Modify lead-out pointer (A2) to a value between 00:01
and 00:07.
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Audio track Data track length 4 see, plus fill to a total CD length.
As a result of these changes, a computer CD-ROM or DVD-ROM drive, upon reading
the
sub-code Q channel data in session 3, will either fail to mount the CD
entirely, or will go into
an endless loop looking for the non-existent data tracks listed in the TOC. If
the drive does
succeed in accessing session 1, which holds the normal audio content, it is
still likely to be
confused by the 1 sec pregap (which is outside the Red Book specification)
and/or by the
distorted A-time. CD audio and DVD players, however, will be largely
unaffected by these
changes. In this regard, session 2 is important, since without it, some Blue
Book-compliant
DVD players might refuse to mount the CD. It will be understood that the
specific
modifications noted in the table are listed by way of example. Alternative
combinations of
modifications and settings of the time values and other parameters will be
apparent to those
skilled in the art.
Preferably, in order to allow an authorized personal computer (PC) to play the
contents
of the protected disk, the original contents are compressed (using any
suitable format known in
the art, such as MP3 or VQF) and encrypted, by audio compressor 35 (Fig. 2).
These
encrypted data are stored using a "digital wrapper" technique, so that the
data are hidden from
conventional CD driver software. For example, the data may be stored in the
reserved
sub-code channel bits, as described above, or in one of the data sessions on
the CD, or in a
lead-in, lead-out or pause area. The data can be decrypted and played back
only by using
authorized software, which is also preferably stored in the second session and
is made
available only to appropriately-licensed users and/or equipment. The
authorized software will
prevent the decryption of the data if its source is not the original CD or if
the rules for
distributing and using the data are otherwise violated.
It will be appreciated that the preferred embodiments described above are
cited by way
of example, and that the present invention is not limited to what has been
particularly shown
and described hereinabove. Rather, the scope of the present invention includes
both
combinations and sub-combinations of the various features described
hereinabove, as well as
variations and modifications thereof which would occur to persons skilled in
the art upon
reading the foregoing description and which are not disclosed in the prior
art.
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