Abstract:
Systems for identifying potentially erroneous and/or erased data are provided. Systems have a bit detector, an accumulator, and a data reconstruction processor. The bit detector assigns values to bits read in a data signal. The bit detector illustratively assigns multiple values to each of the bits. The accumulator accumulates a count of the multiple values assigned by the bit detector for each of the bits. The accumulator associates each bit with a particular value based at least in part on its accumulated count. The data reconstruction processor determines for each of the bits a confidence level of the particular value associated to it. The data reconstruction process sets flags for a portion of the bits. The flags identify the portion of the bits as possible erased or erroneous data. The flags are set based at least in part on the confidence levels of the portion of the bits.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is based on and claims the benefit of U.S. patent application Ser. No. 11/481,078, filed on Jul. 5, 2006, and of U.S. Provisional Patent Application Ser. No. 60/738,716, filed on Nov. 22, 2005, the contents of which are hereby incorporated by reference in their entireties. 
    
    
     BACKGROUND 
     Before data are transmitted over a communications channel to a receiver or a data storage device, that data are typically encoded to allow for error detection and/or correction. The error correction/detection encoding manipulates the data in accordance with a distance d error correction (“ECC”), to produce ECC codewords that include the data and associated redundancy information. To decode the data and associated redundancy information from received or retrieved signals, the decoder first recovers the bits and then groups the bits into symbols or sequences of appropriate length for the ECC, and thus, reproduces the ECC codewords. The system next decodes the ECC codewords using the ECC to produce, if possible, error-free data. Typically, an (n, k) distance d Reed Solomon ECC is used to encode data that are to be stored for later retrieval, and the ECC decoder is an on-the-fly hardware decoder that detects and corrects up to “t” errors using 2t=n−k redundancy symbols, where the minimum distance is D min =2t+1. 
     When a sector of a storage medium is “marginal,” such that retrieval of the data stored therein is impaired by, for example, a defect in the medium or a degradation of the signal that represents the data, the system may determine that the stored data contains more errors than the ECC can correct. The system then tries to recover the data through error recovery operations. Generally, the error recovery operations involve up to a predetermined number of sector re-reads in which the error correction operations are performed independently for the respective re-reads. 
     The error recovery operations may include re-reading the sector with the head at various off-track positions, with an increased bias current, using modified filter responses, and so forth, to improve the quality of the readback signal. However, such attempts may not recover the data such that the number of errors included therein is within the correction capability of the ECC. 
     Another error recovery technique referred to as “majority detection” is described in United States Patent Application Publication US 2005/0262423 entitled Majority Detection In Error Recovery, which has a common assignee and is incorporated herein in its entirety by reference. Using majority detection, the system keeps track of the respective bits for multiple re-reads and determines, that is, votes, which bits are likely to be 1&#39;s based no the values of the recovered bit signals in each of the re-reads. A bit signal that is detected as a 1 in more than, for example, one-half of the re-reads, is “voted” as a 1. Otherwise, the bit is voted as a 0. The results of the voting are used to reconstruct the ECC codeword, which is then presented to the ECC decoder. 
     While the majority detection operations work well, and significantly increase the likelihood that the data are recovered from the marginal sector, the operations are constrained by the error detection/correction capability of the ECC. To further increase the likelihood of data recovery, the system could include more redundancy in the stored ECC codewords, however, fewer information symbols could then be included in the sector. Further, such a system would have to be more complex, to operate with the more powerful ECCs. 
     SUMMARY 
     An aspect of the disclosure relates to systems for identifying potentially erroneous and/or erased data. In one embodiment, systems have a bit detector, an accumulator, and a data reconstruction processor. The bit detector assigns values to bits read in a data signal. The bit detector illustratively assigns multiple values to each of the bits. The accumulator accumulates a count of the multiple values assigned by the bit detector for each of the bits. The accumulator associates each bit with a particular value based at least in part on its accumulated count. The data reconstruction processor determines for each of the bits a confidence level of the particular value associated to it. The data reconstruction processor sets flags for a portion of the bits. The flags identify the portion of the bits as possible erased or erroneous data. The flags are set based at least in part on the confidence levels of the portion of the bits. 
     These and various other features and advantages that characterize the claimed embodiments will become apparent upon reading the following detailed description and upon reviewing the associated drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a functional block diagram of a system constructed in accordance with the invention. 
         FIG. 2  is a flow chart of the operations of an error detection sub-system that is included in the system of  FIG. 1 . 
         FIG. 3  is a more detailed functional block diagram of the error detection sub-system. 
         FIG. 4  is a functional block diagram of an alternative operation of the error detection sub-system. 
     
    
    
     DETAILED DESCRIPTION  
     We describe the system in terms of recovering data that are stored on a storage medium. The system operates in essentially the same manner to recover a buffered transmitted signal. 
     Referring now to  FIG. 1 , an enhanced error correction system  10  that is used for error recovery operations includes a bit detector  110  that operates in a known manner to assign values of 1 or 0 to signals that are read from a sector of a storage medium (not shown). The bit detector  110  provides the bit values over line  114  to an error detection sub-system  120  that accumulates, for the respective bits, counts of the number of times that the bits are determined to be 1&#39;s. The bit detector  110  may also provide the bit values over line  112  to the ECC decoder  140 , which may for each re-reading of the sector perform an error correction operation to determine if the retrieved data can be decoded to error-free data. If the ECC decoder successfully decodes the data to error-free data, the system ends the error recovery operation. Otherwise, the system continues re-reading the sector until the sector has been re-read a predetermined number of times. In the example, the predetermined number of times is 15, though in practice the number of times may be selected to be essentially any number that can be completed within the duration of an associated error recovery time-out. 
     At the end of the predetermined number of re-reads, the error detection sub-system  120  reconstructs the values of the respective bits by comparing the counts to a majority detection threshold. The sub-system also determines the confidence level associated with the bit values using upper and lower thresholds, as discussed in more detail below with reference to  FIGS. 2 and 3 . The error detection sub-system then sets flags for those bits for which the associated confidence levels are sufficiently low. In the example, the system reconstructs a bit with a count of greater than 7 as a 1 and flags the bit if the count is less than or equal to an upper threshold, which in the example is 10. Similarly, in the example, the system sets a bit with a count of 7 or less to a 0 and flags the bit if the count is greater than a lower threshold, which in the example is 4. The upper and lower thresholds are programmable and may be set to any values above and below the value used for the majority detection threshold. Further, the upper and lower thresholds may but need not be equal distances from the majority detection threshold. 
     After all of the bits have been assigned values, or reconstructed, the error detection sub-system  120  regroups the bits into symbols and identifies as erroneous a predetermined number “s” of the symbols that are associated with the highest degree of uncertainty in the detected bits. As discussed below, the system identifies as erasures the s symbols with the largest number of flagged bits or, as appropriate, the s symbols with the largest number of bits that have counts that are associated with the lowest confidence levels. The error detection sub-system then sets erasure pointers that identify the locations of the erroneous symbols in the reconstructed ECC codeword, and provides the reconstructed ECC codeword and the erasure pointers over lines  122  to an error and erasure decoder  130 . The decoder  130  operates in a known manner to detect up to (2t−s)/2 errors in the ECC codeword symbols that are not identified as erasures and correct the identified erasures and detected errors in accordance with an (n, k) distance d error correction code (“ECC”). In the example, the ECC is an (n, k) distance d Reed Solomon code and the decoder  130  corrects up to (2t−s)/2 errors plus s erasures using 2t=n−k codeword redundancy symbols, where d min =2t+1. The decoder  130  preferably utilizes the hardware of the ECC decoder  140  to perform certain of the error detection and error and erasure correction operations. 
     The operations of the error detection sub-system are now described in more detail with reference to  FIGS. 2-4 . 
     In an error recovery operation, sector data for each re-read are supplied to the bit detector  110 , which operates in a known manner to detect the signal values associated with the respective bits and assign bit values, that is, 1s or 0s to the bits. The bit values determined by the bit detector  110  are supplied to accumulators  310 , which for the respective bits accumulate counts that are equal to the number of times the given bits are detected as 1s in the multiple re-reads. At the completion of the predetermined number of retries, the counts in the accumulators  310  are provided to a data reconstruction processor  320 , which compares each of the counts to a majority detection threshold  324 , to determine whether the corresponding bit should be reconstructed as a 1 or a 0. The processor further compares the count to one of either an upper threshold  322  or a lower threshold  326  to determine whether or not the bit should be flagged as most likely erroneous. 
     The data reconstruction processor  320  thus compares the count to a majority detection threshold which is generally equal to one-half the number of re-reads. (step  210 ). If the count exceeds the majority detection threshold, the data reconstruction processor reconstructs the bit as a 1 (step  214 ). The data reconstruction processor also compares the count to an upper threshold, to determine if the confidence level associated with the bit value is sufficiently low that the bit should be considered erroneous, that is, neither a 1 nor a 0 (step  218 ). 
     Thus, in the example, the data reconstruction processor  320  compares the count with an upper threshold of 10, to determine how likely it is that the bit should be considered a 1. If the count exceeds the threshold, that is, if more than 10 of the 15 re-read operations detected the bit as a 1, the confidence level associated with assigning the bit a value of 1 is sufficiently high and the bit is not flagged as erroneous. If, however, the bit is detected as a 1 between 8 and 10 times, the associated confidence level is low, and the processor sets a flag to denote what could be an erroneous bit (step  220 ). 
     If the count for a given bit is less than the majority detection threshold, the data bit reconstruction processor sets the bit to 0. The confidence with which the bit is set to a 0 is then assessed by determining if the associated count is greater than a lower threshold, which in the example is 4 (steps  212 ,  216 ). If the count is greater than 4, the data reconstruction processor sets a flag, to denote what could be an erroneous bit (step  220 ). 
     The data reconstruction processor  320  then groups the bits into multi-bit symbols that are used for error correction, to reconstruct the ECC codeword (step  226 ). Next, the data reconstruction processor determines which s codeword symbols are associated with the highest uncertainty in the detected bits. In this example, the processor determines which s symbols are associated with the highest uncertainty in the detected bits. In this example, the processor determines which s symbols are associated with the largest number of flagged bits (step  228 ). These s symbols are treated as erasures, and the processor sets erasure pointers to identify the locations of the symbols in the ECC codeword. The data reconstruction processor then sends the reconstructed ECC codeword and the erasure pointers to the error and erasure decoder  130 , which operates in a known manner using the ECC decoder hardware to correct the s symbols that are identified as erasures and to detect and correct up to (2t−s)/2 errors in the remaining ECC codeword symbols. 
     Referring now to  FIG. 4 , the data reconstruction processor  320  may, after grouping the bits into symbols (step  226 ,  FIG. 2 ), determine that more than s symbols are associated with the largest number of flagged bits (steps  400 ,  402 ). For example, the processor may determine that g symbols each of f flags set, where g&gt;s. The processor next determines the degrees of uncertainty for the respective g symbols, based on the counts associated with the flagged bits (step  404 ). Thus the process “rates” the symbols based on how close the counts of the respective flagged bits are to the majority detection threshold, that is, to the lowest confidence levels for setting the bit values. In the example, the largest number of flags set per symbol is f=3. The processor gives a highest rating to a symbol with, for example, 3 flagged 0 bits that have each have a count of 7, that is, that each have a count associated with the lowest confidence level for setting a bit to 0. The processor gives a lower rating to a symbol that has, for example, a 0 bit with a count of 7, a 1 bit with a count of 8 and another 1 bit with a count of 9, and so forth. After rating all g of the symbols, the processor identifies as erasures the s symbols with the highest ratings (steps  406 ,  408 ). Thus, the processor identifies as erasures the s symbols most likely to contain bit values that are incorrectly set. The processor then performs the error and erasure correction operations as discussed above with reference to step  232  of  FIG. 2 . 
     The number of erasure pointers that are used in the system is selected to be less than the total erasure correction capability of the ECC. In this way, the system can detect symbols in the ECC codeword that have been reconstructed incorrectly but not identified as erasures. There is thus a trade-off between using the maximum correction capability of the ECC for erasure decoding and the ability to detect erroneously reconstructed symbols that have bits that are associated with higher confidence levels and thus correct smaller number of symbols. 
     In the example, the number of erasure pointers is selected to be one-half the error correction capability of the ECC. In this way, the error correction capability of the ECC is enhanced by the inclusion of the erasures to correct one and one-half times the number of symbols the ECC could correct as undetected errors. Thus, for an ECC that can detect and correct up to 20 erroneous symbols using 40 redundancy symbols, the system utilizes 10 erasure pointers, to correct 10 erasures and detect and correct up to 15 errors, or a total of 25 erroneous symbols. 
     Before performing the error and erasure decoding, the system may attempt to correct the errors in the majority-detected reconstructed ECC codeword by applying the ECC codeword directly to the ECC decoder  140 . However, this majority detection decoding operation need not be performed. Further, the step of providing of the detected bit information to the ECC decoder after each re-read, as described above with reference to  FIG. 1 , may but need not be performed during the error recovery operation. 
     Certain processors are shown as separate devices but may be combined. Various operations may be performed in hardware, software or firmware or any combination thereof. 
     The term sector is used herein generally to refer to a data storage unit of a data storage device. The data storage device may be a disk drive, a tape drive or a solid state device. 
     The foregoing description has been limited to specific embodiments of this invention. It will be apparent, however, that variations and modifications may be made to the invention, such as counting 0s instead of 1s in the re-read operations, flagging only 1s or only 0s as most likely erroneous, and so forth, with the attainment of some or all of its advantages. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.