Patent Publication Number: US-2005117871-A1

Title: Signal processing method and apparatus and disk device using the method and apparatus

Description:
The present application is a continuation of application Ser. No. 09/400,856, filed Sep. 21, 1999; which is a continuation-in-part of application Ser. No. 08/948,942, filed Oct. 10, 1997, now U.S. Pat. No. 6,125,156, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      The present invention relates to a signal processing technique, and a disk device such as a magnetic disk drive or a magneto-optic disk device, or more in particular to a data sync signal detection technique and a disk device using a data sync signal detection and a disk device using a data sync signal detection technique with an improved data sync signal detection rate in which the data sync signal can be detected even in the presence of a data discrimination error in the data sync signal field of the data read from the disk device and discriminated.  
       FIG. 20  shows an example of the recording format for a magnetic disk drive. The data includes an ID field and a DATA field for each sector providing a unit storage area. The ID field and the DATA field each include a PLO (Phase Locked Oscillator) SYNC field  91  for pull-in of a PLL (phase locked loop), a data sync signal  92  for detecting the starting position of an ID (address information) or data for producing a demodulation timing signal of a modulated code, an ID field for recording/reproducing the ID information or a DATA field  93  for recording/reproducing the data actually, and a CRC field or an ECC field  94  for error detection and correction. Also, there is a GAP field  95  providing a pattern for absorbing various delay time between the ID field and the DATA field or between sectors.  
      It is well known that accurate detection of the data sync signal  92  is very important for the subsequent code demodulation of the ID or DATA field  93 . In other words, even in the case where the decode data in the ID or DATA field  93  has a very satisfactory error rate, a detection error of the data sync signal  92  which is normally about several bytes causes inaccurate code demodulation of the ID or DATA field  93  of several tens to several hundred bytes.  
      A method using a pattern having no continuous data inversion as a data sync signal is disclosed in JP-A-8-096312.  
      In the method disclosed in U.S. Pat. No. 5,844,920, there are provided patterns (marks) for data synchronization at two points, between which a gap (no data) or data are filled. In the case where such a gap is filled with data and the data sync detection is effected by the second data sync pattern, the data between the data sync patterns is restored by correcting an erasure pointer for the data error correction code. The provision of data sync patterns at two points makes possible data sync detection even in the case a thermal asperity (TA) occurs in the data sync pattern field.  
      Further, in order to improve the reproduction performance, there has been proposed a MTR (Maximum Transition Run) code in which the number of continuous magnetization inversions is limited, according to the reference “Maximum Transition Run Codes for Data Storage Systems”, IEEE. Trans. Mag. Vol. 32, No. 5, September 1996, written by J. Moon and B. Brickner.  
      In a method of data sync detection for a signal processing apparatus having a configuration as shown in  FIG. 21  examined by the present inventors, input data  511  are discriminated by a data discriminator  501 , and a data discrimination output  512  is subjected to a predetermined post-code processing (bit operation) in a post-coder  502 . In the post-code processing, the processing corresponding to the pre-code processing at the time of recording not shown is performed. This is in order to assure correspondence between the data coding at the time of recording and the decoding at the time of reproduction. According to the method disclosed in JP-A-9-223365, it is possible to perform the processing equivalent to the post-code processing at the time of outputting the result of the state transition in the data discriminator  501 . Therefore, the post coder  502  is not always necessary as an independent component element. Nevertheless, the method of JP-A-9-223365 is also considered to functionally include an independent post coder  502  having the post-coding operation separated from the data discriminator, or a post-code processing means for simply passing through the code from the data discriminator. The post-code output  513  is applied to a decoder  504 . Also, the same post-code output  513  is applied to a data sync signal detector  503  and compared with a predetermined sync pattern  514 . When they coincide with each other, a data sync signal  92  is detected, and applied to the decoder  504  as a sync signal detection output  516 . With this signal as a decode timing signal, the decoder  504  performs the decode operation thereby to produce an output data  517 .  
      The data sync signal detector  503  is so configured, as disclosed in U.S. patent application Ser. No. 08/948,942, that the data-discriminated code string is divided into groups of a bit string of odd numbered bits and a bit string of even numbered bits, and each group is compared with a sync pattern for coincidence. In the case where the number of coincident groups exceeds a predetermined threshold value  515 , it is determined that a data sync signal has been detected. This data sync signal detection processing can exhibit a high ability of data sync signal detection.  
      On the other hand, the MTR code described above is the code in which the recording data is inverted by 1. When using such a code, the pre-code processing is the (1/(1+D)) processing (an input value and an output value delayed by a predetermined time are added in modulo  2  as an output value). The corresponding post-coding process is the (1+D) processing (an input value and an input value delayed by a predetermined time are added in modulo  2  as an output value). The use of the MTR code improves the data reproduction performance and shortens the error length. Even in the case where the error in the data discriminator  501  is one bit, however, it presents itself as an error of two continuous bits after the (1+D) processing of the post-coder  502 . The data sync signal cannot be successfully detected, therefore, even when a code string is divided into a bit string of odd numbered bits and a bit string of even numbered bits.  
      In the case where a one-bit data error of the data sync signal  92  occurs in the configuration shown in  FIG. 21 , therefore, the data sync signal is detected erroneously, followed by the ID and DATA fields  93  all erroneous. (If a permanent bit drop-off in the data sync signal unit occurs due to a defect of the medium, etc. data for one sector cannot be correctly reproduced.)  
      As described above, an erroneous detection (detection not at right position or detection at an erroneous position) of the data sync signal at the head of data causes not merely the erroneous detection of the data sync signal but also all the subsequent decode processing of several hundred bytes become erroneous, resulting in the technical problem that the error rate of the whole apparatus is considerably deteriorated.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to provide a signal processing technique capable of reducing the error in the data sync signal detection.  
      Another object of the invention is to provide a signal processing technique capable of improving the data sync signal detection performance of a data sync signal detector with the improvement in the reproduction performance of the data field.  
      Still another object of the invention is to provide a signal processing technique capable of reducing the circuit size with a simple configuration of the data sync signal detector.  
      A further object of the invention is to provide a magnetic disk drive capable of improving the recording density by employing a signal processing system including a Maximum-Likelihood or Viterbi decoding means and reducing the error rate by the improved detection performance of the data sync signal at the same time.  
      A yet further object of the invention is to provide a magnetic disk drive capable of reducing the production cost by reducing the circuit size of the signal processing system for detecting the data sync signal and reducing the error rate by the improved detection performance of the data sync signal at the same time.  
      According to one aspect of the invention, there is provided a data sync signal detection system for a signal processing apparatus including a data discriminator for outputting a bit string of data, a post-coder for performing a predetermined post-code processing (bit operation processing) on the bit string, a decoder for decoding the post-coded bit string thereby to reproduce the data, the system comprising: a (1+D) processing unit for performing the processing of adding, in modulo- 2 , an input value of the bit string of the code input to the decoder to a value delayed a predetermined time from the input value ((1+D) processing) and producing an output value, a separator for dividing the bit string of the code containing data sync signals after the (1+D) processing into a bit string of odd numbered bits and a bit string of even numbered bits, each bit string being subdivided into one group or two or more groups separated with or without a bit string containing one or more bits of an arbitrary pattern interposed there between, at last one matching unit for comparing or matching the output of each group with or against a corresponding predetermined sync pattern and determining a coincidence or non-coincidence, and a decision unit supplied with the output from each matching unit for outputting a data sync signal detection signal to the decoder in the case where the number of coincident groups is equal to or more than a predetermined threshold value.  
      Also, an error detection/correction unit is interposed between the separator and each pattern matching unit for detecting and correcting an error of the output code separated into a bit string of odd numbered bits and a bit string of even numbered bits and matching the pattern of the code bit string thus corrected against a predetermined sync pattern.  
      As a result, a sync pattern capable of error detection and correction with the data inversion not continuous is selectively used as a predetermined sync pattern.  
      According to another aspect of the invention, there is provided a data sync signal detection apparatus comprising a data discriminator for producing a data bit string, a detector for detecting a data sync signal from the data bit string output from the data discriminator, a separator for dividing the raw data sync pattern into predetermined bit groups, a matching unit for matching each pattern with a predetermined sync pattern, and an error detection and correction unit associated with each group for detecting and correcting an error of the output from each group, wherein a code string for which the discrimination error has been corrected is matched against the data sync pattern thereby to detect a data sync signal. This detection apparatus is not provided with the (1+D) processor described above. Instead, the code bit string containing the data sync signals after post-processing is divided into groups, for each of which the discrimination error is detected and corrected, and then each group is matched against a predetermined data sync pattern.  
      Other objects, features and advantages of the present invention will become apparent from reading the description of the following embodiments of the invention taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram showing a signal processing apparatus according to an embodiment of the invention.  
       FIG. 2  is a block diagram showing a signal processing apparatus according to another embodiment of the invention.  
       FIG. 3  is a block diagram showing a signal processing apparatus according to still another embodiment of the invention related to the embodiment shown in  FIG. 1 .  
       FIG. 4  is a diagram useful for explaining an error pattern in the output of a data discriminator according to a third embodiment.  
       FIG. 5  is a diagram showing a polynomial of degree  5  used with a syndrome calculator according to the embodiment of  FIG. 9 .  
       FIG. 6  is a diagram for explaining a configuration of a 9-bit sync pattern used with the embodiment of  FIG. 9 .  
       FIG. 7  is a diagram for explaining the relation between an error position and a syndrome value for each error pattern according to the embodiment of  FIG. 9 .  
       FIG. 8  is a diagram for explaining the relation between an error position and a syndrome value for each error pattern according to the embodiment of  FIG. 10 .  
       FIG. 9  is a block diagram showing a signal processing apparatus according to a further embodiment of the invention related to the embodiment shown in  FIG. 2 .  
       FIG. 10  is a block diagram showing a signal processing apparatus according to a still further embodiment of the invention related to the embodiment shown in  FIG. 2 .  
       FIG. 11  is a block diagram showing a signal processing apparatus according to a yet further embodiment of the invention related to the embodiment shown in  FIG. 2 .  
       FIG. 12  is a diagram showing an example configuration of a syndrome calculator according to the embodiment shown in  FIG. 9 .  
       FIG. 13  is a diagram showing an example configuration of an error correction circuit according to the embodiment of  FIG. 9 .  
       FIG. 14  is a diagram showing an example configuration of an error correction circuit according to the embodiment of  FIG. 10 .  
       FIG. 15  is a block diagram showing an internal configuration of a magnetic disk drive according to a further embodiment of the invention.  
       FIG. 16  shows an example of a sync pattern used with a signal processing apparatus according to the invention.  
       FIGS. 17A and 17B  are graphs showing an example characteristic of a data sync signal detector according to the embodiment of  FIGS. 3 and 9 .  
       FIGS. 18A and 18B  are graphs showing an example characteristic of a data sync signal detector according to the embodiment of  FIG. 10 .  
       FIGS. 19A and 19B  are graphs showing an example characteristic of a data sync signal detector according to the embodiment of  FIG. 11 .  
       FIG. 20  is a diagram for explaining an example format of the recording data in a magnetic disk device.  
       FIG. 21  is a block diagram showing a configuration of a signal processing apparatus useful for explaining the present invention.  
       FIG. 22  is a block diagram showing a signal processing apparatus according to still another embodiment of the invention.  
       FIG. 23  is a block diagram showing a signal processing apparatus according to yet another embodiment of the invention related to the embodiment of  FIG. 22 .  
       FIG. 24  is a block diagram showing a signal processing apparatus according to still another embodiment of the invention.  
       FIG. 25  is a flowchart showing a method of detecting the data sync signal according to a yet further embodiment of the invention. 
    
    
     DESCRIPTION OF THE EMBODIMENTS  
      Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.  
       FIG. 1  is a block diagram showing a signal processing apparatus according to an embodiment of the invention.  
      In  FIG. 1 , input data  11  are input to a data discriminator  1 , and a data discrimination output  12  providing a code bit output discriminated by the data discriminator  1  is input to a post-coder  2  for performing a predetermined post-code processing. Further, a post-coder output  13  is input to a decoder  4  and a (1+D) processing unit  5 . A (1+D) processing output  18  is input to a data sync signal detector  3 , and is compared with or matched against a sync pattern  14  by a predetermined method. In the case where the number of pattern coincidences is not less than a threshold value  15 , a sync signal detection output  16  is output. The sync signal detection output  16  is applied to the decoder  4  and gives a decode timing of the code string of the post-code output  13 . As a result, a decoded output data  17  is output from the decoder  4 .  
      Alternatively, independently of the data reproduction system including the post-coder  2  and the decoder  4 , it is also possible to configure such that the data sync signal is detected from the code string obtained by subjecting the data discrimination output  12  to the post-code processing and the (1+D) processing. In other words, the code string constituting the input to the decoder  4  is not used. However, this is nothing but a parallel arrangement of post-code processing and apparently equivalent to the configuration of  FIG. 1 .  
      A signal processing apparatus according to another embodiment of the invention will be explained with reference to  FIG. 2 .  
      In  FIG. 2 , input data  11  are input to a data discriminator  1 , and a data discrimination output  12  providing a code bit output discriminated by the data discriminator  1  is input to a post-coder  2  for performing a predetermined post-code processing. Further, a post-code output  13  of the post-coder  2  is input to a decoder  4  and a (1+D) processing unit  5 . A (1+D) processing output  18  of the (1+D) processing unit  5  is input to an error detection/correction unit  6 . The error detection/correction unit  6  separates the (1+D) processing output  18  into a bit string of odd numbered bits and a bit string of even numbered bits, so that an error is detected and corrected of the bit strings thus grouped. The error detection and correction output  19  after error correction is input to a data sync signal detector  3 , and compared with or matched against a predetermined sync pattern. In the case where the number of pattern coincidences is not less than a predetermined threshold value  15 , a sync signal detection output  16  is output. The sync signal detection output  16  is input to the decoder  4  and gives a decode timing of the code string of the post code output  13 , whereupon a decoded output data  17  is output from the decoder  4 .  
      As described above, the (1+D) processing is executed and the bit string is separated into a bit string of odd numbered bits and a bit string of even numbered bits before detection of the data sync signal. In this way, the number of the types of the error pattern can be reduced while at the same time shortening the error pattern length. As a result, the error detection and correction is easily realized, and the data sync signal can be detected with a higher accuracy.  
       FIG. 3  is a block diagram showing a configuration of a signal processing apparatus according to the embodiment of  FIG. 1  used for decoding MTR codes.  
      This embodiment will be explained specifically with reference to  FIG. 3 . An 18-bit sync pattern is used.  
      In  FIG. 3 , the decoder  4  is a MTR decoder with not more than 3 continuous “1s”.  
      The data discriminator  1  is a Maximum-Likelihood decoder or Viterbi decoder of EEPRML (Extended Extended Partial Response with Maximum Likelihood detection) type. This channel response is (1−D)(1+D) 3 . Also, assume that the data discriminator is optimized for use with the MTR code described above. Two sync patterns “001111111100011000” and “110000000011100111” are available for the data discrimination output  12 .  
      The post coder  2  has the characteristic of (1+D). One 18-bit sync pattern “001000000010010100”, is available for the post code output  13 . Also, the operation of the post-coder  2  ((1+D) processing) may be included for outputting the state transition of the data discriminator  1 , and “001000000010010100” may be output as the data discrimination output  12  without providing the post-coder  2 . In such a case, too, the function of the post coder  2  can be considered to be included.  
      The (1+D) processing unit  5  arranged before the data sync signal detector  3  is configured with a unit time delay circuit or cell  31  and an exclusive OR circuit  32 . The post-code output  13  is applied to the unit time delay cell  31  and the exclusive OR circuit  32 . Also, the output of the unit time delay cell  31  is input to the remaining input terminal of the exclusive OR circuit. The output of the exclusive OR circuit  32  constitutes the (1+D) processing output  18 . The sync pattern in the (1+D) processing output  18  is given as a 18-bit pattern of “001100000011011110”.  
      The (1+D) processing output  18  is applied to a shift register  21  in the data sync signal detector  3 . The shift register  21  has a 17-bit configuration. This is in order to selectively use a 9-bit pattern as a sync pattern. Nine bits including every other bit of the shift register  21  are output as a shift register output  22 . By use of the values of every other bit of the shift register  21 , the (1+D) processing output  18  can be divided into two groups of a bit string of odd numbered bits and a bit string of even numbered bits for each operation clock not shown. The sync pattern in the shift register output  22  is one of the two 9-bit patterns of “010001011” and “010001110”.  
      The shift register output  22  is input to a pattern matching unit  27   i  and a pattern matching unit  27   j , and matched against the sync patterns of a sync pattern holder  26   i  and a sync pattern holder  26   j , respectively. Each sync pattern is given as one of the sync patterns  14 . The sync pattern holder  26   i  holds the 9-bit pattern of “010001011”, and the sync pattern holder  26   j  holds the 9-bit pattern of “010001110”. In order to assure the same timing of the outputs of the pattern matching unit  27   i  and the pattern matching unit  27   j , the output of the pattern matching unit  27   i  is delayed through a unit time delay circuit  28   b  and input to a majority decision logic circuit (decision circuit)  29 .  
      The majority decision logic circuit  29  compares the number of coincidences between the two pattern matching results with the value of the threshold level  15 , and in the case where the number of coincidences of the pattern matching result is not less than the threshold value  15 , the sync signal detection output  16  is output to the decoder  4 . In this case, the threshold value  15  is given as 1, and therefore a two-input OR circuit can be used.  
      The sync signal detection output  16  gives a decode timing to the MTR code decoder  4 . As a result, the correct decode operation is realized thereby producing the output data  17 .  
      Now, with reference to  FIG. 4 , the error patterns generated in the embodiment of  FIG. 3  will be explained. In  FIG. 4 , the column to the extreme left lists error patterns of the data discrimination output  12  in the data discriminator  1  (EEPRML), where x designates an error bit, and 0 a non-error bit. Six patterns x, xx, xxx, x0x, x00x and x000x are shown, of which five patterns other than xxx develop an error. Each error pattern (error event) is defined as an error pattern that can occur between the time when the state transition path in the Viterbi decoder is displaced from the original path by error and the time when it again comes to coincide with (returns to) the correct path.  
      The leftmost column but one represents the distance of each error pattern code, which is an indication of the degree of likelihood of error occurrence. The smaller the distance, the easier the error occurs.  
      The third column form the left represents the ratio of error occurrence in the sync pattern used in the embodiment of  FIG. 11  described later and the immediately preceding PLO SYNC pattern, or specifically, the ratio of error occurrence in the 128-bit pattern of “1010101010101010101010101010101010101010 10001001000001010 0 10101010101010101010101010101010 001000000010010100 10001 0101010101010” in the post-code output  13 . The 18 bits (underlined portions) from each of the 43rd bit and the 93rd bit of the pattern described above are the sync patterns. Originally, the pattern xxx is most likely to develop an error. Since the selected patterns have no portion where the data inversion is continuous (i.e. the portion where 1 continues such as “11”), however, the EEPRML optimized to the MTR code with the data inversion limited to 3 bits or less prevents the occurrence of the error pattern xxx. Thus, in this case, the occurrence of the error pattern x represents about 90% of all the error patterns. The bit error rate involved (the ratio of error event to the total number of bits reproduced) is 0.0004. Thus, in the range of 10 −6  to 10 −8  where the bit error rate is lowest, for example, the ratio of occurrence of a long error pattern such as x000x is still lowered to a degree negligible. About the same can be said of the sync patterns used in the embodiments of  FIGS. 3, 9 ,  10 . Also for the embodiments of  FIGS. 9, 10 ,  11  described later, refer to  FIG. 4 .  
      The fourth column from the left indicates each error pattern in the post-code output  13 .  
      The fifth column from the left indicates each error pattern in the (1+D) processing output  18  for data sync signal detection.  
      The sixth column from the left indicates each error pattern in the (1+D) processing output  18  after being divided into a bit string of odd numbered bits and a bit string of even numbered bits for detecting the data sync signal, i.e. each error pattern in the shift register output  22 . The error pattern of x in the data discrimination output  12  indicates that an error (xx) of two continuous bits appears either in the bit string of odd numbered bits or the bit string of even numbered bits in the shift register output  22 .  
      From these facts, it can be understood that the provision of the (1+D) processing unit  5  anew for detecting the data sync signal can remarkably improve the detection rate of the data sync signal  92 , because after division into a bit string of odd numbered bits and a bit string of even numbered bits, one of them contains no error even when a 1-bit error (x) representing about 90% of the error patterns occurs.  
      A specific performance will be explained with reference to  FIGS. 17A, 17B .  FIGS. 17A, 17B  are graphs showing the performance of the embodiment of  FIG. 3  in computer simulation.  
      In  FIG. 17A , the abscissa represents the signal-to-noise ratio of the input to the Maximum-Likelihood or Viterbi decoder, and the ordinate the bit error rate and the detection error rate of the data sync signal. A characteristic curve  175  represents the bit error rate of the data in the data discrimination output  12 . This is the characteristic obtained when the data is regarded as random one. A characteristic curve  171 , on the other hand, represents the characteristic of the detection error rate of the data sync signal in the case where the process of detecting the data sync signal is executed under the condition that all the 18 bits of the sync pattern are coincident. A characteristic curve  172  represents a method of a reference technique not including the (1+D) processing unit  5  for detection of the data sync signal, which is the characteristic of the detection error rate of the data sync signal in the case where the process of detecting the data sync signal is carried out under the condition that one of the 9-bit patterns divided into a bit string of odd numbered bits and a bit string of even numbered bits is coincident. A characteristic curve  173  represents a characteristic of the detection error rate of the data sync signal in the case where the process for detecting the data sync signal is carried out under the conditions of the embodiment of the invention shown in  FIG. 3 . It is seen that an improvement of about 2 dB is attained in terms of signal-to-noise ratio as compared with the method of the reference techniques.  
      In  FIG. 17B , the abscissa represents a bit error rate in the data discrimination output  12 , and the ordinate the detection error rate of the data sync signal. This graph is the result of replotting the graph of  FIG. 17A  with the characteristic curve  175  as the abscissa. The characteristic curve  176  corresponds to the characteristic curve  171 , the characteristic curve  177  corresponds to the characteristic curve  172 , and the characteristic curve  178  corresponds to the characteristic curve  173 . Let Be (abscissa) be the rate of occurrence of the error event to the total number of output bits in the output of the data discriminator  1 , and Se (ordinate) be the rate of occurrence of the data sync signal detection error to the number of data sync signal detection requests. Then, for the range of Be not more than 0.1, the characteristic curve  178  is approximated by equation (1) below. 
 
 S   e =7 B   e   1.20   (1) 
 
       FIG. 9  is a block diagram showing a signal processing apparatus according to another embodiment to which the embodiment of  FIG. 2  is applied for decoding of the MTR code.  
      This embodiment will be explained with reference to  FIG. 9 .  
      The configuration of the data discriminator  1 , the post-coder  2 , the decoder  4  and the (1+D) processing unit  5  in  FIG. 9  is similar to that of the embodiment shown in  FIG. 3 . Also, the same 18-bit pattern is used as in the embodiment of  FIG. 3 . Thus, the sync pattern of each part up to the (1+D) processing output  18  is also the same.  
      The (1+D) processing output  18  is input to a shift register  21  in the error detection/correction unit  6 . The configuration of the shift register  21  is the same as that of the embodiment shown in  FIG. 3 . Thus, the sync pattern in the shift register output  22  is given as either of the two 9-bit patterns of “010001011” and “010001110”. The shift register output  22  is input to a syndrome calculator  23   a , a syndrome calculator  23   b , an error corrector  24   a  and an error corrector  24   b.    
      The 9-bit sync pattern is configured with a 4-bit code and a corresponding 5-bit CRCC (Cyclic Redundancy Check Code) as shown in  FIG. 6 . The CRCC is the 5-bit remainder after dividing the 4-bit code by a generator polynomial. Thus, in the absence of an error, the remainder of the 9-bit sync pattern divided by the generator polynomial is always zero, while in the presence of an error, the remainder of the 9-bit sync pattern divided by the generator polynomial indicates a corresponding value. The value of this remainder is called the syndrome value. Unless the syndrome value is zero, it indicates an error and the error can be detected. According to this syndrome value, the error position can be detected and the error corrected (1 to 0, or 0 to 1).  
      Now, consider the sync pattern “010001011” as used herein. The leading four bits “0100” is the original code. The bit string “010000000” obtained by shifting the leading four bits is divided by the polynomial of degree  5  (X 5 +X 4 +X 2 +1) and the remainder constitutes the 5 bits “01011” of the CRCC. The remainder after dividing the sync pattern “010001011” by the polynomial of degree  5  (X 5 +X 4 +X 2 +1) is zero. This generator polynomial corresponds to e in  FIG. 5 .  
      The syndrome calculator  23   a  uses the polynomial of degree  5  (X 5 +X 4 +X 2 +1) expressed in e of  FIG. 5  as a generator polynomial. In the syndrome calculator  23   a , the dividing operation is performed using the generator polynomial (X 5 +X 4 +X 2 +1), and the remainder thereof is output in five bits as a syndrome value  20   a.    
      A detailed example configuration of the syndrome calculator  23   a  is shown in  FIG. 12 . In this case, the 9-bit input of the shift register output  22  is divided at a time by the generator polynomial using 11 exclusive OR circuits  301  to  311 , and the 5-bit syndrome value  20   a  is output. This calculation can be made by calculation-by-writing using a mathematic operation. As a result, the syndrome value  20   a  can be output for each shift register output  22  grouped and output as a bit string of odd numbered bits and a bit string of even numbered bits according to each operation clock not shown.  
      This is also the case with the syndrome calculator  23   b , which can be configured with h(X 5 +X 4 +X 3 +X 2 +1) of  FIG. 5  corresponding to the sync pattern “010001110” as a generator polynomial.  
      Now, the syndrome value  20   a  for error patterns will be explained with reference to  FIG. 7 .  FIG. 7  shows ten error patterns of one bit or two. These are error patterns of two continuous bits often appearing in the shift register output  22 , and at the end of the 9-bit group, constitutes a one-bit pattern. The syndrome value for these ten error patterns, as shown in the column of the polynomial e of  FIG. 7  corresponding to the polynomial e of  FIG. 5 , assumes ten different values of 22, 29, 20, 10, 5, 24, 12, 6, 3 and 1. The polynomials a to h in  FIG. 5  correspond to the columns a to h of the generator polynomials a to h of  FIG. 7 , respectively. Thus, also in the other generator polynomials a to d and f to h, the syndrome values for ten error patterns indicate ten different values, respectively. Therefore, the eight generator polynomials of  FIG. 5  are seen to be effective for error detection and correction.  
      The syndrome value  20   a  and the syndrome value  20   b  in  FIG. 9  are input to the error corrector  24   a  and the error corrector  24   b , respectively. The corresponding error in the shift register output  22  is corrected by the syndrome value  20   a  in the error corrector  24   a  and by the syndrome value  20   b  in the error corrector  24   b . The process for detecting an error corresponding to the generator polynomial (X 5 +X 4 +X 2 +1) in e of  FIG. 5  and an error corresponding to the generator polynomial (X 5 +X 4 +X 3 +X 2 +1) in h of  FIG. 5  is executed and, upon detection of the error, a corresponding correction is carried out. The result is output as an error detection/correction output  19   a  and an error detection/correction output  19   b , respectively.  
      An example of a more detailed configuration of the error correction unit  24   a  is shown in  FIG. 13 . The syndrome value  20   a  is compared with ten values of 22, 29, 20, 10, 5, 24, 12, 6, 3, 1 by comparators  312  to  321 . Upon coincidence by any comparator in the presence of an error, the result is applied to a corresponding circuit of the exclusive OR circuits  331  to  339  through a corresponding circuit of the OR circuits  322  to  330 . The exclusive OR circuits  331  to  339  are supplied with the information on the error position and the shift register output  22 . Thus, the bit associated with the error is inverted, and the error is corrected. The result is output as an error detection/correction output  19   a.    
      Consider the error correcting operation in more detail. Assume, for example, that the leading two bits of the sync pattern “010001011” are erroneous and the value “100001011” appears in the shift register output  22 . The error is that of the error pattern  2  in  FIG. 7 . The syndrome value  20   a  involved is  29  from  FIG. 7 . In  FIG. 13 , the result of comparison in the comparator  313  coincides and  1  (true value) is output. This value is input to the OR circuit  322  and the OR circuit  323 , the output of which also assumes  1  (true value). As a result, one of the input terminals of the exclusive OR circuits  331 ,  332  is supplied with 1. Thus, the two bits on the MSB side (corresponding to the head of the sync pattern) of the shift register output  22  are inverted, so that “100001011” is corrected to “010001011”. This pattern corrected is output as an error detection/correction output  19   a.    
      The error detection/correction outputs  19   a ,  19   b  are applied to the pattern matching units  27   a ,  27   b  of the data sync signal detector  3 , and matched against the sync patterns of the sync pattern holders  26   a ,  26   b , respectively. Each sync pattern is given as a sync pattern  14 , so that the sync pattern holder  26   a  holds a 9-bit pattern “010001011”, and the sync pattern holder  26   b  a 9-bit pattern “010001110”. For setting the timing of the outputs of the pattern matching units  27   a ,  27   b  in order, the output of the pattern matching unit  27   a  is delayed through the unit time delay circuit  28   a  and then applied to the majority decision logic circuit  29 .  
      In the majority decision logic circuit  29 , the number of coincidences between two patterns is compared with the threshold value  15 , and in the case where the number of coincidences of the pattern matching is not less than the threshold value  15 , the sync signal detection output  16  is produced. In the case under consideration, the threshold value  15  is 2, and therefore a 2-input AND circuit can be used. When an error is detected and corrected by the error detection/correction unit  6 , the fact that the data starting position is unknown increases the possibility of correcting the sync pattern erroneously. Thus the threshold value is required to be 2 or more.  
      The sync signal detection output  16  gives the decode timing to the decoder  4  of the MTR code. As a result, the correct decoding is realized and the output data  17  is obtained.  
      Now, the performance of the embodiment shown in  FIG. 9  will be described again with reference to  FIG. 4 . In the embodiment of  FIG. 3 , the error pattern x could be saved. In the embodiment of  FIG. 9 , however, a continuous 2-bit error can be detected and corrected, and therefore, the error patterns xx, x00x can also be saved. In other words, it can be understood that by providing the (1+D) processing unit  5  and detecting and correcting the continuous 2-bit error after division into a bit string of odd numbered bits and a bit string of even numbered bits, about 98.8% of the errors occurred can be saved and the detection rate of the data sync signal  92  can be improved further.  
      This performance will be explained with reference to  FIGS. 17A, 17B  partially used for reference above. Characteristic curves  174 ,  179  represent the data sync signal detection error rate in the case where the data sync signal s detected under the conditions of the embodiment shown in  FIG. 9 . From  FIG. 17A , it is seen that an improvement of about 0.5 dB can be attained in terms of the signal-to-noise ratio of the Viterbi decoder input as compared with the embodiment of  FIG. 3 . Also, let Be (abscissa) be the ratio of occurrence of an error event to the total number of output bits of the data discriminator  1  and Se (ordinate) be the ratio of occurrence of the data sync signal detection error to the number of requests for data sync signal detection. The characteristic curve  179  for Be in the range of 0.1 or less is approximated by equation (2) below. 
 
 S   e =12 B   e   1.42   (2) 
 
      A signal processing apparatus according to still another embodiment of the invention will be explained with reference to  FIG. 10 . The basic configuration of  FIG. 10  is the same as that of the embodiment shown in  FIG. 9 . Only the difference will be described in detail. The difference lies in a sync pattern  14 , an error detection/correction unit  6  and generator polynomials used for them.  
      The sync pattern used in this case is an 18-bit pattern “000000100101010010” in the post code output  13 . In the (1+D) processing output  18 , on the other hand, an 18-bit pattern “000000110111111011” is involved. In the shift register output  22 , the patterns are “000101111” and “000111101”. The generator polynomials for error detection and correction of these patterns are given as d(X 5 +X 3 +X 2 +X 1 +1) and h(X 5 +X 4 +X 3 +X 2 +1) shown in  FIG. 5 .  
      The syndrome calculators  23   c ,  23   d  can be configured with an exclusive OR circuit as in the embodiment shown in  FIG. 9 . The syndrome calculator  23   c  corresponds to the generator polynomial (X 5 +X 3 +X 2 +X 1 +1), and the syndrome calculator  23   d  corresponds to the generator polynomial (X 5 +X 4 +X 3 +X 2 +1).  
      Now, the syndrome value  20   c  for error patterns will be explained with reference to  FIG. 8 .  FIG. 8  shows  19  error patterns of 1 bit or two. These are error patterns which appear in the shift register output  22 , and include a 2-bit continuous error pattern which frequently appears as explained in the embodiment of  FIG. 9  and becomes a one-bit error pattern occurring at the end of the 9-bit group. Next frequently appearing is the x0x error pattern which becomes one-bit error pattern at the second bit from the end of the 9-bit group. For these 19 error patterns, there are 19 different syndrome values of 9, 26, 13, 17, 31, 24, 12, 6, 3, 1, 19, 23, 28, 14, 7, 20, 10, 5 and 2 as shown in the column of the generator polynomials d of  FIG. 8  corresponding to the polynomials d of  FIG. 5 . Also, for another generator polynomials h, the syndrome values for 19 error patterns assume 19 different values in similar fashion. It is seen therefore that two generator polynomials d and h in  FIG. 5  are effective for two types of error detection and correction.  
      The syndrome values  20   c ,  20   d  of  FIG. 10  are input to the error correction units  25   c ,  25   d , respectively. The corresponding error of the shift register output  22  is corrected by the syndrome value  20   c  in the error correction unit  25   c , and by the syndrome value  20   d  in the error correction unit  25   d . In respective cases, the error detection is carried out in a way corresponding to the generator polynomial (X 5 +X 3 +X 2 +X 1 +1) in d of  FIG. 5 , and corresponding to the polynomial (X 5 +X 4 +X 3 +X 2 +1) in h of  FIG. 5 . Upon detection of an error, the corresponding correction is carried out. The result is output as an error detection/correction outputs  19   c  and  19   d.    
      An example configuration of the error correction unit  25   c  is shown in detail in  FIG. 14 . The syndrome value  20   c  is compared with 19 values including 19, 9, 23, 26, 28, 13, 14, 17, 7, 31, 20, 24, 10, 12, 5, 6, 2, 3 and 1 by comparators  340  to  358 . In the case where the output of any one of the comparators is coincident in the presence of an error, the result is applied to the corresponding one of the exclusive OR circuits  384  to  392  through the OR circuits  359  to  383 . The exclusive OR circuits  384  to  392  are supplied with the error position information and the shift register output  22 , and therefore, the bit associated with the error is inverted thereby to correct the error. The result is output as an error detection/correction output  19   c.    
      Consider the error correction operation in more detail. Assume, for example, that the 2nd and 4th bits from the head of the sync pattern “000101111” are erroneous so that the value “010001111” has appeared as the shift register output  22 . The error is that of the error pattern  13  in  FIG. 8 . In this case, the syndrome value  20   c  is  28  as seen from  FIG. 8 . At the same time, in  FIG. 14 , the result of comparison in the comparator  344  is coincident, and 1 (true value) is output. This value is applied to the OR circuits  362 ,  367 . Further, the signal is output through the OR circuits  363 ,  369 , so that the output also assumes 1 (true value). As a result, 1 is input to one of the input terminals of the exclusive OR circuits  385 ,  387 , and therefore the 2nd and 4th bits from the MSB side (corresponding to the head of the sync pattern) of the shift register output  22  are inverted. Thus, the pattern “010001111” is corrected to “000101111”. The pattern thus corrected is output as an error detection/correction output  19   c.    
      The error detection/correction outputs  19   c ,  19   d  are input to the pattern matching units  27   c ,  27   d  of the data sync signal detector  3 , and compared with the sync patterns of the sync pattern holders  26   c ,  26   d , respectively. Each sync pattern is given as a sync pattern  14 , so that the sync pattern holder  26   c  holds the 9-bit pattern “000101111” and the sync pattern holder  26   d  holds the 9-bit pattern “000111101”. In order to set the outputs of the pattern matching units  26   c ,  26   d  in the same timing, the output of the pattern matching unit  27   c  is delayed through the unit time delay line  28   c  and input to the majority decision logic circuit  29 .  
      In the majority decision logic circuit  29 , the number of coincidences in the result of the comparison between the two patterns obtained is compared with the threshold value  15 . In the case where the number of coincidences as a result of pattern matching is not less than the value given by the threshold level  15 , the sync signal detection output  16  is output. In this case, too, like in the embodiment of  FIG. 9 , 2 is given as the threshold value  15 , and therefore a two-input AND circuit can be used for this purpose.  
      The sync signal detection output  16  gives the decode timing of the decoder  4  of the MTR code. As a result, the correct decoding is realized, thereby producing the output data  17 .  
      Once again, the performance of the embodiment shown in  FIG. 10  is described with reference to  FIG. 4 . In the embodiment of  FIG. 9 , the error pattern x, the error pattern xx and the error pattern x00x can be saved. In the embodiment of  FIG. 10 , on the other hand, it is seen that the error pattern x0x can also be saved. Specifically, by inserting the (1+D) processing unit  5  and by detecting and correcting a 2-bit continuous error and a 3-bit continues error of x0x after division into a bit string of odd numbered bits and a bit string of even numbered bits, it is understood that about 99.9% of the errors occurred can be saved, and the detection rate of the data sync signal  92  is further improved.  
      This performance will be explained with reference to  FIGS. 18A, 18B .  FIGS. 18A, 18B  are graphs mainly indicating the performance of the embodiment of  FIG. 10 , prepared by computer simulation.  
      In  FIG. 18A , the abscissa represents the signal-to-noise ratio in the Viterbi decoder input, and the ordinate the bit error rate and the data sync signal detection error rate. A characteristic curve  185  indicates the bit error rate of the data in the data discrimination output  12 . This is a characteristic obtained when the data are regarded to be random. A characteristic curve  181  indicates the characteristic of the data sync signal detection error rate in the case where the data sync signal is detected under the condition that all the 18 bits of the sync pattern are coincident. A characteristic curve  182  is based on a method using a reference technique not including the (1+D) processing unit for data sync signal detection, and represents a characteristic of the data sync signal detection error rate in the case where the data sync signal detection is carried out under the condition that any one of the 9-bit patterns of the bit string of odd numbered bits and the bit string of even numbered bits are coincident. A characteristic curve  183  represents the characteristic of the data sync signal detection error obtained in the case where the data sync signal detection is carried out under the condition ((1+D) processing unit  5  is included, and the error detection/correction is not effected) of the embodiment of the invention shown in  FIG. 3 . A characteristic curve  184  represents the characteristic of the data sync signal detection error obtained in the case where the data sync signal detection is carried out under the condition of the embodiment of the invention shown in  FIG. 10 . From  FIG. 18A , it is seen that the signal-to-noise ratio is improved by about 1 dB in the input of the Viterbi decoder as compared with the embodiment of  FIG. 3 . This is seen to be an improvement of about 0.5 dB in the signal-to-noise ratio as compared with the embodiment of  FIG. 9 .  
      In  FIG. 18B , the abscissa represents the bit error rate in the data discrimination output  12 , and the ordinate represents the data sync signal detection error rate. This is the graph of  FIG. 18A  replotted with the characteristic curve  185  used as the abscissa. A characteristic curve  186  corresponds to the characteristic curve  181 , a characteristic curve  187  corresponds to the characteristic curve  182 , a characteristic curve  188  corresponds to the characteristic curve  183  and a characteristic curve  189  corresponds to the characteristic curve  184 . Let Be (abscissa) be the ratio of occurrence of an error event to the total number of output bits in the output of the data discriminator  1 , and Se (ordinate) be the ratio of occurrence of a data sync signal detection error to the number of requests for data sync signal detection. The characteristic curve  189  for the value of Be in the range of 0.1 or less is approximated by equation (3) below. 
 
 S   e =20 B   e   1.64   (3) 
 
      A signal processing apparatus according to a further embodiment of the invention will be explained with reference to  FIG. 11 . The basic configuration of  FIG. 11  is the same as that of the embodiment shown in  FIG. 9 . What is different is that four 9-bit patterns are used as a sync pattern  14 . A method of detecting and correcting an error is also the same as that of the embodiment shown in  FIG. 9 , and each sync pattern corresponds to ten error patterns for correction.  
      The sync patterns used in this embodiment include an 18-bit pattern of “100010010000010100” and an 18-bit pattern of “001000000010010100” in the post-code output  13 , and a total of 36-bit patterns are matched. Further, a 32-bit pattern of “10101010101010101010101010101010” for prevention of error propagation is inserted between the aforementioned two bit patterns. In the (1+D) processing output  18 , these patterns are represented as “110011011000011110”, “001100000011011110” and “11111111111111111111111111111111”, respectively. The patterns matched in the shift register output  22  include “101010011”, “101100110”, “010001011” and “010001110”. The generator polynomials for error detection and correction of these patterns are given as f(X 5 +X 4 +X 2 +X 1 +1), h(X 5 +X 4 +X 3 +X 2 +1), e(X 5 +X 4 +X 2 +1) and h (X 5 +X 4 +X 3 +X 2 +1), respectively.  
      The syndrome calculators  23   e  to  23   h  can be configured with exclusive OR circuits as in the embodiment of  FIG. 9 . The syndrome calculator  23   e  corresponds to the generator polynomial (X 5 +X 4 +X 2 +X 1 +1), the syndrome calculator  23   f  corresponds to the polynomial (X 5 +X 4 +X 3 +X 2 +1), the syndrome calculator  23   g  corresponds to the polynomial (X 5 +X 4 +X 2 +1), and the syndrome calculator  23   h  corresponds to the polynomial (X 5 +X 4 +X 3 +X 2 +1). The syndrome calculators  23   f  and  23   h  are for calculating the same generating polynomials, and therefore can alternatively be replaced by a single common syndrome calculator.  
      The syndrome value for ten error patterns for each pattern matched is the value of the corresponding polynomial in the syndrome value column in  FIG. 7 . Specifically, the syndrome value  20   e  is given in the column of the generator polynomial f, the syndrome value  20   f  is given in the column of the generator polynomial h, the syndrome value  20   g  is given in the column of the generator polynomial e, and the syndrome value  20   h  is given in the column of the generator polynomial h.  
      The syndrome values  20   e  to  20   h  in  FIG. 11  are input to the error correction units  24   e  to  24   h , respectively. The corresponding error in the shift register output  22  is corrected by the syndrome value  20   e  in the error correction unit  24   e , by the syndrome value  20   f  in the error correction unit  24   f , by the syndrome value  20   g  in the error correction unit  24   g , and by the syndrome value  20   h  in the error correction unit  24   h . In the respective cases, the error detection is carried out in a manner corresponding to the generator polynomial (X 5 +X 4 +X 2 +X 1 +1) in f of  FIG. 5 , the generator polynomial (X 5 +X 4 +X 3 +X 2 +1) in h of  FIG. 5 , the generator polynomial (X 5 +X 4 +X 2 +1) in e of  FIG. 5 , and the generator polynomial (X 5 +X 4 +X 3 +X 2 +1) in h of  FIG. 5 . Upon detection of an error, the corresponding correction is carried out. The result is output as error detection/correction outputs  19   e  to  19   h . A detailed configuration of the error correction units  24   e  to  24   h  is realized in a similar form to  FIG. 13 . In this case, too, the error correction units  24   f  and  24   h  which perform a similar processing can alternatively be replaced by a single common error correction unit.  
      The error detection/correction outputs  19   e  to  19   h  are input to the pattern matching units  27   e  to  27   h , respectively, of the data sync signal detector  3 , and matched against the sync patterns of the sync pattern holders  26   e  to  26   h , respectively. Each sync pattern is given as the sync pattern  14 , so that the sync pattern holder  26   e  holds a 9-bit pattern “101010011”, the sync pattern holder  26   f  a 9-bit pattern “101100110”, the sync pattern holder  26   g  a 9-bit pattern “010001011”, and the sync pattern holder  26   h  a 9-bit pattern “010001110”. In order to set the output timing of the pattern matching units  27   e  to  27   h  in order, the output of the pattern matching  27   e  is delayed by 51T (1T is a unit time) by the delay cell  28   e , the output of the pattern matching  27   f  by 50T by the delay cell  28   f , and the output of the pattern matching  27   g  by 1T by the unit time delay cell  28   g . The result of each delay is input to the majority decision logic circuit  29 .  
      The majority decision logic circuit  29  compares the number of coincidences of the four pattern matching results with the threshold value  15 , and in the case where the number of coincidences of the pattern matching result is not less than the figure of the threshold value  15 , a sync signal detection output  16  is output. In this case, too, the threshold value  15  is set to 2 as in the embodiment of  FIG. 9 .  
      The sync signal detection output  16  gives the decode timing for the MTR code decoder  4 . As a result, a correct decoding is realized and the output data  17  is obtained.  
      The performance for the embodiment of  FIG. 11  will be explained. In the embodiment shown in  FIG. 9 , the detection cannot be carried out upon occurrence of a single error pattern x0x or x000x. In the configuration under consideration, however, the data signal detection of two or less errors of whatever type is possible for all the error patterns shown in  FIG. 4 . If only the error pattern x occurs, for example, the data sync signal can be detected against five or less errors. Thus, it can be understood that the detection rate of the data sync signal  92  is remarkably improved.  
      The performance will be explained with reference to  FIGS. 19A, 19B .  FIGS. 19A, 19B  are graphs showing the performance of the embodiment of  FIG. 11  as determined by computer simulation.  
      In  FIG. 19A , the abscissa represents the signal-to-noise ratio of the Viterbi decoder input and the ordinate the bit error rate and the data sync signal detection error rate. A characteristic curve  195  represents the bit error rate of the data in the data discrimination output  12 . This a characteristic obtained when the data are considered to be random. A characteristic curve  191  represents the characteristic of the data sync signal detection error rate when the data sync signal detection is carried out under the condition that all the 36 bits of the sync pattern coincide. As compared with the characteristic curve  171  of  FIG. 17A  or the characteristic curve  181  of  FIG. 18A , it is seen that the detection performance is deteriorated somewhat by an amount equivalent to the increase in the number of bits of the matching pattern. A characteristic curve  192  is based on a method of a reference technique not including the (1+D) processing unit  5  for data sync signal detection, and represents the characteristic of the data sync signal detection error rate in the case where the data sync signal detection is carried out under the condition that any one of the four 9-bit patterns separated into a bit string of odd numbered bits and a bit string of even numbered bits is coincident. A characteristic curve  193 , on the other hand, represents a characteristic of the data sync signal detection error rate in the case where the data sync signal detection is carried out under the condition (the (1+D) processor  5  is included, and no error is detected or corrected) of the embodiment of  FIG. 3 . A characteristic curve  194  represents the characteristic of the data sync signal detection error rate in the case where the data sync signal is detected under the condition of the embodiment of  FIG. 11  according to the invention. From  FIG. 19A , it is seen that the signal-to-noise ratio is improved by about two or three dB as compared with the configuration of the reference technique.  
      In  FIG. 19B , the abscissa represents the bit error rate in the data discrimination output  12 , and the ordinate represents the data sync signal detection error rate. This is the result of replotting the graph of  FIG. 19A  with the characteristic curve  195  as an abscissa. A characteristic curve  196  corresponds to the characteristic curve  191 , a characteristic curve  197  corresponds to the characteristic curve  192 , a characteristic curve  198  corresponds to the characteristic curve  193 , and a characteristic curve  199  corresponds to the characteristic curve  194 . Let Be (abscissa) be the ratio of occurrence of an error event to the total number of output bits in the output of the data discriminator  1 , and let Se (ordinate) be the rate of occurrence of the data sync signal detection error to the number of requests for data sync signal detection. The characteristic curve  198  for Be in the range of 0.1 or less is approximated by equation (4), and the characteristic curve  199  for Be in the range of 0.1 or less is approximated by equation (5). 
 
 S   e =90 B   e   2.51   (4) 
 
 S   e =160 B   e   3.15   (5) 
 
      As explained in the embodiments of  FIGS. 3, 9 ,  10 ,  11 , the pattern used as a sync pattern is required to have the remainder of zero in the dividing operation by the generator polynomial shown in  FIG. 4 , and these operations are required to make no error of easily employing other patterns. Such 9-bit patterns are listed in  FIG. 16 . In this case, 44 types of patterns are available. The patterns used in the embodiments of  FIGS. 3 and 9  are Nos.  15  and  17  in  FIG. 16 , the patterns used in the embodiment of  FIG. 10  are Nos.  3  and  7  in  FIG. 16 , and the patterns used in the embodiments of  FIG. 11  are Nos.  15 ,  17 ,  33  and  37  in  FIG. 16 .  
      When an attempt is made to realize the data sync detector in the signal processing apparatus according to the invention with an integrated circuit, the circuit size, when a 2-input NAND gate is converted as one gate, increases about 10 gates for the embodiment of  FIG. 3 , about 200 gates for the embodiment shown in  FIG. 9 , about 350 gates for the embodiment shown in  FIG. 10 , and about 400 gates for the embodiment shown in  FIG. 11 , as compared with the reference technique. This is easily realizable taking the recent progress of the integrated circuit technology into account.  
      The data sync signal detector according to the invention can also be configured and realized in software as described later.  
      As described above, with the signal processing apparatus according to the invention, the (1+D) processing is executed before detection of the data sync signal, and further the bits are divided into a bit string of odd numbered bits and a bit string of even numbered bits. In this way, the types of error patterns can be reduced and the error pattern length shortened. As a result, the error detection and correction is facilitated, thereby making possible more accurate data sync signal detection.  
      As shown in  FIGS. 17A, 17B ,  18 A,  18 B,  19 A,  19 B, the method of detecting the data sync signal for the signal processing apparatus according to the invention, as compared with the method of the reference technique, has an effect of improving the signal-to-noise ratio in the input of the Maximum-Likelihood or Viterbi decoder by about 2 to 3 dB. Thus, it is possible to obtain data sync information with high accuracy. Also, the data error caused by the error of the data sync information of the signal processing circuit, the information recording/reproduction apparatus or the information transmission system using the data sync information can be reduced.  
      Now, a further embodiment of the invention will be explained with reference to  FIG. 22 .  
      In  FIG. 22 , input data  11  are input to a data discriminator  1 , and a data discrimination output  12  providing a code bit output discriminated by the data discriminator  1  is input to a post-coder  2  and subjected to a predetermined post-code processing. Further, the output  13  of the post-coder  2  is input to a decoder  4  and an error detection/correction unit  6 . The error detection/correction unit  6  detects and corrects an error of bit strings grouped in one or more groups of bit string according to a predetermined method. The output  19  thus corrected in error is input to a data sync signal detector  3 , matched against a sync pattern  14 . In the case where the number of pattern coincidences is not less than the threshold value  15 , a sync signal detection output  16  is output. The sync signal detection output  16  is input to the decoder  4 , and gives a decode timing of the code string in the post-code output  13 . Thus, the demodulated output data  17  is produced from the decoder  4 .  
      An error of the data sync signal is detected and corrected before data sync signal detection and therefore the data sync signal can be accurately detected.  
      A yet further embodiment of the invention will be explained with reference to  FIG. 23 .  
      The configuration of  FIG. 23  is similar to that of the embodiment shown in  FIG. 9 . However, the difference lies in that the (1+D) processing unit  5  is not included, the data discriminator  1  is a Viterbi decoder of ordinary EEPRML type not optimized for the MTR code, the post coder  2  has the characteristic of (1+D) 2 , and the code applied to the decoder  4  is of GCR (Group Code Recording) (for example, “RATE 16/17 (0, 6/6)” IBM Technical Disclosure Bulletin Vol. 31, No. 8, January 1989, pp. 21-23). Also, the sync pattern used is the same 18-bit pattern as in the embodiment of  FIG. 9 , and two sync patterns “0011111111100011000” and “110000000011100111” are available in the data discrimination output  12 . Due to the difference in the characteristic of the post coder  2 , however, the sync pattern in the post code output  13  is a 18-bit pattern of “001100000011011110”.  
      The post code output  13  is input to a shift register  21  in the error detection/correction unit  6 . The configuration of the error detection/correction unit is the same as that in the embodiment of  FIG. 9 . Thus, the shift register  21 , the syndrome calculator  2   a , the syndrome calculator  23   b , the error correction unit  24   a  and the error correction unit  24   b  included in the error detector/corrector  6  are also the same as the corresponding parts of the embodiment of  FIG. 9 .  
      The sync signal detection output  16  provides the decode timing for the decoder  4  of the GCR code. As a result, a correct decoding is realized and the output data  17  is produced.  
      Even in the case where the MTR code or the GCR code is used as the data modulation code, a data sync pattern which improves the performance of the data sync signal detection can be selected. The same pattern can be used in this case. In addition, due to the presence of the particular pattern in each code, this embodiment can be configured the same way as the embodiment of FIG.  9 . The performance of the data sync signal detection is also substantially the same. The performance of the data section, however, is varied depending on the code used.  
      In this embodiment, the output of the post coder is grouped into a bit string of even numbered bits and a bit string of odd numbered bits and matched. The data sync signal detector can be configured with the error detection and correction function, however, in which the output of the post-coder is divided into one or more groups of bit string by a method other than dividing it into a bit string of odd numbered bits and a bit string of even numbered bits. Nevertheless, such a configuration involves more error patterns and is complicated as compared with the embodiment under consideration, with the performance thereof inferior to this embodiment as correctable error patterns are limited.  
      A still further embodiment of the invention will be explained with reference to  FIG. 24 .  
      The basic configuration of  FIG. 24  is the same as that of the embodiment shown in  FIG. 13 . The difference, however, lies in the configuration of the shift register  21 , the manner in which the shift register output  22  is produced, the manner in which the shift register output  22  is input to the syndrome calculator  23 , the manner in which the shift register output  22  is applied to the error correction unit  24 , and the absence of the delay circuit. The illustrated arrangement is also different.  
      The shift register  21  has a length of 68 bits.  
      Nine bits including every other bit from the MSB side (farthest from the input of the (1+D) processing output  18 ) of the shift register  21  constitute a shift register output  22   e  and applied to the syndrome calculator  23   e  and the error correction unit  24   e . The 9 bits one bit nearer to the LSB side (the side where the (1+D) processing output  18  is input) of the shift register output  22   e  constitutes the shift register output  22   f  and are applied to the syndrome calculator  23   f  and the error correction unit  24   f . Nine bits including every other bit from the LSB side of the shift register  21  constitute the shift register output  22   h  and are applied to the syndrome calculator  23   h  and the error correction unit  24   h . The 9 bits one bit nearer to the MSB side of the shift register output  22   h  constitutes the shift register output  22   g , and are applied to the syndrome calculator  23   g  and the error correction unit  24   g.    
      The shift register  21  is lengthened and the output retrieve position thereof is selected, so that the shift register  21  can have the function and effect of the delay circuits  28   e  to  28   g  in the embodiment of  FIG. 13 . This effect can eliminate the delay lines  28   e  to  28   g . The shift register may be less than 68 bit length, for example, 36 or 37 or more bit length so that the register output is divided in groups with 0 or 1 or more bits of an arbitrary pattern interposed therebetween.  
      The configuration of  FIG. 24  is exactly equivalent to the configuration of the embodiment shown in  FIG. 11 .  
      With reference to  FIG. 25 , a yet further embodiment of the invention will be explained.  
       FIG. 25  is a flowchart for realizing, by software, a data sync signal detector having the configuration of the embodiment shown in  FIG. 9 . The functions to be realized are the same as those of the embodiment of  FIG. 9 , and so are the data sync pattern and the method of error detection and correction.  
      First, the process starts from step  401 , and the initialization required for data sync detection is performed in step  402 . The storage starting address adr of the memory for storing the value of the post-code output data is set to the value of AD, the program control count cnt to zero and the program control count limit value to L.  
      Then, in step  403 , the post-code output data  13  is stored from address AD of the memory. The memory can sufficiently store the portions before and after the data involved.  
      In step  404 , the data of 18 bits or more are read out of the address AD of the memory and the calculation (1+D) is performed.  
      In step  405 , 9 bits are retrieved from every other position from the address (AD+cnt).  
      In step  406 , the syndrome value is calculated from the particular 9 bits. This calculation is equivalent to  23   a  of  FIG. 9 . It is determined in step  407  whether the syndrome value thus calculated is zero or not. In the case where the syndrome value is zero, the process proceeds to step  410 . In the case where the syndrome value is not zero, on the other hand, it is determined in step  408  whether the error can be corrected from the particular syndrome value. If the error cannot be corrected, on the other hand, the process proceeds to step  419 . The error, if correctable, is corrected in step  409 .  
      Then, the pattern is compared with the pattern A in step  410 . It is determined in step  411  whether the result of comparison is coincident or not, and if not coincident, the process proceeds to step  419 , otherwise to step  412 .  
      In step  412 , 9 bits including every other bit from are retrieved from the address (AD+cnt+1) of the data read from the memory and subjected to the (1+D) calculation in step  404 .  
      In step  413 , the syndrome value is calculated from the particular 9 bits. This calculation is equivalent to  23   b  of  FIG. 9 . It is determined in step  414  whether the calculated syndrome value is zero or not. If the syndrome value is zero, the process proceeds to step  417 . Otherwise, it is determined in step  415  whether the error is correctable from the particular syndrome value. If the error is not correctable, the process proceeds to step  419 , otherwise, the error is corrected in step  416 .  
      Then, the pattern is compared with the pattern B in step  417 . It is determined in step  418  whether the result of comparison is coincident or not, and if not coincident, the process proceeds to step  419 . Otherwise, the process proceeds to step  421 .  
      In step  421 , both the matching patterns A and B are coincident, indicating that the data sync signal detection is possible.  
      Then in step  422 , the head address of the data is determined from the program control count cnt and the process is terminated in step  424 .  
      In the case where the data sync signal is not detected, the process proceeds to step  419 , where 1 is added to the program control count cnt, and it is determined in step  420  whether this program control count cnt is not more than the program control count limit. If it is within the limit, the process returns to step  404  for proceeding with the data sync signal detection. By adding 1 to the program control count cnt, the syndrome value can be calculated or the bit position for pattern matching can be displaced bit by bit.  
      In the case where the determination in step  420  is that the program control count cnt exceeds the limit, on the other hand, the data sync signal detection is impossible, and the fact is reported in step  423 , with the process terminated in step  424 . The program control count limit “limit” represents and defines the range where the data sync signal can be stored in memory.  
      The foregoing description refers to the case where the data sync signal detector according to the embodiment of  FIG. 9  is realized in software. Nevertheless, the other embodiments can of course be realized also in software. It is also possible to realize the software as a computer-readable program embodied on the recording medium.  
       FIG. 15  is a block diagram showing an example configuration of a magnetic disk device according to an embodiment of the invention. The magnetic disk device shown in  FIG. 15  uses a signal processing apparatus according to the invention described above.  
      The magnetic disk device  201  comprises a magnetic disk  211  providing a data recording medium, a magnetic head  212  for recording/reproducing data on the magnetic disk  211 , a R/W amplifier  213  for amplifying the data signal recorded/reproduced, a HDC (Hard Disk Controller) microcomputer  214  for performing the I/F control with a host system  202  and the control operation, etc. for the whole system, a data buffer  215  for temporarily storing the data exchanged with the host system  202 , a servo processing circuit  216  for processing the servo control signal recorded in the magnetic disk  211 , a mechanism driver  217  for controlling a motor  219  for rotationally driving a magnetic disc  211  or a VCM (Voice Coil Motor)  218  which sets the magnetic head  212  in position based on a command from the servo processing circuit  216 , and a signal processing circuit  220  for coding and modulating the data recorded in the magnetic disk  211  and decoding the data read from the magnetic disk  211 .  
      The signal processing circuit  220  is configured with the signal processing apparatus according to the embodiments of  FIGS. 3, 9 ,  10 ,  11 ,  23 ,  24  or  25  or a modification thereof, and includes a data sync signal detector  221 . The magnetic disk device  201  having this configuration can be realized with a small detection error rate of the data sync signal.  
      Specifically, the recording density of the magnetic disk  211  can be improved by employing the signal processing system such as a data discriminator including a Viterbi decoder, and at the same time, the error rate can be reduced by improving the data sync signal detection performance by the employment of the data sync signal detector  221  at the same time.  
      Also, the production cost is reduced by reducing the circuit size of the signal processing system for detecting the data sync signal such as the data sync signal detector  221  while at the same time reducing the error rate by improving the data sync signal detection performance.  
      The invention developed by the present inventor has been described specifically above based on embodiments thereof. The present invention, however, is not limited to the embodiments described above but can be modified in various ways without departing from the scope and spirit of the invention.  
      In the foregoing description, for example, the data sync signal detection system of the signal processing apparatus according to the invention is referred to as an example of a magnetic disk device. In addition to the magnetic disk device, however, the invention can be used with an information processing signal processing circuit, an integrated circuit, a magneto-optic device, a floppy disk drive, etc. with equal effect.  
      The features of the invention other than those described in the appended claims are as follows.  
      A 2-bit continuous error and a one-bit error at the ends of the group can be detected and corrected by the error detection/correction unit.  
      The 2-bit error having an error pattern x0x (x indicates an error bit, and 0 a bit not an error) and the one-bit error at the second position from each end of the group can be detected and corrected by the error detection/correction unit ( FIG. 8 ).  
      The data sync signal is detected in the case where the threshold value for data sync signal detection is set to 2 and the number of coincident bit string groups is 2 or more (FIGS.  9  to  11 ).  
      The signal processing apparatus is formed of integrated circuits.  
      The signal processing system of signal processing apparatus according to this invention is used with the magnetic disk device, the magneto-optical disk device or the optical disk device.  
      In a signal processing apparatus according to the embodiments described above, the detection error can be reduced in the data sync signal detection.  
      Also, the signal processing apparatus described in the embodiments above has the effect of improving the data sync signal detection performance of the data sync signal detector with the improvement of the reproduction performance of the data section.  
      Further, the configuration of the data sync signal detector is simple and the circuit size can be reduced.  
      In the magnetic disk drive according to the aforementioned embodiments, the recording density is improved by the employment of a signal processing system including a Maximum-Likelihood or Viterbi decoder while at the same time reducing the error rate by the improvement of the data sync signal detection performance.  
      Also, in the magnetic disk device according to the above-mentioned embodiments, the production cost is reduced by the reduced circuit size of the signal processing system for detecting the data sync signal and the error rate can be reduced by the improved data sync signal detection performance at the same time.