Abstract:
A waveform equalizer for equalizing a waveform of a reproduction signal. A delay element delays a propagation of the reproduction signal. A plurality of multipliers multiply predetermined coefficients by the reproduction signal and the delayed propagation reproduction signal. A detector detects an asymmetry of the reproduction signal arising from physical profiles of each of the marks and the non-marks, and outputs a detection signal representing an amount of asymmetry. A discriminator outputs a discrimination signal in response to a discrimination of the marks and the non-marks. A calculator calculates a first coefficient multiplied by the reproduction signal of the marks based on the outputted detection signal, and calculates a second coefficient multiplied by the reproduction signal of the non-marks. A selector selects either the first or second coefficient, based upon the outputted discriminating signal.

Description:
This is a continuation of U.S. patent application Ser. No. 09/984,351, filed Oct. 29, 2001 now U.S. Pat. No. 6,678,230, pending, the contents of which are expressly incorporated by reference herein in its entirely. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This present invention relates to a technique for equalizing waveform of signals read from recording medium such as optical discs, etc. 
   2. Description of the Related Art 
   In a information reproducing apparatus for reproducing information recorded on recording medium such as optical discs, a slice method has been adopted, in which a signal waveform level greater than a specified value is judged as “1” and that smaller than a specified value is judged as “0”. However, in this method, it is difficult to reproduce data at a high reliability from the recording medium with remarkably improved recording density. Therefore, in recent years, attention is attracted to Partial Response Maximum Likelihood (hereinafter referred to as PRML) method that can reproduce the data at a high reliability. PRML method is a technique utilized for a signal processing technique of recording medium with increased density, which is utilized in, for example, a digital recording camera integrated type VTR, rewritable optical discs, not to mention a hard disk drive (hereinafter referred to as HDD). As the recording density increases, a need for reproducing data correctly from reproduction signals with low signal-to-noise (S/N) ratio or nonlinear reproduction signals is increased. 
     FIG. 16  is a block diagram of a general configuration of an information reproducing apparatus  181  using PRML method. First of all, an optical pickup  183  irradiates optical disc  182  with laser beam. Information reproducing apparatus  181  detects intensity of the reflected light, reads the information (data) recorded on optical disc  182 , converts into electrical signals, and outputs to Front End Processor (hereinafter referred to as FEP)  184 . FEP 184  amplifies the electrical signals read and adjusts its gain. FEP 184  further processes to remove noise components of unrequired high frequency band and to emphasize the required signal frequency band. The output signals from FEP 184  are converted to digital signals by analog/digital (A/D) converter  185  and entered into waveform equalizer  186 . Waveform equalizer  186  waveform-equalizes waveforms of the digital signals to the preset PR characteristics. Maximum likelihood decoder  187  decodes waveform-equalized signals to PR characteristics and outputs as the reproduction data. 
   Waveform equalizer  186  of information reproducing apparatus  181  generates waveform in such a manner as to achieve the desired PR characteristics, that is, PR ( 3 ,  4 ,  4 ,  3 ) characteristics.  FIG. 17  is a block diagram of an exemplary configuration of waveform equalizer  186 . Waveform equalizer  186  is called as a transversal filter or Finite Impulse Response(FIR) filter. Waveform equalizer  186  generally includes a plurality of delay elements  192 , a plurality of equalizing coefficients (coefficients A through E) for achieving the desired PR characteristics, a plurality of multipliers  193  for multiplying equalizing coefficients by the output of delay element  192 , and an adder  194  for adding outputs of a plurality of multipliers  193 . 
   In order to accurately equalize into desired PR characteristics, a technique to automatically adaptively control equalizing coefficients (taps) of FIR filter is adopted. This technique is effective for various stresses such as tilting of a disc, defocus of laser beam, off-track of optical head. Many adaptive control algorithms are known, including Least-mean square (LMS) algorithm, Normalized LMS algorithm, Recursive Least Square (RLS) algorithm, projection algorithm, neural network algorithm. 
   Now, the adaptive waveform equalizer using LMS algorithm will be briefly described. In this algorithm, a temporary judgment value used for LMS is required in order to calculate adaptive equalizing coefficients. This LMS algorithm utilizes a feedback operation for minimizing square errors between the “desired response” and the “response of transmission line.” This “desired response” is a PR equalization target value. The “response of transmission line” is a digital reproduction signal entered from FIR filter and equalized into PR frequency characteristics. In LMS algorithm, a signal that represents a difference between the temporarily judged value and the digital reproduction signal value after equalization, which is obtained in the block of adaptively controlling the coefficients of FIR filter is called an equalization error signal. 
   The block that adaptively controls coefficients of FIR filter updates the equalizing coefficients of FIR filter as required to minimize the square value of the equalization error signal. This is called as an adaptive equalization. A formula for setting LMS equalizing coefficients is shown in the following formula (for example, S. Heikin: “Introduction to Adaptive Filters” Gendai Kogakusha):
 
 w ( n ( T +1))= w ( nT )+ A·e ( nT )· x ( nT )  Eq. (1)
 
(where, T=0, 1, 2, 3, . . . )
 
   w (nT) represents a present coefficient, w (n(T+1) is a coefficient to be updated, “A” is a tap gain, e (nT) is an equalization error, x (nT) is an input signal to FIR filter. “n” is a parameter for selecting update cycles of the coefficients. Based on Eq. (1) above, the equalizing coefficients of FIR filter is updated. 
   Now, asymmetry of optical disc  182  ( FIG. 16 ) will be described. Asymmetry means absence of symmetry between pits and non-pits of the optical disc. In optical disc  182  (FIG.  16 ), information is recorded in the form of arrangement and length of microscopic emboss sections called pits. The pit has length of, for example, 3T, 5T when T denotes the reference length. Pits are arranged with spaces of 3T, 5T. The pit length is preferably exactly 3T, 5T. However, there are some deviations in pit length. This is because a master disc with deviations in pit length is manufactured as a result of, for example, slight deviation of power of recording light used for mastering an optical disc. When the recording power is not appropriate, each pit formed is slightly longer or shorter in the same amount from the standard value in front and behind in the length direction. That is, there is no symmetry between the pits and the non-pits, which is called as asymmetry. Hereinafter, in the present specification, relationships between the pits and the non-pits of the optical disc should be same as the relationships between recorded portions (marks) and unrecorded portions (spaces) of hard disk. Note that, for a read-only optical discs, terms “pit” and “non-pit” may be used and for recordable optical discs, the portion where the information is recorded (that is, portion intensely irradiated with laser) may be called as a “mark” and the region between marks a “space.” In the present specification, terms “pit” and “mark” are synonymous. Terms “non-pit” and “space” and further “non-mark” are synonymous. In addition, a signal when the optical disc with no symmetry between pits and non-pits (that is, asymmetry) is reproduced is called as an asymmetric signal, and a signal when not an asymmetric optical disc is reproduced is called as a symmetric signal. 
     FIGS. 18A through 18C  show simple models of asymmetry. In  FIGS. 18A through 18C , 3T marks, 3T spaces, 5T marks, and 5T spaces pit arrangements are shown. In these figures, the reference length is 1T and a detection window width is adopted.  FIG. 18B  is a standard pit arrangement, and both marks and spaces are symmetric. As against  FIG. 18B ,  FIG. 18A  shows marks, each of which has length uniformly shorter than that shown in  FIG. 18B  by length x. On the other hand,  FIG. 18C  shows marks, each of which has length uniformly longer than that shown in  FIG. 18B  by length y. In either case shown in  FIGS. 18A and 18C , no symmetry is observed in both marks and spaces. Because this asymmetry is also caused by fluctuations of laser wavelength, in general, it is difficult to adjust and maintain symmetry between pits and non-pits at the time of recording. 
   Now, the description is made on the specific hardware configuration and procedure for binarizing the analog data signal (reproduction signal) read from the optical disc.  FIG. 19  is a block diagram of the configuration of the PRML detector  210 . PRML detector  210  carries out adaptive equalization and updates the equalizing coefficients of FIR filter from time to time. First of all, A/D converter  221  of PRML detector  210  converts the reproduction signal from analog to digital. Phase comparator  222  generates binary data with a certain threshold value used as a reference. Then, PR temporary judging section  223  receives the binary data. PR temporary judging section  223  temporarily judges the desired value of PR and outputs coefficient adaptive controller  224 . The desired value of PR method can be determined based on the amplitude zero-cross information obtained at phase comparator  222  (for example, see Screen Image Information Media Society Technical Report (ITE Technical Report Vol. 24, No. 46, pp. 13-18 MMS2000-14 (July 2000)). Then, coefficient adaptive controller  224  updates the equalizing coefficient (tap) of FIR equalizer  225  using the adaptive algorithm described before. Viterbi decoder  226  converts the waveform equalized into the specified PR at FIR equalizer  225  into the binary data. 
   The clock used at A/D converter  21  is generated as a result of specified processing of phase comparator  22  which detects phase difference from the output of A/D converter  21  and by loop filter, DAC for converting digital signal to analog signal and voltage control transmitter VCO (all not illustrated). 
     FIG. 20  is a block diagram of the configuration of PRML detector  220 . PRML detector  220  outputs the judgment value by PR ( 1 ,  1 ) equalization using the output of the FIR equalizer, and calculates the desired value of PR (a, b, b, a) equalization in the FIR equalizer using the judgment value, which is disclosed in, for example, Japanese Laid-open Publication No. 2000-123487. A/D converter  231  converts the reproduction signal from analog signal to digital signal. FIR equalizer  32  carries out the specified PR equalization for the digital signal. PR temporary judging section  233  temporarily judges the desired value of PR method using binarized data of output of FIR equalizer  32  and outputs to coefficient adaptive controller  234 . Coefficient adaptive controller  234  uses the temporary judgment value and updates taps of FIR equalizer  232 . PRML detector  220  can suppress the probability for occuring judgment errors to a low level by reducing the judgment threshold value. 
   Conventionally, since there used to be questions (1) and (2) as shown below, it was unable to obtain properly binarized reproduction signals. That is, 
   (1) When there is no symmetry between the pits and the non-pits in an optical disc (that is, asymmetry), the performance of PRML degrades. In other words, information reproducing apparatus  181  using the conventional PRML generates errors by asymmetrical reproduction signals. 
   Problem (1) above can be described as follows. FIG.  21 A and  FIG. 21B  show histograms of output signals of A/D converter  185  in information reproducing apparatus  81  (FIG.  16 ). The abscissa is the reproduction signal level, while the frequency of signal level is taken as ordinate. This waveform example uses the 8-16 modulation used in the DVD (Digital Versatile Disc) specifications. That is, the reproduction waveform has 3T-14T (including sync code) mark length and space-length. 
   The phase error is detected with the center of the reproduction signal level used as a reference (for example, central value  64  (40 h) of expressible 0-128 when the number of effective bits of ND converter is 7) and the clock frequency for sampling the reproduction signal and the phase are controlled. Under the control, histogram is separated to generally have five distributions. This is because when PRML has the PR coefficient such as a PR (a, b, b, a) ML, the number of signal levels (signal distributions) becomes five (let a, b denote positive coefficients). Waveform equalizer  86  controls the clock phase and facilitates PR equalization. 
     FIG. 21A  shows a histogram of reproduction signal, which is not asymmetrical, while  FIG. 21B  shows a histogram of asymmetrical reproduction signal.  FIG. 21C  shows the histogram of output signal when waveform equalizer  86  PR-equalizes (in this case, PR ( 3 ,  4 ,  4 ,  3 ) equalizes) the non-asymmetrical reproduction signal (FIG.  21 A). As clear from  FIG. 21C , variance is minimum and each level is accurately separated. 
   On the other hand,  FIG. 21D  shows a histogram of an equalizer output when the asymmetric reproduction waveform is PR-equalized. As clear from the figure, PR-equalizing asymmetric reproduction waveform increases the variance. This is because PRML is originally intended for processing symmetric waveform and automatically equalizes the waveform in such a manner that each level has equal interval. In other words, when an asymmetric signal which has minimum variance with unequal intervals is supplied, the equalizer forcibly equalizes the signal to have equal intervals. As a result, the variance further increases. 
     FIG. 22  is a graph that shows the frequency characteristics of reproduction signal based on the histogram of asymmetric reproduction waveform of FIG.  21 B. In addition,  FIG. 22  also shows the frequency characteristics of the desired PR characteristics. In  FIG. 21B , the left side from the center of the reproduction signal level is designated as mark and the right side as space. In the graph of FIG.,  22 , when the standardized frequency is taken as abscissa and the gain as ordinate, characteristics on the mark side and the characteristics on the space side differ each other due to the influence of asymmetry. As easily understood, in order to equalize the reproduction waveform to have desired PR characteristics, the equalizer must carry out equalization with characteristics that differ on the mark side and on the space side. However, the conventional waveform equalizer  186  cannot equalize the reproduction waveform to have desired PR characteristics highly accurately because the same equalization is carried out on the mark side and the space side. As a result, variance in the output signal of the maximum likelihood decoder  187  ( FIG. 16 ) increases, resulting in degraded performance. 
   (2) When the adaptive equalization processing is automatically carried out, the results of the temporary judgment (temporary judgment value) utilized in LMS may cause errors. To specifically explain, first of all, in the system shown in  FIGS. 19 and 20 , when the reproduction signal contains comparatively less noise and provides good signal quality to a certain extent, satisfactory convergence characteristics can be obtained. However, when noises arising from various stresses as described above mix in the reproduction signal and the jitter increases, the bit error rate BER degrades and the probability for making an erroneous temporary judgment increases. Making an erroneous temporary judgment causes the equalization error signal to become abnormal and the LMS action itself becomes abnormal. Consequently, appropriate adaptive equalizing coefficients are unable to be calculated and correct PR equalization is prevented. This degrades the bit error rate of binary data after Viterbi decoding. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to obtain the correct equalization error even in the asymmetry reproduction waveform or under noisy environment and at the same time to accurately output the temporary judgment value when the coefficients of FIR filter is adaptively controlled. Thus, highly accurate desired PR characteristics can be obtained. 
   The waveform equalizer according to the present invention equalizes a waveform of a reproduction signal obtained by reproducing marks and non-marks recorded on a recording medium, including: a delay element for delaying propagation of the reproduction signal; a plurality of multipliers for multiplying predetermined coefficients by the reproduction signal and the delayed signal from the delay element; a detector for detecting asymmetry of the reproduction signal arising from physical profiles of each of the marks and the non-marks and for outputting a detection signal representing asymmetric amount; a discriminator for discriminating the marks and the non-marks based on the reproduction signal to output a discriminated signal; a selector for calculating a first coefficient multiplied by the reproduction signal of the marks based on the detection signal outputted from the detector, said selector further calculating a second coefficient multiplied by the reproduction signal of the non-marks, which is different from the first coefficient, and said selector selecting one of the first coefficient and the second coefficient based on the discriminating signal outputted from the discriminator; and an adder for adding outputs from the plurality of multipliers. According to the configuration, the above-mentioned object can be achieved. 
   The waveform equalizer according to the present invention equalizes a waveform of a reproduction signal obtained by reproducing marks and non-marks recorded on a recording medium, including: a delay element for delaying propagation of the reproduction signal; a plurality of multipliers for multiplying predetermined coefficients by the reproduction signal and the delayed signal from the delay element; a coefficient learning circuit for adaptively updating the predetermined coefficients for each of the plurality of multipliers; and an adder for adding outputs from the plurality of multipliers, said coefficient learning circuit including: a teaching signal generator for generating a teaching signal that serves as an equalizing target; an error detector for detecting an error between an output signal having impulse response of (a, b, b, a) characteristics by partial response method and the teaching signal generated by the teaching signal generator; a discriminator for discriminating the marks and the non-marks based on the reproduction signal to output a discriminating signal; a plurality of registers for updating and outputting the predetermined coefficients so that mean square of the error becomes minimum, said plurality of registers including a first register for holding a first coefficient multiplied by the reproduction signal of the marks and including a second register for holding a second coefficient multiplied by the reproduction signal of the non-marks, which is different from the first coefficient; and a register selector for selecting one of the first coefficient held in the first register and the second coefficient held in the second register based on the discriminating signal outputted from the discriminator. According to the configuration, the above-mentioned object can be achieved. 
   A PRML detector according to the present invention includes a waveform equalizer for equalizing a waveform of a reproduction signal obtained by reproducing marks and non-marks recorded on a recording medium, and a decoder for generating binary-coded data of the reproduction signal based on the waveform equalized by the waveform equalizer, said decoder outputting a temporary data string which is obtained before the binary-coded data is obtained, said waveform equalizer comprising: an equalizer that includes a delay element for delaying propagation of the reproduction signal, a plurality of multipliers for multiplying predetermined coefficients by the reproduction signal and the delayed signal from the delay element, and an adder for adding outputs from the plurality of multipliers; a target value judging section for determining a target value for equalizing based on the temporary data string outputted from the decoder; and a coefficient adaptive controller for calculating the predetermined coefficients and for adaptively updating the calculated predetermined coefficients for each of a plurality of multipliers. According to the configuration, the above-mentioned object can be achieved. 
   According to the present invention, changing over the equalization characteristics in the waveform equalizer from the mark side to the space side in accordance with the asymmetry amount can suppress displacement and variance at the detection point and can improve the performance of the PRML. Furthermore, by disposing the coefficient learning circuit, even when asymmetry exists, appropriate equalizing coefficients are learned and the target value can be determined. The use of the waveform equalizer according to the present invention can suppress errors at the time of data decoding to a low level in the PRML signal processing of optical discs such as DVD, MO, etc. and magnetic disks of HDD, etc. 
   In addition, according to the present invention, the temporary judgment information for judging the PR equalization target is extracted from the Viterbi decoder. Thus, the temporary judgment error rate can be reduced. As a result, by judging the target value based on the state transition rule of the PR from the temporary judgment information, an accurate equalization error can be found and the satisfactory convergence characteristics can be obtained. That is, errors at the time of data decoding can be suppressed to a low level. Furthermore, in the present invention, detecting whether the output is merged or not in the path memory of the Viterbi decoder can prevent loop performance degradation of the feed back system and can avoid an increase of delay. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments according to the present invention will be described with reference to the accompanying drawings, in which: 
       FIG. 1  is a block diagram of a general configuration of the information reproducing apparatus for carrying out PRML signal processing; 
       FIG. 2  is a block diagram of an exemplary configuration of waveform equalizer; 
       FIG. 3  is a graph of an exemplary impulse response; 
       FIG. 4  is a histogram of reproduction signal when waveform of asymmetrical signal is equalized; 
       FIG. 5  is a block diagram of the configuration of adaptive waveform equalizer that determines and updates appropriate equalizing coefficients; 
       FIG. 6  is a block diagram of the configuration of coefficient learning circuit; 
       FIG. 7  is a block diagram of the configuration of coefficient arithmetic section; 
       FIG. 8A  is a waveform diagram that shows three kinds of impulse responses; 
       FIG. 8B  is a graph of amplitude-frequency characteristics of the three kinds of impulse responses, respectively; 
       FIG. 9A  is a histogram of a asymmetrical reproduction waveform being sampled by an A/D converter when ( 1 ,  7 ) RLL modulation code is used; 
       FIG. 9B  is a histogram of output signals of a conventional waveform equalizer; 
       FIG. 9C  is a histogram of output signals of a waveform equalizer according to the present invention; 
       FIG. 10  is a block diagram of the configuration of the PRML detector according to embodiment 2; 
       FIG. 11  is a state transition diagram of Viterbi decoder when code words, having minimum code length 2T, with PR (a, b, b, a) are adopted; 
       FIG. 12  is a block diagram of a specific configuration of the Viterbi decoder; 
       FIG. 13  is a circuit of the detailed configuration of path memory; 
       FIG. 14  is a block diagram of the detailed configuration of outputting temporary judgment in path memory of the Viterbi decoder; 
       FIG. 15  is a diagram describing the output of the FIR equalizer and the procedure for determining PR equalizing desired value; 
       FIG. 16  is a block diagram of a general configuration of an information reproducing apparatus using PRML; 
       FIG. 17  is a block diagram of an example of the configuration of the waveform equalizer; 
       FIG. 18A  is a diagram of the asymmetric model with the mark width uniformly shortened by length X; 
       FIG. 18B  is a diagram of the asymmetric model; 
       FIG. 18C  is a diagram of the asymmetric model with the mark width uniformly shortened by length y; 
       FIG. 19  is a block diagram of the configuration of the PRML detector; 
       FIG. 20  is a block diagram of the configuration of the PRML detector; 
       FIG. 21A  is a histogram of a reproduction signal which is not asymmetric; 
       FIG. 21B  is a histogram of an asymmetric reproduction signal; 
       FIG. 21C  is a histogram of the output signal when the waveform equalizer PR-equalizes the reproduction signal which is not asymmetric; 
       FIG. 21D  is a histogram of the equalizer output signal when asymmetric reproduction waveform is PR-equalized; and 
       FIG. 22  is a graph of the frequency characteristics of reproduction waveform based on the histogram of asymmetric reproduction waveform of FIG.  21 B. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In embodiment 1, there described is a waveform equalizer that can greatly improve error rate after PRML processing even for asymmetric waveform of reproduction signal arising from asymmetry between pits and non-pits (for example, spaces between pits) equipped in an optical disc. In embodiment 2, there described is a PRML detector that can accurately find a temporary judgment value necessary for adaptively controlling equalizing coefficients (taps) of the FIR filter even under noisy environment and that can accurately obtain equalization error signal indicating the difference between the temporary judgment value and the digital reproduction signal value after equalization. Note that the present application claims priority right pursuant to Japanese Patent Application Nos. 2000-331973 and 2001-196034. The contents of the specifications and drawings of these applications are incorporated herein by references. 
   (First Embodiment) 
     FIG. 1  is a block diagram of a general configuration of an information reproducing apparatus  1  that carries out PRML (Partial Response Maximum Likelihood) signal processing. The PRML signal processing is a technique combining the wavelength equalizing technique for modifying reproduction destortion generated when information is reproduced with a signal processing technique for selecting the mostly likely data string from the reproduction signal containing data errors by positively utilizing the redundancy which the equalized waveform itself possesses. For probability estimation of judging “most likely,” Viterbi decoding is used. In the following description, an example in which PRML signal processing is carried out on a reproduction signal from optical discs such as DVD (Digital Versatile Disc). will be discussed, but this technique can be utilized for the reproduction signal from magnetic disks such as HDD (hard disk drive). 
   Information reproducing apparatus  1  comprises an optical pickup  3 , front end processor (FEP)  4 , analog/digital (AND) converter  5 , waveform equalizer  11 , and maximum likelihood decoder  6 . Optical pickup  3  irradiates optical disc  2  having the information recorded with laser beam, detects the intensity of the light reflected from optical disc  2 , and outputs electrical reproduction signal. FEP 4  amplifies the reproduction signal and adjusts its gain. FEP 4  further carries out noise elimination of the unwanted high frequency band and emphasizing of the required signal band. The output signal from FEP 4  is converted to digital signal by A/D converter  5  and entered in waveform equalizer  11 . Waveform equalizer  11  equalizes waveform of the digital signal to the preset PR characteristics. Maximum likelihood decoder  6  decodes the signal waveform-equalized to PR characteristics and outputs as the reproduction data. In the present specification, maximum likelihood decoder  6  is also called as a Viterbi decoder. When waveform equalizer  11  carries out PR equalization, waveform equalizer  11  and the maximum likelihood decoder  6  are also called a PRML detector in combination. 
   In optical disc  2 , information is recorded as pits or marks. Assume that the pits is formed, so-called, asymmetrically. That is, the space between one pit and the other must not be formed accurately in lengths of 3T, 5T when width of a detection window is designated as the reference length T. For specific examples of asymmetry, see  FIG. 18A  or FIG.  18 C. 
     FIG. 2  is a block diagram of an exemplary configuration of waveform equalizer  11 . Waveform equalizer  11  comprises a plurality of delay elements  12  connected in series, equalizing coefficients (coefficient A-E) for achieving the desired characteristics, a plurality of multipliers  13  for multiplying each of output signals of delay elements  12  by each of equalizing coefficients, and an adder  14  for adding each of multiplier output signals. The delay amount of delay element  12  is a parameter for determining cutoff frequency to input signal X from A/D converter  5  ( FIG. 1 ) and should be appropriately adjusted. Numbers of delay elements  12  corresponds to those of equalizing coefficients (taps) for achieving desired equalization and may be determined appropriately. In the example shown in  FIG. 2 , a filter with  5  taps is used, and four delay elements are therefore inserted. The number of taps can be varied in such a manner to satisfy the required performance. 
   Waveform equalizer  11  further comprises an asymmetry detector  15 , polarity discriminator  16 , and coefficient C selector  17 . Asymmetry detector  15  calculates asymmetry amount from input signal X. Amount of asymmetry is expressed as a ratio of overall amplitude A of input signal X to displacement amount B from a center of the signal waveform, that is, B/A. For example, let 1 be the overall amplitude A and 0.241 be displacement amount B from the center. The asymmetry amount is 24.1%. Polarity discriminator  16  discriminates polarity of input signal X. “Discriminating polarity of input signal X” means to discriminate a mark side and a space side pursuant to input signal X. For example, the polarity may be discriminated by “0” or “1” of a most significant bit (MSB) of input signal X. Coefficient C selector  17  calculates coefficient C for the mark side and C′ for the space side based on the asymmetry amount detected in asymmetry detector  15  and the difference between the waveform and desired values of input signal X. Coefficient C selector  17  changes over output coefficient C and coefficient C′ by the signal from polarity discriminator  16 . Note that difference between the waveform of input signal X and the desired value corresponds to the equalization error in LMS (Least-mean square) algorithm later discussed. The procedure for generating coefficients based on the equalization error will be later discussed. 
   Waveform equalizer  11  according to the present invention has an absolute value of the center coefficient (center tap) greater than absolute values of other coefficients and has impulse response that has nearly symmetrical values on the right and left with the center tap set as the center.  FIG. 3  shows a graph of an exemplary impulse response. Coefficients A through E are shown with open circles. Intervals between coefficients (tap intervals) in the abscissa corresponds to delay amount of delay element  12  (FIG.  2 ). The ordinate shows tap values. In particular, a center tap value adjusts the output signal gain of waveform equalizer  11  ( FIG. 2 ) and each tap ratio determines the boost amount of waveform equalizer  11  (FIG.  2 ). 
   Waveform equalizer  11  ( FIG. 2 ) changes over values of the center tap to coefficient C and coefficient C′, respectively at a mark side of the optical disc and at a space side, for example, on a negative side and a positive side with level “0” of the reproduction signal set as the center in FIG.  9 A. More specifically, waveform equalizer  11  ( FIG. 2 ) varies the gain and boost amount of reproduction signal and equalizes on the mark side and on the space side, respectively. The difference As between coefficient C value and coefficient C′ value is calculated from the asymmetric amount detected by asymmetry detector  15  (FIG.  2 ). Needless to say, when waveform equalizer  11  ( FIG. 2 ) receives non-asymmetric input waveform X, asymmetry detector  15  judges it as “not asymmetric” and coefficient C and coefficient C′ become the same value. Consequently the difference As is 0. 
     FIG. 4  shows a histogram of a reproduction signal when the waveform of the asymmetric signal ( FIG. 21B ) is equalized. As compared to the histogram ( FIG. 21D ) of reproduction signal of the conventional waveform equalizer  186 . (FIG.  16 ), it is understood that a variance becomes smaller. In this way, by adaptively changing over the center tap value on each of mark side and space side and equalizing waveform with varied equalization characteristics, equalized waveforms that have small variance values at detection points can be output even in accordance with the asymmetric reproduction signal. 
   It is extremely important to determine the change amount of center tap of the waveform equalizer in accordance with asymmetry amount obtained from input signal X. In the present embodiment, further discussion will be made on an adaptive waveform equalizer that automatically detects the asymmetry amount and determines appropriate equalizing coefficients for the mark side and the space side by using LMS (Least-mean square) algorithm. 
     FIG. 5  shows a configuration of adaptive waveform equalizer  41  for determining and updating the appropriate equalizing coefficients. Adaptive waveform equalizer  41  comprises a plurality of delay elements  42  connected in series, a coefficient learning circuit  45  for determining equalizing coefficients (coefficients A through E) that achieve the desired PR characteristics, a plurality of multipliers  43  for multiplying the output signal of each delay element  42  by the equalizing coefficients, and an adder  44  for adding each output signal of the multipliers. Because functions and actions of ay elements  42 , coefficients A through E, multipliers  43 , and adder  44  are same as those of delay elements  12 , coefficients A through E, multipliers  13 , and adder  14  explained in waveform equalizer  11  (FIG.  2 ), the explanation will be omitted. 
     FIG. 6  is a block diagram of the configuration of coefficient learning circuit  45 . Coefficient learning circuit  45  comprises an error signal detector  51 , PR equalizing teaching signal generator  52 , correlation detector  53 , loop gain setting section  54 , coefficient arithmetic section  55 , polarity discriminator  56 , and recovery circuit  57 . Each component element of coefficient learning circuit  45  is provided based on the setting formula (Eq. (1)) of the equalizing coefficient of LMS described above. That is, to show it again,
   w ( n ( T+ 1))= w ( nT )+ A·e ( nT )· x ( nT )  Eq. (1) 
(where, T=0, 1, 2, 3, . . . )
 
   w (nT) is a present coefficient, w(n(T+1)) is a coefficient to be updated, “A” is a tap gain, e (nT) is an equalization error, x (nT) is an FIR filter input signal. n is a parameter for selecting update cycles of the coefficient. 
   Error signal detector  51  detects an error between output signal Y of FIR filter  46  and a signal from PR equalizing teaching signal generator  52  that outputs the teaching signal for PR equalization. The teaching signal is targeted for PR equalization. The error signal corresponds to equalization error e (nT) of Eq. (1) above. Correlation detector  53  detects correlation between error signal e(nT) and input signal X. The correlation is expressed by a product of two signals. Consequently, the correlation detecting signal corresponds to e (nT)·x (nT) of Eq. (1). Loop gain setting section  54  adjust the response speed of LMS feedback control. In Eq. (1), this corresponds to the tap gain A. Coefficient arithmetic section  55  adds an updated value (i.e. A·e (nT)·x (nT)) calculated in the preceding block to the present coefficient and calculated the updated equalizing coefficient. 
     FIG. 7  is a block diagram of the configuration of coefficient arithmetic section  55 . Coefficient arithmetic section  55  comprises an adder  61 , selectors  62  through  68 , coefficient updating counter  69 , and a group of registers  70  for holding values of coefficients A through C, C′, D, and E, respectively. First of all, coefficient updating counter  69  controls selectors  62 ,  65 ,  66  to add the value outputted from loop gain setting section  54  and the values held in registers  70 , and updates each of coefficient registers  70 , successively. Since the bit number of this counter can be decided in advance in the design specifications, changes of the coefficient updating speed can be controlled. 
   Now, features of the invention related to the present embodiment will be described. Polarity discriminating circuit  56  discriminates the polarity based on whether the value outputted from loop gain setting section  54  is the calculated value on the mark side of the optical disc or the calculated value on the space side. Selectors  63 ,  64 ,  67  changes over registers to be updated on the mark side and on the space side based on the discrimination results of polarity discriminating circuit  56 . Polarity discriminating circuit  56  can discriminate the polarity from both of input signal X and output signal Y shown in FIG.  6 . The section behind selector  66  has functions of outputting the calculated values held in the register to FIR filter  46  as the tap coefficients. To be more specific, they are functions to hold values of bit-width (i.e. coefficient values) adjusted and functions to hold initial values used in the beginning of learning. Polarity discriminating circuit  56  discriminates and outputs the mark side and the space side based on the input signal X. Selector  68  outputs coefficient C when the discriminating signal represents the mark side and coefficient C′ when it represents the space side. 
   According to the configuration described above, the center taps (coefficient C, coefficient C′) can be learned and updated in each of mark side and the space side for the asymmetric signal and appropriate equalization can be achieved both on the mark side and space side, respectively. In addition, even if great asymmetry exists on the mark side or on the space side, appropriate waveform equalization is possible without taking the asymmetry polarity taken into account. 
   Next discussion will be made on recovery circuit  57  (FIG.  6 ). Waveform equalizer  11  ( FIG. 1 ) of the present embodiment has impulse response in that the absolute value of the center tap is greater than the absolute values of other taps and has impulse response that has nearly symmetric values with the center tap set as the center. However, due to various disturbances including defects, learned and converged values of the coefficients represent to an unexpected impulse response. 
     FIG. 8A  is a waveform of three types of impulse responses, which are A-TAP, B-TAP, C-TAP, respectively. On the other hand,  FIG. 8B  is graphs of relevant amplitude frequency characteristics of three types of impulse responses of 7-tap FIR filter. A-TAP is an impulse response in which the absolute value of the center tap is greater than absolute values of other taps and the impulse response has values closely symmetrical with the center tap set as a center. As against this, at B-TAP and C-TAP, protruded absolute values of two taps are greater than absolute values of other taps and the values of the two taps are nearly the same. As clear from FIG.  8 A and  FIG. 8B , though the impulse responses ( FIG. 8A ) differs each other, it is understood that the amplitude frequency ( FIG. 8B ) indicate nearly same characteristics. Specifically, in 8-16 modulation, which is recording modulation method in the DVD specification, the normalized frequency up to about 0.16 is used. Consequently, at any of A-TAP, B-TAP, C-TAP, the amplitude frequency characteristics are nearly same. Note that in ( 1 , 7 ) RLL (Run Length Limited) modulated code, the normalized frequency up to about 0.25 is used. In this case, at any of A-TAP, B-TAP, C-TAP, nearly the same amplitude frequency characteristics are used. Because waveform equalizer  11  ( FIG. 1 ) is composed on the assumption that impulse responses of the characteristics such as A-TAP are adopted, inconvenience occurs in the impulse response of the characteristics such as B-TAP, C-TAP. Consequently, recovery circuit  57  ( FIG. 6 ) judges that the equalizer falls into the impulse response such as B-TAP, C-TAP when the tap value corresponding to the characteristics of the tap of B-TAP, C-TAP as described above is obtained, and sets the learning results to the initial value and starts re-learning. 
   In the description as described, waveform equalizer  11  ( FIG. 1 ) has an odd number of tap coefficients and varies the equalizing characteristics using the tap coefficient at the center only which has tap values different on the mark side and on the space side. However, the present invention is not restricted to this kind of configuration. For example, the wavelength equalizer may have an even number of tap coefficients. In addition, for tap coefficients used on the mark side and on the space side, a value located at the center may not be adopted. In addition, not only one tap coefficient is varied but a plurality of tap coefficients may be varied on the mark side and on the space side. 
   The present invention is described using an example of 8-16 modulation of DVD as the recording modulation method. However, the present invention can be applied to the case in which, for example, ( 1 ,  7 ) RLL (Run Length Limited) modulation code is used.  FIG. 9A  shows a histogram when a asymmetrical reproduction waveform is sampled by A/D converter  5  (FIG.  1 ). This histogram shows the reproduction signal when phase errors are detected with the reproduction signal level ( 0 ) set as a reference and the clock frequency and phase for sampling the reproduction signal is controlled as is the case of the present embodiment. When the reference for detecting the phase error is varied, histograms shown in  FIGS. 21A ,  21 B, and  9 A are not obtained. 
   Furthermore,  FIG. 9B  shows a histogram of the output signal of the conventional waveform equalizer, which does not change the equalizing characteristics on the mark side and on the space side. According to  FIG. 9B , when the waveform is equalized to PR ( 1 ,  2 ,  2 ,  1 ) characteristics, it is expected that the output of the waveform equalizer is divided into seven signal distributions. However, since asymmetry exists in the reproduction waveform, the output is not successfully distributed into seven signal level bands. Note that “being equalized to PR ( 1 ,  2 ,  2 ,  1 ) characteristics” means that when the signals read from the disc are ( 1 ,  0 ,  0 ,  1 ), the waveform equalizer outputs 1×1+2×0+2×0+1×1=2. Because when width of the mark that expresses 1 and the space that expresses 0 are set to “2T or more,” patterns of ( 1 ,  0 ,  1 ) and ( 0 ,  1 ,  0 ) are not included in the entered signal, the outputs can be restricted to 7 kinds. Thus, the output signal from the wavelength equalizer is assumed to be classified into 7 signal distributions. 
   On the other hand,  FIG. 9C  shows a histogram of output signal of the waveform equalizer according to the present invention. As shown in  FIG. 9C , distribution of the output signal from the waveform equalizer according to the present invention is clearly divided into seven signal level bands and has small variance. The waveform equalizer according to the present invention can suppress displacement and variance at the detection point and can improve the performance of PRML, even when ( 1 , 7 ) RLL demodulation code is used. 
   Not only due to influence of asymmetry shown in  FIG. 9A  but also due to influence of noise and various stresses, variance of each level of the reproduction signal may be increased. When the variance is small, the sample value comes close to the reproduction signal level  0  shown on the abscissa of  FIGS. 9A through C  at the time the tap coefficient is changed over by the control signal from polarity discriminating circuit  56  (FIG.  6 ). Assuming that the reproduction signal level is set to 0, the tap coefficient must be multiplied by 0. As a result, the same value is obtained even before and after the tap coefficient is changed over, which appears to give no detrimental effect. However, due to various causes, there are cases in which the sample value at the time of change-over (reproduction signal level is in the vicinity of  0 ) has a comparatively large value. In such event, the multiplication results may greatly differ before and after the tap coefficient is changed over, resulting in detrimental effects in the waveform equalization. Consequently, the sample value preferably has the value close to the reproduction signal level  0 , at the time the tap coefficient is changed over by the control signal by the polarity discriminating circuit  56  (FIG.  6 ). In order to avoid detrimental effects, the sample value is set to achieve smaller values, such as reducing to ½, ¼, or ⅛ at the time described above. 
   (Second Embodiment) 
     FIG. 10  is a block diagram of the configuration of PRML detector  100  according to embodiment 2. The features of PRML detector  100  lies in Viterbi recorder  112  that outputs temporary judgment results. As described in  FIG. 19  or  FIG. 20 , the conventional PR temporary judgment is made based on the signal before it is entered in the Viterbi decoder (i.e. maximum likelihood decoder). For this PR temporary judgment, more accurate binary-coded data string in the Viterbi decoder is used to obtain highly accurate desired PR characteristics. 
   PRML detector  100  comprises an FIR equalizer  111 , Viterbi decoder  112 , PR equalization target value judging section  113 , and coefficient adaptive controller  114 . PRML detector  100  has functions corresponding to the wavelength equalizer  11  of information reproducing apparatus  1  described referring to FIG.  1  and maximum likelihood decoder  6 . FIR equalizer  111 , PR equalization target value judging section  113 , and coefficient adaptive controller  114  are also called the adaptive equalizer and corresponds to waveform equalizer  11  (FIG.  1 ). More specifically, FIR equalizer  111  of PRML detector  100  corresponds to FIR filter  46  (FIG.  6 ), PR equalization target value judging section  113  corresponds to error signal detector  51  (FIG.  6 ), and coefficient adaptive controller  114  corresponds primarily to correlation detector  53 , loop gain setting section  54 , coefficient arithmetic section  55 , and polarity discriminating circuit  56 . 
   To FIR equalizer  111 , digital signal after AD conversion is entered. By using FIR equalizer  111 , the signal can be equalized to the desired PR characteristics. PR will be described in detail as follows. 
   The recording and reproducing system of the optical disc has various fluctuations of low-frequency components. When the recording density is increased, the upper limit of the frequency band of the recording and reproducing system is used and relevant reproduction waveforms tend to cause interference when adjacent marks are read. When reproduction waveforms cause interference, reading errors occur. This phenomenon is called as intercede interference. PR equalization positively utilizes the intercede interference. Thus, the significance of the data at the sampling point can be determined in accordance with the transmission characteristics. This means that the reproduction signal from the disc can be equalized in the desired form or characteristics. 
   In PR equalization, various method exist. Consequently, it is necessary to select the system that is consistent to the frequency characteristics of the recording medium. In the case of optical discs, particularly, in the case of DVD, it is essential to select PR to conform to the modulation transfer function (MTF), which is the frequency characteristics of the optical system and that takes into account the modulation frequency characteristics of recorded code. In reproducing DVD, codeword of minimum code length 3T (such as EFM (Eight to Fourteen Modulation) or EFM-Plus code) is utilized. If reproduction signals of DVD are PR-equalized to adopt PR (a, b, b, a) of 4 PR length, the signal can be restricted to five levels ( 0 , a, a+b, a+ 2   b ,  2   a + 2   b ). Consequently, the number of states of the Viterbi decoder is  5 . In this “a” and “b,” integers are entered. In addition, if it is the case in which the codeword of 2T minimum code length such as ( 1 ,  7 ) RLL (Run Length Limited) code is utilized for the optical disc and PR (a, b, b, a) of  4  PR length is further adopted, the signal level has 7 values ( 0 , a,  2   a , a+b,  2   b , a+ 2   b ,  2   a + 2   b ) and the number of states of the. Viterbi decoder is 7. As the PR length increases, the signal levels increases and the number of states of the Viterbi decoder increases. That is, still more complicated system results. 
   As described above, when a signal is PR-equalized using the adaptive algorithm, as the probability to make mistakes in temporary judgment increases, it becomes difficult to calculate accurate equalization errors at all the equalization targets and satisfactory convergence characteristics are unable to be obtained. Therefore, as shown in  FIG. 10 , by extracting a binary-coded data string used for temporary judgment from Viterbi decoder  12 , error rate of the temporary judgment can be reduced. 
   In the following section, a system which combines a codeword of 2T minimum code length with PR (a, b, b, a) is taken as an example, and explanation will be made on the fact in that a more accurate equalizing error is calculated for the desired equalization targets, and satisfactory converging characteristics is obtained. 
     FIG. 11  shows a state transition diagram of Viterbi decoder  112  ( FIG. 10 ) when code words, having minimum code length 2T, with PR (a, b, b, a) are adopted. When codeword of 2T minimum code length is used, since patterns of “010” and “101” do not exist in the coding group, the number of states is restricted to 6 and the number of paths is restricted to 10. When a signal level is calculated from this 6 states and 10 paths, an output for signal series input “0000” is “0”, an output for input “0001” is “a”, an output for input “0011” is “a+b”, an output for input “0110” is “ 2   b ”, an output for input “0111” is “a+ 2   b ”, an output for input “1000” is “a”, an output for input “1001” is “ 2   a ”, an output for input “1100” is “a+b”, an output for input “1110” is “a+ 2   b ”, and an output for input “1111” is “ 2   a + 2   b ”. 
   As a result, there are 7 levels exist for the output signal level, namely, “0,” “a,” “a+b,” “ 2   a ,” “ 2   b ,” “a+ 2   b ,” and “ 2   a + 2   b ,” each level becomes the PR equalization target value described above. That is, coefficient adaptive controller  114  ( FIG. 10 ) updates the tap of FIR equalizer  111  ( FIG. 10 ) as is the case in which the input signal is equalized to the equalization target value. Equalization takes place by reducing difference (i.e. equalizing error) between an output signal of FIR equalizer  111  ( FIG. 10 ) and a PR equalization target value. 
   Next, discussion will be made on the action of Viterbi decoder  112  ( FIG. 10 ) with respect to the PR equalizing method described above. Viterbi decoder  112  ( FIG. 10 ) does not carry out judgment of “0” or “1” (so-called hard judgment) at a certain threshold value with respect to the input data as is the case with the level detection system. Viterbi decoder  112  ( FIG. 10 ) carries out judgment of the most probable data string (so-called soft judgment) based on the data strings digitized in the past. 
     FIG. 12  is a block diagram of the specific configuration of Viterbi decoder  112 . Viterbi decoder  112  generally includes a branch metric arithmetic circuit  151 , path metric arithmetic circuit  152 , and path memory  153 . Branch metric arithmetic circuit  151  calculates branch metric which is square errors of signals entered from FIR equalizer  111  ( FIG. 10 ) for respective channel clock (i.e. sample data) and 7 different expected values [d 0 , d 1 , d 2 , d 3 , d 4 , d 5 , d 6 ] entered from coefficient adaptive controller  114  (FIG.  10 ). These 7 expected values correspond to signal levels of “0,” “a,” , “a+b,” “ 2   a ,” “ 2   b ,” “a+ 2   b ,” and “ 2   a + 2   b ,” respectively. Specifically, branch metric arithmetic circuit  151  calculates branch metric BMk(i) by the following Eq.2.
   BMk ( i )=( y   k   −di ) 2   (Eq. 2) 
where, y k  is the signal entered from FIR equalizer  111  ( FIG. 10 ) (i.e. sample data) and di (i=0, 1, . . . , 6) are seven expected values [d 0 , d 1 , d 2 , d 3 , d 4 , d 5 , d 6 ].
 
   Then, path metric arithmetic circuit  152  cumulatively adds branch metrics by every 1 channel clock and calculates path metric. Specifically, path metric arithmetic circuit  152  calculate the path metric by the following Eq. 3.
 
 PM   k   s0 =min [ PM   k-1   s0   +BM   k (0),  PM   k-1   s5   +BM   k (1)]
 
 PM   k   s1 =min [ PM   k-1   s0   +BM   k (1),  PM   k-1   s5   +BM   k (2)]
 
 PM   k   s2   =PM   k-1   s1   +BM   k (3)
 
 PM   k   s3 =min [ PM   k-1   s3   +BM   k (6),  PM   k-1   s2   +BM   k (5)]
 
 PM   k   s4 =min [ PM   k-1   s3   +BM   k (5),  PM   k-1   s2   +BM   k (4)]
 
  PM   k   s5   =PM   k-1   s4   +BM   k (3)  (Eq. 3)
 
   In Eq. 3, “min” is a mathematical symbol. For example, min [a, b] indicates the smaller one in “a” and “b” (either one when a=b). 
   Path metric arithmetic circuit  152  calculates signals [sel 0 , sel 1 , sel 2 , sel 3 ] for selecting data sequence in which path metric becomes the minimum, that is, which is the most probable, based on Eq. 4 through Eq. 7 and outputs results to path memory  153 .
 
 PM   k-1   s0   +BM   k (0)≧ PM   k-1   s5   +BM   k (1) Sel 0 =1
 
 PM   k-1   s0   +BM   k (0)&lt; PM   k-1   s5   +BM   k (1) Sel 0 =0  (Eq. 4)
 
 PM   k-1   s0   +BM   k (1)≧ PM   k-1   s5   +BM   k (2) Sel 1 =1
 
 PM   k-1   s0   +BM   k (1)&lt; PM   k-1   s5   +BM   k (2) Sel 1 =0  (Eq. 5)
 
 PM   k-1   s3   +BM   k (6)≧ PM   k-1   s2   +BM   k (5) Sel 2 =1
 
 PM   k-1   s3   +BM   k (6)&lt; PM   k-1   s2   +BM   k (5) Sel 2 =0  (Eq. 6)
 
 PM   k-1   s3   +BM   k (5)≧ PM   k-1   s2   +BM   k (4) Sel 3 =1
 
 PM   k-1   s3   +BM   k (5)&lt; PM   k-1   s2   +BM   k (4) Sel 3 =0  (Eq. 7)
 
   Path memory  153  stores one or more specified candidate sequences and outputs the data string based on the selection signals [sel 0 , sel 1 , sel 2 , sel 3 ] received from path metric arithmetic circuit  152 . When memory length of path memory  153  in which the data string is stored is increased, the probability of correct selection increases, but conversely, if it is too long, the circuit scale increases. Consequently, correct selection probability and the circuit scale have a trade-off relation and are determined referring to the performance and the circuit scale. Furthermore, in the present embodiment, path memory  153  outputs the temporary judgment data sequence in the midway. The outputted temporary judgment data sequence is used for the temporary judgment value for judging the PR equalization target value. 
     FIG. 13  is a circuit diagram of the detailed configuration of path memory  153  (FIG.  12 ). Path memory  153  includes a plurality of flip-flops (FFs) and selectors. Flip-flops FFs are state registers and are arranged in 6 pieces in the longitudinal direction of the figure and the number “6” corresponds to the number of states. The lateral direction corresponds to the memory length of path memory  153 . Note that one set of four selectors and 6 flip-flops FF arranged in the longitudinal direction in the figure forms one stage. Path memory  153  ( FIG. 12 ) is formed by 20 to 30 stages. Path memory  153  ( FIG. 12 ) receives selection signals sel 0 , sel 1 , sel 2 , sel 3  from path metric arithmetic circuit  152  ( FIG. 12 ) and selects either data “0” or “1” entered in FF based on the received selection signals. In the figure, to the leftmost side flip-flops FF, [011101] are entered successively from the top as initial values. The selection signal is controlled to select the most probable path. As a result, the paths are combined together into one path and the output of flip-flop FF of each stage becomes same in a certain path memory length. That is, at the final stage, the output from any flip-flop FF is the same. The final output is outputted from Viterbi decoder  112  as the Viterbi detection output. 
   The path memory length that outputs the temporary judgment value sequence must be properly selected by combining the codeword used and PR. Specifically, it is essential to select the length that does not greatly degrade the error rate as compared to the final stage of the path memory in the Viterbi decoder. However, excessively long path memory length that outputs the sequence of temporary judgment values increases the delay of the feedback loop until the tap coefficient is updated. Consequently, in view of these two points, the path memory length that outputs the temporary judgment must be properly selected. 
   Now, discussion will be made on the method to facilitate the recovery when loop performance is degraded in the feedback system or the loop performance is likely to be degraded. According to this method, even if the error rate of the temporary judgment group from Viterbi decoder  112  ( FIG. 10 ) in the present invention is increased, unstable operation of PRML detector  100  will be able to be prevented. 
     FIG. 14  is a block diagram of the detailed configuration to carry out temporary judgment output ( FIG. 13 ) in path memory  153  ( FIG. 12 ) of Viterbi decoder  112 . As already discussed, in Viterbi decoder  112  (FIG.  12 ), selecting the most probable paths draws the paths together to one path. That is, in a flip-flop FF (a state register) before path memory  153  ( FIG. 12 ) is all finished to be transferred, the output of flip-flop FF is converged to one same value, which is called “merged.” However, there is a case in which the paths are not drawn together even though the most probable paths are selected, that is, there is a case in which the output of flip-flop FF is not converged. In such event, flip-flop FF is in the condition to hold a plurality of candidate paths, which is called “not merged.” When the output is not merged, flip-flop FF continues transferring the output, and this condition increases the possibility of mistaking temporary judgment output and the final output (Viterbi detected output) of Viterbi decoder  112  (FIG.  12 ). 
   Therefore, in the embodiment, the merge check signal indicating whether merged or not is outputted.  FIG. 14  is a diagram of a circuit for outputting the merge check signal, formed by the use of two NOR circuits and an AND circuit. The output of six flip-flops FFs of the specified stage is entered in the first NOR circuit and the AND circuit, respectively, and the output is entered in the second NOR. The merge check signal is obtained as the output from the second NOR circuit. The merge check signal becomes of the low level when the output is merged, and when the output is not merged, it becomes of the high level. 
   As shown in  FIG. 10 , coefficient adaptive controller  114  ( FIG. 10 ) may determine whether the temporary judgment sequence is fed back or not by the use of the merge check signal. For example, when the merge check signal is of a high level or when the frequency of achieving the high level in one channel clock section exceeds a specified value, coefficient adaptive controller  114  ( FIG. 10 ) may stop updating the tap coefficient or may reset (initialize) the tap coefficient to the specified initial value. Furthermore, it may be changed over to return to the processing by conventional feed-forward (FIG.  19 ). Furthermore, in this case, the binary-coded data outputted from phase comparator  222  ( FIG. 19 ) may be outputted as it is as the final binary-coded data so that PRML detection does not take place. Or processing may be changed over to the processing method ( FIG. 20 ) that further reduces the feedback delay. Further fail-safe measures are achieved by changing the utilized circuit and the output in accordance with the merge check signal. 
   Referring now again to  FIG. 10 , how the PR equalization target value judging section  13  judges the desired PR target value based on the temporary judgment output of Viterbi decoder  112  will be discussed. In this case, PR (a, b, b, a) described above is taken for an example, and the PR equalization target value judging section  13  determines the PR target value based on the state transition diagram ( FIG. 11 ) in which the minimum code length is determined by 2T codeword and PR (a, b, b, a). 
   To be more specific, the PR equalization target value judging section  113  is equipped with a 4-channel-bit table. The value of each channel bit is a temporary judgment output value from Viterbi decoder  12 . In the table, the corresponding relationships between the input values and the output values are defined on the basis of the state transition diagram (FIG.  11 ). That is, the table is defined to recite the following relationships:
         the PR target value for the case of “0000” is “0”,   the PR target value for the case of “0001” is “a”,   the PR target value for the case of “0011” is “a+b”,   the PR target value for the case of “0110” is “ 2   b ”,   the PR target value for the case of “0111” is “a+ 2   b ”,   the PR target value for the case of “1000” is “a”,   the PR target value for the case of “1001” is “ 2   a ”,   the PR target value for the case of “1100” is “a+b”,   the PR target value for the case of “1110” is “a+ 2   b ”, and   the PR target value for the case of “1111” is “ 2   a + 2   b ”.       

     FIG. 15  illustrates outputs of FIR equalizer  111  ( FIG. 1 ) and a deciding procedure of the PR equalization target value. The output of FIR equalizer  111  ( FIG. 1 ) is the signal waveform when 5T mark, 2T space, and 3T mark are read, and let yk[ 0 ] through yk[ 14 ] denote sampling value for each channel clock. On the other hand, let “00111110011100” denote temporary judgment outputs from Viterbi decoder  112  ( FIG. 10 ) obtained on the basis of the outputs of FIR equalizer  111  (FIG.  1 ). According to the table above and the temporary judgment output, PR equalization target value judging section  113  ( FIG. 10 ) judges that 
   (1) the PR target value for “0011” of the first to fourth bits is “a+b”, 
   (2) the PR target value for “0111” of the second to fifth bits is “a+ 2   b”,    
   (3) the PR target value for “1111” of the third to sixth bits is “ 2   a + 2   b”,    
   (4) the PR target value for “1111” of the fourth to seventh bits is “ 2   a + 2   b”, and    
   (5) the PR target value for “1110” of the fifth to eighth bits is “a+ 2   b ”. 
   The PR equalization target value judging section  113  ( FIG. 10 ) decides the PR target value of (1) as the target value for the FIR equalizer output yk[ 2 ]. In addition, the PR equalization target value judging section  113  ( FIG. 10 ) decides the PR target value (2) as the target value for yk[3]. Similarly, as the target value for yk[ 4 ], the PR target value of (3) and as the target value for yk[ 5 ], the PR target value of (4), and as the target value for yk[ 6 ], the PR target value of (5), and as the target value for yk[ 7 ], the PR target value of (5) are decided. 
   Coefficient adaptive controller  114  ( FIG. 10 ) updates taps of FIR equalizer  111  ( FIG. 10 ) in such a manner that the input signal is equalized to the equalization target value, that is, the equalizing error is reduced. The equalizing error can be found from the difference between the PR equalization target value and the output value of FIR equalizer  111  ( FIG. 10 ) against the PR equalization target value. 
   Note that, if any sequence that does not fit to the state transition rule ( FIG. 11 ) is entered when the PR equalization target value is decided by the PR equalization target value judging section  113  (FIG.  10 ), it can be judged apparently an error in the temporary judgment group. In such event, PR equalization target value judging section  113  ( FIG. 10 ) instructs coefficient adaptive controller  114  ( FIG. 10 ) to stop updating the tap coefficients of FIR equalizer  111  (FIG.  10 ). Stopping updating of the tap coefficients can avoid erroneous tap updating. 
   As described above, embodiments 1 and 2 according to the present invention were discussed. Embodiment 1 reduces errors at the time of data decoding caused by the asymmetry of the physical profile of the mark on the recording medium. On the other hand, embodiment 2 improves the accuracy of the temporary judgment when coefficients of the FIR equalizer are adaptively controlled and reduces errors at the time of data decoding. Because the inventions according to embodiments 1 and 2 intend to reduce errors resulting from different causes, both inventions can be combined. Specifically, to FIR equalizer  111  of  FIG. 10 , asymmetry detector  15 , polarity discriminator  16 , and coefficient C selector  17  of  FIG. 2  should be incorporated. Thus, effects of both embodiments 1 and 2 can be obtained and at the same time errors at the time of decoding can be further suppressed to a low level. 
   The present invention was described using specific exemplary embodiments. For those skilled in the art, many different modifications and improvements would be apparent. Consequently, the embodiments described above shall not restrict the scope of the present invention as specified by the claims attached.