RLL ENCODING

TECHNICAL FIELD

The present invention relates generally to RLL encoding.

BACKGROUND ART

Runlength-limited (RLL) codes have been widely used in magnetic and optical data storage to eliminate sequences that are undesired for the processes of recording and reproducing digital data. Various classes of RLL codes are used in practice. For example, peak detection systems employing runlength-limited KLL(d,k) constrained codes such as rate- 1/2 RLL(2,7) and rate-2/3 RLL(1, 7) codes have been predominant in digital magnetic storage at low normalized linear densities. At moderate normalized linear densities, the introduction of partial-response maximum- likelihood (PRML) detection channels into data storage required a different type of constrained codes. This class of codes, which are known as PRML(G,/) or PRML(O ₅ G,/) codes, facilitates timing recovery and gain control, and limits the path memory length of the sequence detector, and therefore the decoding delay, without significantly degrading detector performance. PRML(G,/) codes may also be used in conjunction with 1/(1 ©D ² ) precoders and with noise-predictive, maximum- likelihood

(NPML) channels which generalize the PRML concept ( © stands for the logical exclusive- OR operation). The G constraint limits the maximum runlength of zeros at the modulation encoder output (input of 1/(1 ©D ² ) precoder) to G. The / constraint limits the maximum runlength of zeros in the even and odd interleave of the encoder output (input of 1/(1 ©D ² ) precoder) to /.

The first PRML(G, I) code that was implemented in a data storage device had the code rate 8/9 and satisfied the constraints G=4 and 1=4. In addition, it satisfied a VFO constraint, which is also known as the M constraint, allowing discrimination of encoded data from the synchronization preamble and therefore fast start-up of the PRML receiver. The M constraint limits the maximum runlength of ones at the modulation encoder output (input of 1/(1 ©D ² ) precoder) to M. The class of RLL codes satisfying G, / and M constraints is known as

PRML(G, /, M) codes. The recorded VFO pattern ...++--++--... (the output of a 1/(1 ©D ² ) precoder) is received as a tone with frequency 1/(4T) at the center of the channel. The VFO constraint (M constraint) was generalized to constraints used to construct anti- whistle codes that exclude data patterns with zero or one spectral component in the frequency band (0, 1/(2T)). Hard disk drive (HDD) products often used architectures that did not require the

VFO constraint. Therefore, high-rate PRML codes with rates higher than 8/9, which have been designed, satisfy G and / constraints but not necessarily the M constraint.

The RLL code used in Linear Tape Open (LTO) standards for generations 2 to 4 (hereinafter LTO 2-4) is a twins-constrained maximum-transition-run MTR(/, k,t) code that requires

1/(1 ©D) precoding. It satisfiesy, k and t constraints at the input of the 1/(1 ©D) precoder. The k constraint limits the maximum runlength of zeros at the modulation encoder output (input of 1/(1 ©D) precoder) to k. They constraint limits the maximum runlength of ones at the modulation encoder output (input of 1/(1 ©D) precoder) toy. The twins constraint t limits the maximum number of consecutive twins (pair of zeros or pair of ones) at the modulation encoder output (input of 1/(1 © D) precoder) to t. The G and / constraints at the input of 1/(1 ©D ² ) precoders are equivalent to constraints j= G+\, k=G+\ and t=I at the input of 1/(1 ©D) precoders where both set of constraints PRML(G,/) and MT¥L(j,k,t) give rise to the same set of constraints on recorded patterns (output of precoder). The RLL code in LTO 2-4 is based on a rate-8/9 code that also satisfies the VFO constraint, rules out the Data Set

Separator (DSS) and Resynchronization (Re Sync) patterns and ensures that each RLL codeword has at least one isolated transition. The concatenated rate of the RLL code in LTO 2-4 is 16/17 because a rate-8/9 encoded byte is concatenated with an uncoded byte. The constraints satisfied by the rate- 16/17 RLL code in LTO 2-4 are equivalent to G=I 3, I=I 1 and M=23. Additionally, the RLL code parameter that is very important to error-rate performance with error-correction-coding (ECC) is error propagation. Error propagation is defined as the minimum length of a channel error burst in NRZ bits that causes two erroneous symbols in one Reed-Solomon (RS) codeword at the RS decoder input. Error propagation depends on the modulation code properties and the depth of interleaving of ECC codewords. In LTO 2-4 two-way interleaved RS codewords are RLL encoded and the error propagation of the modulation code is therefore 9 NRZ bits. Since most of the error patterns

at the detector output are 1 to 4 NRZ bits long, most of the time one symbol in a RS codeword is in error as a result of a short (non- fading) error burst in the channel.

Construction of rate-8/9 PRML(G, /, M) codes has previously been described. Additionally, the rate of non-concatenated PRML(G, /, M) codes that have been constructed is less than

16/17.

The next generation LTO standard (LTO 5) will likely have a rate-32/33 or rate-48/49 RLL code in order to increase the format efficiency. One method for the design of such codes is the straightforward extension of the rate- 16/ 17 LTO 2-4 code by inserting two or four more uncoded bytes to obtain a rate-32/33 or rate-48/49 code, respectively. However, this solution results in a rate-32/33 RLL code with parameters G= 29, 7=19, M=39 or a rate- 48/49 RLL code with parameters G= 45, 7=27, M=55. These values are not acceptable because the constraints are too weak. Another solution would be to generate a rate- 16/17 encode and decode table using a computer and use computer-aided design tools to generate

Boolean-based or ROM-based logic for encoding and decoding based on the encode and decode table. However, this solution is too complex and would require more than 500,000 gates per channel to implement. Additionally, the encoding operation needs a compact representation such that it can be included into the LTO 5 standard; specifying 65,536 17-bit codewords in the LTO 5 standard is not acceptable. Therefore, there is a need for an algorithmic approach to design an RLL code that satisfies similar constraints as the LTO 2-4 RLL code does.

SUMMARY OF THE INVENTION

The present invention provides a method for encoding a data input sequence of m bits into an output sequence codeword of m+1 bits, where m is an integer multiple of an ECC symbol size s. The method includes the steps of receiving a data stream of unencoded m-bit input sequences and dividing each m-bit input sequence into a first block of/? bits and a second block ofm-n unencoded bits, where n is an integer multiple of s. The first block of/? bits is divided into a first set of n+\ encoded bits, wherein at least one of Pl subblocks of the first set of n+1 bits satisfies a G constraint, an M constraint and an 7 constraint. The first set of

/7+1 encoded bits is mapped in a one-to-one manner into a second set of n+1 encoded bits wherein at least one of Pl subb locks of the second set of n+1 bits gives rise to at least Q\ transitions after 1/(1+D ² ) precoding. The second set of n+1 encoded bits is divided into P3 encoded subblocks and the P3 encoded subb locks are interleaved among (m-n)/s unencoded symbols so as to form the (m+l)-bit output sequence codeword. The output sequence codeword is then stored on a data storage medium.

In one embodiment, an encoder generates a rate- 16/17 PRML(G= 14, I=W, M=IV) mother code from which higher rate codes may be generated. In various embodiments, such higher rate codes include a rate-32/33 PRML(G=14, 7=11, M=23) code (RLL 1), a rate-48/49

PRML(G=22, 7=15, M=3l) code (RLL 2) and a rate-48/49 PRML(G=14, 7=19, M=39) code (RLL 3).

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 is a high level block diagram of a data storage device in which the present invention may be implemented;

FIG. 2A is a block diagram of a rate- 16/17 encoder to generate the mother code of an embodiment of the present invention;

FIG. 2B is a block diagram of a decoder of an embodiment of the present invention;

FIGs. 3A, 3B and 3C are representations of the output blocks from the first, second and third stages, respectively, of the rate- 16/ 17 encoder of an embodiment of the present invention;

FIG. 4 illustrates the process by which an input of four 8-bit symbols is transformed into a

33-bit codeword for a 32/33 code by multiplexing coded and uncoded bits;

FIGs. 5A, 5B illustrate a 4-way interleaving (multiplexing) scheme of 8-bit symbols at the ouptut of four Cl Reed-Solomon encoders which may be implemented for the rate-32/33 PRML(G= 14, /=11, M=23) code of an embodiment of the present invention;

FIG. 6 illustrates a 4-way Cl interleaving scheme which may be implemented for the rate-

48/49 PRML(G=22, 7=15, M=31) code of an embodiment of the present invention; and

FIG. 7 illustrates a 4-way Cl interleaving scheme which may be implemented for the rate- 48/49 PRML(G=14, 7=19, M=39) code of an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Many of the functional units described in this specification have been illustrated in the Figures as blocks, in order to more particularly emphasize their implementation independence. For example, a block may be implemented as a hardware circuit comprising custom VLSI circuits or gate arrays, off-the-shelf semiconductors such as logic chips, transistors, or other discrete components. A block may also be implemented in programmable hardware devices such as field programmable gate arrays, programmable array logic, programmable logic devices or the like.

Blocks may also be implemented in software for execution by various types of processors. An identified block of executable code may, for instance, comprise one or more physical or logical blocks of computer instructions which may, for instance, be organized as an object, procedure, or function. Nevertheless, the executables of an identified block need not be physically located together, but may comprise disparate instructions stored in different locations which, when joined logically together, comprise the block and achieve the stated purpose for the block.

Indeed, a block of executable code could be a single instruction, or many instructions, and may even be distributed over several different code segments, among different programs, and across several memory devices. Similarly, operational data may be identified and illustrated herein within blocks, and may be embodied in any suitable form and organized

within any suitable type of data structure. The operational data may be collected as a single data set, or may be distributed over different locations including over different storage devices, and may exist, at least partially, merely as electronic signals on a system or network.

Furthermore, the described features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of programming, software blocks, logic blocks, user selections, network transactions, database queries, database structures, hardware blocks, hardware circuits, hardware chips, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, and so forth. In other instances, well-known steps, components or operations are not shown or described in detail to avoid obscuring aspects of the invention.

Embodiments of the present invention provides an algorithmic approach to the design of higher-rate PRML(G, /, M) codes which also enforce a minimum number of transitions per codeword. A rate- 16/17 PRML(G=6, 7=7, M= 15) "mother code" is provided that enforces at least four transitions per codeword. To this end the input to the encoder is partitioned into a first set of blocks which are mapped into a second set of blocks and G, I and M constraints are imposed on at least one of the second set of blocks. The encoding operation is initially performed by detecting violations and indicating in encoded data the type of violations that have occurred. Further stages of encoding by violation detection and substitution tighten the code constraints and rule out DSS and Re Sync patterns. The mother code is then used to construct higher-rate 32/33 or 48/49 PRML(G, /, M) codes by combining uncoded 8-bit bytes with the 17-bit output from the mother code. The present invention also provides a rate-32/33 PRML(G=14, I=I 1, M=23) code (RLL 1), a rate-48/49 PRML(G=22, 7=15, M=31) code (RLL 2) and a rate-48/49 PRML(G=14, 7=19, M=39) code (RLL 3).

NOTATION

The notations x(i) and X ₁ are used interchangeably herein to denote the components of a vector x . The following representations are used for a row vector x with n components x = [x(l) x(2) ... x(n)] = [x _l X ₂ ... x _κ ] = [xl x2 ... xn] =x(\ : n)

Column vectors are often specified by the transpose of a row vector and the superscript T is used to indicate the transpose operation.

Two different notations are used herein for Boolean operations. The following convention has been used to specify Boolean code constraints and to perform Boolean operations involving multiplication of a matrix and a vector: overbar stands for negation, multiplication for AND, and addition for OR. Among these three operations, negation has the highest precedence, AND (multiplication) has second highest precedence and OR (addition) has the lowest precedence. Exclusive-OR (XOR) is indicated by © .

Encoder and decoder hardware based on Boolean logic is specified using the MATLAB notation for Boolean operations. In particular, ~ stands for negation, & for AND, and | for

OR. Again, among these three operations, negation (~) has the highest precedence, followed by AND (&) and OR (|) which has the lowest precedence, i.e., the usual Boolean precedence rules apply.

The next section describes the requirements upon the resulting codes in terms of prohibited output sequences, which therefore determines the requirements on the mother code. The steps in creating the mother code are described both in text and via illustration in compact matrix notation. The section is followed by a section describing a corresponding decoder for the mother code. Next are sections which include Boolean listings in MATLAB format of the requirements for the third encoding and decoding stages for the mother code. Finally, the last three sections describe the design of rate-32/33 PRML(G=14, 7=11, M=23) code, rate-48/49 PRML(G=22, 7=15, M=31) code and rate-48/49 PRML(G=14, 7=19, M=39) code from the mother code.

FIG. 1 is a high level block diagram of a data storage device 100 in which the present invention may be implemented. The device 100 includes an encoder 200 which receives user data from a host 10. The encoder 200 encodes the data, as will be described herein, and

passes the encoded data to a precoder 102 which may include write-equalization functionality as in LTO standards. A controller 104 receives the precoded data and transmits it to a write channel 106 which records the encoded data onto a data storage medium 20.

The storage device 100 further includes a read channel 108 which reads the data from the storage medium 20 and transmits it to the controller 104. The controller 104 sends the data to a detector 110 which processes (e.g., inverse precoding) and passes it on to a decoder 210. The decoder decodes the data, as will described herein, and sends the decoded data to the host 10.

CONSTRUCTION OF RATE-16/17 PRML(G=6, I=I, M=\5) MOTHER CODE WITH MINIMUM TRANSITION DENSITY

FIG. 2A is a block diagram of the encoder 200. The encoder 200 receives unencoded user data a(l : 16) and, using K encoding stages, outputs a rate-16/17 PRML(G=6, I=I , M=I 5) mother code y(l : 17). The example of the encoder 200 which is illustrated and described herein includes K = 3 encoding stages 202, 204, 206, although more or fewer stages may be used. The output of the k-th encoding stage of (binary column vector) can be described by the multiplication of a binary transformation matrix Mf = gf (if ) and a binary partitioning vector nf = hf (if ) (binary column vector) that depend on the binary input if (binary column vector) as follows: where the superscript E refers to the encoding operation and the addition and the multiplication operations in the matrix multiplication are Boolean OR and Boolean AND operations, respectively. For a given 16-bit encoder input, only one of the components of a partitioning vector is one whereas all the other components are zero. This property is related to the fact that the input space is partitioned in a mutually exclusive manner. Finally, concatenation of encoding stages is done such that the output of the k-th encoding stage, for k=\,2 in a three-stage system, is the input of the next encoding stage, i.e., if _+l = of .

FIRST ENCODER STAGE

The first encoder stage 202 ensures that at least one of Pi =4 subblocks of the 17-bit output of the first encoder stage 202 satisfy G, I and M constraints. The output of = b ^τ = [B ₁ b ₂ ... b _l7 ] ^τ of the first encoder stage 202 is a 17 ^χ l binary column vector. The P\ =4 subblocks of the 17-bit output of the stage 202 are illustrated in FIG. 3 A as b _x b ₂ b ₃ b ₄ , b ₅ b ₆ b ₇ b _% , b _g b _w b _n b _l2 b _l3 and O ₁₄ O ₁₅ O ₁₆ O ₁₇ . Concatenation of the four output vectors results in a

PRML(G=6, 1=9, M=20) code. The first four bits b _x b ₂ b ₃ b ₄ are not allowed to be all zero. Similarly, the next four bits b ₅ b ₆ b _η b _% are also not allowed to be all zero. The next five bits b ₉ b _w b _n b _l2 O ₁₃ are not allowed to be any of the following twelve 5-bit combinations {00000,

00010, 01000, 01010, 00001, 00100, 00101, 10000, 10001, 10100, 10101, 11111}, thereby imposing G, I and M constraints on this 5 -bit combination. The last four bits are also not allowed to be all zero. Note that only a G constraint was imposed on the 4-bit combinations.

The output b may be partitioned into two blocks of size 8 bits and 9 bits which will be used to construct a rate-32/33 PRML(G, /, M) code. Similarly, the output b may be partitioned into four blocks of size 4 bits, 4 bits, 5 bits and 4 bits which will be used to construct a rate-

48/49 PRML(G, /, M) code. At the end of the first encoder stage 202 there are at most (2 ⁴ -l) (2 ⁴ -l) (2 ⁵ -12) (2 ⁴ -l)=67,500 codewords. Since 67,500>2 ¹⁶ the first stage may be accomplished with a rate- 16/17 code mapping 16 bits into 17 bits. Note that only three of the twelve excluded bit patterns b ₉ b _w b _n b _l2 b _l3 , viz., {00010, 01010, 11111} have a 1 in the fourth position, i.e., b _l2 = \ . Therefore, if a _γ a ₂ a ₃ a ₄ is nonzero, a ₅ a ₆ a ₇ α ₈ is nonzero, a _g a _w a _n a _l2 is none of the three bit patterns {0000, 0100, 1111} and α ₁₃ a _l4 a _l5 a _l6 is nonzero, then the 16x1 encoder input vector if = a ^τ = [a _γ a ₂ ... a _l6 f is mapped into of = b ^τ = [b _l b ₂ ... b _l7 ] ^τ using the substitutions [b _γ b ₂ b ₃ b ₄ ] = [a _γ a ₂ a ₃ a ₄ ] , [b _s b ₆ b ₇ b _& ] = [a _s a ₆ a ₇ a _& ] , [b ₉ b _w b _n b _l3 ] = [a ₉ a _w a _n a _l2 ] , [b _l4 b _l5 b _l6 b _l7 ] = [a _l3 a _l4 a _l5 a _l6 ] and b _l2 = 1 . Therefore, there are six types of violation that can occur: a _γ a ₂ a ₃ a ₄ is all zero, a ₅ a ₆ a ₇ α ₈ is all zero, a ₉ a _l0 a _n a _l2 is one of the three bit patterns {0000, 0100, 1111 } or α ₁₃ α ₁₄ α ₁₅ a _l6 is all zero. A violation can be indicated by b _l2 = 0 and b _l0 = 1 . The type of a single violation can be indicated by three bits [b ₉ b _n b _l3 ] . These three bits are not allowed to be all zeros in order not to violate the / code constraint. And one of the three-bit

combinations, the all-one bit pattern [b ₉ b _n b _l3 ] = [111] in the example, is used to indicate that more than one violation has occurred. Thus, there are exactly six remaining three bit patterns {001, 010, 011, 100, 101, 110} which can be used to indicate the type of a single violation. If only one of the patterns a _γ a ₂ a ₃ a ₄ , a ₅ a ₆ α ₇ α ₈ and a _l3 a _u a _l5 a _l6 is all zero, then the information in a ₉ a _w a _n a _l2 is placed into the bit pattern that is in violation. For example, if a ₅ a ₆ Ci ₁ α ₈ is all zero, then [b ₅ b ₆ b ₁ b _i \ = [a _g a _w a _n a _l2 ] , etc. One partition for no violations and 6 partitions for a single violation have so far been obtained. There are also 12 possible two violations and 13 possible three or four violations for a total of 1+6+12+13=32 first-stage partitions. If two, three or four violations occur, the type of violation is indicated in the output as in the case of a single violation and the non- violation blocks are reshuffled if necessary. In the next section, the exact expression for the 32x 1 binary partitioning vector «f = /zf (z ^' f ) = p ^τ = [p(\) p(2) ... p(32)] ^τ and binary 17 ^χ 32 matrix My described in terms of two 17 ^χ 16 submatrices B ₁ and B ₂ .

The constraints satisfied by of = b' = [b _y b ₂ ... b _ιη ]' following the first encoder stage 202 are

G constraints:

I constraints:

M constraints:

K K K K ^b u = °

Therefore, the code obtained at the output of the first encoder stage 202 is a PRML(G=6, 1=9, M=20) code.

SECOND ENCODER STAGE

The second encoder stage 204 further tightens the code constraints / and M by detection of prohibited patterns and subsequent substitution with a desired pattern that does not violate the code can be uniquely recognized during the decoding. The first encoder stage 202 ensures that the third of the P ₁ =4 subblocks of the 17-bit output b of the first encoder stage 202 satisfies G, I and M constraints. The first encoder stage 202 further ensures that the remaining three 4-bit subblocks satisfy a G constraint by not allowing the 4-bit allzero pattern to occur as a subblock. The second encoder stage 204 remaps undesired 17-bit outputs b of the first encoder stage 202 into desired 17-bit outputs c. FIG. 3B illustrates the two subblocks of the 17-bit output c of the second encoder stage 204. Specifically, the second encoder stage 204 detects prohibited patterns and replaces them with substitute patterns and therefore ensures that the first of the two subblocks of the 17-bit output of the second encoder stage 204 c satisfies / and M constraints. The first subblock of the 17-bit output of the second encoder stage 204 is C ₁ c ₂ c ₃ c ₄ c ₅ c ₆ C ₁ c ₈ and the second subblock is c ₉ c ₁₀ c _u c ₁₂ C ₁₃ c ₁₄ c ₁₅ c ₁₆ c ₁₇ . Note that the second subblock already satisfies / and M constraints as a result of the encoding in the first stage. TABLE I shows the prohibited patterns and the corresponding substitute patterns in the second encoder stage 204.

TABLE I

The bold pattern within the prohibited pattern b is used to detect violations during the second stage of encoding whereas the bold pattern in the substitute pattern c ox d (if the presence of channel errors and errors in the first decoder stage 212 is considered, then c, the output of the second encoder stage 204, becomes d, the input of the second decoder stage 214) is used to detect required inverse substitution during the second stage of decoding. If none of the three prohibited patterns is detected during the second encoder stage 204, the output of the second

encoder stage 204 is equal to its input, i.e., c=b. Therefore, there are four partitions (three substitutions plus absence of substitution) in the second encoder stage 204.

The constraints satisfied by o ₂ = c ^τ =[q c ₂ ... C ₁₇ ] ^τ following the second encoder stage 204 are

G constraints:

I constraints:

M constraints:

Therefore, the code obtained at the output of the second encoder stage 204 is a PRML(G=6, I=I, M= 15) code.

THIRD ENCODER STAGE

FIG. 3C illustrates the P ₂ = 2 subblocks of the 17-bit output y of the third encoder stage 206. The third encoder stage 206 increases the minimum transition density and ensures that the output of the encoder y gives rise to at least β ₂ =4 transitions in the recorded pattern subsequent to 1/(1 ©D ² ) precoding. The motivation behind this step is to rule out (that is, prevent them from appearing) two low-transition-density patterns from modulation encoded data: the data separator sequence (DSS), a sequence with a period of 24 bits, and the 34-bit Re Sync pattern in LTO 4. It is noted that DSS consists of consecutive 12Tmagnets where T is the channel bit period (magnetization remains the same for an interval of 127 if write

equalization is not taken into account) whereas Re Sync in LTO 4 consists of a 107 magnet, followed by a 117 magnet and a 127 magnet. More specifically, the third encoder stage 206 ensures that each of the encoded blocks [y _γ y ₂ y ₃ y ₄ y ₅ y ₆ y _η J ₈ ] and

[yg Ji o Jn Ji ₂ Ji ₃ Ji ₄ Ji ₅ Ji e Ji ₇ ] ' when converted into NRZI notation, will result in at least two transitions (NRZI ones). Therefore, the output y of the entire encoder gives rise to at least transitions in the recorded pattern subsequent to 1/(1 ©D ² ) precoding. As previously stated, this code property is used to ensure that the low-transition-density patterns DSS and Re Sync are ruled out in modulation encoded data.

Coded data y must be precoded using a 1/(1 ©D ² ) precoder. It is well known that a

1/(1 ©D ² ) precoder can be represented by the serial concatenation of two precoders of type 1/(1 © D). In LTO 1-4 1/(1 © D) precoding is performed in conjunction with write equalization. Therefore, 1/(1 © D) precoding of coded data y would be sufficient. In the following, the notation y' is used for 1/(1 ©D) precoded coded data y. Note that the components of y' are bits in NRZI notation (0 represents no transition, 1 represents transition). Uncoded data may either be not precoded at all, 1/(1 ©D) precoded or 1/(1 ©D ² ) precoded. From an error-rate performance viewpoint, it is preferable to not precode the uncoded data. However, because 1/(1 © D) precoding is already used in conjunction with write equalization in LTO 1-4, it may be more desirable to use the 1/(1 ©D) precoding. Therefore, assuming that 1/(1 © D) (NRZI) precoding is used for uncoded data and the uncoded bit preceding the first coded subblock [^ ₁ y ₂ y ₃ y ₄ y ₅ y ₆ y _η j> ₈ ] is U _\ and the uncoded bit (NRZI) preceding the second coded subblock [y _g y _w y _n y _l2 y _l3 y _l4 y _l5 y _l6 y _l7 ] is U2, the minimum transition density constraints translate into two sets of requirements.

The first set of requirements, which ensure that the first coded subblock gives rise to at least

Q _\ =2 transitions after 1/(1 ©D ² ) precoding, is

where all of the additions in the above equations are integer additions y and [y[ y ₂ y ₃ y _A ' y ₅ y ₆ y _η ' y _s '] is the 1/(1 © D) precoded first subblock.

The second set of requirements, which ensure that the second coded subblock gives rise to at least transitions after 1/(1 ©D ² ) precoding, is:

where all of the additions in the above equations are integer additions

J ₉ = U ₂ © J ₉ and is the 1/(1 © D) precoded second subblock.

The third encoder stage 206 remaps undesired 17-bit outputs of the second encoder stage 204 c into desired 17-bit outputs y. Specifically, the third encoder stage 206 detects prohibited patterns and replaces them with substitute patterns to ensure that each of the subblocks (FIG. 3C) of the 17-bit output;; of the third encoder stage 206 gives rise to at least two transitions in the recorded pattern subsequent to 1/(1 ©D ² ) precoding. The first subblock of the 17-bit output of the third encoder stage 206 is J ₁ y ₂ y ₃ y ₄ y ₅ y ₆ y ₇ J ₈ and the second subblock is y _g y _w y _n y _l2 y _l3 J ₁₄ J ₁₅ J ₁₆ J ₁₇ . TABLE II shows the prohibited patterns and the corresponding substitute patterns in the third (last) encoding stage.

TABLE II

The bold pattern within the prohibited pattern c is used to detect violations during the third stage of encoding whereas the bold pattern in the substitute pattern y or z (if the presence of channel errors is considered, then j>, the output of the third encoder stage 206, becomes z, the input of the first decoder stage 212) is used to detect required inverse substitution during the first stage of decoding. If none of the two prohibited patterns is detected during the third encoder stage 206, the output of the third encoder stage 206 is equal to its input, i.e., y=c. Therefore, there are three partitions (two substitutions plus absence of substitution) in the third encoder stage 206.

The constraints satisfied by of = y = [y _{ y ₂ ... J ₁₇ ]' following the third encoder stage 206 are G constraints:

/ constraints:

M constraints:

Minimum transition density constraints:

The code obtained at the output of the third (and last, when k=l, 2, 3) encoding stage is a PRML(G=6, I=I, M= 15) code that enforces at least transitions per codeword. The second and third encoder stages 204, 206 may optionally be combined into a single remapping stage of undesired patterns into desired patterns where remapping occurs in a manner allowing the inverse operation during decoding.

DECODING

FIG. 2B is a block diagram of the decoder 210. The decoder 210 receives rate- 16/17 PRML(G=6, I=I, M= 15) encoded data z(l :17) and, using k encoding stages, outputs unencoded user data f(l : 16). The example of the decoder 210 which is illustrated and described herein includes K = 3 decoding stages 212, 214, 216, although more or fewer

stages may be used. The three stages 212, 214, 216, provide the inverse of the operations performed at the encoding stages: the first decoder stage 212 inverts the third encoder stage 206, the second decoder stage 214 inverts the second encoder stage 204 and the third decoder stage 216 inverts the first encoder stage 202. The output of the k-th decoding stage of (binary column vector) may be described by the multiplication of a binary transformation matrix M° = g°(i°) and a binary partitioning vector n° = h°(i°) (binary column vector) that depend on the binary input if (binary column vector) where the superscript D refers to the decoding operation. Finally, concatenation of decoding stages is done such that the output of the k-th decoding stage, A=I, 2 (in general, k=\, ..., K-

1) is the input of the next decoding stage, i.e., if _+l = of .

As will be shown in the section on implementing the rate- 16/ 17 encoder and decoder, the encoder and decoder may be implemented with few two-input gates. All of the Boolean equations for encoding and decoding are the result of multiplication of a specific matrix and a partitioning column vector that are specified in the next two sections.

MATRIX- VECTOR DESCRIPTION OF ENCODING STAGES FOR RATE- 16/17 PRML(G=6, I=I, M= 15) CODE WITH MINIMUM TRANSITION DENSITY

In this section, the encoding of 16 bits to 17 bits, that is the generation of codewords of the mother code, is fully described using a matrix- vector notation. A compact representation of all three encoding stages 202, 204, 206 is presented. Sixteen input bits α(l) ... α(16) are input to the first encoder stage 202, then processed through each of the stages described below to produce the 17-bit mother code output bits y{\) ... X 17) at the third level encoder output. The final step to creating the 33 -bit or 49-bit codeword is interleaving the 17-bit encoder output with either 16 or 32 uncoded bits. This final step is briefly described in the last three sections.

Encoding stage 1 :

Input column vector: z ^' f = a ^τ = [a _γ a ₂ ... a _l6 ] ^τ

Transformation matrix: Mf = gf (if) = [Mf ₁ Mf ₂ ]

Partitioning column vector: nf = hf (if) = p ^τ = [p _γ p ₂ ... p ₃₂ \ ^τ where the following six auxiliary variables:

are used to express the components of the partitioning vector:

Output column vector:

Encoding stage 2:

Input column vector:

Transformation matrix:

Partitioning column vector: where

Output column vector:

Encoding stage 3 :

Input column vector: if = c ^τ =[q c ₂ ... c _l7 ] ^τ Transformation matrix: Mf = gf(if)

Partitioning column vector: n where

Output column vector:

MATRIX- VECTOR DESCRIPTION OF DECODING STAGES FOR RATE- 16/17 PRML(G=6, 1=7, M= 15) CODE WITH MINIMUM TRANSITION DENSITY

In this section, decoding 17-bits to 16-bits is described using matrix- vector notation. A compact representation of all three decoding stages 212, 214, 216 is presented. Seventeen input bits z(l) ... z(17) are input to the first decoder stage 212, then processed through each of the stages described below to produce the 16-bit output bits/(l) .../(16) at the output/of the third decoder stage 216. The final step to creating the 32-bit or 48-bit dataword is interleaving the 16-bit output of the decoder 210 with either 16 or 32 uncoded bits. This final step is briefly described in the last three sections.

Decoding stage 1 :

Input column vector:

Transformation matrix:

Partitioning column vector: where

W ₃ W ₁ + W ₂

Output column vector:

Decoding stage 2:

Input column vector:

Transformation matrix:

Partitioning column vector: where

Output column vector:

Decoding stage 3 :

Input column vector:

Transformation matrix:

Partitioning column vector: where

Output column vector:

BOOLEAN LOGIC FOR RATE-16/17 PRML(G=6, I=7, M=15) ENCODER

The encoding of 16-bits to 17-bits to generate the mother code in accordance with the present invention is completely described by the following MATLAB Boolean operations. Sixteen inputs bits a(l) ... a(16) are input to a first encoder stage 202, then processed through each of the following stages described below to produce the 17-bit mother code output bits y(l) ... y(17) at the third level encoder output. The final step to creating the 33 -bit or 49-bit codeword is interleaving the 17-bit encoder output with either 16 or 32 uncoded bits and is briefly described in the last three sections. The following Boolean equations describe the first encoder stage 202 with 16-bit input a and 17-bit output b.

Auxiliary Boolean variables: vl=~(a(l)|a(2)|a(3)|a(4)) v2=~(a(5)|a(6)|a(7)|a(8)) v3=~(a(9)|a(10)|a(l l)|a(12)) v4=~a(9)&a(10)&~a(l l)&~a(12)

First-stage partitions:

p(29)=vl&v2&v3&~v4&~v5&v6 p(30)=vl&v2&~v3&v4&~v5&v6 p(3 I)=vl&v2&~v3&~v4&v5&v6 p(32)=~v 1 &~v2&~v3 &~v4&~v5 &~v6

Auxiliary Boolean variables: v7=p(l)|p(7) v8=p(2)|p(3)|p(4)|p(5)|p(6)|p(8)|p(13)|p(14)|p(15)|p(16)|p(1 7)|p(18)|p(32) v9=p(9)|p(10)|p(l l)|p(12) vlθ=p(l)|p(3)|p(4)|p(5)|p(6)|p(32) vl l=p(14)|p(15)|p(16)|p(17)|p(18) vl2=p(19)|p(20)|p(21)|p(22)|p(23)|p(24)|p(25)|p(26)|p(27)|p( 28)|p(29)|p(30)|p(31) vl3=p(10)|p(l l)|p(12)|p(13) vl4=p(7)|p(8)|v9|p(13)|vl l|vl2 vl5=p(l)|p(2)|p(3)|p(4)|p(5)|p(7)|p(10)|p(l l)|p(12)|p(13)|p(14)|p(15)|p(20)|p(21)|p(22)|p(32) vl6=p(6)|p(8)|p(9)|p(19) vl7=p(16)|p(17)|p(18)|p(23)|p(24)|p(25) vl8=p(26)|p(27)|p(28) vl9=p(29)|p(30)|p(31)

First-stage encoding output: b(l) =a(9)&v7|a(l)&v8|a(5)&v9|p(26)|p(27)|p(28)|p(29) |p(30)|p(31) b(2) = _a (10)&v7|a(2)&v8|a(6)&v9|p(22)|p(23)|p(24)|p(25)| p(30)|p(31) b(3) =a(l l)&v7|a(3)&v8|a(7)&v9|p(20)|p(21)|p(24)|p(25)|p( 28)|p(29) b(4) =a(12)&v7|a(4)&v8|a(8)&v9|p(19)|p(21)|p(23)|p(25 )|p(27)|p(29)|p(31) b(5) =a(5)&vlθ|a(9)&p(2)|vl l|vl2 b(6) =a(6)&vl0|a(10)&p(2)|vl3|p(18)|vl2 b(7) =a(7)&vlθ|a(l l)&p(2)|p(8)|p(9)|p(12)|p(13)|p(16)|p(17) b(8) =a(8)&vlθ|a(12)&p(2)|p(7)|p(9)|p(l l)|p(13)|p(15)|p(17)|vl2 b(9) =a(9)&p(32)|p(4)|p(5)|p(6)|vl4 b(10)=a(10)&p(32)|p(l)|p(2)|p(3)|p(4)|p(5)|p(6)|vl4 b(l l)=a(l I)&p(32)|p(2)|p(3)|p(6)|vl4

b(12)=p(32) b(13)=a(12)&p(32)|p(l)|p(3)|p(5)|vl4 b(14)= _a (13)&vl5|a(9)&vl6|a(5)&vl7|a(l)&vl8 b(15)=a(14)&vl5|a(10)&vl6|a(6)&vl7|a(2)&vl8 b(16)=a(15)&vl5|a(l l)&vl6|a(7)&vl7|a(3)&vl8|vl9 b(17)=a(16)&vl5|a(12)&vl6|a(8)&vl7|a(4)&vl8| vl9

The following Boolean equations describe the second encoder stage 204 with 17-bit input b and 17-bit output c.

Second-stage partitions: q(l)=~(b(l)|b(3)|b(5)|b(7)) q(2)=~(b(2)|b(4)|b(6)|b(8)) q(3)=b(l)&b(2)&b(3)&b(4)&b(5)&b(6)&b (7)&b(8) q(4)=~(q(l)|q(2)|q(3))

Second-stage encoder remapping output: c(l) =b(2)&q(l)|b(14)&q(2)|b(14)&q(3)|b(l)&q(4) c(2) =q(2)|b(15)&q(3)|b(2)&q(4) c(3) =b(4)&q(l)|b(9)&q(2)|q(3)|b(3)&q(4) _c (4) =b(14)&q(l)|b(15)&q(2)|b(9)&q(3)|b(4)&q(4) c(5) =b(6)&q(l)|b(l l)&q(2)|b(10)&q(3)|b(5)&q(4) c(6) =b(10)&q(l)|b(10)&q(2)|b(l l)&q(3)|b(6)&q(4) c(7) =b(8)&q(l)|b(13)&q(2)|b(12)&q(3)|b(7)&q(4) c(8) =b(12)&q(l)|b(12)&q(2)|b(13)&q(3)|b(8)&q(4) c(9) =b(9)&q(l)|b(l)&q(2)|q(3)|b(9)&q(4) c(10)=q(3)|b(10)&q(4) c(l l)=b(l l)&q(l)|b(3)&q(2)|q(3)|b(l l)&q(4) c(12)=q(3)|b(12)&q(4) c(13)=b(13)&q(l)|b(5)&q(2)|q(3)|b(13)&q(4) c(14)=q(l)|q(2)|b(14)&q(4) c(15)=b(15)&q(l)|b(7)&q(2)|q(3)|b(15)&q(4)

c(16)=b(16)&q(l)|b(16)&q(2)|b(16)&q(3)|b(16)& amp;q(4) c(17)=b(17)&q(l)|b(17)&q(2)|b(17)&q(3)|b(17)& ;q(4)

The following Boolean equations describe the third encoder stage 206 with 17-bit input c and 17-bit output y.

Third- stage partitions: u( 1 )=~c(l )&~c(2)&~c(3)&c(4)&c(5)&~c(6)&~c(7)& amp;~c(8) u(2)=~c(9)&~c( 10)&~c( 11 )&~c( 12)&c( 13)&c( 14)&~c( 15)&~c( 16)&~c( 17) u(3)=~(u(l)|u(2))

Auxiliary Boolean variables: v20=u(2)|u(3) v21=u(l)|u(3)

Third- stage encoder remapping output (encoder output): y(l)= c(10)&u(l)|c(l)&v20 y(2)= c(12)&u(l)|c(2)&v20 y(3)= u(l)|c(3)&v20 y(4)= u(l)|c(4)&v20 y(5)= c(5)&v20 y(6)= c(6)&v20 y(7)= u(2)|c(7)&u(3) y(8)= u(2)|c(8)&u(3) y(9)= c(9)&v21 y(10)=u(l)|c(7)&u(2)|c(10)&u(3) y(l l)=c(l l)&v21 y(12)=c(8)&u(2)|c(12)&u(3) y(13)=c(13)&v21 y(14)=c(14)&v21|u(2) y(15)=c(15)&v21|u(2) y(16)=c(16)&v21

y(17)=c(17)&v21|u(2)

BOOLEAN LOGIC FOR RATE-16/17 PRML(G=6, I=I, M=\5) DECODER

The following Boolean equations describe the first decoder stage 212 with 17-bit input z and

17-bit output d. The first decoder stage 212 inverts the third encoder stage 206.

First-stage decoder partitions: w( 1 )=z(3)&z(4)&~z(5)&~z(6)&~z(7)&~z(8)& z( 10)&~z( 12) w(2)=z(7)&z(8)&~z(9)&~z(l l)&~z(13)&z(14)&z(15)&~z(16)&z(17) w(3)=~(w(l)|w(2))

Auxiliary Boolean variables: tl=w(2)|w(3) t2=w(l)|w(3)

First-stage decoder remapping output: d(l)= z(l)&tl d(2)= z(2)&tl d(3)= z(3)&tl d(4)= w(l)|z(4)&tl d(5)= w(l)|z(5)&tl d(6)= z(6)&tl d(7)= z(10)&w(2)|z(7)&w(3) d(8)= z(12)&w(2)|z(8)&w(3) d(9)= z(9)&t2 d( 10)=z( 1 )&w( l)|z(l 0)&w(3) d(l l)=z(l I)&t2 d( 12)=z(2)&w( l)|z(l 2)&w(3) d(13)=z(13)&t2|w(2) d(14)=z(14)&t2|w(2) d(15)=z(15)&t2

d(16)=z(16)&t2 d(17)=z(17)&t2

The following Boolean equations describe the second decoder stage 214 with 17-bit input d and 17-bit output e. The second decoder stage 214 inverts the second encoder stage 204.

Second-stage decoder partitions: r( 1 )=~d(2)&~d( 10)&~d( 12)&d( 14) r(2)=d(2)&~d( 10)&~d( 12)&d( 14) r(3)=d(3)&d(9)&d(10)&d(l l)&d(12)&d(13)&~d(14)&d(15) r(4)=~(r(l)|r(2)|r(3))

Auxiliary Boolean variables: t3=r(l)|r(4) t4=r(l)|r(2)

Second-stage decoder remapping output: e(l)= d(9)&r(2)|r(3)|d(l)&r(4) e(2)= d(l)&r(l)|r(3)|d(2)&r(4) e(3)= d(l l)&r(2)|r(3)|d(3)&r(4) e(4)= d(3)&r(l)|r(3)|d(4)&r(4) e(5)= d(13)&r(2)|r(3)|d(5)&r(4) e(6)= d(5)&r(l)|r(3)|d(6)&r(4) e(7)= d(15)&r(2)|r(3)|d(7)&r(4) e(8)= d(7)&r(l)|r(3)|d(8)&r(4) e(9)= d(9)&t3|d(3)&r(2)|d(4)&r(3) e( 10)=d(6)&t41 d(5 )&r(3 ) | d( 10) &r(4) e(l l)=d(l I)&t3|d(5)&r(2)|d(6)&r(3) e(12)=d(8)&t4|d(7)&r(3)|d(12)&r(4) e(13)=d(13)&t3|d(7)&r(2)|d(8)&r(3) e(14)=d(4)&r(l)|d(l)&(r(2)|r(3))|d(14)&r(4) e(15)=d(15)&t3|d(4)&r(2)|d(2)&r(3)

e(16)=d(16) e(17)=d(17)

The following Boolean equations describe the third (last) decoding stage with 17-bit input e and 16-bit output/ The third decoder stage 216 inverts the first encoder stage 202. In the absence of channel errors, the 16-bit encoder input is equal to the 16-bit decoder output, i.e., a=f.

Auxiliary Boolean variables: t5=e(9)&e(10)&e(l l)&~e(12)&e(13) t6=e(5)&e(6)&~e(7)&e(8)&t5

Third- stage decoder partitions: s( 1 )=~e(9)&e( 10)&~e( 11 )&~e( 12)&e( 13) s(2)=~e(9)&e(10)&e(l l)&~e(12)&~e(13) s(3)=~e(9)&e( 10)&e(l 1 )&~e( 12)&e( 13) s(4)=e(9)&e( 10)&~e( 11 )&~e( 12)&~e( 13) s(5)=e(9)&e( 10)&~e(l 1 )&~e( 12)&e( 13) s(6)=e(9)&e( 10)&e( 11 )&~e( 12)&~e( 13) s(7)=~e(5)&~e(6)&~e(7)&e(8)&t5 s(8)=~e(5)&~e(6)&e(7)&~e(8)&t5 s(9)=~e(5)&~e(6)&e(7)&e(8)&t5 s( 10)=~e(5)&e(6)&~e(7)&~e(8)&t5 s(l I)=~e(5)&e(6)&~e(7)&e(8)&t5 s( 12)=~e(5)&e(6)&e(7)&~e(8)&t5 s( 13)=~e(5)&e(6)&e(7)&e(8)&t5 s( 14)=e(5)&~e(6)&~e(7)&~e(8)&t5 s( 15)=e(5)&~e(6)&~e(7)&e(8)&t5 s( 16)=e(5)&~e(6)&e(7)&~e(8)&t5 s(17)=e(5)&~e(6)&e(7)&e(8)&t5 s( 18)=e(5)&e(6)&~e(7)&~e(8)&t5 s( 19)=~e( 1 )&~e(2)&~e(3)&e(4)&t6

s(20)=~e( 1 )&~e(2)&e(3)&~e(4)&t6 s(21 )=~e( 1 )&~e(2)&e(3)&e(4)&t6 s(22)=~e( 1 )&e(2)&~e(3)&~e(4)&t6 s(23)=~e( 1 )&e(2)&~e(3)&e(4)&t6 s(24)=~e(l)&e(2)&e(3)&~e(4)&t6 s(25)=~e( 1 )&e(2)&e(3)&e(4)&t6 s(26)=e( 1 )&~e(2)&~e(3)&~e(4)&t6 s(27)=e( 1 )&~e(2)&~e(3)&e(4)&t6 s(28)=e( 1 )&~e(2)&e(3)&~e(4)&t6 s(29)=e(l)&~e(2)&e(3)&e(4)&t6 s(30)=e( 1 )&e(2)&~e(3)&~e(4)&t6 s(31 )=e( 1 )&e(2)&~e(3)&e(4)&t6

Auxiliary Boolean variables: t7= s(3)|s(4)|s(5) t8= s(6)|t7 t9= s(10)|s(l l)|s(12) tlθ=s(13)|s(14)|s(15) tl l=s(16)|s(17)|s(18) tl2=s(20)|s(21)|s(22) tl3=s(23)|s(24)|s(25) tl4=s(26)|s(27)|s(28) tl5=s(l)|s(7)

Last third-stage decoder partition: s(32)=~(tl5|s(2)|t8|s(8)|s(9)|t9|tl0|tl l|s(19)|tl2|tl3|tl4|s(29)|s(30)|s(31))

Auxiliary Boolean variables: tl6=s(2)|t8|s(8)|tlθ|tl l|s(32) tl7=s(l)|t8|s(32) tl8=s(9)|t9 tl9=tl l|tl3

t20=s(6)|s(8)|s(9)|s(19)

121=8(5)18(12)18(15)18(18)18(22)18(25)18(28)18(31) t22=tl5|s(2)|t7|t9|tlθ|tl2|s(32)

Third- stage decoding output (decoder output): f(l) =e(l)&tl6|e(14)&tl4 f(2) =e(2)&tl6|e(15)&tl4 f(3) =e(3)&tl6|e(16)&tl4 f(4) =e(4)&tl6|e(17)&tl4 f(5) =e(5)&tl7|e(l)&tl8|e(14)&tl9 f(6) =e(6)&tl7|e(2)&tl8|e(15)&tl9 f(7) =e(7)&tl7|e(3)&tl8|e(16)&tl9 f(8) =e(8)&tl7|e(4)&tl8|e(17)&tl9 f(9) =e(l)&tl5|e(5)&s(2)|t21|e(14)&t20|e(9)&s(32) f(10)=e(2)&tl5|e(6)&s(2)|t21|e(15)&t20|e(10)& ;s(32)|s(4)|s(l l)|s(14)|s(17)|s(21)|s(24)|s(27)|s

(30) f(l l)=e(3)&tl5|e(7)&s(2)|t21|e(16)&t20|e(l l)&s(32) f(12)= _e (4)&tl5|e(8)&s(2)|t21|e(17)&t20|e(13)&s(32) f(13)=e(14)&t22 f(14)=e(15)&t22 f(15)=e(16)&t22 f(16)=e(17)&t22

The encoder and the decoder may be implemented with a total of 936 two-input AND and OR gates (559 two-input AND gates and 377 two-input OR gates).

RATE-32/33 PRML(G= 14, 7=11, M=23) CODE (RLL 1)

In one embodiment of a rate-32/33 PRML code, m=32, n=\6, 5=8, Qι=2, P ₁ =A, P ₂ =I and mother code with minimum transition density Q ₂ =4 may be used to obtain a rate-32/33 PRML(G=14, 7=11, M=23) code by breaking the codewords into pieces (8 bits and 9 bits) and inserting (m-n)/s=2 uncoded

bytes in between (a coded block of 8 bits or 9 bits is always preceded and followed by an uncoded byte), m and n are both integer multiples of the ECC symbol size s. Coded data is always precoded using a 1/(1 ©D ² ) precoder. However, uncoded data can be either not precoded at all, 1/(1 © D) precoded or 1/(1 ©D ² ) precoded. From an error-rate performance viewpoint, it is preferable to not use any precoding for the uncoded data. However, because

1/(1 ©D) precoding is already used in conjunction with write equalization in LTO 1-4, it may be therefore more desirable to use 1/(1 ©D) precoding.

FIG. 4 illustrates the process by which a 32-bit input, comprising four s = 8-bit error correction coding (ECC) symbols BO(1 :8), Bl(1 :8), B2(l :8), B3(l :8), is transformed into a

33-bit rate-32/33 codeword CO(1 :8), Bl(1 :8), C2(l :9), B3(l:8) for the 32/33 code by multiplexing coded and uncoded bits. The 32-bit input is separated by a demultiplexer 400 into two sets of two 8-bit symbols BO(1 :8), Bl(1 :8) and B2(l :8), B3(l :8). 16 bits in one of the sets, BO(1 :8) and Bl(1 :8) for example, are encoded by the rate-16/17 mother code encoder 402 which outputs 17 encoded bits CO(1 :8), C2(l :9). The 16 bits in the other set,

B2(l :8), B3(l :8), remain unencoded. A multiplexer 404 multiplexes the four 8-bit symbols into a 33-bit codeword CO(1 :8), Bl(1 :8), C2(l :9), B3(l :8). Although not illustrated in the Figures, a 48-bit input is transformed into a 49-bit codeword by a comparable process.

The rate-32/33 PRML(G=14, /=11 , M=23) code rules out both DSS and Re Sync and has an error propagation of 8 NRZ bits assuming 4-way Cl interleaving. FIG. 5 A illustrates a 4- way interleaving (multiplexing) mechanism of 8-bit symbols at the output of four Cl Reed- Solomon encoders which may be implemented for the rate-32/33 PRML(G= 14, I=W, M=IV) code of the present invention. FIG. 5B is a block diagram of such an interleaving scheme. Four Cl encoders 500A, 500B, 500C, 500D process separate inputs and their outputs input into a multiplexer 502. The least and most significant bits of a sequence of bits 502 is also input into the multiplexer 502. The output of the multiplexer 502 BO, Bl, B2, B3 provides a 32-bit input to a rate-32/33 RLL encoder 504. These four s = 8-bit symbols are encoded by the encoder 504 (in the manner illustrated in FIG. 4) and the 4-way interleaved 33-bit codeword, CO, Bl, C2, B3, is output. Although not illustrated in the Figures, the block diagram illustrating an interleaving mechanism for the rate-48/49 code would include comparable functional blocks.

RATE-48/49 PRML(G=22, 7=15, M=31) CODE (RLL 2)

In an embodiment of a rate-48/49 PRML code, m=48, n=16, 5=8, Qι=2, P _x =A, P ₁ =I and 15) mother code may be used to obtain a rate- 48/49 PRML(G=22, 7=15, M=31) code by breaking the codewords into i ^> ₃ =2 pieces (8 bits and 9 bits) and inserting (m-n)/s=4 uncoded bytes in between (a coded block of 8 bits or 9 bits is always preceded and followed by two uncoded bytes). Coded data is precoded using a 1/(1 ©7) ² ) precoder and uncoded data may either be not precoded at all, 1/(1 ©7)) precoded or 1/(1 ©T) ² ) precoded. From an error-rate performance viewpoint, it may be preferable to not use any precoding for the uncoded data. However, because 1/(1 ©7)) precoding is already used in conjunction with write equalization in LTO 1-4, it may be therefore more desirable to use 1/(1 ©7)) precoding. FIG. 6 illustrates a 4-way Cl interleaving scheme which may be implemented for the rate-48/49 PRML(G=22, 7=15, M=31) code.

RATE-48/49 PRML(G=14, 7=19, M=39) CODE (RLL 3)

In another embodiment of a rate-48/49 PRML code, P ₁ =I and P ₃ =A. A rate-48/49 PRML(G=14, 7=19, M=39) code may be obtained by breaking the codewords of the rate-16/17 PRML(G=6, 7=7, M=I 5) code into i ^> ₃ =4 coded blocks (4 bits + 4 bits + 5 bits + 4 bits) and inserting (m-n)/s=4 uncoded bytes in between (a coded block of 4 bits or 5 bits is always preceded and followed by an uncoded byte). However, this code may require redefining DSS and Re Sync patterns to ensure that the maximum length of modulation-encoded data that looks like DSS or Re Sync is not prohibitively large. FIG. 7 illustrates a 4-way Cl interleaving scheme which may be implemented for the rate-48/49 PRML(G=14, 7=19, M=39) code.

It is important to note that while the present invention has been described in the context of a fully functioning data processing system, those of ordinary skill in the art will appreciate that the processes of the present invention are capable of being distributed in the form of a computer readable medium of instructions and a variety of forms and that the present invention applies regardless of the particular type of signal bearing media actually used to carry out the distribution. Examples of computer readable media include recordable-type

media such as a floppy disk, a hard disk drive, a RAM, and CD-ROMs and transmission- type media.

The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. Moreover, although described above with respect to methods and systems, the need in the art may also be met with a computer program product containing instructions for encoding a data input sequence of m bits into an output sequence codeword of m+1 bits or a method for deploying computing infrastructure comprising integrating computer readable code into a computing system for encoding a data input sequence of m bits into an output sequence codeword of m+1 bits.