US20030067998A1 - Method for evaluating the quality of read signal and apparatus for reading information - Google Patents

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

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US20030067998A1
US20030067998A1 US10/198,604 US19860402A US2003067998A1 US 20030067998 A1 US20030067998 A1 US 20030067998A1 US 19860402 A US19860402 A US 19860402A US 2003067998 A1 US2003067998 A1 US 2003067998A1
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state transition
read signal
probable
paths
signal
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Takeshi Nakajima
Harumitsu Miyashita
Hiromichi Ishibashi
Shigeru Furumiya
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Panasonic Corp
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Matsushita Electric Industrial Co Ltd
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    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B20/00—Signal processing not specific to the method of recording or reproducing; Circuits therefor
    • G11B20/10—Digital recording or reproducing
    • G11B20/18—Error detection or correction; Testing, e.g. of drop-outs
    • G11B20/1816—Testing

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  • the present invention relates to a method for evaluating the quality of a digital signal that has been read out from a storage medium and then decoded by a maximum likelihood decoding technique, and also relates to an apparatus for reading information from a storage medium and performing such quality evaluation on the read signal.
  • HDD hard disk drive
  • optical disk drive optical disk drive
  • magneto-optical disk drive for reading digital information from a storage medium
  • FIG. 1 is a block diagram showing a configuration for a part of a conventional optical disk drive 900 .
  • a light beam that has been reflected from an optical disk 1 is converted by an optical head 2 into a read signal.
  • the read signal has its waveform shaped by a waveform equalizer 3 and then digitized by a comparator 4 .
  • the threshold value of the comparator 4 is normally subjected to a feedback control so that the output digital signals of the comparator 4 equals zero when integrated together.
  • a phase-locked loop (PLL) circuit In the optical disk drive 900 , normally a phase-locked loop (PLL) circuit generates a clock signal that is synchronized with a read signal.
  • a clock signal of that type is termed “a read clock signal”.
  • the PLL circuit includes a phase detector 5 , a low-pass filter (LPF) 6 and a voltage controlled oscillator (VCO) 7 .
  • the phase detector 5 detects a difference in phase between the output digital signal of the comparator 4 and an output clock signal of the VCO 7 .
  • the phase difference detected is averaged by the LPF 6 .
  • the control voltage of the VCO 7 is determined.
  • the oscillation frequency of the VCO 7 is subjected to a feedback control so that the phase difference output from the phase detector 5 always equals zero.
  • the VCO 7 can output a clock signal that is synchronized with the read signal.
  • the read clock signal is used to determine whether the recorded code (i.e., digital information) is one or zero. More specifically, the digital information can be read out by determining whether or not each detection pulse of the comparator 4 falls within a window width defined by the read clock signal.
  • the “detection pulse” of the comparator 4 refers to a portion of the output digital signal of the comparator 4 that exceeds the predetermined threshold value.
  • the output detection pulse of the comparator 4 might deviate from the window width of the read clock signal due to various factors including intersymbol interference occurring in the read signal, the distortion of a recording mark, circuit noise and a control residual of the PLL. In that case, an error occurs.
  • Such a time lag created between the detection pulse of the comparator 4 and the read clock signal is called a “jitter”.
  • the quality (which is represented in terms of an error rate) of the read signal can be evaluated by using the distribution of jitter.
  • the jitter distribution may be supposed to form a normal distribution having a mean of zero.
  • ⁇ is the standard deviation of the jitter distribution that is supposed to be a normal distribution and Tw is the window width.
  • FIG. 2 is a graph showing a relationship between the jitter and the bit error rate (BER).
  • BER bit error rate
  • the jitter of a read signal can be actually measured with a time interval analyzer (TIA). Accordingly, even if no errors have actually occurred, the quality of the signal can also be evaluated by the jitter standard deviation ⁇ per the window width Tw. Thus, it is possible to predict the probability of occurrence of errors (which will be herein referred to as an “error probability”). For that reason, by measuring the standard deviation of the jitter, the performance of a given drive, a storage medium or an optical head can be checked and tested. Also, if the parameters of an equalizer are controlled in such a manner as to decrease the standard deviation of the jitter, then a read operation can be performed even more constantly.
  • digital information is directly obtained from the output digital signal of the comparator 4 .
  • digital information may also be obtained by a maximum likelihood decoding method.
  • known maximum likelihood decoding methods include a partial response maximum likelihood (PRML) method.
  • PRML partial response maximum likelihood
  • data is read or written from/on a storage medium having a high storage capacity with the potential occurrence of intersymbol interference fully taken into account. More specifically, a signal that has been read out from such a high-capacity storage medium is subjected, by a waveform equalizer, a digital filter and so on, to a partial response equalization so as to have a predetermined frequency characteristic.
  • the PR equalized and filtered signal is decoded into most likely (or most probable) digital data by a Viterbi decoding technique, for example.
  • data can be decoded at a low error rate even from a read signal with a low signal-to-noise ratio (SNR) or a read signal that is affected by the intersymbol interference relatively seriously.
  • SNR signal-to-noise ratio
  • y 1 is the actual value of the read signal (or digital sample data) at a time i and level v is an expected ideal value of the read signal.
  • a state transition path having the minimum probability quantity as represented by Equation (3) is selected.
  • a Euclidean distance of (y k -level v ) 2 is obtained from the data that is sampled at each point in time k by reference to a read clock signal according to the maximum likelihood decoding method.
  • the data is decoded based on the Euclidean distance. Accordingly, the decoded result obtained by the maximum likelihood decoding method is also affected by a past sampled value y k of a read signal.
  • a method for evaluating the quality of a signal that has been decoded by the maximum likelihood decoding method is disclosed in Japanese Laid-Open Publication No. 10-21651, for example.
  • the apparatus disclosed in Japanese Laid-Open Publication No. 10-21651 obtains a difference in likelihood between two state transition paths, having a minimum Euclidean distance between them, and then processes this difference by a statistical method, thereby evaluating the quality of the signal.
  • preferred embodiments of the present invention provide a method and apparatus for evaluating the quality of a read signal by using indices that are correlated with the error rate of digital data decoded by the maximum likelihood decoding method.
  • a preferred embodiment of the present invention provides a method for evaluating the quality of a read signal that has been decoded by a maximum likelihood decoding method.
  • a most probable state transition path is preferably 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 preferably includes the step of 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.
  • the method preferably further includes the step of evaluating the reliability of the decoded signal, obtained in the predetermined period j, by using
  • 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 are 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 detected.
  • the step of evaluating the reliability preferably 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.
  • the step of evaluating the reliability preferably 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.
  • the number n is preferably two.
  • a Euclidean distance between the first and second state transition paths is preferably a minimum value.
  • the method preferably further includes the step of detecting a variation in the reliability of the decoded signal by measuring
  • the step of detecting the variation in the reliability may include the step of deriving a standard deviation of a
  • the step of detecting the variation in the reliability may include the step of deriving a standard deviation and an average of a
  • the step of detecting the variation in the reliability may include the step of detecting a frequency of occurrence at which
  • the method may further include 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.
  • the method may further include 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.
  • the method may further include 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.
  • the step of evaluating the reliability may include the step of obtaining
  • the apparatus preferably includes gain controller, first waveform equalizer, read clock signal generator, A/D converter, maximum likelihood decoder and differential metric calculator.
  • the gain controller preferably adjusts an amplitude value of a read signal.
  • the first waveform equalizer preferably shapes the waveform of the read signal so that the read signal has a predetermined equalization characteristic.
  • the read clock signal generator preferably generates a read clock signal that is synchronized with the read signal.
  • the A/D converter preferably generates and outputs sampled data by sampling the read signal in response to the read clock signal.
  • the maximum likelihood decoder preferably decodes the sampled data into most likely digital information.
  • the differential metric calculator preferably obtains
  • 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 are estimated by the maximum likelihood decoder to be the most probable and the second most probable, respectively.
  • the apparatus preferably further includes 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.
  • the read clock signal is preferably generated from the read signal that has had its waveform shaped by the second waveform equalizer.
  • FIG. 1 is a block diagram illustrating a configuration for a conventional optical disk drive.
  • FIG. 2 is a graph showing a relationship between the jitter and the bit error rate (BER).
  • FIG. 3 is a state transition diagram that is defined by the constraints, including a minimum polarity inversion interval of two and the use of a PR (1, 2, 2, 1) equalization technique, according to a preferred embodiment of the present invention.
  • FIG. 4 is a trellis diagram that is defined by the constraints, including the minimum polarity inversion interval of two and the use of the PR (1, 2, 2, 1) equalization technique, in the preferred embodiment of the present invention.
  • FIG. 5 is a diagram showing two possible state transition paths between states SO k and SO k ⁇ 5 that are extracted from the trellis diagram shown in FIG. 4.
  • FIGS. 6A and 6B are graphs schematically showing the distributions of the reliability Pa ⁇ Pb of the decoded result.
  • FIG. 7 is a block diagram illustrating a configuration for an optical disk drive as an exemplary apparatus for evaluating the quality of a read signal according to a third specific preferred embodiment of the present invention.
  • FIG. 8 is a block diagram illustrating detailed configurations of the Viterbi circuit and differential metric analyzer of the optical disk drive shown in FIG. 7.
  • FIG. 9 is a diagram illustrating a detailed configuration of the path memory of the optical disk drive shown in FIG. 7.
  • FIG. 10 is a block diagram illustrating a configuration for another optical disk drive according to the third preferred embodiment.
  • FIG. 11 is a block diagram illustrating a configuration for still another optical disk drive according to the third preferred embodiment.
  • FIG. 12 is a block diagram illustrating a configuration for yet another optical disk drive according to the third preferred embodiment.
  • FIG. 13 is a block diagram illustrating a configuration for an optical disk drive according to a fourth specific preferred embodiment of the present invention.
  • FIG. 14 is a block diagram illustrating a configuration for another optical disk drive according to the fourth preferred embodiment.
  • FIG. 15 is a block diagram illustrating a configuration for still another optical disk drive according to the fourth preferred embodiment.
  • FIG. 16 is a graph showing a relationship between the PRML error index MLSA and the bit error rate (BER).
  • FIG. 17 is a state transition diagram that is defined by the constraints, including a minimum polarity inversion interval of two and the use of a PR (C0, C1, C0) equalization technique, according to another preferred embodiment of the present invention.
  • FIG. 18 is a trellis diagram that is defined by the constraints, including the minimum polarity inversion interval of two and the use of the PR (C0, C1, C0) equalization technique, in the preferred embodiment of the present invention.
  • FIG. 19 is a state transition diagram that is defined by the constraints, including a minimum polarity inversion interval of two and the use of a PR (C0, C1, C2, C1, C0) equalization technique, according to another preferred embodiment of the present invention.
  • FIG. 20 is a trellis diagram that is defined by the constraints, including the minimum polarity inversion interval of two and the use of the PR (C0, C1, C2, C1, C0) equalization technique, in the preferred embodiment of the present invention.
  • a read signal quality evaluating method according to a preferred embodiment of the present invention will be described.
  • a code having a minimum polarity inversion interval of two e.g., a code defined by a ( 1 , 7 ) RLL modulation method
  • the recorded code is used as the recorded code. That is to say, any recorded code always has two or more consecutive zeros or ones.
  • a signal is supposed to be decoded by a PRML method in which the frequency characteristics of read and write systems substantially correspond to a PR (1, 2, 2, 1) equalization characteristic as a whole.
  • PRML method the frequency characteristics of read and write systems substantially correspond to a PR (1, 2, 2, 1) equalization characteristic as a whole.
  • a state at the time k is represented by S (b k ⁇ 2 , b k ⁇ 1 , b k ) and a state at the previous time k ⁇ 1 is represented by S (b k ⁇ 3 , b k ⁇ 2 , b k ⁇ 1 ).
  • the following Table 1 is a table of state transitions that is compiled by obtaining possible combinations of states at the times k ⁇ 1 and k.
  • a modulation technique that defines the minimum inversion interval at two (i.e., at least two zeros or ones appear consecutively) is adopted in this preferred embodiment.
  • the states S (0, 0, 0) k , S (0, 0, 1) k , S (0, 1, 1) k , S (1, 1, 1) k , S (1, 1, 0) k , S (1, 0, 0) k and so on at the time k will be identified by S 0 k , S 1 k , S 2 k , S 3 k , S 4 k , S 5 k and so on, respectively, for the sake of simplicity.
  • the state transitions that may occur in the period between the time k ⁇ 1 and the time k are represented by the state transition diagram shown in FIG. 3.
  • the state transition diagram shown in FIG. 3 When the state transition diagram shown in FIG. 3 is expanded with respect to the time axis, the trellis diagram shown in FIG. 4 is obtained.
  • Level v is supposed to be any value between ⁇ 3 and 3. That is to say, the Level v values of ⁇ 3, ⁇ 2, ⁇ 1, 0, 1, 2 and 3 correspond to Level 0 , Level 1 , Level 2 , Level 3 , Level 4 , Level 5 , and Level 6 , respectively.
  • the sum of squared errors Pa obtained in this manner is an index indicating the probability of state transitions of the path A in the predetermined period between the times k ⁇ 5 and k. That is to say, the smaller the Pa value, the more probable the path A will be.
  • the sum of squared errors Pb obtained in this manner is an index indicating the probability of state transitions of the path B in the predetermined period between the times k ⁇ 5 and k. That is to say, the smaller the Pb value, the more probable the path B will be. Also, if the Pa or Pb value is zero, then the path A or B is estimated to be the most probable one.
  • Pa ⁇ Pb the difference between the Pa and Pb values means.
  • a maximum likelihood decoder does not hesitate to choose the path A if Pa ⁇ Pb or the path B if Pa>>Pb.
  • the Pa ⁇ Pb value may be used as a measure of the reliability of the decoded result. That is to say, the greater the absolute value of Pa ⁇ Pb, the higher the reliability of the decoded result should be. On the other hand, the closer to zero the absolute value of Pa ⁇ Pb, the lower the reliability of the decoded result should be.
  • the Pa ⁇ Pb value corresponding to the zero Pa value will be herein identified by ⁇ Pstd and the Pa ⁇ Pb value corresponding to the zero Pb value will be herein identified by Pstd.
  • Pstd is subtracted from the absolute value of Pa ⁇ Pb (i.e., when
  • the distribution shown in FIG. 6B is obtained.
  • the standard deviation ⁇ and the average Pave of the distribution are obtained.
  • the standard deviation ⁇ and average Pave of this distribution may be used to estimate a bit error rate. For example, if the estimated
  • 0 is zero), decoding errors may be regarded as occurring at a frequency of occurrence that corresponds to the probability at which the function becomes zero or less. In that case, the error probability P ( ⁇ , Pave) may be defined by the following Equation (7) using the standard deviation ⁇ and the average Pave:
  • the error rate of the digital decoded result obtained by a maximum likelihood decoding method can be estimated by using the average Pave and the standard deviation ⁇ that have been derived from the distribution of Pa ⁇ Pb.
  • the average Pave and the standard deviation ⁇ may be used as indices to the quality of the read signal.
  • distribution is supposed to be a normal distribution. But if it is difficult to regard the
  • a state transition has occurred from a first predetermined state (e.g., S 0 k ⁇ 5 ) into a second predetermined state (e.g., S 0 k ) during a predetermined period
  • between the probabilities of two possible paths in the predetermined period is calculated, thereby evaluating the reliability of the decoded result.
  • of the decoded result can be obtained. In this manner, the quality of the read signal can be evaluated (i.e., the bit error rate of the read signal can be estimated).
  • Pa ⁇ Pb ( A k ⁇ 4 ⁇ B k ⁇ 4 )+( A k ⁇ 3 ⁇ D k ⁇ 3 )+( A k ⁇ 2 ⁇ E k ⁇ 2 )+( A k ⁇ 1 ⁇ D k ⁇ 1 )+( B k ⁇ C k ); (8.2)
  • Pa ⁇ Pb (A k ⁇ 3 ⁇ B k ⁇ 3 )+( B k ⁇ 2 ⁇ D k ⁇ 2 )+( D k ⁇ 1 ⁇ F k ⁇ 1 )+( E k ⁇ F k ); (8.3)
  • Pa ⁇ Pb ( F k ⁇ 3 ⁇ G k ⁇ 3 )+( D k ⁇ 2 ⁇ F k ⁇ 2 )+( B k ⁇ 1 ⁇ D k ⁇ 1 )+( A k ⁇ B k ); (8.13)
  • a k (y k ⁇ 0) 2
  • B k (y k ⁇ 1) 2
  • C k (y k ⁇ 2) 2
  • D k (y k ⁇ 3) 2
  • E k (y k ⁇ 4) 2
  • F k (y k ⁇ 5) 2
  • G k (y k ⁇ 6) 2 .
  • Pa ⁇ Pb ( B k ⁇ 4 ⁇ C k ⁇ 4 )+( A k ⁇ 3 ⁇ D k ⁇ 3 )+( A k ⁇ 2 ⁇ E k ⁇ 2 )+( A k ⁇ 1 ⁇ D k ⁇ 1 )+( A k ⁇ B k ) (10.5)
  • the error rate can be estimated for each pattern of the most likely decoded results.
  • the standard deviation a ⁇ 10 and average Pave 10 or the standard deviation a ⁇ 36 and average Pave 36 may be used as indices to the quality of the read signal.
  • may be used effectively as an index to the quality of the read signal by detecting only state transition patterns having relatively high error probabilities (or error rates). That is to say, an index correlated with the error rate can be obtained without detecting all state transition patterns.
  • the “state transition patterns having relatively high error probabilities” refer to state transition patterns of which the maximum value of the reliability values
  • the quality of the read signal may be evaluated by detecting patterns representing predetermined state transitions in a predetermined period and by using, as indices, the standard deviation a ⁇ 10 and average Pave 10 of the
  • the error rate may be estimated by using the standard deviation a ⁇ 10 .
  • a maximum likelihood sequence amplitude (MLSA), which is an error index for use in PRML processing (which will be herein simply referred to as an “MLSA index”), may also be used as an index to the signal quality (or error rate).
  • d 2 min is the square of the minimum Euclidean distance between two possible paths.
  • d 2 min 10.
  • This MLSA index is obtained by Equation (14) on the supposition that the average Pave 10 used in Equation (11) is zero (i.e., while leaving the average Pave 10 out of consideration). This is because the average Pave 10 is typically approximately zero and normally does not constitute a major factor of decreasing the correlation between the index and the error rate.
  • FIG. 16 shows a relationship between the MLSA index as defined by Equation (14) and a bit error rate BER as derived by Equation (11). It can be seen that just like the jitter-error rate relationship shown in FIG. 2, as the MLSA index increases, the error rate increases. That is to say, it can be seen that the error rate to be obtained after the PRML processing may be estimated by using the MLSA index.
  • a PR (1, 2, 2, 1) equalization technique is used as an exemplary (C0, C1, C1, C0) equalization technique (where C0 and C1 are arbitrary positive integers).
  • C0, C1, C1, C0 are arbitrary positive integers.
  • an index correlated with the error rate can also be obtained through a similar procedure.
  • a recorded code having a minimum polarity inversion interval of two is used as in the preferred embodiment described above.
  • a PR (C0, C1, C0) (where C0 and C1 are arbitrary positive integers) equalization technique is applied to the following preferred embodiment.
  • the states S (0, 0) k, S (0, 1) k , S (1, 1) k , S (1, 0) k and so on at the time k will be identified by S 0 k , S 1 k , S 2 k , S 3 k and so on, respectively, for the sake of simplicity.
  • the state transitions that may occur in the period between the time k ⁇ 1 and the time k are represented by the state transition diagram shown in FIG. 17.
  • the state transition diagram shown in FIG. 17 is expanded with respect to the time axis, the trellis diagram shown in FIG. 18 is obtained.
  • each recorded code has a minimum polarity inversion interval of two and the PR (C0, C1, C0) equalization technique is used.
  • there are six possible state transition patterns i.e., possible combinations of states as for state transitions occurring from a predetermined state at a time into another predetermined state at a different time along two paths (i.e., paths A and B) as shown in the following Table 5: TABLE 5 Recording code Recording code State (b k-1 , . . . , b k ) of (b k-1 , . . .
  • transition path A path B S0 k-3 ⁇ S2 k (0, 0, 0, 1, 1) (0, 0, 1, 1, 1) S2 k-3 ⁇ S0 k (1, 1, 0, 0, 0) (1, 1, 1, 0, 0) S0 k-3 ⁇ S0 k (0, 0, 0, 0, 0) (0, 0, 1, 1, 0, 0) S2 k-3 ⁇ S2 k (1, 1, 0, 0, 1, 1) (1, 1, 1, 1, 1, 1) S0 k-4 ⁇ S0 k (0, 0, 0, 1, 1, 0, 0) (0, 0, 1, 1, 0, 0, 0) S2 k-4 ⁇ S2 k (1, 1, 0, 0, 0, 1, 1) (1, 1, 1, 0, 0, 1, 1)
  • the sum of squared errors between the expected values and the actual values y k ⁇ 2 , y k ⁇ 1 and y k of the read signal in the period between the times k ⁇ 2 and k is identified by Pa.
  • the path A is estimated to be the more probable one.
  • the path B is estimated to be the more probable one. That is to say, even when a recorded code having a minimum polarity inversion interval of two is combined with the PR (C0, C1, C0) equalization technique, the reliability of the decoded result can also be evaluated by
  • a state transition having the highest error probability should have a minimum Euclidean distance between the paths A and B.
  • the two state transition patterns shown in the following Table 6 should have the minimum Euclidean distance between their two paths: TABLE 6 Recording code Recording code State (b k-1 , . . . , b k ) (b k-1 , . . . , b k ) transition of path A of path B S0 k-3 ⁇ S2 k (0, 0, 0, 1, 1) (0, 0, 1, 1, 1) S2 k-3 ⁇ S0 k (1, 1, 0, 0, 0) (1, 1, 1, 0, 0)
  • Pa ⁇ Pb ( CC k ⁇ 2 ⁇ DD k ⁇ 2 )+(BB k ⁇ 1 ⁇ CC k ⁇ )+( AA k ⁇ BB k ) (18.2)
  • c k is the decoded result
  • k is an integer
  • AA k , BB k , CC k and DD k are given by:
  • the standard deviation ⁇ and average Pave may be used to estimate the error rate of the read signal or evaluate the quality of the read signal.
  • the quality of the read signal can also be evaluated by the difference in probability
  • each recorded code has a minimum polarity inversion interval of two and the PR (C0, C1, C2, C1, C0) equalization technique is used.
  • PR C0, C1, C2, C1, C0
  • there are 90 possible state transition patterns i.e., possible combinations of states for state transitions occurring from a predetermined state at a time into another predetermined state at a different time along two paths (i.e., paths A and B) as shown in the following Table 8: TABLE 8 Recording code Recording code State (b k-1 , . . . b k ) (b k-1 , . . .
  • the sum of squared errors between the expected values shown by the path A and the actual values y k ⁇ 4 , y k ⁇ 3 , y k ⁇ 2 , y k ⁇ 1 and y k of the read signal in the period between the times k ⁇ 4 and k is identified by Pa.
  • the sum of squared errors between the expected values shown by the path B and the actual values y k ⁇ 4 , y k ⁇ 3 , y k ⁇ 2 ) y k ⁇ 1 and y k of the read signal in the period between the times k ⁇ 4 and k is identified by Pb.
  • the path A is estimated to be the more probable one.
  • the path B is estimated to be the more probable one. That is to say, even when a recorded code having a minimum polarity inversion interval of two is combined with the PR (C0, C1, C2, C1, C0) equalization technique, the reliability of the decoded result can also be evaluated by
  • a state transition having the highest error probability should have a minimum Euclidean distance between the paths A and B.
  • the sixteen state transition patterns shown in the following Table 9 should have the minimum Euclidean distance between their two paths: TABLE 9 State Recording code Recording code transition (b k-1 , . . . , b k ) of path A (b k-1 , . . .
  • Pa ⁇ Pb ( CC k ⁇ 4 ⁇ II k ⁇ 4 )+( HH k ⁇ 3 ⁇ II k ⁇ 3 )+( CC k ⁇ 2 ⁇ EE k ⁇ 2 )+( EE k ⁇ 1 ⁇ GG k ⁇ 1 )+( FF k ⁇ GG k ); (23.14)
  • c k is the decoded result
  • k is an integer and AA k , BB k , CC k , DD k , EE k , FF k , GG k , HH k , II k and JJ k are given by:
  • GG k ( y k ⁇ ( C 0+2 ⁇ C 1 +C 2)) 2 ,
  • JJ k ( y k ⁇ (2 ⁇ C 0+2 ⁇ C 1 +C 2)) 2
  • the standard deviation ⁇ and average Pave may be used to estimate the error rate of the read signal or evaluate the quality of the read signal.
  • the quality of the read signal can also be evaluated by the difference in probability
  • the second preferred embodiment relates to a specific method of calculating the probabilities of respective states and the reliability Pa ⁇ Pb of the decoded result where the read signal is decoded by a PRML decoding method (e.g., the PR (1, 2, 2, 1) equalization technique described above).
  • a PRML decoding method e.g., the PR (1, 2, 2, 1) equalization technique described above.
  • L k S0 min [ L k ⁇ 1 S0 +( y k +3) 2 , L k ⁇ S5 +( y k +2) 2 ]
  • L k S1 min [ L k ⁇ 1 S0 +( y k +2) 2 , L k ⁇ 1 S5 +( y k +1) 2 ]
  • L k S3 min [ L k ⁇ 1 S3 +( y k ⁇ 3) 2 , L k ⁇ 1 S2 +( y k ⁇ 2) 2 ] (25)
  • L k S4 min [ L k ⁇ 1 S3 +( y k ⁇ 2) 2 , L k ⁇ 1 S2 +( y k ⁇ 1) 2 ]
  • L k ⁇ 1 S0 through L k ⁇ 1 S5 are the probabilities of the respective states S 0 through S 5 at the previous time k- ⁇ 1
  • y k is the actual sample value at the time k
  • min [xxx, zzz] is an operator indicating that the smaller one of xxx and zzz should be selected.
  • each branch metric e.g., (y k +3) 2
  • its associated probability e.g., L k ⁇ 1 S0
  • y k 2 /2 is always subtracted from the sum.
  • the smallest one of the probabilities L k S0 through L k S5 may be selected by comparing them with each other. Accordingly, if these calculation rules are applied to all of the equations for obtaining L k S0 through L k S5 , then the decoded result will not be affected at all.
  • the probabilities L k S0 through L k S5 of the respective states S 0 through S 5 at the time k may be given by the following Equations (26):
  • L k S0 min [ L k ⁇ 1 S0 +( y k +3) 2 /2 ⁇ y k 2 /2 , L k ⁇ 1 S5 +( y k +2) 2 /2 ⁇ y k 2 /2]
  • L k S1 min [ L k ⁇ 1 S0 +( y k +2) 2 /2 ⁇ y k 2 /2 , L k ⁇ 1 S5 +( y k +1) 2 /2 ⁇ y k 2 /2]
  • L k S3 min [ L k ⁇ 1 S3 +( y k ⁇ 3) 2 /2 ⁇ y k 2 /2 , L k ⁇ 1 S2 +( y k ⁇ 2) 2 /2 ⁇ y k 2 /2]
  • L k S4 min [ L k ⁇ 1 S3 +( y k ⁇ 2) 2 /2 ⁇ y k 2 /2 , L k ⁇ 1 S2 +( y k ⁇ 1) 2 /2 ⁇ y k 2 /2]
  • Equations (26) may be expanded into the following Equations (27):
  • L k S0 min [ L k ⁇ 1 S0 +3 y k + ⁇ fraction (9/2) ⁇ , L k ⁇ 1 S5 +2 y k +2]
  • L k S1 min [ L k ⁇ 1 S0 +2 y k +2 , L k ⁇ 1 S5 +y k +1 ⁇ 2]
  • L k S3 min [ L k ⁇ 1 S3 ⁇ 3 y k + ⁇ fraction (9/2) ⁇ , L k ⁇ 1 S2 ⁇ 2 y k +2]
  • L k S4 min [ L k ⁇ 1 S3 ⁇ 2 y k +2 , L k ⁇ 1 S2 ⁇ y k +1 ⁇ 2]
  • a k , B k , C k , D k , E k , F k and G k are defined as follows:
  • th 1 ⁇ fraction (5/2) ⁇
  • th 2 ⁇ fraction (3/2) ⁇
  • th 3 1 ⁇ 2
  • th 4 ⁇ 1 ⁇ 2
  • th 5 ⁇ fraction (3/2) ⁇
  • th 6 ⁇ fraction (5/2) ⁇ .
  • the probabilities L k S0 through L k S5 of the respective states S 0 through S 5 at the time k may be obtained by calculating A k through G k through simple multiplications and additions following the Equations (27), i.e., without calculating the squared errors between the ideal values and the actual sample values.
  • the circuit configuration of the ML decoder does not have to be so complicated.
  • the quality of the read signal may be evaluated by obtaining the difference in probability
  • calculation may also be a relatively simple one that includes no square calculations.
  • an alternative simplified method of calculating
  • the Pa ⁇ Pb values are preferably obtained for such pairs of paths A and B as having the minimum Euclidean distance between them.
  • the path A includes state transitions of S 0 ⁇ S 0 ⁇ S 1 ⁇ S 2 ⁇ S 4 and the path B includes state transitions of S 0 ⁇ S 1 ⁇ S 2 ⁇ S 3 ⁇ S 4 .
  • the probability Pa of the path A may be given by:
  • the probability Pb of the path B may be given by:
  • the Pa ⁇ Pb may be obtained by:
  • Pa ⁇ Pb ( A k ⁇ 3 ⁇ B k ⁇ 3 )+ B k ⁇ 2 ⁇ F k ⁇ 1 +( E k ⁇ F k )
  • the Pa ⁇ Pb value can be calculated by using the A k through G k values that are obtained through simple additions and subtractions on the sample value y k and the preset values th 1 through th 6 .
  • the Pa ⁇ Pb value can be obtained relatively easily without performing the square calculations.
  • the ML decoder may have a simplified circuit configuration.
  • the Pa ⁇ Pb values may also be calculated by using the A k through G k values in a similar manner for the other state transitions.
  • the Pa ⁇ Pb values of some of the other state transitions may be obtained in the following manner:
  • Pa ⁇ Pb ( A k ⁇ 3 ⁇ B k ⁇ 3 )+ B k ⁇ 2 ⁇ F k ⁇ 1 +( F k ⁇ G k )
  • Pa ⁇ Pb ( E k ⁇ 3 ⁇ F k ⁇ 3 ) ⁇ F k ⁇ 2 +B k ⁇ 1 +( A k ⁇ B k )
  • Pa ⁇ Pb ( E k ⁇ 3 ⁇ F k ⁇ 3 ) ⁇ F k ⁇ 2 +B k ⁇ 1 +( B k ⁇ C k )
  • Pa ⁇ Pb ( B k ⁇ 3 ⁇ C k ⁇ 3 )+ B k ⁇ 2 ⁇ F k ⁇ 1 +( E k ⁇ F k )
  • Pa ⁇ Pb ( B k ⁇ 3 ⁇ C k ⁇ 3 )+ B k ⁇ 2 ⁇ F k ⁇ 1 +( F k ⁇ G k )
  • Pa ⁇ Pb ( F k ⁇ 3 ⁇ G k ⁇ 3 ) ⁇ F k ⁇ 2 +B k ⁇ 1 +( A k ⁇ B k )
  • Pa ⁇ Pb ( F k ⁇ 3 ⁇ G k ⁇ 3 ) ⁇ F k ⁇ 2 +B k ⁇ 1 +( B k ⁇ C k )
  • the third preferred embodiment relates to an optical disk drive 100 for use to decode a read signal by a PRML decoding method.
  • a read signal which has been read out from an optical disk 8 by an optical head 50 , is amplified by a preamplifier 9 .
  • the pre-amplified signal is subjected to AC coupling and then input to an automatic gain controller (AGC) 10 .
  • AGC 10 controls the gain of its input signal so that the output of a waveform equalizer 11 on the next stage will have predetermined amplitude.
  • the gain-controlled output signal of the AGC 10 has its waveform shaped by the waveform equalizer 11 . Then, the waveform-shaped output signal of the waveform equalizer 11 is supplied to both a PLL circuit 12 and an A/D converter 13 .
  • the PLL circuit 12 generates a read clock signal that is synchronized with the read signal.
  • the PLL circuit 12 may have the same configuration as the conventional PLL circuit shown in FIG. 1 (including the phase detector 5 , LPF 6 and VCO 7 ).
  • the A/D converter 13 samples the read signal.
  • the A/D converter 13 outputs the sampled data obtained in this manner to a digital filter 14 .
  • the digital filter 14 has a frequency characteristic that has been defined so as to match the frequency characteristic of the read/write systems with the characteristic required by a Viterbi circuit 15 .
  • the characteristic required by the Viterbi circuit 15 is a PR (1, 2, 2, 1) equalization characteristic.
  • the output filtered data of the digital filter 14 is input to the Viterbi circuit 15 , which decodes the data by a maximum likelihood decoding method. More specifically, the Viterbi circuit 15 decodes the PR (1, 2, 2, 1) equalized signal by the maximum likelihood decoding, thereby outputting digital data.
  • the Viterbi circuit 15 outputs not only the decoded digital data but also Euclidean distances that have been calculated at respective points in time (i.e., branch metrics) to a differential metric analyzer 16 .
  • the differential metric analyzer 16 estimates possible state transitions from the digital data that has been supplied from the Viterbi circuit 15 . Also, the differential metric analyzer 16 derives Pa ⁇ Pb, representing the reliability of the decoded result, from the estimated state transitions and the branch metrics, thereby estimating the error rate of the decoded result.
  • FIG. 8 is a block diagram illustrating an exemplary configuration for the Viterbi circuit 15 and differential metric analyzer 16 .
  • Sample values y k that have been output from the digital filter 14 are input to a branch metric calculator 17 of the Viterbi circuit 15 .
  • the branch metric calculator 17 calculates respective branch metrics corresponding to the distances between the sample values y k and their associated expected values Level v . Since the PR (1, 2, 2, 1) equalization technique is adopted in this preferred embodiment, the expected values Level v have seven values of 0 through 6.
  • the branch metrics A k , B k , C k , D k , E k , F k and G k representing the respective distances between the expected values and sample values y k at the time k are defined by the following Equations (28):
  • the branch metrics that have been calculated in this manner are input to an adder/comparator/selector 18 .
  • the probabilities (i.e., metric values) of the respective states S 0 through S 5 (see FIG. 4) at a current time k are obtained from the branch metrics at the current time k and the probabilities of those states S 0 through S 5 at the previous time k ⁇ 1 .
  • the probabilities of the respective states SO through S 5 at the current time k are given by the following Equations (29):
  • L k S0 min [ L k ⁇ 1 S0 +A k , L k ⁇ 1 S5 +B k ]
  • L k S1 min [ L k ⁇ 1 S0 +B k , L k ⁇ 1 S5 +C k ]
  • L k S3 min [ L k ⁇ 1 S3 +G k , L k ⁇ 1 S2 +F k]
  • L k S4 min [ L k ⁇ 1 S3 +F k , L k ⁇ 1 S2 +E k ]
  • min [xxx, zzz] is an operator indicating that the smaller one of the two values xxx and zzz should be selected.
  • the metric values L k S0 through L k S5 at the time k are stored in a register 19 and will be used to calculate metric values of the respective states S 0 through S 5 at the next time k+1.
  • the adder/comparator/selector 18 selects state transitions that have the minimum metric values in accordance with Equations (29). Also, based on the results of selection, the adder/comparator/selector 18 outputs control signals Sel 0 through Sel 3 to a path memory 20 , which has a circuit configuration such as that shown in FIG. 9, in accordance with the following Inequalities (30):
  • the path memory 20 estimates most probable state transition paths according to the state transition rule and outputs digital decoded data c k corresponding to the estimated state transition paths.
  • the branch metrics that have been output from the branch metric calculator 17 are input to a delay circuit 21 .
  • the output of the branch metrics to a differential metric calculator 22 is delayed for the amount of time corresponding to the time it takes for the adder/comparator/selector 18 and the path memory 20 to perform their signal processing.
  • the output digital data c k of the path memory 20 is input to a state transition detector 23 , which detects predetermined patterns from the digital data c k .
  • the state transition detector 23 detects data patterns corresponding to the eight state transitions given by Equations (9.1) though (9.8).
  • the differential metric calculator 22 calculates the Pa ⁇ Pb values of those detected state transitions in accordance with the Equations (9.1) through (9.8).
  • the Pa ⁇ Pb values may be calculated by a method including no square calculations as described for the second preferred embodiment.
  • the Pa ⁇ Pb values may be obtained without using the branch metrics that have been calculated by the branch metric calculator 17 . Accordingly, in that case, the sample values y k that have been output from the digital filter 14 may be directly input to the differential metric calculator 22 by way of the delay circuit 21 only.
  • the differential metric calculator 22 may obtain the Pa ⁇ Pb values from the sample values y k by the method described for the second preferred embodiment.
  • the Pa ⁇ Pb values that have been calculated in this manner for the predetermined state transitions detected are input to an average/standard deviation calculator 24 .
  • the average/standard deviation calculator 24 obtains and outputs the average Pave 10 and the standard deviation ⁇ 10 of the distribution of the input Pa ⁇ Pb values. It should be noted that the average Pave 10 and the standard deviation ⁇ 10 to be output in this case are obtained for predetermined state transitions, each having two possible paths with a minimum Euclidean distance between them (i.e., having relatively high error probabilities). According to Equation (11), the error rate of the read signal can be estimated by using the average Pave 10 and the standard deviation ⁇ 10 .
  • the standard deviation and the average obtained by the average/standard deviation calculator 24 may be used as indices that indicate the quality of the read signal and that are correlated with the error rate. It should be noted that the error rate may also be obtained with the average Pave 10 supposed to be zero because the average is expected to be approximately equal to zero.
  • the optical disk drive 100 has a configuration such as that shown in FIG. 7.
  • the optical disk drive 100 may further include another waveform equalizer 28 having such an equalization characteristic as to allow the PLL circuit 12 to generate a clock signal more appropriately as shown in FIG. 10.
  • the optical disk drive 100 shown in FIG. 10 can also obtain the standard deviation and the average and can evaluate the quality of the read signal by using them.
  • a read clock signal may also be generated based on the output of the A/D converter 13 (i.e., digital signal) as shown in FIG. 11. Even so, just like the optical disk drive 100 shown in FIG. 7, the optical disk drive 100 shown in FIG. 11 can also obtain the standard deviation and the average and can also evaluate the quality of the read signal by using them.
  • the quality of the read signal is evaluated by using the standard deviation ⁇ and average Pave of the Pa ⁇ Pb distribution, which are output from the differential metric analyzer 16 , as respective indices.
  • a control operation may also be carried out by using these indices (i.e., the standard deviation ⁇ and average Pave) to improve the quality of the read signal.
  • the frequency characteristic of the waveform equalizer 11 may be modified by the frequency characteristic controller 29 shown in FIG. 12 so that the average or the standard deviation output from the differential metric analyzer 16 becomes zero or minimized. Then, the quality of the read signal can also be improved.
  • recording parameters can be optimized by controlling the recording power or the degree of recording compensation (e.g., recording pulse width) so that the average or standard deviation, output from the differential metric analyzer 16 , becomes zero or minimized.
  • the degree of recording compensation e.g., recording pulse width
  • the PRML error index MLSA is obtained by dividing the standard deviation (or root mean square) ⁇ of the most probable state transition path from the read signal by the Euclidean distance between the most probable and the second most probable state transition paths.
  • the PRML error index MLSA is an index that can be used to evaluate the quality of the read signal appropriately when the PRML decoding technique is adopted.
  • the error index MLSA that has been output from the differential metric analyzer 160 is supplied to a frequency characteristic controller 290 .
  • the frequency characteristic controller 290 optimizes the characteristics of the waveform equalizer 11 (e.g., the boost level and the boost center frequency thereof) so as to minimize the error index MLSA.
  • the frequency characteristic controller 290 may change the boost level slightly and then compare the PRML error index MLSA resulting from the original boost level with the PRML error index MLSA resulting from the slightly changed boost level. Based on the result of the comparison, the frequency characteristic controller 290 may select one of the two boost levels that has resulted in the smaller MLSA. By performing such an operation repeatedly, the frequency characteristic controller 290 can optimize the characteristics of the waveform equalizer 11 and converge the PRML error index MLSA to a minimum value.
  • the PRML error index MLSA that has been generated by the differential metric analyzer 160 may also be supplied to a focus offset searcher 291 as shown in FIG. 14.
  • the optical disk drive 100 performs a focus servo control so that the light beam emitted from the optical head 50 can always scan the information recording plane of the optical disk 8 .
  • This focus servo control is carried out by subjecting the focus actuator (not shown) of the optical head 50 to a feedback control so that the focus error signal that has been detected by a servo amplifier 91 is equalized with a predetermined target value X 0 by way of a subtractor 92 .
  • the focus offset searcher 291 may output a value corresponding to the smallest PRML error index MLSA as the predetermined target value X 0 to the subtractor 92 . Then, the focus servo control may be carried out in such a manner as to minimize the PRML error index MLSA (i.e., to minimize the error rate). It should be noted that such a target value X 0 may be searched for by detecting the PRML error index MLSA corresponding to a slightly changed target value X 0 and comparing the MLSA value detected with the original MLSA value.
  • the focus target value is optimized by using the PRML error index MLSA.
  • the PRML error index MLSA may also be used to optimize any other servo target value.
  • the PRML error index MLSA may also be used for tracking servo control, disk tilt control, lens spherical aberration correction and so on.
  • the present invention is also applicable to an optical disk drive including two optical heads 50 and 51 for reading a signal from the optical disk 8 and writing a signal on the optical disk 8 , respectively, as shown in FIG. 15.
  • the recording power may be controlled by reference to the PRML error index MLSA that is output from the differential metric analyzer 160 .
  • a signal to be written on the optical disk 8 is generated by a write signal generator 103 and then supplied to the signal writing optical head 51 by way of a modulator 102 .
  • the modulator 102 multiplies the write signal by an appropriate recording power P and then supplies the product to the optical head 51 .
  • the PRML error index MLSA that has been generated by the differential metric analyzer 160 may be supplied to a recording power controller 292 . Then, the recording power controller 292 may determine the recording power P in such a manner that the PRML error index MLSA is minimized.
  • the optical disk drive 100 shown in FIG. 15 gets the read and write operations performed by the two different heads 50 and 51 .
  • a single head may be switched to perform the read or write operation selectively.
  • the recording power is controlled by using the PRML error index MLSA.
  • the width or the phase of write pulses may also be controlled by reference to the PRML error index MLSA.
  • Various preferred embodiments of the present invention described above provide 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 of state transition paths that represent n probable transitions from a first state at a time k ⁇ j into a second state at a time k.

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CN1306514C (zh) 2007-03-21
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