Abstract:
A recording/reproducing apparatus records and reproduces, over a partial response channel, a recording signal produced by encoding data according to a convolutional code and reproduces the data from a reproduction signal by iterative decoding using likelihood information. A burst error detector detects a burst error part in the reproduction signal. A substituting part substitutes, for a sampling value included in the burst error part, a predetermined value according to a detected result of the burst error detector.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to data recording/reproducing apparatuses, and more particularly, to a data recording/reproducing apparatus having a substituting part substituting for a burst error and to a method of substituting for a burst error.  
           [0003]    2. Description of the Related Art  
           [0004]    Apparatuses that record and reproduce data include various recording/reproducing apparatuses, such as recording/reproducing apparatuses of magnetic disks, magnetic tapes, optical disks, magnetic optical disks, and the like. In order to record data on such media, magnetic recording marks are mainly used. It is possible to save data permanently and at lower cost than semiconductor memories by magnetic recording. Nowadays, recording/reproducing apparatuses are essential as information recording apparatuses for computers, for recording such as images and image information having a lot of information.  
           [0005]    [0005]FIG. 1 shows the construction of a conventional data recording apparatus.  
           [0006]    First, a description will be given of a case where data are recorded. User data U k  are input to an encoder  101  that modulates the user data U k  to codes that can be iteratively decoded. Then, data interleaved via a puncture part (MUX puncture)  102  and an interleaver (π)  103  are supplied to an LD driver  104 . The LD driver  104  modulates a laser beam based on the supplied data and records the data on an information recording medium  105 . In an example shown in FIG. 1, a magnetic optical disk is used as the information recording medium  105  (hereinafter referred to as the “magnetic optical disk  105 ”). However, a magnetic disk, an optical disk, and other information recording media may also be used. In the case of a magnetic disk, the data are supplied to a magnetic head suitable for the recording medium.  
           [0007]    Next, a description will be given of a case where data are reproduced from the magnetic optical disk  105 . Recording marks are reproduced from the magnetic optical disk  105  by a head and a MO reproduction signal is obtained. A recording/reproducing system  106  constructed by a writing head, the magnetic optical disk  105 , and the reproducing head forms a partial response channel (PR channel) having characteristics such as PR ( 1 ,  1 ). The reproduced MO reproduction signal is amplified by an amplifier  110 . Then, the amplitude of the signal is controlled by an AGC  111 , and thereafter waveform equalization is performed on the signal by a low-pass filter (LPF)  112  and an equalizer (EQ)  113 . The MO reproduction signal Yi subjected to waveform equalization as described above is converted to a digital signal by an A/D converter  114  by using a clock synchronized with the reproduction signal. Then, the digital signal thus converted is accumulated in a memory  115 .  
           [0008]    Next, based on the data accumulated in the memory  115 , the user data are reproduced by a iterative decoder  116  such as a turbo decoder. The iterative decoder  116  is controlled by a controller  117  (for example, an ODC in the case of a magnetic optical disk apparatus). The iterative decoder  116  decodes the user data through iterative decoding of the number of times determined by the controller  117 .  
           [0009]    [0009]FIG. 2 shows an example of the encoder  101  that encodes the user data into codes for performing iterative decoding. The encoder shown in FIG. 2 is an iterative convolutional encoder and is constructed by registers  201  and  202 , and exclusive ORs  203  and  204 . The encoder shown in FIG. 2 generates a parity sequence p k  from the user data sequence U k .  
           [0010]    [0010]FIG. 3 shows an example of a conventional construction of the iterative decoder  116  in FIG. 1. Data (a reception signal sequence) y i  represent a reception signal digitized by the A/D converter  114  and accumulated in the memory  115  shown in FIG. 1. The sampling data y i  are supplied to an a posteriori probability decoder (PR Channel APP)  301 . The a posteriori probability decoder  301  calculates, under the condition where input sampling value Y (y 1 , y 2 , y 3 , . . . y n ) is detected, a logarithmic likelihood ratio L(c i *) between the probability P (ci=1|y) that the next input bit ci is 1 and the probability P (ci=0|y) that ci is 0. When iteration is made for the first time, a priori information La(c i ) input to the a posteriori probability decoder  301  is all zeros. This represents that the probability that all of the bits ci are “1” and the probability that all of the bits ci are “0” are the same probability (are equal).  
           [0011]    Then, the a priori information La(c i ) is subtracted from L(c i *), which is the output of the a posteriori probability decoder  301 , by a subtractor  302  so as to obtain extrinsic likelihood information Le(c). The extrinsic likelihood information Le(c) is converted by a deinterleaver  303  and thereafter sent to a depuncture part  304 . The depuncture part  304  converts the deinterleaved extrinsic likelihood information Le(c) to likelihood information L(u k ) corresponding to a data bit U k  and likelihood information L(P k ) corresponding to a parity bit P k  and supplies the information to a code decoder (Code APP)  305 . The code decoder  305  outputs a logarithmic likelihood ratio L(u*) with respect to the data bit u k  and a logarithmic likelihood ratio L(p*) with respect to the parity bit p k  from L(u k ) and L(p k ), respectively. When performing iterative decoding, L(u*) and L(p*) are sent to a puncture part  306  and converted to likelihood information L(c*)(the result of combining and thinning out L(u*) and L(p*)). A priori information Le(c) is subtracted from L(c*) by a subtractor  307 . Then, interleaving is performed by an interleaver  308  on the output of the subtractor  307  so as to obtain La(c i ). La(c i ) is supplied to the a posteriori probability decoder (PR Channel APP)  301  as a priori information and iteration is repeatedly performed. Data detection is performed such that a hard decision part  309  determines whether L(u*) obtained from the code decoder  305  is “1” or “0” and outputs the user data sequence U k .  
           [0012]    However, the above-described conventional example suffers from the following problems.  
           [0013]    Generally, there are local defects in recording media such as optical disks (including magnetic optical disks), magnetic disks, and magnetic tapes. Especially, in optical disks and magnetic tapes that are replaceable media, defective parts are increased by the influence of adhesion of dust and scratches made when handling them. The iterative decoding described above operates very effectively for reduced SNR associated with recording media and apparatuses of higher density. When a reproduction signal (burst error signal) of a defective part in a recording medium is input, however, likelihood information that is made vastly different via a priori information is propagated to data of a part(s) other than the burst error part, and an error in the burst error part is propagated to the data of the other part(s). This is because the likelihood information obtained from the data of the burst error part is greatly different from the likelihood information obtained from the original data. Hence, there is a problem in that the effect of error correction by iterative decoding cannot be obtained sufficiently.  
         SUMMARY OF THE INVENTION  
         [0014]    It is a general object of the present invention to provide an improved and useful data recording/reproducing apparatus and method in which the above-mentioned problems are eliminated.  
           [0015]    It is another and more specific object of the present invention to provide a data recording/reproducing apparatus using iterative decoding and capable of correctly demodulating data even in a case where a reproduction signal includes a burst error signal, that is, capable of adequately obtaining the effect of error correction by iterative decoding.  
           [0016]    In order to achieve the above-mentioned objects according to one aspect of the present invention, there is provided a recording/reproducing apparatus that records and reproduces, over a partial response channel, a recording signal produced by encoding data according to a convolutional code and reproduces the data from a reproduction signal by iterative decoding using likelihood information, the recording/reproducing apparatus including:  
           [0017]    a burst error detector detecting a burst error part in the reproduction signal; and  
           [0018]    a substituting part substituting, for a sampling value included in the burst error part, a predetermined value according to a detected result of the burst error detector.  
           [0019]    According to the present invention, by detecting a burst error part and substituting, for the burst error part, a value that does not exert influence on a part(s) other than the burst error part, it is possible to control the influence of wrong likelihood information in iterative decoding. Thus, it is possible to maintain the decoding ability of iterative decoding.  
           [0020]    As described above, according to the present invention, wrong likelihood information is not propagated even if a reproduction signal includes a burst error part. Hence, it is possible to obtain a recording/reproducing apparatus having high decoding ability even with low S/N ratios by iterative decoding.  
           [0021]    Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the following drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is a block diagram showing the construction of a conventional data recording apparatus using iterative decoding;  
         [0023]    [0023]FIG. 2 is a block diagram showing an example of the construction of an encoder that encodes user data to codes for performing iterative decoding;  
         [0024]    [0024]FIG. 3 is a block diagram showing an example of a conventional construction of the iterative decoder shown in FIG. 1;  
         [0025]    [0025]FIG. 4 is a block diagram showing a first embodiment of the present invention;  
         [0026]    [0026]FIG. 5 is a block diagram showing a second embodiment of the present invention;  
         [0027]    [0027]FIG. 6 is a block diagram showing a third embodiment of the present invention;  
         [0028]    [0028]FIG. 7 is a block diagram showing a fourth embodiment of the present invention;  
         [0029]    [0029]FIG. 8 is a block diagram showing a fifth embodiment of the present invention;  
         [0030]    [0030]FIG. 9 is a block diagram showing a sixth embodiment of the present invention  
         [0031]    [0031]FIG. 10 is a timing diagram showing an operation example of a burst error waveform;  
         [0032]    [0032]FIG. 11 is a block diagram showing a seventh embodiment of the present invention;  
         [0033]    [0033]FIG. 12 is a graph showing simulation results of the error rate with respect to the number of times of iteration of iterative decoding using the present invention;  
         [0034]    [0034]FIG. 13 is a block diagram showing one embodiment of a burst error detector of the present invention; and  
         [0035]    [0035]FIG. 14 is a timing diagram for explaining the operation of the burst error detector of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0036]    A description will be given of preferred embodiments of the present invention.  
         [0037]    [0037]FIG. 4 shows a first embodiment of the present invention. The first embodiment of the present invention shown in FIG. 4 differs from the recording/reproducing system of optical disks using conventional iterative decoding shown in FIG. 1 in that a burst error detector  401  as burst detecting means and a substituting circuit  402  as substituting means are provided in the first embodiment shown in FIG. 4. Fundamental recording and reproducing of data in the first embodiment are the same as those explained with reference to FIG. 1.  
         [0038]    In the first embodiment shown in FIG. 4, the A/D converter  114  converts the MO reproduction signal subjected to waveform equalization into digital data, and, from this value, the burst error detector  401  detects a burst error region from the value. Then, the substituting circuit  402  substitutes, for the data corresponding to the burst error part, likelihood information that does not affect (that hardly propagates the error to) data of parts other than the burst error part when iterative decoding is performed by the iterative decoder  116 . The data whose value in the burst error part is substituted for are accumulated in the memory  115 . The data are substituted for with likelihood information representing that the probability of “1” and the probability of “0” are the same probabilities. For example, in a case where highest likelihood information of probability that data are “1” (lowest likelihood information of probability that data are “0”) is +1, and lowest likelihood information of probability that data are “1” (highest likelihood information of probability that data are “0”) is −1, the data to be substituted for are replaced by the intermediate value “0” of likelihood information. Hence, it is possible to exert the least influence of the burst error part on parts other than the burst error part. In this manner, the iterative decoder  116  performs iterative decoding on the data accumulated in the memory  115 , including the data in the burst error part whose values are substituted. It should be noted that the reason for accumulating data in the memory  115  is that the iterative process of the iterative decoder  116  is an operation having a lower rate than the channel transfer rate. Moreover, another reason is that, in iterative decoding, it is necessary to perform a backward operation with respect to pathmetric. In some cases, the memory  115  may not be required depending on the execution method of the following iterative decoding.  
         [0039]    Next, FIG. 5 shows a second embodiment of the present invention. In this embodiment, the MO reproduction signal subjected to waveform equalization is converted to a digital signal by the A/D converter  114 , and thereafter the digital value is temporarily accumulated in the memory  115 . Then, using the accumulated values, a burst error is detected by the burst error detector  401 , and the substitution of data is performed by the substituting circuit  402 . The data to be substituted are the same as those in the first embodiment shown in FIG. 4.  
         [0040]    In this embodiment, while reading the data from the memory  115 , the read data are substituted and then supplied to the iterative decoder  116 . It is possible to replace burst error data in this manner.  
         [0041]    [0041]FIG. 6 shows a third embodiment of the present invention. In this embodiment, while reading the data from the memory  115 , a burst error is detected by the burst error detector  401 , the read data are substituted by the substituting circuit  402 , and then the data are written again in the memory  115 . The data to be substituted are the same as those in the first embodiment shown in FIG. 4. It is possible to substitute the data in the memory  115  in this manner.  
         [0042]    [0042]FIG. 7 shows a fourth embodiment of the present invention. In FIG. 7, those parts that are designated by the same reference numerals in FIG. 3 are the same as those corresponding parts in FIG. 3. In this embodiment, the output of the a posteriori probability decoder (PR Channel APP)  301  is substituted for. In FIG. 7, using the data output from the memory  115 , which data are the input to the a posteriori probability decoder  301 , a burst error position is detected by the burst error detector  401 , and the output of the a posteriori probability decoder  301  is replaced by the substituting circuit  701 . It is possible to substitute for burst error data in this manner. The data to be replaced are substituted for with likelihood information representing that the probability of “1” and the probability of “0” are the same probability. For example, in a case where highest likelihood information of probability that data are “1” (lowest likelihood information of probability that data are “0”) is “+1”, and lowest likelihood information of probability that data are “1” (highest likelihood information of probability that data are “0”) is “−1”, the data are replaced with the intermediate value “0” of likelihood information. Hence, it is possible to exert the least influence of the burst error part on parts other than the burst error part.  
         [0043]    Next, a description will be given of a fifth embodiment of the present invention. FIG. 8 shows the fifth embodiment of the present invention. In the embodiment shown in FIG. 8, those parts that are designated by the same reference numerals in the fourth embodiment shown in FIG. 7 are the same as those corresponding parts in FIG. 7. The fifth embodiment of the present invention shown in FIG. 8 differs from the fourth embodiment of the present invention shown in FIG. 7 in that a select circuit  801  is provided in the fifth embodiment.  
         [0044]    In the initial stage of iterative decoding, such as the number of times of iteration is one and two, likelihood information of the PR channel corresponding to the burst error part exerts great influence on likelihood information of parts other than the burst error part. In order to control this, in this embodiment, based on control information  118  of the number of times of iteration supplied to the iterative decoder  116  from the controller  117  shown in FIG. 1, whether to select and send, to the subtractor  302 , L(c i *) that is output from the a posteriori probability decoder  301  or to select and send, to the subtractor  302 , the output of the substituting circuit  701  is controlled in accordance with the number of times of iterative decoding.  
         [0045]    Next, a description will be given of a sixth embodiment of the present invention. FIG. 9 shows the sixth embodiment of the present invention. In this embodiment, data of a burst error part and vicinity are replaced through performing a predetermined operation by an operation part  901  with respect to the data accumulated in the memory  115  and corresponding to the burst error part and vicinity detected by the burst error detector  401 .  
         [0046]    [0046]FIG. 10 shows one embodiment of the operation with respect to a burst error waveform. FIG. 10-A represents a reproduction waveform of a burst error part, FIG. 10-B represents an operation coefficient k, and FIG. 10-C represents the waveform after the operation by the operation part  901 . In FIG. 10-A, yt indicates each sampling value, a time period T indicates the burst error part, B 1  indicates a threshold value on the positive side of a burst error detection level, B 2  indicates a threshold value on the negative side of the burst error detection level, and C indicates the center value. The operation of the operation part  901  is performed according to: 
           yt′=k*yt+C (1 −k )  (1) 
         [0047]    where yt′ is the sampling value after the operation.  
         [0048]    First, the burst error detector  401  shown in FIG. 9 reads the accumulated data from the memory  115  and detects the burst error part T. Then, with a central focus on the range of the burst error part, the sampling value is calculated according to the equation (1) by using the operation coefficient k represented by FIG. 10-B. For example, in FIG. 10-A, when the sampling value yt has an amplitude greater than the threshold value B 1  in the time period  25 - 32 , the burst error detector  401  detects that a burst error part due to scratches of dust exists. Usually, the influence of such as scratches is exerted also on parts before and after the burst error part. Therefore, while reading the data from the memory  115 , the operation coefficient k is varied as indicated by FIG. 10-B, including the parts before and after the burst error part T.  
         [0049]    When the operation is executed according to the equation (1) by using the operation coefficient k, as represented by FIG. 10-C, the amplitude of the signal of the burst error part becomes small and assumes values close to the center value C. In the case where the reproduction waveform of FIG. 10-A is the waveform of PR( 1 ,  1 ), the center value C is a value at which whether data are “1” or “0” cannot be determined. Thus, according to the operation of this embodiment, it is possible to substitute, for the burst error part, likelihood information of the iterative decoding process that does not exert influence on other data.  
         [0050]    As described above, in the embodiments of the present invention explained with reference to FIGS. 4 through 10, the values of the burst error part in the sampling values of the MO reproduction waveform digitized by the A/D converter  114  are directly replaced or replaced through the operation, with values that do not exert influence on likelihood information of parts other than the burst error part. That is, the values of the burst error part are replaced by other values in the part corresponding to the PR channel data.  
         [0051]    Next, a description will be given of a seventh embodiment of the present invention. FIG. 11 shows the seventh embodiment of the present invention. In this embodiment, those parts that are designated by the same reference numerals in FIG. 7 are the same as those corresponding parts in FIG. 7. This embodiment shows an embodiment where data corresponding to Code data are replaced. In this embodiment, first, a burst error part is detected from the sampling value y i  that is output from the memory  115 . Then, deinterleaving is performed by a deinterleaver  1101  on the position of the detected burst error part, and the position corresponding to the burst error part on the PR channel is converted so as to correspond to the output of the deinterleaver  303  and supplied to a substituting circuit  1102  as substituting means.  
         [0052]    The substituting circuit  1102  substitutes, for likelihood information of the part corresponding to the burst error part, the deinterleaved extrinsic likelihood information Le(c) output from the deinterleaver  303 . In this case, the likelihood information Le(c) is a likelihood information ratio. Thus, if the probability that data are “1” is 100%, then Le(c)=1, and if the probability that data are “0” is 100%, then Le(c)=−1. In addition, if the probability that data are “1” and the probability that data are “0” are the same, then Le(c)=0. Accordingly, the likelihood information Le(c) corresponding to the burst error part is substituted as the value 0. In this manner, by substituting the likelihood information representing that the probability that data are “1” and the probability that data are “0” are same, the influence of the burst error part is not propagated to parts other than the burst error part.  
         [0053]    Next, a description will be given of simulation results of the error rate with respect to the number of times of iteration of the iterative decoding according to the present invention, in a case where a burst error part was generated. FIG. 12 shows the simulation results of the error rate with respect to the number of times of iteration of iterative decoding using the present invention. In a result  1201  of the case where a burst error part did not exist, the error rate at the beginning of the iterative decoding starts from 4.0×10  −4 , and as the number of times of iteration increases, the error rate falls. Then, in the third iteration of decoding, the error rate is stabilized at 1.0×10 −5 .  
         [0054]    On the other hand, in a result  1202  of the case where data of a burst error part were not replaced, the error rate does not fall in accordance with the increase of the number of times of iteration. This is because wrong likelihood information of the burst error part was propagated to parts other than the burst error part. Thus, the error rate fluctuated.  
         [0055]    In a result  1203  of the case where data of the burst error part were replaced according to the present invention, compared with the result  1201  of the case where the burst error part did not exist, a greater number of times of iteration is required for convergence. As the number of times of iteration of the iterative decoding increases, however, the error rate falls. In the fourth iteration of decoding, the error rate reaches an equivalent error rate of the result  1201  of the case where the burst error part did not exist.  
         [0056]    As described above, with the iteration decoding method according to the present invention, it is possible to obtain a system that does not propagate wrong likelihood information to parts outside of the burst error part and, by iterative decoding, possesses high decoding ability even for low S/N ratios.  
         [0057]    Next, by referring to FIGS. 13 and 14, a description will be given of one embodiment of the burst error detector of the present invention. FIG. 13 shows the embodiment of a burst error detector  1300  as burst detecting means of the present invention. FIG. 14 is a timing diagram for explaining the operation of the burst error detector  1300  of the present invention.  
         [0058]    [0058]FIG. 13 shows the embodiment of the burst error detector  1300 . The burst error detector  1300  includes comparators  1301  and  1302 , shift registers  1303  and  1304 , and an OR circuit  1305 . Each of the comparators  1301  and  1302  includes an input a and an input b, and it is assumed that when a is equal to or greater than b, the output is at a high level, and when a&lt;b, the output is at a low level. The comparator  1301  compares the sampling value yi with B 1  shown in FIG. 10-A, and determines whether the sampling value yi is in a burst error part. The comparator  1302  compares the sampling value yi with B 2  shown in FIG. 10-A, and determines whether the sampling value yi is in a burst error part. The output of the two comparators  1301  and  1302  are input to the N-stage shift registers  1303  and  1304 , respectively, which represent a burst error position. All output of each of the shift registers  1303  and  1304  is input to the OR circuit  1305 .  
         [0059]    The output of the OR circuit  1305  is a burst error gate signal (BG), that is, the burst error period T in FIG. 10-A. However, when the sampling value yi is delayed for N/ 2  stages of the shift register in a shifting circuit such as the shifting circuit  402  shown in FIG. 5, the BG is opened before N/ 2  of the BP. Thus, it is possible to deal with even the small influence of a burst error occurring in the hem (tail ends) of a Gaussian distribution of an optical beam due to dust and scratches.  
         [0060]    [0060]FIG. 14 shows the operation of the burst error detector  1300 . FIG. 14-A shows the sampling values obtained by sampling a signal  1401  that does not include a burst error part in reproduction data, and the sampling values obtained by sampling a signal  1402  that includes a burst error part in reproduction data.  
         [0061]    As explained by referring to FIG. 13, the burst error detector  1300  determines that yi is at the burst error position BP when yi is greater than the threshold value B 1  on the positive side of the burst error detection level, or when yi is smaller than the threshold value B 2  on the negative side of the burst error detection level.  
         [0062]    In this embodiment shown in FIG. 14, the case is shown where the number of stages of the shift registers  1303  and  1304  shown in FIG. 13 is, for example, N= 4 . As indicated by FIG. 14-B, when yi is greater than the threshold value B 1  on the positive side of the burst error detection level, the outputs of the shift register  1303  are high levels  1403  through  1406 . On the other hand, as indicated by FIG. 14-C, when yi is smaller than the threshold value B 2  on the negative side of the burst error detection level, the outputs of the shift register  1304  are high levels  1407  and  1408 . When all of the outputs of the N stages of each of the shift registers  1303  and  1304  are input to the OR circuit  1305 , as indicated by FIG. 14-D, a signal comprising high levels  1409  through  1411  in the burst error periods is obtained as the output of the OR circuit  1305 . In this manner, it is possible to generate the burst error gate signal BG having a time period of the intervals under the influence of the burst error.  
         [0063]    As described above, by supplying the burst error signal generated by the burst error detector  1300  to a substituting circuit as substituting means such as the substituting circuit  402  shown in FIG. 5, it is possible to replace the burst error part with a predetermined signal or by an operation.  
         [0064]    In addition, FIG. 14-E indicates sampling values  1412  obtained by delaying, for N/2=2 clocks in the substituting circuit, the signal  1401  that does not include the burst error part in the reproduction data, and sampling values  1413  obtained by delaying, for N/2=2 clocks in the substituting circuit, the signal  1402  that includes the burst error part in the reproduction data. In this manner, by delaying the sampling value yi for N/2 stages in the substituting circuit with respect to the burst error gate signal BG generated by the burst error detector  1300 , it is also possible to substitute the predetermined signal or by an operation, for the sampling value yi under the influence of the burst error part in a part before the burst error position BP.  
         [0065]    As described above, according to the present invention, by detecting a burst error part and substituting for the burst error part a value that does not affect parts other than the burst error part, it is possible to control the influence of wrong likelihood information in iterative decoding. Accordingly, it is possible to maintain the decoding ability of iterative decoding.  
         [0066]    In addition, according to the present invention, wrong likelihood information is not propagated even if a reproduction signal includes a burst error part. Hence, it is possible to obtain a recording/reproducing apparatus having high decoding ability even with low S/N ratios by iterative decoding.  
         [0067]    The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.  
         [0068]    The present application is based on Japanese priority application No. 2002-246841 filed on Aug. 27, 2002, the entire contents of which are hereby incorporated by reference.