Record carrier containing a signal having a sequence of successive information signal portions

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BACKGROUND OF THE INVENTION

The invention relates to a record carrier, and more particularly, to a record carrier in which a signal having a sequence of successive information signal portions is recorded in a track.

A record carrier and related methods and devices for recording are published by K. A. Schouhamer Immink in the book entitled "Coding Techniques for Digital Recorders" (ISBN 0-13-140047-9). In said title, for example, the so-called EFM modulation system is described which is used for recording information on so-called Compact Discs. The EFM-modulated signal is obtained by converting a series of 8-bit information words to a series of 14-bit code words, three merging bits being inserted into the code words. The code words are selected such that the minimum number of "0" bits situated between the "1" bits is d (2) and the maximum number is k (10). This constraint is also referenced dk-constraint. The series of code words is converted, via a modulo-2 integration operation, to a corresponding signal formed by bit cells having a high or low signal value, a "1"-bit being represented in the modulated signal by a change from the high to the low signal value or vice versa. A "0"-bit is represented by the lack of a change of signal value at a transition between two bit cells. The merging bits are selected such that even in the regions of transition between two code words the dk-constraint is satisfied and that in the corresponding signal the so-called running digital sum value remains substantially constant. The running digital sum value at a specific instant is understood to mean the difference between the number of bit cells having the high signal value and the number of bit cells having the low signal value, calculated over the modulated signal portion situated before this instant. A substantially constant running digital sum value means that the frequency spectrum of the signal does not comprise frequency components in the low frequency area. Such a signal is also referenced a DC-free signal. The lack of low-frequency components in the signal is highly advantageous when the signal is read from a record carrier on which the signal is recorded in the track, because then continuous tracking control unaffected by the recorded signal is possible. Information recording has a constant need for enhancing the information density on the record carrier.

A possible solution to this is a reduction of the number of bit cells per information word in the modulated signal. However, the problem occurring then is that as a result of the reduction of this number of bit cells per information word the number of unique bit combinations which may represent the information words will decrease, due to which less strict constraints can be imposed on the modulated signal, for example, constraints as regards low-frequency contents of the modulated signal.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a record carrier in which the number of bit cells per information word recorded thereon is reduced while counteracting a reduction in the number of unique bit combinations which may represent the information words

In accordance with an aspect of the invention, a record carrier has a signal recorded in a track. The signal includes a sequence of successive information signal portions. Each signal portion represents an information word wherein each of the information signal portions includes n bit cells having a first or second signal value. A plurality of track information patterns represent the signal portions. The information signal portions are spread over at least one group of a first type and at least one group of a second type. Each information signal portion belonging to a group of the first type uniquely represents an information word and each information signal portion belonging to a group of the second type in combination with the signal values of p bit cells at predetermined positions in a following information signal portion represent a unique information word. Consequently, one information signal portion belonging to a group of the second type represents a plurality of information words among which the respective information word is distinguishable by the signal values.

It is a feature of the invention that each number of successive bit cells having a same signal value ranges from a minimum of d+1 to a naximum of k+1. At any arbitrary point in the signal the running value of the difference between the number of bit cells having the first signal value and the bit cells having the second signal value in the signal portion preceding this point is limited.

It is another feature of the invention that n is equal to 16, d is equal to 2 and k is equal to 10. The signal can include sync signal portions which have bit cell patterns that do not occur in the sequence of successive information signal portions, while a unique information word is established by each of the information signal portions of the second group combined with either an adjacent sync signal portion or an adjacent information signal portion.

It is yet another feature of the invention that p can be equal to 2. The information signal portions from the at least one group of the first type end in s bit cells can have a same signal value The information signal portions from the a&t least one group of the second type can end in t bit cells having the same signal value, wherein s and t can assume different values and wherein s and t are different in value, The value of t can be greater than or equal to 2 and smaller than or equal to 5.

It is still another feature of the invention that the track information patterns can include first and second parts alternating in the direction of the track. The first parts present detectable first properties. The second parts present second properties distinguishable from the first properties. Those parts having the first properties represent bit cells having the first signal value and chose parts having the second properties represent the bit cells having the second signal value. These properties can be optically detectable.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further explained with reference to the drawings, in which:

FIG. 1 shows a series of information words, a corresponding series of code words and a modulated signal;

FIGS. 2A-J and 3A-C show tables in which the relation between the information words and code words is established;

FIG. 4 shows the values of various parameters as they are when a series of information words is converted to a series of code words;

FIGS. 5a and 5b show the low-frequency portions of frequency spectra of various signals;

FIGS. 6 and 8 show various embodiments for coding devices;

FIG. 7 shows an embodiment for a selection circuit to be used in the coding device shown in FIG. 6;

FIG. 9 shows possible bit patterns of suitable sync words;

FIG. 10 shows an adaptation of the coding device of FIG. 6 for the insertion of sync words;

FIG. 11 shows a decoding device;

FIG. 12 shows a record carrier;

FIG. 13 shows a considerably enlarged portion of the record carrier of FIG. 12;

FIG. 14 shows a recording device;

FIG. 15 shows a reading device;

FIG. 16 shows parts of a modulated signal and its corresponding code words; and

FIG. 17 gives a diagrammatic representation of the spreading of code words over groups and sets.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows three consecutive m-bit information words, in this case, 8-bit information words referenced 1. The three information words 1 have the respective word values "24", "121" and "34". This series of 3 information words 1 is converted to three consecutive n-bit code words, in this case, 16-bit code words referenced 4. The code words 4 form a bit string of bits having a logical "0" value and bits having a logical "1" value. The conversion of the information words is such that in the bit string the minimum number of bits having a logical "0" value positioned between two bits having a logical "1" value is d and the maximum is k, where d is equal to 2 and k is equal to 10. Such a bit string is often referenced a RLL string (RLL=Run Length Limited) with a dk-constraint. The individual bits of the code words will further be referenced x1, . . . , x16, where x1 denotes the first bit (from the left) of the code word and x16 denotes the last bit of the code word.

The bit string formed by the code words 4 is converted to a modulated signal 7 by means of a modulo-2 integration operation. This modulated signal comprises three information signal portions 8 representing the code words 4. The information signal portions comprise bit cells 11 which may have a high signal value H or a low signal value L. The number of bit cells per information signal portion is equal to the number of bits of the associated code word. Each code word bit having a logical "1" value is indicated in the modulated signal 7 by a transition from a bit cell having the high signal value to a bit cell having the low signal value, or vice versa. Each code word bit having, the logical "0" value is indicated in the modulated signal 7 by the absence of a change of signal value at a bit cell transition.

Furthermore, the frequency spectrum of the modulated signal 7 is required to include substantially no low-frequency components. Worded differently, the modulated signal 7 is to be DC-free.

In the following an embodiment of the method according to the invention by which the modulated signal can be obtained will be described in detail.

First there is a requirement with respect to the code words that within the code words the dk-constraint is satisfied. FIG. 17 diagrammatically shows the set of all the possible code words satisfying said dk-constraint in the zone enclosed by frame 170. The code words are divided into at least a group of a first type and at least a group of a second type. When a code word is delivered from one of the groups of the first type, a coding state is established which exclusively depends on the group of the first type to which the delivered code word belongs. When one of the code words of the group of the first type is delivered, a coding state is established which depends both on the group of the first type and on the information word represented by the delivered code word. In the embodiment described herein, two groups of the first type can be distinguished i.e. a first group G11 which comprises code words ending in a bits having a logical "0" value, where a is an integer equal to 0 or 1, and a second group G12 of code words ending in b bits having a logical "0" where with b is an integer smaller than or equal to 9 and greater than or equal to 6.

In FIG. 17 the code words belonging to group G11 lie in a frame 171. The code words belonging to group G12 lie in a frame 172.

The coding state established by the first group G11 of the first type will henceforth be referenced S1. The coding state established by the second group G12 of the first type will henceforth be referenced S4. The embodiment to be described here only knows one group of the second type. This group comprises code words ending in c bits having a logical "0" value, where c is an integer greater than or equal to 2 and smaller than or equal to 5. This group will henceforth be referenced group G2. In FIG. 17 the code words of group G2 lie in a frame 173. In the example to be described here, two coding states i.e. S2 and S3 can be established by the combination of a code word and associated information word.

When the information words are converted to code words, a code word belonging to a set of code words depending on the coding state is assigned to the information word to be converted. The sets of code words belonging to the coding states S1, S2, S3 and S4 will henceforth be referenced V1, V2, V3 and V4, respectively. The code words of the sets V1, V2, V3 and V4 lie in the frames 174, 175, 176 and 177. The code words in the sets are selected such that each bit string that can be formed by a code word from the group that has established a coding state and an arbitrary code word from the set established by this coding state satisfies the dk-constraint. In the case where the coding state S4 is established by the delivery of the previously delivered code word and the coding state thus denotes that the previous code word ends in a bit string having a logical "0" value greater than or equal to 6 and smaller than or equal to 9, code word set V4 which is established by the coding state S4 is only allowed to comprise code words beginning with a maximum of 1 bit having the logical "0" value. For that matter, code words beginning with a larger number of bits having the logical "0" value will have transitional areas between the previously delivered code word and the code word to be delivered, in which areas the number of successive bits having the logical "0" value will not always be smaller than or equal to 10 and thus not satisfy the dk-constraint. For similar reasons, set V1 comprises only code words beginning with a number of bits having the logical "0" value that is greater than or equal to 2 and smaller than or equal to 9.

Sets V2 and V3 of code words belonging to the coding states S2 and S3 contain only code words beginning with a number of bits having a logical "0" value greater than or equal to 0 and smaller than or equal to 5. The code words satisfying this condition are spread over the two sets V2 and V3, so that sets V2 and V3 do not contain any common code words at all. Sets V2 and V3 will be referenced disjunct sets irk the following. The spreading of the code words over sets V2 and V3 is preferably such that on the basis of the logical values of a limited number of p bits there can be determined to what set a code word belong. In the example described above, the bit combination x1.x13 is used for this purpose. Code words from set V2 are recognisable from the bit combination x1.x13=0.0. Code words from set V3 are then recognisable from the combination x1.x13 which is unequal to 0.0. A distinction is made between code words establishing coding state S1 (group G11) on delivery, code words establishing coding state S2 or S3 (group G2) on delivery, and code words establishing the coding state S4 (group G12) on delivery. Set V1 comprises 138 code words from group G11, 96 code words from group G2 and 22 code words from group G12. It will be evident that the number of different code words in set V1 is smaller than the number of different 8-bit information words.

Since the code words from group G2 are always followed by a code word from set V2 or a code word from set V3, and, in addition, based on the code word following a code word from group G2 there may be established what set this code word belongs to, a code word from group G2 followed by a code word from set V2 can be unequivocally distinguished from the same code word from group G2, but followed by a code word from set V3. Worded differently, when code words are assigned to an information word, each code word from group G2 can be used twice. Each code word from group G2 together with a random code word from set V2 forms a unique bit combination which is inseparable from the bit combination formed by the same code word and a random code word from the same set V3. This means that 138 unique bit combinations (code words) from group G11 can be used for set V1, 22 unique bit combinations (code words) from group G12 and 2*96 unique bit combinations (code words from group G2 combined with subsequent code words) from group G2. This brings the total number of useful unique bit combinations to 352. The number of unique bit combinations formed with the code words from. sets V2, V3 and V4 are 352, 351 and 415, respectively.

By way of illustration FIG. 17 shows a code word 178 belonging to group G2. This means that the next code word belongs either to set V2 or set V3. Code word 178 and the next code word are thus capable of unambiguously establishing two different information words. In FIG. 17 code word 178 followed by a code word from set V2, for example, code word 179, establishes a different information word from the one established by code word 178 followed by a code word from set V3, for example, code word 180. Code word 179 belongs to group G11, resulting in that code word 179 is always followed by a code word from set V1, regardless the information word to be coded next, so that code word 179 is capable of establishing not more than a single information word. The same holds for code word 180. The conversion of information words takes place as follows:

Let us assume that the code word delivered last is code word 178 from group G2, the next code word will then belong either to set V2 or set V3, depending on the information word to be converted. Assuming that this information word establishes code word 179, this means that the next code word will belong to set V1. Which code word from set V1 is used is determined by the information word to be converted. In this example this is code word 181. Code word 181 belongs to group G12, so that the next code word will belong to set V4. Which code word this will be will again be established by the information word to be converted. In this example this is code word 182. Code word 182 belongs to group G2. This means that, depending on the information word corresponding to code word 182, the next code word comes either from set V2 or from set V3. Which of the code words from set V2 or V3 is used depends on the information word to be converted. In this example code word 182 is followed by code word 183. Code word 183 also belongs to group G2, so that, depending on the information word corresponding to code word 183, the next code word will come either from set V2 or V3. Which of the code words in the set is used again depends on the information word to be converted. In this case this is code word 184. In the manner described above any random series of information words can be uniquely converted to a series of code words.

In the foregoing an explanation has been given of the number of available code words extended by a subdivision of code words into groups of a first and a second type which establish a coding state, which coding states per se establish a set of code words from which a code word is to be selected for the conversion of a next information word. It is then essential that the sets of code words from which a selection is to be made do not have code words in common in the event of coding states laid down by code words from a group of the second type. As a result, it is possible to assign the same code word from a set of code words to different information words, provided that due care is taken that the code words following this same code word belong to different sets that do not hare code words in common. It will be obvious to a person skilled in the art that said subdivision of code words into sets and groups for obtaining code words to which more than one information word can be assigned can also be applied to code words having a different random number of bits. Neither is it necessary for the series of code words to satisfy a specific dk-constraint. Other constraints are possible, for example, as described in EP-A 0.319.101 (PHN 12.339).

As explained hereinbefore, a larger number of available unique bit combinations arises from the fact that more than one unique bit combination can be established with code words from the group(s) of the second type (G2). Generally, the subdivision of code words into groups and sets will be selected such that the number of available unique bit combinations is larger than the number of different information words. This surplus of unique bit combinations provides the possibility of imposing additional constraints on the conversion.

One possibility is utilizing only as many available unique bit combinations as there are different information words. In that case the surplus of unique bit combinations allows of imposing specific additional constraints on the code words.

However, it is to be preferred for one or more of the sets to assign a pair formed by two code words from the associated set to each of a number of information words, and then select either of the available code words from the pair according to a certain criterion on conversion, so as to influence a specific property of the modulated signal.

A highly attractive possibility is influencing the low-frequency component in the modulated signal. This influence preferably consists of minimizing the DC components. This may be effected by determining the digital sum value at the end of each information signal portion and selecting such code words when the information is converted, so that the digital sum value determined at the end of each information portion continues to be in the neighborhood of a certain reference value. This may be effected by assigning to a number of information words a pair of code words which effect different changes of the digital sum value. Preferably, each pair of code words comprises not more than two code words for which the changes of the digital sum values have opposite signs. For a given signal level at the end of the last information signal portion, the code word can then be selected for which the digital sum value will be nearest the reference value once the code word has been delivered.

Another possibility of selecting code words is selecting the code word for which, at the given signal level at the end of the code word delivered last, the sign of the digital sum value change caused by the associated code word will be opposite to that of the difference between the digital sum value prior to the delivery of the code word and the reference value. The selection of the code word to be delivered when a selection is possible from two code words having opposite influence on the digital sum value may then be simply made on the basis of the signal value at the end of each information signal portion and the sign of the difference between the digital sum value associated to this end and the reference value.

FIG. 2 shows by way of illustration for each of the sets V1, V2, V3 and V4 a code word assigned to each of the possible information words. In this Figure the first (left) column shows the word values of all possible information words. The second, fourth, sixth and eighth columns show the code words assigned to the information words from the respective sets V1, V2, V3 and V4. The third, fifth, seventh and ninth columns show by way of the respective digits 1, 2, 3 and 4 which of the coding states S1, S2, S3 and S4 are established by the associated code word. In FIG. 2 not more than 256 of the available code words are used for each of the sets V1, V2, V3 and V4. FIG. 3 shows, similarly to FIG. 2, the code words of the sets not shown in the table of FIG. 2 for 88 information words to which a pair of two code words is assigned. The code words represented in FIG. 3 will henceforth be referenced alternative code words. The assigning of code words to the information words is such that the change of the digital sum value caused by the alternative code words is the opposite to the change of the digital sum value caused by the code words of FIG. 2 which are assigned to the word values "0" to "87" inclusive.

It should be noted that all all the sets in FIG. 3 contain equally many code words. It will be obvious to a man of ordinary skill in the art that this is not a necessity. It is equally possible that these sets are not equally large.

Furthermore, there is observed that the assignment of code words to the information words is chosen to be such that the relation between, on the one hand, the combination of a code word and the bits x1 and x13 of the next code word and, on the other hand, the information words, is unique, so that the decoding can exclusively be effected based unpon a received code word and the bits x1 and x13 of the next code word. For the code word assignment this means that if a code word occurs in different sets, the same code words in different sets represent the same information words. For example, the information word having the word value "2" is represented by "0010000000100100" in the sets V0 and V2 shown in FIG. 2 and by "1000000000010010" in the sets V2 and V3.

Needless to observe that it is not necessary that code words from different sets represent the same information words. However, this does mean that the coding state is to be recovered on decoding to reconstitute the original information word.

The conversion of a series of information words into a series of code words will be further explained with reference to FIG. 4.

Column IW shows from top to bottom the word values of a series of successive m-bit information words. For each of the information words for which a word value is included in column IW are shown a number of data. The column SW represents the coding state laid down when the code word was delivered, which code word was obtained as a result of the conversion of the preceding information word. This code word will henceforth be referenced preceding code word. The coding state in column SW denotes which of the sets V1, V2, V3 and V4 of code words is to be used for the conversion of the information word. Column LB shows the signal value of the modulated signal at the end of the information signal portion which portion corresponds with the code word obtained when the preceding information word was converted. This signal value will henceforth be referenced running information signal value. In the column D S V the digital sum value is shown which belongs to the running signal value of the modulated signal, the running modulated signal value.

Column CW shows the code words assigned to the information words of column IW according to the columns of FIGS. 2 and 3. In the case where a pair of code words is assigned to an information word, the two code words of the pair are shown, the upper code word of the pair corresponding to the table of FIG. 2 and the lower code word of the pair corresponding to the table of FIG. 3. Column dDSV shows the change in the digital sum value caused by the code word, assuming that the running modulated signal value would have had value "H".

Column DSVN shows the new digital sum value for the associated code word as this value would be for the case where the associated code word is delivered. Column L B N represents via a logical "1" that the signal value at the beginning and end of the information signal portion belonging to the code word are different. A logical "0" indicates that the signal values at the beginning and end of the associated information signal portion are equal. The signal value at the beginning and end of an information signal portion are different if the associated code word contains an odd number of "1" bits, which corresponds to an odd number of changes of signal levels in the information signal portion. With an even number of "1" bits in the code word, the signal value at the beginning and end of the information signal portion is the same. In the column SWN the coding state is shown which would be established in the case where the relevant code word is delivered.

Furthermore, column CS shows by an asterisk "*" which code word is actually delivered for the associated information word.

The first (top) word from the series of code words shown in column IW has a word value of "2". Let us assume that the coding state (column SW) is S1 when the conversion of the series of information words is initiated, and that the modulated signal begins with the signal level H and that the digital sum value D S V is equal to 0. In that case the associated DSVN value is equal to -6 for the upper code word, whereas the DSVN value is +10 for the lower code word of the pair. When the criterion is applied that the code word is delivered for which the DSVN value is nearest possible a reference value of 0, the upper of the two code words of the pair is delivered for the information word having the word value of "2". This means that the coding state for the next information word (word value "8") becomes S2. At the end of the information signal portion corresponding to the delivered code word, the signal value is L and the signal value at the beginning of the next information portion is thus L as is shown in column LB. The value of dDSV for the upper code word of the pair belonging to the information word having the word value of "8" is equal to -6. This value of -6 applies to the case where the signal value at the beginning of the associated information signal portion would be H. Since this signal value is L in the situation shown, the change of the digital sum value caused by the code word is not equal to -6, but +6. This means that DSVN becomes equal to 0. For the lower code word of the pair DSVN is equal to -18. The value of DSVN for the upper code word is nearest the value of 0, so that the upper code word is delivered. Subsequently, the information word having the word value of "100" is to be converted. Not more than one code word is assigned to this information word, so that a selection depending on DSVN is impossible for this information word. Similarly to the manner described above, the information words having the word values "230", "0", "61" and "255" are converted. Each time a conversion is to take place of an information word to which a pair of code words is assigned, that particular code word is selected from the pair for which the value of DSVN is nearest zero. In this manner the DC voltage level of the modulated signal is maintained at a substantially constant level and the frequency spectrum of the modulated signal will not show any low-frequency components. Although a set of code words is not available for each information word, an influencing of the digital sum value will nevertheless be possible for 88/256 of all the information words to be converted on average. In practice this appears to be amply sufficient to provide that the low-frequency component is absent in the modulated signal. It is to be preferred to include in the code word pairs those code words for which the change caused in the digital sum value is greatest. On the one hand, this is advantageous in that the digital sum value can be changed to its maximum. On the other hand, this means that the change caused in the digital sum value is relatively small for code words not belonging to the pair and that the influence of these code words on the digital sum value is relatively small.

By way of illustration FIG. 5a shows the low-frequency portion of the frequency spectrum of a modulated signal obtained by implementing the method according to the invention. In FIG. 5b the corresponding low-frequency portion of the frequency spectrum of an EFM-modulated signal is plotted. As appears from the FIGS. 5a and 5b, the frequency spectra for the two signals are substantially the same. The dk-constraint for the EFM-modulated signal and the modulated signal obtained by implementing the method according to the invention is also substantially the same. The number of bit cells per information word in an EFM-modulated signal is equal to 17, whereas this is equal to 16 in a modulated signal according to the invention. This means that if the method according to the invention is implemented, an increase of information density of about 7% is obtained relative to an EFM-modulated signal, without this being at the cost of an increase of the low-frequency contents and without any concessions to the dk-constraint.

FIG. 6 shows an embodiment for a coding device 140 according to the invention by which the method described above can be carried out. The coding device is arranged for converting the m-bit information words 1 to the n-bit code words 4 and the number of different coding states can be indicated by s bits. The coding device comprises a converter 60 for converting (m+s+1) binary input signals to (n+s+t) binary output signals. From the inputs of the converter m inputs are connected to a bus 61 For receiving m-bit information words. From the outputs of the converter n outputs are connected to a bus 62 for delivering n-bit code words. Furthermore, s inputs are connected to an s-bit bus 63 for receiving a state word denoting the current coding state. A state word is delivered by a buffer memory 64, for example, in the form of s flip-flops. The buffer memory 64 has s inputs connected to a bus 58 for receiving a state word to be stored in the buffer memory. For delivering the state words to be stored in the buffer memory, s outputs of the converter 60 are used which are connected to bus 58.

Bus 62 is connected to the parallel inputs of a parallel-to-serial converter 66 which converts code words 4 received over bus 62 to a serial bit string to be supplied over a signal line 67 to a modulator circuit 68 which converts the bit string to the modulated signal 7 to be delivered over signal line 70. The modulator circuit 68 may be one of a customary type, for example, a so-termed modulo-2 integrator.

In addition to the code words and state words, the converter applies to a bus 75 for each received combination of information word and state word information which

denotes whether for the associated state word the code word or a pair of code words is assigned to the associated information word,

denotes for each of these assigned code words the change dDSV of the digital sum value caused by the code word as this change would be for a high signal value at the beginning of an information signal portion corresponding to this code word, <