Patent Publication Number: US-2003227838-A1

Title: Optical disk and optical disk recording/reproducing apparatus

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to an optical disk with a sample servo method, and more particularly to a technical field of an optical disk having a pit configuration for tracking servo that is suitable for a high recording density, and an optical disk recording/reproducing apparatus for performing at least one of recording and reproducing data on and from the optical disk.  
       [0003] 2. Description of the Related Art  
       [0004] Conventionally, an information recording medium that records information at high density to store a large amount of information, and its recording or reproducing apparatus have been developed. Particularly, with the expansion of a computerized society, the increasing amount of information is dealt with, and the portable uses are spread, whereby there is a demand for the information recording medium of small size, high density and large capacity. In such a situation, an optical disk for optically recording and reproducing data having excellent operability has been vigorously employed, such as CD-ROM (Compact Disc-Read Only Memory), MD (Mini Disc), DVD, and DVD-ROM (Digital Versatile Disc-Read Only Memory).  
       [0005] To control the recording and reproducing of the optical disk, there is a need for a tracking servo for enabling a light beam spot to follow a track, a focus servo for focusing, and a spindle motor servo for controlling the number of revolutions of the optical disk. Accordingly, an error signal is detected for each servo control. For example, for the spindle servo, an error signal is detected on the basis of a synchronizing signal extracted from a reproduced signal and an output from a revolution number detector provided on the spindle motor. For the tracking servo or focus servo, error signals are detected from the reproduced signal of the optical disk, on the basis of control pits formatted in accordance with a sampled servo method, or on the basis of the composition of an optical pickup.  
       [0006] An optical disk system for recording and reproducing the information by enabling a light beam spot to be traced on a center between two tracks in which a pit (here, the “pit” means a concavity corresponding to the recording information, formed along a track direction of the optical disk) for the higher recording density is made smaller, and an interval of recording track is made narrower has been offered and implemented. In such an optical disk, control pits are provided at a predetermined position in the track for the tracking servo, apart from data pits, and a tracking error signal is produced by reading the control pits.  
       [0007] However, with the conventional method, the control pits with periodicity that is provided on the track restricts the area for data pits, so that the data recording capacity is decreased. Also, if the pit is made shorter to suppress a decrease in the data recording capacity to the minimum, the pit period approaches a diffraction limit, so that the amplitude of reproduced signal is decreased. Therefore, the reproduced signal for two pits for tracking servo located on adjacent two tracks has a smaller amplitude difference, resulting in a problem that a tracking error can not be accurately detected.  
       SUMMARY OF THE INVENTION  
       [0008] Accordingly, the present invention has been achieved in the light of the above-mentioned problems, and it is an object of the invention to provide an optical disk having a pit format in which the area occupied by a pit for tracking servo is reduced and the recording capacity of information is kept large, and an optical disk recording/reproducing apparatus for performing at least one of recording and reproducing data on and from the optical disk.  
       [0009] In one aspect of the present invention, an optical disk having a synchronous pit serving as a synchronizing reference for recording/reproducing, guard pits and data pits on each track, is provided with: on an odd track, a first land having a first length between the synchronous pit and the guard pit provided forward of the synchronous pit; and a second land having a second length between the synchronous pit and the guard pit provided backward of the synchronous pit; on an even track, a third land having a third length between the synchronous pit and the guard pit provided forward of the synchronous pit; and a fourth land having a fourth length between the synchronous pit and the guard pit provided backward of the synchronous pit, wherein a tracking error signal is detected on the basis of a reflected light from the first land, the third land, and a land between the odd track and the even track adjacently provided, and a reflected light from the second land, the fourth land, and a land between the odd track and the even track adjacently provided.  
       [0010] The optical disk of the present invention employs a sample-and-hold method, in which a synchronous pit as a control pit for recording or reproducing data on or from the optical disk, and guard pits before and after the synchronous pit are arranged. The guard pits with respect to the synchronous pit are arranged differently depending on whether an odd track or an even track. Depending on whether the guard pit is arranged before or after the synchronous pit, a distance between the synchronous pit and the guard pit is different, namely, a length of land between them is different. Due to the difference in the arrangement or distance, when the beam spot is traced off the track, the amount of reflected light is changed, whereby it is discriminated whether or not the beam spot is deviated in accordance with a signal based on this amount of change, and to which side it is deviated, if any. Then, a tracking error signal is detected.  
       [0011] The optimal positional relation between the synchronous pit and the guard pits for the optical disk of the present invention, the optimal relation with the beam spot, and the detection of tracking error in employing the optical disk of the present invention will be apparent from the following description.  
       [0012] In this application, the “recording direction or reproducing direction of the optical disk” means a direction from left to right in FIG. 1, the “forward” means the left side of the reference position in FIG. 1, and the “backward” means the right side of the reference position in FIG. 1. Also, the “leading part of pit” means a pit edge on the left side of pit, and the “trailing part of pit” means a pit edge on the right side of pit. Also, in this application, the “pit composed of concavity” means a concave portion as seen from one surface of the optical disk, but a convex portion as seen from the opposite surface. That is, the concave and convex of pit have the relative meaning. For example, the pit is concave as seen from the side where a recording light or a reproducing light is radiated.  
       [0013] In another aspect of the present invention, the first length of the first land is equal to the fourth length of the fourth land, and the second length of the second land is equal to the third length of the third land.  
       [0014] In this aspect, the distance between the synchronous pit and the guard pit arranged forward of it, or the land length, on the odd track, and the distance between the synchronous pit and the guard pit arranged backward of it, or the land length, on the even track are equal. Also, the distance between the synchronous pit and the guard pit arranged backward of it, or the land length, on the odd track, and the distance between the synchronous pit and the guard pit arranged forward of it, or the land length, on the even track are equal.  
       [0015] In this aspect, “equal” in the “land length is equal” means literally equal, or slightly different in length so far as the lands or pits are regarded as the same land or pit in the relation with the diameter of beam spot of the recording light or reproducing light practically used, and in the relation with the resolution.  
       [0016] In further aspect of the present invention, the first length of the first land is not equal to the second length of the second land.  
       [0017] In this aspect, the second length is different from the first length. Hence, when the beam spot is traced off the track, a tracking error signal is detected.  
       [0018] In further aspect of the present invention, the second length of the second land is two times longer than the first length of the first land, and the second length of the second land is two times longer than an average length of the data pits.  
       [0019] In this aspect, the second length is double the first length as well as double the average length of data pits corresponding to the recorded information. The average length of data pits is one when the information is recorded in multi-values corresponding to the edges of data pits.  
       [0020] In this aspect, “equal” means literally equal or slightly different in length so far as the lands or pits are regarded as the same land or pit in the relation with the diameter of beam spot of the recording light or reproducing light practically used, and in the relation with the resolution of recording or reproducing.  
       [0021] In further aspect of the present invention, the synchronous pit on said each track is arranged at the same position in a radial direction of the optical disk.  
       [0022] In this aspect, the synchronous pit is linearly provided in a radial direction of the optical disk. Synchronism is detected in whole area of the optical disk, and used for the recording and reproducing control of the optical disk. Also, the reversal of the odd track and the even track is detected for every synchronous pit, and a tracking error is detected from the relation between the synchronous pit and the guard pits corresponding to each track.  
       [0023] In this aspect, the “same position” means literally the identical position or a slightly different position so far as the lands or pits are regarded as the same land or pit in the relation with the diameter of beam spot of the recording light or reproducing light practically used, and in the relation with the resolution of recording or reproducing.  
       [0024] In further aspect of the present invention, the synchronous pit has a longer shape than the guard pits and the data pits.  
       [0025] In this aspect, the synchronous pit is formed longer than other pits, and can detect the synchronism correctly.  
       [0026] In further aspect of the present invention, a trailing part of the guard pit arranged backward of the synchronous pit on the odd track is modulated in accordance with the recorded data.  
       [0027] In this aspect, the guard pit is employed as a part of recorded information, whereby the recording capacity is increased.  
       [0028] In further aspect of the present invention, a leading part of the guard pit arranged forward of the synchronous pit on the even track is modulated in accordance with the recorded data.  
       [0029] In this aspect, the guard pit is also employed as a part of recorded information, whereby the recording capacity is increased.  
       [0030] In further aspect of the present invention, the data pit is recorded in multi-valued digital information as the positional information of the pit edge.  
       [0031] In this aspect, the data pit records the information corresponding to the position of its edge, whereby more information can be represented by one data pit, and the recording capacity is increased.  
       [0032] In further aspect of the present invention, a material capable of optically forming the data pit is provided at least between the adjacent two tracks of the optical disk.  
       [0033] In this aspect, one type of the optical disk of this invention employs a material capable of optically forming the pit on an entire surface of the optical disk, or at least between adjacent two tracks. For example, this optical disk may be a write-once disk having a pigment film, and a rewritable disk having a phase change film or MO film.  
       [0034] In one aspect of an optical disk recording/reproducing apparatus of the present invention, which is apparatus for performing at least one of recording and reproducing data on and from an optical disk having a synchronous pit as a synchronization reference for recording/reproducing, guard pits and data pits on each track, the optical disk recording/reproducing apparatus is provided with: an optical device which performs at least one of recording and reproducing data by radiating a beam spot of laser beam over two adjacent tracks of the optical disk; a tracking error detecting device which detects a tracking error signal on the basis of a reflected light from a land between the synchronous pit and the guard pit and a land provided between the adjacent two tracks; and a tracking servo device which enables the beam spot of laser beam to trace the center between the adjacent two tracks on the basis of the tracking error signal by the tracking error detecting device.  
       [0035] The optical disk recording/reproducing apparatus of the invention records or reproduces the information on or from the optical disk having the pit configuration of the invention, in which a tracking error is detected on the basis of a reflected light from the land between the synchronous pit and the guard pits and the land provided between adjacent two tracks by radiating a beam spot of laser beam over adjacent two tracks, and the beam spot is traced on the center between adjacent two tracks with the tracking servo. In this tracing state, the information is reproduced from one data pit of adjacent two tracks, or the data pit is formed or information is recorded on the land portion between adjacent two tracks.  
       [0036] In another aspect of the present invention, the tracking error detecting device samples, at a predetermined position, a reproduced signal level on the basis of a reflected light from the land between the synchronous pit and the guard pit arranged forward of the synchronous pit and the land provided between the adjacent two tracks, and a reproduced signal level on the basis of a reflected light from the land between the synchronous pit and the guard pit arranged backward of the synchronous pit and the land provided between the adjacent two tracks, and detects a tracking error by comparing the sampled values.  
       [0037] In this aspect, the detection of tracking error is made by detecting the reflected light from the land portion provided between the synchronous pit and the guard pits before and after the synchronous pit and the land which is located between adjacent two tracks and at the same position as the land portion at each position before and after the synchronous pit. The output of detection is sampled at a predetermined position of the land portion, and by comparing two sampled values, it is discriminated whether or not there is any tracking error, and if so, to which side the tracking is deviated. If there is any tracking error, the tracking servo is made on the basis of a comparison result of two sampled values.  
       [0038] This detection way of tracking error allows the tracking error signal to be detected at high sensitivity by appropriately setting the land lengths L 1  and L 2 . When the track pitch is narrow, or the pit period is short, the tracking servo is made stably and correctly.  
       [0039] In further aspect of the present invention, a comparison result of the sampled values by the tracking error detecting device is supplied to the tracking servo device, with the polarity reversed for each track.  
       [0040] In this aspect, since the arrangement of the synchronous pit and the guard pits is changed every track, the relation between the direction of track deviation and the polarity of the tracking error signal is reversed every track. Thus the polarity should be reversed every track to make the correct tracking.  
       [0041] In further aspect of the present invention, a comparison result of the sampled values by the tracking error detecting device is supplied to the tracking servo device, with the polarity reversed for each frame.  
       [0042] In this aspect, since the arrangement of the synchronous pit and the guard pits is changed every frame, the relation between the direction of track deviation and the polarity of the tracking error signal is reversed every frame. Thus the polarity should be reversed every frame to make the correct tracking.  
       [0043] The operation and other benefits of the present invention will be clarified from the following description of the embodiments. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0044]FIG. 1 is a view showing a pit configuration and its arrangement of a ROM type disk involving an optical disk according to the present invention;  
     [0045]FIG. 2 is a view showing a pit configuration and its arrangement of a conventional ROM type disk;  
     [0046]FIG. 3A is a view showing a relation between a pit arrangement and a light beam, and a sample extracting point for tracking servo, in which a beam spot lies on an odd centerline;  
     [0047]FIG. 3B is a view showing a relation between a pit arrangement and a light beam, and a sample extracting point for tracking servo, in which a beam spot lies on an even centerline;  
     [0048]FIG. 4A is a view showing an output signal at a sample extracting point for tracking servo, corresponding to FIG. 3A, in which a beam spot lies on a centerline;  
     [0049]FIG. 4B is a view showing an output signal at a sample extracting point for tracking servo, corresponding to FIG. 3A, in which the beam spot is deviated upward from a centerline;  
     [0050]FIG. 4C is a view showing an output signal at a sample extracting point for tracking servo, corresponding to FIG. 3A, in which the beam spot is deviated downward from a centerline;  
     [0051]FIG. 5A is a view showing an output signal at a sample extracting point for tracking servo, corresponding to FIG. 3B, in which a beam spot lies on a centerline;  
     [0052]FIG. 5B is a view showing an output signal at a sample extracting point for tracking servo, corresponding to FIG. 3B, in which the beam spot is deviated upward from a centerline;  
     [0053]FIG. 5C is a view showing an output signal at a sample extracting point for tracking servo, corresponding to FIG. 3B, in which the beam spot is deviated downward from a centerline;  
     [0054]FIG. 6A is a view showing a relation between the arrangement of the synchronous pit and the guard pit and the beam spot for the optical disk according to the present invention;  
     [0055]FIG. 6B is a view showing a relation between the arrangement of the synchronous pit and the guard pit and the beam spot for the optical disk according to the present invention;  
     [0056]FIG. 6C is a view showing a relation between the arrangement of the synchronous pit and the guard pit and the beam spot for the optical disk according to the present invention;  
     [0057]FIG. 7 is a view showing a pit configuration and its arrangement involving an optical disk for recording, and a relation between the beam spot and data pits to be recorded; and  
     [0058]FIG. 8 is a block diagram of an optical disk recording/reproducing apparatus for recording data on and reproducing data from the optical disk of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0059] First Embodiment of Optical Disk  
     [0060] Referring to FIGS.  1  to  6 , a first embodiment of an optical disk according to the present invention will be described below. FIG. 1 shows a pit configuration of the optical disk for reproduction using a sampled servo method. A pit means a concavity formed along a track direction of the optical disk, and consists of a synchronous pit as a synchronization reference for recording and reproduction of the optical disk, a guard pit, and a data pit corresponding to information to be recorded. In FIG. 1, the digital data is recorded as the information corresponding to the position of a pit edge in accordance with a multi-valued recording method.  
     [0061] The optical disk of this embodiment has a servo area composed of a synchronous pit (Psync)  41 , a guard pit (Pgrdf)  42  and a guard pit (Pgrdr)  43  provided on each track (TRK)  1 ,  2 ,  3 ,  4 , . . . and a data area composed of data pits (Pdat)  44 , as indicated by a solid line in FIG. 1. A region between two adjacent tracks is referred to as centerlines (CL)  12 ,  23 ,  34 , . . .  
     [0062] In reproducing the digital data of the optical disk according to the present invention, a beam spot  40  of the laser beam is traced on the centerlines CL 12 , CL 23 , CL 34 , . . . , from left to right in FIG. 1. In reproducing the digital data, a laser beam is applied on the centerline between two adjacent tracks to cover them to detect the quantity of light that is varied in accordance with a pattern of the data pits  44 .  
     [0063] The data pits  44  are recorded in accordance with a pit edge multi-valued recording method. In the pit edge multi-valued recording, three valued data of 0, 1 and 2 are recorded corresponding to each pit edge of the data pit  44 . Namely, a pit sequence at a fixed period is formed on the track, in which the position of the pit edge is varied at three steps in accordance with the digital data. Specifically, if the recording data is “0”, the position of the pit edge is shifted a certain distance in a direction where the pit length is shorter, if the recording data is “1”, the position of the pit edge is unchanged, or if the recording data is “2”, the position of the pit edge is shifted a certain distance in a direction where the pit length is longer. In this manner, the three valued digital data are recorded as the positional information of the pit edge. In FIG. 1, three positions of the pit edge are illustrated, but the actual pit edge takes one position.  
     [0064] The synchronous pit  41  serves as the synchronization reference at the time of reproduction. The synchronous pit  41  on each track is longer than other pits (e.g., P=2×D) to allow the reproducing apparatus to easily detect it, and aligned at the same position in a radial direction of the disk. Also, the synchronous pit  41  is a clock phase reference at the time of reproduction.  
     [0065] The guard pits  42  and  43  have the pit position changed in a tangential direction of the disk for every track. For example, in the odd tracks  1 ,  3 , . . . , a land having a length L 1  is provided between a leading part of the synchronous pit  41  and a trailing part of the guard pit  42 , and a land having a length L 2  is provided between a trailing part of the synchronous pit  41  and a leading part of the guard pit  43  (L 1 ≠L 2 ). A trailing part of the guard pit  43  is dealt with as the data, and modulated with the multi-value recording.  
     [0066] On the other hand, in the even tracks  2 ,  4 , . . . , a land having the length L 2  is provided between the leading part of the synchronous pit  41  and the trailing part of the guard pit  42 , and a land having the length L 1  is provided between the trailing part of the synchronous pit  41  and the leading part of the guard pit  43 . The leading part of the guard pit  42  is dealt with as the data, and modulated with the multi-value recording.  
     [0067] The reproducing apparatus detects a tracking error from a signal on the basis of a reflected light from a land portion between the guard pits  42 ,  43  and the synchronous pit  41 , and a land portion between two adjacent tracks, and performs a tracking servo so that the tracking error may be zero. Consequently, a beam spot  40  is traced on the centerlines CL 12 , CL 23 , CL 34 , . . . between two adjacent tracks, as shown in FIG. 1.  
     [0068] Referring to FIG. 2 to clarify features of the optical disk of the present invention, the pit configuration of the conventional optical disk using the same sampled servo method will be described below. A servo area consists of a synchronous pit (Psync)  51 , a tracking pit (Ptrk)  52 , and guard pits (Pgrd)  53  on the tracks (TRK)  1 ,  2 ,  3 , . . . and a data area consists of data pits (Pdat)  54 .  
     [0069] Herein, a tracking pit  52  for detecting the tracking error is provided, unlike this invention. The tracking pit  52  is disposed backward of the synchronous pit  51 , in which the pit position in a tangential direction of the disk is varied at a period of four tracks. The reproducing apparatus detects a tracking error from an amplitude difference of reproduced signal between two tracking pits  52  located on two adjacent tracks, and performs a tracking servo so that the tracking error may be zero.  
     [0070] This tracking pit  52  is required for the tracking servo, whereby a space for providing the tracking pit  52  is required. Also, the space is larger due to the periodicity, whereby an area for recording the digital data is reduced. In the optical disk of the present invention, the tracking pit  52  can be omitted to increase the data recording capacity.  
     [0071] Referring to FIGS.  3  to  5 , the pit arrangement of the optical disk of the present invention described above and a method of detecting the tracking error will be described below.  
     [0072]FIG. 3 is a view showing an array of the synchronous pit  41  and the guard pits  42  and  43 , the beam spot  40 , and sample extracting points r and s for the tracking servo. In FIG. 3A, the beam spot  40  lies on the odd centerline CL 12 , and in FIG. 3B, the beam spot  40  lies on the even centerline CL 23 .  
     [0073]FIG. 4 is a view showing an output signal at the sample extracting points r and s for the tracking servo corresponding to the odd centerline of FIG. 3A. In FIG. 4A, the beam spot  40  lies on the centerline CL 12 , in FIG. 4B, the beam spot  40  is deviated upward from the centerline CL 12 , and in FIG. 4C, the beam spot  40  is deviated downward from the centerline CL 12 .  
     [0074]FIG. 5 is a view showing an output signal at the sample extracting points r and s for the tracking servo corresponding to the even centerline of FIG. 3B. In FIG. 5A, the beam spot  40  lies on the centerline CL 23 , in FIG. 5B, the beam spot  40  is deviated upward from the centerline CL 23 , and in FIG. 4C, the beam spot  40  is deviated downward from the centerline CL 23 . A sample extracting point r is provided at an intermediate position between the leading part of the synchronous pit  41  and the trailing part of the guard pit  42  in the track  2 , and a sample extracting point s is provided at an intermediate position between the trailing part of the synchronous pit  41  and the leading part of the guard pit  43  in the track  1 . The points r and s are points at which the reproduced output with the reflected lights from respective lands is at peak, in which a tracking error is detected by sampling and holding the peak of this output. Also, it is desirable to control a diameter of the beam spot  40  to be almost equal to a distance between the leading part of the synchronous pit  41  and the trailing part of the guard pit  42  in the track  2 .  
     [0075] Firstly, in the case where the beam spot  40  is traced on the centerline CL 12 , the beam spot  40  lies at the points r and s, as shown in FIG. 3A, whereby the sample is extracted at the points r and s. At this time, the beam spot  40  is applied evenly on two adjacent tracks  1  and  2 .  
     [0076] Also, in the case where the beam spot  40  is moved upward from the centerline CL 12  for tracing, or a tracking error occurs above, the beam spot  40  lies at points r 1  and s 1 , whereby more radiation is made on the track  1  than the track  2 , as indicated by the beam spot  40   r   1  and the beam spot  40   s   1 .  
     [0077] On the other hand, in the case where the beam spot  40  is moved downward from the centerline CL 12  for tracing, or a tracking error occurs below, the beam spot  40  lies at points r 2  and s 2 , whereby more radiation is made on the track  2  than the track  1 , as indicated by the beam spot  40   r   2  and the beam spot  40   s   2 .  
     [0078] Next, in the case where the beam spot  40  is traced on the centerline CL 23 , the beam spot  40  lies at the points r and s, as shown in FIG. 3B, whereby the sample is extracted at the points r and s. At this time, the beam spot  40  is radiated evenly on two adjacent tracks  2  and  3 .  
     [0079] Also, in the case where the beam spot  40  is moved upward from the centerline CL 23  for tracing, or a tracking error occurs above, the beam spot  40  lies at points r 1  and s 1 , whereby more radiation is made on the track  2  than the track  3 , as indicated by the beam spot  40   r   1  and the beam spot  40   s   1 .  
     [0080] On the other hand, in the case where the beam spot  40  is moved downward from the centerline CL 23  for tracing, or a tracking error occurs below, the beam spot  40  lies at points r 2  and s 2 , whereby more radiation is made on the track  3  than the track  2 , as indicated by the beam spot  40   r   2  and the beam spot  40   s   2 .  
     [0081] As described above, with the position of the beam spot  40 , there occurs a change in the amount of reflected light from the land with the arrangement of the synchronous pit  41  and the guard pits  42  and  43  and the land between two tracks, whereby a tracking error can be detected.  
     [0082] Referring to FIG. 4, the detection of tracking error in the state of FIG. 3A will be described below. FIG. 4A shows an output signal when the beam spot  40  is traced on the centerline CL 12 , as shown in FIG. 3A, in which an output at point r and an output at point s take the almost same level. No tracking error occurs in this case.  
     [0083]FIG. 4B shows an output signal when the beam spot  40  is traced above the centerline CL 12 , in which the output at point s is larger than the output at point r. This is because the point s is less affected by the guard pit  43 , and has a greater amount of reflected light, while the point r is greatly affected by the guard pit  42 , and has a smaller amount of reflected light. Accordingly, a comparison between the output at point r and the output at point s reveals that the tracking is deviated upward, whereby a tracking error is detected.  
     [0084]FIG. 4C shows an output signal when the beam spot  40  is traced below the centerline CL 12 , in which the output at point r is larger than the output at point s. This is because the point r is less affected by the guard pit  42 , and has a greater amount of reflected light, while the point s is greatly affected by the guard pit  43 , and has a smaller amount of reflected light. Accordingly, a comparison between the output at point r and the output at point s reveals that the tracking is deviated downward, whereby a tracking error is detected.  
     [0085] Referring to FIG. 5, the detection of tracking error in the state of FIG. 3B will be described below. FIG. 5A shows an output signal when the beam spot  40  is traced on the centerline CL 23 , as shown in FIG. 3B, in which the output at point r and the output at point s take the almost same level. No tracking error occurs in this case.  
     [0086]FIG. 5B shows an output signal when the beam spot  40  is traced above the centerline CL 23 , in which the output at point r is larger than the output at point s. This is because the point r is less affected by the guard pit  42 , and has a greater amount of reflected light, while the point s is greatly affected by the guard pit  43 , and has a smaller amount of reflected light. Accordingly, a comparison between the output at point r and the output at point s reveals that the tracking is deviated upward, whereby a tracking error is detected.  
     [0087]FIG. 5C shows an output signal when the beam spot  40  is traced below the centerline CL 23 , in which the output at point s is larger than the output at point r. This is because the point s is less affected by the guard pit  43 , and has a greater amount of reflected light, while the point r is greatly affected by the guard pit  42 , and has a smaller amount of reflected light. Accordingly, a comparison between the output at point r and the output at point s reveals that the tracking is deviated downward, whereby a tracking error is detected.  
     [0088] As will be apparent from FIGS. 4 and 5, the polarity of the tracking error in the same direction is changed for every centerline to be traced. Accordingly, when the tracking servo is practically performed, it is required that the polarity of the tracking error signal is reversed for every centerline, namely, for every track. In the case where the recording is made in a format where the polarity is changed for every frame, it is naturally required that the polarity is reversed for every frame.  
     [0089] Referring to FIG. 6, a relation between the arrangement of the synchronous pit  41  and the guard pits  42  and  43  for detecting the tracking error efficiently and at high quality and the beam spot  40  will be described below.  
     [0090]FIG. 6A shows an optimal relation. Assuming that a distance between the synchronous pit  41  and the guard pit  42  is L 2  and a distance between the synchronous pit  41  and the guard pit  43  is L 1  in the even track (e.g., TRK 2 ), and the distance between the synchronous pit  41  and the guard pit  42  is L 1  and the distance between the synchronous pit  41  and the guard pit  43  is L 2  in the odd track (e.g., TRK 3 ), L 2 =2×L 1 . Also, the point r for sample extraction is set at an intermediate position of L 2 , and the diameter of the beam spot  40  is configured almost equal to L 2 . Thereby, the reflected light of the beam spot  40  is affected only by the guard pit  42  on the odd track, but not affected by other pits, whereby the tracking error signal with high S/N ratio is detected. At point s, the reflected light is affected only by the guard pit  43  on the even track, whereby the tracking error signal with high S/N ratio is detected, like point r. The even track and the odd track are called simply to distinguish the tracks for convenience, and characterized by the arrangement of the synchronous pit  41  and the guard pits  42  and  43 .  
     [0091] In FIG. 6B, the distance between the synchronous pit  41  and the guard pits  42  and  43  is shorter, and the tracking error signal is affected by other pits to cause the S/N ratio to be degraded, although the recording density is increased.  
     [0092] Further, in FIG. 6C, the distance between the synchronous pit  41  and the guard pits  42  and  43  is longer, and the S/N ratio of the tracking error signal remains the same, but the recording density is decreased.  
     [0093] As above described, if the beam spot is traced off the centerline between two adjacent tracks, a tracking error is obtained corresponding to a deviation of the beam spot. By feeding this deviation back to the pick-up, the deviation of the beam spot is corrected, so that the correct tracking servo is made. That is, the beam spot is accurately traced on the centerline between two adjacent tracks. Since this detection method for the tracking error signal has a high detection sensitivity, it is possible to make a stable and accurate tracking servo, when the track pitch is narrow, or the pit period is short.  
     [0094] Second Embodiment of Optical Disk  
     [0095] Referring to FIG. 7, a second embodiment of the optical disk will be described below. This second embodiment involves a write-once disk having a pigment film, and a rewritable disk having a phase change film or MO film.  
     [0096] The pit configuration has a servo area composed of a synchronous pit (Psync)  41 , a guard pit (Pgrdf)  42  and a guard pit (Pgrdr)  43  provided on each track (TRK)  1 ,  2 ,  3 ,  4 , . . . and a recording area for recording data pits (Pdat)  44  corresponding to the digital data in a region between two adjacent tracks, as shown in FIG. 7. This recording area corresponds to a region of the centerlines (CL)  12 ,  23 ,  34 , . . . .  
     [0097] The arrangement and size of the synchronous pit  41  and the guard pits  42  and  43 , the detection of tracking error, and the formation of error signal for tracking servo are the same as in the first embodiment, and are not described here.  
     [0098] The beam spot  40  is traced on the centerlines  12 ,  23 ,  34 , . . . on the basis of the tracking error signal obtained by the arrangement of pits on two adjacent tracks to record the data pits  44  corresponding to the digital data in the recording area.  
     [0099] The data pits  44  recorded on the centerlines  12 ,  23 ,  34 , . . . are recorded by the pit edge multi-valued recording method. In the pit edge multi-valued recording, three valued data of 0, 1 and 2 is recorded corresponding to each pit edge of the data pit  44 . In FIG. 7, the positions of three pit edges are illustrated, but the pit edge of the data pit  44  actually recorded takes any one position.  
     [0100] Thus, the first and second embodiments of the optical disk according to the present invention have been described in detail. However, by exchanging the pit and the land, the tracking servo may be made on the basis of the reproduced signal of pits located between the synchronous land and the guard lands.  
     [0101] Embodiment of Optical Disk Recording/Reproducing Apparatus  
     [0102] Referring to FIG. 8, an optical disk recording/reproducing apparatus for recording and reproducing data on and from the optical disk of FIG. 7 using a sampled servo method will be described below.  
     [0103] First of all, a configuration and an operation of a reproducing system will be described. An optical pick-up  103  irradiates a laser beam of low power on a disk surface of an optical disk  101  in reproducing the digital data, and detects the amount of reflected light changing in accordance with the data pits  44  to output a reproduced signal.  
     [0104] An amplification circuit  104  amplifies a reproduced signal from the optical pick-up  103 . An A/D conversion circuit  105  converts the reproduced signal from analog to digital form in synchronism with a reproduction clock to output a sample sequence. A reproduced signal processing circuit  123  makes the digital signal processing for the sample sequence output from the A/D conversion circuit  105  to restore the digital data recorded on the optical disk  101  as the reproduced digital data.  
     [0105] A synchronous detection circuit  106  detects a sample corresponding to the synchronous pit from the sample sequence output from the A/D conversion circuit  105  to output a synchronous detection signal. A timing generating circuit  107  generates various kinds of timing signals in reference to the synchronous detection signal output from the synchronous detection circuit  106  and supplies them to a clock phase error detecting circuit  108 , a tracking error detecting circuit  112  and a polarity switching circuit  113 .  
     [0106] The clock phase error detecting circuit  108  extracts the samples for the pit edges on both sides of the synchronous pit from the sample sequence in accordance with a timing signal output from the timing generating circuit  107 , and calculates a level difference between both samples to output the phase error data.  
     [0107] A D/A conversion circuit  109  converts the phase error data output from the clock phase error detecting circuit  108  from digital to analog form to output a phase error signal. An LPF (Low Pass Filter)  110  removes the high frequency components contained in the phase error signal output from the D/A conversion circuit  109  to smooth the waveform. A VCO (Voltage Controlled Oscillation)  111  changes the oscillation frequency of clock in accordance with a voltage passed through the LPF  110  to generate a reproduction clock phased with the reproduced signal.  
     [0108] The A/D conversion circuit  105 , the clock phase error detecting circuit  108 , the D/A conversion circuit  109 , the LPF  110 , and the VCO  111  make up a PLL (Phase Locked Loop). The PLL is a servo loop for phasing the reproduction clock to the reproduced signal.  
     [0109] A tracking error detecting circuit  112  extracts the samples corresponding to points r and s of FIG. 3 from the sample series according to the timing signal, and calculates a level difference between both samples to output the tracking error data. A tracking error has the polarity changed for every frame or every track in accordance with a recording format. A polarity switching circuit  113  reverses the polarity of the tracking error for every frame or every track, depending on the timing signal to output a tracking error.  
     [0110] A D/A conversion circuit  114  converts the tracking error data from digital to analog form to output a tracking error signal. An LPF  115  removes the high frequency components contained in the tracking error signal output from the D/A conversion circuit  114  to smooth the waveform. A tracking servo circuit  116  controls the position of the optical pick-up  103  in the radial direction of the disk to trace the beam spot of the optical pick-up  103  along the predetermined line in accordance with the tracking error signal passed through the LPF  115 .  
     [0111] The amplification circuit  104 , the A/D conversion circuit  105 , the tracking error detecting circuit  112 , the D/A conversion circuit  114 , the LPF  115 , the tracking servo circuit  116 , and the optical pick-up  103  make up a tracking servo loop. The tracking servo loop is a servo loop for causing the beam spot of laser beam to trace on the centerline between two adjacent tracks.  
     [0112] To constitute a focus servo loop, a focus error detecting circuit  117  detects a focus error on the basis of an output of the A/D conversion circuit  105 . A D/A conversion circuit  118  converts the focus error from digital to analog form to output a focus error signal. An LPF  119  removes the high frequency components contained in the focus error signal to smooth the waveform. A focus servo circuit  120  controls the beam spot of the optical pick-up  103  to be focused on the disk surface in accordance with a focus error signal passed through the LPF  119 .  
     [0113] Moreover, to control the number of revolutions of the spindle motor  102  for revolving the optical disk  101 , a spindle servo circuit is provided to control the number of revolutions of the spindle motor  102  on the basis of an error signal from a revolution number error detecting circuit  121 . The information regarding the number of revolutions from a signal reproduced by the optical pick-up  103  may be detected in the revolution number error detecting circuit  121  and applied to the spindle servo circuit  122 . Particularly, this is required for the revolution control at a constant linear velocity.  
     [0114] A recording system comprises a recording signal processing circuit  124  for producing a recording signal by digitizing the signal to be recorded, converting it into a predetermined format, and adding the necessary information for error correction or the like, and a laser driving circuit  125  for driving a laser of high power for recording on the basis of an output signal from the recording signal processing circuit  124 . The recording system can employ the tracking servo, the focus servo and the spindle servo of the reproducing system.  
     [0115] The optical disk recording/reproducing apparatus has been described above. However, a reproduction dedicated apparatus and a recording dedicated apparatus may be constituted by extracting the respectively corresponding functions of the optical disk recording/reproducing apparatus.  
     [0116] This invention is not limited to the above embodiments, but may be modified or varied appropriately without departing from the scope and spirit of the invention as defined by the appended claims and described in the entire specification. It is intended that the modified optical disk and the modified optical disk recording/reproducing apparatus may also fall within the technical scope of the invention.  
     [0117] As described above, since the tracking servo is made by detecting a tracking error from a reflected light from the land portion between the synchronous pit and the guard pits and the land portion between two adjacent tracks, the conventional tracking pit is unnecessary, whereby the area for recording the data can be widely kept.  
     [0118] If the interval between the synchronous pit and the guard pits disposed before and after the synchronous pit, namely, the distance between the synchronous pit and the guard pit, is set with a certain relation in view of the odd track and the even track, the amplitude of the signal due to the reflected light from the land portion can be fully increased. Accordingly, even when the pit period is short, a correct tracking error can be detected, whereby the optical disk recording/reproducing apparatus has the enhanced performance.  
     [0119] The entire disclosure of Japanese Patent Application No. 2002-130072 filed on May 1, 2002 including the specification, claims, drawings and summary is incorporated herein by reference in its entirety.