Method for evaluating the quality of read signal and apparatus for reading information

Abstract

A most probable path is selected from a number n (where n≧2) of paths of state transitions occurring from a first state S k−3 (where k≧3 and j≧2) at a time k−j into a second state S k at a time k. The method includes the steps of detecting predetermined combinations of the first and second states S k−j and S k defining the n probable paths in a predetermined period j between the times k−j and k, and evaluating the reliability of a read signal, decoded in the period j, by |Pa−Pb|. Pa and Pb indicate the probabilities of state transition of first and second state transition paths in the period j. The first and second state transition paths are estimated to be the most probable and the second most probable, respectively, among the n probable paths defined by the combinations detected.

What is claimed is:  
     
         1 . A method for evaluating the quality of a read signal that has been decoded by a maximum likelihood decoding method, in which a most probable state transition path is selected from a number n (where n is an integer equal to or greater than two) of state transition paths that represent n probable transitions from a first state S k−j  (where k is an integer equal to or greater than three and j is an integer equal to or greater than two) at a time k−j into a second state S k  at a time k, the method comprising the steps of: 
 (a) detecting predetermined combinations of the first and second states S k−j  and S k  that define the n probable state transition paths in a predetermined period j between the times k−j and k; and  
 (b) evaluating the reliability of the decoded signal, which has been obtained in the predetermined period j, by using |Pa−Pb|, where Pa and Pb are indices indicating the respective probabilities of state transition of first and second state transition paths in the predetermined period j, the first and second state transition paths being estimated to be the most probable and the second most probable, respectively, among the n probable state transition paths that are defined by the predetermined combinations that have been detected in the step (a).  
 
     
     
         2 . The method of  claim 1 , wherein the step (b) includes the steps of: 
 defining the index Pa by differences between expected values shown by the first state transition path and actual sample values in the predetermined period j; and    defining the index Pb by differences between expected values shown by the second state transition path and the actual sample values in the predetermined period j.    
     
     
         3 . The method of  claim 2 , wherein the step (b) includes the steps of: 
 obtaining the index Pa as a sum of squares of differences between the expected values l k−j , . . . , l k−1  and l k  shown by the first state transition path and the actual sample values y k−j , . . . , y k−1  and y k  in the predetermined period j; and    obtaining the index Pb as a sum of squares of differences between the expected values m k−j , . . . , m k−1  and m k  shown by the second state transition path and the actual sample values y k−j , . . . , y k−1  and y k  in the predetermined period j.    
     
     
         4 . The method of  claim 1 , wherein the number n is two.  
     
     
         5 . The method of  claim 1 , wherein a Euclidean distance between the first and second state transition paths is a minimum value.  
     
     
         6 . The method of  claim 1 , further comprising the step of detecting a variation in the reliability of the decoded signal by measuring |Pa−Pb| a number of times.  
     
     
         7 . The method of  claim 6 , wherein the step of detecting the variation in the reliability includes the step of deriving a standard deviation of a |Pa−Pb| distribution as the variation.  
     
     
         8 . The method of  claim 6 , wherein the step of detecting the variation in the reliability includes the step of deriving a standard deviation and an average of a |Pa−Pb| distribution as the variation.  
     
     
         9 . The method of  claim 6 , wherein the step of detecting the variation in the reliability includes the step of detecting a frequency of occurrence at which |Pa−Pb| exceeds a predetermined range.  
     
     
         10 . The method of  claim 1 , further comprising the step of decoding a read signal in which a recorded code has a minimum polarity inversion interval of two and which has been subjected to a PR (C0, C1, C0) equalization.  
     
     
         11 . The method of  claim 1 , further comprising the step of decoding a read signal in which a recorded code has a minimum polarity inversion interval of two and which has been subjected to a PR (C0, C1, C1, C0) equalization.  
     
     
         12 . The method of  claim 1 , further comprising the step of decoding a read signal in which a recorded code has a minimum polarity inversion interval of two and which has been subjected to a PR (C0, C1, C2, C1, C0) equalization.  
     
     
         13 . The method of  claim 2 , wherein the step (b) includes the step of obtaining |Pa−Pb| without calculating squares of the actual sample values.  
     
     
         14 . An apparatus for reading information, comprising: 
 a gain controller for adjusting an amplitude value of a read signal;    a first waveform equalizer for shaping the waveform of the read signal so that the read signal has a predetermined equalization characteristic;    a read clock signal generator for generating a read clock signal that is synchronized with the read signal;    an A/D converter for generating and outputting sampled data by sampling the read signal in response to the read clock signal;    a maximum likelihood decoder for decoding the sampled data into most likely digital information; and    a differential metric calculator for obtaining |Pa−Pb|, where Pa and Pb are indices indicating respective probabilities of state transition of first and second state transition paths in a predetermined period, the first and second state transition paths being estimated by the maximum likelihood decoder to be the most probable and the second most probable, respectively.    
     
     
         15 . The apparatus of  claim 14 , further comprising a second waveform equalizer for shaping the waveform of the read signal differently from the first waveform equalizer so that the read signal has another predetermined equalization characteristic, 
 wherein the read clock signal is generated from the read signal that has had its waveform shaped by the second waveform equalizer.