Method and Means for Correcting Random and Burst Errors

1. Technical Field

This invention relates to a method and means for locating and correcting both random and burst errors in data recorded on a storage medium in blocks comprising subblocks, and more specifically relates to such a method and means which includes error correction code (ECC) for which decoded and estimated block check syndromes are generated.

2. Background of the Invention

In U.S. Pat. No. 4,706,250 assigned to the assignee of the present invention, there is disclosed a two-level error correction code structure in an improved multibyte error-correcting subsystem. Data is formatted on a disk track into a number of subblocks, each within a respective block. There are also two sets of three subblock check bytes. One set is associated with the even phases and the other with the odd phase, this being due to two-phase interleaving.

With this arrangement, it is possible to correct one error in each subblock at a first (subblock) level of correction, and correct two errors in one of the subblocks of the block at a second (block) level of correction. Thus, if there are two or more errors in a subblock, due for example to an error burst, the ECC will be unable to correct all the errors. To partially address this problem, the size of the subblocks could be reduced from say 100 bytes to 50 bytes to reduce the likelihood that more than one error would occur in each subblock. But, associated with each subblock (as shown in FIG. 1 of U.S. Pat. No. 4,706,250) is a set of subblock check bytes which, as illustrated, are two-way interleaved. Hence doubling the number of subblocks doubles the number of subblock check bytes, thus increasing the overhead.

In the copending application of Eggenberger et. al., U.S. Ser. No. 247,461, filed Sept. 21, 1988, assigned to the assignee of the present invention, a method and means are disclosed capable of correcting both random conventional errors and a long burst error of up to N consecutive bytes in a two-level ECC system. This is achieved by adding at the end of each block a series of N sequential parity check bytes. During reading, after the generation of the ECC syndromes, parity syndromes are generated by comparing parity check bytes computed from the data bytes and check bytes as read with the parity check bytes as written. Each syndrome is a byte in length. In response to a long-burst error, a pointer is generated to the first of N consecutive bytes in a block that could have been influenced by the error. Then those ECC syndromes not dependent on the N bytes identified by the pointer are used to correct any correctable errors. Following this, the parity syndromes are adjusted according to the errors thus corrected. The adjusted parity syndromes are then used to correct correctable errors in the N bytes indicated by the pointer. Any unused ECC syndromes are adjusted according to the errors thus corrected by the adjusted parity syndromes; whereupon the adjusted ECC syndromes are used to correct all correctable errors then remaining. This method and means operates satisfactorily but requires that N parity bytes be added at the end of each block in order to correct an error burst N bytes long, and requires a pointer to each error burst.

It is desirable to provide an improved two-level ECC system of the above general type capable of locating any preselected number of subblocks in error containing up to a preselected number of bytes in error, whether nonconsecutive or in bursts. Also, it would be desirable to provide such a system wherein the location of the subblocks in error may be specified either by an externally generated pointer or the location may be unknown, without reducing first (subblock) level correction capability.

Toward this end, and according to the invention, a method and means are provided for correcting multiple burst errors in a storage medium on which data is recorded in blocks that comprise subblocks of data bytes and redundant check bytes for which decoded and estimated block syndromes are generated during reading. After reading the data, the decoded block syndromes for each block are algebraically summed with the estimated block syndromes for that block to provide a set of syndromes for a code which locates subblocks in that block having error bursts. This set of syndromes is then decoded to generate algebraic pointer(s) to identify each subblock having an error burst. Thereafter block syndromes for each subblock having an error burst are computed at 33 and used in conjunction with previously computed subblock syndromes from 20 for those subblocks and both sets of syndromes are decoded to identify the locations and the values of the errors within those subblocks having error bursts.

FIG. 1 is a schematic block diagram of the encoder portion of an apparatus embodying the invention;

FIG. 2 is a schematic block diagram showing in more detail portions of the apparatus of FIG. 1 which computes block check bytes; and

FIGS. 3A-3D, when taken such that the right edge of FIG. 3A is matched to the left edge of FIG. 3B, the right edge of FIG. 3B to the left edge of FIG. 3C, and the right edge of FIG. 3C matched to the left edge of FIG. 3D constitute a block diagram of the decoder portion of the apparatus embodying the invention.

The method and apparatus embodying the invention are shown applied to a two-level ECC system of the type disclosed in the above-cited U.S. Pat. No. 4,706,250. As shown in FIG. 1 of said patent, data is recorded in blocks of variable (or fixed) length each comprising a plurality of subblocks, of the same length except possibly the last. Each subblock comprises a variable (or fixed) number of data bytes and six subblock check bytes for first level (subblock) error correction. At the end of the last subblock in the block are four CRC bytes for data integrity checking, followed by two block check bytes for second level (block) error correction.

In this two-level ECC system, the data is encoded on the subblock (first) level by a Reed-Solomon (RS) [n, n-r.sub.1, r.sub.1 +1]-encoder, where n is the number of bytes in the encoded subblock, r.sub.1 is the number of subblock check bytes in the encoded subblock, and r.sub.1 +1 is the minimum Hamming distance or the separation achieved among subblocks. This distance suffices for the correction of up to t.sub.1 = r.sub.1 /2 byte errors in a subblock. The fundamental feature of this prior art two-level architecture is to compute r.sub.2 block level syndromes for a second [n, n-r.sub.2, r.sub.2 +1]-RS code. These syndromes are computed for each subblock in a block and added cumulatively to produce r.sub.2 block check bytes which are shared by all subblocks. These block check bytes are stored after the last subblock in a block.

If the location of a subblock with an error burst is specified by an externally generated pointer and the number of errors in that subblock does not exceed r.sub.1 /2, , then the errors can be corrected by a subblock leve decoder. If the number of errors does not exceed (r.sub.1 +r.sub.2)/2 , then the errors may be corrected by combining the correction power of the subblock and block level check bytes.

If the location of the subblock with an error burst is unknown, however, this prior art two-level ECC system can locate the subblock with the error burst and correct the error burst provided the number of errors (i.e. its weight) does not exceed r.sub.1 /2 +c. This burst location and correction is achieved at the cost of lowering the maximal subblock error correction capability in all other subblocks from r.sub.1 /2 -c where c is a fixed design parameter which is a positive integer such that

It should be noted that, in FIGS. 1-3, a diagonal slash across an input or output signifies that it consists of the specified number of bytes in parallel. Also, as hereinafter used, the term "error burst" is defined as any number of bytes with errors, whether or not the errors are consecutive, where this number exceed the first level (i.e., subblock) correction capability of the apparatus.

According to the invention, there are four parameters that establish the error correction capability of the two-level ECC apparatus. These are r.sub.1, r.sub.2, r.sub.3 and B where:

r.sub.1 is the number of subblock (first level) check bytes per subblock;

r.sub.2 is the number of block (second level) check bytes per block;

r.sub.3 is the number of the check bytes for the block check bytes, the number being sufficient so that all of the bytes in a block are equally protected; and

B.gtoreq.2T+S, where T and S denote the numbers of correctable subblocks with error bursts without and with, respectively, externally generated pointers to the correctable subblocks with error bursts. Thus, if B=4, correction can be achieved if T=0, with S.ltoreq.4; if T=1, with S.ltoreq.2; and with T=2, with S=0. The maximal number of errors in a subblock that can be corrected is ##EQU1##

As illustrated in FIG. 1, an encoder 10 encodes (n-r.sub.1) bytes of data. The encoded n bytes of data are stored in a data field on a disk track 9. The encoded data is also supplied to syndrome computation circuitry 11 which generates r.sub.2 block syndromes for each of the subblocks of the block. As earlier noted, each syndrome is one byte in length. These r.sub.2 syndromes are supplied to block check computation circuitry 12 which, as shown in FIG. 2, comprises circuitry 13 and 14.

Circuitry 13 comprises means for multiplying the r.sub.2 block syndromes for each subblock by a series of preselected cumulative weighting factors {.alpha..sup.l, . . . , .alpha..sup.l(B-1) } according to the location index 1 of the subblock within the block, where .alpha. is the primitive generator of the Galois finite field.

The different factor-weighted syndromes from each of the B buffers 15 are cumulatively summed by circuitry 14 for all the subblocks within the block to generate a total of r.sub.2 B block syndromes, which are reordered in a preselected fashion as shown at 16. After the last subblock of the block is written, encoder 17 (FIG. 1) encodes the r.sub.2 B block check syndromes and appends to them r.sub.3 additional check bytes. The r.sub.2 B block check syndromes plus the r.sub.3 additional check bytes are then stored at the end of the block on disk track 9 following the last subblock, N, in the block.

During reading of the data, and as illustrated in FIG. 3A, the n data bytes as read from each subblock of a block are stored in a buffer 18 and also concurrently fed to subblock syndrome computation circuitry 19. Circuitry 19 computes, for each subblock, subblock syndromes that are stored in a buffer 20. Those subblock syndromes that do not correspond to a subblock with an error burst are passed through a subblock data traffic controller 21 to a decoder 22. Externally generated pointers to subblocks with error bursts, if available, are also supplied to controller 21, stored in a location register 42. Decoder 22 produces in bus 23 an estimated error pattern for each of the subblocks without error bursts. These estimated error patterns in bus 23 are algebraically summed at 24 with the subblock data stored in buffer 18 to correct all correctable errors in these subblocks without error bursts and thereby provide in bus 25 corrected subblock data for those subblocks without error bursts.

Block level syndromes are then computed for each subblock from the estimated error pattern (in 23) by circuitry 26 (FIG. 3B) and from the corrected subblock data (in 25) by circuitry 27. Using block check byte computation circuitry 28 identical with that described for circuitry 12, the block level syndromes for the corrected subblock data are multiplied by the aforementioned series of preselected cumulative weighting factors according to their order of occurrence within the block and each block level syndrome as multiplied by a different weighting factor is stored in a respective one of the B buffers 15; and then these factor-weighted syndromes from the B buffers 15 are cumulatively summed to produce estimated block check syndromes covering all the subblocks of that block.

Decoder 30 (FIG. 3C) decodes the encoded block check syndromes generated during writing for all subblocks of each block. Each of these decoded block check syndromes is then algebraically summed with its corresponding estimated block check syndrome by a respective summing circuit 31 to provide a set of syndromes for a code for locating those subblocks having error bursts. Decoder 32 then decodes this set of syndromes to produce up to T algebraic pointers to the subblocks with error bursts. Concurrently, for each subblock with an error burst, decoder 32 produces a set of r.sub.2 block level syndromes. Due to the effect of miscorrection at the subblock level, these r.sub.2 block level syndromes must be updated. This is done by algebraically summing at 33 the block level syndromes computed by decoder 32 with the block level syndromes computed by circuitry 26 from the estimated error pattern syndromes to produce block level syndromes in bus 34 for the true error pattern for those subblocks having error burst.

Subblock data traffic controller 35 has as inputs: n representing the data bytes as read; S representing updated externally generated pointers from updated pointer set 42 to subblocks with error bursts; T representing algebraic pointers obtained from decoder 32 to subblocks with error bursts; the estimated crror pattern from bus 23 that had been stored in a buffer 36 (FIG. 3B), and the block level syndromes for the true error pattern from bus 34.

For those subblocks without an error burst, controller 35 directs the n data bytes as read and the buffered 2r.sub.1 bytes for the estimated error pattern from buffer 36 to an algebraic summing circuit 37 to correct all subblocks without error bursts.

For those subblocks with error bursts, controller 35 directs to decoder 39 r.sub.2 block syndromes for the true error pattern from bus 34 and also the r.sub.1 subblock syndromes from buffer 20 for only those subblocks with error bursts. From these inputs decoder 39 produces an error pattern that is algebraically summed at 40 with only those of the subblocks as read which contain error bursts to provide a block of data in bus 41 in which all errors and erasures have been corrected up to the preselected capability of the apparatus.

As illustrated, all subblocks as read are buffered at 18 and directed to the traffic controller 35; and said controller channels those subblocks with error bursts to algebraic summing circuit 40 and those without error bursts via controller 35 to summing circuit 37. However, if preferred, the apparatus could be modified to buffer at 18 only those subblocks believed to have error bursts and to send only these via controller 35 to summing circuit 40.

The functions performed by computation circuitry 19 and C.sub.1 -decoder 22; by circuitry 26, 27 and C.sup.B -decoder 32; by C.sub.3 -decoder 30; and by C.sub.1,2 -decoder 39 may be provided by respective separate semiconductor chips, such as the WD60C80 Error Detection and Correction Chip (EDAC) marketed by Western Digital Corp. of Irvine, Calif.

The encoder portion and decoder portion of the apparatus embodying the invention will now be described in greater detail, followed by an Appendix setting forth the proof establishing the operability and efficacy of the method and means herein disclosed.

Assume that the data alphabet is formally represented by a finite, Galois Field with q symbols that is denoted by GF(q). For practical storage applications q is most commonly taken as 2.sup.8, i.e., each symbol in GF(2.sup.8) corresponds to a byte.

The l-th data subblock consists of the symbols m.sub.j,l GF(q), where 0.ltoreq.j.ltoreq.n-r.sub.1 -1 and 0.ltoreq.l.ltoreq.N-1. This subblock is first encoded by the subblock level [n, n-r.sub.1, r.sub.1 +1]-RS encoder 10. Mathematically, it is convenient to describe this data subblock in terms of the polynomial ##EQU2##

The dummy variable x is introduced so that its powers can represent the order of appearance of the bytes in the subblock. Thus the j-th byte m.sub.j,l appears as the coefficient of x.sup.j, 0.ltoreq.j.ltoreq.n-r.sub.1 -1, in the polynomial m.sub.1 (x) that represents the l-th subblock. The polynomial formulation is convenient since it allows for the description of the encoding operation as polynomial multiplication. Encoder 10 for the subblock level code (C.sub.1 -code) is characterized by the encoding polynomial ##EQU3##

The symbol .alpha. in GF(q) denotes a root of a primitive irreducible polynomial whose distinct powers form a representation of the nonzero elements of GF(q). Such a polynomial is a divisor of x.sup.q-1 -1. The l-th encoded subblock is represented by the polynomial

The encoding operation can be described in the frequency transform domain as forcing r.sub.1 zeroes at the Galois Field symbol frequencies {.alpha..sup.i }, i.e. c.sub.l (.alpha..sup.i)=0, for 0.ltoreq.i.ltoreq.r.sub.1 -1.

The C.sub.l -coded subblocks {c.sub.l (x)},0.ltoreq.i.ltoreq.r.sub.2 -1, are written on disk track 9 and, as shown in FIG. 1, are also simultaneously processed to produce shared block check syndromes using circuitry 12. The computation of these check syndromes is performed in two steps. In the first step, (see FIG. 1) the block C.sub.2 -syndrome values {(c.sub.1 (.alpha..sup.r 1.sup.+i)},0.ltoreq.i.ltoreq.r.sub.2 -1, are evaluated at the Galois Field frequencies at which the C.sub.2 -zeroes are located. These nulls are at {.alpha..sup.r 1.sup.+i }, 0.ltoreq.i.ltoreq.r.sub.2 -1, the zeroes of the encoding polynomial g.sub.2 (x) for the C.sub.2 -code which is an [n, n-r.sub.2, r.sub.2 +1]-RS code. The explicit expression for this encoding polynomial is ##EQU4##

The encoding process as thus far described is known from the prior art. However, according to an important feature of the invention, in the second step each syndrome value c.sub.l (.alpha..sup.r.sbsp.1.sup.+i),0.ltoreq.i.ltoreq.r.sub.2 -1, is weighed by a factor .alpha..sup.bl and cumulatively-weighted syndrome sums are generated as follows: ##EQU5##

In the weight factor .alpha..sup.bl, the parameter 1, 0.ltoreq.l.ltoreq.N-1 provides the subblock location inside the block, and the parameter b, 0.ltoreq.b.ltoreq.B-1 indexes the B buffers 15 in which these cumulatively-weighted syndromes are computed and which are shown in FIG. 2. The total number of cumulatively-weighted C.sub.2 -syndromes is r.sub.2 B bytes, which are shared among all the subblocks in a block and referred to as block check syndromes. These block check syndromes are generally stored after the last written subblock in a block.

The block check syndromes have a special status that has to be addressed in the encoding portion. The r.sub.2 B block check syndromes computed in Equation (5) differ from the rest of the stored data because they are not protected from errors. This however is easily overcome by encoding them by using [r.sub.2 B+r.sub.3, r.sub.2 B, r.sub.3 +1]-RS encoder 17. The parameter r.sub.3 is chosen such that equal error protection for all stored data bytes is achieved.

If random and burst error statistics for the storage channel are available, then r.sub.3 can be chosen such that equal error protection in a probabilistic sense is achieved. However, if accurate statistics are not available, a worst-case choice for r.sub.3 can be made as follows. Typically r.sub.2 B is much smaller than n and hence the block check syndromes can be considered as constituting a shortened data subblock. Assume that the channel can cause, in the worst case, up to t errors and s erasures in any subblock, such that 2t+s.ltoreq.r.sub.1 +r.sub.2, where an error is defined as a perturbation, the location and value of which are unknown, and an erasure is defined as a perturbation whose location is known by an external pointer, such that only the value of the error must be computed. Then r.sub.3 =r.sub.1 +r.sub.2 ensures equal error protection for all data bytes. Encoder 17 thus protects the block check syndromes.

The enhanced error correction capability of applicants' improved two-level architecture composed of the subblock level [n, n-r.sub.1, r.sub.1 +1] and block level [n, n-r.sub.2, r.sub.2 +1]-RS codes C.sub.1 and C.sub.2, respectively, will now be discussed. Let t denote the desired number of correctable errors and s the desired number of correctable erasures in a subblock. With these values of the desired performance parameters s and t established, the design parameters r.sub.1 and r.sub.2 of the two-level architecture must satisfy the inequality 2t+s.ltoreq.r.sub.1 +r.sub.2 for each subblock to correct all errors.

A subblock is declared to have an error burst if its number of errors and erasures satisfy the inequalities r.sub.1 <2t+s.ltoreq.r.sub.1 +r.sub.2. Let us denote by T the number of subblocks with error bursts whose locations are unknown and by S the number of subblocks with error bursts whose locations are given by externally generated pointers from set 42, both of which types of errors are to be corrected. If the number of B buffers of length r.sub.2 each, which is the third design parameter of the proposed two-level architecture, is chosen to satisfy the inequality 2T+S.ltoreq.B, then T+S will be the total number of correctable subblocks with error bursts. All the other subblocks for which 2t+s.ltoreq.r.sub.1 are without burst errors and are correctable by the subblock level code C.sub.l.

The read back l-th subblock c.sub.l (x) differs from the written C.sub.l -encoded subblock c.sub.l (x) by the error pattern polynomial e.sub.l (x) as follows:

where ##EQU6##

The first step in the decoding process is computing the C.sub.1 -syndromes c.sub.l (.alpha..sup.i),0.ltoreq.i.ltoreq.r.sub.1 -1 with circuitry 19. Since by definition c.sub.l (.alpha..sup.i)=0,0.ltoreq.i.ltoreq.r.sub.1 -1, the C.sub.1 -syndromes are as follows:

c.sub.l (.alpha..sup.i)=e.sub.l (.alpha..sup.i), 0.ltoreq.i.ltoreq.r.sub.1 -1 (7)

These syndromes are computed for all read back subblocks. In the next step C.sub.1 -decoder 22 processes these syndromes to produce a minimal Hamming distance estimate e.sub.l (x) of the error pattern polynomial which is algebraically summed at 24 with the buffered read back subblock polynomial c.sub.l (x). This computation is performed only on those subblocks which do not have pointers. The processing of the C.sub.1 -syndromes of all the other subblocks is deferred to the C.sub.1,2 -decoder 39. C.sub.1 -decoder 22 produces an estimated C.sub.1 -codeword

where e.sub.l (x) is the estimated error pattern.

If the l-th subblock has no error bursts, C.sub.1 -decoder 22 generates an estimated error pattern e.sub.l (x) that is identical with the actual error pattern e.sub.l (x). Hence, c.sub.l (x)=c.sub.l (x). If the l-th subblock has an error burst and no pointer, attempting to correct it the C.sub.1 -decoder 22 will either fail or the error burst will be miscorrected. A decoder failure 43 generates a pointer in 42 that locates the l-th subblock as a subblock previously unknown to have an error burst. To include all the subblocks of a block in the subsequent discussion, it is assumed that when the location of a subblock with an error burst is given either by a pointer from 42 or from a failure (see 43) of C.sub.1 -decoder 22, then

Assume now that the l-th subblock is miscorrected, i.e., the estimated error pattern e.sub.l (x) in 23 differs from the actual error pattern e.sub.l (x). There is the possibility that the miscorrection is a valid C.sub.2 -codeword and hence will pass through the decoder 39 shown in FIGS. 3D and invalidate the correction capability of the two-level architecture. To prove that this cannot happen, the Appendix includes Lemma 1, an analysis of the properties of the difference error pattern polynomial obtained as the difference between the actual and the estimated error pattern polynomials:

Following the processing in C.sub.1 -decoder 22, S, the number of subblocks with error bursts whose locations are known is generally increased according to the proportional relationship of 2T+S.ltoreq.B as T, the number of subblocks having error bursts with unknown locations is decreased. The new erasures correspond to C.sub.1 -decoder failures that are transmitted via 43 to update location register 42.

The next step in the decoding process is the computation (at 26, 27) of the C.sub.2 -syndromes of the estimated C.sub.1 -codeword c.sub.1 (x) provided (in 25) by the C.sub.1 -decoder 22. These syndromes are used as inputs to the computation of the block check syndrome estimates (at 28). This computation is identical to the one described in FIG. 2 for circuitry 12, and the equations for the block syndrome byte estimates, similar to Equation (5), are as follows: ##EQU7##

The read back block check syndromes, stored after the last subblock are restored by the RS C.sub.3 -decoder 30. The inputs to this decoder are the noisy r.sub.2 B block check syndromes A.sub.i (b), 0.ltoreq.b.ltoreq.B-1, 0.ltoreq.i.ltoreq.r.sub.2 -1 and the r.sub.3 check bytes which may also have incurred errors. The output of the decoder 30 is the restored r.sub.2 B block check syndromes which are subtracted from the estimates computed in (10) to produce the differences .DELTA.A.sub.i (b)=A.sub.i (b)-A.sub.i (b) explicitly given as follows: ##EQU8##

The cumulative block level syndromes are e.sub.1.sup.i (.alpha..sup.r.sbsp.1.sup.+i), 0.ltoreq.l.ltoreq.N-1, 0.ltoreq.i.ltoreq.r.sub.2 -1. By Lemma 1 these are zero if and only if the l-th subblock has no error bursts. If this l-th subblock has an error burst, then e.sub.l.sup.i is nonzero for some i in the range 0.ltoreq.i.ltoreq.r.sub.2 -1. In terms of these syndromes the i-th cumulative block syndrome polynomial 0.ltoreq.i.ltoreq.r.sub.2 -1 is defined as follows: ##EQU9##

The block check differences .DELTA.A.sub.i (b) are the values assumed by the polynomial defined in (12) at the locations .alpha..sup.b i.e.

Assume {l'.sub.1, . . . ,l'.sub.T } to be the unknown locations and (l".sub.l, . . . ,l".sub.S) to be the known locations of the subblocks with error bursts. The problem to be solved in the sequel is the computation of the unknown burst locations {l'.sub.1, . . ,l'.sub.T } and of the cumulative block syndromes e.sub.l (a.sup.r.sub.1.sup.+i),0.ltoreq.l.ltoreq.N-1, 0.ltoreq.i.ltoreq.r.sub.2 -1, for each 1 in the union set of known and unknown locations 1.epsilon. {l'.sub.1,l'.sub.2, . . . ,l'.sub.T } {l".sub.1,l".sub.2, . . . ,l".sub.S } of subblocks with error bursts. This problem is solved by introducing a novel interpretation of the cumulative block syndrome polynomials e.sup.i (x), 0.ltoreq.i.ltoreq.r.sub.2 -1. For a fixed i, e.sup.i (x) may be viewed as an error pattern polynomial for the whole block which consists of N subblocks, now viewed as a codeword in a [N, N-B, B+1]-RS code, denoted by C.sup.B. The unknown locations {l'.sub.1, . . ,l'.sub.T } of subblocks with error bursts, as well as the nonzero syndrome values e.sub.1, 0.ltoreq.i.ltoreq.r.sub.2 -1 defined previously, are obtained from C.sup.B -decoder 32. This decoder has the required decoding capability because the cumulative block syndrome polynomials e.sup.i (x) used at its input represent error patterns composed of at most S erasures and T random errors, and the design parameter B satisfies the inequality 2T+S.ltoreq.B. By Lemma 1 it is known that the l-th subblock has no error bursts if the block syndromes e.sub.l.sup.i, 0.ltoreq.i.ltoreq.r.sub.2 -1 are all zero and in this case the C.sub.1 -estimated codeword c.sub.l (x) is the decoded output which is passed through controller 35 to algebraic summing circuit 37. However, if this is not the case, i.e. the l-th subblock has an error burst, then a further step is required.

This step, which is the last step in the deooding process of a subblock with an error burst, is the processing of the complete string of syndromes e.sub.l (.alpha..sup.i), 0.ltoreq.i.ltoreq.r.sub.1 +r.sub.2 -1 by the decoder 39 for the RS-code C.sub.1,2 which has the parameters [n, n-r.sub.1 -r.sub.2, r.sub.1 +r.sub.2 +1], This complete string of syndromes is obtained by combining (at 40) the two disjoint strings of syndromes that have been computed in the steps previously described. The first substring of syndromes, e.sub.l (.alpha..sup.i),0.ltoreq.i.ltoreq.r.sub.1 -1, is obtained in the first step of the decoding process as the C.sub.1 -syndromes. The second substring, e.sub.l (.alpha..sup.r.sbsp.1.sup.+i),0.ltoreq.i.ltoreq.r.sub.2 -1, is known as can easily be shown by writing the explicit equation for these values using (9):

The terms e.sub.l (.alpha..sup.r.sbsp.1.sup.+1),0.ltoreq.i.ltoreq.r.sub.2 -1, at the right side of (14) are precisely the cumulative block syndromes e.sub.1.sup.i, 0.ltoreq.i.ltoreq.r.sub.2 -1, which are obtained from the C.sup.B -decoder 32. The remaining terms at the right side of (14), e.sub.l (.alpha..sup.r.sbsp.1.sup.+i),0.ltoreq.i.ltoreq.r.sub.2 -1 are obtained by evaluating the error pattern estimate e.sub.l (x) which is computed by the C.sub.1 -decoder 22 at the locations .alpha..sup.r.sbsp.1.sup.+i,0.ltoreq.i.ltoreq.r.sub.2 -1.

It will thus be seen that the decoding process described in this section and illustrated in FIGS. 3A-D provides the complete set of syndromes {e.sub.l (.alpha..sup.i)}, 0.ltoreq.i.ltoreq.r.sub.1 +r.sub.2 -1 required by the [n, n-r.sub.1 -r.sub.2, r.sub.1 +r.sub.2 +1]-decoder 39, designated the C.sub.1,2 -decoder This decoder completes the process of correcting subblocks with error bursts by producing the error pattern e.sub.l (x) which is subtracted (at 40) from c.sub.1 (x) as shown in FIG. 3D.

It will now be apparent that if S and T denote the number of subblocks with error bursts whose locations are respectively known and unknown, the method and means of our architecture embodying the invention and above-described, is capable of locating and correcting all the combinations of subblocks with error bursts in a block for which the condition 2T+S.ltoreq.B is true, where B is the number of buffers 15. This is achieved without lowering the subblock level error correction capability by using the differences between the stored and the recomputed block check syndromes as block syndrome inputs to the RS-decoder 32.

It will also be seen that the architecture above-described requires the minimum number of check bytes for a selected error correction capability; namely, Nr.sub.1 +Br.sub.2 +r.sub.3 where, as earlier stated, N is the number of subblocks per block, r.sub.1 is the number of subblock check bytes in the encoded subblock, r.sub.2 is the number of block check bytes in the block, and r.sub.3 is that number of check bytes for the block check bytes sufficient to equally protect all of the bytes of the block, and where all combinations of T and S such that 2T+S.ltoreq.B must be provided for.

If, however, the constraint S.dbd.O is imposed (i.e., no externally generated pointers to subblocks are available), then this architecture may be modified and, as proved by Lemma 2 in the Appendix, the total number of check bytes can be decreased to 2t.sub.1 N+3(t.sub.2 -t.sub.1)T+2t.sub.3 where t.sub.1 and t.sub.2 denote the number of desired correctable random errors at the subblock and block levels, respectively; t.sub.1 .ltoreq.t.sub.2 ; t.sub.3 is the maximum number of errors in the block check syndromes; and T is the desired number of correctable subblocks having burst errors to which no pointers are available.

The difference between the actual and the estimated error patterns produced by the C.sub.1 -decoder 22 represented as the error difference polynomial e.sub.l (x) is always a C.sub.1 -codeword. However, if it is nonzero because a miscorrection due to an error burst (i.e., an error exceeding the first level correction capability) has occurred, it is not a C.sub.2 -codeword and hence its C.sub.2 -syndrome bytes will be nonzero. These nonzero C.sub.2 -syndromes will be used to undo this miscorrection.

From Equations (6) and (8), it is known that c.sub.l (x)=c.sub.l (x)+e.sub.l (x)-(x) and in conjuction with (9) e.sub.l (x)=c.sub.1 (x)-c.sub.1 (x) is easily obtained. Since both c.sub.1 (x) and c.sub.1 (x) are codewords in C.sub.1, their difference e.sub.l (x) is also a codeword because C.sub.1 is a linear code. As stated above, if the l-th subblock has no error bursts, then the actual and the estimated error patterns are identical and the difference is zero. In case the l-th subblock has an error burst, its actual error pattern exceeds the correction capability of the C.sub.1 -ecoder 22 and the estimated error pattern, in case of miscorrection, will differ from the actual one thus resulting in a nonzero difference.

Assume now that the actual pattern e.sub.l (x) consists of s erasures and t' random errors, where 2t'+s.ltoreq.r.sub.1 +r.sub.2. In case of miscorrection the C.sub.1 -decoder 22 estimates an error pattern polynomial e.sub.l (x) whose errors are confined to the s erasures and furthermore to t' random errors such that 2t's.ltoreq.r.sub.1. The error pattern after miscorrection, e.sub.l (x), is the difference of the two; and thus its Hamming weight is at most s+t'+t".ltoreq.r.sub.1 + r.sub.2 /2 .ltoreq.r.sub.1 +r.sub.2. Now consider the linear code C.sub.1,2 (in 39) which is specified by the encoder polynomial =g.sub.1,2 (x)=(x-1)(x-.alpha.)(x-.alpha..sup.2) . . . (x-.alpha..sup.r.sbsp.1.sup.+r.sbsp.2.sup.-1) It is thus defined as the RS-code with the parameters [n, n-r.sub.1 -r.sub.2, r.sub.1 +r.sub.2 +1]; i.e., its minimal Hamming weight is r.sub.1 +r.sub.2 +1. Since the Hamming weight of e.sub.l (x) is at most r.sub.1 +r.sub.2 it cannot be a codeword in C.sub.1,2. However e.sub. l (x) is a codeword in C.sub.1 and since C.sub.1,2 is specified in terms of the nonoverlapping spectral nulls of C.sub.1 and C.sub.2, it has been proved that it cannot be a codeword in C.sub.2.

LEMMA 2

The total number of check bytes can be reduced to 2t.sub.1 N+3(t.sub.2 -t.sub.1)T+2t.sub.3 because the block syndrome values e.sub.l (.alpha..sup.r.sbsp.1.sup.+i) of an error difference polynomial e.sub.l (x) for a subblock with an error burst are not all zero in the range 0.ltoreq.i.ltoreq. r.sub.2 /2 -1.

From the proof of Lemma 1, it has been shown that e.sub.l (x) is a codeword in C.sub.1 and furthermore that its Hamming weight is at most r.sub.1 + r.sub.2 2 . Now assume that the converse is true; i.e. e.sub.l (.alpha..sup.r.sbsp.1.sup.+i)=0,0.ltoreq.i.ltoreq. r.sub.2 /2 -1. Since e.sub.l (x) is a codeword in C.sub.1, e(.alpha..sup.i)=0,0.ltoreq.i.ltoreq.r.sub.1 + r.sub.2 /2 -1. Since e.sub.l (x) may be considered as a codeword in a [n,n-r.sub.1 - r.sub.2 /2 ,r.sub.1 + r.sub.2 /2 +1]-RS code. But this leads to a contradiction since from the proof of Lemma 1, it has been shown that the Hamming weight of e.sub.l (x) is at most r.sub.1 + r.sub.2 /2 and thus we have a nonzero value e.sub.l.sup.i (.alpha..sup.r.sbsp.1.sup.+i).noteq.0,0.ltoreq.i.ltoreq. r.sub.2 /2 - 1.

Lemma 2 suggests that using the C.sup.B -decoder 32, r.sub.2 /2 instead of r.sub.2 times is sufficient to determine the locations of the subblocks with error bursts as well as their block syndromes e.sub.l.sup.i, 0.ltoreq.i.ltoreq. r.sub.2 /2 -1. To compute the remaining half e.sub.l, r.sub.2 /2 .ltoreq.i.ltoreq.r.sub.2 -1, of these syndromes, since their locations are a known, the C.sup.B -decoder 32 requires only e.sub.l.sup.i (.alpha..sup.b)=.DELTA.A.sub.i (b), r.sub.2 /2 i.ltoreq.r.sub.2 -1,0.ltoreq.b.ltoreq.T-1 rather than 0.ltoreq.b.ltoreq.B-1. Because B=2T and r.sub.2 =2(t.sub.2-t.sub.1), it is unecessary to compute the T(.sub.2 -t.sub.1) block check bytes .DELTA.A.sub.i (b), r.sub.2 /2 .ltoreq.i.ltoreq.r.sub.2 -1T .ltoreq.b.ltoreq.B-1 and the total number of check bytes in the architecture embodying the invention can be reduced to 2t.sub.1 N+3(t.sub.2 -t.sub.1)T+2t.sub.3.