Patent Publication Number: US-8111474-B2

Title: Eccentricity correction method, signal processing circuit, magnetic storage apparatus and perpendicular magnetic recording medium

Description:
This application is a Divisional of application Ser. No. 11/474,156, filed Jun. 23, 2006. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to eccentricity correction methods, signal processing circuits, magnetic storage apparatuses and perpendicular magnetic recording media, and more particularly to an eccentricity correction method for correcting eccentricity by reproducing an eccentricity correction data that is recorded by a recoding head from a perpendicular magnetic recording medium that is prerecorded with servo information by a magnetic transfer, a signal processing circuit and a magnetic storage apparatus that employ such an eccentricity correction method, and a perpendicular magnetic recording medium that is recorded with an eccentricity correction data in an easily reproducible manner. 
     2. Description of the Related Art 
     A magnetic disk is prerecorded with servo information for controlling a position of a head with respect to the magnetic disk. The servo information may be recorded by a recording head or, by a magnetic transfer that applies magnetization in an in-plane direction or a perpendicular direction with respect to the magnetic disk. The magnetic transfer itself is known, as may be seen from a Japanese Laid-Open Patent Application No. 2004-342297. 
     After the magnetic disk that is prerecorded with the servo information is assembled into a magnetic disk drive, an eccentricity correction data for correcting eccentricity of the magnetic disk is recorded on the magnetic disk by a recording head. The eccentricity correction is carried out in real-time by reproducing the eccentricity correction data from the magnetic disk by a reproducing head. 
       FIG. 1  is a system block diagram showing an important part of a conventional magnetic disk drive. A magnetic disk drive  1  shown in  FIG. 1  includes a preamplifier part  2 , a read and write part  3 , a hard disk controller (HDD)  4 , and a servo controller (SVC)  5 . The preamplifier part  2  includes an amplifier  21  and a driver  22 . The read and write part  3  includes a synchronizing circuit  31 , a prefilter  32 , a switching circuit  33 , a data demodulating circuit  34 , a servo demodulating circuit  35 , a post processor  36 , a recording compensation circuit  37 , and a driver  38 . 
       FIG. 2  is a diagram showing a format on a conventional perpendicular magnetic disk. The perpendicular magnetic disk is provided with a servo region in which servo information is prerecorded by a recording head. Data are recorded in a data region which follows the servo region. A preamble # 1 , servo mark and address, and servo burst are prerecorded in the servo region. After the perpendicular magnetic disk is assembled into the magnetic disk drive  1 , an eccentricity correction data is recorded in the servo region by the recording head subsequent to the servo burst. At the time of a recording, the data is recorded subsequent to a preamble # 2 . At the time of a reproduction, the recorded data is reproduced from the data region. The data that are reproduced and recorded with respect to the data region are shown as R/W (read/write) data in  FIG. 2 . 
     Information reproduced from the perpendicular magnetic disk (not shown) by the reproducing head (not shown) is supplied to the data demodulating circuit  34  and the servo demodulating circuit  35  via the amplifier  21 , the synchronizing circuit  31  and the prefilter  32 . The synchronizing circuit  31  generates a clock and a servo mark from the reproduced information, and supplies the clock and the servo mark to the switching circuit  33 . The switching circuit  33  controls switching of the demodulating circuits  34  and  35  based on the clock and the servo mark, so that the output of the prefilter  32  is demodulated by the servo demodulating circuit  35  while reproducing the information from the servo region and the output of the prefilter  32  is demodulated by the data demodulating circuit  34  while reproducing the information from the data region. Output reproduced data from the data demodulating circuit  34  are supplied to the HDC  4 , and output reproduced servo information from the servo demodulating circuit  35  is supplied to the SVC  5 . The reproduced data are supplied to other parts within the magnetic disk drive  1  or, output outside the magnetic disk drive  1 , via the HDC  4 . The reproduced servo information is used for various kinds of control operations of the magnetic disk drive  1  in the SVC  5 . 
     At the time of the recording, the recording data are supplied to the recording head (not shown) via the HDC  4 , the post processor  36 , the recording compensation circuit  37  and the drivers  38  and  22 . Hence the recording head records the recording data within the data region on the perpendicular magnetic disk subsequent to the preamble # 2 . 
     According to the format shown in  FIG. 2 , both the information within the servo region and the information within the data region that are reproduced by the reproducing head are reproduced in the form of a rectangular wave. Hence, the prefilter  32  has a differentiating characteristic. 
     However, the method of recording the servo information on the magnetic disk by the magnetic transfer is more efficient that the method of recording the servo information by the recording head, in that the servo information can be recorded simultaneously, that is, in a batch. In addition, in the case of the magnetic transfer that applies the magnetization in the in-plane direction with respect to the perpendicular magnetic disk, it is further desirable in that the signal-to-noise ratio (SNR) improves.  FIG. 3  is a diagram showing a format on a perpendicular magnetic disk that is recorded with the servo information by the magnetic transfer which applies the magnetization in the in-plane direction with respect to the perpendicular magnetic disk, and is also recorded with the eccentricity correction data by the recording head. 
     But in the case of the perpendicular magnetic disk that is recorded with the servo information by the magnetic transfer which applies the magnetization in the in-plane direction with respect to the perpendicular magnetic disk and is recorded with the eccentricity correction data by the recording head, the preamble # 1 , the servo mark and address, and the servo burst that are reproduced by the reproducing head are reproduced in the form of differentiated waveforms and cannot be subjected to the same process as the eccentricity correction data within the servo region and the information within the data region that are reproduced in the form of rectangular waveforms. In other words, if the prefilter  32  has a differentiating characteristic, this prefilter  32  will not be suited for processing the preamble # 1 , the servo mark and address, and the servo burst within the servo region that are reproduced in the form of differentiated waveforms, and cannot carry out a suitable prefiltering with respect to the servo information that is to be supplied to the servo demodulating circuit  35 . On the other hand if the prefilter  32  has a non-differentiating characteristic, this prefilter  32  will not be suited for processing the eccentricity correction data within the servo region and the information within the data region that are reproduced in the form of rectangular waveforms, and cannot carry out a suitable prefiltering with respect to the eccentricity correction data to be supplied to the servo demodulating circuit  35  and the data to be supplied to the data demodulating circuit  34 . 
     For this reason, in the case of the perpendicular magnetic recording medium that is prerecorded with the servo information by the magnetic transfer which applies the magnetization in the in-plane direction with respect to the perpendicular magnetic recording medium, there was a problem in that the eccentricity correction cannot be made by reproducing the eccentricity correction data that has been recorded by the recording head. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide a novel and useful eccentricity correction method, signal processing circuit, magnetic storage apparatus and perpendicular magnetic recording medium, in which the problems described above are suppressed. 
     Another and more specific object of the present invention is to provide an eccentricity correction method, a signal processing circuit, a magnetic storage apparatus and a perpendicular magnetic recording medium, which can carry out an eccentricity correction by reproducing an eccentricity correction data that has been recorded by a recording head from a perpendicular magnetic recording medium that is prerecorded with servo information by a magnetic transfer which applies a magnetization in an in-plane direction with respect to the perpendicular magnetic recording medium. 
     Still another object of the present invention is to provide an eccentricity correction method comprising a reproducing step reproducing information from a perpendicular magnetic recording medium that is recorded with servo information, eccentricity correction data and read/write data, by a reproducing head; a filtering step filtering a reproduced output of the reproducing head, by filtering the servo information which has a differentiated waveform by a non-differentiating filter characteristic, and by filtering the eccentricity correction data and the read/write data which have rectangular waveforms by a differentiating filter characteristic; a demodulating step demodulating the servo information, the eccentricity correction data and the read/write data that are filtered by the filtering step; and an output step outputting the demodulated servo information and eccentricity correction data to a servo system which carries out a control process including an eccentricity correction, and the demodulated read/write data to a data processing system. According to the eccentricity correction method of the present invention, it is possible to carry out an eccentricity correction by reproducing the eccentricity correction data that has been recorded by the recording head from the perpendicular magnetic recording medium that is prerecorded with the servo information by the magnetic transfer which applies the magnetization in the in-plane direction with respect to the perpendicular magnetic recording medium. 
     A further object of the present invention is to provide a magnetic storage apparatus comprising a reproducing head configured to reproduce information from a perpendicular magnetic recording medium that is recorded with servo information, eccentricity correction data and read/write data; a filter part configured to filter a reproduced output of the reproducing head, by filtering the servo information which has a differentiated waveform by a non-differentiating filter characteristic, and by filtering the eccentricity correction data and the read/write data which have rectangular waveforms by a differentiating filter characteristic; a demodulating part configured to demodulate the servo information, the eccentricity correction data and the read/write data that are filtered by the filter part; a servo system configured to carry out a control process including an eccentricity control based on the servo information and the eccentricity correction data that are demodulated; and a data processing system configured to process the read/write data that are demodulated. According to the magnetic storage apparatus of the present invention, it is possible to carry out an eccentricity correction by reproducing the eccentricity correction data that has been recorded by the recording head from the perpendicular magnetic recording medium that is prerecorded with the servo information by the magnetic transfer which applies the magnetization in the in-plane direction with respect to the perpendicular magnetic recording medium. 
     Another object of the present invention is to provide a perpendicular magnetic recording medium which is prerecorded with servo information for controlling a position of a head with respect to the perpendicular magnetic recording medium, by a magnetic transfer that applies a magnetization in an in-plane direction with respect to the perpendicular magnetic recording medium, comprising a servo region recorded with a first preamble, the servo information, a second preamble and an eccentricity correction data; and a data region recorded with a third preamble and read/write data, wherein the first, second and third preambles are made up of patterns having a constant period and mutually different frequencies, and the second preamble, the eccentricity correction data, the third preamble and the read/write data are recorded by a recording head. According to the perpendicular magnetic recording medium of the present invention, it is possible to carry out an eccentricity correction by reproducing the eccentricity correction data that has been recorded by the recording head from the perpendicular magnetic recording medium that is prerecorded with the servo information by the magnetic transfer which applies the magnetization in the in-plane direction with respect to the perpendicular magnetic recording medium. 
     Still another object of the present invention is to provide a perpendicular magnetic recording medium which is prerecorded with servo information for controlling a position of a head with respect to the perpendicular magnetic recording medium, by a magnetic transfer that applies a magnetization in an in-plane direction with respect to the perpendicular magnetic recording medium, comprising a servo region recorded with a first preamble and the servo information; and a data region recorded with a second preamble, an eccentricity correction data and read/write data, wherein the first and second preambles are made up of patterns having a constant period and mutually different frequencies, and the second preamble, the eccentricity correction data, the third preamble and the read/write data are recorded by a recording head. According to the perpendicular magnetic recording medium of the present invention, it is possible to carry out an eccentricity correction by reproducing the eccentricity correction data that has been recorded by the recording head from the perpendicular magnetic recording medium that is prerecorded with the servo information by the magnetic transfer which applies the magnetization in the in-plane direction with respect to the perpendicular magnetic recording medium. 
     A further object of the present invention is to provide a signal processing circuit comprising a filter part configured to filter a reproduced output of a reproducing head, by filtering servo information which has a differentiated waveform by a non-differentiating filter characteristic, and by filtering an eccentricity correction data and read/write data which have rectangular waveforms by a differentiating filter characteristic; a demodulating part configured to demodulate the servo information, the eccentricity correction data and the read/write data that are filtered by the filter part; a servo system configured to carry out a control process including an eccentricity control based on the servo information and the eccentricity correction data that are demodulated; and a data processing system configured to process the read/write data that are demodulated. According to the signal processing circuit of the present invention, it is possible to carry out an eccentricity correction by reproducing the eccentricity correction data that has been recorded by the recording head from the recording medium that is prerecorded with the servo information by the magnetic transfer which applies the magnetization in the in-plane direction with respect to the perpendicular magnetic recording medium. 
     Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a system block diagram showing an important part of a conventional magnetic disk drive; 
         FIG. 2  is a diagram showing a format on a conventional perpendicular magnetic disk; 
         FIG. 3  is a diagram showing a format on a perpendicular magnetic disk that is recorded with servo information by a magnetic transfer that applies magnetization in an in-plane direction with respect to the perpendicular magnetic disk; 
         FIG. 4  is a system block diagram showing a first embodiment of a magnetic storage apparatus according to the present invention; 
         FIG. 5  is a diagram showing a format of a first embodiment of a perpendicular magnetic recording medium according to the present invention; 
         FIG. 6  is a plan view showing the format shown in  FIG. 5 ; 
         FIG. 7  is a diagram for explaining an output of a reproducing head; 
         FIG. 8  is a system block diagram showing a second embodiment of the magnetic storage apparatus according to the present invention; 
         FIG. 9  is a system block diagram showing a third embodiment of the magnetic storage apparatus according to the present invention; 
         FIG. 10  is a diagram showing a format of a third embodiment of the perpendicular magnetic recording medium according to the present invention; 
         FIG. 11  is a plan view showing the format shown in  FIG. 10 ; 
         FIG. 12  is a system block diagram showing a fourth embodiment of the magnetic storage apparatus according to the present invention; and 
         FIG. 13  is a system block diagram showing a fifth embodiment of the magnetic storage apparatus according to the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A description will be given of various embodiments of the eccentricity correction method, the signal processing circuit, the magnetic storage apparatus and the perpendicular magnetic recording medium according to the present invention, by referring to  FIG. 4  and the subsequent drawings. 
     First Embodiment 
       FIG. 4  is a system block diagram showing a first embodiment of the magnetic storage apparatus according to the present invention. This first embodiment of the magnetic storage apparatus employs a first embodiment of the signal processing circuit according to the present invention, a first embodiment of the eccentricity correction method according to the present invention, and a first embodiment of the perpendicular magnetic recording medium (having a format shown in  FIG. 5 ) according to the present invention. 
     A magnetic disk drive  41 - 1  shown in  FIG. 4  includes a reproducing head  40 R, a recording head  40 W, a preamplifier part  42 , a read and write part  43 - 1 , a hard disk controller (HDC)  44 , and a servo controller (SVC)  45 . The preamplifier part  42  includes an amplifier  421  and a driver  422 . The read and write part  43 - 1  includes a synchronizing circuit  431 - 1 , a prefilter part  432 - 1 , switching circuits  433 - 1  and  434 - 1 , a data demodulating circuit  435 , a servo demodulating circuit  436 , a post processor  437 , a recording compensation circuit  438 , and a driver  439 . The prefilter part  432 - 1  includes a prefilter  51  having a non-differentiating characteristic, and a prefilter  52  having a differentiating characteristic. In this embodiment, each of the prefilters  51  and  52  is made up of a finite impulse response (FIR) filter. The signal processing circuit includes at least the prefilter part  432 - 1 , the data demodulating circuit  435 , the servo demodulating circuit  436 , the HDC  44  and the SVC  45 . 
       FIG. 5  is a diagram showing a format of the first embodiment of the perpendicular magnetic recording medium according to the present invention. A perpendicular magnetic disk  500 - 1  has a servo region and a data region which follows the servo region. A servo mark and address, and a servo burst are prerecorded subsequent to a preamble P 1  within the servo region by a known magnetic transfer which applies a magnetization in an in-plane direction of the perpendicular magnetic disk  500 - 1 . The perpendicular magnetic disk  500 - 1  after the magnetic transfer is assembled into the magnetic disk drive  41 - 1 , and an eccentricity correction data (or a post code) is recorded within the servo region by the recording head  40 W subsequent to the a preamble P 2  following the servo burst within the servo region. The preamble P 1  is made up of patterns having a constant period and is recorded for use in controlling read timings of the servo mark and address, and the servo burst. The preamble P 2  is made up of patterns having a constant period and is recorded for use in controlling a read timing of the eccentricity correction data. The eccentricity correction data itself is a correction data that is used to carry out a known eccentricity correction, that is, a repeatable run out (RRO) correction in real-time, and is obtainable by a known method. Data are recorded subsequent to a preamble P 3  within the data region which follows the servo region. In the data region, the data are recorded by the recording head  40 W subsequent to the preamble P 3  at the time of the recording, and the recorded data are reproduced by the reproducing head  40 R at the time of the reproduction. The data that are reproduced and recorded with respect to the data region are shown as R/W (read/write) data in  FIG. 5 . The preamble P 3  is made up of patterns having a constant period and is recorded for use in controlling a read timing of the R/W data. Since patterns of the preambles P 1 , P 2  and P 3  have mutually different frequencies, it is possible to easily distinguish the information following each of the preambles P 1 , P 2  and P 3 . 
       FIG. 6  is a plan view showing the format shown in  FIG. 5 . For example, an upward direction in  FIG. 6  corresponds to an inner peripheral direction of the perpendicular magnetic disk  500 - 1 , and a downward direction in  FIG. 6  corresponds to an outer peripheral direction of the perpendicular magnetic disk  500 - 1 . In addition, one-dot chain lines in  FIG. 6  indicate centerlines of two mutually adjacent tracks. The tracks are formed concentrically or, as track turns of a spiral track, on the perpendicular magnetic disk  500 - 1 . 
       FIG. 7  is a diagram for explaining an output of the reproducing head  40 R. As shown in FIG.  7 , the reproducing head  40 R outputs differentiated waveforms when reproducing the preamble P 1 , the servo mark and address, and the servo burst within the servo region that are recorded by the magnetic transfer. In addition, the reproducing head  40 R outputs rectangular waveforms when reproducing the preamble P 2  and the eccentricity correction data within the servo region that are recorded by the recording head  40 W. Furthermore, the reproducing head  40 R outputs rectangular waveforms when reproducing the preamble P 3  and the R/W data within the data region that are recorded by the recording head  40 W. The servo information frequency (including the eccentricity correction data) within the servo region is constant for the entire region on the perpendicular magnetic disk  500 - 1 . On the other hand, the R/W data frequency within the data region is different for each zone (or region) that is formed in a ring-shape in a radial direction of the perpendicular magnetic disk  500 - 1 . 
     The information that is reproduced from the perpendicular magnetic disk  500 - 1  by the reproducing head  40 R is supplied to the data demodulating circuit  435  and the servo demodulating circuit  436  via the amplifier  421 , the synchronizing circuit  431 - 1  and the prefilter part  432 - 1 . The synchronizing circuit  431 - 1  generates clocks CK 1  through CK 3  and the servo mark from the reproduced information, supplies the clocks CK 1  and CK 2  (or the clocks CK 1  and CK 3 ) and the servo mark to the switching circuit  433 - 1 , and supplies the clocks CK 2  and CK 3  (or only the clock CK 2 ) and the servo mark to the switching circuit  434 - 1 . Moreover, the synchronizing circuit  431 - 1  supplies the clocks CK 1  and CK 2  to the servo demodulating circuit  436 , and supplies the clock CK 3  to the data demodulating circuit  435 . 
     For example, the synchronizing circuit  431 - 1  generates from the patterns of the preamble P 1  a clock CK 1  having twice the fundamental frequency thereof, generates from the patterns of the preamble P 2  a clock CK 2  having twice the fundamental frequency thereof, and generates from the patterns of the preamble P 3  a clock CK 3  having twice the fundamental frequency thereof. The synchronizing circuit  431 - 1  reads the preamble P 1  and the servo mark and address and the servo burst immediately after the preamble P 1  within the servo region, in synchronism with the clock CK 1 , and reads the preamble P 2  and the eccentricity correction data within the servo region in synchronism with the clock CK 2 . In addition, the synchronizing circuit  431 - 1  reads the preamble P 3  and the R/W data within the data region in synchronism with the clock CK 3 . 
     The switching circuit  433 - 1  controls the switching of the prefilter part  432 - 1  so as to selectively output an output of the prefilter  51  while reading the preamble P 1 , the servo mark and address, and the servo burst within the servo region, based on the clock CK 1  and the servo mark, and to selectively output an output of the prefilter  52  while reading the preamble P 2  and the eccentricity correction data within the servo region and the preamble P 3  and the R/W data within the data region, based on the clock CK 2  and the servo mark. For example, a first switching signal that is output from the switching circuit  433 - 1  to the prefilter part  432 - 1  becomes active to switch the output of the prefilter part  432 - 1  from the output of the prefilter  51  to the output of the prefilter  52  at a timing which is m1 periods after the servo mark by counting the clock CK 1  using the servo mark as a trigger, and becomes inactive to switch the output of the prefilter part  432 - 1  from the output of the prefilter  52  to the output of the prefilter  51  at a timing which is m2 periods after the servo mark by counting the clock CK 1  using the servo mark as a trigger. 
     In addition, the switching circuit  434 - 1  controls the switching of the demodulating circuits  435  and  436  based on the clocks CK 2  and CK 3  (or only the clock CK 2 ) and the servo mark, so that the output of the prefilter part  432 - 1  is demodulated by the servo demodulating circuit  436  while reading the servo region and the output of the prefilter part  432 - 1  is demodulated by the data demodulating circuit  435  while reading the data region. For example, a second switching signal that is output from the switching circuit  434 - 1  to the demodulating circuits  435  and  436  becomes active to make the data demodulating circuit  435  active (or to enable the data demodulating circuit  435 ) and the servo demodulating circuit  436  inactive (or to disable the servo demodulating circuit  436 ) at a timing which is n1 periods after the servo mark by counting the clock CK 1  using the servo mark as a trigger, and becomes inactive to make the data demodulating circuit  435  inactive and the servo demodulating circuit  436  active at a timing which is n2 periods after the servo mark by counting the clock CK 1  using the servo mark as a trigger. 
     When active, the servo demodulating circuit  436  demodulates the preamble P 1 , the servo mark and address, and the servo burst within the servo region in synchronism with the clock CK 1 , demodulates the preamble P 2  and the eccentricity correction data within the servo region in synchronism with the clock CK 2 , and supplies the demodulated servo information and the demodulated eccentricity correction data to the SVC  45 . The demodulated servo information and the demodulated eccentricity correction data are used for various kinds of control processes, including the eccentricity control process, in the SVC  45  which forms a servo system of the magnetic disk drive  41 - 1 . When active, the data demodulating circuit  435  demodulates the preamble P 3  and the R/W data within the data region in synchronism with the clock CK 3 , and supplies the demodulated R/W data to the HDC  44 . The demodulated R/W data is output via the HDC  44  which forms a data processing system of the magnetic disk drive  41 - 1 , to other parts within the magnetic disk drive  41 - 1  or, to the outside of the magnetic disk drive  41 - 1 . 
     At the time of the recording, the recording data is supplied to the recording head  40 W via the HDC  44 , the post processor  437 , the recording compensation circuit  438  and the drivers  439  and  422 . Hence, the recording data are recorded subsequent to the preamble P 3  within the data region of the perpendicular magnetic disk  500 - 1  by the recording head  40 W. 
     According to this embodiment, the servo mark and address and the servo burst are prerecorded subsequent to the preamble P 1  within the servo region of the perpendicular magnetic disk  500 - 1  by the magnetic transfer which applies the magnetization in the in-plane direction with respect to the perpendicular magnetic disk  500 - 1 , while the eccentricity correction data within the servo region are recorded by the recording head  40 W. However, it is possible to satisfactorily reproduce the servo information within the servo region that is recorded by different recording systems (or recording techniques). As a result, it is possible to simultaneously improve the SNR by the magnetic transfer which applies the magnetization in the in-plane direction and carry out a satisfactory eccentricity correction based on the eccentricity correction data. 
     Second Embodiment 
       FIG. 8  is a system block diagram showing a second embodiment of the magnetic storage apparatus according to the present invention. This second embodiment of the magnetic storage apparatus employs a second embodiment of the signal processing circuit according to the present invention, a second embodiment of the eccentricity correction method according to the present invention, and a second embodiment of the perpendicular magnetic recording medium according to the present invention. The format of this second embodiment of the perpendicular magnetic recording medium may be the same as the format of the first embodiment shown in  FIG. 5 , and an illustration and description thereof will be omitted. In  FIG. 8 , those parts that are the same as those corresponding parts in  FIG. 4  are designated by the same reference numerals, and a description thereof will be omitted. A signal processing circuit includes at least a prefilter part  432 - 2 , a data demodulating circuit  435 , a servo demodulating circuit  436 , an HDC  44  and a SVC  45 . 
     As shown in  FIG. 8 , a magnetic disk drive  41 - 2  includes a read and write part  43 - 2 . The read and write part  43 - 2  has a prefilter part  432 - 2 , and the prefilter part  432 - 2  has a filter coefficient storage part  61  and a prefilter  62 . The filter coefficient storage part  61  stores first filter coefficients for operating the prefilter  62  as a FIR having a non-differentiating characteristic, and second filter coefficients for operating the prefilter  62  as a FIR having a differentiating characteristic. The first or second filter coefficients stored in the filter coefficient storage part  61  are set in the prefilter  62  based on the first switching signal from the switching circuit  433 - 1 . 
     The switching circuit  433 - 1  controls the switching of the prefilter part  432 - 2  so as to set the first filter coefficients in the prefilter  62  while reading the preamble P 1 , the servo mark and address, and the servo burst within the servo region, based on the clock CK 1  and the servo mark, and to set the second filter coefficients in the prefilter  62  while reading the preamble P 2  and the eccentricity correction data within the servo region and the preamble P 3  and the R/W data within the data region, based on the clock CK 2  and the servo mark. For example, the first switching signal that is output from the switching circuit  433 - 1  to the prefilter part  432 - 2  becomes active to switch the filter coefficients that are set in the prefilter  62  from the first filter coefficients to the second filter coefficients at the timing which is m1 periods after the servo mark by counting the clock CK 1  using the servo mark as a trigger, and becomes inactive to switch the filter coefficients set in the prefilter  62  from the second filter coefficients to the first filter coefficients at the timing which is m2 periods after the servo mark by counting the clock CK 1  using the servo mark as a trigger. Hence, the output of the prefilter part  432 - 2  becomes the same as the output of the prefilter part  432 - 1  of the first embodiment described above. 
     According to this embodiment, it is possible to obtain effects similar to those obtainable in the first embodiment described above. 
     Third Embodiment 
       FIG. 9  is a system block diagram showing a third embodiment of the magnetic storage apparatus according to the present invention. This third embodiment of the magnetic storage apparatus employs a third embodiment of the signal processing circuit according to the present invention, a third embodiment of the eccentricity correction method according to the present invention, and a third embodiment of the perpendicular magnetic recording medium (having a format shown in  FIG. 10 ) according to the present invention. In  FIG. 9 , those parts that are the same as those corresponding parts in  FIG. 4  are designated by the same reference numerals, and a description thereof will be omitted. A signal processing circuit includes at least a prefilter part  432 - 1 , a data demodulating circuit  435 , a servo demodulating circuit  436 , an HDC  44  and a SVC  45 . 
     As shown in  FIG. 9 , a magnetic disk drive  41 - 3  includes a read and write part  43 - 3 . The read and write part  43 - 3  has a synchronizing circuit  431 - 2 , switching circuits  433 - 2  and  434 - 2 , a data demodulating circuit  435 , a servo demodulating circuit  436 , and a selector circuit  441 . 
       FIG. 10  is a diagram showing the format of the third embodiment of the perpendicular magnetic recording medium according to the present invention. A perpendicular magnetic disk  500 - 2  has a servo region and a data region which follows the servo region. A servo mark and address, and a servo burst are prerecorded subsequent to a preamble P 1  within the servo region by a known magnetic transfer which applies a magnetization in an in-plane direction of the perpendicular magnetic disk  500 - 2 . The perpendicular magnetic disk  500 - 2  after the magnetic transfer is assembled into the magnetic disk drive  41 - 3 , and an eccentricity correction data (or a post code) is recorded within the data region by the recording head  40 W subsequent to the a preamble P 4  following the servo burst within the servo region. The preamble P 1  is made up of patterns having a constant period and is recorded for use in controlling read timings of the servo mark and address, and the servo burst. The preamble P 4  is made up of patterns having a constant period and is recorded for use in controlling read timings of the eccentricity correction data and the R/W data. The eccentricity correction data itself is a correction data that is used to carry out a known eccentricity correction, that is, an RRO correction in real-time, and is obtainable by a known method. Data are recorded in the data region subsequent to the eccentricity correction data within the data region. In the data region, the data are recorded by the recording head  40 W subsequent to the eccentricity correction data at the time of the recording, and the recorded data are reproduced by the reproducing head  40 R at the time of the reproduction. The data that are reproduced and recorded with respect to the data region are shown as R/W (read/write) data in  FIG. 10 . Since patterns of the preambles P 1  and P 4  have mutually different frequencies, it is possible to easily distinguish the information following each of the preambles P 1  and P 4 . 
     In this case, the reproducing head  40 R outputs differentiated waveforms when reproducing the preamble P 1 , the servo mark and address, and the servo burst within the servo region that are recorded by the magnetic transfer. In addition, the reproducing head  40 R outputs rectangular waves when reproducing the preamble P 4 , the eccentricity data and the R/W data within the data region that are recorded by the recording head  40 W. 
       FIG. 11  is a plan view showing the format shown in  FIG. 10 . For example, an upward direction in  FIG. 11  corresponds to an inner peripheral direction of the perpendicular magnetic disk  500 - 2 , and a downward direction in  FIG. 11  corresponds to an outer peripheral direction of the perpendicular magnetic disk  500 - 2 . In addition, one-dot chain lines in  FIG. 11  indicate centerlines of two mutually adjacent tracks. The tracks are formed concentrically or, as track turns of a spiral track, on the perpendicular magnetic disk  500 - 2 . 
     The R/W data frequency within the data region (including the eccentricity correction data) is different for each zone (or region) that is formed in a ring-shape in a radial direction of the perpendicular magnetic disk  500 - 2 . 
     For example, the synchronizing circuit  431 - 2  generates from the patterns of the preamble P 1  a clock CK 1  having twice the fundamental frequency thereof, and generates from the patterns of the preamble P 4  a clock CK 4  having twice the fundamental frequency thereof. The synchronizing circuit  431 - 2  reads the preamble P 1  and the servo mark and address and the servo burst immediately after the preamble P 1  within the servo region, in synchronism with the clock CK 1 , and reads the preamble P 4  and the eccentricity correction data and the R/W data within the data region in synchronism with the clock CK 4 . 
     The switching circuit  433 - 2  controls the switching of the prefilter part  432 - 1  so as to selectively output an output of the prefilter  51  while reading the preamble P 1 , the servo mark and address, and the servo burst within the servo region, based on the clock CK 1  and the servo mark, and to selectively output an output of the prefilter  52  while reading the preamble P 4 , the eccentricity correction data and the R/W data within the data region, based on the clock CK 4  and the servo mark. For example, the first switching signal that is output from the switching circuit  433 - 2  to the prefilter part  432 - 1  becomes active to switch the output of the prefilter part  432 - 1  from the output of the prefilter  51  to the output of the prefilter  52  at a timing which is m3 periods after the servo mark by counting the clock CK 1  using the servo mark as a trigger, and becomes inactive to switch the output of the prefilter part  432 - 1  from the output of the prefilter  52  to the output of the prefilter  51  at a timing which is m4 periods after the servo mark by counting the clock CK 1  using the servo mark as a trigger. 
     In addition, the switching circuit  434 - 2  controls the switching of the demodulating circuits  435  and  436  based on the clocks CK 1  and CK 4  and the servo mark, so that the output of the prefilter part  432 - 1  is demodulated by the servo demodulating circuit  436  while reading the servo region and the output of the prefilter part  432 - 1  is demodulated by the data demodulating circuit  435  while reading the data region. For example, the second switching signal that is output from the switching circuit  434 - 2  to the demodulating circuits  435  and  436  becomes active to make the data demodulating circuit  435  active (or to enable the data demodulating circuit  435 ) and the servo demodulating circuit  436  inactive (or to disable the servo demodulating circuit  436 ) at a timing which is n3 periods after the servo mark by counting the clock CK 1  using the servo mark as a trigger, and becomes inactive to make the data demodulating circuit  435  inactive and the servo demodulating circuit  436  active at a timing which is n4 periods after the servo mark by counting the clock CK 1  using the servo mark as a trigger. 
     The output of the data demodulating circuit  435  is supplied to the selector circuit  441 . For example, the selector circuit  441  selectively outputs to the SVC  45  the output of the data demodulating circuit  435  (that is, the eccentricity correction data) from a timing which is n3 periods after the servo mark to a timing which is n5 periods after the servo mark by counting the clock CK 1  using the servo mark from the synchronizing circuit  431 - 2  as a trigger, and selectively outputs to the HDC  44  the output of the data demodulating circuit  435  (that is, the R/W data) from a timing which is n5 periods after the servo mark to a timing which is n4 periods after the servo mark. Hence, the demodulated R/W data from the data demodulating circuit  435  are supplied to the HDC  44 , and the demodulated servo information (servo mark and address, and servo burst) from the servo demodulating circuit  436  and the demodulated eccentricity correction data from the data demodulating circuit  435  are supplied to the SVC  45 . 
     According to this embodiment, it is possible to obtain effects similar to those obtainable in the first embodiment described above. In addition, since the common preamble P 4  is used to identify the eccentricity correction data and the R/W data in the case of the perpendicular magnetic disk  500 - 2 , it is possible to improve the format efficiency compared to that of the perpendicular magnetic disk  500 - 1 . 
     Fourth Embodiment 
       FIG. 12  is a system block diagram showing a fourth embodiment of the magnetic storage apparatus according to the present invention. This fourth embodiment of the magnetic storage apparatus employs a fourth embodiment of the signal processing circuit according to the present invention, a fourth embodiment of the eccentricity correction method according to the present invention, and a fourth embodiment of the perpendicular magnetic recording medium according to the present invention. The format of this fourth embodiment of the perpendicular magnetic recording medium may be the same as the format of the third embodiment shown in  FIG. 10 , and an illustration and description thereof will be omitted. In  FIG. 12 , those parts that are the same as those corresponding parts in  FIGS. 8 and 9  are designated by the same reference numerals, and a description thereof will be omitted. A signal processing circuit includes at least a prefilter part  432 - 2 , a data demodulating circuit  435 , a servo demodulating circuit  436 , an HDC  44  and a SVC  45 . 
     The switching circuit  433 - 2  controls the switching of the prefilter part  432 - 2  so as to set the first filter coefficients in the prefilter  62  while reading the preamble P 1 , the servo mark and address, and the servo burst within the servo region, based on the clock CK 1  and the servo mark, and to set the second filter coefficients in the prefilter  62  while reading the preamble P 4 , the eccentricity correction data and the R/W data within the data region, based on the clock CK 4  and the servo mark. For example, the first switching signal that is output from the switching circuit  433 - 2  to the prefilter part  432 - 2  becomes active to switch the filter coefficients that are set in the prefilter  62  from the first filter coefficients to the second filter coefficients at the timing which is m3 periods after the servo mark by counting the clock CK 1  using the servo mark as a trigger, and becomes inactive to switch the filter coefficients set in the prefilter  62  from the second filter coefficients to the first filter coefficients at the timing which is m4 periods after the servo mark by counting the clock CK 1  using the servo mark as a trigger. 
     According to this embodiment, it is possible to obtain effects similar to those obtainable in the first embodiment described above. In addition, since the common preamble P 4  is used to identify the eccentricity correction data and the R/W data in the case of the perpendicular magnetic disk  500 - 2 , it is possible to improve the format efficiency compared to that of the perpendicular magnetic disk  500 - 1 . 
     Fifth Embodiment 
       FIG. 13  is a system block diagram showing a fifth embodiment of the magnetic storage apparatus according to the present invention. This fifth embodiment of the magnetic storage apparatus employs a fifth embodiment of the signal processing circuit according to the present invention, a fifth embodiment of the eccentricity correction method according to the present invention, and a fifth embodiment of the perpendicular magnetic recording medium according to the present invention. The format of this fifth embodiment of the perpendicular magnetic recording medium may be the same as the format of the first embodiment shown in  FIG. 5  or, the format of the third embodiment shown in  FIG. 10 , and an illustration and description thereof will be omitted. A signal processing circuit includes at least a prefilter part  432 , a data demodulating circuit  435 , a servo demodulating circuit  436 , an HDC  44  and a SVC  45 . 
       FIG. 13  only shows an important part of the fifth embodiment of the magnetic storage apparatus. In  FIG. 13 , those parts that are the same as those corresponding parts in  FIG. 4  are designated by the same reference numerals, and a description thereof will be omitted. As shown in  FIG. 13 , in a read and write part  43 - 5  of a magnetic disk drive  41 - 5 , the prefilter part  432  constantly supplies an output of a synchronizing circuit  431  to a selector circuit  600  via prefilters  51 A and  51 B. The prefilter  51 A has a non-differentiating filter characteristic, and the prefilter  52 A has a differentiating filter characteristic. The selector circuit  600  inputs one of outputs of the prefilters  51 A and  52 A at a timing based on the clocks CK 1  through CK 3  from the synchronizing circuit  431 , and selectively outputs the one of the outputs input to the selector circuit  600  to one of the data demodulating circuit  435  and the servo demodulating circuit  436 . 
     In the case where the fifth embodiment of the perpendicular magnetic recording medium uses the same format as the first embodiment, the perpendicular magnetic disk  500 - 1  has the format shown in  FIG. 5 . In this case, the input of the selector circuit  600  is switched and controlled so as to selectively input the output of the prefilter  51 A while reading the preamble P 1 , the servo mark and address, and the servo burst within the servo region, based on the clock CK 1  and the servo mark from the synchronizing circuit  431 , and to selectively input the output of the prefilter  52 A while reading the preamble P 2  and the eccentricity correction data within the servo region and the preamble P 3  and the R/W data within the data region, based on the clock CK 2  and the servo mark from the synchronizing circuit  431 . In addition, the output of the selector circuit  600  is switched and controlled so as to selectively output the output of the prefilter part  432  to the servo demodulating circuit  436  while reading the servo region, and to selectively output the output of the prefilter part  432  to the data demodulating circuit  435  while reading the data region, based on the clocks CK 1  through CK 3  and the servo mark from the synchronizing circuit  431 . 
     On the other hand, in the case where the fifth embodiment of the perpendicular magnetic recording medium uses the same format as the third embodiment, the perpendicular magnetic disk  500 - 2  has the format shown in  FIG. 10 . In this case, the input of the selector circuit  600  is switched and controlled so as to selectively input the output of the prefilter  51 A while reading the preamble P 1 , the servo mark and address, and the servo burst within the servo region, based on the clock CK 1  and the servo mark from the synchronizing circuit  431 , and to selectively input the output of the prefilter  52 A while reading the preamble P 4 , the eccentricity correction data within the servo region and the R/W data within the data region, based on the clock CK 4  and the servo mark from the synchronizing circuit  431 . In addition, the output of the selector circuit  600  is switched and controlled so as to selectively output the output of the prefilter part  432  to the servo demodulating circuit  436  while reading the servo region, and to selectively output the output of the prefilter part  432  to the data demodulating circuit  435  while reading the data region, based on the clocks CK 1  and CK 4  and the servo mark from the synchronizing circuit  431 . 
     According to this embodiment, it is possible to obtain effects similar to those obtainable in the first or third embodiment described above. Furthermore, because the selection of the outputs of the prefilters  51 A and  52 A within the prefilter part  432  and the selection of the demodulating circuits  435  and  436  to which the output of the prefilter part  432  is to be input are made using the single selector circuit  600 , it is unnecessary to provide a mechanism for controlling the demodulating circuits  435  and  436  to the active or inactive states (or to enable or disable the demodulating circuits  435  and  436 ), it is possible to simplify the circuit structure. 
     In each of the embodiments described above, the various timings are generated by counting the clock CK 1 . However, it is of course possible to generate the various timings by counting clocks other than the clock CK 1 . 
     In addition, although the present invention is applied to the perpendicular magnetic disk in the described embodiments, the present invention is of course applicable to perpendicular magnetic recording media other than the perpendicular magnetic disk. For example, the perpendicular magnetic recording medium may have a card shape, and the card-shaped perpendicular magnetic recording medium may have concentric tracks or a spiral track formed thereon. 
     This application claims the benefit of a Japanese Patent Application No. 2006-080580 filed Mar. 23, 2006, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference. 
     Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention.