Abstract:
A storage unit optically reproduces information from an optical recording medium of a type having a control region which is prerecorded with control information with an embossed shape and a data region in which data is recorded optically. The control information includes medium information peculiar to the recording medium, and the control region and the data region have mutually different recording densities. The storage unit is provided with a control device for switching a frequency of a read clock which is used when reproducing the control information and the data from the optical recording medium between a case where the control information is reproduced and a case where the data is reproduced.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to storage units, optical recording mediums and information recording methods, and more particularly to a storage unit for reproducing information which is recorded with a high density from a recording medium, an optical recording medium recorded with information with a high density, and an information recording method for recording information on a recording medium at with a high density. 
     As one type of optical recording medium, there is the magneto-optical recording medium typified by a magneto-optical disk. The magneto-optical disk has a substrate, and a recording layer made of a magnetic material and formed on the substrate, and records information using changes in magnetic field and heat caused by light. In addition, a magneto-optical effect is used when reproducing information from the magneto-optical disk. A data track for recording data, and a control track for recording medium information peculiar to the magneto-optical disk are provided on such a magneto-optical disk, where each track includes an identification (ID) part for identifying a sector which is a recording region and a data part for recording the data. In order to prevent rewriting of the information, the manufacturer of the magneto-optical disk records the control track by forming concavo-convex parts (embossed pits) on the substrate by use of a stamper or, records the control track simultaneously as the formation of guide grooves (lands/grooves) on the substrate by injection molding. For similar reasons, the ID part is recorded by forming the concavo-convex parts on the substrate by the same production process. 
     Various methods have been proposed conventionally to improve the recording density of the magneto-optical disk described above. According to one method which uses the magnetic super resolution (MSR), although the minimum recorded information which can be reproduced is generally determined by the wavelength, it is possible to reproduce information smaller than such a limit. In other words, by forming a magnetic mask by utilizing a temperature distribution of a laser power at the time of the reproduction, it is possible to reproduce only the necessary information from the magneto-optical disk. 
     FIG. 1 is a diagram for explaining the operating principle of the method which uses the MSR. In FIG. 1, the upper part shows a plan view of a part of  1  track on the magneto-optical disk, and the lower part shows a cross sectional view of the magneto-optical disk. A recording layer  101 , an intermediate layer  102  and a reproducing layer  103  are provided on a substrate (not shown) of the magneto-optical disk. Arrows within these layers  101  through  103  indicate the magnetization direction. In FIG. 1, BM denotes a moving direction of the laser beam, DM denotes a moving direction (rotating direction) of the magneto-optical disk, RM denotes a reproducing magnetic field, and the hatching indicates an interface magnetic domain wall  104 . 
     The intermediate layer  102  transfers or blacks the information recorded in the recording layer  101  to the reproducing layer  103  depending on the temperature. When reproducing the information recorded in the data part of the track in this manner, the temperature distribution of the laser power at the time of the reproduction is utilized to form a magnetic front mask  105  and a rear mask  106  at parts other than the reproducing position, so that it is possible to reproduce only the necessary information from the magneto-optical disk. In other words, in a case where the information recorded in the data part has a minimum mark length of 0.38 μm and this information is reproduced using a laser beam having a wavelength of 680 nm, for example, it is possible to reproduce only the necessary information from the magneto-optical disk by forming the masks  105  and  106 , even if the spot diameter of the laser beam is approximately 1 μm and is approximately 3 times the minimum mark length. 
     However, the ID part of the control track is recorded by forming the concavo-convex parts (embossed pits) on the substrate of the magneto-optical disk. For this reason, even if an attempt is made to record the information in the ID part of the control track with the same density as the data part of the control track, the MSR cannot be used, and there was a problem in that the information recorded in the ID part cannot be reproduced accurately. In other words, in the case where the spot diameter of the laser beam is approximately 1 μm as described above, for example, approximately 3 pits fall within the beam spot even if an attempt is made to reproduce the pit having the minimum mark length of 0.38 μm, and it is possible to reproduce the information from only the necessary one of the 3 pits. This is because there is no known means for masking the information from the pits other than the necessary pit from among the 3 pits which fall within the beam spot. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide a novel and useful storage unit, optical recording medium and information recording method in which the problems described above are eliminated. 
     Another and more specific object of the present invention to provide a storage unit, an optical recording medium and an information recording method which can accurately reproduce information recorded in the form of concavo-convex parts, even when reproducing the information from the recording medium by the magneto-optical effect utilizing the MSR. 
     Still another object of the present invention is to provide a storage unit for optically reproducing information from an optical recording medium of a type having a control region which is prerecorded with control information with an embossed shape and a data region in which data is recorded by an optical means,.where the control information includes medium information peculiar to the recording medium, the control region and the data region have mutually different recording densities, and the storage unit comprises control means for switching a frequency of a read clock which is used when reproducing the control information and the data from the optical recording medium between a case where the control information is reproduced and a case where the data is reproduced. According to the storage unit of the present invention, because the data part and the identification part of the control region are both prerecorded with the embossed shape, it is possible to simplify the control of the read clock frequency, by making the read clock frequency the same for the data part and the identification part of the control region. For this reason, even if the recording density of the data region increases, it is possible to read the control information from the control region. 
     In the storage unit, the recording density of the control region may be 1/N times the recording density of the data region, where N&gt;1. 
     In addition, in the. storage unit, the control means may switch the frequency of the read clock when reproducing the control information to 1/N times the frequency of the read clock at a time of reproducing the data, where N&gt;1. 
     In these cases, when the read clock frequency for reading the data part of the data region is set to 1/N times the read clock frequency for reading the identification part of the data region and the control region, it becomes possible to obtain the required read clock frequencies with a high accuracy. 
     On the other hand, the storage unit may further comprise a first generator which generates a reference clock, and a second generator which generates first and second clocks based on the reference clock, where the control means switches the frequency of the read clock to a frequency of the first clock when reproducing the control information from the control region, and to a frequency of the second clock when reproducing the data from the data region. In this case, by generating the first and second clocks based on the reference clock, it is possible to possible easily synchronize the first and second clocks. In addition, the first and second clocks can easily be generated by dividing the reference clock by different frequency dividing ratios, thereby making it unnecessary to provide a plurality of clock generating circuits, and the circuit scale is simplified. 
     In this case also, the frequency of the first clock may be 1/N times the frequency of the second clock, where N&gt;1. In this case, when the read clock frequency for reading the data part of the data region is set to 1/N times the read clock frequency for reading the identification part of the data region and the control region, it becomes possible to obtain the required read clock frequencies with a high accuracy. 
     A further object of the present invention is to provide an optical recording medium comprising a control region prerecorded with control information with an embossed shape, and a data region recorded with data by an optical means, where the control region has a recording density which is 1/N times a recording density of the data region, where N&gt;1. According to the optical recording medium of the present invention, because the data part and the identification part of the control region are both prerecorded with the embossed shape, it is possible to simplify the control of the read clock frequency, by making the read clock frequency the same for the data part and the identification part of the control region. For this reason, even if the recording density of the data region increases, it is possible to read the control information from the control region. 
    
    
     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 diagram for explaining the operating principle of a method which uses MSR; 
     FIG. 2 is a system block diagram showing the general construction of an embodiment of a storage unit according to the present invention; 
     FIG. 3 is a cross sectional view showing the general construction of an enclosure; 
     FIG. 4 is a system block diagram for explaining parameter setting control and settling wait functions with respect to a read LSI, ODC and a DSP of a MPU in an optical disk unit; 
     FIG. 5 is a system block diagram for explaining a read system of the ODC and the read LSI associated with the MPU; 
     FIG. 6 is a flow chart for explaining the switching of the clock frequency; 
     FIG. 7 is a perspective view showing a portion of an optical disk on an enlarged scale; 
     FIGS. 8A and 8B are diagrams showing a track format of a conventional optical disk; 
     FIGS. 9A and 9B are diagrams showing a sector format of a conventional optical disk; 
     FIGS. 10A and 10B are diagrams showing a track format of a Specific Example 1 of the optical disk; 
     FIGS. 11A,  11 B and  11 C are diagrams showing a sector format of the Specific Example 1 of the optical disk; 
     FIGS. 12A and 12B are diagrams showing a track format of a Specific Example 2 of the optical disk; 
     FIGS. 13A,  13 B and  13 C are diagrams showing a sector format of the Specific Example 2 of the optical disk; 
     FIGS. 14A and 14B are diagrams showing a track format of a Specific Example 3 of the optical disk; 
     FIGS. 15A,  15 B and  15 C are diagrams showing a sector format of the Specific Example 3 of the optical disk; 
     FIGS. 16A and 16B are diagrams showing a track format of a Specific Example 4 of the optical disk; and 
     FIGS. 17A,  17 B and  17 C are diagrams showing a sector format of the Specific Example 4 of the optical disk. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A description will be given of an embodiment of a storage unit according to the present invention. FIG. 2 is a system block diagram showing the general construction of this embodiment of the storage unit. In this embodiment of the storage unit, the present invention is applied to an optical disk unit. This embodiment of the storage unit employs an embodiment of an information recording method according to the present invention, and creates an embodiment of an optical recording medium according to the present invention. 
     As shown in FIG. 2, the optical disk unit generally includes a control unit  10  and an enclosure  11 . The control unit  10  includes a microprocessor unit (MPU)  12  which generally controls the operation of the optical disk unit, an interface  17  which exchanges commands and data between a host unit (not shown), an optical disk controller (ODC)  14  which carries out processes required to read/write data with respect to an optical disk (not shown), a digital signal processor (DSP)  16 , and a buffer memory  18 .. The buffer memory  18  is used in common by the MPU  12 , the ODC  14  and the interface  17 , and includes a dynamic random access memory (DRAM), for example. A crystal oscillator  101  which is used to generate clocks is coupled to the MPU  12 . 
     The ODC  14  includes a formatter  14 - 1  and error correction code (ECC) processor  14 - 2 . At the time of a write access, the formatter  14 - 1  divides NRZ write data into sector units of the optical disk and generates a recording format, and the ECC processor  14 - 2  generates and adds an ECC with respect to sector write data units and also generates and adds if necessary a cyclic redundancy check (CRC) code. Further, the ECC processor  14 - 2  converts the sector data with the encoded ECC into a 1-7 run length limited (RLL) code, for example. 
     At the time of a read access, a reverse conversion of the 1-7 RLL is carried out with respect to the sector data, and after carrying out the CRC, the error detection and error correction using the ECC are carried out in the ECC processor  14 - 2 . Further, the NRZ data in sector units are connected in the formatter  14 - 2  so as to transfer a NRZ read data stream to the host unit. 
     A write large scale integrated (LSI) circuit  20  is provided with respect to the ODC  14 . This write LSI circuit  20  includes a write modulator  21  and( a laser diode control circuit  22 . A control output of the laser diode control circuit  22  is supplied to a laser diode unit  30  which is provided in an optical unit of the enclosure  11 . The laser diode unit  30  integrally includes a laser diode  30 - 1  and a monitoring detector  30 - 2 . The write modulator  21  converts the write data into the data format of the pit position modulation (PPM) recording (or mark recording) or, a pulse width modulation (PWM) recording (or edge recording). 
     When recording and reproducing data with respect to the optical disk using the laser diode unit  30 , this embodiment can use any one of writable magneto-optical (MO) cartridge mediums having the storage capacities of 128 MB, 230 MB, 540 MB and 640 MB. In the MO cartridge mediums having the storage capacities of 128 MB and 230 MB, the PPM recording is employed to record the data in correspondence with the existence and non-existence of marks on the optical disk. In addition, a constant angular velocity (CAV) is used as the recording format of the optical disk having the storage capacity of 128 MB, and a zone constant angular velocity (ZCAV) is used as the recording format of the optical disk having the storage capacity of 230 MB, where 1 zone is provided as a user region in the case of the optical disk having the storage capacity of 128 MB and 10 zones are provided as the user region in the case of the optical disk having the storage capacity of 230 MB. 
     In the MO cartridge mediums having the storage capacities of 540 MB and 640 MB and used for the high density recording, the PWM recording is employed to record the data in correspondence with the edges of the marks, that is, leading and trailing edges, on the optical disk. The difference between the storage capacities of the optical disk having the storage capacity of 540 MB and the optical disk having the storage capacity of 640 MB is due to the different sector capacities. The optical disk has the storage capacity of 540 MB if the sector capacity is 512 bytes, and the optical disk has the storage capacity of 640 MB if the sector capacity is 2048 bytes. In addition, the zone CAV is used as the recording format of the optical disk, where 18 zones are provided as the user region in the case of the optical disk having the storage capacity of 540 MB and 11 zones are provided as the user region in the case of the optical disk having the storage capacity of 640 MB. 
     Hence, this embodiment can cope with the optical disks having the storage capacities of 128 MB, 230 MB, 540 MB and 640 MB, and also cope with direct overwrite type optical disks having the storage capacities of 230 MB, 540 MB and 640 MB. Accordingly, when the optical disk is loaded into the optical disk unit, an identification (ID) part of the optical disk is first read so as to recognize the type of the optical disk by the MPU  12  from the intervals of the pits, and a recognition result of the type of loaded optical disk is notified to the ODC  14 . 
     A read LSI circuit  24  is provided as a read system with respect to the ODC  14 . A read demodulator  25  and a frequency synthesizer  26  are provided in the read LSI circuit  24 . An ID/MO detector  32  of the enclosure  11  detects a laser beam emitted from the laser diode  30 - 1  and returned via the optical disk, and a detection signal from this ID/MO detector  32  is input as an ID signal and a MOD signal to the read LSI circuit  24  via a head amplifier  34 . 
     The read demodulator  25  of the read LSI circuit  24  includes the functions of circuits such as an automatic gain control (AGC) circuit, a filter and a sector mark detection circuit. Hence, the read demodulator  25  generates a read clock and read data from the input ID signal and MO signal, and demodulates the PPM data or the PWM data back into the original NRZ data. In addition, since the zone CAV is employed, the MPU  12  controls a setting of a frequency dividing ratio with respect to the frequency synthesizer  26  of the read LSI circuit  24  in order to generate a clock frequency in correspondence with the zone. 
     The frequency synthesizer  26  is a phase locked loop (PLL) circuit having a programmable frequency divider, and generates as a read clock a reference clock having a predetermined specific frequency depending on the zone position on the optical disk. In other words, the frequency synthesizer  26  is formed by the PLL circuit having the programmable frequency divider, and generates the reference clock having a frequency fo based on fo=(m/n)·fi according to a frequency dividing ratio m/n which is set by the MPU  12  depending on a zone number. 
     A denominator n of the frequency dividing ratio m/n is a specific value depending on the type of optical disk having the storage capacity of 128 MB, 230 MB, 540 MB or 640 MB. In addition, a numerator m of the frequency dividing ratio m/n is a value which changes depending on the zone position on the optical disk, and table information of the values corresponding to the zone numbers are prepared in advance with respect to each type of optical disk. Moreover, fi denotes a reference clock frequency generated outside the frequency synthesizer  26 . 
     The read data demodulated in the read LSI circuit  24  is supplied to the read system of the ODC  14 , and after carrying out the reverse conversion of the 1-7 RLL, the CRC and the ECC processes are carried out by the encoding function of the ECC processor  14 - 2  so as to restore the original NRZ data. Next, the formatter  14 - 1  connects and converts the NRZ sector data into the NRZ read data stream, and this NRZ read data stream is transferred to the host unit via the buffer memory  18  and the interface  17 . 
     A detection signal from a temperature sensor  36  provided in the enclosure  11  is supplied with respect to the MPU  12  via the DSP  16 . Based on an environmental temperature within the optical disk unit detected by the temperature sensor  36 , the MPU  12  controls the light emission powers for the read, write and erase in the laser diode control circuit  22 . 
     The MPU  12  controls a spindle motor  40  provided in the enclosure  11  via the DSP  16  and a driver  38 . In this embodiment, since the zone CAV is employed as the recording format of the optical disk, the spindle motor  40  is rotated at a constant speed of 3000 rpm, for example. 
     In addition, the MPU  12  controls an electromagnet  44  provided in the enclosure  11  via the DSP  16  and a driver  42 . The electromagnet  44  is arranged on a side opposite to the side of the optical disk on which the laser beam is irradiated within the optical disk unit which is loaded with this optical disk. This electromagnet  44  supplies an external magnetic field on the optical disk at the time of the recording and erasure. 
     The DSP  16  is provided with a servo function for positioning the laser beam from the laser diode  30 - 1  with respect to the optical disk, and functions as a seek controller and an on-track controller which enable the laser beam to seek a target track and to be positioned on the target track. The seek control and the on-track control may be carried out simultaneously in parallel with the write access or the read access of the MPU  12  with respect to a host command. 
     In order to realize the servo function of the DSP  16 , a focus error signal (FES) detector  45  is provided in the optical unit of the enclosure  11  so as to detect the laser beam emitted from the laser diode  30 - 1  and returned via the optical disk. A FES detection circuit  46  generates a FES El from a detection signal received from the FES detector  45 , and inputs this FES E 1  to the DSP  16 . 
     A tracking error signal (TES) detector  47  is also provided in the optical unit of the enclosure  11  so as to detect the laser beam emitted from the laser diode  30 - 1  and returned via the optical disk. A TES detection circuit  48  generates a TES E 2  from a detection signal received from the TES detector  47 , and inputs this TES E 2  to the DSP  16 . The TES E 2  is also input to a track zero crossing (TZC) detection circuit  50 , and this TZC detection circuit  50  generates a TZC signal E 3  which is input to the DSP  16 . 
     A lens position sensor  52  is provided in the enclosure  11 . This lens position sensor  52  detects a position of an objective lens through which the laser beam is irradiated on the optical disk. A lens position detection signal (LPOS) E 4  from the lens position sensor  52  is input to the DSP  16 . The DSP  16  controls and drives a focus actuator  60 , a lens actuator  64  and a voice coil motor (VCM)  68  via corresponding drivers  58 ,  62  and  66 , so as to control the position of a beam spot formed by the laser beam on the optical disk. 
     FIG. 3 is a cross sectional view showing the general construction of the enclosure  11 . As shown in FIG. 3, the spindle motor  40  is provided within a housing  67 . By inserting a MO cartridge  70  into the housing  67  from the side of an inlet door  69 , an optical disk (MO disk)  72  accommodated within the MO cartridge  70  engages a rotary shaft of the spindle motor  40 , thereby loading the optical disk  72  with respect to the optical disk unit. The optical disk  72  has the layer structure shown in FIG. 1, for example, when utilizing the MSR. 
     A carriage  76  is provided below loaded the optical disk  72  within the MO cartridge  70 . This carriage  76  is freely movable in a direction which traverses tracks on the optical disk  72  when driven by the VCM  64 . An objective lens  80  is mounted on the carriage  76 . The laser beam emitted from the laser diode  30 - 1  which is provided within a fixed optical system  78 .is reflected by a mirror  82 , and is irradiated on the recording surface of the optical disk  72  via the objective lens  80 , thereby forming a beam spot on the recording surface. 
     The movement of the objective lens  80  along an optical axis is controlled by the focus actuator  60  of the enclosure  11  shown in FIG.  2 . In addition, the objective lens  80  is movable in a radial direction which traverses the tracks on the optical disk  72  by the lens actuator  64 , within a range of several tens of tracks. The position of the objective lens  80  mounted on the carriage  76  is detected by the lens position sensor  54  shown in FIG.  2 . The lens position sensor  54  outputs the lens position detection signal E 4  which takes a value zero at a neutral position where the optical axis of the objective lens  80  is perpendicular to the recording surface of the optical disk  72 , and has a magnitude and a polarity depending on the amount the optical axis of the objective lens  80  moves towards the inner or outer peripheral side of the optical disk  72 . 
     FIG. 4 is a system block diagram for explaining the parameter setting control and settling wait functions of the MPU  12  of the optical disk unit shown in FIG. 2 with respect to the read LSI circuit  24 , the ODC  14  and the DSP  16 . 
     The MPU  12  includes a parameter setting controller  90  which operates based on a read command from the host unit, and a settling wait processor  92  for waiting the settling after the parameter setting. The parameter setting controller  90  controls the setting of the parameters which are required to make various kinds of accesses by use of a parameter table  94  which is developed in a RAM or the like included in the buffer memory  18 . 
     Elements the-parameters of which are to be set by the parameter setting controller  90  of the MPU  12  include the frequency synthesizer  26  and an equalizer circuit  95  within the read LSI circuit  24 . The equalizer circuit  95  is provided with respect to the MO signal which is obtained from the ID/MO detector  32 . In this embodiment,  3  control registers  96 ,  98  and  100  are provided with respect to the frequency synthesizer  26 . 
     The parameters related to the frequency dividing ratio m/n, the frequency setting of a voltage controller oscillator (VCO) and the selection of a PLL damping resistance are respectively set in the control registers  96 ,  98  and  100  by the parameter setting controller  90  of the MPU  12 . A control register  102  is provided with respect to the equalizer circuit  95 . Parameters related to an equalizer cutoff frequency are set in the control register  102  by the parameter setting controller  90  of the MPU  12 . Furthermore, a control register  106  is provided with respect to a sector mark detection circuit  104  of the ODC  14 . Parameters related to a sector mark detection cutoff frequency are set in the control register  106  by the parameter setting controller  90  of the MPU  12 . 
     A seek command is transferred to the DSP  16  when the MPU  12  executes a read command from the host unit. The DSP  16  is provided with a seek controller  108 . Based on the seek command, the seek controller  108  of the DSP  16  carries out a seek control for positioning the beam spot to a target track on the optical disk  72  simultaneously in parallel with the process of the MPU  12 . 
     Therefore, the parameter setting controller  90  of the MPU  12  can optimize the cutoff frequency of the MO signal equalizer circuit  95  within the read LSI circuit  24  by controlling the setting of the control register  102 . In addition, the parameter setting controller  90  can optimize the frequency dividing ratio m/n, the VCO frequency setting and the PLL damping resistance selection of the frequency synthesizer  26  within the read LSI circuit  24  by controlling the setting of the control registers  96 ,  98  and  100 . Furthermore, the parameter setting controller  90  can optimize the cutoff frequency of the sector mark detection circuit  104  within the ODC  14  by controlling the setting of the control register  106 . 
     The firmware of the control unit  10  is installed, for example, by reading the firmware from the optical disk  72  inserted into the enclosure  11  and storing the firmware in the buffer memory  18  under the control of the host unit, and the firmware stored in the buffer memory  18  is executed. In addition, programs executed by the MPU  12  are similarly read from the optical disk  72  inserted into the enclosure  11  and stored in the buffer memory  18  by the MPU  12  under the control of the host unit, for example, and the programs stored in the buffer memory  18  are executed. In other words, The programs of the MPU  12  for realizing an identification information recording method according to the present invention may be recorded on a storage medium according to the present invention, and in this case, the storage medium according to the present invention is not limited to the optical disk  72 . The storage medium according to the present invention may be made selected from various kinds of disks including magnetic disks, various kinds of semiconductor memory devices, various kinds of memory cards, and the like. 
     When installing the firmware, a version number of the firmware is stored in a version number memory within the buffer memory  18  by a known means. In addition, a number of times this firmware is installed in the past in the storage unit shown in FIG.  2  and in other storage units is stored in a version number counter within the buffer memory  18 . 
     FIG. 5 is a system block diagram for explaining the read system of the ODC  14  and the read LSI  24  associated with the MPU  12  of the optical disk unit shown in FIG.  2 . In FIG. 5, those parts which are the same as those corresponding parts in FIGS. 2 and 4 are designated by the same reference numerals, and a description thereof will be omitted. 
     A MO signal (data signal) and an ID signal from the ID/MO detector  32  which detects the light returned via the optical disk  72  are input to the read LSI  24 . The ID signal is obtained by detecting the existence of embossed pits as a change in a light quantity on the ID/MO detector  32 . Since the data of the ID part and the control track are recorded in the form of the embossed pits, the ID signal is read from the ID part and the control track. The MO signal is subjected to a waveform equalization in an equalizer circuit  94 , and is thereafter amplified in an automatic gain control (AGC) circuit  110 . On the other hand, the ID signal is amplified in an AGC circuit  112 . 
     With respect to the equalizer circuit  94  for the MO signal, the parameter setting controller  90  of the MPU  12  shown in FIG. 4 sets an optimizes an equalizer cutoff frequency of the control register  102  depending on the zone position on the optical disk  72 . An output of the AGC circuit  110  for the MO signal and an output of the AGC circuit  112  for the ID signal are input to a multiplexer (MUX)  114 , and an output of this multiplexer  114  is selected in response to an ID/MO switching signal from the MPU  12  and successively supplied to a differentiating circuit  116 , so as to detect a peak level by a zero-crossing. 
     An output of the differentiating circuit  116  is supplied to a data demodulating circuit  117  which generates a read clock and a read data. During a seek operation or the like when no ID signal or MO signal is obtained, the frequency synthesizer  26  generates a frequency of a target reference clock based on a clock from a frequency dividing circuit  119 , responsive to a setting of a frequency dividing ratio corresponding to a zone ratio of a target track. For example, the clock from the crystal oscillator  101  shown in FIG. 2 is supplied to the frequency dividing circuit  119 , and this clock is frequency-divided by a frequency dividing ratio which depends on a data/control track switching signal obtained from the MPU  12  or the ODC  14  before being supplied to the frequency synthesizer  26 . In addition, when an on-track state is reached upon completion of the seek operation, the frequency synthesizer  26  generates a reference clock which follows a peak detection pulse of the ID signal or the MO signal from the differentiating circuit  116 , based on the clock received from the frequency dividing circuit  119 . 
     The data demodulating circuit  117  generates read data by synchronizing the ID signal or the MO signal obtained in the on-track state after completion of the seek operation to the read clock generated from the frequency synthesizer  26 . In this state, the data demodulating circuit  117  carries out a demodulation to restore the PPM modulated data or the PWM modulated data which is obtained as the read data to a read data before the modulation. 
     The output of the differentiating circuit is further differentiated by a differentiating circuit  118 , and an output of the differentiating circuit  118  is compared with a predetermined threshold level in a comparator circuit  120 , so as to output a sector mark pulse signal from the comparator circuit  120 . This sector mark pulse signal indicates a sector mark recorded in the ID region. 
     The read system of the ODC  14  is formed by a RLL data demodulating circuit  122 , a sync byte detection circuit  124 , an address mark detection circuit  126 , an ECC circuit  128 , a CRC check circuit  130 , an ID detection circuit  132 , and the sector mark detection circuit  104 . 
     The read data-and the read clock demodulated in the read LSI  24  are input to the RLL data demodulating circuit  122 , the sync byte detection circuit  124  and the address mark detection circuit  126 . 
     With respect to the read data of the ID signal at the sector head which is first obtained, the sync byte detection circuit  124  carries out a sync byte detection, and the address mark detection circuit  126  then carries out an address mark detection. Outputs indicative of the detections made in the sync byte detection circuit  124  and the address mark detection circuit  126  are supplied to the RLL data demodulating circuit  122 , so as to recognize the read data of the data part (MO part) subsequent to the ID part and to demodulate the read data by the reverse conversion of the 1-7 RLL. 
     The read data demodulated in the RLL data demodulating circuit  122  is supplied to the ECC circuit  128 , the CRC check circuit  130  and the ID detection circuit  132 . The CRC check circuit  130  detects an error of a data stream made up of the data and the ECC, and supplies a result of the error detection to the ECC circuit  128 . The ECC circuit  128  carries out an error detection and correction with respect to the read data based on the ECC, and outputs a NRZ data. 
     The ID detection circuit  132  detects ID information of the read data, and outputs an ID detection update notification signal. The address mark detection circuit  126  outputs an address mark detection signal with respect to the MPU  12 , and the sector mark detection circuit  104  outputs a sector mark detection signal with respect to the MPU  12 . 
     The control register  106  shown in FIG. 4 is provided with respect to the sector mark detection circuit  104  which is provided in the read system of the ODC  14 . A cutoff frequency is set and controlled with respect to the control register  106  from the setting controller  90  of the MPU  12  depending on the zone position, and a cutoff frequency characteristic of the sector mark detection circuit  104  is optimized. 
     FIG. 6 is a flow chart for explaining the switching of the clock frequency in this embodiment. A process shown in FIG. 6 corresponds to the operations of the MPU  12  or the ODC  14 , the frequency dividing circuit  119  and the frequency synthesizer  26 . In addition, FIG. 7 is a perspective view showing a portion of the optical disk  72  on an enlarged scale. 
     The physical format is determined in advance with respect to a data track (data region)  201  and a control track (control region)  202  on the optical disk  72  shown in FIG.  7 . Hence, this embodiment switches the clock frequency based which one of the tracks (regions) is accessed. FIG. 7 also shows an ID part  205  and a data part  206  on the optical disk  72 . In this embodiment, the information recorded on the control track  202  is recorded in the form of the concavo-convex parts (embossed pits) within both the ID part  205  and the data part  206 . On the control track  202 , information for identifying the recording region of the data part  206  is recorded in the ID part  205 , and medium information peculiar to the optical disk  72  or the like is recorded in the data part  206 . On the other hand, the information recorded on the data track  201  is recorded in the form of the concavo-convex parts (embossed pits) within the ID part  205 , but is recorded using the magneto-optical effect within the data part  206 . On the data track  201 , information for identifying the recording region of the data part  206  is recorded in the ID part  205 , and the data is recorded in the data part  206 . 
     In FIG. 6, a step S 1  decides whether or not the track which is to be read is the control track  202 . If the decision result in the step S 1  is YES, a step S 2  judges that the switching of the read clock frequency is unnecessary between the ID part  205  and the data part  206 , and the read operation is continued. On the other hand, if the decision result in the step S 1  is NO, a step S 3  judges that the switching of the read clock frequency is necessary between the ID part  205  and the data part  206  and carries out the necessary switching operation. More particularly, the read clock frequency used to read the ID part  205  is switched to 1/N (N&gt;1) times the read clock frequency used to read the data part  206 . 
     In other words, when reading the data track  201 , the MPU  12  or the ODC  14  supplies to the frequency dividing circuit  119  a data/control switching signal for setting the same read clock frequency when reading the ID part  205  and when reading the data part  206 . On the other hand, when reading the control track  202 , the MPU  12  or the ODC  14  supplies to the frequency dividing circuit  119  a data/control switching signal for setting the read clock frequency when reading the ID part  205  to a frequency which is 1/N (N&gt;1) times the read clock frequency at the time of reading the data part  206 . Therefore, the read clock frequency used to read the information recorded in the ID part  205  and the data part  206  of the control track  202  and the ID part  205  of the data track  201  is switched and set to 1/N times the read clock frequency used to read the information recorded in the data part  206  of the data track  201 . In order to simplify the circuit construction and the like, it is preferable that N is set to an integer greater than or equal to 2. 
     In addition, when writing information in the data part  206  of the data track  201  on the optical disk  72 , a write clock frequency within the write LSI  20  is switched and set similarly at the time of the read operation. In this case, when writing the information on the data track  201 , the MPU  12  or the ODC  14  controls the write LSI  20  so that a write clock frequency used to write the information in the data part  206  is set to N (N&gt;1) times the write clock frequency used to write the information in the ID part  205 . In this case, it is also preferably that N is set to an integer greater than or equal to 2. 
     Of course, the switching of the clock frequency in the process shown in FIG. 6 may be enabled, for example, when the optical disk  72  loaded into the enclosure  11  during the load process of the control unit  10  is recognized as a medium utilizing the MSR. 
     Next, a description will be given of a particular format of the optical disk  72  used in this embodiment. For the sake of comparison, FIGS. 8A and 8B show a track format of a conventional optical disk having a diameter of 90 mm, a memory capacity of 640 MB and in conformance with the ISO/IEC15041, and FIGS. 9A and 9B show a sector format of this conventional optical disk. In the optical disk having the diameter of 90 mm, the memory capacity of 640 MB and in conformance wit the ISO/IEC15041, the minimum mark length of the ID part and the data part is set to 0.64 μm for both the control track and the data track, and approximately the limit mark length is reached when the wavelength of the laser beam used is 680 nm. 
     FIG. 8A shows the format of the control track, and FIG. 8B shows the format of the data track. FIG. 9A shows the structure of a 63-byte ID part (header), and FIG. 9B shows the structure of a 2584-byte sector. In FIG. 9A, the ID part includes a sector mark SM, a VFO 1  field, an address mark AM, an ID 1  field, a VFO 2  field, an address mark AM, an ID 2  field, and a postamble PA, and the number of bytes of each of these portions is indicated in correspondence with these portions. On the other hand, in FIG. 9B, the sector includes, in addition to the ID part described above, fields denoted by Gap, VFO 3 , Sync, Data Field, PA and Buffer, and the number of bytes of each of these fields is indicated in correspondence with these fields. In the case of the optical disk having the diameter of 90 mm, the memory capacity of 640 MB and in- conformance with the ISO/IEC15041, the sector format shown in FIGS. 9A and 9B is used in common with respect to both the data track and the control track. 
     Specific Example 1 of the Format 
     In order to achieve a memory capacity of 1.3 GB which is approximately 2 times the memory capacity of 640 MB using the above described optical disk having the diameter of 90 mm, the memory capacity of 640 MB and in conformance with the ISO/IEC15041, it is necessary to set the minimum mark length to 0.32 μm. In the data part  206  of the data track  201  on the optical disk  72 , it is possible to realize a minimum mark length of 0.32 μm utilizing the MSR. But in the ID part  205  of the data track  201  and the ID part  205  and the data part  206  of the control track  202 , it becomes impossible to reproduce the information if the minimum mark length is set to 0.32 μm. Hence, the minimum mark length is set to 0.64 μm as in the conventional case in the ID part  205  of the data track  201  and the ID part  205  and the data part  206  of the control track  202 . The ratio with respect to the entire storage capacity of the optical disk  72  occupied by the ID part  205  of the data track  201  and the control track  202  is small, and thus, the storage capacity of the optical disk  72  as a whole will not be greatly reduced by such an arrangement. The track format for this case is shown in FIGS. 10A and 10B, and the sector format for this case is shown in FIGS. 11A,  11 B and  11 C. In FIGS. 11A through 11C, those parts which are the same as those corresponding parts in FIGS. 9A and 9B are designated by the same reference numerals. 
     FIG. 10A shows the format of the control track, and FIG. 10B shows the format of the data track. In addition, FIG. 11A shows the structure of a 126-byte ID part, FIG. 11B shows the structure of a 5264-byte sector on the control track  202 , and FIG. 11C shows the structure of a 2694-byte sector on the data track  201 . In FIG. 11A, the ID part includes a sector mark SM, a VFO 1  field, an address mark AM, an ID 1  field, a VFO 2  field, an address mark AM, an ID 2  field, and a postamble PA, and the number of bytes of each of these portions is indicated in correspondence with these portions. On the other hand, in FIG. 11B, the sector includes, in addition to the ID part described above, fields denoted by Gap, VFO 3 , Sync, Data Field, PA and Buffer, and the number of bytes of each of these fields is indicated in correspondence with these fields. Furthermore, in FIG. 11C, the sector includes, in addition to the ID part described above, fields denoted by Gap, VFO 3 , Sync, Data Field, PA and Buffer, and the number of bytes of each of these fields is indicated in correspondence with these fields. 
     In this Specific Example 1, the number of bytes of the control track  202  is approximately 2 (N=2) times the number of bytes of the data track  201  based on the recording frequency of the data part  206  of the data track  201 . Except for the fact that the storage capacity is approximately 2 times that of the optical disk having the diameter of 90 mm, the storage capacity of 640 MB and in conformance with the ISO/IEC15041, the optical disk  72  is basically in conformance with the ISO/IEC15041. 
     In this case, the recording density in the track longitudinal direction is 0.57 μm, for example, on the control track  202  and in the ID part  205  of the data track  201 . The recording density in the track longitudinal direction is 0.29 μm, for example, in the data part  206  of the data track  201 . 
     Specific Example 2 of the Format 
     In order to achieve a memory capacity of 1.3 GB which is approximately 2 times the memory capacity of 640 MB using the above described optical disk having the diameter of 90 mm, the memory capacity of 640 MB and in conformance with the ISO/IEC15041, it is necessary to set the minimum mark length to 0.32 μm. In the data part  206  of the data track  201  on the optical disk  72 , it is possible to realize a minimum mark length of 0.32 μm utilizing the MSR. But in the ID part  205  of the data track  201  and the ID part  205  and the data part  206  of the control track  202 , it becomes impossible to reproduce the information if the minimum mark length is set to 0.32 μm. Hence, the minimum mark length is set to 0.64 μm as in the conventional case in the ID part  205  of the data track  201  and the ID part  205  and the data part  206  of the control track  202 . The ratio with respect to the entire storage capacity of the optical disk  72  occupied by the ID part  205  of the data track  201  and the control track  202  is small, and thus, the storage capacity of the optical disk  72  as a whole will not be greatly reduced by such an arrangement. The track format for this case is shown in FIGS. 12A and 12B, and the sector format for this case is shown in FIGS. 13A,  13 B and  13 C. In FIGS. 13A through 13C, those parts which are the same as those corresponding parts in FIGS. 9A and 9B are designated by the same reference numerals. 
     FIG. 12A shows the format of the control track, and FIG. 12B shows the format of the data track. In addition, FIG. 13A shows the structure of a 126-byte ID part, FIG. 13B shows the structure of a 5388-byte sector on the control track  202 , and FIG. 13C shows the structure of a 2694-byte sector on the data track  201 . In FIG. 13A, the ID part includes a sector mark SM, a VFO 1  field, an address mark AM, an ID 1  field, a VFO 2  field, an address mark AM, an ID 2  field, and a postamble PA, and the number of bytes of each of these portions is indicated in correspondence with these portions. On the other hand, in FIG. 13B, the sector includes, in addition to the ID part described above, fields denoted by Gap, VFO 3 , Sync, Data Field, PA and Buffer, and the number of bytes of each of these fields is indicated in correspondence with these fields. Furthermore, in FIG. 13C, the sector includes, in addition to the ID part described above, fields denoted by Gap, VFO 3 , Sync, Data Field, PA and Buffer, and the number of bytes of each of these fields is indicated in correspondence with these fields. 
     In this Specific Example 2, the number of bytes of the control track  202  is approximately 2 (N=2) times the number of bytes of the data track  201  based on the recording frequency of the data part  206  of the data track  201 . Except for the fact that the storage capacity is approximately 2 times that of the optical disk having the diameter of 90 mm, the storage capacity of 640 MB and in conformance with the ISO/IEC15041, the optical disk  72  is basically in conformance with the ISO/IEC15041. 
     Specific Example 3 of the Format 
     In order to achieve a memory capacity of 2.0 GB which is approximately 3 times the memory capacity of 640 MB using the above described optical disk having the diameter of 90 mm, the memory capacity of 640 MB and in conformance with the ISO/IEC15041, it is necessary to set the minimum mark length to 0.21 μm. In the data part  206  of the data track  201  on the optical disk  72 , it is possible to realize a minimum mark length of 0.21 μm utilizing the MSR. But in the ID part  205  of the data track  201  and the ID part  205  and the data part  206  of the control track  202 , it becomes impossible to reproduce the information if the minimum mark length is set to 0.21 μm. Hence, the minimum mark length is set to 0.64 μm as in the conventional case in the ID part  205  of the data track  201  and the ID part  205  and the data part  206  of the control track  202 . The ratio with respect to the entire storage capacity of the optical disk  72  occupied by the ID part  205  of the data track  201  and the control track  202  is small, and thus, the storage capacity of the optical disk  72  as a whole will not be greatly reduced by such an arrangement. The track format for this case is shown in FIGS. 14A and 14B, and the sector format for this case is shown in FIGS.  15 A,  15 B and  15 C. In FIGS. 15A through 15C, those parts which are the same as those corresponding parts in FIGS. 9A and 9B are designated by the same reference numerals. 
     FIG. 14A shows the format of the control track, and FIG. 14B shows the format of the data track. In addition, FIG. 15A shows the structure of a 189-byte ID part, FIG. 15B shows the structure of a 7893-byte sector on the control track  202 , and FIG. 15C shows the structure of a 2694-byte sector on the data track  201 . In FIG. 15A, the ID part includes a sector mark SM, a VFO 1  field, an address mark AM, an ID 1  field, a VFO 2  field, an address mark AM, an ID 2  field, and a postamble PA, and the number of bytes of each of these portions is indicated in correspondence with these portions. On the other hand, in FIG. 15B, the sector includes, in addition to the ID part described above, fields denoted by Gap, VFO 3 , Sync, Data Field, PA and Buffer, and the number of bytes of each of these fields is indicated in correspondence with these fields. Furthermore, in FIG. 15C, the sector includes, in addition to the ID part described above, fields denoted by Gap, VFO 3 , Sync, Data Field, PA and Buffer, and the number of bytes of each of these fields is indicated in correspondence with these fields. 
     In this Specific Example 3, the number of bytes of the control track  202  is approximately 3 (N=3) times the number of bytes of the data track  201  based on the recording frequency of the data part  206  of the data track  201 . Except for the fact that the storage capacity is approximately 3 times that of the optical disk having the diameter of 90 mm, the storage capacity of 640 MB and in conformance with the ISO/IEC15041, the optical disk  72  is basically in conformance with the ISO/IEC15041. 
     Specific Example 4 of the Format 
     In order to achieve a memory capacity of 2.0 GB which is approximately 3 times the memory capacity of 640 MB using the above described optical disk having the diameter of 90 mm, the memory capacity of 640 MB and in conformance with the ISO/IEC15041, it is necessary to set the minimum mark length to 0.21 μm. In the data part  206  of the data track  201  on the optical disk  72 , it is possible to realize a minimum mark length of 0.21 μm utilizing the MSR. But in the ID part  205  of the data track  201  and the ID part  205  and the data part  206  of the control track  202 , it becomes impossible to reproduce the information if the minimum mark length is set to 0.21 μm. Hence, the minimum mark length is set to 0.64 μm as in the conventional case in the ID part  205  of the data track  201  and the ID part  205  and the data part  206  of the control track  202 . The ratio with respect to the entire storage capacity of the optical disk  72  occupied by the ID part  205  of the data track  201  and the control track  202  is small, and thus, the storage capacity of the optical disk  72  as a whole will not be greatly reduced by such an arrangement. The track format for this case is shown in FIGS. 16A and 16B, and the sector format for this case is shown in FIGS. 17A,  17 B and  17 C. In FIGS. 17A through 17C, those parts which are the same as those corresponding parts in FIGS. 9A and 9B are designated by the same reference numerals. 
     FIG. 16A shows the format of the control track, and FIG. 16B shows the format of the data track. In addition, FIG. 17A shows the structure of a 189-byte ID part, FIG. 17B shows the structure of a 8082-byte sector on the control track  202 , and FIG. 17C shows the structure of a 2694-byte sector on the data track  201 . In FIG. 17A, the ID part includes a sector mark SM, a VFO 1  field, an address mark AM, an ID 1  field, a VFO 2  field, an address mark AM, an ID 2  field, and a postamble PA, and the number of bytes of each of these portions is indicated in correspondence with these portions. On the other hand, in FIG. 17B, the sector includes, in addition to the ID part described above, fields denoted by Gap, VFO 3 , Sync, Data Field, PA and Buffer, and the number of bytes of each of these fields is indicated in correspondence with these fields. Furthermore, in FIG. 17C, the . sector includes, in addition to the ID part described above, fields denoted by Gap, VFO 3 , Sync, Data Field, PA and Buffer, and the number of bytes of each of these fields is indicated in correspondence with these fields. 
     In this Specific Example 4, the number of bytes of the control track  202  is approximately 3 (N=3) times the number of bytes of the data track  201  based on the recording frequency of the data part  206  of the data track  201 . Except for the fact that the storage capacity is approximately 3 times that of the optical disk having the diameter of 90 mm, the storage capacity of 640 MB and in conformance with the ISO/IEC15041, the optical disk  72  is basically in conformance with the ISO/IEC15041. 
     In the Specific Examples 1 through 4 described above, the recording density is improved by setting the mark length to {fraction (1/2, 1/3)}, . . . , based on the optical disk having the diameter of 90 mm, the storage capacity of 640 MB and in conformance with the ISO/IEC15041. However, it is also possible to realize an optical disk having a storage capacity of 1.3 GB by reducing the track pitch from 1.1 μm to 0.9 μm and reducing the mark length from 0.64 μm to 0.38 μm, for example. In this case, the sector format may be the same as that shown in FIGS. 13A through 13C. 
     In the present invention, even when the read/write clock frequency for the ID part (control track) is switched to 1/N (for example, {fraction (1/2. 1/3, 1/4, 1/5)}, . . . ) times the read/write clock frequency for the data part (data track), it is possible to increase the data storage capacity of the optical recording medium without deteriorating the read accuracy of the concavo-convex parts (embossed pits) of the ID part (control track). 
     In the embodiments described above, the present invention is applied to the magneto-optical disk. However, the application of the present invention is not limited to the magneto-optical disk, and the present invention is similarly applicable to various kinds of recording mediums including optical disks such as a phase change type optical disk and a card shaped recording medium, as long as the recording medium is provided with a first region which is recorded with first information in the form of the concavo-convex shape and a second region which is recorded with second information by an optical means. 
     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.