Abstract:
A magneto-optical reproducing apparatus for reproducing from a magnetic film medium having at least a displacement layer, a switching layer, and a memory layer. A magnetic wall displacement is generated in the displacement layer in any region where the temperature exceeds the Curie temperature of the switching layer to effectively enlarge any recorded magnetic domain. The reproduced signal from the magnetic film medium is equalized with regard to waveform in an equalizer circuit and then fed to a magnetic wall displacement detection circuit that produces a magnetic wall displacement signal by using a differential signal or a secondary differential signal of the reproduced signal so as to provide a low bit error rate despite any sudden DC level variation in the reproduced signal.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a data reproducing apparatus and method handling a magneto-optical recording medium which has a magnetic three-layer film consisting at least of a displacement layer, a switching layer and a memory layer so formed that, in any region where the temperature of the magnetic film exceeds the Curie temperature of the switching layer, displacement of a magnetic wall is generated in the displacement layer to dimensionally enlarge the effectively recorded magnetic domain. More particularly, the present invention relates to a data reproducing apparatus and method capable of detecting generation of a magnetic wall displacement from a differential signal of a reproduced signal or a difference signal thereof in the time base direction and detecting data by the use of such detection output, hence performing proper reproduction of the data at a sufficiently low bit error rate even if any sudden DC level variation peculiar to a DWDD mode is caused in the reproduced signal. 
     There are known magneto-optical recording media employed as rewritable high-density recording media. And among such magneto-optical recording media, one type is attracting notice recently. It has a magnetic three-layer film consisting at least of a displacement layer, a switching layer and a memory layer, wherein a magnetic wall displacement of the displacement layer is generated in any region where the magnetic film temperature is rendered higher than the Curie temperature of the switching layer, so that the size of an effectively recorded magnetic domain is enlarged. A reproduction method handling such a magneto-optical recording medium is termed a DWDD (Domain Wall Displacement Detection) mode. According to this DWDD mode, a very large signal can be reproduced also from a tiny recording domain of a period below the optical limit resolution of a light beam, whereby a high density is attainable without the necessity of changing the wavelength of light, the numerical aperture NA of an objective lens and so forth. 
     Now a further detailed explanation will be given on such DWDD mode. 
     As shown in FIG. 9A, a magneto-optical recording medium  10  has a three-layer film of switched connection consisting of a displacement layer  11 , a switching layer  12  and a memory layer  13  formed in this order. The memory layer  13  is composed of a perpendicular magnetizing film indicating a great magnetic wall reluctance. The displacement layer  11  is composed of another perpendicular magnetizing film indicating a small magnetic wall reluctance and having a high magnetic wall displaceability. The switching layer  12  is composed of a magnetic layer whose Curie temperature Ts is lower than those of the displacement layer  11  and the memory layer  13 . Each arrow  14  in the individual layers denotes the direction of atomic spin. A magnetic wall  15  is formed in the boundary between regions where the atomic spin directions are mutually reverse. 
     If the surface of a recording film is locally heated by the use of a reproducing light beam (laser beam)  16 , there is formed a distribution of temperature T as shown in FIG. 9B, and accordingly a distribution of magnetic wall energy density σ is formed as shown in FIG.  9 C. Since the magnetic wall energy density σ generally becomes lower in accordance with a temperature rise, the distribution is such that the magnetic wall energy density σ becomes minimum at the position of the highest temperature. As a result, a magnetic wall driving force F(x) for displacement toward the high temperature side, where the magnetic wall energy density σ is low, is generated as shown in FIG.  9 D. FIG. 9E shows the positional relationship between a spot  16 P of the light beam  16  and a region  17  whose temperature is higher than the Curie temperature Ts of the switching layer  12 . 
     In any area of the medium  10  where the temperature is lower than the Curie temperature Ts of the switching layer  12 , the magnetic layers are mutually in switched connection, so that even when the magnetic wall driving force F(x) due to the above-described temperature gradient is applied, it is checked by the great magnetic wall reluctance of the memory layer  13  to eventually cause no displacement of the magnetic wall  15 . However, in any area of the medium  10  where the temperature is higher than the Curie temperature Ts, the switched connection between the displacement layer  11  and the memory layer  13  is cut off, so that the magnetic wall  15  of the displacement layer  11  having a small magnetic wall reluctance is rendered displaceable by the magnetic wall driving force F(x) due to the temperature gradient. Consequently, upon entrance of the magnetic wall  15  into the connection cut-off region beyond the position of the Curie temperature Ts with scanning of the medium  10  by the light beam  16 , then the displacement layer  12  begins to be displaced toward the higher temperature side of the magnetic wall  15 . 
     Whenever any of the magnetic walls  15  formed at intervals corresponding to the recorded signal on the medium  10  passes the position of the Curie temperature Ts with scanning of the medium by the light beam  16 , there occurs a displacement of the magnetic wall  15  of the displacement layer  11 . Since the effectively recorded magnetic domain is enlarged dimensionally by such displacement, it becomes possible to reproduce a very large signal as well even from tiny recorded domains of a period below the optical limit resolution of the light beam  16 . 
     As the light beam  16  scans the medium  10  at a fixed speed, the above-described magnetic wall displacement is generated at a temporal interval corresponding to the spatial interval of the recorded magnetic walls  15 . The generation of such magnetic wall displacement can be detected as a change in the polarization plane of the reflected light of the light beam (laser beam)  16 . 
     As shown by broken lines in FIG. 9A, a magnetic wall displacement is generated from the rear of the region  17  as well, so that the signal due to such magnetic wall displacement from the rear is superimposed as a ghost signal on the reproduced signal due to the magnetic wall displacement from the front. Although an explanation on this ghost signal is omitted here, the problem arising therefrom can be solved by properly contriving the application of a reproducing magnetic field or the recording film. 
     The DWDD type magneto-optical disk apparatus mentioned above is substantially similar in structure to any general magneto-optical disk recording/reproducing apparatus. FIG. 10 shows a partial structure of a conventional reproducing section in such an apparatus. A reproduced signal S MO  obtained from an optical head (not shown) is supplied to an equalizer circuit  21  where the frequency characteristic thereof is compensated. A reproduced signal S MO , obtained after such frequency characteristic compensation is supplied to a binary coding circuit  22 , which then converts the input signal into a binary signal S 2 . 
     The binary signal S 2  outputted from the binary coding circuit  22  is supplied to a data detection circuit  23  and a PLL (phase-locked loop) circuit  24 . In the PLL circuit  24 , a clock signal CLK synchronized with the leading and trailing edges of the binary signal S 2  is produced, and then the clock signal CLK is supplied to the data detection circuit  23 . Subsequently in the data detection circuit  23 , data are detected from the binary signal S 2  by the use of the clock signal CLK and then are outputted as reproduced data PD. 
     On a DWDD type magneto-optical disk, signals are recorded in such a manner that data bit strings are first converted into, e.g., RLL modulated bits and then are processed in NRZI (Non-Return to Zero Inverted) mode where a portion with data inversion is expressed as  1  while a portion without data inversion is expressed as  0 . In this case, the data detection circuit  23  converts, e.g., NRZI data into NRZ data, whereby RLL modulated data are obtained as reproduced data PD. 
     The binary coding circuit  22  consists of a comparator  22   a  having a fixed threshold value as shown in FIG. 11A, or a comparator  22   b  of FIG. 11B for integrating the binary signal S 2  by an integrator  23  and feeding back the same to a threshold value, or a comparator  22   c  of FIG. 11C equipped with, on its input side, a DC controller  24  for calculating the envelope center value by the use of a peak hold circuit or a bottom hold circuit and then feeding back the center value. 
     In the above-described DWDD type, however, there still exist many problems to be solved. For example, some variation is caused suddenly in the DC level of the reproduced signal S MO . Such a phenomenon is supposed to be derived from that the direction of magnetization is indefinite and may be inverted with a certain probability in any region other than those contributing to detection of the signal by magnetic wall displacement in the light beam spot. 
     FIG. 12 graphically shows an actual reproduced waveform representing the above phenomenon. On a DWDD type magneto-optical disk, signal is recorded in the NRZI mode as described, and it is seen in this diagram that the DC level of the reproduced signal is suddenly varied upward in the portion indicated by an arrow P. Upon occurrence of such a phenomenon, it is impossible to obtain a proper binary signal in the binary coding circuit of FIG. 11A which converts the input signal into binary one with a fixed threshold value. Even in using the binary coding circuit of FIGS. 11B or  11 C, the relevant DC level variations are extremely faster than the response speed of the coding circuit and may become continuous bit errors during a period prior to the follow-up of the coding circuit. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a data reproducing apparatus and method capable of reproducing data at a sufficiently low bit error rate despite any sudden DC level variation caused in the reproduced signal. 
     According to one aspect of the present invention, there is provided a DWDD type data reproducing apparatus which includes a signal reproducing means for irradiating a light beam from the side of a displacement layer onto a magneto-optical recording medium while moving the light beam relatively to the medium, and displacing a magnetic wall which has a gradient on the magnetic recording medium in the direction of motion of the light beam spot and is formed in the displacement layer with a temperature distribution having a temperature region higher than the Curie temperature of at least the switching layer, thereby obtaining a reproduced signal which corresponds to the change on a polarization plane of the reflected light of the light beam; a magnetic wall displacement detection means for detecting generation of the magnetic wall displacement by the use of a differential signal of the reproduced signal obtained from the signal reproducing means, or by the use of a difference signal thereof in the time base direction; and a data detection means for detecting data by the use of the detection signal obtained from the magnetic wall displacement detection means. 
     According to another aspect of the present invention, there is provided a DWDD type data reproducing method which includes a first step of irradiating a light beam from the side of a displacement layer onto a magneto-optical recording medium while moving the light beam relatively to the medium, and displacing a magnetic wall which has a gradient on the magnetic recording medium in the direction of motion of the light beam spot and is formed in the displacement layer with a temperature distribution having a temperature region higher than the Curie temperature of at least the switching layer, thereby obtaining a reproduced signal which corresponds to the change on a polarization plane of the reflected light of the light beam; a second step of detecting generation of the magnetic wall displacement by the use of a differential signal of the reproduced signal obtained at the first step, or by the use of a difference signal thereof in the time base direction; and a third step of detecting data by the use of a detection signal which represents generation of the magnetic wall displacement detected at the second step. 
     In the present invention, every time any of the magnetic walls formed in the magneto-optical recording medium at intervals corresponding to the recorded signal passes the Curie temperature position in accordance with scanning of the medium by the light beam, the magnetic wall of the displacement layer is displaced, so that the level of the reproduced signal is changed sharply and suddenly in conformity with such magnetic wall displacement. Therefore, upon generation of the magnetic wall displacement, the level of the differential signal of the reproduced signal or that of the difference signal in the time base direction is raised. Consequently, generation of the magnetic wall displacement can be detected by the use of such differential signal or difference signal. 
     As the light beam scans the medium at a fixed speed, the aforementioned magnetic wall displacements are generated at a temporal interval corresponding to the spatial interval of the recorded magnetic walls. Therefore, it becomes possible to perform detection of data by the use of a detection signal representing the generation of such magnetic wall displacement. In this case, generation of the magnetic wall displacement is detected by using the differential signal of the reproduced signal or the difference signal thereof in the time base direction, so that detection of the displacement can be executed without being harmfully affected by any variation caused in the DC level of the reproduced signal. Thus, reproduction of the data can be performed at a sufficiently low bit error rate despite any sudden DC level variation of the reproduced signal peculiar to the DWDD type. 
     The above and other features and advantages of the present invention will become apparent from the following description which will be given with reference to the illustrative accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing an exemplary structure of a magneto-optical disk apparatus represented as a preferred embodiment of the invention; 
     FIG. 2 is a block diagram showing a structural example of a magnetic wall displacement detection circuit; 
     FIG. 3 is a block diagram showing a structural example of a difference circuit; 
     FIGS. 4A to  4 J are timing charts of signals for explaining the operation of the magnetic wall displacement detection circuit; 
     FIG. 5 is a block diagram showing a structural example of a data detection circuit; 
     FIGS. 6A to  6 G are timing chart of signals for explaining the operation of the data detection circuit; 
     FIG. 7 is a block diagram showing another structural example of the magnetic wall displacement detection circuit; 
     FIG. 8 graphically shows input-output characteristics of a comparator having hysteresis; 
     FIGS. 9A to  9 E are explanatory diagrams of a DWDD mode; 
     FIG. 10 is a block diagram partially showing a reproducing section of a conventional magneto-optical disk apparatus; 
     FIGS. 11A to  11 C show some structural examples of a binary coding circuit; and 
     FIG. 12 graphically shows an exemplary reproduced waveform with occurrence of DC level variations peculiar to the DWDD type. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter some preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 shows a DWDD type magneto-optical disk apparatus  100  as one embodiment. 
     A magneto-optical disk  111  handled in this disk apparatus  100  is so composed that a magneto-optical recording medium  10  described already in connection with FIG. 9A is deposited on a substrate of glass or plastic material, and a protective film is formed thereon. 
     The disk apparatus  100  has a spindle motor  113  for driving the magneto-optical disk  111  to rotate the same. The magneto-optical disk  111  is driven to be rotated at a constant angular velocity in both recording and reproducing modes. A frequency generator  114  is attached to the rotary shaft of the spindle motor  113  for detecting the rotation speed thereof. 
     The disk apparatus  100  further has a magnetic head  115  for generating an external magnetic field; a magnetic head driver  116  for controlling generation of the magnetic field from the magnetic head  115 ; an optical head  117  consisting of a semiconductor laser, an objective lens, an optical detector and so forth; and a laser driver  118  for controlling emission of the light from the semiconductor laser in the optical head  117 . The magnetic head  115  and the optical head  117  are disposed opposite to each other in a manner to interpose the magneto-optical disk  111  therebetween. 
     A laser power control signal S PC  is supplied from an undermentioned servo controller  141  to the laser driver  118 , and the power of the laser. beam emitted from the semiconductor laser of the optical head  117  serves as a recording power P W  in a recording mode, or as a reproducing power P R , which is lower than the recording power P W , in a reproducing mode. 
     In a data write mode (recording mode), record data D r  is supplied as NRZI data to the magnetic head driver  116  as will be described later, and a magnetic field corresponding to such record data D r  is generated from the magnetic head  115 , so that the record data D r  is recorded on the magneto-optical disk  111  in cooperation with the light beam (laser beam) obtained from the optical head  117 . 
     The disk apparatus  100  further has a servo controller  141  equipped with a CPU (central processing unit). To this servo controller  141 , there are supplied a focus error signal S FE  and a tracking error signal S TE  produced in the optical head  117 , and also a frequency signal S FG  outputted from the aforementioned frequency generator  114 . 
     The operation of the servo controller  141  is controlled by an undermentioned system controller  151 . The servo controller  141  controls an actuator  145  which includes a tracking coil, a focus coil and a linear motor for radially moving the optical head  117 , whereby tracking and focus servo control actions are executed, and the radial motion of the optical head  117  is also controlled. The servo controller  141  further controls the spindle motor  113 , thereby controlling the magneto-optical disk  111  in a manner to rotate the same at a constant angular velocity in both the recording and reproducing modes. 
     The disk apparatus  100  further has a system controller  151  equipped with a CPU; a data buffer  152 ; and an SCSI (Small Computer System Interface)  153  for transferring data and commands therefrom or to a host computer. The system controller  151  serves to control the entire system. 
     The disk apparatus  100  further has an ECC (error correction code) circuit  154  for additionally attaching an error correction code to the write data supplied from the host computer via the SCSI  153  and serving to correct any error in the output data of an undermentioned data demodulator  159 ; and a data modulator  155  for converting bit strings of the write data where the error correction code has been attached by the ECC circuit  154 , into RLL modulated bits and then converting the same into NRZI data to thereby obtain record data D r . 
     The disk apparatus  100  further has an equalizer circuit  156  for compensating the frequency characteristic of the reproduced signal S MO  obtained from the optical head  117 ; a magnetic wall displacement detection circuit  157  for detecting generation of a magnetic wall displacement from the reproduced signal S MO , whose frequency characteristic has been compensated by the equalizer circuit  156 ; a PLL circuit  158  for obtaining a clock signal CLK synchronized with the leading edge of a detection signal (pulse signal) which signifies generation of the magnetic wall displacement outputted from the magnetic wall displacement detection circuit  157 ; a data detection circuit  159  for obtaining reproduced data (NRZ data) from the detection signal P DS  by the use of the clock signal CLK; and a data demodulator  160  for obtaining read data through demodulation of the reproduced data D P  (e.g., RLL modulated data). 
     Now the structure of the magnetic wall displacement detection circuit  157  will be described below. FIG. 2 shows a structural example of the magnetic wall displacement detection circuit  157 . This detection circuit  157  comprises a differentiator  171  for obtaining a differential signal S 11  through differentiation of the frequency-compensated reproduced signal S MO′ ; a differentiator  172  for obtaining a secondary differential signal S 12  through differentiation of the differential signal S 11 ; and a delay circuit  173  for delaying the differential signal S 11  to thereby obtain a differential signal S 11 ′ which is synchronized with the output timing of the secondary differential signal S 12 . Practically, either of the differentiators  171  and  172  need not be one that gives mathematically strict differentiation characteristic, and may be replaced with a difference circuit  165  of FIG. 3 consisting of a delay circuit  166  and a subtracter  167 . 
     The detection circuit  157  comprises a comparator  174  for comparing the differential signal S 11 ′ with the positive threshold value +V and detecting the periphery of the leading edge of the reproduced signal S MO ; a comparator  175  for comparing the differential signal S 11 ′ with the negative threshold value −V and detecting the periphery of the trailing edge of the reproduced signal S MO ; and a comparator  176  for comparing the secondary differential signal S 12  with a zero level and detecting the leading and trailing edges of the reproduced signal S MO . 
     The detection circuit  157  further comprises an AND circuit  177  for taking a logical product of the output signal S 13  of the comparator  174  and the output signal S 15  of the comparator  176 ; an AND circuit  178  for taking a logical product of the output signal S 14  of the comparator  175  and the output signal S 15  of the comparator  176 ; and an OR circuit  179  for taking a logical sum of the output signals S 16 , S 17  of the AND circuits  177 ,  178  to obtain a detection signal P DS  which represents generation of the magnetic wall displacement. 
     Now the operation of the magnetic wall displacement detection circuit  157  shown in FIG. 2 will be described below with reference to timing charts of FIGS. 4A to  4 J. FIG. 4A shows the reproduced signal S MO  outputted from the optical head  115 , wherein it is supposed that a sudden DC level variation peculiar to the DWDD type is generated in a portion indicated by an arrow P. The reproduced signal S MO  is processed in the equalizer circuit  156  for compensation of its frequency characteristic, so that the waveform-equalized reproduced signal S MO′  of FIG. 4B is obtained. 
     This reproduced signal S MO′  is differentiated in the differentiator  171 , so that a signal S 11 ′ shown in FIG.  4 C is obtained from the delay circuit  173 . The signal S 11 ′ is turned to a positive level correspondingly to the leading edge (portion with generation of one magnetic wall displacement) of the reproduced signal S MO′ , or is turned to a negative level correspondingly to the trailing edge (portion with generation of the other magnetic wall displacement) of the reproduced signal S MO′ . 
     The differential signal S 11  outputted from the differentiator  171  is further differentiated in the differentiator  172 , so that a secondary differential signal S 12  of FIG. 4D is obtained. In this secondary differential signal S 12 , zero-crossing is induced in synchronism with the leading edge and the trailing edge of the reproduced signal S MO . For briefing the explanation, it is assumed here that the process of differentiation in each of the differentiators  171  and  172  is executed without any delay, and therefore the delay amount required in the differentiator  173  is zero. 
     The differential signal S 11 ′ outputted from the delay circuit  173  is supplied to the comparator  174  and then is compared with the threshold value +V, whereby a signal S 13  of FIG. 4E turned to its high level in the periphery of the leading edge of the reproduced signal S MO  is outputted from the comparator  174 . Similarly, the differential signal S 11 ′ outputted from the delay circuit  173  is supplied also to the comparator  175  and then is compared with the threshold value −V, whereby a signal S 14  of FIG. 4F turned to its high level in the periphery of the trailing edge of the reproduced signal S MO  is outputted from the comparator  175 . As will be described later, the signals S 13  and S 14  are used as gate signals, since these signals are turned to a high level thereof in the periphery of the edge of the reproduced signal S MO  as mentioned above. 
     The secondary differential signal S 12  outputted from the differentiator  172  is supplied to the comparator  176  and then is compared with the zero level, whereby a signal S 15  of FIG. 4G is outputted from the comparator  176 . This output signal S 15  falls in synchronism with the leading edge of the reproduced signal S MO  and rises in synchronism with the trailing edge of the reproduced signal S MO . Each of portions denoted by “X” in FIG. 4G is an indefinite area where the threshold value (zero level) of the comparator  176  and the level of the secondary differential signal S 12  are approximately equal to each other. 
     The output signal S 15  of the comparator  176  is inverted and supplied to the AND circuit  177 , to which the output signal S 13  of the comparator  174  is also supplied as a gate signal. Therefore a pulse signal S 16  of FIG. 4H rising in synchronism with the leading edge of the reproduced signal S MO  is obtained from the AND circuit  177 . Similarly, the output signal S 15  of the comparator  176  is supplied to the AND circuit  178 , to which the output signal S 14  of the comparator  175  is also supplied as a gate signal. Therefore a pulse signal S 17  of FIG. 4I rising in synchronism with the trailing edge of the reproduced signal S MO  is obtained from the AND circuit  178 . 
     Subsequently, these signals S 16  and S 17  are supplied to the OR circuit  179 , which then outputs a pulse detection signal P DS  of FIG. 4J rising in synchronism with the leading and trailing edges of the reproduced signal S MO . In this stage, as the magneto-optical disk  111  is scanned by the light beam emitted from the optical head  117 , magnetic wall displacements are generated at the temporal interval corresponding to the spatial interval of the recorded magnetic walls  15 . The reproduced signal S MO  rises in response to generation of a displacement of the magnetic wall  15  where the atomic spin direction  14  (see FIG. 9A) is changed from one to the other, or the reproduced signal S MO  falls in response to generation of a displacement of the magnetic wall  15  where, contrary to the above, the atomic spin direction  14  is changed from the other to one. Consequently, the aforementioned detection signal P DS  represents the result of detecting the magnetic wall displacement from the reproduced signal S MO . 
     As obvious from the waveform of each signal, if any sudden DC level variation peculiar to the DWDD type is caused in the portion indicated by an arrow P in FIG. 4A, such variation brings about substantially no harmful influence on the output signals S 11 , S 12  of the differentiators  171 ,  172 . Consequently, despite any sudden DC level variation caused in the reproduced signal S MO , the detection signal P DS  properly represents the result of detecting the edge of the reproduced signal S MO , i.e., generation of the magnetic wall displacement. 
     Next, the structure of the data detection circuit  159  will be described below. FIG. 5 shows a structural example of the data detection circuit  159 . This data detection circuit  159  comprises four D flip-flops  181 - 184  and an exclusive OR circuit  185 . The detection signal P DS  outputted from the magnetic wall displacement detection circuit  157  is supplied to a clock terminal of the flip-flop  181 , and an inverted output terminal Q bar of the flip-flop  181  is connected to a data terminal D. Thus, the flip-flop  181  constitutes a T flip-flop. 
     An output terminal Q of the flip-flop  181  is connected to a data terminal D of the flip-flop  182 , while an output terminal Q of the flip-flop  182  is connected to a data terminal D of the flip-flop  183 . And output terminals Q of the flip-flops  182  and  183  are connected respectively to the input side of the exclusive OR circuit  185 . An output terminal Q of the flip-flop  184  serves as a reproduced-data output terminal. And a clock signal CLK produced in the PLL circuit  158  is supplied to the respective clock terminals of the flip-flops  182 - 184 . 
     Hereinafter the operation of the data detection circuit  159  shown in FIG. 5 will be described with reference to timing charts of FIGS. 6A to  6 G. FIG. 6A shows an example of the detection signal PDS outputted from the magnetic wall displacement detection circuit  157 , and FIG. 6C shows a clock signal CLK outputted from the PLL circuit  158  and synchronized with the leading edge of the detection signal P DS . 
     When the detection signal P DS  is supplied to the clock terminal of the flip-flop  181 , there is obtained, from its output terminal Q, a signal S 21  whose state is inverted at every leading edge of the detection signal P DS , as shown in FIG.  6 B. Then the signal S 21  is supplied to the data terminal D of the flip-flop  182 , and a signal S 22  synchronized by the clock signal CLK as shown in FIG. 6D is obtained from the output terminal Q of the flip-flop  182 . The signal S 22  thus obtained is supplied to the data terminal D of the flip-flop  183 , and a signal S 23  delayed correspondingly to the duration of one clock pulse as shown in FIG. 6E is obtained from the output terminal Q of the flip-flop  183 . 
     The signals S 22  and S 23  obtained respectively from the output terminals Q of the flip-flops  182  and  183  are supplied to the exclusive OR circuit  185 , which then takes an exclusive logical sum of the input signals. Consequently, there is obtained, from the OR circuit  185 , a signal S 24  which becomes 1 at the edge position while being kept in synchronism with the clock signal CLK as shown in FIG.  6 F and becomes 0 at any other position than the edge, whereby the NRZI waveform is converted into an NRZ waveform. The signal S 24  thus obtained is then supplied to the data terminal D of the flip-flop  184 , and reproduced data (NRZ data) D P  synchronized by the clock signal CLK as shown in FIG. 6G is obtained from the output terminal Q of the flip-flop  184 . 
     Next, the operation of the magneto-optical disk apparatus shown in FIG. 1 will be described below. In case a data write command is supplied from the host computer to the system controller  151 , a data write (recording) mode is executed. In this case, an error correction code is attached, in the ECC circuit  154 , to the write data received from the host computer via the SCSI  153  and stored in the data buffer  152 , and then the data are converted into RLL modulated bits or NRZI data in the data modulator  155 . Subsequently, record data D r  in the form of NRZI data are supplied from the data modulator  155  to the magnetic head driver  116 , and the data D r  are recorded in the data area at a target position on the magneto-optical disk  11 . 
     In case a data read command is supplied from the host computer to the system controller  151 , a data read (reproducing) mode is executed. In this case, a reproduced signal S MO  is obtained from the data area at a target position on the magneto-optical disk  111 . The reproduced signal S MO  is processed in the equalizer circuit  156  for compensation of its frequency characteristic, and then a reproduced signal S MO′  after such compensation is supplied to the magnetic wall displacement detection circuit  157 . Subsequently, a detection signal P DS  obtained from the detection circuit  157  to represent generation of a magnetic wall displacement is supplied to the PLL circuit  158  and the data detection circuit  159 . 
     In the PLL circuit  158 , a clock signal CLK synchronized with the leading edge of the detection signal P DS  is reproduced. Meanwhile in the data detection circuit  159 , reproduced data (NRZ data) D P  are obtained from the detection signal P DS  by the use of the clock signal CLK reproduced in the PLL circuit  158 . The reproduced data D P  are demodulated in the data demodulator  160  and then are corrected in the ECC circuit  154 , whereby read data are obtained. Subsequently the read data are once stored in the data buffer  152 , and thereafter are transmitted at predetermined timing to the host computer via the SCSI  153 . 
     In this embodiment, as described above, generation of the magnetic wall displacement is detected from the reproduced signal S MO  by the magnetic wall displacement detection circuit  157 , and detection of the data is performed in the data detection circuit  159  by the use of the detection signal P DS  to obtain reproduced data D p . In this case, generation of the magnetic wall displacement is detected in the magnetic wall displacement detection circuit  157  by using the differential signal of the reproduced signal S MO  or the difference signal thereof in the time base direction. Consequently, even if any sudden DC level variation peculiar to the DWDD type is caused in the reproduced signal S MO , it brings about substantially none of harmful influence on the process of detection, so that the detection signal P DS  properly represents the result of detecting the generation of the magnetic wall displacement, hence realizing desired reproduction of the data at a sufficiently low bit error rate. 
     It is to be understood that the present invention is not limited merely to the above embodiment alone. For example, the equalization target waveform of the equalizer circuit  156  may be set to a differential one, and the equalizer circuit  156  may be used also as the differentiator  171  (see FIG. 2) of the magnetic wall displacement detection circuit  157 . 
     In order to avoid unnecessary power consumption or generation of noise in any region where the output signal from the comparator  176  of the magnetic wall displacement detection circuit  157  is rendered unstable, the comparator  176  may be replaced with two comparators each having hysteresis. FIG. 7 shows a magnetic wall displacement detection circuit  157 ′ where such a modified structure is employed. In FIG. 7, any component parts corresponding to those in FIG. 2 are denoted by like reference numerals or symbols, and a detailed explanation thereof is omitted here. 
     In an equalizer circuit  156 ′, its equalization target is set to a differential waveform, thereby eliminating the necessity of the differentiator  171  shown in FIG.  2 . Each of comparators  176   a  and  176   b  has hysteresis. A secondary differential signal S 12  outputted from a differentiator  172  is supplied to a non-inverting input terminal of the comparator  176   a  and then is compared with a zero level supplied to the non-inverting input terminal. When the input to the comparator  176   a  is IN + , the input-output characteristics thereof are such as shown in FIG.  8 . An output signal S 15   a  obtained from the comparator  176   a  is supplied to an AND circuit  178 . 
     Each of the comparators  176   a  and  176   b  has hysteresis. A secondary differential signal S 12  from the differentiator  172  is supplied to a non-inverting input terminal of the comparator  176   b  and then is compared with a zero level supplied to the non-inverting input terminal. When the input to the comparator  176   b  is IN − , the input-output characteristics thereof are such as shown in FIG.  8 . An output signal S 15   b  from the comparator  176   b  is supplied to an AND circuit  177 ′. This AND circuit  177 ′ is a normal one differently from the AND circuit  177  in FIG.  2  where one input is a negative logic, because the signal S 15   b  obtained from the comparator  176   b  need not be inverted. 
     The magneto-optical disk apparatus  100  shown in FIG. 1 is equipped with the SCSI  153  for transferring data from or to the host computer. However, the present invention is applicable also to an apparatus equipped with, for example, an MPEG encoder/decoder instead to perform recording and reproduction of video and audio signal data. 
     Thus, according to the present invention, generation of a magnetic wall displacement is detected from a differential signal or a difference signal of the reproduced signal, and the data are detected on the basis of the result of detecting the displacement. Therefore, even if any sudden DC level variation peculiar to the DWDD type is induced in the reproduced signal, such DC level variation causes substantially none of harmful influence on the process of detecting generation of the magnetic wall displacement, hence achieving exact detection of the magnetic wall displacement with high precision. Consequently, it becomes possible to reproduce the data at a sufficiently low bit error rate while ensuring a superior operation with a systematic margin. Further, occurrence of some sudden DC level variations can be permitted to a certain extent with regard to the DWDD type magneto-optical recording medium used in the apparatus, thereby attaining contribution to improvement of the medium yield rate and also to reduction of the manufacturing cost.