# Patent

## Title
Correcting errors in a disk drive read back signals by iterating with the Reed-Solomon decoder

## Abstract

A signal detector includes a detector to generate raw decisions based on a read back signal. A post-processor identifies possible defects in the raw decisions. A selector selects a portion of the possible defects, and generates modified decisions by operating on the portion of the possible defects. A decoder generates final decisions based on the modified decisions. An estimator evaluates the final decisions to determine whether a sequence in the read back signal has been decoded to a correct codeword. In response to the sequence in the read back signal being decoded to the correct codeword, the final decisions are output from the signal detector. In response to the sequence in the read back signal not being decoded to the correct codeword, the selector selects another portion of the possible defects for processing by the selector, the decoder, and the estimator.

## Background
This application is a continuation of U.S. patent application Ser. No. 11/217,349, filed Sep. 2, 2005, now U.S. Pat. No. 7,372,652 which application is a continuation of U.S. patent application Ser. No. 10/135,422, filed Apr. 29, 2002, now U.S. Pat. No. 6,961,197 which claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application Nos. 60/344,407, filed Dec. 28, 2001, and 60/353,756, filed Jan. 30, 2002, the entire contents of each of which are incorporated by reference herein.


BACKGROUND
1. Field of the Invention
This invention relates to a detection scheme for input signals of communication system channels.
2. Background Information
Data stored on magnetic media, such as hard disk drives, is typically encoded using error correction code so that errors that occur in storage and read back of the data might be detected and corrected. In conventional systems, the read back signal is typically detected by a Viterbi detector that makes bit-by-bit (hard) decisions of the read back signal. The decisions from the Viterbi usually include errors. A post-processor may be used in an attempt to correct the errors before sending them to the error correction code decoder. However, during post-processing of the Viterbi decisions, the post-processor sometimes makes mis-corrections leading to potentially more errors in the post-processed signal than in the Viterbi detected signal. A decoder applies an error correction routine to the processed signal to retrieve codewords in the signal. If the processed signal contains too many errors, the decoder will decode the sequence to an incorrect codeword, which may result in a retry being initiated. During a retry, the entire process repeats beginning with the Viterbi detector in an attempt to reduce the number of errors generated in the bit decisions so that after post-processing the errors are low enough for the decoder to generate accurate codewords. Each retry is computationally complex and may consume a significant amount of computation time. In addition, during each retry a different set of errors may occur in the Viterbi decisions and post-processed signal resulting in further retries.
SUMMARY OF THE INVENTION
A signal detector to detect symbols in a read back signal. The signal detector includes a first detector to generate raw decisions as a function of the read back signal. A post processor identifies possible defects in the raw decisions. A selector selects a portion of the possible defects and generates modified decisions based upon correcting the portion of the possible defects. At least one signal decoder generates final decisions as a function of the modified and raw decisions. A decision block returns control to the selector in response to detecting excess errors in the final decisions.



BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and advantages of the present invention will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments, in conjunction with the accompanying drawings, wherein like reference numerals have been used to designate like elements, and wherein:
FIG. 1A is a block diagram of one aspect of an error correction system for read signals from a disk drive.
FIG. 1B is a block diagram of one aspect of an error correction system for noisy input signals.
FIG. 2 is a block diagram of another aspect of an error correction system for read back signals.
FIG. 3 is a flow diagram of one aspect of an error correction system for read back signals.
FIG. 4 is a data graph illustrating the performance improvement of one aspect of an error correction system for read back signals.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1A shows a disk drive system 10 including a read channel device 11 for detecting a read back signal from a hard disk drive 12. Although, the invention may preferably be employed as part of a disk drive system, any communication system is suitable and in particular communication systems having a noisy communication channel. The read channel device 11 may include a signal receiver 13 to receive and condition the read back signal. A signal detector 14 detects and decodes the conditioned read back signal to generate a codeword. The signal detector 14 may attempt to decode a detected symbol multiple times to obtain a correct codeword. An interface circuit 15 coupled to the signal detector 21 outputs the codeword.
FIG. 1B shows a read channel device 16 for detecting an input signal. The read channel device 16 may include a signal preconditioner 17 to receive the input signal. A signal detector 21 detects and decodes the conditioned input signal to generate a codeword. The signal detector 21 may attempt to decode a detected symbol multiple times to obtain a correct codeword. An interface circuit 22 coupled to the signal detector 21 outputs the codeword. Any form of preconditioner 17 may be employed. One example includes a variable gain amplifier (VGA) 18 coupled to an analog-to-digital converter (ADC) 19 and finite-impulse-response filter (FIR) 20.
FIG. 2 shows a signal detector 30 in accordance with the principles of the invention. The signal detector 30 detects and decodes a read back signal from a hard disk drive (not shown). The signal detector 30 includes a detector 32 to generate raw decisions based on the read back signal. The detector 32 may be any Viterbi-like detector including selectable noise-predictive detectors. A post-processor 34 evaluates the raw decisions to determine whether any possible errors are included within the decisions. The post-processor 34 generates a list 36 of the possible defects. The possible defects may be grouped in any manner in the list 36 such as being categorized as either possible errors or possible erasures, or being listed by likelihood of being an error or erasure. A selector 38 selects one or more of the possible defects from the list 36 and applies a correction technique or marks an erasure to generate modified decisions. A decoder 40 decodes the modified decisions to generate final decisions. The decoder 40 is preferably a single Reed-Solomon decoder, however other decoders such as Bose, Chaudhuri, Hocquerghen (BCH) Code, Hamming Code, and Trellis Code decoders may be used. Also, more than one decoder may operate on modified decisions. For example, the selector 38 may select several groupings of errors/erasures and generate modified decisions corresponding to each grouping. Each of the modified decisions may then be sent to separate decoders for decoding. The final decisions from the decoder 40 are evaluated by an estimator 42 to determine whether the sequence in the read back signal has been decoded to the correct codeword.
FIG. 3 shows a detection scheme for a hard disk drive read back signal. At block 50, channel symbols in a read back signal are detected. Preferably, maximum likelihood decisions are made based on the read back signals to generate raw decisions, block 52. Continuing to block 54, the raw decisions are evaluated for possible defects such as possible errors and possible erasures. At block 56, a list of the possible defects is generated. The list may group the defects in any manner such as by possible errors, possible erasures, and by the likelihood of the possible defect being an actual defect. The length of the list may be fixed or variable including limiting the length to a predetermined quantity of possible defects and including all possible defects that exceed a likelihood threshold. Continuing to block 58, a subset of the possible defects associated with the raw decisions is selected. The subset of possible defects may include possible errors, possible erasures, or a combination of possible errors and erasures. At block 60, the subset of possible defects and corresponding raw decisions are operated on to attempt to correct the possible errors and mark the possible erasures. Continuing to block 62, modified decisions are generated as a function of operating on the subset of possible defects and corresponding raw decisions. At block 64, the modified decisions are decoded to generate final decisions. At decision block 66, the final decisions are evaluated to determine whether the sequence was decoded to a correct codeword. If the decode is successful and the correct codeword is decoded, the final decisions are output. But, if a wrong codeword is decoded, control is transferred to block 58, and another subset of possible defects is selected. The detection scheme may continue to loop through blocks 58 to 66 until a correct codeword is decoded, the loop is executed a predetermined number of times, or until a specified threshold is reached such as computation time. Computation speed is enhanced by not looping back to the detector to eliminate errors by redetecting the sequence within the read back signal. Instead, the detection scheme operates on the originally detected sequence and uses the selector and decoder to correct or mark subsets of the defects to obtain a correct codeword.
As an example of one implementation, the 12 most-likely defects in the raw decisions may be listed and the selector may correct/mark up to 3 defects at a time from the list of 12 defects. In this case, the decoder may need to perform up to 299 decoding attempts, with an average number of decoding attempts of about 150.
FIG. 4 shows a comparison simulation of the sector error rate for a conventional detector 70 versus a detector 72 in accordance with the principles of the invention. For the simulation, it is assumed the decoder employs a Reed-Solomon Code with 10-bit symbols and can correct 3 symbol errors. The simulation is performed with a user bit density (UBD) of 2.747 and 100% additive white Gaussian noise.
A number of embodiments of the invention have been described. The invention may be implemented in any known means including hardware, software, firmware, middleware and combinations thereof. It will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.

## Claims

1. A signal detector, comprising:
a detector to generate raw decisions based on a read back signal;
a post-processor to identify possible defects in the raw decisions;
a selector to
select a portion of the possible defects, and
generate modified decisions by operating on the portion of the possible defects;

a decoder to generate final decisions based on the modified decisions; and
an estimator to evaluate the final decisions to determine whether a sequence in the read back signal has been decoded to a correct codeword, wherein
in response to the sequence in the read back signal being decoded to the correct codeword, the final decisions are output from the signal detector, and
in response to the sequence in the read back signal not being decoded to the correct codeword, the selector selects another portion of the possible defects for processing by the selector, the decoder, and the estimator.

2. The signal detector of claim 1, wherein the possible defects include possible errors and possible erasures.

3. The signal detector of claim 2, wherein:
the post-processor generates a list of possible defects based on an evaluation of the raw decisions; and
the selector selects the portion of the possible defects from the list of possible defects.

4. The signal detector of claim 1, wherein the detector includes a Viterbi detector.

5. The signal detector of claim 1, wherein the decoder comprises a Reed Solomon decoder, a Trellis Code decoder, a Hamming Code decoder, or a Bose, Chaudhuri, Hocquerghen (BCH) Code decoder.

6. The signal detector of claim 1, wherein the detector searches for a predetermined number of possible defects in the raw decisions.

7. The signal detector of claim 2, wherein the selector operating on the portion of the possible defects includes:
the selector correcting the possible errors and marking the possible erasures.

8. The signal detector of claim 2, wherein the selector marks a predetermined number of possible errors as possible erasures.

9. A read channel device comprising:
the signal detector of claim 1;
an interface circuit that communicates with the decoder; and
a signal preconditioner that communicates with the interface circuit.

10. A disk drive comprising:
the signal detector of claim 1;
a medium to store information; and
a receiver to
read the information, and
generate the read back signal based on the information read from the medium.

11. A method for operating a signal detector, the method comprising:
generating raw decisions based on a read back signal and identifying possible defects in the raw decisions;
selecting a portion of the possible defects;
generating modified decisions by operating on the portion of the possible defects;
generating final decisions based on the modified decisions; and
evaluating the final decisions to determine whether a sequence in the read back signal has been decoded to a correct codeword, wherein
in response to the sequence in the read back signal being decoded to the correct codeword, outputting the final decisions from the signal detector, and
in response to the sequence in the read back signal not being decoded to the correct codeword, selecting another portion of the possible defects for generating second modified decisions, generating second final decisions, and evaluating the second final decisions.

12. The method of claim 11, wherein the possible defects include possible errors and possible erasures.

13. The method of claim 12, further comprising:
generating a list of possible defects based on an evaluation of the raw decisions,
wherein selecting a portion of the possible defects comprises selecting the portion of the possible defects from the list of possible defects.

14. The method of claim 11, wherein generating raw decisions includes using a Viterbi detector to generate the raw decisions.

15. The method of claim 11, wherein generating final decisions comprises decoding the modified decisions using a Reed Solomon decoder, a Hamming Code decoder, Trellis Code decoder, or a BCH Code decoder.

16. The method of claim 12, wherein operating on the portion of the possible defects includes:
correcting the possible errors and marking the possible erasures.

17. The method of claim 11, further comprising searching for a predetermined number of possible defects in the raw decisions.

18. The method of claim 16, further comprising selecting a predetermined number of the possible errors for correction.

19. The method of claim 16, further comprising marking a predetermined number of possible errors as possible erasures.