Abstract:
An information recording and reproducing apparatus for increasing a speed of tilt control, including a portion for storing reference data, a portion for storing sample values data, a portion for obtaining positional information, a stilt servo generator, a tilt adjuster.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an information recording and reproducing apparatus for recording and/or reproducing information by irradiating a disk with laser light. In particular, the present invention is preferably used in an optical disk apparatus for recording/reproducing information with respect to a laminated disk such as a digital versatile disk (DVD). 
     2. Description of the Related Art 
     A conventional optical disk apparatus is provided with focus control means and tracking control means as means for allowing laser light to follow a track. In addition, an optical disk apparatus for recording/reproducing information with respect to a high-density disk such as a DVD drive is provided with so-called tilt control means for controlling an optical axis of laser light to be vertical to a disk surface. 
     Regarding such tilt control, for example, JP 2001-23213 A describes an optical disk apparatus for controlling a tilt using a focus control signal. More specifically, by inputting a focus control signal at a current recording/reproducing position in a tilt control circuit, and controlling a tilt correction mechanism based on the focus control signal, the influence of a tilt is cancelled in synchronization with a tilt amount due to disk rotation. This enables stable recording/reproduction to be performed with respect to an optical disk having surface wobbling. 
     The tilt control means shown in JP 2001-23213 A is preferably used in the case where the state of surface wobbling varies dynamically owing to the change in temperature in a drive and the like. However, in a laminated disk such as a DVD, the state of surface wobbling does not vary so much even when the temperature in a drive changes. Thus, the dynamic tilt control as in JP2001-23213 A is not necessarily required. 
     Furthermore, in the case of seeking a recording/reproducing position during recording/reproduction, when a tilt is detected from, for example, the state of a reflected beam one by one, a period of time from the end of a seek to the start of recording/reproduction becomes long. In the field of an optical disk apparatus, there is a demand for an increase in speed of a recording/reproducing operation. In this respect, a further increase in speed is required even in tilt control during a seek operation. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to increase the speed of tilt control while suppressing a useless tilt control operation. 
     To achieve such an object, according to the present invention, there is provided an information recording and reproducing apparatus for recording and/or reproducing information by irradiating a disk with laser light, including: reference value data storing means for previously storing reference value data in accordance with a focus bias voltage when a predetermined radial position on a reference disk is focused; sample value data obtaining means for detecting a focus bias voltage when a predetermined radial position on a target disk with respect to which information is to be recorded/reproduced is focused, prior to a commencement of a recording/reproducing operation, and obtaining sample value data in accordance with the detected focus bias voltage under a condition that the sample value data is associated with the radial position; positional information obtaining means for obtaining positional information on a recording/reproducing position on the target disk; tilt servo signal generating means for generating a tilt servo signal at the recording/reproducing position on the target disk, based on a differential value between the reference value data stored in the reference value data storing means and the sample value data obtained by the sample value data obtaining means, and the positional information obtained by the positional information obtaining means; and tilt adjusting means for adjusting a tilt of laser light with respect to a disk surface in accordance with the tilt servo signal generated by the tilt servo signal generating means. 
     In the invention, the sample value data obtaining means may obtain sample value data under a condition that the sample value data are associated with a plurality of radial positions, and the tilt servo signal generating means may obtain a value in accordance with a tilt amount at the radial position from a differential value between the sample value data obtained from the radial position on the target disk and the reference value data, and generate a tilt servo signal at the recording/reproducing position based on the obtained value. 
     At this time, the tilt servo signal generating means may generate a tilt servo signal at the recording/reproducing position, with a value in accordance with a tilt amount at a radial position immediately before or immediately after the recording/reproducing position among the radial positions on the target disk being a value in accordance with a tilt amount at the recording/reproducing position. 
     In addition, the tilt servo signal generating means may obtain a value in accordance with a tilt amount at the recording/reproducing position by linearly approximating a value in accordance with a tilt amount at the recording/reproducing position from values in accordance with tilt amounts at two radial positions sandwiching the recording/reproducing position among the radial positions on the target disk, and generate a tilt servo signal based on the value in accordance with the obtained tilt amount. 
     In addition, in the present invention, a radial position set by the sample value data obtaining means may be set so as to correspond to a radial position set by the reference value data storing means. At this time, the tilt servo signal generating means may obtain a value in accordance with a tilt amount at the radial position based on a differential value between the sample value data and the reference value data corresponding to the sample value data. 
     Furthermore, in the present invention, when a recording/reproducing position on the target disk is sought, the tilt servo signal generating means may generate a tilt servo signal with a target position of a seek destination being the recording/reproducing position. 
     In the present invention, a “value in accordance with a tilt amount” widely includes a value varied in accordance with a tilt amount such as a tilt servo signal, as well as a tilt amount itself. 
     According to the present invention, an appropriate tilt servo signal is predicted and generated at a recording/reproducing position from information regarding the recording/reproducing position. Therefore, it is not necessary to actually measure a tilt amount at a recording/reproducing position, which can simplify and speed the tilt control. 
     In particular, if a tilt servo signal is generated in accordance with a target position of a seek destination as described above, a tilt amount at a recording/reproducing position after a seek can be predicted, so tilt control can be remarkably increased in speed during a seek. For example, if a tilt amount at a recording/reproducing position after a seek is predicted and set during a seek period, an operation can be shifted rapidly to a recording/reproducing operation after a seek. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other objects and novel features of the present invention will be more completely clear upon reading the following description of embodiments with reference to the following attached drawings in which: 
         FIG. 1  shows a configuration of an optical disk apparatus according to an embodiment of the present invention; 
         FIG. 2  shows a configuration of an objective lens actuator according to the embodiment of the present invention; 
         FIG. 3  shows a configuration of the objective lens actuator according to this embodiment of the present invention; 
         FIG. 4  shows a configuration of a pickup feeding mechanism according to this embodiment of the present invention; 
         FIG. 5  is a view illustrating how to obtain a tilt amount Ti according to Embodiment 1 of the present invention; 
         FIG. 6  is a view illustrating an approximate example of the tilt amount Ti according to Embodiment 1 of the present invention; 
         FIG. 7  is a processing flowchart of tilt control according to Embodiment 1 of the present invention; 
         FIG. 8  is a processing flowchart of the tilt control according to Embodiment 1 of the present invention; 
         FIG. 9  shows a modified example of the processing chart of the tilt control according to Embodiment 1 of the present invention; 
         FIG. 10  shows a modified example of the processing chart of the tilt control according to Embodiment 1 of the present invention; 
         FIG. 11  is a view illustrating an approximate example of a tilt amount Ti according to Embodiment 2 of the present invention; 
         FIG. 12  is a processing flowchart of tilt control according to Embodiment 2 of the present invention; 
         FIG. 13  shows a modified example of the processing flow chart of the tilt control according to Embodiment 2 of the present invention; 
         FIG. 14  is a view illustrating how to obtain a tilt amount Ti according to Embodiment 3 of the present invention; and 
         FIG. 15  is a processing flow chart of tilt control according to Embodiment 3 of the present invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinafter, the present invention will be described by way of examples with reference to the drawings. It should be noted that the following embodiments are shown merely for an illustrative purpose, and do not limit the scope of the present invention. 
     In this embodiment, a DVD+RW (hereinafter, referred to as a disk  100 ) is used as a disk (target disk) with respect to which information is to be recorded/reproduced. The disk  100  is divided into a lead-in area, a data area, and a lead-out area in this order from an inner circumference. The disk  100  is provided with a spiral groove from the inner circumference to an outer circumference, and data is recorded with respect to the groove. 
     Herein, the groove is wobbled in a radial direction, and this wobbling allows address information to be held. That is, a phase modulation section called an address in pre-groove (ADIP) is inserted in a monotonous wobbling section at a constant period, and when such a phase modulation section is scanned with a beam, address information on the groove is read to be reproduced from a change in the intensity of reflected light. In the ADIP of the lead-in area, various pieces of control data with respect to the disk are recorded by phase modulation. 
       FIG. 1  shows a configuration of an optical disk apparatus according to an embodiment of the present invention. 
     As shown in the figure, the optical disk apparatus includes an ECC encoder  101 , a modulation circuit  102 , a laser driving circuit  103 , a laser power adjustment circuit  104 , an optical pickup  105 , a signal amplification circuit  106 , a demodulation circuit  107 , an ECC decoder  108 , a servo circuit  109 , an ADIP reproduction circuit  110 , and a controller  111 . 
     The ECC encoder  101  adds an error correction code to input recording data, and outputs it to the modulation circuit  102 . The modulation circuit  102  subjects the input recording data to predetermined modulation, and generates and outputs a recording signal to the laser driving circuit  103 . The laser driving circuit  103  outputs a driving signal in accordance with a recording signal from the modulation circuit  102  to a semiconductor laser  105   a  during recording, and outputs a driving signal for irradiation of laser light with a single intensity to the semiconductor laser  105   a  during reproduction. Herein, a laser power is set to be the one adjusted and set by the laser power adjustment circuit  104 . 
     The laser power adjustment circuit  104  adjusts and sets a laser power at an optimum value based on the state of a reproduction RF signal detected during trial writing. Herein, the adjustment of a laser power is performed using a known γ-method. The γ-value of the disk is included in the ADIP in the lead-in area. 
     The optical pickup  105  includes the semiconductor laser  105   a  and a photodetector  105   b , and writes/reads data with respect to a disk by condensing laser light onto the groove. The optical pickup  105  includes: an objective lens actuator (described later) for adjusting an irradiation state of laser light with respect to the groove; and an optical system for guiding laser light emitted from the semiconductor laser  105   a  to an objective lens and guiding light reflected from the disk  100  to the photodetector  105   b.    
     The signal amplification circuit  106  amplifies a signal received from the photodetector  105   b , generates various kinds of signals as a result of operation processing, and outputs them to the corresponding circuit. The demodulation circuit  107  demodulates a reproduction RF signal input from the signal amplification circuit  106  to generate reproduction data, and outputs it to the ECC decoder  108 . The ECC decoder  108  subjects the reproduction data input from the demodulation circuit  107  to an error correction, and outputs the resultant data to a circuit in a later stage. 
     The servo circuit  109  generates a focus servo signal and a tracking servo signal from a focus error signal and a tracking error signal input from the signal amplification circuit  106 , and outputs them to an objective lens actuator of the optical pickup  105 . Furthermore, the servo circuit  109  generates a motor servo signal from a wobble signal input from the signal amplification circuit  106 , and outputs it to a disk drive motor. Furthermore, the servo circuit  109  generates a tilt servo signal from a tilt error signal supplied from the controller  111 , and outputs it to the objective lens actuator of the optical pickup  105 . 
     The servo circuit  109  also has a function of driving a pickup feeding mechanism (not shown), and feeding the optical pickup  105  in a disk radial direction. The controller  111  controls the servo circuit  109  to allow it to seek a scanning position of the optical pickup  105 , and allows a recording/reproducing position to access the predetermined position on the disk. 
     The ADIP reproduction circuit  110  reproduces address information and various kinds of pieces of control information from the wobble signal input from the signal amplification circuit  106 , and outputs them to the controller  111 . The controller  111  stores various kinds of data in a storage memory, and controls each portion in accordance with a previously set program. The control operation during tilt servo by the controller  111  will be described later in detail. 
       FIGS. 2 and 3  each show a configuration of the objective lens actuator. 
     As shown in  FIG. 3 , a coil assembly is composed of a lens holder  202 , a focusing coil  203 , four tracking coils  204 , and four tilt coils  205 . In  FIG. 3 , a solid arrow provided to each coil represents a winding direction of each coil. 
     The focusing coil  203  is wound in the same shape as that of an outer circumferential shape of the lens holder  202  so that an inner circumferential frame thereof is slightly larger than an outer circumference of the lens holder  202 , and thereafter, cemented with a resin. Then, the focusing coil  203  is fitted to the lens holder  202  from above, and attached thereto with an adhesive. 
     Each tracking coil  204  is wound in the same shape as an outer circumferential shape of a projection formed on an outer circumference of the lens holder  202  so that an inner circumferential frame thereof is slightly larger than an outer circumference of the projection formed on the outer circumference of the lens holder  202 , and thereafter, cemented with a resin. Then, the tracking coil  204  is fitted to the projection from a side and attached thereto with an adhesive. 
     Each tilt coil  205  is wound so that an inner circumferential frame thereof has such a size as to come into contact with a pair of nail portions formed on a reverse surface side of the lens holder  202 , and thereafter, cemented with a resin. Then, the tilt coil  205  is fitted to the nail portions from below, and attached thereto with an adhesive. 
       FIG. 2  shows a schematic perspective view of the objective lens actuator. 
     The lens holder  202  with each coil mounted thereon as described above is suspended with wires  208  on a support  207  set on a base  206 , in such a manner that each coil is inserted in a magnetic gap between a magnet  209  and a yoke  210 . 
     The wires  208  are electrically connected to corresponding coils, and a servo signal is supplied to each coil via the wires. By changing the bias value of a servo signal flowing through each coil, a displacement amount of the objective lens  201  can be changed in a focus direction, a tracking direction, and a tilt direction. Furthermore, by inverting the direction of a servo signal flowing through each coil, the driving direction of the objective lens  201  can be inverted appropriately. 
       FIG. 4  shows a main configuration of a support mechanism for guiding and supporting the optical pickup  105  in a disk radial direction.  FIG. 4  is a view of the support mechanism seen from above. 
     The optical pickup  105  is supported by a pair of support shafts  301  slidably in a radial direction of a turntable  302 . Herein, the objective lens  201  of the optical pickup  105  is provided so that laser light is moved in a radial direction of the disk  100  along with the movement of the optical pickup  105 , when the disk  100  is mounted on the turntable  302 . 
     Each support shaft  301  is provided so that one end thereof can be displaced in a vertical direction by adjusting a tangential tilt adjustment screw  303  and a radial tilt adjustment screw  304 . By adjusting the tangential tilt adjustment screw  303  and the radial tilt adjustment screw  304  during assembly of the optical disk apparatus, the tilt state of the optical pickup  105  is adjusted mechanically. 
     Such an adjustment is performed by: placing a reference disk (having the same disk format as that of the disk  100 ) having a substrate with a high flatness precision, such as a glass substrate, on the turntable  302 , setting a focus servo and a tracking servo in an ON state and setting a tilt servo in an OFF state; and, under this condition, adjusting the tangential tilt adjustment screw  303  and the radial tilt adjustment screw  304  so that a reproduced signal becomes most satisfactory. More specifically, the tangential tilt adjustment screw  303  and the radial tilt adjustment screw  304  are adjusted so that a reproduction RF signal, when information is reproduced from the reference disk, becomes maximum, or a jitter of a reproduced signal becomes minimum. 
     After the above-mentioned mechanical adjustment is performed, the optical pickup  105  is fed to a predetermined radial position on the reference disk, and subjected to focus servo at that position. At this time, the tilt serve is turned off (the tracking servo may be turned on/off). Then, the bias value of the focus servo signal in a focus ON state is output from the servo circuit  109  to the controller  111 , and stored in a storage memory as a reference bias value. 
     Based on the reference bias value, tilt control is performed with respect to the disk  100  (target disk). Hereinafter, various embodiments in which such tilt control is embodied will be described sequentially. 
     Embodiment 1 
       FIG. 5  shows an outline of a tilt control operation in this embodiment. 
     In this embodiment, during assembly, a reference bias value V 0  is obtained from only one radial position on the above-mentioned reference disk and stored in the controller  111 . Herein, the position at which the reference bias value V 0  is obtained is set, for example, at a point positioned substantially at the center of a region from a disk innermost circumference to a disk outermost circumference among radial positions of the disk  100 . Furthermore, the reference bias value V 0  is obtained by sequentially sampling bias values of a focus servo signal of one round of the disk at that point, and averaging them. 
     In this embodiment, when the disk  100  (target disk) is placed on the optical disk apparatus, the optical pickup  105  accesses previously determined 6 radial positions P 1  to P 6  prior to a recording/reproducing operation, and bias values vs 1  to vs 6  of a focus servo signal are obtained at respective radial positions. More specifically, bias values of a focus servo signal when the optical pickup  105  is subjected to focus servo at the radial positions P 1  to P 6  are output from the servo circuit  109  to the controller  111 . The controller  111  sequentially samples bias values of a focus servo signal of one round of the disk, and average them to obtain focus bias values vs 1  to vs 6  with respect to the respective radial positions P 1  to P 6 , in the same as in the above-mentioned reference bias value V 0 . Then, the obtained focus bias values vs 1  to vs 6  are associated with the respective radial positions P 1  to P 6  and stored in a sample value memory. 
     After that, the controller  111  calculates tilt angles θ 1  to θ 6  of the disk  100  at the respective radial positions P 1  to P 6  based on the reference bias value V 0  and the focus bias values vs 1  to vs 6 . More specifically, bias differences Δvs 1  to Δvs 6  (Δvs n =vs n −V 0 ) between the reference bias value V 0  and the focus bias values vs 1  to vs 6  are obtained, and the obtained bias differences Δvs 1  to Δvs 6  are converted to displacement amounts Δd 1  to Δd 6  of the disk surface of the disk  100  with respect to the disk surface of the reference disk. At this time, Δdn has a positive/negative polarity in accordance with Δvsn. Then, based on distances r 1  to r 6  from the disk inner most circumferential position to the respective radial positions P 1  to P 6  and the above-mentioned displacement amounts Δd 1  to Δd 6 , the tilt angles θ 1  to θ 6  of the disk  100  at the respective radial positions P 1  to P 6  are obtained. 
     Herein, the tilt angle θ n  is obtained, for example, by calculating θ n =tan −1 {(Δd n+1 −Δd n )/(r n+1 −r n )}. Alternatively, the tilt angle θ n  may be obtained by calculating θ n =tan −1 (Δd n /r n ). In any case, the tilt angle θ n  has a positive/negative polarity in accordance with Δd n . 
     As described above, the optical pickup  105  is mechanically adjusted with respect to the reference disk with a high flatness precision, so the obtained tilt angles θ 1  to θ 6  are substantially matched with the tilt amounts of laser light at the respective radial positions P 1  to P 6 . The controller  111  detects tilt amounts Ti 1  to Ti 6  at the respective radial positions P 1  to P 6  from the obtained tilt angles θ 1  to θ 6 . Then, the controller  111  approximates the tilt amount Ti with respect to the entire area of the disk  100  based on the detected tilt amounts Ti 1  to Ti 6 , and during recording/reproduction, outputs a tilt error signal in accordance with the approximated value Ti to the servo circuit  109 . 
     The above-mentioned radial positions P 1  to P 6  are set, for example, so that the radial position P 1  and the radial position P 6  are placed at an innermost circumferential position and an outermost circumferential position of the disk  100 , respectively, and the radial positions P 2  to P 5  are placed at positions substantially equally dividing a region between the radial position P 1  and the radial position P 6 . Alternatively, the radial positions P 2  to P 5  may be set so that an interval between two adjacent positions becomes smaller toward an outer circumferential portion. Because of this, the tilt amount Ti can be approximated with relatively satisfactory precision even with respect to large surface wobbling that easily occurs in the outer circumferential portion. 
       FIG. 6  shows an approximate example of the tilt amount Ti at a recording/reproducing position. 
     As shown in  FIG. 6 , in this approximate embodiment, when the recording/reproducing position is placed between adjacent two radial positions among the above-mentioned radial positions P 1  to P 6 , the tilt amount Ti of the position among the radial positions P 1  to P 5 , which is placed immediately before the recording/reproducing position, is set to be the tilt amount of the recording/reproducing position. At this time, the tilt amount Ti at each of the radial positions P 1  to P 5  is obtained, for example, from the above-mentioned calculation expression: θ n =tan −1 {(Δd n+1 −Δd n )/(r n+1 −r n )}. 
     The tilt amount Ti of the position among the radial positions P 2  to P 6 , which is placed immediately after the recording/reproducing position, may be set to be the tilt amount of the recording/reproducing position, in place of the tilt amount of the position among the radial positions P 1  to P 5  which is placed immediately before the recording/reproducing position. At this time, the tilt amount Ti of each of the radial positions P 2  to P 6  is obtained, for example, from a calculation expression: θ n =tan −1 {(Δd n −Δd n−1 )/(r n −r n−1 )}. 
       FIG. 7  shows a processing flow of tilt control in this embodiment. The processing flow may use an approximation method of a tilt amount shown in  FIG. 6 . 
     When a disk is mounted on the optical disk apparatus, the controller  111  allows the optical pickup  105  to access the above-mentioned radial positions P 1  to P 6 , obtains the focus bias values vs 1  to vs 6  at the respective radial positions from the servo circuit  109 , and stores them in the storage memory under the condition that they are associated with the radial positions (S 101 ). Then, the above-mentioned arithmetic operation is performed based on the reference bias value V 0  stored in the storage memory during assembly and the focus bias values vs 1  to vs  6  obtained in S 101 , and the tilt amounts Ti 1  to Ti 5  at the respective radial positions P 1  to P 5  are calculated (S 102 ). Then, the calculated tilt amounts Ti 1  to Ti 5  are stored under the condition that they are associated with the respective radial positions P 1  to P 5  (S 103 ). 
     In the above-mentioned calculation in S 102 , as described above, while the bias differences Δvs 1  to Δvs 6 , the displacement amounts Δd 1  to Δd 6  of the disk surface, and the tilt angles θ 1  to θ 5  are calculated, the tilt amounts Ti 1  to Ti 5  may be obtained finally. Alternatively, a calculation algorithm from which such intermediate operations are omitted is set in the controller  111 , and the tilt amounts Ti 1  to Ti 5  may be directly obtained from the focus bias values vs 1  to vs 6  obtained in S 101  in accordance with the algorithm. 
     Thus, when the tilt amounts Ti 1  to Ti 5  at the radial positions P 1  to P 5  are stored in the controller  111 , and thereafter, a recording/reproducing instruction is input (S 104 ), the controller  111  generates a tilt error signal based on the tilt amounts Ti 1  to Ti 5  stored in S 103 , and outputs the tilt error signal to the servo circuit  109  (S 105 ) during the recording/reproducing operation (S 106 : NO). When the recording/reproducing operation is completed (S 106 : YES), if the disk is not ejected (S 107 : NO), the process returns to S 104  and stands by until the commencement of the subsequent recording/reproducing operation. When the disk is ejected (S 107 : YES), the focus bias values vs 1  to vs  6  and the tilt amounts Ti 1  to Ti 5  stored in the storage memory are deleted (S 108 ), and the tilt control with respect to the disk is completed. 
       FIG. 8  shows a processing flow of the tilt control in S 105 . 
     During the recording/reproducing operation, the controller  111  detects whether or not a seek occurs at the recording/reproducing position (S 201 ). When a seek does not occur, the radial position in accordance with the current recording/reproducing position is detected based on the address data from the ADIP reproduction circuit  110  (S 202 ). Then, a radial position Pk immediately before the current recording/reproducing position among the radial positions P 1  to P 6  is detected (S 203 ), and a tilt amount Tik stored in the storage memory as the tilt amount corresponding to the radial position Pk is set as the tilt amount at the current recording/reproducing position (S 204 ). Furthermore, a tilt error signal is generated from the set tilt amount Tik, and supplied to the servo circuit  109  (S 205 ). Thus, the tilt control at the current recording/reproducing position is performed. 
     On the other hand, when the controller  111  detects that a seek occurs at the recording/reproducing position during the recording/reproducing operation (S 201 : YES), the controller  111  detects a radial position in accordance with the recording/reproducing position after the seek based on a seek target address (S 206 ). Then, a radial position Pk placed immediately before the radial position after the seek among the radial positions P 1  to P 6  is detected (S 203 ), and the tilt amount Tik stored in the storage memory as the tilt amount corresponding to the radial position Pk is set as the tilt amount at the recording/reproducing position after the seek (S 204 ). Furthermore, a tilt error signal is generated from the set tilt amount Tik, and supplied to the servo circuit  109  (S 205 ). Thus, the tilt control at the recording/reproducing position after the seek is performed. 
     In such a processing flow, the tilt amount at the radial position close to the current recording/reproducing position among the tilt amounts Ti 1  to Ti 5  at the radial positions P 1  to P 5  obtained prior to the recording/reproducing operation is set as the tilt amount at the current recording/reproducing position. Therefore, the actual measurement of the tilt amount at the current recording/reproducing position can be omitted, and the tilt control at the current recording/reproducing position can be made easy and increased in speed. 
     In particular, in the case where a seek occurs at the recording/reproducing position, the tilt amount at the recording/reproducing position after the seek can be predicted from a seek target address, so the tilt control at a time of occurrence of the seek can be increased in speed remarkably. For example, if the tilt amount at the recording/reproducing position after the seek is predicted during a seek period, the process can be shifted rapidly to the recording/reproducing operation after the seek. 
     In the above description, the tilt amount of the position among the radial positions P 1  to P 5 , which is placed immediately before the current recording/reproducing position or the recording/reproducing position after the seek, is set to be the tilt amount at the recording/reproducing position. However, as described above, the tilt amount Ti at the position among the radial positions P 2  to P 6 , which is placed immediately after the recording/reproducing position, may be set as the tilt amount at the recording/reproducing position. At this time, the tilt amount Ti at each of the radial positions P 2  to P 6  is obtained, for example, from a calculation expression: θn=tan −1 {(Δd n −Δd n−1 )/(r n −r n−1 )}. 
     Furthermore, in the above description, the tilt amounts Ti 1  to Ti 5  at the radial positions P 1  to P 5  are previously calculated and stored in the storage memory. However, only the focus bias values vs 1  to vs 6  at the radial positions P 1  to P 6  may be stored in the storage memory, and the tilt amounts Ti 1  to Ti 5  may be calculated each time, based on the corresponding focus bias value and the reference bias value during the recording/reproducing operation. 
       FIGS. 9 and 10  each show a processing flow in such a case. 
     The processing flow in  FIG. 9  corresponds to a processing flow in  FIG. 7 , and in this flow, S 102  and S 103  that correspond to the processing of calculating and storing the tilt amounts Ti 1  to Ti 5  are omitted, compared with the processing flow in  FIG. 7 . That is, in the processing flow in  FIG. 9 , only the bias values vs 1  to vs 6  at the radial positions P 1  to P 6  are obtained in S 101 , and stored in the storage memory. 
     Furthermore, the processing flow in  FIG. 10  corresponds to the processing flow in  FIG. 8 , and in this flow, S 210  that corresponds to the processing of calculating the tilt amount at the recording/reproducing position is added, compared with the processing flow in  FIG. 8 . More specifically, in the processing flow in  FIG. 10 , after the radial position Pk immediately before the recording/reproducing position is detected (S 203 ), the tilt amount Tik at the radial position Pk is calculated from the reference bias value V 0  stored in the storage memory and the corresponding focus bias value (S 210 ). At this time, the tilt amount Tik is obtained based on the above-mentioned calculation expression: θ n =tan −1 {(Δd n+1 −Δd n )/(r n+1 −r n )}. Then, the calculated tilt amount Tik is set as the tilt amount at the recording/reproducing position (S 204 ), and the tilt control is performed (S 205 ). 
     Embodiment 2 
     In Embodiment 1, the tilt amount Ti at a position placed immediately before (or immediately after) a recording/reproducing position among the radial positions P 1  to P 6  is set as the tilt amount at the recording/reproducing position. In this embodiment, the tilt amount at the recording/reproducing position is set by linear approximation of tilt amounts Ti, Ti+1 at positions immediately before and immediately after the recording/reproducing position among the radial positions P 1  to P 6 , as shown in  FIG. 11 . 
       FIG. 12  shows a processing flow of tilt control during recording/reproduction. 
     The processing flow in  FIG. 12  corresponds to the processing flow in  FIG. 8 , and compared with the processing flow in  FIG. 8 , S 301  to S 303  that correspond to the processing of setting the tilt amount at a recording/reproducing position by linear approximation of tilt amounts Ti, Ti+1 at positions immediately before and immediately after the recording/reproducing position among the radial positions P 1  to P 6  are added. More specifically, in the processing flow in  FIG. 12 , after radial positions Pk, Pk+1 immediately before and immediately after the recording/reproducing position are detected (S 301 ), the tilt amount at the recording/reproducing position is calculated by linear approximation of Tik, Tik+1 at the radial positions Pk, Pk+1 stored in a storage memory (S 302 ). Then, the calculated tilt amount is set as the tilt amount at the recording/reproducing position (S 303 ), and tilt control is performed (S 205 ). 
     According to this embodiment, the processing becomes more complicated than that in Embodiment 1 of the present invention in accordance with the calculation of a tilt amount by linear approximation. However, the setting precision of a tilt amount can be enhanced compared with the case where the tilt amount is set in stages as in Embodiment 1 of the present invention. 
     Even in this embodiment, in the same way as in the modified example of Embodiment 1 of the present invention, only the focus bias values vs 1  to vs 6  at the radial positions P 1  to P 6  may be stored in a storage memory, and the tilt amounts Ti 1  to Ti 6  may be calculated every time based on the corresponding focus bias values and the reference bias value during a recording/reproducing operation. More specifically, as shown in  FIG. 13 , S 310  is added immediately after S 301 , and tilt amounts Tik, Tik+1 at radial positions Pk, Pk+1 are calculated from the focus bias values vsk, vsk+1 at the radial positions Pk, Pk+1, and the reference bias value V 0 . Then, the tilt amount at the recording/reproducing position is linearly approximated based on the calculated tilt amounts Tik, Tik+1 (S 302 ). 
     In this embodiment, the tilt amount at a recording/reproducing position is linearly approximated based on the tilt amounts Ti, Ti+1 at positions immediately before and immediately after the recording/reproducing position among the radial positions P 1  to P 6 . Therefore, when the tilt amount Tin is obtained based on the calculation expression: θn=tan 1 {(Δd n+1 −Δd n )/(r n+1 −r n )}, the tilt amount Ti 6  at the radial position P 6  cannot be obtained, and in the case where the recording/reproducing position is placed between the radial positions P 5  and P 6 , the tilt amount Ti at the recording/reproducing position cannot be obtained by linear approximation. The same holds true for the case where the tilt amount Tin is obtained based on the above-mentioned calculation expression: θn=tan −1  {(Δd n−1 −Δd n )/(r n+1 −r n )}. 
     Thus, the tilt amount Tin in this embodiment is preferably calculated, for example, based on the above-mentioned calculation expression: θn=tan −1 (Δd n /r n ). Because of this, the tilt amounts Ti 1  to Ti 6  can be obtained regarding all the radial positions P 1  to P 6 , and hence, the tilt amount Tin can be linearly approximated even in the vicinity of an outer most circumferential portion (section of P 5  to P 6 ). 
     Embodiment 3 
     In Embodiments 1 and 2 described above, only one reference bias value V 0  is obtained from a reference disk and stored in a storage memory. In contrast, in this embodiment, as shown in  FIG. 14 , reference bias values V 1  to V 6  are obtained from positions substantially corresponding to the radial positions P 1  to P 6  and stored, and the tilt amounts Ti 1  to Ti 5  at the respective radial positions P 1  to P 5  are obtained from the reference bias value Vn and the focus bias value vsn corresponding to each other. 
       FIG. 15  shows a processing flow in this embodiment. The processing flow in  FIG. 15  corresponds to that in  FIG. 7 , and compared with the processing flow in  FIG. 7 , S 102  that corresponds to the processing of calculating the tilt amounts Ti 1  to Ti 5  is changed to S 110 . More specifically, in the processing flow in  FIG. 15 , a tilt amount Ti 1  at a radial position P 1  is calculated from the above-mentioned calculation expression: θn=tan −1 {(Δd n+1 −Δd n )/(r n+1 −r n )}, based on the reference bias values V 1 , V 2  and the focus bias values vs 1 , vs 2 . Similarly, tilt amounts Ti 2  to Ti 6  at the radial positions P 2  to P 5  are calculated based on the reference bias values V 2  to V 6  and the focus bias values vs 2  to vs 6 . Then, the calculated tilt amounts Ti 1  to Ti 5  are stored in a storage memory under a condition that they are associated with the respective radial positions P 1  to P 5  (S 103 ). 
     In this embodiment, only the processing of calculating the tilt amounts Ti 1  to Ti 5  is different from those in Embodiments 1 and 2 of the present invention, and the other processing steps are performed in the same way as in the processing flow described in each of Embodiments 1 and 2 of the present invention. More specifically, in the case where the tilt amounts Ti 1  to Ti 5  are stored in the storage memory prior to the recording/reproducing operation, the processing flow in  FIG. 8  or  FIG. 12  is performed as it is during a recording/reproducing operation. In the case where the tilt amounts Ti 1  to Ti 5  are calculated sequentially during a recording/reproducing operation ( FIGS. 10 and 13 ), the tilt amounts Ti 1  to Ti 5  are calculated based on the reference bias value Vn and the focus bias value vsn corresponding to each other. 
     In this embodiment, the reference bias value Vn and the focus bias value vsn are obtained at a substantially corresponding radial position, and the tilt amounts Ti 1  to Ti 5  at the respective radial positions P 1  to P 5  are calculated based on the reference bias value Vn and the focus bias value vsn. Therefore, compared with the case where the tilt amounts Ti 1  to Ti 5  are calculated based on one bias value V 0  as in Embodiments 1 and 2 of the present invention, the calculation precision of the tilt amount at each radial position can be enhanced. 
     More specifically, in the mechanical adjustment during assembly, as described above, tilt adjustment is performed while an adjustment screw is being adjusted so that the state of a reproduced signal becomes most satisfactory. Therefore, it is difficult to completely remove tilt by such mechanical adjustment. Owing to the error in mechanical adjustment, the reference bias value to be originally set is not the same at all the radial positions, and there may be a slight difference in accordance with a radial position. In such a case, when the tilt amount Tin is calculated based on one bias value V 0  as in Embodiments 1 and 2 of the present invention, the bias value V 0  may largely shift with respect to the bias value to be originally set, depending upon a radial position. Therefore, the calculated tilt amount Tin may largely shift from an actual tilt amount. 
     In contrast, if the reference bias value Vn and the focus bias value vsn are obtained at a substantially corresponding radial position, and the tilt amount Tin at each radial position Pn is calculated based on the reference bias value Vn and the focus bias value vsn, as in this embodiment of the present invention, even when there is an error in mechanical adjustment as described above, the reference bias value Vn can be closer to the original bias value than the case to obtain it by using one reference bias value V 0  as in Embodiment 1 and 2. Hence, the calculated tilt amount Tin can be suppressed from largely shifting from the actual tilt amount. Thus, according to this embodiment, compared with the case where the tilt amount Tin is calculated based on one bias value V 0  as in Embodiments 1 and 2 of the present invention, the calculation precision of a tilt amount at each radial position can be enhanced further. 
     The embodiment of the present invention has been described above. It should be noted that the present invention is not limited to the above embodiment, and may be variously modified. The embodiment of the present invention can be appropriately varied within a scope of a technical concept shown in the claims.