Modifying the recording process to extend the life of a rewritable information carrier

The device according to the invention is suitable for recording a rewritable information carrier such a CD-E covered by a phase-change recording layer. As is known, such an information carrier has a limited useful life because the recorded, optically readable patterns start deviating ever more from the desired form when recorded recurrently. The invention is also based on the recognition that this degradation is accelerated if a certain part of the track on the information carrier constantly accommnodates the same patterns, for example, if information blocks are recorded starting from fixed initial positions and the information is represented by the patterns in a specific manner. The device comprises means for shifting the initial position along the track over a distance randomly selected within predefined boundaries and/or for adapting the manner of representation. This causes constantly different patterns to be recorded, even if the same information is to be recorded in a specific part of the track. The information carrier described according to the invention has an additional margin between the information blocks to allow of the shifting of the initial position.

BACKGROUND OF THE INVENTION 
1 Field of the Invention 
The invention relates to a device for recording an information carrier of a 
rewritable type, the information carrier comprising a track and the device 
comprising recording means for recording an information block in a part of 
the track from an initial position, the information block representing 
information in a predefined manner. 
The invention further relates to a method of recording an information 
carrier of a rewritable type, the information carrier comprising a track, 
and an information block being recorded in a part of the track from an 
initial position, the information block representing information in a 
predefined manner. 
The invention also relates to an information carrier of a rewritable type 
recorded in accordance with the method. 
The invention also relates to a device for reading an information carrier 
of a rewritable type recorded in accordance with the method, the device 
comprising reading means for reading and decoding the information blocks. 
2. Description of the Related Art 
Such an arrangement, method and information carrier are known from U.S. 
Pat. No. 5,388.105. The device disclosed therein is suitable for an 
information carrier of an optically rewritable and readable type, such as, 
for example, a Compact Disc Erasable (CD-E), which has a rewritable layer 
of phase-change material. Such information carrier has a preformed track, 
a so-called pregroove. The pregroove comprises address information and is 
intended for positioning a write head during the recording process for 
recording in a track optically readable patterns which represent 
information. The information is contained in blocks which are recorded in 
specific parts of the track, so-called sectors, from an initial position 
depending on the address information. The information is then represented 
in a certain manner by the optically readable patterns which form the 
block. This representation includes, for example, a sector identification 
(ID), a synchronization zone, a data block and error detection and 
correction data, which information is contained in the data block under a 
specific coding. A problem with the known device is that the optically 
readable patterns start differing from the desired form when the 
information carrier is recorded recurrently, so that the reading quality 
is degraded. This reduces the useful life of an information carrier. 
It is an object of the invention to provide a device in which the 
deviations of the patterns are counteracted. 
SUMMARY OF THE INVENTION 
For this purpose, the device according to the invention is characterized in 
that the recording means are arranged for substantially shifting the 
initial position along the track over a randomly selected distance within 
predefined boundaries and/or for adapting the manner of representation. 
The invention is also based on the recognition that the deviation of the 
patterns formed in recurrent recording strongly depends on the correlation 
between blocks recorded on the same part of the track, the recording of 
the same or very similar blocks considerably accelerating the pattern 
degradation. A small shift of the patterns, such as a few (2 to 3) times 
the longest written mark used in therein was expected to be sufficient. 
Surprisingly, however, such a small shift, e.g. of around 32 channelbits, 
proved insufficient, while a substantial shift, e.g. of at least around 
128 bits, was found to be effective. 
For this purpose, a method according to the invention is characterized in 
that the initial position is substantially shifted along the track over a 
distance arbitrarily chosen within predefined boundaries and/or the manner 
of representation is adapted. 
An information carrier according to the invention is recorded in accordance 
with said method. 
A reading device according to the invention is characterized in that the 
reading means are arranged for reading the information blocks from 
positions substantially shifted over a distance arbitrarily chosen within 
predefined boundaries. Also, or alternatively decoding the information 
blocks is carried out in accordance with the adapted manner of 
representation of the information therein. 
As a result of the aforesaid measures according to the invention, 
constantly varying patterns will be recorded on every part of the track, 
thus counteracting the accelerated degradation. An advantageous effect of 
this is the lengthening of the useful life of the information carrier. 
This advantageous effect particularly occurs when customary file 
management systems are used that rewrite file management information 
always on the same parts of the track. 
An embodiment for the device according to the invention is characterized in 
that the manner of representation of the recorded information may be 
recovered from the information block itself when read out. This is 
advantageous in that the recovery of the information from a read 
information block is independent of information about the manner of 
representation stored elsewhere. 
A further embodiment for the device according to the invention is 
characterized in that the manner of representation of the recorded 
information comprises scrambling with a scramble key which scramble key is 
included in the information block. This is advantageous in that any 
correlation between information blocks to be recorded on the same location 
is reduced with scrambling. 
A further embodiment for the device according to the invention is 
characterized in that the manner of representation of the recorded 
information comprises including a variable amount of dummy data in the 
information block, which dummy data is recognizable when read out. The 
advantage of this is that by recurrently adding a variable amount of dummy 
data, possibly similar equal patterns in an information block will be 
recorded each time at different positions. If the of dummy data is added 
prior to the information in an information block, old information blocks 
will be largely overwritten and so invalid portions will not continue to 
be readable. 
A further embodiment for the device according to the invention is 
characterized in that the manner of representation if the recorded 
information comprises including a first variable amount of dummy data 
before and a second variable amount of dummy data following the 
information in the block, while the sum of the first and second variable 
amount is substantially constant. The advantage of this is, that the total 
length of the information block will be substantially, constant allowing 
recording in a fixed location, while the information content is 
nevertheless recorded in randomly changing locations. 
A further embodiment of the device according to the invention is 
characterized in that the mode of representation of recorded information 
comprises rotating the information over a randomly selected distance in 
the information block. Information which extends beyond one side of the 
information block is added to the other side. This is advantageous in that 
no additional space on the disc is necessary for rotating the information. 
This is particularly advantageous when synchronization patterns are 
supplied which shift along with the information and are therefore not 
always located in the same position. 
A further embodiment of the device according to the invention is 
characterized in that the mode of representation of recorded information 
comprises including the randomly selected distance in the respective 
information block. This is advantageous in that the distance of the 
rotation of the information can then be simply derived. This is 
particularly advantageous when synchronization patterns are used which 
shift along with the information, because in that case the amount of 
information before the first synchronization pattern and/or after the last 
synchronization pattern can be determined when read out. 
These and other aspects of the invention will be apparent from and 
elucidated with reference to the embodiments described hereinafter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1a shows a disc-shaped information carrier 1 of a rewritable type. The 
information carrier includes a track 9 intended for recording, which track 
is arranged in a helical pattern of windings. The windings may also be 
arranged concentrically in lieu of helically. The track 9 is indicated on 
the information carrier by a servo pattern in which, for example, a 
pregroove 4 enables a read/write head to follow the track 9 during the 
scanning operation. The servo pattern may also includes, for example, 
regularly divided sub-patterns which periodically cause signals to occur 
in a tracking servo. FIG. 1b shows a section along b--b of the information 
carrier 1, in which a substrate 5 is covered by a rewritable recording 
layer 6 and a transparent layer 7. The pregroove 4 may also be arranged as 
a raised part or as a material property different from its surroundings. 
The recording layer 6 may be recorded optically or magneto-optically by a 
device for recording information blocks. Information on the information 
carrier is represented by patterns of marks. in the track 9, each mark 
being is formed by one or more write pulse of a constant or variable write 
power which depends on, for example, the length of the mark. The write 
parameters, such as the write power, the number of pulses, the variation, 
the duty cycle etc. are to be attuned to the information carrier, whose 
material properties play a large role. An example of a writable 
information carrier is the CD erasable which is covered by a layer of 
phase-change material. A description of the reading of a CD and the use of 
a pregroove may be found in the title "Principles of Optical Disc Systems" 
by Bouwhuis et al., ISBN 0-85274-785-3. 
FIGS. 1c and 1d show two examples of a periodic modulation (wobble) of the 
pregroove. This wobble causes an extra signal to be developed in the 
tracking servo pick-up. The wobble is, for example, frequency modulated 
with an auxiliary signal which auxiliary signal contains encoded auxiliary 
information. The auxiliary information may be address information 
indicating position in the longitudinal direction along the track. A 
description of an information carrier containing such auxiliary 
information may be found in EP-0 397 238. The initial position in the 
track for writing information blocks may be derived from the address 
information on this information carrier. 
The invention is likewise applicable to information carriers on which the 
position along the track is determined differently, such as, for example, 
via a separate reference pattern or via patterns previously recorded on 
the information carrier. The invention may also be applied to rewritable 
information carriers of another structure, such as, for example, an 
optical tape. These information carriers may be provided with address 
information in a different manner, for example, via information along an 
auxiliary track. 
FIG. 2 shows a graph of the effect of recurrent recording versus shifting 
of the initial position. The graph represents the situation in which 
always the same information block is recurrently recorded on a specific 
part of a rewritable information carrier. The information block is formed 
by optically readable marks of different lengths, for which the length 
differences are always a multiple of a fixed distance, a so-called channel 
bit. The quality of the read signal is always assessed, for example, on 
the basis of jitter or an eye pattern of the read signal. When recorded 
recurrently, the initial position of the information block in the 
longitudinal direction of the track is shifted each time over a random 
distance within specified boundaries. The number along the X-axis 31 
denotes the distance between these boundaries in channel bits. Along the 
Y-axis 32 (logarithmically) is shown the number of times a specific part 
of a track has been recorded. A measuring point is found by recording 
until a specific degradation of quality occurs. In pulse position 
modulation (PPM) customary for known rewritable recording systems, only 
brief marks are recorded in a distributed manner. However when pulse 
length modulation (PLM) is used, which makes a higher information density 
possible, long marks often occur. An example of PLM with high information 
density is the EFM+ channel code described in PCT/IB95/00070, used in the 
high-density CD. Measurements have shown that an intensified version of 
the phenomenon occurs, if the patterns comprise stretched out, 
contiguously recorded marks. Furthermore, the phenomenon appears to occur 
especially if the frequency spectrum of the running average of the 
recorded patterns includes low-frequency components. The invention is 
therefore eminently suitable for use in combination with PLM. The curve 33 
running through the measured points reflects a relation between the number 
of times a recording is made and the degree to which the recorded patterns 
are different each time they are repeated. This relation has surprisingly 
shown that the usable number of times a rerecording can be made diminishes 
if the same patterns are recurrently recorded on a (substantially) fixed 
position on the information carrier. Furthermore, a considerable shift of 
the initial position is necessary for attaining the maximum attainable 
useful life. Contrary to expectations, a small shift of usable around 32 
channelbits (first measurement value 34) did show a rather low number of 
rewrites. Such a shift, which is a few times the longest used mark in PLM, 
would distribute such long marks well beyond overlapping. In a practical 
example in CD-E having patterns of 17 channel bits for one information 
byte the longest mark is 11 channel bits. A shift of 2 information bytes, 
i.e. 34 channelbits, was expected to be sufficient. However only at a 
substantially larger shift of around 100 bits (second measurement value 35 
at 128 bits) a considerable improvement was found. Beyond a shift of the 
order of 1000 channel bits (fifth measurement value 36) no further 
improvement was found for the measured information carrier. 
FIG. 3 shows a device according to the invention for reading and/or 
recording the disc-shaped information carrier. The device comprises coding 
means 52 and a read/write unit 57 for writing and/or reading information 
blocks on the information carrier 1. Information is applied to input 51 
and converted into a write signal in coding means 52, the information 
being represented in the information blocks in a predefined manner. The 
write signal is applied to the read/write unit 57. The information carrier 
1 rotates, driven by drive means 58. The read/write unit 57 scans via beam 
56 the track 9 and writes patterns of marks in it which represent the 
information. During the scanning operation, the read/write unit 57 is 
positioned over track 9 by a servo system of a customary type (not shown). 
The system controller 59 controls the scanning of the information carrier 
1 via the drive means 58 and the servo system. Generally, such a device 
will also comprise decoding means 53 for recovering the information from 
the patterns read out via the read/write unit 57. The recovered 
information is then produced on the output 54. A device for recording only 
may not include the decoding means 53 and output 54, while a device for 
reading only may not include the coding means 52 and input 51. The device 
further may include demodulator means 55 for recovering the address 
information. The servo signals generated while the track is being scanned, 
are applied to the demodulator means 55 which is arranged for demodulating 
the servo signals for recovering the auxiliary signal. The demodulator 
means 55 derives the address information from the auxiliary signal which 
information is passed on to the system controller 59. 
According to the invention, the write means 52, 57, 59 are arranged for 
writing the information blocks in such a way that the writing of same 
patterns at the same position in a specific area of the track is 
minimized. A first measure to prevent this is that the system controller 
59 shifts the initial position of an information block to be recorded 
always by a random distance relative to the nominally defined initial 
position. This distance is chosen at will but does lie within predefined 
boundaries, because otherwise it would be possible for preceding or 
succeeding information blocks in the track to be partly overwritten. 
Furthermore, it will be a large distance compared to pattern distances 
found in channel coding. With the respective rewritable information 
carrier, ample room should be left open between the blocks to make such a 
shift in position possible. This may be realized, for example, by 
reserving ample space for an information block and the maximum 
(peak-to-peak) shift between the nominal initial positions, for example, 
laid down in a standard system-specification. A shift by 1,000 bits may be 
taken as an example, which leads to a loss of information density of about 
3% for the EFM+ code (8/16 data/channel bits) and a 2 kbyte information 
block and ID synchronization zones etc. and about 0.2% for a 32 kbyte 
information block. The effect of the first measure is further shown in 
FIG. 4. 
A second measure according to the invention is the constant adaptation of 
the manner of representation of the information in the information blocks. 
For this purpose, the coding means 52 is arranged to provide a selected 
adaptation which is always different. A key which is indicative of the 
mode of representation, and which is therefore necessary for decoding the 
information block again when read out, is generated by the system 
controller 59. For example, a random byte creating 256 different keys may 
be used as a key. Alternatively, it is possible to use consecutive keys. 
They may then also function as a repetition counter, which shows by the 
key how often a part of the track has already been recorded. When a 
certain maximum is reached, that particular part of the track may then be 
blocked from further use, before errors will develop during reading. For 
generating the consecutive keys, a system may be used in which a large 
part of the bits between the consecutive keys is different. The key is 
stored either in the information block or somewhere else on the 
information carrier, for example, in a central region. In this respect it 
is important for the key to have maximum protection against errors during 
reading, since the information is no longer recoverable in the event of an 
incorrect key. For this purpose, the key may be protected by the customary 
errorcorrecting codes, but may also be repeated at several points in the 
information block. It is alternatively possible for the keys to be stored 
in a memory (for example, in the system controller) in the device. The 
memory may be a permanent memory; the information carrier can then only be 
read by the respective device. Alternatively, it is possible for the 
contents of the memory to be recorded on the information carrier at 
specific instants. 
For adapting the manner of representation of the recorded information, 
scrambling may be used, for example. For such an adaptation, the write 
signal is processed in known fashion with a bit stream (for example, by an 
EX-OR operation). For generating the bit stream, use is made of a 
generator circuit in which the generated bit stream depends on the key. 
This key may be included in the information block, for example, in the ID. 
It is attractive for the ID to be recoverable in a simple manner from the 
read signal, since the ID should to be known rapidly to the system 
controller when a desired information block is to be jumped to. For this 
purpose, a simpler scrambling operation may be used for the ID. So-called 
self-synchronizing scramble operations may also be used for scrambling. 
This does not require any explicit key, but the descrambler will again be 
in line after a specific number of bits of a scrambled signal have been 
processed. It should be observed that when information is scrambled whose 
key is not changed for each writing operation, but is derived, for 
example, in information from the ID, the desired useful effect will not be 
achieved. 
A further measure for adapting the manner of representation if the recorded 
information is the addition of an amount of dummy data to the information 
block, for example, by including a variable length zone filled with a 
randomly selected, but fixed dummy byte for each recording at the 
beginning of an information block. After this dummy zone then follows the 
ID zone which can be recognized in a fixed manner. As a result, the ID 
zone will always be found during the reading of the information block and 
still the whole track zone is recorded, so that no old or no longer valid 
parts of previously recorded information blocks continue to be present. 
Alternatively, at the end of the information block, such a zone with 
variable dummy data can be added while the length of the zone to be 
recorded (dummy zone +block contents +dummy zone) may then be constant. 
The manner of representation then comprises including a first variable 
amount of dummy data before and a second variable amount of dummy data 
following the information of the block, while the sum of the first and 
second variable amount is substantially constant. 
FIG. 7 schematically shows three possible situations between successive 
information blocks having dummy data included therein as described above. 
In the upper situation a first block 71 and a second block 72 are shown 
having a nominal (longitudinal) position of the information contents. When 
recorded in the track the blocks would be aligned laterally and therefor 
overlapping in a small margin 80. The overlapping area has the advantage, 
that the track will be recorded continuously also in the event of small 
positioning inaccuracies caused by actual timing errors. In a track 
uninterrupted recording marks prevent disturbance of servo systems and 
signal processing. The information content of the first block 71 ends with 
a synchronisation mark 73, as indicated by the thick line. After this 
synchronisation mark 73 an end link area 78 comprising dummy data follows 
until the physical end of recording 74. The second block 72 starts from a 
physical starting point 75 with start link area 79 comprising dummy data, 
followed by a synchronisation mark 77 and the information contents of the 
second block. The area between the last synchronisation mark 73 of the 
first block and the first synchronisation mark 77 of the second block is 
usually called a linking frame. Further linking frames may be present in 
the second block before the information contents to enable the signal 
processing of a reading device to adjust to the new block, which may have 
been recorded using slightly different parameters. The synchronisation 
marks preceding linking frames may have a special value indicating a 
linking frame. The linking frame between two successive information blocks 
has a nominal length 76, including the overlapping margin 80. In the 
middle situation two further successive blocks 81, 82 are shown having a 
shifted position of the information contents. The first block 81 has a 
large amount of dummy data at the beginning (not shown) and a 
complementary small amount 84 of dummy data at the end, resulting in the 
total length and the ending point of the first block 81 being equal to the 
upper situation, while the actual information contents have been shifted a 
distance +.DELTA.. The second block 82 in the middle situation has a 
minimal amount of dummy data at the beginning, so the linking frame 83 has 
the shortest possible length, while the information contents of the second 
block is shifted a distance -.DELTA.. The lower situation shows a first 
block 85 having a shift of -.DELTA. and therefore a large amount of dummy 
data at the end. The second block 86 here has a shift of +.DELTA., 
resulting in a maximum linking frame 87. The maximum shift should be 
sufficiently large as described above. In a practical example the frames 
between synchronisation marks could be 91 bytes, one byte being 
represented by 16 channel bits as in EFM+(see above). A shift of 62 bytes 
(992 bits) would result in a .DELTA. of maximum 31. The end link area 78 
of a linking frame is 49 bytes, while the start link area 79 is 50 bytes, 
and an overlapping area of 8 bytes allowing a jitter in the positioning of 
4 bytes of both information blocks. This results in the lower situation in 
a maximum end link area of 49+31=80 bytes and a maximum start link area of 
50+31=81 bytes and therefore in a maximum linking frame 87 of 80+81-8=153 
bytes, which equals the nominal length augmented by the total shift 
91+62=153. Correspondingly in the middle situation a minimum end link area 
of 49-31=18 bytes and a minimum start link area of 50-31=19 bytes result 
in a minimum linking frame 83 of 19+18-8=91-62=29 bytes. These limits of 
153 and 29 bytes are the worst case situations above circumstances. It has 
to be noted, that one or more linking frames will be necessary in a 
practical system anyway for allowing the servo and signal processing 
electronics to adjust to the following information block, which very well 
may be written under different conditions in a different recorder from the 
previous block. So these frames can be useful for preparing reading or 
recording in a device. Applying the substantial shift according to the 
invention by adding dummy data as described here in combination with 
linking frames does not require any additional disk space, and so 
advantageously allows a high information density without loss of storage 
capacity. 
The aforesaid two different measures may be implemented separately or in 
combination. An advantage of exclusively applying shift is that a reading 
device may not need to have specific measures for reading a rewritable 
information carrier recorded in accordance with the invention. For that 
matter, reading is then substantially the same as reading a Read-Only or 
Write-Once information carrier. This is an advantage for computer and 
multimedia applications in which information carriers of different types 
are used. If relatively small information blocks are recorded, the 
shifting may considerably the information density. In that case it is 
attractive, for example, to combine a limited shift with a scrambling 
operation. The scrambling strongly reduces the autocorrelation of the 
information block to be recorded. Depending on the selected channel code, 
certain patterns may continue to be fixed, for example, such as 
synchronization patterns in the EFM+ channel code. A limited shift 
operation will then be sufficient to record these patterns always at a 
different position on the track. By selecting a limited shift, it is 
naturally possible to achieve a higher information density on the 
information carrier. 
According to the invention, the read means including the read/write unit 57 
and the decoding means 53 as shown in FIG. 3 are arranged for reading the 
information blocks from positions substantially shifted over a distance 
arbitrarily chosen within predefined boundaries and/or for decoding the 
information blocks in accordance with the adapted manner of 
representation. The measures as described above for adapting the manner of 
representation are implemented in a complementary way in the decoding 
means, while the substantially shifted position and/or linking frames as 
described with FIG. 7 are handled by the read/write unit in combination 
with the servo system and the decoding means 53. 
FIG. 4 gives a diagrammatic representation (and not to scale) of a track in 
which information blocks are recorded on shifted initial positions. The 
nominal initial positions are denoted by the dashed lines 41, 42 and 43. 
They are fixed mutual distances 40 apart. Furthermore, the system of 
shifting may also be applied to a subdivision of the track with different 
spacings between the nominal initial positions. The recorded information 
blocks are referenced 44, 45 and 46. There may be noticed that the first 
block 44 commences at the nominal initial position, the second block 45 at 
a specific distance after the nominal initial position and the third block 
46 before the nominal initial position. The spacing between the blocks is 
therefore variable. After several recordings, the spacing will partly 
contain old information as is shown at the start of the last block 47. 
However, there should be observed that in known recording systems for 
writable information carriers some margin is given between the blocks for 
a variation of the initial position of an information block. This margin 
(for example, 16 bits) may be used to compensate for mechanical and 
electrical tolerances in the initial position, which occur when the 
recording operation is started. It has been found that such a margin is 
not sufficient to effectively counteract detonation, as described with 
FIG. 2. 
FIG. 5 gives a diagrammatic representation of an adaptation procedure. Two 
measures are combined for this purpose, scrambling and shifting. The 
system controller 59 carries out the following steps: 
S1: the system controller receives the order for recording an information 
block and determines a nominal initial position. 
S2: the system controller generates a random key and supplies same to the 
coding means 52 and orders the coding means to scramble with this key. The 
key itself, for example, is also included in the block. 
S3: the system controller adds a random distance to the nominal initial 
position. 
S4: the system controller positions the read/write unit 57 over the desired 
zone of the track 9 and waits for the computed initial position to be 
located under the read/write unit, for example in dependence of the 
address information read by the demodulator means 55. 
S5: the system controller orders the write means 52, 57 to write. The 
reading operation (not shown) is carried out in similar manner, in which 
the key is first recovered from the read signal and applied to the 
descrambler means in the decoding means 53. Subsequently, the decoding is 
carried out as in known reading devices. 
FIG. 6 gives a diagrammatic representation of rotating information in an 
information block. In a first information block 61 the information is 
placed in the nominal position. As is customary, the information block 61 
starts with a RUN-IN part 63 which includes a run-in pattern for the 
reading means to run in, more particularly, for a Phase Locked Loop (PLL) 
to lock on to the regeneration of the data clock, and the information 
block is closed by a GUARD part 67 for avoiding an abrupt transition to an 
unrecorded part of the information carrier or patterns recorded 
previously. Between RUN-IN 63 and GUARD 67 there is located an information 
area 60 containing one or more data blocks Dn and synchronization patterns 
S; in FIG. 6 are shown 4 data blocks D0 (65), D1, D2 and D3 (66) and each 
data block is preceded by a synchronization pattern 64. To avoid data 
blocks and synchronization patterns S always being recorded in the same 
position when recorded recurrently, the information in the information 
area 60 is rotated by a random distance a each time it is recorded, as is 
shown in the second information block 62 in FIG. 6. Rotation, 
alternatively referenced wrap-around, is meant to be understood as the 
shifting of all the bits in the information area 60 in a particular 
direction, for which the bits that are shifted over the boundary of the 
data area are shifted to the vacant space on the other boundary of the 
information area. In the example of the information block shown 62, the 
data blocks and synchronization patterns are shifted over a channel bits 
and the information of the last data block D3 is split up into two parts 
68, 69. The second part 69 of the last data block D3 would end up past the 
terminal boundary 70 after the shift and is added to the first data block 
after the initial boundary of the information area 60. Needless to observe 
that the rotation can also be effected in the other direction, while 
particularly D0 is split up into two parts. During the reading operation, 
the shift a is to be derived from the read signal, and the beginning 
and/or end of the information area 60 are to be derived too. This may be 
effected, for example, by choosing a specific format for the RUN-IN 
patterns and/or the GUARD patterns. Also the value of a can be explicitly 
included in the information block, preferably in a predefined position 
relative to the synchronization patterns. Preferably a variable pattern is 
used for the RUN-IN and/or GUARD part of the information block, because 
otherwise the quality of these parts will degrade faster than the parts of 
the data blocks. For example, by using the (variable) value of a in the 
RUN-IN pattern, the RUN-IN is variable and a can be recovered when read 
out. Alternatively, it is possible to vary only the position of the 
synchronization patterns by a and scramble the remaining data with a 
scramble key which is then included, for example, in the RUN-IN part. For 
reading blocks that display shifted synchronization patterns, the reading 
device preferably comprises a memory for storing the signal that precedes 
the first synchronization pattern, the contents of the memory being 
decoded after synchronization has been established. In the second 
information block 62 shown, the decoding may, for example, be effected, 
from the memory that stores the part 69 of the block which part is added 
to the first data block, subsequent to the decoding of the first part 68 
of the last data block D3. 
By selecting the distance a sufficiently large (of the order of 1000 
channel bits, cf. FIG. 2), there is achieved that the rewritability for 
information blocks with synchronization patterns is high. This obviously 
holds likewise for information blocks without fixed synchronization 
patterns for which the synchronization of the decoding is effected 
differently when read out. It is also possible to use information blocks 
without a RUN-IN or GUARD area, for example, if a plurality of information 
blocks are recorded continuously. In all cases there is the advantage that 
rotation in this fashion does not ask for additional space between the 
information blocks, because they can always be recorded in the same 
positions.