GENERATION OF A RUNLENGTH LIMITED DIGITAL INFORMATION SIGNAL

Abstract: An apparatus is disclosed for generating a runlength limited (RLL) digital information signal, the digital information signal having a minimum runlength of d' and a maximum runlength of k', d' and k' being integers larger than zero and where k' is larger than d'. In accordance with the invention, the apparatus is adapted to generate the runlength limited digital information signal, such that a minimum runlength in the digital information signal occurs isolated from other minimum runlengths in the digital information signal.

Description: Generation of a runlength limited digital information signal. The invention relates to an apparatus for generating a runlength limited (RLL) digital information signal, the digital information signal having a minimum runlength of d' and a maximum runlength of k\ d' and k' being integers larger than zero and where k' is larger than d' and to a method for generating such runlength limited digital information signal. The invention further relates to an apparatus for and a method of generating a (d,k) sequence. An apparatus as defined in the opening paragraph is eg. known from USP 5,477,222, document Dl in the list of related documents that can be found at the end of this description. Runlength limited digital information signals obtained from channel coding an information signal are described in the book 'Coding techniques for digital recorders' by K.A. Schouhamer Immink, Prentice Hall, 1991, chapter 5, document D2. Such runlength limited digital information signals are generally derived from so-called (d,k) constrained sequences. In such sequences, expressed in NRZ-I (non-return-to-zero-inverse) notation, two logical 'ones' in the sequence are separated by a run of consecutive 'zeros' of length at least d, and any run of consecutive 'zeros' is at most k. A (d,k) sequence is normally converted in a precoding step so as to obtain the runlength limited information signal. In such runlength limited information signal obtained from the above (d,k) sequence, minimum runlengths of d+1 of consecutive like symbols occur and maximum runlengths of k+1 consecutive like symbols occur in sequences expressed in NRZ notation. This precoding of a (d,k) sequence, for d=l and k=7 as an example, results in a runlength limited information signal having a minimum runlength of 2 and a maximum runlength of 8. A choice of a specific channel code, properly adopted to the physical channel characteristics, is limited by the fact that the d-constraint is restricted to the natural numbers (d=0, d=l, d=2, ...). It has been proposed in the past to remove this limitation, by using rational d-constraints. Reference is made in this respect to the publication by P. Funk, document D3 in the list of related documents at the end of this description. The invention has for its object to provide a different alternative for an apparatus which is capable of generating channel codes which fill up the gap between d and d+1 for the traditional (d,k)-based codes. In accordance with a first aspect of the invention, the apparatus is characterized in that the apparatus is adapted to generate the runlength limited digital information signal, such that a minimum runlength in the digital information signal occurs isolated from other minimum runlengths in the digital information signal. In accordance with a second aspect of the invention, the apparatus for generating a runlength limited digital information signal, the digital information signal having a minimum runlength of d' and a maximum runlength of k', d' and k' being integers larger than zero and where k' is larger than d', characterized in that the apparatus is adapted to generate the runlength limited digital information signal, such that a minimum runlength occurring in the digital information signal follows only a directly preceding runlength larger than d' . In accordance with a third aspect of the invention, the apparatus for generating a runlength limited digital information signal, the digital information signal having a specific minimum runlength and a maximum runlength of k', the apparatus comprising means for generating runlengths in the digital information signal of length d'+l at minimum, where d' and k' are integers larger than zero and where k' is larger than d'+l, characterized in that the apparatus further comprises means for generating runlengths of d' in the runlength limited digital information signal, such that such runlengths of d' occur isolated from other such runlengths of d' in the digital information signal. In accordance with a fourth aspect of the invention, the apparatus for generating a runlength limited digital information signal , the digital information signal having a specific minimum runlength and a maximum runlength of k', the apparatus comprising means for generating runlengths in the digital information signal of length d'+l, where d' and k' are integers larger than zero and where k' is larger than d'+l, characterized in that the apparatus further comprises means for generating runlengths of d' in the runlength limited digital information signal, such that such runlengths of d' occur only following a directly preceding runlength larger than d' . The invention is based on the objective to generate (d,k)-based RLL channel codes, with the additional allowance of isolated Id runlengths (that is: isolated runlengths of length d), which are normally forbidden in a pure (d,k) code. An isolated Id runlength has to be surrounded by two other runlengths In with n > d. In contrast, for a pure (d-l,k) RLL channel code, the appearance of Id's is unconditional. Therefore, the capacity of the new (d,k) code with isolated Id's (called for short an iso-Id(d,k) code), is between the capacity of the traditional (d,k) and (d-l,k) channel codes. The increase in capacity of the iso-Id(d,k) code compared to the pure (d,k)-code, goes together with an increase in the highest frequency in the bit-sequence, which equals l/(2d+l) for the iso-(d,k) code, which is between l/(2d+2) and l/2d (for the pure (d,k) and (d-l,k) codes, respectively). It is to be understood that, where the claims define a RLL digital information signal with a minimum runlength of d', d' should be taken equal to the parameter d introduced above, in order to bring the claims in conformity with the above argumentation. These and other objects of the invention will be elucidated below in the following figure description, in which figure 1 shows a state-transition diagram for an iso-Il(d=l,k=4) code, figure 2 shows the state-transition diagram for an iso-Il(d=l,k=7) code, figure 3 shows the state-transition diagram for an iso-I2(d=2,k) code, and figure 4 shows an embodiment of the apparatus in accordance with the invention, In the following figure description, practical examples are worked out for a number of channel codes. Iso-Id(d,k) code implementations with mapping of n user bits to m channel bits, have been constructed with the parity preserving property, so that DC-control can be applied. Reference is made in this respect to USP 5,477,222, document Dl in the list of related documents. Parity preserving implies that user words of n bits long, and channel words of m bits long, have the same parity (which is the number of l's in a word, modulo 2). This property must also hold for the substitution encoding tables of the code (with 2n-to-2m mapping), which are used when runlength violations appear upon concatenation of channel words by use of the standard encoding tables (with n-to-m mapping). Next a Rate R=4/5, iso-Il(d=l,k=4) Code will be described. The Finite-State Runlength Diagram of the code is shown in Fig.l. The states numbered 1 to 5 in the state diagram of figure 1 describe the normal (d=l,k=4) code. By going from the state 1 to the state 2 and back to the state 1, a sequence ...01010... is obtained, which is a representation of the d(=l) constraint. That is: at minimum one 'zero' occurs between two subsequent ' ones' in the sequence, expressed in NRZ-I notation. Further, by going through the states numbered 1, 2, 3, 4, 5 and back to the state 1, a sequence ...01000010 ... is generated, which represents the k constraint: four 'zeroes' at most occur between two consecutive ' ones' in the sequence. It can be seen that, upon IT precoding this NRZ-I sequence, a runlength limited sequence is obtained having runlengths of at minimum 2 and at maximum 5 runs of consecutive like symbols (in NRZ notation), or . . .+ — b + . . . . and . . . + + . . . , respectively. It can further be seen that, apart from the traditional runlength constraints on the number of successive zeroes, there is now also a constraint on the number of successive ones, which should not exceed 2. This is realized by adding the state numbered 1'. By going through one of the states numbered 2, 3, 4 or 5, via the state 1 and the state 1', and going back to the state 2, a sequence ....0110.... has been obtained. Upon IT precoding, this results in the occurrence of a run of 1, or . . . + + - + + . . . . The capacity of the iso-Il(d=l,k=4) code equals C=0.8376. The code implementation has a rate R=0.8, thus yielding a code efficiency of 95.5 %. An iso-Il(d=l,k=3) code,which has a more severe constraint on k, leads to a capacity of C=0.7947. Such code can thus not be realised with a 4-to-5 mapping. The standard conversion rules for the iso-Il(d=l,k=4) code are shown in table I A and table I B. The table I A is the conversion table for the conversion of the even parity words and the table I B is the conversion table for the odd parity words. TABLE I A: Standard encoding table for even-parity channel words of iso-Il (d=l, k=4) code. TABLE I B: Standard encoding table for odd-parity channel words of iso-Il (d=l, k=4) code. Further, substitution tables are needed to maintain the various constraints across the boundaries of consecutive converted words. The table II A corrects for the runlength violation otherwise occurring when concatenating some of the even parity input words, more specifically when concatenating the input words numbered 1 and 7, or when concatenating the input words 1 and 8, or when concatenating the input words 6 and 1 or when concatenating the input words 8 and 1, of the table I A. The middle column in the table II A shows that the runlength violations are in the form of three directly successive ' ones' or in the form of five directly successive 'zeroes', assuming the conversion in accordance with the rules of the table I A would have been carried out. The right column gives the substitution sequence. It should be noted that the second 5-bit word in the substitution table is not included in the converted words of the tables I A and I B. Therefore, those words can be used as an identification, upon decoding, that the two 5-bit converted words should be reconverted in accordance with the table II A. Table II B corrects for the runlength violation otherwise occurring when concatenating some of the even parity input words from the table I A with some of the odd parity input words from the table I B. Table II C corrects for the runlength violation otherwise occurring when concatenating some of the odd parity input words from the table I B with some of the even parity input words from the table I A. Table LT D corrects for the runlength violation otherwise occurring when concatenating some of the odd parity input words from the table I B. TABLE II A: Substitution encoding table: EE-concatenation of iso-Il (d=l,k=4) code. TABLE II B: Substitution encoding table: EO-concatenation of iso-Il (d=l,k=4) code. TABLE II C: Substitution encoding table: OE-concatenation of iso-Il (d=l,k=4) code. TABLE II D: Substitution encoding table: OO-concatenation of iso-Il (d=l,k=4) code. Summarizing, the advantages of the R=4/5 iso-Il(d=l,k=4) code are: - standard mapping (4-to-5) and substitutional mapping (8-to-10) compatible with byte- oriented error correction, and error-propagation only occurs when the substitution tables during decoding involve two successive bytes - low k-value, thus limited intersymbol-interference (ISI), and proper clocking. - DC-control via parity preserve principle. - relatively simple encoding tables. Next, a Rate R=6/7 iso-Il(d=l,k=7) code will be described. The finite-state runlength diagram of the code is shown in Fig.2. The states numbered 1 to 8 in the state diagram of figure 2 describe the normal (d=l,k=7) code. By going from the state 1 to the state 2 and back to the state 1, a sequence ...01010... is obtained, which is a representation of the d(=l) constraint. That is: at minimum one 'zero' occurs between two subsequent ' ones' in the sequence, expressed in NRZ-I notation. Further, by going through the states numbered 1, 2, 3, , 7, 8 and back to the state 1, a sequence ...01000000010 ... is generated, which represents the k constraint: seven 'zeroes' at most occur between two consecutive ' ones' in the sequence. It can be seen that, upon IT precoding this NRZ-I sequence, a runlength limited sequence is obtained having runlengths of at minimum 2 and at maximum 8 runs of consecutive like symbols (in NRZ notation), or . . . . + - - + . . . and . . . + + . . , respectively. It can further be seen that, apart from the traditional runlength constraints on the number of successive zeroes, there is now also a constraint on the number of successive ones, which should not exceed 2. This is realized by adding the state numbered 1'. By going through one of the states numbered 2, 3, 4 or 5, via the state 1 and the state 1', and going back to the state 2, a sequence ....0110.... has been obtained. Upon IT precoding, this results in the occurrence of a run of 1. The capacity of the RLL code equals C=0.8732. The code implementation has a rate R=0.8571, thus yielding a code efficiency of 98.2 %. The standard encoding rules for the code is shown in the tables III A and III B. The table III A discloses the conversion of even parity input words into converted words and the table III B discloses the conversion of the odd parity input words. TABLE III A: Standard encoding table for even-parity words for iso-I2 (d=l,k=7) code. TABLE III B: Standard encoding table for odd-parity words for iso-I2 (d=l,k=7) code. Further, the runlength violation problems which would arise upon free concatenation by the standard table, are removed by the substitution tables, shown in the table IV A to IV D, where the table IV A solves the problem when concatenating even parity input words from the table III A, the table IV B solves the problem when concatenating even parity words from the table III A with odd parity input words from the table III B, the table IV C solves the problem when concatenating odd parity input words from the table III B with even parity input words from the table III A and the table IV D solves the problem when concatenating odd parity words from the table III B. TABLE IV A: Substitution encoding table: EE-concatenation for iso-Il (d=l,k=7) code. TABLE IV B: Substitution encoding table: EO-concatenation for iso-Il (d=l,k=7) code. TABLE IV C: Substitution encoding table: OE-concatenation for iso-Il (d=l,k=7) code. TABLE IV D: Substitution encoding table: OO-concatenation for iso-Il (d=l,k=7) code. Next, a rate R=3/5, iso-I2(d=2,k=10) Code is described. This code has a rate between R=l/2 of EFMPlus (d=2,k=10), see document D4 in the list of documents at the end of the description, and R=2/3 of the code disclosed in document Dl. The Finite-State Runlength Diagram of the code is shown in Fig.3. It can be seen that apart from the traditional runlengths 13, 14, ... I(k+1), now also runlengths 12 are allowed, on condition that they are not preceeded nor succeeded by another 12. This is outlined in the finite state runlength diagram by going from state 2 to state k+2, emitting a 1, after which two zeroes are emitted on passing along state k+3 on the way to state 3. Continuing from state 3 on, the minimum runlength is an 13. The capacity of the RLL code (for k=10) equals C=0.646057. The code implementation has a rate R=0.6, thus yielding a code efficiency of 92.9 %. The standard encoding rules are shown in the table V. TABLE V: Standard encoding rules for iso-I2 (d=2, k=10) code. Runlength violation problems could arise upon free concatenation of some of the 3-bit input words upon conversion by use of the standard table. Those combinations of 3-bit input words are given in the table VI and the runlength violations are removed by the combinations of 5-bit channel words given in this table VI. TABLE VI: Substitution encoding table iso-I2 (d=2, k=10) code. It can be summarized that the advantages of the R=3/5 iso-I2(d=2,k=10) code are: rate between that of EFMPlus (d=2) and (d=l) codes. DC-control via parity preserve principle. - relatively simple encoding tables. A point of attention is that - standard mapping (3-to-5) and substitutional mapping (6-to-10) are not compatible with byte-oriented error correction. In order to avoid extra error propagation, an error correction code may be devised on formats using 9 bit or 12 bit symbols. Figure 4 shows schematically an apparatus for generating one of the (d,k) sequences described above. The apparatus is identified by the encoding unit 40, which has an input 42 for receiving the sequence of input words. The encoding unit 40 converts the sequence of input words into the (d,k) sequence with the properties described above, and supplies the (d,k) sequence to its output 46. The output 46 is coupled to an input 48 of an aT precoder 50, more preferably a IT precoder, well known in the art. The IT precoding carried out on the (d,k) sequence, results in a RLL signal of the type (d,k), which is supplied to an output 52. The output 52 is coupled to an input 54 of a writing unit 56 for writing the RLL signal on a record carrier 60, such as a magnetic record carrier eg. in the form of a tape. The writing unit 56 comprises at least one write head 58 for realizing this. Instead of a magnetic record carrier, the RLL signal could have been recorded on an optical record carrier, such as an optical disk 60a. Whilst the invention has been described with reference to preferred embodiments thereof, it is to be understood that these are not limitative examples. Thus, various modifications may become apparent to those skilled in the art, without departing from the scope of the invention, as defined by the claims. As an example. The invention has been explained above with reference to embodiments where binary signals have been encoded and decoded. It will be clear that the invention is equally well applicable on m-ary valued signals, where m is larger than 2. Further, the invention lies in each and every novel feature and combination of features. LIST OF RELATED DOCUMENTS (Dl) USP 5,477,222 (PHN 14.448) (D2) K.A. Schouhamer Immink, Coding Techniques for Digital Recorders, Prentice Hall, 1991. (D3) P. Funk, Run-Length-Limited Codes with Multiple Spacing, LEEE Transactions on Magnetics, Vol. MAG- 18, 1982, pp. 772-775. (D4) USP 5,696,505 (PHN 14.746)

Claims: CLAIMS: 1. Apparatus for generating a runlength limited (RLL) digital information signal, the digital information signal having a minimum runlength of d' and a maximum runlength of k', d' and k' being integers larger than zero and where k' is larger than d' , characterized in that the apparatus is adapted to generate the runlength limited digital information signal, such that a minimum runlength in the digital information signal occurs isolated from other minimum runlengths in the digital information signal. 2. Apparatus for generating a runlength limited digital information signal, the digital information signal having a minimum runlength of d' and a maximum runlength of k', d' and k' being integers larger than zero and where k' is larger than d', characterized in that the apparatus is adapted to generate the runlength limited digital information signal, such that a minimum runlength occurring in the digital information signal follows only a directly preceding runlength larger than d' . 3. Apparatus for generating a runlength limited digital information signal, the digital information signal having a specific minimum runlength and a maximum runlength of k', the apparatus comprising means for generating runlengths in the digital information signal of length d'+l at minimum, where d' and k' are integers larger than zero and where k' is larger than d'+l, characterized in that the apparatus further comprises means for generating runlengths of d' in the runlength limited digital information signal, such that such runlengths of d' occur isolated from other such runlengths of d' in the digital information signal. 4. Apparatus for generating a runlength limited digital information signal , the digital information signal having a specific minimum runlength and a maximum runlength of k', the apparatus comprising means for generating runlengths in the digital information signal of length d'+l, where d' and k' are integers larger than zero and where k' is larger than d'+l, characterized in that the apparatus further comprises means for generating runlengths of d' in the runlength limited digital information signal, such that such runlengths of d' occur only following a directly preceding runlength larger than d'. 5. Apparatus as claimed in anyone of the preceding claims, characterized in that it further comprises means for recording the runlength limited digital information signal on a record carrier. 6. Apparatus as claimed in claim 5, characterized in that the recording means are adapted to record the information signal on a magnetic record carrier. 7. Apparatus as claimed in claim 5, characterized in that the recording means are adapted to record the information signal on an optical record carrier. 8. Method of generating a runlength limited digital information signal, the digital information signal having a minimum runlength of d' and a maximum runlength of k', d' and k' being integers larger than zero and where k' is larger than d', characterized in that the method comprises the step of generating the runlength limited digital information signal, such that a minimum runlength in the digital information signal occurs isolated from other minimum runlengths in the digital information signal. 9. Method of generating a runlength limited digital information signal, the digital information signal having a minimum runlength of d' and a maximum runlength of k', d' and k' being integers larger than zero and where k' is larger than d', characterized in that the method comprises the step of generating the runlength limited digital information signal, such that a minimum runlength occurring in the digital information signal follows only a directly preceding runlength larger than d' . 10. Method of generating a runlength limited digital information signal, the digital information signal having a specific minimum runlength and a maximum runlength of k', the method comprising the step of generating runlengths in the digital information signal of length d'+l at minimum, d' and k' being integers larger than zero and where k' is larger than d'+l, characterized in that the method further comprises the step of generating runlengths of d' in the runlength limited digital information signal, such that such runlengths of d' occur isolated from other such runlengths of d' in the digital information signal. 11. Method of generating a runlength limited digital information signal, the digital information signal having a specific minimum runlength and a maximum runlength of k', the method comprising the step of generating runlengths in the digital information signal of length d'+l, d' and k' being integers larger than zero and where k' is larger than d'+l, characterized in that the method further comprises the step of generating runlengths of d' in the runlength limited digital information signal, such that such runlengths of d' occur only following a directly preceding runlength larger than d'. 12. Method as claimed in any of the claims 9, 10, 11 or 12, further comprising the step of recording the runlength limited digital information signal on a record carrier. 13. Apparatus for generating a (d,k) sequence, two logical 'ones' in said sequence being separated by a run of consecutive 'zeros' of at least d, characterized in that said runs of d consecutive 'zeros' occur isolated from each other in said sequence. 14. Method of generating a (d,k) sequence, two logical 'ones' in said sequence being separated by a run of consecutive 'zeros' of at least d, characterized in that said runs of d consecutive 'zeros' occur isolated from each other in said sequence. 15. Apparatus for generating a modified (d,k) sequence, characterized in that minimum runs of d-1 consecutive 'zeros' between two subsequent 'ones' are allowed and occur isolated isolated from each other in said modified sequence. 16. Method of generating a modified (d,k) sequence, characterized in that minimum runs of d-1 consecutive 'zeros' between two subsequent 'ones' are allowed and occur isolated from each other in said modified sequence.

Inventors: COENE WILLEM M J, COENE, WILLEM, M., J.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1999
Main classification: H03M5
IPC codes: G11B20, H03M5, H03M7