Patent Publication Number: US-8996970-B2

Title: Systems and methods for positive feedback short media defect detection

Description:
BACKGROUND 
     Embodiments are related to systems and methods for transferring information, and more particularly to systems and methods for determining problems related to a medium associated with a data transfer. 
     Various data transfer systems have been developed including storage systems, cellular telephone systems, radio transmission systems. In each of the systems data is transferred from a sender to a receiver via some medium. For example, in a storage system, data is sent from a sender (i.e., a write function) to a receiver (i.e., a read function) via a storage medium. The effectiveness of any transfer is impacted by any defects associated with the transfer medium. In some cases, data loss caused by defects in the transfer medium can make recovery of data from the transfer medium difficult even for data received from non-defective areas or times. Various approaches have been developed for identifying defects in the transfer medium. Such approaches provide a general ability to identify defects, but in many cases are inaccurate. In the best case, this inaccuracy limits the effectiveness of any defect identification. In the worst case, inaccurate defect detection may actually hamper the data recovery process. 
     Hence, for at least the aforementioned reasons, there exists a need in the art for advanced systems and methods for defect detection. 
     BRIEF SUMMARY 
     Embodiments are related to systems and methods for transferring information, and more particularly to systems and methods for determining problems related to a medium associated with a data transfer. 
     Various embodiments of the present invention provide data processing systems that include: a data detector circuit, a data decoder circuit, a media defect detector circuit, and a threshold detection circuit. The data detector circuit is operable to apply a data detection algorithm to a detector input to yield a first detected output, and to apply the data detection algorithm to the detector input to yield a second detected output. The data decoder circuit is operable to apply a data decoding algorithm to a first decoder input derived from the first detected output to yield a first decoded output, and to apply the data decoding algorithm to a second decoder input derived from the second detected output to yield a second decoded output. The media defect detector circuit is operable to generate a defect value based on a combination of the first detected output and the first decoded output, and to compare the defect value with a first defect threshold to yield a defect indicator for a region of a medium from which the detector input is derived for the first global iteration. The threshold selection circuit is operable to select a second defect threshold based at least in part on the defect indicator indicating a media defect for the first global iteration. 
     This summary provides only a general outline of some embodiments of the invention. The phrases “in one embodiment,” “according to one embodiment,” “in various embodiments”, “in one or more embodiments”, “in particular embodiments” and the like generally mean the particular feature, structure, or characteristic following the phrase is included in at least one embodiment of the present invention, and may be included in more than one embodiment of the present invention. Importantly, such phases do not necessarily refer to the same embodiment. Many other embodiments of the invention will become more fully apparent from the following detailed description, the appended claims and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A further understanding of the various embodiments of the present invention may be realized by reference to the figures which are described in remaining portions of the specification. In the figures, like reference numerals are used throughout several figures to refer to similar components. In some instances, a sub-label consisting of a lower case letter is associated with a reference numeral to denote one of multiple similar components. When reference is made to a reference numeral without specification to an existing sub-label, it is intended to refer to all such multiple similar components. 
         FIG. 1  shows a storage system including a read channel with feedback based short media defect detection threshold modification circuitry in accordance with various embodiments of the present invention; 
         FIG. 2  depicts a data processing circuit including a feedback based short media defect detection threshold modification circuit in accordance with various embodiments of the present invention; 
         FIG. 3  is a flow diagram showing a method in accordance with some embodiments of the present invention for data processing including feedback based short media defect detection threshold modification; 
         FIG. 4  shows another data processing circuit including a feedback based short media defect detection threshold modification circuit in accordance with various embodiments of the present invention; and 
         FIG. 5  is a flow diagram showing a method in accordance with other embodiments of the present invention for data processing including feedback based short media defect detection threshold modification. 
     
    
    
     DETAILED DESCRIPTION OF SOME EMBODIMENTS 
     Embodiments are related to systems and methods for transferring information, and more particularly to systems and methods for determining problems related to a medium associated with a data transfer. 
     Various embodiments of the present invention provide data processing circuitry that include a data detection circuit operable to apply a data detection algorithm to a data input to yield a detected output, and a data decoding circuit operable to apply a data decoding algorithm to a decoder input derived from the detected output to yield a decoded output. As used herein, application of both the data detection algorithm and the data decoding algorithm is referred to as a “global iteration”. In some cases, the data decoding algorithm may be repeatedly applied during a give global iteration. Each application of the data decoding algorithm during a given global iteration is referred to herein as a “local iteration”. 
     As part of processing a data set, a media defect detection is applied. This media defect detection includes generated a value indicative of a media defect and comparing the value against a threshold. Where a either a first global iteration is being applied or a preceding global iteration did not indicate a media defect, the threshold is a default threshold. Alternatively, where a previous global iteration resulted in an indication of a media defect, a modified threshold is used. The modified threshold may be set such that a media defect is more easily identified. In one or more cases, the modified threshold is a value based upon threshold values for a preceding bit periods. 
     Various embodiments of the present invention provide data processing systems that include: a data detector circuit, a data decoder circuit, a media defect detector circuit, and a threshold detection circuit. The data detector circuit is operable to apply a data detection algorithm to a detector input to yield a first detected output, and to apply the data detection algorithm to the detector input to yield a second detected output. The data decoder circuit is operable to apply a data decoding algorithm to a first decoder input derived from the first detected output to yield a first decoded output, and to apply the data decoding algorithm to a second decoder input derived from the second detected output to yield a second decoded output. The media defect detector circuit is operable to generate a defect value based on a combination of the first detected output and the first decoded output, and to compare the defect value with a first defect threshold to yield a defect indicator for a region of a medium from which the detector input is derived for the first global iteration. The threshold selection circuit is operable to select a second defect threshold based at least in part on the defect indicator indicating a media defect for the first global iteration. 
     In some cases, the systems are implemented as an integrated circuit. In various cases, the systems are implemented as part of a storage device. In some such cases, the medium is a magnetic storage medium. In one or more cases, the data decoder circuit is a low density parity check decoder circuit. In particular cases, the data detector circuit may be a Viterbi algorithm data detector circuit, while in other cases the data detector circuit may be a maximum a posteriori data detector circuit. 
     In some instances of the aforementioned embodiments, the second defect threshold is selected to be the first defect threshold. In one or more instances of the aforementioned embodiments, the second defect threshold is selected to be the first defect threshold multiplied by a scalar value when the defect indicator indicates a media defect for the first global iteration. The scalar value is greater than unity. In some cases, the scalar value is programmable. 
     In one or more instances of the aforementioned embodiments, the defect value is a number of mismatches between the first detected output and the first decoded output within a first range of an instant location. The second defect threshold is selected as one the first defect threshold, or a number of mismatches between the first detected output and the first decoded output within a second range of the instant location. In some such instances, the number of mismatches between the first detected output and the first decoded output within a second range of the instant location is selected as the second defect threshold when the defect indicator indicates a media defect for the first global iteration. 
     In various instances of the aforementioned embodiments, the defect indicator is a first defect indicator and the defect value is a first defect value. In such instances, the media defect detector circuit is further operable to generate a second defect value based on a combination of the second detected output and the second decoded output, and to compare the second defect value with the second defect threshold to yield a second defect indicator for a region of a medium from which the detector input is derived for the second global iteration. 
     Other embodiments provide methods for media defect detection that include: applying a data detection algorithm to a detector input by a data detector circuit to yield a first detected output; applying a data decoding algorithm to a first decoder input derived from the first detected output to yield a first decoded output; applying a media defect detection algorithm to a combination of the first detected output and the first decoded output to yield a defect value; comparing the defect value with a first defect threshold to yield a defect indicator for a region of a medium from which the detector input is derived for a first global iteration; applying the data detection algorithm to the detector input to yield a second detected output; applying the data decoding algorithm to a second decoder input derived from the second detected output to yield a second decoded output; and selecting a second a second defect threshold based at least in part on the defect indicator indicating a media defect for the first global iteration. 
     Turning to  FIG. 1 , a storage system  100  including a read channel circuit  110  having feedback based short media defect detection threshold modification circuitry in accordance with various embodiments of the present invention. Storage system  100  may be, for example, a hard disk drive. Storage system  100  also includes a preamplifier  170 , an interface controller  120 , a hard disk controller  166 , a motor controller  168 , a spindle motor  172 , a disk platter  178 , and a read/write head assembly  176 . Interface controller  120  controls addressing and timing of data to/from disk platter  178 . The data on disk platter  178  consists of groups of magnetic signals that may be detected by read/write head assembly  176  when the assembly is properly positioned over disk platter  178 . In one embodiment, disk platter  178  includes magnetic signals recorded in accordance with either a longitudinal or a perpendicular recording scheme. 
     In operation, read/write head assembly  176  is accurately positioned by motor controller  168  over a desired data track on disk platter  178 . Motor controller  168  both positions read/write head assembly  176  in relation to disk platter  178  and drives spindle motor  172  by moving read/write head assembly to the proper data track on disk platter  178  under the direction of hard disk controller  166 . Spindle motor  172  spins disk platter  178  at a determined spin rate (RPMs). Once read/write head assembly  176  is positioned adjacent the proper data track, magnetic signals representing data on disk platter  178  are sensed by read/write head assembly  176  as disk platter  178  is rotated by spindle motor  172 . The sensed magnetic signals are provided as a continuous, minute analog signal representative of the magnetic data on disk platter  178 . This minute analog signal is transferred from read/write head assembly  176  to read channel circuit  110  via preamplifier  170 . Preamplifier  170  is operable to amplify the minute analog signals accessed from disk platter  178 . In turn, read channel circuit  110  decodes and digitizes the received analog signal to recreate the information originally written to disk platter  178 . This data is provided as read data  103 . 
     As part of processing data accessed from disk platter  178 , read channel circuit  110  performs a media defect detection process operable to determine whether received data is associated with a defective region of disk platter  178 . This media defect detection includes selectively modifying a defect threshold based at least in part on whether a defect from the corresponding region was detected during a previous global iteration. In some embodiments of the present invention, the data processing circuit including the feedback based short media defect detection threshold modification circuitry may be implemented similar to that discussed below in relation to  FIG. 2  or  FIG. 4 , and/or may apply data processing similar to that discussed below in relation to  FIG. 3  or  FIG. 5 . 
     It should be noted that storage system  100  may be integrated into a larger storage system such as, for example, a RAID (redundant array of inexpensive disks or redundant array of independent disks) based storage system. Such a RAID storage system increases stability and reliability through redundancy, combining multiple disks as a logical unit. Data may be spread across a number of disks included in the RAID storage system according to a variety of algorithms and accessed by an operating system as if it were a single disk. For example, data may be mirrored to multiple disks in the RAID storage system, or may be sliced and distributed across multiple disks in a number of techniques. If a small number of disks in the RAID storage system fail or become unavailable, error correction techniques may be used to recreate the missing data based on the remaining portions of the data from the other disks in the RAID storage system. The disks in the RAID storage system may be, but are not limited to, individual storage systems such as storage system  100 , and may be located in close proximity to each other or distributed more widely for increased security. In a write operation, write data is provided to a controller, which stores the write data across the disks, for example by mirroring or by striping the write data. In a read operation, the controller retrieves the data from the disks. The controller then yields the resulting read data as if the RAID storage system were a single disk. 
     A data decoder circuit used in relation to read channel circuit  110  may be, but is not limited to, a low density parity check (LDPC) decoder circuit as are known in the art. Such low density parity check technology is applicable to transmission of information over virtually any channel or storage of information on virtually any media. Transmission applications include, but are not limited to, optical fiber, radio frequency channels, wired or wireless local area networks, digital subscriber line technologies, wireless cellular, Ethernet over any medium such as copper or optical fiber, cable channels such as cable television, and Earth-satellite communications. Storage applications include, but are not limited to, hard disk drives, compact disks, digital video disks, magnetic tapes and memory devices such as DRAM, NAND flash, NOR flash, other non-volatile memories and solid state drives. 
     In addition, it should be noted that storage system  100  may be modified to include solid state memory that is used to store data in addition to the storage offered by disk platter  178 . This solid state memory may be used in parallel to disk platter  178  to provide additional storage. In such a case, the solid state memory receives and provides information directly to read channel circuit  110 . Alternatively, the solid state memory may be used as a cache where it offers faster access time than that offered by disk platter  178 . In such a case, the solid state memory may be disposed between interface controller  120  and read channel circuit  110  where it operates as a pass through to disk platter  178  when requested data is not available in the solid state memory or when the solid state memory does not have sufficient storage to hold a newly written data set. Based upon the disclosure provided herein, one of ordinary skill in the art will recognize a variety of storage systems including both disk platter  178  and a solid state memory. 
     Turning to  FIG. 2 , a data processing circuit  200  including a feedback based short media defect detection threshold modification circuit  250  is depicted in accordance with various embodiments of the present invention. Feedback based short media defect detection threshold modification circuit  250  includes a short media defect detector circuit  270 , a defect threshold selector circuit  260 , a multiplexer circuit  266 , and a scalar circuit  265 . Data processing circuit  200  includes a data detector circuit  210  that applies a data detection algorithm to a data input  201  guided by a soft decoded output  232 . Soft decoded output  232  is denoted La(i), where ‘i’ represents the iteration number. In particular, ‘i’ indicates data derived from the most recent iteration, and ‘i−1’ indicates data derived from a preceding iteration. 
     In some embodiments of the present invention, data input  201  is derived from a storage medium. Based upon the disclosure provided herein, one of ordinary skill in the art will recognize a variety of sources for data input  201 . Data detector circuit  210  may be, but is not limited to, a maximum a posteriori data detector circuit, or a Viterbi algorithm data detector circuit. Of note, the general phrases “Viterbi data detection algorithm” or “Viterbi algorithm data detector circuit” are used in their broadest sense to mean any Viterbi detection algorithm or Viterbi algorithm detector circuit or variations thereof including, but not limited to, bi-direction Viterbi detection algorithm or bi-direction Viterbi algorithm detector circuit. Also, the general phrases “maximum a posteriori data detection algorithm” or “maximum a posteriori data detector circuit” are used in their broadest sense to mean any maximum a posteriori detection algorithm or detector circuit or variations thereof including, but not limited to, simplified maximum a posteriori data detection algorithm and a max-log maximum a posteriori data detection algorithm, or corresponding detector circuits. Based upon the disclosure provided herein, one of ordinary skill in the art will recognize a variety of data detector circuits that may be used in relation to different embodiments of the present invention. Application of the data detection algorithm by data detector circuit  210  yields a detected output  211 . Detected output  211  is denoted Le(i), where ‘i’ represents the iteration number. In particular, ‘i’ indicates data derived from the most recent iteration, and ‘i−1’ indicates data derived from a preceding iteration. An iteration denoted by ‘i’ is a global iteration which is defined as processing a data set through both data detector circuit  210  and a data decoder circuit  230 . 
     Detected output  211  and soft decoded output  232  are provided to both a selective scaling circuit  220 , and short media defect detector circuit  270  that performs a short media defect detection. Short media defect detector circuit  270  may be any circuit operable to generate a defect value (i) based upon a combination of detected output  211  and soft decoded output  232 , where the defect value (i) corresponds to a likelihood of a media defect on a storage medium from which data input  201  is derived. Short media defect detector circuit  270  compares the generated defect value (i) with a defect threshold  267 . Where the generated defect value (i) is greater than defect threshold  267 , a defect indicator  271  is asserted. Otherwise, defect indicator  271  is not asserted. In some embodiments of the present invention, the defect value (i) is a number of consecutive, preceding instances of detected output  211  and soft decoded output  232  that do not match. In one particular embodiment of the present invention, short media defect detector circuit  270  may be implemented similar to that disclosed in U.S. Pat. No. 8,201,051 entitled “Method for Detecting Short Burst Errors in LDPC System”, issued Jun. 12, 2012 to Tan et al. The entirety of the aforementioned reference is incorporated herein by reference for all purposes. 
     Defect threshold  267  is adjusted based upon whether defect indicator  271  was asserted to indicate a defect was found during a preceding global iteration processing the corresponding instance i. In particular, defect indicator  271  is provided to defect threshold selector circuit  260 . Where defect indicator  271  is asserted indicating the occurrence of a defect at the location corresponding to the index i for the preceding global iteration, defect threshold selector circuit  260  asserts a selector output  261  to a multiplexer circuit  266  such that a scaled value  264  is selected as defect threshold  267 . Scaled value  264  is a default defect threshold  262  multiplied by a scalar value  263  that is greater than unity (i.e., scalar value &gt;1). Otherwise, where defect indicator  271  is de-asserted indicating the no occurrence of a defect at the location corresponding to the index i for the preceding global iteration, default defect threshold  262  is selected by multiplexer circuit  266  as defect threshold  267 . In some embodiments of the present invention, scalar value  263  and default defect threshold  262  are programmable. In other embodiments of the present invention, one or both of scalar value  263  and default defect threshold  262  are fixed. In one particular embodiment of the present invention, scaled value  264  is a user programmable input or a fixed value in which case scalar circuit  265  can be eliminated. 
     The following pseudocode represents the operation of feedback based short media defect detection threshold modification circuit  250 : 
     
       
         
           
               
             
               
                   
               
             
            
               
                 If(global iterations == 0) { //first global iteration 
               
               
                    for (i= 0 to end of data set){ // for each instance in the processing data set 
               
               
                       apply short media defect detection algorithm to yield a defect value (i); 
               
               
                       If (defect value (i) &gt; default defect threshold){ 
               
               
                          defect indicator (i) for the global iteration = asserted 
               
               
                       } 
               
               
                       Else { 
               
               
                          defect indicator (i) for the global iteration =de- asserted 
               
               
                       } 
               
               
                    } 
               
               
                 } 
               
               
                 Else If (global iterations &gt; 0) { 
               
               
                    for (i= 0 to end of data set){ // for each instance in the processing data set 
               
               
                       apply short media defect detection algorithm to yield a defect value (i); 
               
               
                       If (defect indicator (i) == asserted for the prior global iteration){ 
               
               
                          If (defect value (i) &gt; scaled value){ 
               
               
                             defect indicator (i) for the global iteration = asserted 
               
               
                          } 
               
               
                          Else { 
               
               
                             defect indicator (i) for the global iteration = de-asserted 
               
               
                       } 
               
               
                       Else If (defect indicator (i) == de-asserted for the prior global iteration){ 
               
               
                          If (defect value (i) &gt; default defect threshold){ 
               
               
                             defect indicator (i) for the global iteration = asserted 
               
               
                          } 
               
               
                          Else { 
               
               
                             defect indicator (i) for the global iteration = de-asserted 
               
               
                       } 
               
               
                    } 
               
               
                 } 
               
               
                   
               
            
           
         
       
     
     Defect indicator  271  is provided to a selective scaling circuit  220 . Selective scaling circuit  220  delays detected output  211  to align it with defect indicator  271 . Where defect indicator  271  is asserted, selective scaling circuit  220  applies a symbol by symbol scaling to each symbol in soft decoded output  232  that corresponds to defect indicator  271  and provides the scaled values as a scaled output  221 , and passes detected output  211  through without scaling as scaled output  221  on a symbol by symbol basis where defect indicator  271  is not asserted. In another embodiment of the present invention, where defect indicator  271  is asserted, selective scaling circuit  220  applies a symbol by symbol scaling to each symbol in detected output  211  and provides the scaled values as a scaled output  221 , and passes detected output  211  through without scaling as scaled output  221  on a symbol by symbol basis where defect indicator  271  is not asserted. In either case, this selective scaling operates to modify soft data associated with the effected symbols or bits to reduce the probability that the symbol is considered properly found. By doing this, the likelihood that an effected symbol or bit negatively impacts processing of the data set is reduced and the likelihood that the symbol will be modified by later processing is increased. Selective scaling circuit  220  provides scaled output  221  to a data decoder circuit  230 . For the purposes of this document, a symbol may be either a non-binary symbol or a binary symbol depending upon the particular implementation. 
     Data decoder circuit  230  applies a data decode algorithm to scaled output  221  to yield a decoded output that includes soft decoded output  232 . In some embodiments of the present invention, data decoder circuit  230  is a low density parity check decoder circuit as are known in the art. Where the decoded output converges (i.e., yields the original data set as indicated by the lack of remaining errors), it is provided as a data output  231 . 
     Turning to  FIG. 3 , a flow diagram  300  shows a method in accordance with some embodiments of the present invention for data processing including feedback based short media defect detection threshold modification. Following flow diagram  300 , it is determined whether a data detector circuit is available to process a newly received data set or a data set that has already been subject to one or more prior global iterations (block  310 ). Where a data detector circuit is available (block  310 ), a processing data input is accessed and it is determined whether a decoded output corresponding to the accessed processing data input exists (block  320 ). Such a decoded output is available as a result from a preceding global iteration applied to the same processing data input. The processing data input may be, for example, derived from a storage medium. Where a corresponding decoded output is not available (block  320 ), a data detection algorithm is applied to the processing data input to yield a detected output (block  325 ). The data detection algorithm may be, but is not limited to, a Viterbi data detection algorithm or a maximum a posteriori data detection algorithm as are known in the art. Based upon the disclosure provided herein, one of ordinary skill in the art will recognize a variety of data detection algorithms that may be used in relation to different embodiments of the present invention. A short media detect detection is applied to the corresponding detected output (Le(i)) and decoded output (La(i)) to yield a defect value (i) (block  327 ). In some embodiments of the present invention, the defect value (i) is a number of consecutive, preceding instances of mismatches between corresponding instances of the detected output and the decoded output. 
     It is then determined whether the detected value (i) exceeds a non-scaled threshold value (block  342 ). In some embodiments of the present invention, the non-scaled threshold value is user programmable, while in other embodiments it is fixed. Where the detected value (i) exceeds a non-scaled threshold value (block  342 ), a media defect is indicated at the location corresponding to instance i (bock  346 ). Otherwise, where the detected value (i) does not exceed the non-scaled threshold value (block  342 ), no defect is indicated. 
     Alternatively, where a corresponding decoded output (La(i)) is available (block  320 ), it is accessed (block  315 ) and a data detection algorithm is applied to the processing data input guided by the corresponding decoded output (La(i)) to yield a detected output (Le(i)) (block  330 ). Such a corresponding decoded output is available for the second or later global iterations (i.e., i&gt;=1) for a given processing data input. The short media detect detection is applied to the corresponding detected output (Le(i)) and decoded output (La(i)) to yield a defect value (i) (block  335 ). Again, in some embodiments of the present invention, the defect value (i) is a number of consecutive, preceding instances of mismatches between corresponding instances of the detected output and the decoded output. 
     It is then determined whether the previous global iteration processing the currently processing data set indicated a media defect at the location corresponding to instance i (block  340 ). Where a defect was indicated in the preceding global iteration (block  340 ), it is determined whether the defect value (i) exceeds a scaled threshold value (block  344 ). The scaled threshold value may be derived, for example, by multiplying the non-scaled threshold value by a scalar value greater than unity (i.e., scalar value &gt;1). In other cases, the scaled threshold value may be user programmable, or a pre-defined fixed value. Where the detected value (i) exceeds the scaled threshold value (block  344 ), a media defect is indicated at the location corresponding to instance i (bock  346 ). Otherwise, where the detected value (i) does not exceed the scaled threshold value (block  344 ), no defect is indicated. 
     Otherwise, where a defect was not indicated in the preceding global iteration (block  340 ), it is then determined whether the detected value (i) exceeds the non-scaled threshold value (block  342 ). Where the detected value (i) exceeds the non-scaled threshold value (block  342 ), a media defect is indicated at the location corresponding to instance i (bock  346 ). Otherwise, where the detected value (i) does not exceed the non-scaled threshold value (block  342 ), no defect is indicated. 
     In any case, the detected output is modified at locations corresponding an indicated media defect to yield a modified detected output (block  348 ). This modification may include, but is not limited to, multiplying instances of the detected output corresponding to an indicated media defect by a scaling value less than unity (i.e., scaling factor &lt;1). By doing this, the likelihood that an effected symbol negatively impacts processing of the data set is reduced and the likelihood that the symbol will be modified by later processing is increased. Otherwise, where no defect is indicated, the detected output is left unmodified. As another example, this modification may include replacing the instance of the detected output corresponding to the indicated media defect by a scaled version of the corresponding instance of the decoded output. 
     All local iterations of a data decode algorithm are then applied to the detected output guided by a previous decoded output where available (block  350 ). This process generates an updated decoded output that may be used during subsequent global iterations where it does not converge (i.e., have no remaining unsatisfied checks). In some embodiments of the present invention, the data decode algorithm is a low density parity check algorithm as is known in the art. The decoded output (La(i)) is stored to the prior iteration buffer circuit for later use as a prior iteration decoded output (La(i−1)) (block  352 ). 
     It is determined whether the data decode algorithm converged (block  355 ). Where the data decode algorithm converged (block  355 ), the decoded output is provided as an output codeword, and the next processing data input is selected for processing (block  370 ). Alternatively, where the data decode algorithm failed to converge (block  355 ), it is determined whether another global iteration is allowed (block  360 ). Where another global iteration is not allowed (block  360 ), a failure is indicated (block  361 ) and the decoded output is provided as an output codeword, and the next processing data input is selected for processing (block  370 ). In contrast, where another global iteration is allowed (block  360 ), a subsequent location iteration is applied to the same processing data input guided by the results of the preceding global iteration. 
     Turning to  FIG. 4 , a data processing circuit  400  including a feedback based short media defect detection threshold modification circuit  450  is shown in accordance with various embodiments of the present invention. Feedback based short media defect detection threshold modification circuit  450  includes a short media defect detector circuit  470 , a defect threshold selector circuit  460 , a modified defect threshold calculator circuit  480 , and a multiplexer circuit  466 . Data processing circuit  400  includes a data detector circuit  410  that applies a data detection algorithm to a data input  401  guided by a soft decoded output  432 . Soft decoded output  432  is denoted La(i), where ‘i’ represents the iteration number. In particular, ‘i’ indicates data derived from the most recent iteration, and ‘i−1’ indicates data derived from a preceding iteration. 
     In some embodiments of the present invention, data input  401  is derived from a storage medium. Based upon the disclosure provided herein, one of ordinary skill in the art will recognize a variety of sources for data input  401 . Data detector circuit  410  may be, but is not limited to, a maximum a posteriori data detector circuit, or a Viterbi algorithm data detector circuit. Of note, the general phrases “Viterbi data detection algorithm” or “Viterbi algorithm data detector circuit” are used in their broadest sense to mean any Viterbi detection algorithm or Viterbi algorithm detector circuit or variations thereof including, but not limited to, bi-direction Viterbi detection algorithm or bi-direction Viterbi algorithm detector circuit. Also, the general phrases “maximum a posteriori data detection algorithm” or “maximum a posteriori data detector circuit” are used in their broadest sense to mean any maximum a posteriori detection algorithm or detector circuit or variations thereof including, but not limited to, simplified maximum a posteriori data detection algorithm and a max-log maximum a posteriori data detection algorithm, or corresponding detector circuits. Based upon the disclosure provided herein, one of ordinary skill in the art will recognize a variety of data detector circuits that may be used in relation to different embodiments of the present invention. Application of the data detection algorithm by data detector circuit  410  yields a detected output  411 . Detected output  411  is denoted Le(i), where ‘i’ represents the iteration number. In particular, ‘i’ indicates data derived from the most recent iteration, and ‘i−1’ indicates data derived from a preceding iteration. An iteration denoted by ‘i’ is a global iteration which is defined as processing a data set through both data detector circuit  410  and a data decoder circuit  430 . 
     Detected output  411  and soft decoded output  432  are provided to both a selective scaling circuit  420 , and short media defect detector circuit  470  that performs a short media defect detection. Short media defect detector circuit  470  may be any circuit operable to generate a defect value (i) based upon a combination of detected output  411  and soft decoded output  432 , where the defect value (i) corresponds to a likelihood of a media defect on a storage medium from which data input  401  is derived. Short media defect detector circuit  470  compares the generated defect value (i) with a defect threshold  467 . Where the generated defect value (i) is greater than defect threshold  467 , a defect indicator  471  is asserted. Otherwise, defect indicator  471  is not asserted. In some embodiments of the present invention, the defect value (i) is a number of consecutive, preceding instances of detected output  411  and soft decoded output  432  that do not match. In one particular embodiment of the present invention, short media defect detector circuit  470  may be implemented similar to that disclosed in U.S. Pat. No. 8,201,051 entitled “Method for Detecting Short Burst Errors in LDPC System”, issued Jun. 12, 2012 to Tan et al. The entirety of the aforementioned reference is incorporated herein by reference for all purposes. 
     Defect threshold  467  is adjusted based upon whether defect indicator  471  was asserted to indicate a defect was found during a preceding global iteration processing the corresponding instance i. In particular, defect indicator  471  is provided to defect threshold selector circuit  460 . Where defect indicator  271  is asserted indicating the occurrence of a defect at the location corresponding to the index i for the preceding global iteration, defect threshold selector circuit  460  asserts a selector output  461  to multiplexer circuit  466  such that a modified threshold value (i)  481  is selected as defect threshold  467 . Modified defect threshold calculator circuit  480  averages the defect values  473  generated by short media defect detector circuit  470  for a preceding number of instances i. In one particular embodiment of the present invention, the preceding number of instances is twelve (12) (i.e., defect values (i−11 to i)). Based upon the disclosure provided herein, one of ordinary skill in the art will recognize other numbers of preceding instances that may be used in relation different embodiments of the present invention. This averaged value is provided as modified threshold value (i)  481  to multiplexer circuit  466 . 
     Otherwise, where defect indicator  471  is de-asserted indicating the no occurrence of a defect at the location corresponding to the index i for the preceding global iteration, a default defect threshold  462  is selected by multiplexer circuit  466  as defect threshold  467 . In some embodiments of the present invention, default defect threshold  462  is programmable. In other embodiments of the present invention, default defect threshold  462  is fixed. 
     The following pseudocode represents the operation of feedback based short media defect detection threshold modification circuit  450 : 
     
       
         
           
               
             
               
                   
               
             
            
               
                 If(global iterations == 0) { //first global iteration 
               
               
                    for (i= to end of data set){ // for each instance in the processing data set 
               
               
                       apply short media defect detection algorithm to yield a defect value (i); 
               
               
                       If (defect value (i) &gt; default defect threshold){ 
               
               
                          defect indicator (i) for the global iteration = asserted 
               
               
                       } 
               
               
                       Else { 
               
               
                          defect indicator (i) for the global iteration =de- asserted 
               
               
                       } 
               
               
                       If (i &gt; 10){ // make modified threshold value as the average of the last 12 
               
               
                          temp value = 0; //initialize temp value 
               
               
                          for (n=0 to 11){ 
               
               
                             temp value = defect value (i−n) 
               
               
                          } 
               
               
                          modified threshold value (i) = temp value/12 
               
               
                       } 
               
               
                    } 
               
               
                 } 
               
               
                 Else If (global iterations &gt; 0) { 
               
               
                    for (i= 0 to end of data set){ // for each instance in the processing data set 
               
               
                       apply short media defect detection algorithm to yield a defect value (i); 
               
               
                       If (defect indicator (i) = = asserted for the prior global iteration){ 
               
               
                          If (defect value (i) &gt; modified threshold value){ 
               
               
                             defect indicator (i) for the global iteration = asserted 
               
               
                          } 
               
               
                          Else { 
               
               
                             defect indicator (i) for the global iteration = de-asserted 
               
               
                       } 
               
               
                       Else If (defect indicator (i) == de-asserted for the prior global iteration){ 
               
               
                          If (defect value (i) &gt; default defect threshold){ 
               
               
                             defect indicator (i) for the global iteration = asserted 
               
               
                          } 
               
               
                          Else { 
               
               
                             defect indicator (i) for the global iteration = de-asserted 
               
               
                       } 
               
               
                       If (i &gt; 10){ // make modified threshold value as the average of the last 12 
               
               
                          temp value = 0; //initialize temp value 
               
               
                          for (n=0 to 11){ 
               
               
                             temp value = defect value (i−n) 
               
               
                          } 
               
               
                          modified threshold value (i) = temp value/12 
               
               
                       } 
               
               
                    } 
               
               
                 } 
               
               
                   
               
            
           
         
       
     
     Defect indicator  471  is provided to a selective scaling circuit  420 . Selective scaling circuit  420  delays detected output  411  to align it with defect indicator  471 . Where defect indicator  471  is asserted, selective scaling circuit  420  applies a symbol by symbol scaling to each symbol in soft decoded output  432  that corresponds to defect indicator  471  and provides the scaled values as a scaled output  421 , and passes detected output  411  through without scaling as scaled output  421  on a symbol by symbol basis where defect indicator  471  is not asserted. In another embodiment of the present invention, where defect indicator  471  is asserted, selective scaling circuit  420  applies a symbol by symbol scaling to each symbol in detected output  411  and provides the scaled values as a scaled output  421 , and passes detected output  411  through without scaling as scaled output  421  on a symbol by symbol basis where defect indicator  471  is not asserted. In either case, this selective scaling operates to modify soft data associated with the effected symbols or bits to reduce the probability that the symbol is considered properly found. By doing this, the likelihood that an effected symbol or bit negatively impacts processing of the data set is reduced and the likelihood that the symbol will be modified by later processing is increased. Selective scaling circuit  420  provides scaled output  421  to a data decoder circuit  430 . For the purposes of this document, a symbol may be either a non-binary symbol or a binary symbol depending upon the particular implementation. 
     Data decoder circuit  430  applies a data decode algorithm to scaled output  421  to yield a decoded output that includes soft decoded output  432 . In some embodiments of the present invention, data decoder circuit  430  is a low density parity check decoder circuit as are known in the art. Where the decoded output converges (i.e., yields the original data set as indicated by the lack of remaining errors), it is provided as a data output  431 . 
     Turning to  FIG. 5 , a flow diagram  500  shows a method in accordance with some embodiments of the present invention for data processing including feedback based short media defect detection threshold modification. Following flow diagram  500 , it is determined whether a data detector circuit is available to process a newly received data set or a data set that has already been subject to one or more prior global iterations (block  510 ). Where a data detector circuit is available (block  510 ), a processing data input is accessed and it is determined whether a decoded output corresponding to the accessed processing data input exists (block  520 ). Such a decoded output is available as a result from a preceding global iteration applied to the same processing data input. The processing data input may be, for example, derived from a storage medium. Where a corresponding decoded output is not available (block  520 ), a data detection algorithm is applied to the processing data input to yield a detected output (block  525 ). The data detection algorithm may be, but is not limited to, a Viterbi data detection algorithm or a maximum a posteriori data detection algorithm as are known in the art. Based upon the disclosure provided herein, one of ordinary skill in the art will recognize a variety of data detection algorithms that may be used in relation to different embodiments of the present invention. A short media detect detection is applied to the corresponding detected output (Le(i)) and decoded output (La(i)) to yield a defect value (i) (block  527 ). In some embodiments of the present invention, the defect value (i) is a number of consecutive, preceding instances of mismatches between corresponding instances of the detected output and the decoded output. 
     An averaged threshold value is calculated (block  529 ). The averaged threshold value is calculated by averaging a defined number of preceding instances of the defect value (i) generated in block  527 . In one particular embodiment of the present invention, the defined number is twelve. Based upon the disclosure provided herein, one of ordinary skill in the art will recognize a variety of numbers that may be used in relation to different embodiments of the present invention. 
     It is then determined whether the detected value (i) exceeds a default threshold value (block  542 ). In some embodiments of the present invention, the default threshold value is user programmable, while in other embodiments it is fixed. Where the detected value (i) exceeds the default threshold value (block  542 ), a media defect is indicated at the location corresponding to instance i (bock  546 ). Otherwise, where the detected value (i) does not exceed the default threshold value (block  542 ), no defect is indicated. 
     Alternatively, where a corresponding decoded output (La(i)) is available (block  520 ), it is accessed (block  515 ) and a data detection algorithm is applied to the processing data input guided by the corresponding decoded output (La(i)) to yield a detected output (Le(i)) (block  530 ). Such a corresponding decoded output is available for the second or later global iterations (i.e., i&gt;=1) for a given processing data input. The short media detect detection is applied to the corresponding detected output (Le(i)) and decoded output (La(i)) to yield a defect value (i) (block  535 ). Again, in some embodiments of the present invention, the defect value (i) is a number of consecutive, preceding instances of mismatches between corresponding instances of the detected output and the decoded output. 
     An averaged threshold value is calculated (block  537 ). The averaged threshold value is calculated by averaging a defined number of preceding instances of the defect value (i) generated in block  535 . In one particular embodiment of the present invention, the defined number is twelve. Based upon the disclosure provided herein, one of ordinary skill in the art will recognize a variety of numbers that may be used in relation to different embodiments of the present invention. 
     It is then determined whether the previous global iteration processing the currently processing data set indicated a media defect at the location corresponding to instance i (block  540 ). Where a defect was indicated in the preceding global iteration (block  540 ), it is determined whether the defect value (i) exceeds the averaged threshold value calculated during the preceding global iteration (block  544 ). Where the detected value (i) for the current global iteration exceeds the averaged threshold value calculated during the preceding global iteration (block  544 ), a media defect is indicated at the location corresponding to instance i (bock  546 ). Otherwise, where the detected value (i) does not exceed averaged threshold value (block  544 ), no defect is indicated. 
     Otherwise, where a defect was not indicated in the preceding global iteration (block  540 ), it is then determined whether the detected value (i) exceeds the default threshold value (block  542 ). Where the detected value (i) exceeds the default threshold value (block  542 ), a media defect is indicated at the location corresponding to instance i (bock  546 ). Otherwise, where the detected value (i) does not exceed the default threshold value (block  542 ), no defect is indicated. 
     In any case, the detected output is modified at locations corresponding an indicated media defect to yield a modified detected output (block  548 ). This modification may include, but is not limited to, multiplying instances of the detected output corresponding to an indicated media defect by a scaling value less than unity (i.e., scaling factor &lt;1). By doing this, the likelihood that an effected symbol negatively impacts processing of the data set is reduced and the likelihood that the symbol will be modified by later processing is increased. Otherwise, where no defect is indicated, the detected output is left unmodified. As another example, this modification may include replacing the instance of the detected output corresponding to the indicated media defect by a scaled version of the corresponding instance of the decoded output. 
     All local iterations of a data decode algorithm are then applied to the detected output guided by a previous decoded output where available (block  550 ). This process generates an updated decoded output that may be used during subsequent global iterations where it does not converge (i.e., have no remaining unsatisfied checks). In some embodiments of the present invention, the data decode algorithm is a low density parity check algorithm as is known in the art. The decoded output (La(i)) is stored to the prior iteration buffer circuit for later use as a prior iteration decoded output (La(i−1)) (block  552 ). 
     It is determined whether the data decode algorithm converged (block  555 ). Where the data decode algorithm converged (block  555 ), the decoded output is provided as an output codeword, and the next processing data input is selected for processing (block  570 ). Alternatively, where the data decode algorithm failed to converge (block  555 ), it is determined whether another global iteration is allowed (block  560 ). Where another global iteration is not allowed (block  560 ), a failure is indicated (block  561 ) and the decoded output is provided as an output codeword, and the next processing data input is selected for processing (block  570 ). In contrast, where another global iteration is allowed (block  560 ), a subsequent location iteration is applied to the same processing data input guided by the results of the preceding global iteration. 
     It should be noted that the various blocks discussed in the above application may be implemented in integrated circuits along with other functionality. Such integrated circuits may include all of the functions of a given block, system or circuit, or only a subset of the block, system or circuit. Further, elements of the blocks, systems or circuits may be implemented across multiple integrated circuits. Such integrated circuits may be any type of integrated circuit known in the art including, but are not limited to, a monolithic integrated circuit, a flip chip integrated circuit, a multichip module integrated circuit, and/or a mixed signal integrated circuit. It should also be noted that various functions of the blocks, systems or circuits discussed herein may be implemented in either software or firmware. In some such cases, the entire system, block or circuit may be implemented using its software or firmware equivalent. In other cases, the one part of a given system, block or circuit may be implemented in software or firmware, while other parts are implemented in hardware. 
     In conclusion, the invention provides novel systems, devices, methods and arrangements for performing defect detection. While detailed descriptions of one or more embodiments of the invention have been given above, various alternatives, modifications, and equivalents will be apparent to those skilled in the art without varying from the spirit of the invention. For example, one or more embodiments of the present invention may be applied to various data storage systems and digital communication systems, such as, for example, tape recording systems, optical disk drives, wireless systems, and digital subscriber line systems. Therefore, the above description should not be taken as limiting the scope of the invention, which is defined by the appended claims.