PATENT ABSTRACT
A tilt detector is adapted to be used for a storage medium provided with a recording track on which information is recorded and a first and a second header portions, each arranged in a manner shifted in opposite directions to each other from a center line of the recording track. The detector includes a light irradiation unit for irradiating a light beam onto the first header portion, the second header portion and the recording track; a light receiving unit having a first light receiving surface and a second light receiving surface arranged adjacently and for receiving the light beam reflected by the storage medium, the first light receiving surface outputting a first output and the second light receiving surface outputting a second output; an operation unit for executing an arithmetic operation of the first output and the second output to generate an operation result signal; and an error signal generation unit for generating a tilt error signal based on the operation result signal. The tilt error signal indicates a tilt between the storage medium and an optical axis of the light beam.

PATENT DESCRIPTION
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a tilt detector for detecting a tilt which occurs between an information recording surface of a storage medium and an optical axis of a light beam used for recording and/or reproduction, when information is optically recorded on the storage medium and/or information is optically reproduced from the storage medium. 
     2. Description of Related Art 
     Recently, there is broadly known a technique in which a light beam such as a laser beam and the like is applied to a disc-shaped storage medium to optically record information onto the storage medium and/or to optically reproduce information already recorded on the storage medium therefrom. At the time of recording and/or reproducing information on and/or from a disc-shaped storage medium using the light beam, the angle between the information recording surface of the storage medium and the optical axis of the light beam may sometimes shift from the right angle (i.e., 90 degrees) to induce a tilt in a radial direction of the storage medium (this tilt occurring in the radial direction of the disc-shaped storage medium will be hereinafter referred to as “radial tilt”). For example, a centrifugal force in the storage medium revolution and a deflection of the storage medium itself due to an aged change may be the causes of the radial tilt. If information is recorded or reproduced in the presence of the radial tilt, aberration (mainly coma-aberration) takes place within the light beam irradiated range on the information recording surface of the storage medium. This disables the precise control of light-spot size, and hence high-density information recording becomes difficult. Therefore, it is necessary to detect the quantity and the direction of the radial tilt and compensate for it. For example, in a most general radial tilt detection method employing a tilt sensor, a dedicated light beam for radial tilt detection is irradiated on the information recording surface, separately from the light beam used for information recording and/or reproduction. The light beam reflected by the surface is received by a light detector including multiple light detecting portions divided by a divisional line arranged in parallel with the circumferential direction of the storage medium, and the quantity and the direction of the radial tilt are obtained from the difference of the received light quantities of the respective light detecting portions. Namely, if the difference is equal to zero, there is occurring no radial tilt. If the difference is not zero, there is occurring a radial tilt in the light beam, which has the direction corresponding to the polarity of the difference and the quantity corresponding to the absolute value of the difference. 
     There is known a disc-shaped information storage medium so-called DVD-RAM (DVD-Random Access Memory) which was standardized recently and has an ability to record and/or reproduce information repeatedly for many times. The DVD-RAM is an improvement of DVD which has much larger recording capacity than CD and enables repetitive recording and reproduction for multiple times. In an apparatus for recording and/or reproducing information on and/or from DVD-RAM, no radial tilt detection and compensation function has been employed. This is mainly because DVD-RAM has the same size as CD, and large radial tilt which needs its compensation rarely took place. However, in order to further improve the accuracy of information recording/reproduction for the DVD-RAM, it is preferred to compensate for the radial tilt even if it is small. 
     Supposing that the conventional tilt sensor (i.e., the above-mentioned dedicated tilt sensor which additionally irradiates dedicated light beams for tilt detection) is used to detect the radial tilt taking place in relation to the DVD-RAM, the tilt sensor needs high-accuracy detection function due to the fact that the tilt itself is small, thereby increasing the cost of the information recording and/or reproducing apparatus. Further, additionally providing the dedicated tilt sensor increases the adjustment steps of the tilt sensor itself in the manufacturing process of the information recording and/or reproducing apparatus, thereby declining the productivity. Still further, the detection accuracy of the radial tilt may decline due to aging of the tilt sensor. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a tilt detector capable of accurately detecting the radial tilt without providing a dedicated tilt sensor. 
     According to one aspect of the present invention, there is provided a tilt detector adapted to be used for a storage medium provided with a recording track on which information is recorded and a first and a second header portions, each arranged in a manner shifted in opposite directions to each other from a center line of the recording track, the detector including: a light irradiation unit for irradiating a light beam onto the first header portion, the second header portion and the recording track; a light receiving unit having a first light receiving surface and a second light receiving surface arranged adjacently to each other on both sides of a divisional line which is in parallel with the direction of the center line and for receiving the light beam reflected by the storage medium, the first light receiving surface outputting a first output and the second light receiving surface outputting a second output; an operation unit for executing an arithmetic operation of the first output and the second output to generate an operation result signal; and an error signal generation unit for generating a tilt error signal eased on the operation result signal, the tilt error signal indicating a tilt between the storage medium and an optical axis of the light beam. 
     The above tilt detector receives the light beam reflected by the first header portion and the second header portion formed on the storage medium with the shifts in opposite directions to each other from the center line of the recording track, and detects the tilt between the storage medium and the optical axis of the light beam using the reflected light. Therefore, the tilt can be detected without providing a dedicated tilt sensor. 
     The error signal generation unit may include a low-pass filter which extracts a low-frequency component of the operation result signal as the tilt error signal. By this, the quantity and the direction of the tilt can be accurately detected with a simple configuration. 
     The error signal generation unit may include an averaging circuit which extracts a D.C. component of the operation result signal as the tilt error signal. Thus, the quantity and the direction of the tilt can be accurately detected. 
     In a preferred embodiment, the storage medium may include a disc-shaped storage medium, the recording track may include a pre-groove portion and a land portion, the first header portion may be arranged in a manner shifted by a half track pitch in a first radial direction of the disc-shaped recording medium from a center line of the pre-groove portion, and the second header portion may be arranged in a manner shifted by the half track pitch in a second radial direction, opposite to the first radial direction, of the disc-shaped recording medium from the center line of the pre-groove portion. With this arrangement, the tilt in the radial direction of the disc-shaped storage medium can be accurately detected without the use of a dedicated tilt sensor. 
     According to another aspect of the present invention, there is provided a tilt detector adapted to be used for a disc-shaped storage medium provided with a recording track on which information is recorded and header areas on which predetermined address information is recorded, the recording track including a pre-groove portion and a land portion, each of the header areas including a first header portion arranged in a manner shifted by a half track pitch in a first radial direction of the storage medium from a center line of the pre-groove portion and a second header portion arranged in a manner shifted by a half track pitch in a second radial direction, opposite to the first radial direction, of the storage medium from the center line of the pre-groove portion, the tilt detector including: an irradiation unit for irradiating a light beam onto the header areas and the recording track; a light receiving unit having a first light receiving surface and a second light receiving surface arranged adjacently to each other on both sides of a divisional line which is in parallel with the direction of the center line and for receiving the light beam reflected by the storage medium, the first light receiving surface outputting a first output and the second light receiving surface outputting a second output; a reproduction signal generation unit for generating a first reproduction signal corresponding to the address information recorded in the first header portion and a second reproduction signal corresponding to the address information recorded in the second header portion based on the first output and the second output; a delay unit for delaying the first reproduction signal by a predetermined time period to generate a delay signal; an operation unit for executing an arithmetic operation of the delay signal and the second reproduction signal to generate an operation result signal; and an error signal generation unit for generating a tilt error signal based on the operation result signal, the tilt error signal indicating a tilt between the storage medium and an optical axis of the light beam. 
     The above tilt detector receives the light beam reflected by the first header portion and the second header portion formed on the disc-shaped storage medium with the shifts in opposite directions to each other from the center line of the recording track, and detects the tilt between the storage medium and the optical axis of the light beam using the reflected light. Therefore, the tile can be detected without providing a dedicated tilt sensor. 
     The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiment of the invention when read in conjunction with the accompanying drawings briefly described below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view showing the recording format of the DVD-RAM. 
     FIG. 2 is an magnified view showing the recording format of the DVD-RAM. 
     FIG. 3A is a diagram showing the detailed structure of the header area. 
     FIG. 3B is a diagram showing the data structure of each header portion. 
     FIG. 4 is a block diagram showing the overall configuration of the information reproduction apparatus according to the first embodiment. 
     FIG. 5 is a plan view showing the configuration of the detector provided within the-pickup of the first embodiment. 
     FIG. 6 is a block diagram showing the configuration of the servo signal generation circuit of the first embodiment. 
     FIG. 7A is a graph showing the waveforms of the subtraction signals according to the first embodiment. 
     FIG. 7B is a graph showing the relation between the radial tilt and the tilt error signal according to the first embodiment. 
     FIG. 8 is a block diagram showing the servo signal generation circuit of the second embodiment. 
     FIG. 9A is a graph showing the waveforms of the multiplication signals according to the second embodiment. 
     FIG. 9B is a graph showing the relation between the radial tilt and the tilt error signal according to the second embodiment. 
     FIG. 10 is a block diagram showing the configuration of the servo signal generation circuit of the third embodiment. 
     FIG. 11 is a block diagram showing the configuration of the servo signal generation circuit of the fourth embodiment. 
     FIG. 12A is a graph showing the relation between the radial tilt and the tilt error signal according to the third embodiment. 
     FIG. 12B is a graph showing the relation between the radial tilt and the tilt error signal according to the fourth embodiment. 
     FIG. 13 is a block diagram showing the overall configuration of the information reproduction apparatus according to the fifth embodiment. 
     FIG. 14 is a block diagram showing the configuration of the servo signal generation circuit of the fifth embodiment. 
     FIG. 15A is a graph showing the waveforms of the addition signals according to the fifth embodiment. 
     FIG. 15B is a graph showing the relation between the radial tilt and the tilt error signal according to the fifth embodiment. 
     FIG. 16A is a graph showing the waveforms of the multiplication signals according to the sixth embodiment. 
     FIG. 16B is a graph showing the relation between the radial tilt and the tilt error signal according to the sixth embodiment. 
     FIG. 17 is a block diagram showing the configuration the servo signal generation circuit of the seventh embodiment. 
     FIG. 18A is a graph showing the relation between the radial tilt and the tilt error signal according to the seventh embodiment. 
     FIG. 18B is a graph showing the relation between the radial tilt and the tilt error signal according to the eighth embodiment. 
     FIG. 19 is a block diagram showing the overall configuration of the information reproduction apparatus according to the ninth embodiment. 
     FIG. 20 is a block diagram showing the configuration of the servo signal generation circuit of the ninth embodiment. 
     FIG. 21A is a graph showing the waveforms of the subtraction signals according to the ninth embodiment. 
     FIG. 21B is a graph showing the relation between the radial tilt and the tilt error signal according to the ninth embodiment. 
     FIG. 22 is a block diagram showing the configuration the servo signal generation circuit of the tenth embodiment. 
     FIG. 23A is a graph showing the waveforms of the multiplication signals according to the tenth embodiment. 
     FIG. 23B is a graph showing the relation between the radial tilt and the tilt error signal according to the tenth embodiment. 
     FIG. 24 is a block diagram showing the configuration of the servo signal generation circuit of the eleventh embodiment. 
     FIG. 25 is a block diagram showing the configuration of the servo signal generation circuit of the twelfth embodiment. 
     FIG. 26A is a graph showing the relation between the radial tilt and the tilt error signal according to the eleventh embodiment. 
     FIG. 26B is a graph showing the relation between the radial tilt and the tilt error signal according to the twelfth embodiment. 
     FIG. 27 is a block diagram showing the configuration of the servo signal generation circuit of the thirteenth embodiment. 
     FIG. 28A is a graph showing the waveforms of the subtraction signals according to the thirteenth embodiment. 
     FIG. 28B is a graph showing the relation between the radial tilt and the tilt error signal according to the thirteenth embodiment. 
     FIG. 29 is a block diagram showing the configuration of the servo signal generation circuit of the fourteenth embodiment. 
     FIG. 30A is a graph showing the waveforms of the multiplication signals according to the fourteenth embodiment. 
     FIG. 30B is a graph showing the relation between the radial tilt and the tilt error signal according to the fourteenth embodiment. 
     FIG. 31 is a block diagram showing the configuration of the servo signal generation circuit of the fifteenth embodiment. 
     FIG. 32 is a block diagram showing the configuration of the servo signal generation circuit of the sixteenth embodiment. 
     FIG. 33A is a graph showing the relation between the radial tilt and the tilt error signal according to the fifteenth embodiment. 
     FIG. 33B is a graph showing the relation between the radial tilt and the tilt error signal according to the sixteenth embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention will now be described below with reference to the attached drawings. It is noted that the following embodiments are directed to the case where the present invention is applied to an information reproduction apparatus which reproduces digital information recorded on an information recording surface of a DVD-RAM in the form of pits. 
     [I] DVD-RAM 
     Prior to the description of the configuration of the information reproduction apparatus, the recording format adopted in the DVD-RAM according to the present invention will be described with reference to FIGS. 1 to  3 . A DVD-RAM has multiple zones divided in its radial direction. FIG. 1 is a plan view showing the track structure included in one of those zones, FIG. 2 is a magnified view of a part of the structure shown in FIG. 1, and FIG. 3 is a plan view showing a detailed structure of a header area described later. 
     As shown in FIG. 1, the DVD-RAM  1  adopts a so-called land/groove (L/G) recording system in which digital information is recorded on a groove track  1 G and a land track  1 L, both being formed in advance. In FIG. 1, the land track  1 L is shown as the hatched portion while the groove track  1 G is shown as the remaining white portion. In the DVD-RAM  1 , a single land track  1 L and a single groove track  1 G are spirally formed with the header areas S 0  to S 7  being formed in the radial direction. The disc of this type is generally called as “Single Spiral-Land/Groove (SS-L/G)” recording system. The details of the SS-L/G recording system is disclosed, for example, in “THE ACCESS SYSTEM for SINGLE SPIRAL-LAND/GROOVE RECORDING, Nakano et al., SINGAKU TECHNICAL REPORT OF IEICE, MR95-88, CPM95-126(1996-02), ELECTRONIC INFORMATION COMMUNICATION INSTITUTION”. Each of the land track  1 L and the groove track  1 G is partitioned into sectors, which are the predetermined information unit of the digital information to be recorded. As shown in FIG. 1, at the partitioning points, there are provided the header areas S 0  to S 7  carrying address information substantially indicating the recording position of digital information on the DVD-RAM  1  such as the sector number or the physical recording position on the DVD-RAM  1  of the following and/or proceeding sector on the land track  1 L or the groove track  1 G. The headers S 0  to S 7  are arranged with the same interval angle therebetween such that one header is located within one zone, as shown in FIG.  1 . Reproducing the digital information recorded on the land track  1 L or the groove track  1 G between two neighboring header areas takes identical time period for all areas sandwiched by the header areas S 0  to S 7 . 
     Next, the description will be given of the detailed configuration of the header. areas S 0  to S 7  by referring to FIGS. 2 and 3. FIG. 2 is a magnified view of the portions around the header area S 0  and the header area S 1 . As shown, since the land track  1 L and the groove track  1 G have the single spiral form, the relative positions in the track direction of the land track  1 L and the groove track  1 G shift in the radial direction so that the land track  1 L on the right side of the header area S 0  is on the extension line of the groove track  1 G on the left side of the header area S 0  and the groove track  1 G on the right side of the header area S 0  is on the extension line of the land track  1 L on the left side of the header area S 0 . It is noted that, unlike the header area S 0 , the relative positions of the land track  1 L and the groove track  1 G do not change suddenly around other header areas S 1  to S 7 . 
     As shown in FIG. 2, each of the header areas S 0  and S 1  includes the header portions  10  and  11  on which the address information is recorded, and the non-recorded portions  112  and  113 . The header portions  10  and  11  and the non-recorded portions  112  and  113  have the same width as the groove track  1 G and the land track  1 L. Each of the header portions  10  and  11  and the non-recorded portions  112  and  113  has the ½ length of the header areas S 0  or S 1  in the track (i.e., rotation) direction of the DVD-RAM  1 . The header portions  10  and  11  and the non-recorded portions  112  and  113  are arranged in the zigzag manner as shown in FIG. 2, each being shifted with respect to the groove track  1 G or the land track  1 L by the length of ½ track in the radial direction of the DVD-RAM  1 . The header portions  10  and  11  have the same structure as the groove track  1 G, for example, and is provided with pit array P indicating the address information corresponding to the respective positions. The non-recorded portions  112  and  113  have the mirror-finished surface having the height at the same level as the surface of the land track  1 L. 
     Supposing a virtual track T 1  from the land track  1 L to the groove track  1 G along the track direction of the DVD-RAM  1 , the header portion  10  is provided in the header area S 0  with being shifted by the length of ½ track in the inner direction of the DVD-RAM  1  from the virtual track T 1 , and the header portion  11  is provided in the header area S 0  with being shifted by the length of ½ track in the outer direction of the DVD-RAM  1  from the virtual track T 1 . Likewise, supposing a virtual track T 2  from the groove track  1 G to the land track L 1  along the track direction, the header  10  is provided in the header area S 0  with being shifted by the length of ½ track in the outer direction of the DVD-RAM  1  from the virtual track T 2 , and the header portion  11  is provided in the header area S 0  with being shifted by the length of ½ track in the inner direction of the DVD-RAM  1  from the virtual track T 2 . 
     On the contrary, with respect to the other header areas S 1  to S 7 , supposing a virtual track T 3  from the land track  1 L to the next land track  1 L along the track direction of the DVD-RAM  1 , the header  10  is provided with being shifted by the length of ½ track in the outer direction of the DVD-RAM  1  from the virtual track T 3 , and the header portion  11  is provided with being shifted by the length of ½ track in the inner direction of the DVD-RAM  1  from the virtual track T 3 . Likewise, supposing a virtual track T 4  from the groove track  1 G to the next groove track  1 G along the track direction, the header  10  is provided with being shifted by the length of ½ track in the inner direction of the DVD-RAM  1  from the virtual track T 4 , and the header portion  11  is provided with being shifted by the length of ½ track in the outer direction of the DVD-RAM  1  from the virtual track T 4 . 
     In this way, the positions of the header portions  10  and  11  are different between the header area S 0  and the other header areas S 1  to S 7 . By this, an information reproduction apparatus described later can recognize whether or not the track changes from the land track  1 L to the groove track  1 G or from the groove track  1 G to the land track  1 L before and after the header area. 
     In FIG. 2, the land track  1 L and the groove track  1 G are formed in a wobbling manner. This wobbling is adopted to record a synchronizing signal for controlling the revolution speed of the spindle motor (described later) which rotates the DVD-RAM  1  at the time of recording/reproducing digital information. Namely, at the time of recording/reproducing digital information, the wobbling is detected and used to generate a reference signal which controls the rotation of the spindle motor. It is noted that such wobbling feature is omitted from the illustration in FIG. 1 for the sake of brevity. When digital information recorded on the DVD-RAM  1  is optically reproduced, the light beam of the laser light is converged to produce a light spot on the DVD-RAM  1 . Then, the light reflected by the DVD-RAM  1  is received by a detector having a light receiving portions divided by a divisional line in parallel with the track direction to produce the reproduction signal. 
     Next, the description will be given of the structure of the control information (including the aforementioned address information) recorded in the respective header portions  10  and  11  with reference to FIG.  3 . FIG. 3 shows the structure of the header portions  10  and  11  within the header area S 0  (in which the land track  1 L and the groove track  1 G are not on the same line in the track direction). As shown in FIG. 3A, the header portion  10  includes the first header portion  10   a  preceding in the rotation direction of the DVD-RAM  1  and the second header portion  10   b  following the first header portion  10   a  in the rotational direction. The header portion  11  includes the third header portion  1   a  preceding in the rotational direction and the fourth header portion  11   b  following the third header portion  11   a  in the rotational direction. The first header portion  10   a  has the same length as the third header portion  11   a,  and the second header portion  10   b  has the same length as the fourth header portion  11   b.  The first header portion  10   a  and the second header portion  10   b  are shifted by the ½ track length in the outer direction of the DVD-RAM  1  with respect to the center line of the groove track  1 G, and the third header portion  11   a  and the fourth header portion  1   b  are shifted by the ½ track length in the inner direction of the DVD-RAM  1  with respect to the center line of the groove track  1 G. 
     Next, the description will be given of the structure of address information recorded, in advance, in the header portions by referring to FIG.  3 B. As shown in FIG. 3B, the first header portion  10   a  includes the first VFO (Variable Frequency Oscillator (channel bit synchronization signal)) data  120  of 36-byte length. The first VFO data  120  includes the pit array P of a constant period used for controlling the revolution speed of the spindle motor (described later) which rotates the DVD-RAM  1 . The pit array P has the period corresponding to 8×T if the unit length of the pits formed on the DVD-RAM  1  is defined as “T”. Namely, in this case, plural pits having the length of 4×T are formed with the interval of the length 4×T therebetween. Further, the first header portion  10   a  includes 3-byte AM (Address Mark) data  121 , 4-byte first PID (Physical Identification Data)  122 , 2-byte first IED (ID Error Detection code) data  123  and 1-byte first PA (Post Amble) data  124 . The AM data  121  is a synchronization signal used to read the following first PID data  122 . The first PID data  122  includes address information (specifically, sector information and sector numbers) indicating the recorded position of the header area S 1  on the DVD-RAM  1 . The first IED data  123  includes error detection codes for detecting the first PID data  122 . The first PA data  124  indicates the end of the first IED data  123 . 
     The second header portion  10   b  includes 8-byte second VFO data  125 , AM data  121 , second PID data  126 , second IED data  127  and second PA data  128 . The second VFO data includes other pit array of a constant period used to control the revolution speed of the spindle motor. The second PID data  126  includes other address information indicating the recording position of the header area S 1  on the DVD-RAM  1 . The second IED data  127  includes error detection codes for detecting the second PID data  126  and the second PA data  128  indicates the end of the second IED data  127 . 
     The third header portion  11   a  includes the first VFO data  120 , the AM data  121 , 4-byte third PID data  129  including still other address information indicating the recording position of the header area S 1  on the DVD-RAM  1 , 2-byte third IED data  130  including error detection codes for detecting the third PID data  129 , and the first PA data  124 . The fourth header portion  11   b  includes the second VFO data  125 , the AM data  121 , 4-byte fourth PID data  131  including still other address information indicating the recording position of the header area S 1  on the DVD-RAM  1 , 2-byte fourth IED data  132  including error detection codes for detecting the fourth PID data  126 , and the second PA data  128 . 
     The same data is recorded at the head of the first header portion  10   a  and at the head of the third header portion  11   a,  and the same data is recorded at the end of the first header portion  10   a  and at the end of the third header portion  11   a.  Likewise, the same data is recorded at the head of the second header portion  10   b  and at the head of the fourth header portion  11   b,  and the same data is recorded at the end of the second header portion  10   b  and at the end of the fourth header portion  11   b.  Since the data recorded at the heads of the first header portion  10   a  and the third header portion  11   a  are the same and the data recorded at the ends of the first header portion  10   a  and the third header portion  11   a  are the same (or, the data recorded at the heads of the second header portion  10   b  and the fourth header portion  11   b  are the same and the data recorded at the ends of the second header portion  10   b  and the fourth header portion  11   b  are the same), the same data can be reproduced at those portions if no radial tilt is taking place. By utilizing this, in the following embodiments, the radial tilt taking place in relation with the DVD-RAM  1  is detected and compensated for. 
     [II] 1st Embodiment of Information Reproduction Apparatus 
     Next, the first embodiment of the information reproduction apparatus according to the present invention will be described with reference to FIGS. 4 to  7 . FIG. 4 is a block diagram showing the schematic configuration of the information reproduction apparatus according to the first embodiment, and FIG. 5 is a plan view showing the detailed configuration of the detector described later. FIG. 6 is a block diagram showing the servo signal generation circuit according to the first embodiment, and FIG. 7 is a waveform diagram explaining the generation of the tilt error signal according to the first embodiment. 
     First of all, the configuration of the information reproduction apparatus according to the first embodiment will be described. As shown in FIG. 4, the information reproduction apparatus SS 1  of the first embodiment includes the spindle motor  2 , the shaft  3 , the arm  4 , the motor  5 , the screw  6 . the pickup  7 , the adder  8 , the subtracter  9 , the reproduction circuit  110 , the tracking servo circuit  111 , the servo signal generation circuit  12 , and the driver  13 . The pickup  7  includes a laser diode (not shown), the detector D divided into two half-detectors  7   a  and  7   b  by a divisional line in parallel with the rotation direction of the DVD-RAM  1  as shown in FIG. 5, a polarization beam splitter (not shown), and an objective lens (not shown). As shown in FIG. 6, the servo signal generation circuit  12  includes the delay circuit  12   a,  the subtracter  12   b,  and the error signal generation circuit  12   c.  The error signal generation circuit  12   c  may be designed as an averaging circuit or the like. It is noted that FIG. 4 shows only the components related to the present invention. In practice, the information reproduction apparatus SS 1  includes servo control circuits such as a so-called focus servo control circuit and a so-called spindle servo control circuit in addition to the components shown in FIG.  4 . 
     Next, the operation of the information reproduction apparatus SS 1  will be described with reference to FIG. 4 to  7 . First, the spindle motor  2  rotates the DVD-RAM  1  at a given revolution speed. Simultaneously, the pickup  7  irradiates the light beam B for information reproduction onto the DVD-RAM  1  at the position where information to be reproduced is recorded, and receives the reflected light by the detector D. All optical parts in the pickup  7  are arranged such that the reflected light produces the spot range SP on the detector D as shown in FIG.  5 . The half-detectors  7   a  and  7   b  receive the reflected light and generate the received light signals Spr and Spl corresponding to the received light quantities, respectively. In addition, the pickup  7  is designed so as to be transferred on the arm  4  in the radial direction of the DVD-RAM  1  under the control of a carriage servo circuit (not shown), and the arm  4  is arranged to swing about the shaft  3  in the up-down direction indicated by the arrow in FIG.  4 . The other end of the arm  4  moves up and down in the direction of the arrow by rotating the screw  6 . With this arrangement, when the motor  5  is driven by a drive signal Sd (described later) to rotate the screw  6 , the arm  4  swings in the direction of the arrow together with the pickup  7 . By this movement, the radial tilt existing between the optical axis of the light beam B and the information recording surface of the DVD-RAM  1  is compensated for. 
     The adder  8  adds the received light signals Spr and Spl to each other to generate the detection signal Ss corresponding to information to be reproduced. Then, the adder  8  supplies the detection signal Ss to the reproduction circuit  110  and the servo signal generation circuit  12 . The reproduction circuit  110  applies amplification and demodulation onto the detection signal Ss to generate the reproduction signal Spu corresponding to information to be reproduced, and supplies the reproduction signal Spu to a display and/or speakers (not shown). Simultaneously, the servo signal generation circuit  12  detects the radial tilt, which is now taking place between the optical axis of the light beam B and the information recording surface of the DVD-RAM  1 , by using the detection signal Ss according to the detection processing described later. Then, the servo signal generation circuit  12  generate the tilt error signal Ste which indicates the quantity and the direction (i.e., in which direction the information recording surface is tilting with respect to the optical axis) of the detected radial tilt, and supplies the tilt error signal Ste to the driver  13 . The driver  13  generates the drive signal Sd, which is used to compensate for the radial tilt now taking place, based on the tilt error signal Ste, and supplies it to the motor  5  so that the drive signal Sd drives the motor  5  to compensate for the radial tilt. 
     The subtracter  9  subtracts the received light signal Spl from the received light signal Spr to generate the push-pull signal Spp, which is a tracking error signal according to so-called push-pull method, and supplies it to the tracking servo circuit  111 . The tracking servo circuit  111  generates the tracking drive signal Sdt which is used to compensate for the deviation in tracking direction of the position of the light beam B on the DVD-RAM  1  indicated by the push-pull signal Spp. Then, the tracking servo circuit  111  supplies the tracking drive signal Sdt to an actuator (not shown) provided within the pickup  7  to perform tracking servo control. Namely, the actuator moves the objective lens in the tracking direction of the DVD-RAM  1  based on the tracking drive signal Sdt to control the position of the light beam B. 
     Next, the description will be given of how the servo signal generation circuit  12  of the present invention generates the tilt error signal Ste with reference to FIGS. 6 and 7. As shown in FIG. 6, the detection signal Ss inputted to the servo signal generation circuit  12  is supplied to both the delay circuit  12   a  and the negative input terminal of the subtracter  12   b.  The delay circuit  12   a  detects the detection signal Ss corresponding to the first VFO data  120  within the header portion  10 . Then, the delay circuit  12   a  delays the detection signal Ss for a time period in which the light beam B irradiated on the DVD-RAM  1  passes through the header portion  10  shown in FIG. 3 (namely, the time period required to detect all of the 64-byte data included in the header portion  10 ) to produce the delayed signal Sds, and supplied it to the positive input terminal of the subtracter  12   b.  By this, the detection signal Ss corresponding to the first VFO data  120  in the header portion  10  and the detection signal Ss corresponding to the first VFO data  120  in the header portion  11  are simultaneously supplied to the positive input terminal and the negative input terminal of the subtracter  12   b,  respectively. One method for detecting the detection signal Ss corresponding to the first VFO data  120  in the header portion  10  from the inputted detection signal Ss is as follows. In the DVD-RAM  1 , the header area S 0  to S 7  are periodically arranged on the DVD-RAM  1  as shown in FIG.  1 . Hence, the detection signal Ss corresponding to the first VFO data  120  in the header portion  10  may be detected by detecting the period of the header area arrangement in relation to the revolution speed of the DVD-RAM  1 . 
     The subtracter  12   b  subtracts the detection signal Ss corresponding to the first VFO data  120  in the header portion  11  from the delay signal Sds (i.e., the detection signal Ss corresponding to the first VFO data  120  in the header portion  10 ) to generate the subtraction signal Sm, and supplies it to the error signal generation circuit  12   c.  The actual waveform of the subtraction signal Sm will be described with reference to FIG.  7 A. FIG. 7A shows the waveforms of the subtraction signals Sm generated when the radial tilt are 0.9 degree (hereinafter indicated as “deg”), 0.45 deg., 0 deg. (i.e., no radial tilt existing), −0.45 deg., −0.9 deg. In addition, FIG. 7A shows the waveforms detected within a time period corresponding to a part of the first VFO data  120  in the header portion  10  or a part of the first VFO data  120  in the header portion  11 . Therefore, in FIG. 7A, one period of each subtraction signal Sm corresponds to 8×T. As shown in FIG. 7A, when the quantity and the direction of the radial tilt occurring in relation with the DVD-RAM  1  change, the level and the waveform of the subtraction signal Sm change correspondingly. Specifically, when the radial tilt is 0 deg., the level of the subtraction signal Sm becomes zero level. As the absolute value of the radial tilt increases, the level of the subtraction signal Sm increases. If the radial tilt changes such that its direction remains the same but its absolute value changes, the subtraction signal Sm of the same polarity and different level is generated. 
     The reason why the subtraction signal Sm changes when the quantity and the direction of the radial tilt changes will be described below. As already mentioned, within the first VFO data  120  in the header portion  10  and the first VFO data  120  in the header portion  11 , the same periodical signal is recorded. The periodical signal has a constant period and is used to extract the reference clock signal to control the revolution speed of the spindle motor  2 . The header portions  10  and  11  are formed in a manner being shifted by the ½ track length in the opposite directions along the radial direction of the DVD-RAM  1  with respect to the center line of the groove track  1 G, for example. Therefore, supposing that no radial tilt is occurring in relation to the DVD-RAM  1 , the quantity of light that the half-detector  7   a  receives from the reflected light beam B reflected by the first VFO data  120  in the header portion  10  and the quantity of light that the half-detector  7   b  receives from the reflected light beam B reflected by the first VFO data in the header portion  11  are equal to each other, and hence the detection signals Ss outputted at respective timings have completely the same waveform. The subtraction signal Sm is produced by subtracting the non-delayed detection signal Ss (i.e., the detection signal Ss corresponding to the first VFO data  120  in the header  11 ) from the detection signal Ss delayed by the time period corresponding to the header portion  10  by means of the delay circuit  12   a,  and hence the subtraction signal Sm has zero level. However, if there is radial tilt in either direction in relation with the DVD-RAM  1 , the quantity of light that the half-detector  7   a  receives from the reflected light beam B reflected by the first VFO data  120  in the header portion  10  and the quantity of light that the half-detector  7   b  receives from the reflected light beam B reflected by the first VFO data  120  in the header portion  11  are different from each other. This difference results from the fact that the optical path of the light beam reflected by the DVD-RAM  1  changes due to the tilt of the DVD-RAM  1 . Hence, the detection signals Ss outputted at respective timings have different waveforms from each other, and the difference varies in correspondence with the quantity and the direction of the radial tilt occurring. Thus, the subtraction signal Sm, which is produced by subtracting the non-delayed detection signal Ss from the detection signal Ss delayed by the time period corresponding to the header portion  10 , has different level and polarity in correspondence with the quantity and the direction of the existing radial tilt, respectively. 
     Next, the error signal generation circuit  12   c  averages the subtraction signal Sm, having different level and polarity corresponding to the quantity and the direction of the radial tilt, using a predetermined averaging time period to extract the Direct Current (hereinafter referred to as “D.C.”) component of the subtraction signal Sm. The level and the polarity of the D.C. component thus extracted changes in correspondence with the change of the level and the polarity of the subtraction signal Sm. Then, the error signal generation circuit  12   c  supplies the D.C. component, which level and polarity change in correspondence with the change of the subtraction signal Sm, to the driver  13  as the tilt error signal Ste. As shown in FIG. 7B, the level and the polarity of the tilt error signal thus generated have a linear function relation with the quantity and the direction of the existing radial tilt. Thus, by generating the drive signal Sd based on the tilt error signal Ste, it is possible to generate the drive signal Sd with which the existing radial tilt can be reliably compensated for. The above mentioned predetermined averaging time period is set to be long enough, compared with the one revolution time period of DVD-RAM  1 , to remove the influence by the off-track component generated by the deviation of tracking of the light beam B that occurs due to the eccentricity of the DVD-RAM  1  itself (if this off-track exists, the subtraction signal does not become zero even if no radial tilt exists.). The reason is as follows. The off-track component has the period of one revolution of the DVD-RAM  1  while the radial tilt keeps on existing during plural revolutions of the DVD-RAM  1 . Therefore, by averaging the subtraction signal Sm in the time period sufficiently longer than the one revolution time period of the DVD-RAM  1 , the D.C. component changing only due to the radial tilt can be extracted as the tilt error signal Ste. Thereafter, as described above, the drive signal Sd is generated based on the tilt error signal Ste thus produced, and the motor  5  is driven by the drive signal Sd to compensate for the existing radial tilt. 
     As described above, according to the radial tilt compensation performed by the information reproduction apparatus SS 1  of the first embodiment, the reflected light beam B from the first VFO data  120  in the header portion  10  and the first VFO data  120  in the header portion  11 , which are formed in a shifted manner in the opposite directions along the radial direction of the DVD-RAM  1  with respect to the center line of the groove track  1 G, are received, and then the radial tilt is detected based on the reflected lights thus received. Therefore, the radial tilt may be detected without separately providing a dedicated radial tilt sensor. Further, since the D.C. component in the subtraction signal Sm is extracted as the tilt error signal Ste indicative of the radial tilt, the quantity and the direction of the radial tilt may be accurately detected. Still further, since the subtraction signal Sm is averaged using the averaging time period corresponding to the revolution speed of the DVD-RAM  1  to extract the D.C. component, the quantity and the direction of the radial tilt may be reliably detected. As a result, the radial tilt existing in reproduction of information recorded on the DVD-RAM  1  may be detected and compensated for, without separately providing a dedicated tilt sensor, thereby enabling accurate information reproduction. 
     It is noted that the above described first embodiment is directed to an example in which the error signal generation circuit  12   c  is an averaging circuit for averaging the subtraction signal Sm with the averaging time period. Alternatively, the error signal generation circuit  12   c  may be a low-pass filter which has a cut-off frequency sufficiently lower than the revolution period of the DVD-RAM  1 . With this alternative arrangement, the error signal generation circuit  12   c  may be the low-pass filter of simple configuration, and hence the quantity and the direction of the radial tilt can be accurately detected with simple configuration. 
     [III] 2nd Embodiment of Information Reproduction Apparatus 
     Next, the second embodiment of the information reproduction apparatus will be described with reference to FIGS. 8,  9 A and  9 B. FIG. 8 is a block diagram showing the configuration of the servo signal generation circuit of the second embodiment, and FIG. 9A shows the waveforms for explaining the generation of the tilt error signal according to the second embodiment. The information reproduction apparatus of the second embodiment differs, in configuration, from that of the first embodiment only in the configuration of the servo signal generation circuit, and other configuration is the same as that of the first embodiment. Therefore, the same components are indicated by the same reference numerals and their description will be omitted. 
     In the first embodiment, the detection signal Ss corresponding to the first VFO data  120  in the header portion  11  is subtracted from the detection signal Ss corresponding to the first VFO data  120  in the header portion  10  to generate the subtraction signal Sm, and then the error signal generation circuit  12   c  generates the tilt error signal Ste based on the subtraction signal Sm. In contrast, in the second embodiment, other operation is applied to the detection signal Ss corresponding to the first VFO data  120  in the header portion  10  and the detection signal Ss corresponding to the first VFO data  120  in the header portion  11 , and the tilt error signal Ste is generated based on the operation result. Namely, as shown in FIG. 8, the servo signal generation circuit  20  includes the delay circuit  12   a  and the error signal generation circuit  12   c,  which have the same functions as those in the first embodiment, the subtracter  20   a,  the adder  20   b,  and the multiplier  20   c.  The subtracter  20   a  subtracts the non-delayed detection signal Ss from the delayed signal Sds outputted by the delay circuit  12   a  to generate the subtraction signal Sg, and supplies it to the multiplier  20   c.  The adder  20   b  adds the delay signal Sds to the non-delayed detection signal Ss to generate the addition signal Su, and supplies it to the multiplier  20   c.  The multiplier  20   c  multiplies the subtraction signal Sg by the addition signal Su to generate the multiplication signal Sk, and supplies it to the error signal generation circuit  12   c.  The error signal generation circuit  12   c  may be the averaging circuit or the low-pass filter like the case of the first embodiment, and extracts the D.C. component of the multiplication signal Sk to generate the tilt error signal Ste, and supplies it to the driver  13 . 
     Next, the actual waveform of the multiplication signal Sk will be described with reference to FIG.  9 A. FIG. 9A show the waveforms of the multiplication signal Sk generated when the radial tilt is 0.9 deg., 0.45 deg., 0 deg., −0.45 deg., −0.9 deg., respectively, similarly the case of the first embodiment shown in FIG. 7A, and one period of the respective multiplication signal Sk corresponds to 8×T. As seen in FIG. 9A, when the quantity and the direction of the radial tilt occurring in relation with the DVD-RAM  1  change, the level and the waveform of the multiplication signal Sk changes, similarly to the subtraction signal Sm of the first embodiment, and the tendency or characteristic of the change is the same as that of the subtraction signal Sm in the first embodiment. The reason why the multiplication signal Sk changes as shown in FIG. 9A when the quantity and the direction of the radial tilt change is identical to the case of the first embodiment. Namely, the first VFO data  120  in the header portion  10  and the first VFO data  120  in the header portion  11  are formed in a manner being shifted by ½ track length in the opposite directions along the radial direction of the DVD-RAM  1  with respect to the center line of the groove track  1 G, and hence the received light quantity of the reflected light beam B from the respective first VFO data  120  changes in accordance with the quantity and the direction of the radial tilt. 
     As shown in FIG. 9B, the relation of the level and the polarity of the tilt error signal Ste obtained by extracting the D.C. component of the respective multiplication signals Sk by means of the error signal generation circuit  12   c  with respect to the quantity and the direction of the radial tilt is substantially the linear function, like the case of the first embodiment. Thus, by generating the drive signal Sd based on the tilt error signal Ste, the reliable drive signal Sd can be generated and the existing radial tilt can be accurately compensated for by driving the motor  5  using the drive signal Sd. As described above, according to the radial tilt compensation performed by the information reproduction apparatus of the second embodiment, the advantageous effect similar to that in the first embodiment can be achieved. 
     [IV] 3rd Embodiment of the Information Reproduction Apparatus 
     Next, the third embodiment of the information reproduction apparatus according to the present invention will be described with reference to FIGS. 10 and 12B. FIG. 10 is a block diagram showing the configuration of the servo signal generation circuit according to the third embodiment, and FIG. 12A is a graph showing the relation of the level and the polarity of the tilt error signal with respect to the quantity and the direction of the radial tilt. The information reproduction apparatus of the third embodiment differs, in configuration, from that of the first embodiment only in the configuration of the servo signal generation circuit, and other configuration is the same as that of the first embodiment. Therefore, the same components are indicated by the same reference numerals and their description will be omitted. 
     In the first embodiment, the detection signal Ss corresponding to the first VFO data  120  in the header portion  11  is subtracted from the detection signal Ss corresponding to the first VFO data  120  in the header portion  10  to generate the subtraction signal Sm, and then the error signal generation circuit  12   c  generates the tilt error signal Ste based on the subtraction signal Sm. In contrast, in the third embodiment, the amplitude of the subtraction signal Sm is detected, and the error signal generation circuit  12   c  generates the tilt error signal Ste based on the change of the amplitude thus detected. Namely, as shown in FIG. 10, the servo signal generation circuit  21  of the third embodiment includes the delay circuit  12   a,  subtracter  12   b  and the error signal generation circuit  12   c,  which have the same functions as those in the first embodiment, and the amplitude detection circuit  21   a.  The amplitude detection circuit  21   a  detects the amplitude of the subtraction signal Sm (having completely the same waveform as in the case of first embodiment shown in FIG. 7A) corresponding to the radial tilt and outputted by the subtracter  12   b,  and supplies the detected amplitude to the error signal generation circuit  12   c  as the amplitude signal Sa. The error signal generation circuit  12   c  may be the averaging circuit or the low-pass filter like the case of the first embodiment, and extracts the D.C. component of the amplitude signal Sa to generate the tilt error signal Ste, and supplies it to the driver  13 . 
     As shown in FIG. 12A, the relation of the level and the polarity of the tilt error signal Ste obtained by extracting the D.C. component of the respective amplitude signals Sa by means of the error signal generation circuit  12   c  with respect to the quantity and the direction of the radial tilt is substantially the linear function, like the case of the previous embodiments. Thus, by generating the drive signal Sd based on the tilt error signal Ste, the reliable drive signal Sd can be generated and the existing radial tilt can be accurately compensated for by driving the motor  5  using the drive signal Sd. As described above, according to the radial tilt compensation performed by the information reproduction apparatus of the third embodiment, the advantageous effect similar to that in the first embodiment can be achieved. 
     [V] 4th Embodiment of Information Reproduction Apparatus 
     Next, the fourth embodiment of the information reproduction apparatus according to the present invention will be described with reference to FIGS. 11 and 12B. FIG. 11 is a block diagram showing the configuration of the servo signal generation circuit of the fourth embodiment, and FIG. 12B is a graph showing the relation of the level and the polarity of the tilt error signal with respect to the quantity and the direction of the radial tilt. 
     The information reproduction apparatus of the fourth embodiment differs, in configuration, from that of the first embodiment only in the configuration of the servo signal generation circuit, and other configuration is the same as that of the first embodiment. Therefore, the same components are indicated by the same reference numerals and their description will be omitted. 
     In the second embodiment, the detection signal Ss corresponding to the first VFO data  120  in the header portion  11  is subtracted from the detection signal Ss corresponding to the first VFO data  120  in the header portion  10  to generate the subtraction signal Sg, and those detection signals Ss are added to each other to produce the addition signal Su. Then, the D.C. component in the multiplication signal Sk, obtained by multiplying the subtraction signal Sg by the addition signal Su, is extracted by the error generation circuit  12   c  and outputted as the tilt error signal Ste. In the fourth embodiment, the amplitude of the multiplication signal Sk is detected, and the error signal generation circuit  12   c  generates the tilt error signal Ste based on the change of the amplitude thus detected. 
     Namely, as shown in FIG. 11, the servo signal generation circuit  20  includes the delay circuit  12   a,  the subtraction circuit  20   a,  the adder  20   b,  the multiplier  20   c  and the error signal generation circuit  12   c,  which have the same functions as those in the second embodiment, respectively. The servo signal generation circuit  22  further includes the amplitude detection circuit  22   a  which extracts the amplitude of the multiplication signals Sk corresponding to the radial tilt and outputted by the multiplier  12   c,  and supplies it to the error signal generation circuit  12   c.  The error signal generation circuit  12   c  extracts the D.C. component of the amplitude signal Sa for the respective radial tilts, similarly to the case of the second embodiment, to produce the tilt error signal Ste, and supplies it to the driver  13 . 
     As shown in FIG. 12B, the relation of the level and the polarity of the tilt error signal Ste obtained by extracting the D.C. component of the respective amplitude signals Sa by means of the error signal generation circuit  12   c  with respect to the quantity and the direction of the radial tilt is substantially the linear function in the range from −0.45 deg. to +0.45 deg., like the case of the previous embodiments. Thus, by generating the drive signal Sd based on the tilt error signal Ste, the reliable drive signal Sd can be generated and the existing radial tilt can be accurately compensated for by driving the motor  5  using the drive signal Sd. As described above, according to the radial tilt compensation performed by the information reproduction apparatus of the fourth embodiment, the amplitude of the multiplication signal Sk is detected and the tilt error signal Ste is produced based on the amplitude thus detected. Therefore, the advantageous effect similar to that in the first embodiment can be achieved. 
     [VI] 5th Embodiment of Information Reproduction Apparatus 
     Next, the fifth embodiment of the information reproduction apparatus according to the present invention will be described with reference to FIGS. 13 to  15 . FIG. 13 is a block diagram showing the schematic configuration of the information reproduction apparatus of the fifth embodiment, FIG. 14 is a block diagram showing the configuration of the servo signal generation circuit of the fifth embodiment, and FIGS. 15A and 15B are graphs for explaining the generation of the tilt error signal. 
     First, the configuration of the information reproduction apparatus of the fifth embodiment will be described. As shown in FIG. 13, the information reproduction apparatus SS 5  of the fifth embodiment includes the same components as those of the first embodiment except for the servo signal generation circuit  23 . Further, the information reproduction apparatus SS 5  is different from that of the first embodiment in that the push-pull signal Spp outputted by the subtracter  9  is supplied to the servo signal generation circuit  23 . In FIGS. 13 and 14, the same components as those in the information reproduction apparatus SS 1  are indicated by the same reference numerals and the detailed description therefor will be omitted. The servo signal generation circuit  23  includes, as shown in FIG. 14, the delay circuit  12   a  and the error signal generation circuit  12   c,  which are the same as those in the first embodiment, and the adder  23   a.    
     Next, the operation of the servo signal generation circuit  23  according to the fifth embodiment will be described with reference to FIGS. 14 and 15. As shown in FIG. 14, the servo signal generation circuit  23  detects the radial tilt presently occurring by the later-described processing using the push-pull signal Spp, generates the tilt error signal Ste indicating the quantity and the direction of the radial tilt, and supplies it to the driver  13 . Specifically, the push-pull signal Spp inputted to the servo signal generation circuit  23  is supplied to one input terminal of the adder  23   a  and the delay circuit  12   a.  The delay circuit  12   a  delays the push-pull signal Spp corresponding to the first VFO data  120  in the header portion  10  for the delay time period by which the irradiated range of the light beam B on the DVD-RAM  1  passes through the region of the header portion  10  shown in FIG. 3 according to the same delaying operation as that of the first embodiment, and supplies it to the other input terminal of the adder  23   a  as the delay signal Sdpp. By this, the push-pull signal Spp corresponding to the first VFO data  120  in the header portion  10  and the push-pull signal Spp corresponding to the first VFO data  120  in the header portion  11  are simultaneously supplied to the input terminals of the adder  23   a,  respectively. The method of detecting the push-pull signal Spp corresponding to the first VFO data  120  in the header portion  10  from the inputted push-pull signal Spp may be the same as that employed in the first embodiment. The adder  23   a  adds the delay signal Sdpp (i.e., the push-pull signal corresponding to the first VFO data  120  in the header  10 ) to the push-pull signal Spp corresponding to the VFO data  120  in the header portion  11  to generate the addition signal Supp, and supplies it to the error signal generation circuit  12   c.    
     The actual waveform of the addition signal Supp will be described with reference to FIG.  15 A. It is noted that FIG. 15A shows the waveforms of the addition signal Supp when the radial tilt is 0.9 deg., 0.45 deg., 0 deg., −0.45 deg., −0.9 deg., respectively, like the cases of the previous embodiments. In addition, one period in each waveform corresponds to 8×T. 
     As seen in FIG. 15A, when the quantity and the direction of the radial tilt occurring in relation with the DVD-RAM  1  change, the level and the waveform of the addition signal Supp change, similarly to the subtraction signal Sm of the first embodiment, and the tendency or characteristic of the change is the same as that of the subtraction signal Sm in the first embodiment. The reason why the addition signal Supp changes as shown in FIG. 15A when the quantity and the direction of the radial tilt changes is identical to the case of the first embodiment. Namely, the first VFO data  120  in the header portion  10  and the first VFO data  120  in the header portion  11  are formed in a manner being shifted by ½ track length in the opposite directions along the radial direction of the DVD-RAM  1  with respect to the center line of the groove track  1 G, and hence the received light quantity of the reflected light beam B from the respective first VFO data  120  changes in accordance with the quantity and the direction of the radial tilt. 
     As shown in FIG. 15B, the relation of the level and the polarity of the tilt error signal Ste obtained by extracting the D.C. component of the respective addition signals Supp by means of the error signal generation circuit  12   c  with respect to the quantity and the direction of the radial tilt is substantially the linear function like the case of the first embodiment. Thus, by generating the drive signal Sd based on the tilt error signal Ste, the reliable drive signal Sd can be generated and the existing radial tilt can be accurately compensated for by driving the motor  5  using the drive signal Sd. As described above, according to the radial tilt compensation performed by the information reproduction apparatus of the fifth embodiment, the advantageous effect similar to that in the first embodiment can be achieved. 
     [VII] 6th Embodiment of Information Reproduction Apparatus 
     Next, the sixth embodiment of the information reproduction apparatus according to the present invention will be described with reference to FIGS. 16A and 16B. FIG. 16A shows the waveforms for explaining the generation of the tilt error signal according to the sixth embodiment. The information reproduction apparatus of the sixth embodiment differs, in configuration, from that of the fifth embodiment only in the configuration of the servo signal generation circuit, and other configuration is the same as that of the fifth embodiment. Therefore, the same components are indicated by the same reference numerals and their description will be omitted. 
     In the fifth embodiment, the push-pull signal Spp corresponding to the first VFO data  120  in the header portion  10  is added to the push-pull signal Spp corresponding to the first VFO data  120  in the header portion  11  to generate the addition signal Supp, and then the error signal generation circuit  12   c  generates the tilt error signal Ste based on the addition signal Supp. In contrast, in the sixth embodiment, other operation is applied to the push-pull signal Spp corresponding to the first VFO data  120  in the header portion  10  and the push-pull signal Spp corresponding to the first VFO data  120  in the header portion  11 , and the tilt error signal Ste is generated based on the operation result. The configuration of the servo signal generation circuit according to the sixth embodiment is obtained by replacing the detection signal Ss inputted in the configuration of the servo signal generation circuit of the second embodiment with the above-mentioned push-pull signal Spp, and hence the configuration will be described below by referring to FIG. 8 to simplify the illustration. 
     Namely, the servo signal generation circuit of the sixth embodiment includes the delay circuit  12   a  and the error signal generation circuit  12   c,  which have the same functions as those in the fifth embodiment, the subtracter, the adder, and the multiplier. The subtracter subtracts the non-delayed push-pull signal Spp from the delayed signal of the sixth embodiment (i.e., the push-pull signal Spp delayed by the delay circuit  12   a ) outputted by the delay circuit  12   a  to generate the subtraction signal of the sixth embodiment, and supplies it to the multiplier. The adder adds the delay signal of the sixth embodiment to the non-delayed push-pull signal to generate the addition signal of the sixth embodiment, and supplies it to the multiplier. The multiplier multiplies the subtraction signal of the sixth embodiment by the addition signal of the sixth embodiment to generate the multiplication signal of the sixth embodiment, and supplies it to the error signal generation circuit  12   c.  The error signal generation circuit  12   c  may be the averaging circuit or the low-pass filter like the case of the fifth embodiment, and extracts the D.C. component of the multiplication signal of the sixth embodiment to generate the tilt error signal Ste, and supplies it to the driver  13 . 
     Next, the actual waveform of the multiplication signal of the sixth embodiment will be described with reference to FIG.  16 A. FIG. 16A shows the waveforms of the multiplication signals of the sixth embodiment generated when the radial tilt is 0.9 deg., 0.45 deg., 0 deg., −0.45 deg., −0.9 deg., respectively, similarly the case of the previous embodiments, and one period of the respective multiplication signal corresponds to 8×T. As seen in FIG. 16A, when the quantity and the direction of the radial tilt occurring in relation with the DVD-RAM  1  change, the level and the waveform of the multiplication signal of the sixth embodiment change, similarly to the cases in the previous embodiments, and the tendency or characteristic of the change is the same as that of the addition signal Supp in the fifth embodiment. The reason why the multiplication signal changes as shown in FIG. 16A when the quantity and the direction of the radial tilt changes is identical to the case of the fifth embodiment. Namely, the first VFO data  120  in the header portion  10  and the first VFO data  120  in the header portion  11  are formed in a manner being shifted by ½ track length in the opposite directions along the radial direction of the DVD-RAM  1  with respect to the center line of the groove track  1 G, and hence the received light quantity of the reflected light beam B from the respective first VFO data  120  changes in accordance with the quantity and the direction of the radial tilt. 
     As shown in FIG. 16B, the relation of the level and the polarity of the tilt error signal Ste obtained by extracting the D.C. component of the multiplication signal of the sixth embodiment by means of the error signal generation circuit  12   c  with respect to the quantity and the direction of the radial tilt is substantially the linear function like the case of the fifth embodiment. Thus, by generating the drive signal Sd based on the tilt error signal Ste, the reliable drive signal Sd can be generated and the existing radial tilt can be accurately compensated for by driving the motor  5  using the drive signal Sd. As described above, according to the radial tilt compensation performed by the information reproduction apparatus of the sixth embodiment, the advantageous effect similar to that in the fifth embodiment can be achieved. 
     [VIII] 7th Embodiment of Information Reproduction Apparatus Next, the seventh embodiment of the information reproduction apparatus according to the present invention will be described with reference to FIGS. 17 and 18A. FIG. 17 is a block diagram showing a servo signal generation circuit according to the seventh embodiment, and FIG. 18A shows the waveforms for explaining the generation of the tilt error signal according to the seventh embodiment. The information reproduction apparatus of the seventh embodiment differs, in configuration, from that of the fifth embodiment only in the configuration of the servo signal generation circuit, and other configuration is the same as that of the fifth embodiment. Therefore, the same components are indicated by the same reference numerals and their description will be omitted. 
     In the fifth embodiment, the push-pull signal Spp corresponding to the first VFO data  120  in the header portion  10  is added to the push-pull signal Spp corresponding to the first VFO data  120  in the header portion  11  to generate the addition signal Supp, and then the error signal generation circuit  12   c  extracts the D.C. component to generate the tilt error signal Ste. In contrast, in the seventh embodiment, the amplitude of the addition signal Supp is detected, and the error signal generation circuit  12   c  generates the tilt error signal Ste based on the amplitude thus detected. As shown in FIG. 17, the servo signal generation circuit  24  of the seventh embodiment includes the delay circuit  12   a,  the adder  23   a  and the error signal generation circuit  12   c,  which have the same functions as those in the fifth embodiment. In addition, the servo signal generation circuit  24  includes the amplitude detection circuit  24   a  which detects the amplitude of the addition signal Supp (having the completely same waveform as that in the case of the fifth embodiment) corresponding to the radial tilt outputted by the adder  23   a,  and supplies it to the error signal generation circuit  12   c  as the amplitude signals Sa. The error signal generation circuit  12   c  may be the averaging circuit or the low-pass filter like the case of the fifth embodiment, and extracts the D.C. component of the amplitude signals Sa to generate the tilt error signal Ste, and supplies it to the driver  13 . 
     As shown in FIG. 18A, the relation of the level and the polarity of the tilt error signal Ste obtained by extracting the D.C. component of the amplitude signal Sa by means of the error signal generation circuit  12   c  with respect to the quantity and the direction of the radial tilt is substantially the linear function relation of reverse polarity, as compared with the previous embodiments. Thus, by generating the drive signal Sd based on the tilt error signal Ste, the reliable drive signal Sd can be generated and the existing radial tilt can be accurately compensated for by driving the motor  5  using the drive signal Sd. As described above, according to the radial tilt compensation performed by the information reproduction apparatus of the seventh embodiment, since the amplitude of the addition signal Supp is detected to generate the tilt error signal Ste, the advantageous effect similar to that in the fifth embodiment can be achieved. 
     [IX] 8th Embodiment of the Information Reproduction Apparatus 
     Next, the eighth embodiment of the information reproduction apparatus according to the present invention will be described with reference to FIG.  18 B. FIG. 18B is a graph for explaining the relation of the level and the polarity of the tilt error signal with respect to the quantity and the direction of the radial tilt according to the eighth embodiment. The information reproduction apparatus of the eighth embodiment differs, in configuration, from that of the fifth embodiment only in the configuration of the servo signal generation circuit, and other configuration is the same as that of the fifth embodiment. Therefore, the same components are indicated by the same reference numerals and their description will be omitted. 
     In the sixth embodiment, the push-pull signal Spp corresponding to the first VFO data  120  in the header portion  11  is subtracted from the push-pull signal Spp corresponding to the first VFO data  120  in the header portion  10  to generate the subtraction signal of the sixth embodiment, those two push-pull signals are added to each other to produce the addition signal of the sixth embodiment, the subtraction signal is multiplied by the addition signal to produce the multiplication signal, and then the error signal generation circuit  12   c  extracts the D.C. component of the multiplication signal to generate the tilt error signal Ste. In contrast, in the eighth embodiment, the amplitude of the multiplication signal is detected, and the error signal generation circuit  12   c  generates the tilt error signal Ste based on the change of the amplitude thus detected. 
     The configuration of the servo signal generation circuit according to the eighth embodiment is obtained by replacing the detection signal Ss inputted in the configuration of the servo signal generation circuit  22  of the fourth embodiment with the push-pull signal Spp, and hence the following description will temporarily refer to the configuration of the circuit  22  shown in FIG.  11 . Namely, the servo signal generation circuit of the eighth embodiment includes the delay circuit  12   a,  the subtracter, the adder, the multiplier, and the error signal generation circuit  12   c,  which have the same functions as those in the fourth embodiment. In addition, the servo signal generation circuit of the eighth embodiment includes the amplitude detection circuit which detects the amplitude of the multiplication signal of the eighth embodiment (having the completely same waveform as that in the case of the sixth embodiment shown in FIG. 16A) corresponding to the radial tilt and outputted by the multiplier, and supplies it to the error signal generation circuit  12   c  as the amplitude signals of the eighth embodiment. The error signal generation circuit  12   c  may be the averaging circuit or the low-pass filter like the case of the sixth embodiment, and extracts the D.C. component of the amplitude signals Sa to generate the tilt error signal Ste, and supplies it to the driver  13 . 
     As shown in FIG. 18B, the relation of the level and the polarity of the tilt error signal Ste obtained by extracting the D.C. component of the respective amplitude signals Sa by means of the error signal generation circuit  12   c  with respect to the quantity and the direction of the radial tilt is substantially the linear function like the previous embodiments. Thus, by generating the drive signal Sd based on the tilt error signal Ste, the reliable drive signal Sd can be generated and the existing radial tilt can be accurately compensated for by driving the motor  5  using the drive signal Sd. As described above, according to the radial tilt compensation performed by the information reproduction apparatus of the eighth embodiment, the amplitude of the multiplication signal of the eighth embodiment is detected thereby to produce the tilt error signal Ste. Therefore, the advantageous effect similar to the radial tilt compensation operation in the fifth embodiment can be obtained. 
     [X] 9th Embodiment of Information Reproduction Apparatus 
     Next, the ninth embodiment of the information reproduction apparatus according to the present invention will be described with reference to FIGS. 19 to  21 . FIG. 19 is a block diagram showing the schematic configuration of the information reproduction apparatus of the ninth embodiment, FIG. 20 is a block diagram showing the configuration of the servo signal generation circuit of the ninth embodiment, and FIGS. 21A and 21B are graphs for explaining the generation of the tilt error signal. 
     First, the configuration of the information reproduction apparatus of the ninth embodiment will be described. As shown in FIG. 19, the information reproduction apparatus SS 9  of the ninth embodiment includes the same components as those of the first embodiment except for the servo signal generation circuit  25 . Further, the information reproduction apparatus SS 9  is different from the first embodiment in that the received light signals Spr and Spl outputted by the half-detectors  7   a  and  7   b  (see. FIG. 5) are directly supplied to the servo signal generation circuit  25 . In FIGS. 19 and 20, the same components as those in the information reproduction apparatus SS 1  are indicated by the same reference numerals and the detailed description therefor will be omitted. 
     The half-detectors are arranged as follows. Namely, the half-detector  7   a  is positioned to receive the reflected light from the first VFO data  120  while the light spot of the light beam B is scanning the first VFO data  120  in the header portion  10 , and hence the received light signal Spr includes a lot of information corresponding to the first VFO data  120  in the header portion  10 . The half-detector  7   b  is positioned to receive the reflected light from the first VFO data  120  while the light spot of the light beam B is scanning the first VFO data  120  in the header portion  11 , and hence the received light signal Spl includes a lot of information corresponding to the first VFO data  120  in the header portion  11 . As shown in FIG. 20, the servo signal generation circuit  25  includes the delay circuit  12   a  and the error signal generation circuit  12   c,  which are identical to those in the first embodiment, and the subtracter  25   a.    
     Next, the operation of the servo signal generation circuit  25  according to the ninth embodiment will be described with reference to FIGS. 20,  21 A and  21 B. As shown in FIG. 20, the servo signal generation circuit  25  detects the radial tilt presently occurring by the later-described processing using the received light signals Spr and Spl, generates the tilt error signal Ste indicating the quantity and the direction of the radial tilt, and supplies it to the driver  13 . The received light signal Spl inputted to the servo signal generation circuit  12  is supplied to the negative-input terminal of the subtracter  25 . The received light signal Spr corresponds to the first VFO data  120  in the header portion  10  and is supplied to the delay circuit  12   a.  The delay circuit  12   a  delays the received light signal Spr thus inputted for the delay time period by which the irradiated range of the light beam B on the DVD-RAM  1  passes through the region of the header portion  10  shown in FIG. 3 according to the same delaying operation as that of the first embodiment, and supplies it to the positive-input terminal of the subtracter  25   a  as the delay signal Sdpr. By this, the received light signals Spr corresponding to the first VFO data  120  in the header portion  10  and the received light signal Spl corresponding to the first VFO data  120  in the header portion  11  are simultaneously supplied to the subtracter  25   a,  respectively. The method of detecting the received light signal Spr corresponding to the first VFO data  120  in the header portion  10  from the inputted received light signal Spr may be the same as the method employed in the first embodiment for the detection signal Ss. The subtracter  25   a  subtracts the received light signal Spl corresponding to the first VFO data  120  in the header portion  11  from the delay signal Sdpr (i.e., the received light signal corresponding to the first VFO data  120  in the header  10 ) to generate the subtraction signal Sm, and supplies it to the error signal generation circuit  12   c.    
     The actual waveform of the subtraction signal Sm will be described with reference to FIG.  21 A. It is noted that FIG. 21A shows the waveforms of the subtraction signal Sm when the radial tilt is 0.9 deg., 0.45 deg., 0 deg., −0.45 deg., −0.9 deg., respectively, like the cases of the previous embodiments. In addition, one period in each waveform corresponds to 8×T. 
     As seen in FIG. 21A, when the quantity and the direction of the radial tilt occurring in relation with the DVD-RAM  1  change, the level and the waveform of the subtraction signal Sm changes, and the tendency or characteristic of the change is the same as that of the subtraction signal Sm in the first embodiment. The reason why the subtraction signal Sm changes as shown in FIG. 21A when the quantity and the direction of the radial tilt changes is identical to the case of the first embodiment. 
     As shown in FIG. 21B, the relation of the level and the polarity of the tilt error signal Ste obtained by extracting the D.C. component of the subtraction signal Sm by means of the error signal generation circuit  12   c  with respect to the quantity and the direction of the radial tilt is substantially the linear function like the case of the first embodiment. Thus, by generating the drive signal Sd based on the tilt error signal Ste, the reliable drive signal Sd can be generated and the existing radial tilt can be accurately compensated for by driving the motor  5  using the drive signal Sd. As described above, according to the radial tilt compensation performed by the information reproduction apparatus SS 9  of the ninth embodiment, the advantageous effect similar to that in the first embodiment can be achieved. 
     [XI] 10th Embodiment of Information Reproduction Apparatus 
     Next, the tenth embodiment of the information reproduction apparatus will be described with reference to FIGS. 22,  23 A and  23 B. FIG. 22 is a block diagram showing the configuration of the servo signal generation circuit of the tenth embodiment, and FIGS. 23A and 23B are graphs for explaining the generation of the tilt error signal according to the tenth embodiment. The information reproduction apparatus of the tenth embodiment differs, in configuration, from that of the ninth embodiment only in the configuration of the servo signal generation circuit, and other configuration is the same as that of the ninth embodiment. Therefore, the same components are indicated by the same reference numerals and their description will be omitted. 
     In the ninth embodiment, the received light signal Spl corresponding to the first VFO data  120  in the header portion  11  is subtracted from the received light signal Spr corresponding to the first VFO data  120  in the header portion  10  to generate the subtraction signal Sm, and then the error signal generation circuit  12   c  generates the tilt error signal Ste based on the subtraction signal Sm. In contrast, in the tenth embodiment, other operation is applied to the received light signal Spr corresponding to the first VFO data  120  in the header portion  10  and the received light signal Spl corresponding to the first VFO data  120  in the header portion  11 , and the tilt error signal Ste is generated based on the operation result. Namely, as shown in FIG. 22, the servo signal generation circuit  26  of the tenth embodiment includes the delay circuit  12   a  and the error signal generation circuit  12   c,  which have the same functions as those in the first embodiment, the subtracter  26   a,  the adder  26   b,  and the multiplier  26   c.  The subtracter  26   a  subtracts the received light signal Spl from the delay signal Sdpr outputted by the delay circuit  12   a  to generate the subtraction signal Sg, and supplies it to the multiplier  26   c.  The adder  26   b  adds the delay signal Sdpr to the received light signal Spl to generate the addition signal Su, and supplies it to the multiplier  26   c.  The multiplier  26   c  multiplies the subtraction signal Sg by the addition signal Su to generate the multiplication signal Sk, and supplies it to the error signal generation circuit  12   c.  The error signal generation circuit  12   c  may be the averaging circuit or the low-pass filter like the case of the ninth embodiment, and extracts the D.C. component of the multiplication signal Sk to generate the tilt error signal Ste, and supplies it to the driver  13 . 
     Next, the actual waveform of the multiplication signal Sk will be described with reference to FIG.  23 A. FIG. 23A shows the waveforms of the multiplication signal Sk generated when the radial tilt is 0.9 deg., 0.45 deg., 0 deg., −0.45 deg., −0.9 deg., respectively, similarly the case of the ninth embodiment, and one period of the respective multiplication signal Sk corresponds to 8×T. As seen in FIG. 23A, when the quantity and the direction of the radial tilt occurring in relation with the DVD-RAM  1  change, the level and the waveform of the multiplication signal Sk change, similarly to the subtraction signal Sm of the ninth embodiment, and the tendency or characteristic of the change is the same as that of the subtraction signal Sm in the ninth embodiment. The reason why the multiplication signal Sk changes as shown in FIG. 23A when the quantity and the direction of the radial tilt change is identical to the case of the subtraction signal Sm in the ninth embodiment. 
     As shown in FIG. 23B, the relation of the level and the polarity of the tilt error signal Ste obtained by extracting the D.C. component of the respective multiplication signals Sk by means of the error signal generation circuit  12   c  with respect to the quantity and the direction of the radial tilt is substantially the linear function like the case of the ninth embodiment. Thus, by generating the drive signal Sd based on the tilt error signal Ste, the reliable drive signal Sd can be generated and the existing radial tilt can be accurately compensated for by driving the motor  5  using the drive signal Sd. As described above, according to the radial tilt compensation performed by the information reproduction apparatus of the tenth embodiment, the advantageous effect similar to that in the ninth embodiment can be achieved. 
     [XII] 11th Embodiment of Information Reproduction Apparatus 
     Next, the eleventh embodiment of the information reproduction apparatus according to the present invention will be described with reference to FIGS. 24 and 26A. FIG. 24 is a block diagram showing the configuration of the servo signal generation circuit according to the eleventh embodiment, and FIG. 26B is a graph showing the relation of the level and the polarity of the tilt error signal with respect to the quantity and the direction of the radial tilt. The information reproduction apparatus of the eleventh embodiment differs, in configuration, from that of the ninth embodiment only in the configuration of the servo signal generation circuit, and other configuration is the same as that of the ninth embodiment. Therefore, the same components are indicated by the same reference numerals and their description will be omitted. 
     In the ninth embodiment, the received light signal Spl corresponding to the first VFO data  120  in the header portion  11  is subtracted from the received light signal Spr corresponding to the first VFO data  120  in the header portion  10  to generate the subtraction signal Sm, and then the error signal generation circuit  12   c  generates the tilt error signal Ste extracts the D.C. component to generate the subtraction signal Sm. In contrast, in the eleventh embodiment, the amplitude of the subtraction signal Sm is detected, and the error signal generation circuit  12   c  generates the tilt error signal Ste based on the change of the amplitude thus detected. Namely, as shown in FIG. 24, the servo signal generation circuit  27  according to the eleventh embodiment includes the delay circuit  12   a,  subtracter  25   a  and the error signal generation circuit  12   c,  which have the same functions as those in the ninth embodiment, and the amplitude detection circuit  27   a.  The amplitude detection circuit  27   a  detects the amplitude of the subtraction signal Sm (having completely the same waveform as in the case of ninth embodiment shown in FIG. 21A) corresponding to the radial tilt and outputted by the subtracter  25   a,  and supplies the detected amplitude to the error signal generation circuit  12   c  as the amplitude signal Sa. The error signal generation circuit  12   c  may be the averaging circuit or the low-pass filter like the case of the ninth embodiment, and extracts the D.C. component of the amplitude signal Sa to generate the tilt error signal Ste, and supplies it to the driver  13 . 
     As shown in FIG. 26A, the relation of the level and the polarity of the tilt error signal Ste obtained by extracting the D.C. component of the amplitude signal Sa by means of the error signal generation circuit  12   c  with respect to the quantity and the direction of the radial tilt is substantially the linear function like the cases of the previous embodiments. Thus, by generating the drive signal Sd based on the tilt error signal Ste, the reliable drive signal Sd can be generated and the existing radial tilt can be accurately compensated for by driving the motor  5  using the drive signal Sd. As described above, according to the radial tilt compensation performed by the information reproduction apparatus of the eleventh embodiment, since the amplitude of the subtraction signal Sm is detected to generate the tilt error signal Ste indicative of the radial tilt, the advantageous effect similar to that in the ninth embodiment can be achieved. 
     [XIII] 12th Embodiment of Information Reproduction Apparatus 
     Next, the twelfth embodiment of the information reproduction apparatus according to the present invention will be described with reference to FIGS. 25 and 26B. FIG. 25 is a block diagram showing the configuration of the servo signal generation circuit according to the twelfth embodiment, and FIG. 26B is a graph showing the relation of the level and the polarity of the tilt error signal with respect to the quantity and the direction of the radial tilt. The information reproduction apparatus of the twelfth embodiment differs, in configuration, from that of the ninth embodiment only in the configuration of the servo signal generation circuit, and other configuration is the same as that of the ninth embodiment. Therefore, the same components are indicated by the same reference numerals and their description will be omitted. 
     In the tenth embodiment, the received light signal Spl corresponding to the first VFO data  120  in the header portion  11  is subtracted from the received light signal Spr corresponding to the first VFO data  120  in the header portion  10  to generate the subtraction signal Sg. The received light signal Spr and the received light signal Spl are added to each other to produce the addition signal Su, the subtraction signal Sg is multiplied by the addition signal Su to produce the multiplication signal Sk, and then the error signal generation circuit  12   c  extracts the D.C. component of multiplication signal Sk to generate the tilt error signal Ste. In contrast, in the twelfth embodiment, the amplitude of the multiplication signal Sk is detected, and the error signal generation circuit  12   c  generates the tilt error signal Ste based on the change of the amplitude thus detected. Namely, as shown in FIG. 25, the servo signal generation circuit  28  according to the twelfth embodiment includes the delay circuit  12   a,  subtracter  26   a,  the adder  26   b,  the multiplier  26   c,  and the error signal generation circuit  12   c,  which have the same functions as those in the tenth embodiment, and the amplitude detection circuit  28   a.  The amplitude detection circuit  28   a  detects the amplitude of the multiplication signal Sk (having completely the same waveform as in the case of tenth embodiment shown in FIG. 23A) corresponding to the radial tilt outputted by the multiplier  26   c,  and supplies the detected amplitude to the error signal generation circuit  12   c  as the amplitude signal Sa. Like the case of the tenth embodiment, the error signal generation circuit  12   c  extracts the D.C. component of the amplitude signal Sa to generate the tilt error signal Ste, and supplies it to the driver  13 . 
     As shown in FIG. 26B, the relation of the level and the polarity of the tilt error signal Ste obtained by extracting the D.C. component of the respective amplitude signals Sa by means of the error signal generation circuit  12   c  with respect to the quantity and the direction of the radial tilt is substantially the linear function of the reverse polarity, as compared with the cases of the previous embodiments. Thus, by generating the drive signal Sd based on the tilt error signal Ste, the reliable drive signal Sd can be generated and the existing radial tilt can be accurately compensated for by driving the motor  5  using the drive signal Sd. As described above, according to the radial tilt compensation performed by the information reproduction apparatus of the twelfth embodiment, since the amplitude of the multiplication signal Sk is detected to generate the tilt error signal Ste indicative of the radial tilt, the advantageous effect similar to that in the ninth embodiment can be achieved. 
     [XIV] 13th Embodiment of Information Reproduction Apparatus 
     Next, the thirteenth embodiment of the information reproduction apparatus according to the present invention will be described with reference to FIGS. 27,  28 A and  28 B. FIG. 27 is a block diagram showing the schematic configuration of the information reproduction apparatus of the thirteenth embodiment, and FIGS. 28A and 28B are graphs for explaining the generation of the tilt error signal. 
     First, the configuration of the information reproduction apparatus of the thirteenth embodiment will be described. The information reproduction apparatus of the thirteenth embodiment includes the same components as those of the ninth embodiment shown in FIG. 19, except for the servo signal generation circuit. Therefore, the same components as those in the information reproduction apparatus SS 9  are indicated by the same reference numerals and the detailed description therefor will be omitted. 
     In the information reproduction apparatus SS 9  of the ninth embodiment, the half-detectors constituting the detector D of the pickup  7  are arranged as follows. Namely, the half-detector  7   a  is positioned to receive the reflected light from the first VFO data  120  while the light spot of the light beam B is scanning the first VFO data  120  in the header portion  10 . The half-detector  7   b  is positioned to receive the reflected light from the first VFO data  120  while the light spot of the light beam B is scanning the first VFO data  120  in the header portion  11 . Then, the received light signal Spr corresponding to the first VFO data  120  in the header  10  and the received light signal Spl corresponding to the first VFO data  120  in the header  11  are used to generate the tilt error signal Ste. In the following thirteenth to sixteenth embodiment, the positions of the half-detectors  7   a  and  7   b  are maintained unchanged. However, the received light signal Spl corresponding to the first VFO data  120  in the header portion  10  and the received light signal Spr corresponding to the first VFO data  120  in the header portion  11  (i.e., the received light signal Spl including less information corresponding to the first VFO data  120  in the header portion  10  and the received light signal Spr including less information corresponding to the first VFO data  120  in the header portion  11 ) are used to generate the tilt error signal Ste. Namely, as shown in FIG. 27, the servo signal generation circuit  29  according to the thirteenth embodiment includes the delay circuit  12   a  and the error signal generation circuit  12   c,  which are the same as those in the ninth embodiment, and the subtracter  29   a.  However, the received light signal Spl inputted to the servo signal generation circuit  29  is supplied to the delay circuit  12   a,  and the received light signal Spr is supplied to the negative-input terminal of the subtracter  29   a.    
     Next, the operation of the servo signal generation circuit according to the thirteenth embodiment will be described with reference to FIGS. 27,  28 A and  28 B. As shown in FIG. 27, the servo signal generation circuit  29  detects the radial tilt presently occurring by the later-described processing using the received light signals Spr and Spl, generates the tilt error signal Ste indicating the quantity and the direction of the radial tilt, and supplies it to the driver  13 . The received light signal Spr inputted to the servo signal generation circuit  12  is supplied to the negative-input terminal of the subtracter  29   a.  The received light signal Spl is supplied to the delay circuit  12   a.  The delay circuit  12   a  delays the received light signal Spl thus inputted (i.e., the received light signal Spl corresponding to the first VFO data  120  in the header portion  10 ) for the delay time period by which the irradiated range of the light beam B on the DVD-RAM  1  passes through the region of the header portion  10  shown in FIG. 3 according to the same delaying operation as that of the ninth embodiment, and supplies it to the positive-input terminal of the subtracter  29   a  as the delay signal Sdpl. By this, the received light signals Spl corresponding to the first VFO data  120  in the header portion  10  and the received light signal Spr corresponding to the first VFO data  120  in the header portion  11  are simultaneously supplied to the subtracter  29   a,  respectively. The method of detecting the received light signal Spl corresponding to the first VFO data  120  in the header portion  10  from the inputted received light signal Spl may be the same as the method employed in the first embodiment for the detection signal Ss. The subtracter  29   a  subtracts the received light signal Spr corresponding to the first VFO data  120  in the header portion  11  from the delay signal Sdpl (i.e., the received light signal Spl corresponding to the first VFO data  120  in the header  10 ) to generate the subtraction signal Sm, and supplies it to the error signal generation circuit  12   c.    
     The actual waveform of the subtraction signal Sm will be described with reference to FIG.  28 A. It is noted that FIG. 28A shows the waveforms of the subtraction signal Sm when the radial tilt is 0.9 deg., 0.45 deg., 0 deg., −0.45 deg., −0.9 deg., respectively, like the cases of the previous embodiments. In addition, one period in each waveform corresponds to 8×T. 
     As seen in FIG. 28A, when the quantity and the direction of the radial tilt occurring in relation with the DVD-RAM  1  change, the level and the waveform of the subtraction signal Sm change, and the tendency or characteristic of the change is the same as that of the subtraction signal Sm in the ninth embodiment. The reason why the subtraction signal Sm changes as shown in FIG. 28A when the quantity and the direction of the radial tilt changes is identical to the case of the subtraction signal Sm in the ninth embodiment. As shown in FIG. 28B, the relation of the level and the polarity of the tilt error signal Ste obtained by extracting the D.C. component of the subtraction signal Sm by means of the error signal generation circuit  12   c  with respect to the quantity and the direction of the radial tilt is substantially the linear function like the cases of the previous embodiments. Thus, by generating the drive signal Sd based on the tilt error signal Ste, the reliable drive signal Sd can be generated and the existing radial tilt can be accurately compensated for by driving the motor  5  using the drive signal Sd. As described above, according to the radial tilt compensation performed by the information reproduction apparatus of the thirteenth embodiment, the advantageous effect similar to that in the ninth embodiment can be achieved. 
     [XI] 14th Embodiment of Information Reproduction Apparatus 
     Next, the fourteenth embodiment of the information reproduction apparatus will be described with reference to FIGS. 29,  30 A and  30 B. FIG. 29 is a block diagram showing the configuration of the servo signal generation circuit of the fourteenth embodiment, and FIGS. 30A and 30B show graphs for explaining the generation of the tilt error signal according to the fourteenth embodiment. The information reproduction apparatus of the fourteenth embodiment differs, in configuration, from that of the thirteenth embodiment only in the configuration of the servo signal generation circuit, and other configuration is the same as that of the thirteenth embodiment. Therefore, the same components are indicated by the same reference numerals and their description will be omitted. 
     In the thirteenth embodiment, the received light signal Spr corresponding to the first VFO data  120  in the header portion  11  is subtracted from the received light signal Spl corresponding to the first VFO data  120  in the header portion  10  to generate the subtraction signal Sm, and then the error signal generation circuit  12   c  generates the tilt error signal Ste based on the subtraction signal Sm. In contrast, in the fourteenth embodiment, other operation is applied to the received light signal Spl corresponding to the first VFO data  120  in the header portion  10  and the received light signal Spr corresponding to the first VFO data  120  in the header portion  11 , and the tilt error signal Ste is generated based on the operation result. Namely, as shown in FIG. 29, the servo signal generation circuit  30  of the fourteenth embodiment includes the delay circuit  12   a  and the error signal generation circuit  12   c,  which have the same functions as those in the thirteenth embodiment, the subtracter  30   a,  the adder  30   b,  and the multiplier  30   c.  The subtracter  30   a  subtracts the non-delayed received light signal Spr from the delay signal Sdpl outputted by the delay circuit  12   a  to generate the subtraction signal Sg, and supplies it to the multiplier  30   c.  The adder  30   b  adds the delay signal Sdpl to the received light signal Spr to generate the addition signal Su, and supplies it to the multiplier  30   c.  The multiplier  30   c  multiplies the subtraction signal Sg by the addition signal Su to generate the multiplication signal Sk, and supplies it to the error signal generation circuit  12   c.  The error signal generation circuit  12   c  may be the averaging circuit or the low-pass filter like the case of the thirteenth embodiment, and extracts the D.C. component of the multiplication signal Sk to generate the tilt error signal Ste, and supplies it to the driver  13 . 
     Next, the actual waveform of the multiplication signal Sk will be described with reference to FIG.  30 A. FIG. 30A show the waveforms of the multiplication signal Sk generated when the radial tilt is 0.9 deg., 0.45 deg., 0 deg., −0.45 deg., −0.9 deg., respectively, similarly the case of the previous embodiments, and one period of the multiplication signal Sk corresponds to 8×T. As seen in FIG. 30A, when the quantity and the direction of the radial tilt occurring in relation with the DVD-RAM  1  change, the level and the waveform of the multiplication signal Sk change, similarly to the subtraction signal Sm of the thirteenth embodiment, and the tendency or characteristic of the change is the same as that of the subtraction signal Sm in the thirteenth embodiment. The reason why the multiplication signal Sk changes as shown in FIG. 30A when the quantity and the direction of the radial tilt change is identical to the case of the subtraction signal Sm in the thirteenth embodiment. 
     As shown in FIG. 30B, the relation of the level and the polarity of the tilt error signal Ste obtained by extracting the D.C. component of the multiplication signal Sk by means of the error signal generation circuit  12   c  with respect to the quantity and the direction of the radial tilt is substantially the linear function like the case of the thirteenth embodiment. Thus, by generating the drive signal Sd based on the tilt error signal Ste, the reliable drive signal Sd can be generated and the existing radial tilt can be accurately compensated for by driving the motor  5  using the drive signal Sd. As described above, according to the radial tilt compensation performed by the information reproduction apparatus of the fourteenth embodiment, the advantageous effect similar to that in the thirteenth embodiment can be achieved. 
     [XVI] 15th Embodiment of Information Reproduction Apparatus 
     Next, the fifteenth embodiment of the information reproduction apparatus according to the present invention will be described with reference to FIGS. 31 and 33A. FIG. 31 is a block diagram showing the configuration of the servo signal generation circuit according to the fifteenth embodiment, and FIG. 33A is a graph showing the relation of the level and the polarity of the tilt error signal with respect to the quantity and the direction of the radial tilt according to the fifteenth embodiment. The information reproduction apparatus of the fifteenth embodiment differs, in configuration, from that of the thirteenth embodiment only in the configuration of the servo signal generation circuit, and other configuration is the same as that of the thirteenth embodiment. Therefore, the same components are indicated by the same reference numerals and their description will be omitted. 
     In the thirteenth embodiment, the received light signal Spr corresponding to the first VFO data  120  in the header portion  11  is subtracted from the received light signal Spl corresponding to the first VFO data  120  in the header portion  10  to generate the subtraction signal Sm, and then the error signal generation circuit  12   c  extracts the D.C. component of the subtraction signal Sm to generate the tilt error signal Ste. In contrast, in the fifteenth embodiment, the amplitude of the subtraction signal Sm is detected, and the error signal generation circuit  12   c  generates the tilt error signal Ste based on-the change of the amplitude thus detected. Namely, as shown in FIG. 31, the servo signal generation circuit  31  according to the fifteenth embodiment includes the delay circuit  12   a,  subtracter  29   a  and the error signal generation circuit  12   c,  which have the same functions as those in the thirteenth embodiment, and the amplitude detection circuit  31   a.  The amplitude detection circuit  31   a  detects the amplitude of the subtraction signal Sm (having completely the same waveform as in the case of thirteenth embodiment shown in FIG. 28A) corresponding to the radial tilt and outputted by the subtracter  29   a,  and supplies the detected amplitude to the error signal generation circuit  12   c  as the amplitude signal Sa. The error signal generation circuit  12   c  may be the averaging circuit or the low-pass filter like the case of the thirteenth embodiment, and extracts the D.C. component of the amplitude signal Sa to generate the tilt error signal Ste, and supplies it to the driver  13 . 
     As shown in FIG. 33A, the relation of the level and the polarity of the tilt error signal Ste obtained by extracting the D.C. component of the respective amplitude signals Sa by means of the error signal generation circuit  12   c  with respect to the quantity and the direction of the radial tilt is substantially the linear function like the cases of the previous embodiments. Thus, by generating the drive signal Sd based on the tilt error signal Ste, the reliable drive signal Sd can be generated and the existing radial tilt can be accurately compensated for by driving the motor  5  using the drive signal Sd. As described above, according to the radial tilt compensation performed by the information reproduction apparatus of the fifteenth embodiment, since the amplitude of the subtraction signal Sm is detected to generate the tilt error signal Ste indicative of the radial tilt, the advantageous effect similar to that in the thirteenth embodiment can be achieved. 
     [XVII] 16th Embodiment of Information Reproduction Apparatus 
     Next, the sixteenth embodiment of the information reproduction apparatus according to the present invention will be described with reference to FIGS. 32 and 33B. FIG. 32 is a block diagram showing the configuration of the servo signal generation circuit according to the sixteenth embodiment, and FIG. 33B is a graph showing the relation of the level and the polarity of the tilt error signal with respect to the quantity and the direction of the radial tilt according to the sixteenth embodiment. The information reproduction apparatus of the sixteenth embodiment differs, in configuration, from that of the thirteenth embodiment only in the configuration of the servo signal generation circuit, and other configuration is the same as that of the thirteenth embodiment. Therefore, the same components are indicated by the same reference numerals and their description will be omitted. 
     In the fourteenth embodiment, the received light signal Spr corresponding to the first VFO data  120  in the header portion  11  is subtracted from the received light signal Spl corresponding to the first VFO data  120  in the header portion  10  to generate the subtraction signal Sg. The received light signal Spl and the received light signal Spr are added to each other to produce the addition signal Su, the subtraction signal Sg is multiplied by the addition signal Su to produce the multiplication signal Sk, and then the error signal generation circuit  12   c  extracts the D.C. component of multiplication signal Sk to generate the tilt error signal Ste. In contrast, in the sixteenth embodiment, the amplitude of the multiplication signal Sk is detected, and the error signal generation circuit  12   c  generates the tilt error signal Ste based on the change of the amplitude thus detected. Namely, as shown in FIG. 32, the servo signal generation circuit  32  according to the sixteenth embodiment includes the delay circuit  12   a,  subtracter  30   a,  the adder  30   b,  the multiplier  30   c,  and the error signal generation circuit  12   c,  which have the same functions as those in the fourteenth embodiment, and the amplitude detection circuit  32   a.  The amplitude detection circuit  32   a  detects the amplitude of the multiplication signal Sk (having completely the same waveform as in the case of fourteenth embodiment shown in FIG. 30A) corresponding to the radial tilt and outputted by the multiplier  30   c,  and supplies the detected amplitude to the error signal generation circuit  12   c  as the amplitude signal Sa. The error signal generation circuit  12   c  may be the averaging circuit or the low-pass filter like the case of the fourteenth embodiment, and extracts the D.C. component of the amplitude signal Sa to generate the tilt error signal Ste, and supplies it to the driver  13 . 
     As shown in FIG. 33B, the relation of the level and the polarity of the tilt error signal Ste obtained by extracting the D.C. component of the respective amplitude signals Sa by means of the error signal generation circuit  12   c  with respect to the quantity and the direction of the radial tilt is substantially the linear function like the cases of the previous embodiments. Thus, by generating the drive signal Sd based on the tilt error signal Ste, the reliable drive signal Sd can be generated and the existing radial tilt can be accurately compensated for by driving the motor  5  using the drive signal Sd. As described above, according to the radial tilt compensation performed by the information reproduction apparatus of the sixteenth embodiment, since the amplitude of the multiplication signal Sk is detected to generate the tilt error signal Ste indicative of the radial tilt, the advantageous effect similar to that in the thirteenth embodiment can be achieved. 
     The above described embodiments are directed to the application of the present invention to the information reproduction apparatus for reproducing information from the DVD-RAM  1 . However, the present invention is applicable to an information recording apparatus for recording information on the DVD-RAM  1 . In such a case, for example, the address information recorded at each header area S 0  to S 7  are read out based on the reproduction signal Spu outputted by the reproduction circuit  110  shown in FIG.  4 . Simultaneously, information to be recorded is encoded, and then the encoded information is recorded on the DVD-RAM  1  at the recording position determined by the address information. In this application to the information recording apparatus, the existing radial tilt may be accurately detected and compensated for, and hence information can be accurately recorded. 
     In the above embodiments, the first VFO data  120  in the header portion  10  and the first VFO data  120  in the header portion  11  are used to detect the radial tilt. Alternatively, the second VFO data  25  in the header portion  10  and the second VFO data  25  in the header portion  11  may be used to detect the radial tilt because completely the same periodical pit arrays are formed on those areas. 
     The invention may be embodied on other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning an range of equivalency of the claims are therefore intended to embraced therein. 
     The entire disclosure of Japanese Patent Application No. 10-309193 filed on Oct. 29, 1998 including the specification, claims, drawings and summary is incorporated herein by reference in its entirety.