Abstract:
Methods and apparatus are provided for improved physical re-read operations in a hard disk drive. The disclosed methods and apparatus selectively retain data in a hard disk drive. A signal is read in an iterative read channel by assigning a reliability metric to each of a plurality of segments in a read signal; repeating the assigning step for a plurality of read operations; and selectively retaining the segments based on the assigned reliability metric. The read signal can be obtained by positioning a transducer over a storage media. The reliability metric may be based on soft bit decisions; log likelihood ratios or a noise estimation of a given segment.

Description:
FIELD OF THE INVENTION 
     The present invention is related to techniques for re-try operations in a hard disk drive and, more particularly, to techniques for physical re-read operations in such hard disk drives. 
     BACKGROUND OF THE INVENTION 
     In storage systems, as tracks and sectors are spaced closer together and as the spin rates produced by the spindle motor are increased, the likelihood of an error occurring while reading the storage media increase significantly. Storage systems employ a number of different techniques to eliminate or reduce such errors that may occur when reading data from a storage media. Most error detection and correction techniques in storage systems employ redundant information and special hardware. Error correction codes, for example, such as Low Density Parity Check (LDPC) codes, are often used to detect and correct such errors. LDPC codes are block codes based on a parity check matrix, H. See, for example, R. G. Gallagher, “Low-Density Parity-Check Code,” IEEE Trans. Inform. Theory, vol. IT-8, 21-28 (January 1962). 
     When an error is encountered during a read operation, a retry operation is typically implemented, whereby a signal may be re-read from a buffer or from the storage media itself (or both). When a reread operation is performed from the storage media, the operating conditions may be varied, for example, by repositioning the transducer, to obtain a different signal. The reread operations typically continue until the erroneous data is correctly read from the storage media or until a predefined number of retry operations have been performed. 
     In one existing retry technique, an “on-the-fly” soft channel detector performs the initial read operation. If the “on-the-fly” detector encounters an error, a second soft channel detector may process buffered samples to perform an electronic retry that typically employs additional iterations than the “on-the-fly” detector. If the electronic retry fails, a physical re-read operation will be initiated, whereby the transducer is repositioned over the target area to obtain a new signal. 
     A need exists for improved physical re-read operations in a hard disk drive. A further need exists for improved methods and apparatus for selective data retention decoding in a hard disk drive. 
     SUMMARY OF THE INVENTION 
     Generally, methods and apparatus are provided for improved physical re-read operations in a hard disk drive. According to one aspect of the invention, the disclosed methods and apparatus selectively retain data in a hard disk drive. A signal is read in an iterative read channel by assigning a reliability metric to each of a plurality of segments in a read signal; repeating the assigning step for a plurality of read operations; and selectively retaining the segments based on the assigned reliability metric. For example, the read signal can be obtained by positioning a transducer over a storage media. 
     The reliability metric may be based on, for example, soft bit decisions associated with a given segment; log likelihood ratios associated with a given segment; or a noise estimation of a given segment. The noise estimation can be obtained, for example, by subtracting a reference signal from the segments. 
     A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of an exemplary conventional LDPC code based iterative hard disk drive; and 
         FIG. 2  is a schematic block diagram of an exemplary LDPC based iterative hard disk drive  200  incorporating features of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides improved techniques for physical re-read operations in a hard disk drive. According to one aspect of the present invention, during a physical re-read operation, the hard disk drive can selectively retain the most reliable signal segments from among multiple read operations. In one exemplary embodiment, a reliability metric assigned to each segment identifies the most reliable signal segments. 
       FIG. 1  is a schematic block diagram of an exemplary conventional LDPC code based iterative hard disk drive  100 . As shown in  FIG. 1 , an analog front end  110  generates equalized samples, Y. For example, the equalized samples, Y, may be obtained after a finite impulse response (FIR) filtering. For a detailed discussion of conventional iterative decoding techniques, see for example, E. Yeo et al., “VLSI Architectures for Iterative Decoders in Magnetic Recording Channels,” IEEE Trans. on Magnetics, Vol. 37, No. 2, March 2001, incorporated by reference herein. 
     A first soft channel detector  120 , often referred to as an “on-the-fly” detector, processes the equalized samples, referred to as Y New , and generates a series of reliability information for each Y sample in the form of soft bit decisions, such as log likelihood ratios (LLRs). The LLR values are written to a memory  140  and processed by an LPDC decoder  160 , in a known iterative manner, to generate hard decisions that are stored in a memory  180 . Generally, a hard bit decision is obtained based on the sign of the LLR value. Likewise, the magnitude of the LLR value provides an indication of the reliability of the decision. 
     As previously indicated, if the on-the-fly detection fails, a second soft channel detector  130  will process the buffered Y samples, obtained from a memory  125 , to perform an electronic retry operation. The electronic retry operation typically comprises more iterations between the second soft channel detector  130  and the LPDC decoder  160  than employed by the first soft channel detector  120  during the on-the fly detection. As shown in  FIG. 1 , the LPDC decoder  160  stores decoded information in an external memory  150  that is employed by the second soft channel detector  130  during the electronic retry operation. 
     If the electronic retry operation fails, the hard disk drive  100  will perform a physical re-read operation. During a physical re-read operation, the head or transducer (not shown) is re-positioned over the target area to read the signal from the storage media again. While conventional physical re-read techniques read an entirely new signal from the storage media, the present invention selectively retains portions of the signal, based on a reliability metric. 
     Selective Data Retention During Physical Re-Read 
     According to one aspect of the present invention, during a physical re-read operation, the hard disk drive can selectively retain the most reliable signal segments from among multiple read operations. In one exemplary embodiment, a reliability metric is used to identify the most reliable signal segments. A reliability metric can be assigned to each signal segment for multiple read operations and the more reliable signal segments are selected by comparing the reliability metric values of each segment. In this manner, the present invention selectively retains signal segments based on a signal quality or reliability value assigned to each segment. For example, a signal segment may comprise N samples of the Y signal, such as N equal to 32 or 16 samples. According to one exemplary hard disk format, the number of samples per sector is on the order of 5,000. 
       FIG. 2  is a schematic block diagram of an exemplary LDPC based iterative hard disk drive  200  incorporating features of the present invention. The analog front end  210 , soft channel detectors  220 ,  230 , memories  240 ,  250 , and LPDC decoder  260 , operate in a similar manner to the corresponding elements in  FIG. 1 , discussed above. As shown in  FIG. 2 , the iterative hard disk drive  200  also includes a multiplexer  270  controlled by a selection signal generated by a selection circuit  280 . The selection signal determines whether the multiplexer  270  selects a new signal, Y new , or a retained signal, Y old , read from the Y memory  225 , based on the reliability metric assigned to the segment. 
     Generally, the selection signal selects the new signal segment, Y new , or the retained signal segment, Y old , for storage in the Y memory  225  for subsequent use based on the the reliability metric assigned to each segment. In one exemplary embodiment, after each re-read operation and for each segment in the re-read signal, the new signal, Y new , or the retained signal, Y old , is stored in the Y memory  225  with the corresponding reliability metric assigned to the selected segment. In this manner, the present invention selectively retains the signal segments based on the assigned reliability metric. 
     While the present invention is illustrated using LDPC decoding techniques, the present invention may apply to turbo codes and other iterative codes in a hard disk drive that processes soft (reliability) information in an iterative manner, as would be apparent to a person of ordinary skill in the art. 
     As shown in  FIG. 2 , the LPDC decoder  260  also generates a reference signal. Generally, the reference signal is a reliability indicator based on the noise in the signal samples. In one exemplary embodiment, the reference signal is the convolution of the hard decisions generated by the LPDC decoder  260  with the partial response target. 
     Exemplary Reliability Metrics 
     As indicated above, the reliability value assigned to each segment provides an indication of the reliability of the corresponding segment. As previously indicated, the soft channel detectors  220 ,  230  process the equalized samples, Y New  or Y Old , based on the selection signal and generate a series of reliability information for each Y sample in the form of soft bit decisions, such as LLRs. In one exemplary embodiment, the reliability value assigned to each segment can be based on the soft bit decisions or LLRs associated with each sample in the segment. For example, the reliability value, R Si , for each segment, S i , can be expressed as follows:
 
 R   Si =Σabs(soft bit decisions) or Σabs(LLR),
 
where the sum is over the samples in a given segment. Generally, when based on the soft bit decisions or LLRs, the larger the reliability value, R Si , the more reliable the segment.
 
     In a further variation, the reliability value, R Si , for each segment, S i , can be based on the reference signal generated by the LPDC decoder  260 , as follows:
 
 R   Si =Σabs( Y −reference signal),
 
where the sum is again over the samples in a given segment and the larger the reliability value, R Si , the less reliable the segment (since more noise indicates a less reliable signal).
 
     As previously indicated, after each re-read operation and for each segment in the re-read signal, the new signal, Y new , or the retained signal, Y old , is stored in the Y memory  225  with the corresponding reliability metric assigned to the selected segment. In this manner, the present invention selectively retains the signal segments based on the assigned reliability metric. The final signal sets stored in the Y memory  225  may contain segments read in different retry attempts. After each read operation, the iterative hard disk drive  200  detects and decodes the signal based on the final set of signal segments stored in the Y memory  225 . 
     CONCLUSION 
     While exemplary embodiments of the present invention have been described with respect to digital logic blocks, as would be apparent to one skilled in the art, various functions may be implemented in the digital domain as processing steps in a software program, in hardware by circuit elements or state machines, or in combination of both software and hardware. Such software may be employed in, for example, a digital signal processor, micro-controller, or general-purpose computer. Such hardware and software may be embodied within circuits implemented within an integrated circuit. 
     Thus, the functions of the present invention can be embodied in the form of methods and apparatuses for practicing those methods. One or more aspects of the present invention can be embodied in the form of program code, for example, whether stored in a storage medium, loaded into and/or executed by a machine, or transmitted over some transmission medium, wherein, when the program code is loaded into and executed by a machine, such as a computer, the machine becomes an apparatus for practicing the invention. When implemented on a general-purpose processor, the program code segments combine with the processor to provide a device that operates analogously to specific logic circuits. The invention can also be implemented in one or more of an integrated circuit, a digital signal processor, a microprocessor, and a micro-controller. 
     A plurality of identical die are typically formed in a repeated pattern on a surface of the wafer. Each die includes a device described herein, and may include other structures or circuits. The individual die are cut or diced from the wafer, then packaged as an integrated circuit. One skilled in the art would know how to dice wafers and package die to produce integrated circuits. Integrated circuits so manufactured are considered part of this invention. 
     It is to be understood that the embodiments and variations shown and described herein are merely illustrative of the principles of this invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.