Aberration correcting apparatus and information reproducing apparatus having the same

An aberration correcting apparatus is provided with: a light detection device for irradiating an information record medium, on which record information is recorded, with a light beam, and outputting a light detection signal corresponding to the record information on the basis of a reflection light of the light beam reflected from the information record medium; a tilt detection device for detecting a tilt between an information record surface of the information record medium and an optical axis of the light beam on the basis of the light detection signal; a correction device for correcting a wavefront aberration generated in the light beam due to the tilt; and a driving device for driving the correction device on the basis of the detected tilt.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a technical field of an aberration
 correcting apparatus for correcting a wavefront aberration (mainly a coma
 aberration) generated on an information recording surface of an
 information record medium for an optical information reproduction due to
 an inclination of an angle formed between the information recording
 surface and an optical axis of an optical beam for the information
 reproduction from the right angle. (Hereafter, this inclination is
 referred to as a tilt, and the amount of this inclination is referred to
 as a tilt amount.)
 2. Description of the Related Art
 In such an aberration correcting device, the wavefront aberration caused,
 for example, in the direction of a radius of an optical disc functioning
 as an information record medium (hereafter referred to as radial
 direction) due to the tilt is corrected by: emitting an optical beam for a
 tilt amount detection onto the optical disc besides the above described
 optical beam for the information reproduction; receiving the reflected
 light of the optical beam for the tilt amount detection by using a
 photodetector divided into two partial detection portions by a division
 line perpendicular to the radial direction (hereafter, the emitting device
 for emitting the optical beam for the tilt amount detection and the
 photodetector are referred to collectively as a tilt sensor); calculating
 the above described tilt amount from the difference between the detected
 signals supplied from the respective partial detection portions; and
 inclining the optical axis of the optical beam for the information
 reproduction so as to cancel the tilt on the basis of the calculated tilt
 amount.
 In this aberration correcting apparatus, however, it is necessary to cancel
 the tilt by inclining the optical axis by using a mechanical drive device.
 Therefore, the aberration correcting apparatus has problems of an
 increased manufacturing cost, a lowered reliability and a difficulty in a
 size reduction. These problems occur for the tilt in the rotation
 direction of the optical disc (hereafter, which referred to as a
 tangential direction) as well in the same way.
 Therefore, as an improvement of the above described mechanical aberration
 correcting apparatus, there is a method of giving a phase difference or
 the like to the optical beam for the information reproduction on the basis
 of the detected tilt amount without using the mechanical tilt canceling
 device, to thereby optically cancel the wavefront aberration caused by the
 tilt.
 However, according to the above described method of optically canceling the
 aberration, the tilt itself is not canceled but the wavefront aberration
 caused by the tilt is canceled. Thus, even if the wavefront aberration is
 canceled, the tilt itself is not changed. As a result, the detected signal
 supplied from the tilt sensor is not changed, either.
 Therefore, it cannot be determined from the detected signal supplied from
 the tilt sensor whether the optical correction is insufficient or
 superfluous. Unlike the above described mechanical aberration correcting
 apparatus, therefore, a so-called feedback control based on the detected
 signal of the tilt sensor cannot be conducted. Instead, a so-called
 feedforward control of estimating the wavefront aberration (i.e., the tilt
 amount) on the basis of the detected signal supplied from the tilt sensor
 and conducting the correction is thus execution.
 Therefore, if the tilt amount cannot be detected accurately by the tilt
 sensor, the aberration correction cannot be conducted properly. Thus,
 there is a problem that the dispersion of sensitivities and offsets or
 nonlinear characteristics of the partial detection portions in the tilt
 sensor must be adjusted accurately.
 Furthermore, there is a problem that a change is caused in some cases by an
 elapse of time even or aging even if these values are adjusted accurately
 in the initial state.
 Furthermore, there is a problem that, in the case where both of the tilt in
 the radial tilt direction and the tilt in the tangential direction are to
 be corrected, two tilt sensors must be used resulting in a large sized
 pickup portion.
 SUMMARY OF THE INVENTION
 The present invention is made in view of the above described problems. It
 is therefore an object of the present invention to provide an aberration
 correcting apparatus, which has a high precision and a high reliability
 and which can be reduced in size without employing an additional separated
 sensor such as the above described tilt sensor, and an information
 reproducing apparatus having such an aberration correcting apparatus.
 The above object of the present invention can be achieved by an aberration
 correcting apparatus provided with: a light detection device for
 irradiating an information record medium, on which record information is
 recorded, with a light beam, and outputting a light detection signal
 corresponding to the record information on the basis of a reflection light
 of the light beam reflected from the information record medium; a tilt
 detection device for detecting a tilt between an information record
 surface of the information record medium and an optical axis of the light
 beam on the basis of the light detection signal; a correction device for
 correcting a wavefront aberration generated in the light beam due to the
 tilt; and a driving device for driving the correction device on the basis
 of the detected tilt.
 According to the aberration correcting apparatus, an information record
 medium is irradiated with the light beam by the light detection device,
 and the light detection signal corresponding to the record information is
 outputted on the basis of a reflection light. Then, on the basis of the
 light detection signal, the tilt between the information record surface of
 the information record medium and the optical axis of the light beam is
 detected by the tilt detection device. Then, the wavefront aberration
 generated in the light beam due to the tilt is corrected by the correction
 device. Finally, the corrected device is driven by the driving device on
 the basis of the detected tilt.
 Accordingly, since the tilt is detected on the basis of the light detection
 signal obtained by the irradiation of the light beam for the information
 reproduction, it is not necessary to emit another light beam exclusive for
 the tilt detection besides the light beam for the information
 reproduction, so that the structure of the aberration correcting apparatus
 can be simplified.
 Therefore, it is possible to correct the aberration due to the tilt of the
 optical axis of the light beam precisely by the structure which is
 simplified and is reduced in size.
 In one aspect of the aberration correcting apparatus, the information
 record medium is provided with a disc type record medium on which the
 record information is recorded by forming a spiral or coaxial track. The
 light detection device outputs as the light detection signal a center
 detected signal corresponding to the record information forming a center
 turn of the track on which the record information to be reproduced is
 recorded, an inner detected signal corresponding to the record information
 forming an inner turn of the track located adjacent to the center turn at
 an inner side thereof and an outer detected signal corresponding to the
 record information forming an outer turn of the track located adjacent to
 the center turn at an outer side thereof. And that, the tilt detection
 device provided with at least one of (i) a tangential tilt detection
 device for detecting a tangential tilt, which is the tilt in a tangential
 direction of the track on the disc type record medium, on the basis of the
 center detected signal and (ii) a radial tilt detection device for
 detecting a radial tilt, which is the tilt in a radial direction of the
 track on the disc type record medium on the basis of the inner, center and
 outer detected signals.
 According to this aspect, the center detected signal, the inner detected
 signal and the outer detected signal are outputted by the light detection
 device. Then, the tangential tilt is detected by the tangential tilt
 detection device on the basis of the center detected signal, or the radial
 tilt is detected by the radial tilt detection device on the basis of the
 inner, center and outer detected signals.
 Thus, since the tilt in the radial direction is detected by use of the
 light detection signals from the adjacent turns of the track while the
 tilt in the tangential direction is detected by use of the light detection
 signal from the center turn of the track, it is possible to detect the
 tilt in the respective directions precisely so as to correct the wavefront
 aberration.
 In this aspect, the radial tilt detection device may be provided with a
 crosstalk detection device for detecting (a) an inner crosstalk, which is
 a crosstalk from the inner detected signal to the center detected signal,
 and (b) an outer crosstalk, which is a crosstalk from the outer detected
 signal to the center detected signal, respectively, so as to detect the
 radial tilt as a difference between the detected inner crosstalk and the
 detected outer crosstalk. And that, the driving device may drive the
 correction device so that a value of the radial tilt is reduced to
 approach a zero.
 Thus, the inner and outer crosstalks are detected by the crosstalk
 detection device, so that the radial tilt is detected as the difference
 between the detected inner and outer crosstalks. Then, the correction
 device is driven by the driving device so that the value of the radial
 tilt is reduced to approach a zero. Accordingly, since the wavefront
 aberration in the radial direction is corrected by reducing the difference
 of the inner and outer crosstalks, it is possible to precisely correct the
 wavefront aberration in the radial direction by use of a simple structure.
 In this aspect also, the radial tilt detection device may be provided with
 a crosstalk detection device for detecting (a) an inner crosstalk, which
 is a crosstalk from (b) the inner detected signal to the center detected
 signal, and an outer crosstalk, which is a crosstalk from the outer
 detected signal to the center detected signal, respectively, so as to
 detect the radial tilt as a sum of the detected inner crosstalk and the
 detected outer crosstalk. And that, the driving device may drive the
 correction device so that a value of the radial tilt is reduced to a
 minimum.
 Thus, the inner and outer crosstalks are detected by the crosstalk
 detection device, so that the radial tilt is detected as the sum of the
 detected inner and outer crosstalks. Then, the correction device is driven
 by the driving device so that the value of the radial tilt is reduced to a
 minimum. Accordingly, since the wavefront aberration in the radial
 direction is corrected by reducing the sum of the inner and outer
 crosstalks, it is possible to precisely correct the wavefront aberration
 in the radial direction by use of a simple structure.
 In this aspect also, the tangential tilt detection device may be provided
 with a crosstalk detection device for detecting (a) a forward crosstalk,
 which is a crosstalk from a forward detected signal, which is the center
 detected signal detected at a time t1, to an intermediate detected signal,
 which is the center detected signal detected at a time T2 later than the
 time t1, and (b) a backward crosstalk, which is a crosstalk from a
 backward detected signal, which is the center detected signal detected at
 a time T3 later than the time t2, to the intermediate detected signal,
 respectively, so as to detect the tangential tilt as a difference between
 the detected forward crosstalk and the detected backward crosstalk. And
 that, the driving device may drive the correction device so that a value
 of the tangential tilt is reduced to approach a zero.
 Thus, the forward crosstalk and the backward crosstalk are detected by the
 crosstalk detection device respectively, and the tangential tilt is
 detected as the difference between the detected forward and backward
 crosstalks. Then, the correction device is driven by the driving device so
 that the value of the tangential tilt is reduced to approach a zero.
 Accordingly, since the wavefront aberration in the tangential direction is
 corrected by reducing the difference of the forward and backward
 crosstalks, it is possible to precisely correct the wavefront aberration
 in the tangential direction by use of a simple structure.
 In this aspect also, the tangential tilt detection device may be provided
 with a crosstalk detection device for detecting (a) a forward crosstalk,
 which is a crosstalk from a forward detected signal, which is the center
 detected signal detected at a time t1, to an intermediate detected signal,
 which is the center detected signal detected at a time T2 later than the
 time t1, and (b) a backward crosstalk, which is a crosstalk from a
 backward detected signal, which is the center detected signal detected at
 a time T3 later than the time t2, to the intermediate detected signal,
 respectively, so as to detect the tangential tilt as a sum of the detected
 forward crosstalk and the detected backward crosstalk. And that, the
 driving device may drive the correction device so that a value of the
 tangential tilt is reduced to a minimum.
 Thus, the forward crosstalk and the backward crosstalk are detected by the
 crosstalk detection device respectively, and the tangential tilt is
 detected as the sum of the detected forward and backward crosstalks. Then,
 the correction device is driven by the driving device so that the value of
 the tangential tilt is reduced to a minimum. Accordingly, since the
 wavefront aberration in the tangential is corrected by reducing the sum of
 the forward and backward crosstalks, it is possible to precisely correct
 the wavefront aberration in the tangential direction by use of a simple
 structure.
 In another aspect of the aberration correcting apparatus, the correction
 device is provided with a liquid crystal panel disposed in an optical path
 of the light beam for correcting the wavefront aberration.
 According to this aspect, the wavefront aberration is corrected by the
 liquid crystal panel disposed in the optical path of the light beam. Thus,
 it is possible to construct the correcting device by employing a rather
 simple structure. Further, since no mechanical driving part is necessary
 in the correcting device, the reliability as the aberration correcting
 apparatus is improved, and the reduction in size is promoted.
 In this aspect, the liquid crystal panel may be provided with: a liquid
 crystal for giving a phase difference to the light beam so as to correct
 the wavefront aberration; and an electrode for applying a voltage to the
 liquid crystal so as to give the phase difference to the light beam
 transmitted though the liquid crystal. And that, the driving device may be
 provided with a voltage applying device for applying the voltage to the
 electrode on the basis of the detected tilt.
 Thus, the voltage is applied to the electrode by the voltage applying
 device, on the basis of the detected tilt. Then, the voltage is applied to
 the liquid crystal by the electrode. Then, the phase difference is given
 to the light beam transmitted through the liquid crystal, so that the
 wavefront aberration is corrected. Accordingly, it is possible to correct
 the wavefront aberration efficiently by giving the phase difference to the
 light beam.
 In this case further, the electrode may be provided with a plurality of sub
 electrodes having a shape corresponding to a distribution of the wavefront
 aberration generated in the light beam in correspondence with the tilt,
 and the voltage applying device may individually apply the voltage to each
 of the sub electrodes.
 Thus, the sub electrodes have the shape corresponding to the distribution
 of the wavefront aberration. Here, the voltage is individually applied to
 each of the sub electrodes by the voltage applying device. Accordingly, it
 is possible to correct the wavefront aberration efficiently by use of the
 sub electrodes in correspondence with the distribution of the wavefront
 aberration.
 In another aspect of the aberration correcting apparatus, the correction
 device is provided with an inclination device for inclining the optical
 axis of the light beam on the basis of the detected tilt, so as to
 canceling the detected tilt.
 According to this aspect, the optical axis of the light beam is inclined by
 the inclination device on the basis of the detected tilt, so that the
 detected tilt is canceled and thus the tilt is corrected. Accordingly, it
 is not necessary to emit another light beam exclusive for the tilt
 detection besides the light beam for the information reproduction, and it
 is possible to correct the wavefront aberration by canceling the
 inclination by use of a simple structure.
 The above object of the present invention can be also achieved by an
 information reproducing apparatus provided with (I) the above described
 aberration correcting apparatus of the present invention, (II) a light
 collecting device for collecting the light beam onto the information
 record medium, and (III) a reproducing device for reproducing the record
 information on the basis of the light detection signal.
 According to the information reproducing apparatus, while the wavefront
 aberration is corrected by the above described aberration correcting
 apparatus of the present invention, the light beam is collected or
 condensed onto the information record medium by the light collecting
 device, and the record information is reproduced by the reproducing device
 on the basis of the light detection signal. Therefore, it is possible to
 precisely reproduce the record information with correcting the wavefront
 aberration due to the tilt by use of the simple structure.
 In one aspect of the information reproducing apparatus, the information
 record medium is provided with a disc type record medium on which the
 record information is recorded by forming a spiral or coaxial track. The
 light detection device outputs as the light detection signal a center
 detected signal corresponding to the record information forming a center
 turn of the track on which the record information to be reproduced is
 recorded, an inner detected signal corresponding to the record information
 forming an inner turn of the track located adjacent to the center turn at
 an inner side thereof and an outer detected signal corresponding to the
 record information forming an outer turn of the track located adjacent to
 the center turn at an outer side thereof. And that, the tilt detection
 device provided with at least one of (i) a tangential tilt detection
 device for detecting a tangential tilt, which is the tilt in a tangential
 direction of the track on the disc type record medium, on the basis of the
 center detected signal and (ii) a radial tilt detection device for
 detecting a radial tilt, which is the tilt in a radial direction of the
 track on the disc type record medium on the basis of the inner, center and
 outer detected signals.
 According to this aspect, the center detected signal, the inner detected
 signal and the outer detected signal are outputted by the light detection
 device. Then, the tangential tilt is detected by the tangential tilt
 detection device on the basis of the center detected signal, or the radial
 tilt is detected by the radial tilt detection device on the basis of the
 inner, center and outer detected signals.
 Thus, since the tilt in the radial direction is detected by use of the
 light detection signals from the adjacent turns of the track while the
 tilt in the tangential direction is detected by use of the light detection
 signal from the center turn of the track, it is possible to detect the
 tilt in the respective directions precisely so as to correct the wavefront
 aberration. Therefore, it is possible to precisely reproduce the record
 information with correcting the wavefront aberration in each of the radial
 direction and the tangential direction due to the tilt.
 In this aspect, (I) the radial tilt detection device may be provided with a
 crosstalk detection device for detecting (a) an inner crosstalk, which is
 a crosstalk from the inner detected signal to the center detected signal,
 and (b) an outer crosstalk, which is a crosstalk from the outer detected
 signal to the center detected signal, respectively, so as to detect the
 radial tilt as a difference between the detected inner crosstalk and the
 detected outer crosstalk. The tangential tilt detection device may be
 provided with a crosstalk detection device for detecting (a) a forward
 crosstalk, which is a crosstalk from a forward detected signal, which is
 the center detected signal detected at a time t1, to an intermediate
 detected signal, which is the center detected signal detected at a time T2
 later than the time t1, and (b) a backward crosstalk, which is a crosstalk
 from a backward detected signal, which is the center detected signal
 detected at a time T3 later than the time t2, to the intermediate detected
 signal, respectively, so as to detect the tangential tilt as a difference
 between the detected forward crosstalk and the detected backward
 crosstalk. The driving device may drive the correction device so that each
 value of the radial tilt and the tangential tilt is reduced to approach a
 zero. And that, (II) the reproducing device may be provided with a
 subtracter device for subtracting the inner crosstalk, the outer
 crosstalk, the forward crosstalk and the backward crosstalk from the light
 detection signal to generate a subtracted light detection signal, and may
 reproduce the record information on the basis of the subtracted light
 detection signal.
 Thus, the radial tilt is detected as the difference between the detected
 inner and outer crosstalks. On the other hand, the tangential tilt is
 detected as the difference between the detected forward and backward
 crosstalks. Then, while the correction device is driven by the driving
 device so that the value of each of the radial and tangential tilts is
 reduced to approach a zero, the inner crosstalk, the outer crosstalk, the
 forward crosstalk and the backward crosstalk are subtracted from the light
 detection signal by the subtracter device. Finally, on the basis of this
 subtracted light detection signal, the record information is reproduced.
 Accordingly, it is possible to precisely reproducing the record
 information by removing the crosstalks respectively while correcting the
 wavefront aberration due to the tile by use of a simple structure.
 In this case also, (I) the radial tilt detection device may be provided
 with a crosstalk detection device for detecting (a) an inner crosstalk,
 which is a crosstalk from the inner detected signal to the center detected
 signal, and (b) an outer crosstalk, which is a crosstalk from the outer
 detected signal to the center detected signal, respectively, so as to
 detect the radial tilt as a sum of the detected inner crosstalk and the
 detected outer crosstalk. The tangential tilt detection device may be
 provided with a crosstalk detection device for detecting (a) a forward
 crosstalk, which is a crosstalk from a forward detected signal, which is
 the center detected signal detected at a time t1, to an intermediate
 detected signal, which is the center detected signal detected at a time T2
 later than the time t1, and (b) a backward crosstalk, which is a crosstalk
 from a backward detected signal, which is the center detected signal
 detected at a time T3 later than the time t2, to the intermediate detected
 signal, respectively, so as to detect the tangential tilt as a sum of the
 detected forward crosstalk and the detected backward crosstalk. The
 driving device may drive the correction device so that each value of the
 radial tilt and the tangential tilt is reduced to a minimum. And that,
 (II) the reproducing device may be provided with a subtracter device for
 subtracting the inner crosstalk, the outer crosstalk, the forward
 crosstalk and the backward crosstalk from the light detection signal to
 generate a subtracted light detection signal, and may reproduce the record
 information on the basis of the subtracted light detection signal.
 Thus, the radial tilt is detected as the sum of the detected inner and
 outer crosstalks. On the other hand, the tangential tilt is detected as
 the sum of the detected forward and backward crosstalks. Then, while the
 correction device is driven by the driving device so that the value of
 each of the radial and tangential tilts is reduced to a minimum, the inner
 crosstalk, the outer crosstalk, the forward crosstalk and the backward
 crosstalk are subtracted from the light detection signal by the subtracter
 device. Finally, on the basis of this subtracted light detection signal,
 the record information is reproduced. Accordingly, it is possible to
 precisely reproducing the record information by removing the crosstalks
 respectively while correcting the wavefront aberration due to the tile by
 use of a simple structure.
 The nature, utility, and further features of this invention will be more
 clearly apparent from the following detailed description with respect to
 preferred embodiments of the invention when read in conjunction with the
 accompanying drawings briefly described below.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Preferred embodiments of the present invention will now be described by
 referring to the drawing.
 The embodiments described hereafter are embodiments in the case where the
 present invention is applied to an information reproducing apparatus for
 reproducing record information from an optical disc as one example of a
 disc-like recording medium on which the record information is recorded by
 forming concentrically circular or spiral tracks of pits corresponding to
 the record information.
 (I) First Embodiment
 First of all, a first embodiment of the present invention will now be
 described by referring to FIGS. 1 to 13.
 The entire configuration of an information reproducing apparatus of the
 first embodiment will first be described by referring to FIG. 1.
 As shown in FIG. 1, an information reproducing apparatus S of the first
 embodiment includes: a laser diode 7; a diffraction grating 6; a beam
 splitter 4; a liquid crystal panel 3 serving as one example of a
 correction device; an objective lens 2 serving as one example of a
 condenser device; a detector 5 serving as one example of a detection
 device; three amplifiers 8; delay circuits 9 and 10; a signal process unit
 11 serving as one example of a tilt detection device, a voltage applying
 device and a driving device; and a demodulation unit 12 serving as one
 example of a reproducing device.
 Operations of respective units will now be described. Incidentally, as
 shown in FIG. 1, record information recorded on an optical disc 1 is
 recorded on tracks formed of pits P, which have a plurality of kinds of
 length corresponding to the record information and are arranged in columns
 in a longitudinal direction. The rotation velocity of the optical disc 1
 in the tangential direction (i.e., the beam scan direction) is represented
 by V.sub.L.
 The laser diode 7 emits an optical beam B which is a laser light.
 The diffraction grating 6 separates the optical beam B into a main beam BM
 and a first subsidiary beam BS.sub.1 and a second subsidiary beam
 BS.sub.2.
 The beam splitter 4 transmits a part of each of the main beam BM, the first
 subsidiary beam BS.sub.1 and the second subsidiary beam BS.sub.2 so as to
 make it arrive at the liquid crystal panel 3.
 At this time, the liquid crystal panel 3 give a phase difference to each of
 the main beam BM, the first subsidiary beam BS.sub.1 and the second
 subsidiary beam BS.sub.2 on the basis of a tilt correction control signal
 Sc supplied from the signal process unit 11, to thereby correct a
 wavefront aberration caused by a tilt occurring in the optical disc 1.
 The objective lens 2 applies the main beam BM, the first subsidiary beam
 BS.sub.1 and the second subsidiary beam BS.sub.2 each provided with the
 phase difference on to the optical disc 1. Here, the main beam BM is
 applied to a track having the record information to be reproduced recorded
 thereon (hereafter referred to a reproduction track). On the reproduction
 track, a central optical spot C is thus formed. The first subsidiary beam
 BS.sub.1 is applied to a track located one track inner than the
 reproduction track (hereafter referred to as an inner track). An inner
 optical spot IN is thus formed on the inner track. The second subsidiary
 beam BS.sub.2 is applied to a track located one track outer than the
 reproduction track (hereafter referred to as an outer track). An outer
 optical spot OUT is thus formed on the outer track.
 Thereafter, the main beam BM, the first subsidiary beam BS.sub.1 and the
 second subsidiary beam BS.sub.2 applied onto the respective tracks are
 intensity-modulated by the pits P located on the respective tracks. Due to
 the reflection at the optical disc 1, the plane of polarization of each
 beam is rotated. The main beam BM, the first subsidiary beam BS.sub.1 and
 the second subsidiary beam BS.sub.2 arrive at the beam splitter 4 again
 through the objective lens 2 and the liquid crystal panel 3, and are
 reflected toward a light receiving surface of the detector 5 by the beam
 splitter 4.
 Next, the main beam BM, the first subsidiary beam BS.sub.1 and the second
 subsidiary beam BS.sub.2 received by the detector 5 are converted to
 electric signals respectively separately, are outputted to the amplifiers
 8 respectively separately as a central detected signal Scent corresponding
 to the central beam BM, an inner detected signal Sin corresponding to the
 first subsidiary beam BS.sub.1, and an outer detected signal Sout
 corresponding to the second subsidiary beam BS.sub.2, and are amplified
 respectively.
 Among these detected signals amplified by the amplifiers 8, the outer
 detected signal Sout is outputted to the signal process unit 11 as it is.
 On the other hand, the amplified central detected signal Scent is outputted
 to the signal process unit 11 after being delayed with a delay amount DL
 by the delay circuit 9.
 Furthermore, the amplified inner detected signal Sin is outputted to the
 signal process unit 11 after being delayed with a delay amount 2.times.DL
 by the delay circuit 10.
 The delay amount DL (or 2.times.DL) in the delay circuits 9 and 10 is set
 according to the following equation:
EQU DL=L/V.sub.L
 wherein L represents a distance between the inner optical spot IN and the
 central optical spot C and also between the central optical spot C and the
 outer optical spot OUT measured along the tracks of the optical disc 1.
 The reason why the delay circuits 9 and 10 are provided in the information
 reproducing apparatus S are as following.
 Namely, in the signal process unit 11, a crosstalk caused on the central
 detected signal Scent by the inner detected signal Sin and a crosstalk
 caused on the central detected signal Scent by the outer detected signal
 Sout are derived on the basis of the inner detected signal Sin, the
 central detected signal Scent, and the outer detected signal Sout, so that
 the tilt correction control signal Sc is generated as described later. At
 this time, the respective detected signals Sin, Scent and Sout must be
 generated on the basis of the inner optical spot IN, the central optical
 spot C, and the outer optical spot OUT lining up on a straight line in the
 radial direction of the optical disc 1. However, in fact, in the case
 where the optical beam B is separated by using the diffraction grating 6
 in the actual information reproducing apparatus S, it is difficult to line
 up the optical spots IN, C and OUT on a straight line in the radial
 direction of the optical disc 1. In the information reproducing apparatus
 S, therefore, the inner detected signal Sin generated from the preceding
 inner optical spot IN is delayed with the delay amount 2.times.DL, and the
 central detected signal Scent generated from the central optical spot C is
 delayed with the delay amount DL. At the time point when the outer
 detected signal Sout is generated from the outer optical spot OUT,
 therefore, three detected signals Sin, Scent and Sout are simultaneously
 outputted to the signal process unit 11. Since the optical disc 1 itself
 is moving at the velocity of V.sub.L even during the time duration of this
 delay process, the respective detected signals based upon the pits P
 located in positions lined up on a straight line in the radial direction
 of the optical disc 1 are eventually inputted to the signal process unit
 11 at the same time, resulting in that the above described crosstalk
 amounts are detected accurately.
 On the basis of these inputted inner detected signal Sin, central detected
 signal Scent and outer detected signal Sout, the signal process unit 11
 generates the tilt correction control signal Sc by conducting processing
 described later and outputs it to the liquid crystal panel 3. As a result,
 the liquid crystal panel 3 gives the phase difference to each optical beam
 to thereby correct the wavefront aberration caused on the optical disc 1
 by the tilt.
 On the other hand, the signal process unit 11 outputs the central detected
 signal Scent to the demodulation unit 12 as it is, in parallel with the
 generation of the tilt correction signal Sc.
 The demodulation unit 12 demodulates the central detected signal Scent, and
 generates a reproduced signal Ss corresponding to the record information
 on the optical disc 1 to be reproduced.
 Next, the configuration of the signal process unit 11 is described by
 referring to FIG. 2.
 As shown in FIG. 2, the signal process unit 11 includes: A/D converters 24,
 25 and 26; a radial tilt detector 20 serving as one example of a radial
 tilt detection device; a tangential tilt detector 21 serving as one
 example of a tangential tilt detection device; a radial tilt controller
 22; and a tangential tilt controller 23.
 On the other hand, the radial tilt detector 20 includes: crosstalk amount
 detection units 30 and 31; and a subtracter 32.
 Furthermore, the tangential tilt detector 21 includes: crosstalk amount
 detection units 33 and 34; a subtracter 35; and delay circuits 36 and 37.
 Next, the operations of the respective components is described.
 The A/D converter 26 converts the central detected signal Scent outputted
 from the delay circuit 9 to a digital signal, and outputs the digital
 signal to the demodulation unit 12 and the crosstalk amount detection
 units 30, 31 and 33.
 On the other hand, the A/D converter 24 converts the inner detected signal
 Sin outputted from the delay circuit 10 to a digital signal, and outputs
 the digital signal to the crosstalk amount detection unit 30.
 Furthermore, the A/D converter 25 converts the outer detected signal Sout
 outputted from the amplifier 8 to a digital signal, and outputs the
 digital signal to the crosstalk amount detection unit 31.
 Then, by using these inputted inner detected signal Sin and central
 detected signal Scent, the crosstalk amount detection unit 30 detects a
 crosstalk amount caused on the central detected signal Scent by the inner
 detected signal Sin (hereafter referred to as an inner crosstalk amount).
 The crosstalk amount detection unit 30 outputs the inner crosstalk amount
 to the subtracter 32 as an inner crosstalk signal Scti.
 On the other hand, by using these inputted outer detected signal Sout and
 central detected signal Scent, the crosstalk amount detection unit 31
 detects a crosstalk amount caused on the central detected signal Scent by
 the outer detected signal Sout (hereafter referred to as an outer
 crosstalk amount). The crosstalk amount detection unit 31 outputs the
 outer crosstalk amount to the subtracter 32 as an outer crosstalk signal
 Scto.
 As a result, the subtracter 32 calculates a difference between the inner
 crosstalk signal Scti and the outer crosstalk signal Scto on the basis of
 a detection principle described later, generates a difference signal Ssr,
 and outputs it to the radial tilt controller 22. This difference signal
 Ssr becomes a signal indicating the tilt amount of the radial direction
 (details will be described later).
 Upon being supplied with the difference signal Ssr, the radial tilt
 controller 22 generates a tilt correction control signal Sc.sub.1 for
 driving the liquid crystal panel 3 to correct the radial tilt on the basis
 of the difference signal Ssr, and applies the tilt correction control
 signal Sc.sub.1 to an electrode of the liquid crystal panel 3 for the
 radial tilt correction described later.
 On the other hand, the central detected signal Scent inputted to the
 tangential tilt detector 21 is delayed with delay amounts described later,
 in the delay circuits 36 and 37, and the delayed signals are outputted
 respectively as delayed signals Sd.sub.1 and Sd.sub.2. The central
 detected signal Scent and the delayed signal Sd.sub.1 are inputted to the
 crosstalk amount detection unit 33. The delayed signals Sd.sub.1 and
 Sd.sub.2 are inputted to the crosstalk amount detection unit 34.
 Thereafter, the delayed signal Sd.sub.1, the delayed signal Sd.sub.2, and
 the central detected signal Scent are treated as follows in the tangential
 tilt detector 21. Namely, information contained in the delayed signal
 Sd.sub.2 is treated as information corresponding to information obtained
 from a position located backward in time on a certain track of the optical
 disc 1. Information contained in the delayed signal Sd.sub.1 is treated as
 information corresponding to information obtained from a position located
 centrally in time on the certain track of the optical disc 1. Information
 contained in the central detected signal Scent is treated as information
 corresponding to information obtained from a position located forward in
 time on the certain track of the optical disc 1. By using the delayed
 signal Sd.sub.1, the delayed signal Sd.sub.2, and the central detected
 signal Scent, the tangential tilt detector 21 calculates a crosstalk
 amount caused on the central position by a position located forward and a
 crosstalk amount caused on the central position by a position located
 backward.
 In other words, the crosstalk amount detection unit 33 detects a crosstalk
 amount caused on the delayed signal Sd.sub.1 by the central detected
 signal Scent (i.e., the crosstalk amount caused on the central position by
 the position located forward on the same track, which is hereafter
 referred to as a front crosstalk amount). The crosstalk amount detection
 unit 33 outputs the detected crosstalk amount to the subtracter 35 as a
 front crosstalk signal Sctf.
 On the other hand, the crosstalk amount detection unit 34 detects a
 crosstalk amount caused on the delayed signal Sd.sub.1 by the delayed
 signal Sd.sub.2 (i.e., the crosstalk amount caused on the central position
 by the position located backward on the same track, which is hereafter
 referred to as a rear crosstalk amount). The crosstalk amount detection
 unit 34 outputs the detected crosstalk amount to the subtracter 35 as a
 rear crosstalk signal Sctr.
 As a result, the subtracter 35 calculates a difference between the front
 crosstalk signal Sctf and the rear crosstalk signal Sctr on the basis of a
 detection principle described later, generates a difference signal Sst,
 and outputs it to the tangential tilt controller 23. This difference
 signal Sst becomes a signal indicating the tilt amount of the tangential
 direction (details will be described later).
 Upon being supplied with the difference signal Sst, the tangential tilt
 controller 23 generates a tilt correction control signal Sc.sub.2 for
 driving the liquid crystal panel 3 to correct the tangential tilt on the
 basis of the difference signal Sst, and applies the tilt correction
 control signal Sc.sub.2 to an electrode of the liquid crystal panel 3 for
 the tangential tilt correction described later.
 Incidentally, a combination of the tilt correction control signal Sc.sub.1
 and the tilt correction control signal Sc.sub.2 shown in FIG. 2
 corresponds to the tilt correction control signal Sc shown in FIG. 1.
 Next, the operation of the radial tilt detection unit 20 will now be
 described by referring to FIGS. 3 to 5.
 First of all, a principle of the tilt amount detection in the radial
 direction in the present embodiment is described by referring to FIGS. 3A
 and 3B.
 When there is no tilt in the radial direction, the optical axis of the
 optical beam BM emitted through the objective lens 2 is perpendicular to
 an information recording surface of the optical disc 1 as shown in an
 upper part of FIG. 3B. (In the following description, the optical beams
 BM, BS.sub.1 and BS.sub.2 are represented by the optical beam BM.)
 Therefore, the shape of the central optical spot C itself becomes nearly a
 true circle. Thus, as shown in a lower part of FIG. 3B, a beam profile
 (i.e., an intensity distribution of the optical beam BM on the optical
 disc 1) also becomes symmetrical laterally in the radial direction with
 respect to the central track to which the optical beam BM is applied.
 Therefore, the inner crosstalk amount on the central track is equal to the
 outer crosstalk amount on the central track. In this case, therefore, the
 difference between the two crosstalk amounts becomes zero.
 On the other hand, in the case where the optical axis of the optical beam
 BM is deviated by an angle .theta. in the direction to the inner track as
 shown in FIG. 3A, the central optical spot C has a shape widened in the
 direction to the outer track which is opposite to the direction of the
 inclination of the optical axis. Thus, as shown in a lower part of FIG.
 3A, the beam profile also has a stronger beam intensity in the direction
 to the outer track, and the beam profile does not become symmetrical
 laterally in the radial direction about the central track. Therefore, the
 outer crosstalk amount on the central track becomes larger than the inner
 crosstalk amount on the central track. In this case, therefore, the
 difference between the two crosstalk amounts does not become zero. For
 example, the value obtained by subtracting the outer crosstalk amount from
 the inner crosstalk amount becomes a negative value.
 In contrast, in the case where the optical axis of the optical beam BM is
 deviated by an angle .theta. in the direction to the outer track as shown
 in FIG. 3C, the central optical spot C has a shape widened in the
 direction to the inner track. Thus, as shown in a lower part of FIG. 3C,
 the beam profile also has a stronger beam intensity in the direction to
 the inner track, and the beam profile does not become symmetrical
 laterally in the radial direction with respect to the central track. In
 the case of FIG. 3C, therefore, the inner crosstalk amount on the central
 track becomes larger than the outer crosstalk amount on the central track.
 In this case as well, therefore, the difference between the two crosstalk
 amounts does not become zero. For example, the value obtained by
 subtracting the outer crosstalk amount from the inner crosstalk amount
 becomes a positive value.
 By deriving the difference between the inner crosstalk amount and the outer
 crosstalk amount, the direction of the tilt with respect to the radial
 direction is found based upon whether the difference is positive or
 negative and the tilt amount is found from the amount of the difference,
 as evident from the foregoing description. Furthermore, by driving the
 liquid crystal panel 3 so as to make the difference equal to zero, the
 wavefront aberration caused by the tilt in the radial direction can be
 corrected.
 Next, the configurations of the crosstalk amount detection units 30 and 31,
 and concrete crosstalk amount detection operation will now be described by
 referring to FIGS. 4 and 5.
 As shown in FIG. 4, the crosstalk amount detection unit 30 includes: a
 reference signal generator 40; a subtracter 42; and a multiplier 44.
 Furthermore, the crosstalk amount detection unit 31 includes: a reference
 signal generator 41; a subtracter 43; and a multiplier 44.
 Next, the operation will now be described by referring to FIGS. 4 and 5. By
 conducting the same operation, the crosstalk amount detection unit 30 and
 the crosstalk amount detection unit 31 detect the inner crosstalk amount
 or the outer crosstalk amount, and output the inner crosstalk signal Scti
 or the outer crosstalk signal Scto. In the ensuing description, therefore,
 the operation will be described by taking the crosstalk amount detection
 unit 30 as a representative one.
 First of all, as a premise, the detection of the inner crosstalk amount in
 the crosstalk amount detection unit 30 is executed when the optical beam
 BM is applied to a reference region, which is formed on the optical disc 1
 in advance. In the reference region, the pits corresponding to the
 original record information to be reproduced are not formed in advance,
 but preset pits such as a plurality of pits having a fixed length are
 formed so as to be contiguous at fixed intervals.
 First of all, the reference signal generator 40 generates a reference
 signal Sr.sub.1 (which is represented by a broken line waveform shown in a
 second uppermost stage of FIG. 5) having the same waveform as that of the
 central detected signal Scent which is supposed to be obtained when the
 optical beam BM is applied to the reference region in such a state that
 there is no crosstalk at all (i.e., the central detected signal Scent
 having an ideal waveform). The reference signal generator 40 then outputs
 the reference signal Sr.sub.1 to the subtracter 42.
 Subsequently, the subtracter 42 subtracts the reference signal Sr.sub.1
 from the central detected signal Scent which is being inputted (which is
 represented by a solid line waveform shown in a second uppermost stage of
 FIG. 5), thus generates an error signal Srr.sub.1 (shown in a third
 uppermost stage of FIG. 5), and outputs the error signal Srr.sub.1 to the
 multiplier 44. This error signal Srr.sub.1 indicates the value of each
 sample of the crosstalk amount included in the central detected signal
 Scent.
 Subsequently, the multiplier 44 multiplies the generated error signal
 Srr.sub.1 by the inner detected signal Sin which is being inputted, and
 thereby generates the inner crosstalk signal Scti (shown in a lowermost
 stage of FIG. 5). The process conducted in the multiplier 44 is a process
 for extracting only the inner crosstalk amount caused by the inner
 detected signal Sin out of the error (the inner crosstalk or the outer
 crosstalk) included in the error signal Srr.sub.1 and generating the inner
 crosstalk as the inner crosstalk signal Scti.
 The inner crosstalk signal Scti outputted from the crosstalk amount
 detection unit 30 by the above described operation and the outer crosstalk
 signal Scto outputted from the crosstalk amount detection unit 31 by the
 same operation are inputted to the subtracter 32. Then, in accordance with
 the above described principle, the outer crosstalk signal Scto is
 subtracted from the inner crosstalk signal Scti. The difference signal Ssr
 indicating the tilt amount of the radial direction is thus generated.
 By referring to FIGS. 6A to 8, the operation of the tangential tilt
 detection unit 21 will now be described.
 First, the principle of the tilt amount detection in the tangential
 direction in the present embodiment will now be described by referring to
 FIGS. 6A to 7C.
 As shown in FIG. 6A, the above described radial tilt detector 20 detects
 the tilt amount of the radial direction from (i) the inner crosstalk
 signal Scti indicating the inner crosstalk amount caused on the central
 optical spot C by the inner optical spot IN and (ii) the outer crosstalk
 signal Scto indicating the outer crosstalk amount caused on the central
 optical spot C by the outer optical spot OUT.
 By diverting this method, the tangential tilt detector 21 described
 hereafter virtually forms a front optical spot FR, a rear optical spot RE,
 and a central optical spot C' on three different positions on the same
 central track Tcent as shown in FIG. 6B. By using the detected signals
 from these three optical spots (detected signals form three preceding and
 subsequent positions on the central track Tcent), the tangential tilt
 detector 21 detects the tilt amount of the tangential direction in
 accordance with the same principle as that of the radial tilt detector 20.
 As for detected signals from the above described virtual front optical spot
 FR, rear optical spot RE, and central optical spot C', the actual
 tangential tilt detector 21 regards the central detected signal Scent
 itself as the detected signal obtained from the front optical spot FR,
 regards the delayed signal Sd.sub.1 obtained by delaying the central
 detected signal Scent as the detected signal from the central optical spot
 C', and regards the delayed signal Sd.sub.2 obtained by further delaying
 the delayed signal as the detected signal from the rear optical spot RE.
 Next, the concrete principle of the tangential tilt detection is described
 by referring to FIGS. 7A to 7C.
 When there is no tilt in the tangential direction, the optical axis of the
 optical beam BM emitted from the objective lens 2 is perpendicular to an
 information recording surface of the optical disc 1 as shown in an upper
 part of FIG. 7B. Therefore, the shape of the central optical spot C'
 itself becomes nearly a true circle. As shown in a lower part of FIG. 7B,
 the beam profile (i.e., an intensity distribution of the optical beam BM
 on the optical disc 1) also becomes symmetrical longitudinally in the
 tangential direction with respect to the central track Tcent to which the
 optical beam BM is applied. Therefore, the front crosstalk amount on the
 central track Tcent is equal to the rear crosstalk amount on the central
 track Tcent. In this case, therefore, the difference between the two
 crosstalk amounts becomes zero.
 On the other hand, in the case where the optical axis of the optical beam
 BM is deviated by an angle .gamma. in a direction opposite to the beam
 scan direction of the optical beam BM as shown in FIG. 7A, the central
 optical spot C' has a shape widened forward in the scan direction of the
 optical beam BM which is opposite to the direction of the inclination of
 the optical axis. As shown in a lower part of FIG. 7A, therefore, the beam
 profile also has a stronger beam intensity forward in the scan direction
 of the optical beam BM, and the beam profile does not become symmetrical
 longitudinally in the tangential direction of the central track Tcent.
 Therefore, the front crosstalk amount becomes larger than the rear
 crosstalk amount. In this case, therefore, the difference between the two
 crosstalk amounts does not become zero. For example, the value obtained by
 subtracting the rear crosstalk amount from the front crosstalk amount
 becomes a positive value.
 In contrast, in the case where the optical axis of the optical beam BM is
 deviated by an angle .gamma. in the same direction as the beam scan
 direction as shown in FIG. 7C, the central optical spot C' has a shape
 widened backward in the beam scan direction of the optical beam BM. As
 shown in a lower part of FIG. 7C, therefore, the beam profile also has a
 stronger beam intensity backward in the scan direction of the optical beam
 BM, and the beam profile does not become symmetrical longitudinally in the
 tangential direction of the central track Tcent. Therefore, the front
 crosstalk amount becomes smaller than the rear crosstalk amount. In this
 case, therefore, the value obtained by subtracting the rear crosstalk
 amount from the front crosstalk amount becomes a negative value.
 By deriving the difference between the front crosstalk amount and the rear
 crosstalk amount, the direction of the tilt (forward or backward) with
 respect to the tangential direction is found based upon whether the
 difference is positive or negative and the tilt amount is found from the
 amount of the difference, as evident from the foregoing description.
 Furthermore, by driving the liquid crystal panel 3 so as to make the
 difference equal to zero, the wavefront aberration caused by the tilt in
 the tangential direction can be corrected.
 Comparing the case where the delay amounts in the delay circuits 36 and 37
 (which typically have the same delay amount) are small (as represented by
 white circles in FIGS. 7A to 7C) with the case where the delay amounts are
 large (as represented by black circles in FIGS. 7A to 7C), the following
 is found. Namely, in the case where the delay amount is small, the front
 crosstalk amount becomes larger than the rear crosstalk amount when the
 optical axis of the optical beam BM is deviated in a direction opposite to
 the scan direction of the optical beam BM (as shown in FIG. 7A), whereas
 the front crosstalk amount becomes smaller than the rear crosstalk amount
 when the optical axis of the optical beam BM is deviated in the same
 direction as the scan direction of the optical beam BM (as shown in FIG.
 7C).
 On the other hand, in the case where the delay amount is large, the front
 crosstalk amount becomes smaller than the rear crosstalk amount when the
 optical axis of the optical beam BM is deviated in a direction opposite to
 the scan direction of the optical beam BM, whereas the front crosstalk
 amount becomes larger than the rear crosstalk amount when the optical axis
 of the optical beam BM is deviated in the same direction as the scan
 direction of the optical beam BM.
 Therefore, depending upon the delay amounts of the delay circuits 36 and
 37, the polarity of the tilt is different. As a matter of fact, however,
 the crosstalk amount of the tangential direction can be detected by using
 either the crosstalk amounts in the white circle positions or the
 crosstalk amounts in the black circle positions. Alternatively, the
 polarity of the tilt amount derived by either the crosstalk amounts in the
 white circle positions or the crosstalk amounts in the black circle
 positions may be inverted and both of the cross talk amounts may be added
 together. Incidentally, a concrete example of a delay time .tau. in the
 delay circuits 36 and 37 is as following. It is now assumed that .lambda.
 represents the wavelength of the optical beam BM and NA represents the
 numerical aperture of the objective lens 2. When detecting the crosstalk
 amount in the white circle positions, for example, the delay time .tau. is
 expressed by a following expression.
EQU .tau.&lt;0.6.times.(.lambda./NA).times.(1/V.sub.L)
 Alternatively, when detecting the crosstalk amount in the black circle
 positions, the delay time .tau. is expressed by a following expression.
EQU .tau..gtoreq.0.6.times.(.lambda./NA).times.(1/V.sub.L)
 Next, the configurations of the crosstalk amount detection units 33 and 34,
 and the concrete crosstalk amount detection operation is described by
 referring to FIG. 8.
 As shown in FIG. 8, the crosstalk amount detection unit 33 includes: a
 reference signal generator 50; a subtracter 52; and a multiplier 54.
 Furthermore, the crosstalk amount detection unit 34 includes: a reference
 signal generator 51; a subtracter 53; and a multiplier 54.
 Next, the operation is described by referring to FIG. 8. Incidentally, the
 crosstalk amount detection unit 33 and the crosstalk amount detection unit
 34 detect the front crosstalk amount or rear crosstalk amount, and output
 the front crosstalk signal Sctf or the rear crosstalk signal Sctr by
 conducting the same operation. In the ensuing description, therefore, the
 operation will be described by taking the crosstalk amount detection unit
 33 as a representative one.
 As for the crosstalk amount detection unit 33 and the crosstalk amount
 detection unit 34, the foregoing description holds true by replacing the
 inner detected signal Sin in the crosstalk amount detection unit 30 with
 the central detected signal Scent in the crosstalk amount detection unit
 33, replacing the central detected signal Scent in the crosstalk amount
 detection unit 30 or the crosstalk amount detection unit 31 with the
 delayed signal Sd.sub.1 in the crosstalk amount detection unit 33 or the
 crosstalk amount detection unit 34, and replacing the outer detected
 signal Sout in the crosstalk amount detection unit 31 with the delayed
 signal Sd.sub.2 in the crosstalk amount detection unit 34. As for the
 waveform diagram of various positions in the crosstalk amount detection
 unit 33 or 34, therefore, FIG. 5 is used instead.
 First of all, as a premise, the detection of the front crosstalk amount in
 the crosstalk amount detection unit 33 is executed when the optical beam
 BM is applied to the above described reference region in the same way as
 the case of the crosstalk amount detection unit 30.
 At first, the reference signal generator 50 generates a reference signal
 Sr.sub.1 having the same waveform as that of the delayed signal Sd.sub.1
 which is supposed to be obtained when the optical beam BM is applied to
 the reference region in such a state that there is no crosstalk at all.
 The reference signal generator 50 then outputs the reference signal
 Sr.sub.1 to the subtracter 52.
 Subsequently, the subtracter 52 subtracts the reference signal Sr.sub.1
 from the delayed signal Sd.sub.1 which is being input, thus generates an
 error signal Srr.sub.1 and outputs the error signal Srr.sub.1 to the
 multiplier 54. This error signal Srr.sub.1 indicates the value of each
 sample of the crosstalk amount contained in the delayed signal Sd.sub.1.
 Subsequently, the multiplier 54 multiplies the generated error signal
 Srr.sub.1 by the central detected signal Scent which is being inputted,
 and thereby generates the front crosstalk signal Sctf (shown in the
 lowermost stage of FIG. 5). The process conducted in the multiplier 54 is
 a process for extracting only the front crosstalk amount caused by the
 central detected signal Scent out of the error (front crosstalk or rear
 crosstalk) included in the error signal Srr.sub.1 and generating the front
 crosstalk as the front crosstalk signal Sctf.
 The front crosstalk signal Sctf output from the crosstalk amount detection
 unit 33 by the above described operation and the rear crosstalk signal
 Sctb output from the crosstalk amount detection unit 34 by the same
 operation are inputted to the subtracter 35. In accordance with the above
 described principle, the rear crosstalk signal Sctb is subtracted from the
 front crosstalk signal Sctf. The difference signal Sst indicating the tilt
 amount of the tangential direction is thus generated.
 Thereafter, the difference signal Ssr and the difference signal Sst are
 outputted respectively to the radial tilt controller 22 and the tangential
 tilt controller 23 as described above.
 Next, the configurations and operations of the radial tilt controller 22
 and the tangential tilt controller 23 are described by referring to FIG.
 9.
 As shown in FIG. 9, the radial tilt controller 22 includes an integrator
 22A and a driver 22B. The tangential tilt controller 23 includes an
 integrator 23A and a driver 23B.
 Next, the operations of the radial tilt controller 22 and the tangential
 tilt controller 23 are described.
 The integrator 22A supplied with the difference signal Ssr averages sample
 values included in the difference signal Ssr every predetermined interval
 and outputs the averaged sample values to the driver 22B. Then, the driver
 22B conducts processing such as amplification on the averaged difference
 signal Ssr, and outputs a resultant signal to the liquid crystal panel 3
 as the tilt correction control signal Sc.sub.1.
 On the other hand, the integrator 23A supplied with the difference signal
 Sst averages sample values included in the difference signal Sst every
 predetermined interval and outputs the averaged sample values to the
 driver 23B. The driver 23B conducts processing such as amplification on
 the averaged difference signal Sst, and outputs a resultant signal to the
 liquid crystal panel 3 as the tilt correction control signal Sc.sub.2.
 Then, the liquid crystal panel 3 is driven by the tilt correction control
 signal Sc.sub.1 and the tilt correction control signal Sc.sub.2. The
 wavefront aberration caused in the optical beam B by the tilt is thus
 corrected.
 Next, the configuration and operation of the liquid crystal panel 3 will
 now be described by referring to FIGS. 10 to 13.
 As shown in its longitudinal section view of FIG. 10, the liquid crystal
 panel 3 has a two-layer structure. With a glass substrate 3h in between, a
 subsidiary liquid crystal panel 3' for correcting the wavefront aberration
 caused by the tilt in the radial direction, and a subsidiary liquid
 crystal panel 3" for correcting the wavefront aberration caused by the
 tilt in the tangential direction constitute the liquid crystal panel 3.
 With liquid crystal 3g containing liquid crystal molecules M in between,
 orientation films 3e and 3f for providing the liquid crystal 3g with
 predetermined molecule orientation are formed in the subsidiary liquid
 crystal panel 3'. Outside respective orientation films 3e and 3f,
 transparent electrodes 3c and 3d made of ITO (Indium-tin oxide) or the
 like are formed. On the outmost side of the subsidiary liquid crystal
 panel 3' which is connected to the subsidiary liquid crystal panel 3", a
 glass substrate 3a functioning as a protection layer is formed.
 In this configuration, the transparent electrode 3c is divided into pattern
 electrodes corresponding to distribution of the wave front aberration in
 the radial direction as described later. The transparent electrode 3d is a
 uniform planar electrode having no pattern electrodes.
 On the other hand, with liquid crystal 3m containing liquid crystal
 molecules M in between, orientation films 3k and 31 are formed in the
 subsidiary liquid crystal panel 3". Outside respective orientation films
 3k and 31, transparent electrodes 3i and 3j made of ITO or the like are
 formed. On the outmost side of the subsidiary liquid crystal panel 3"
 which is connected to the subsidiary liquid crystal panel 3', a glass
 substrate 3b functioning as a protection layer is formed.
 In this configuration, the transparent electrode 3j is divided into pattern
 electrodes corresponding to distribution of the wavefront aberration in
 the tangential direction as described later. The transparent electrode 3i
 is a uniform planar electrode having no pattern electrodes.
 As the liquid crystals 3g and 3m, those having a refractive index in the
 optical axis direction of the liquid crystal molecule M different from
 that in a direction perpendicular to the optical axis direction, i.e.,
 those having a double refraction effect are used as shown in FIG. 10. By
 changing voltage values applied to the transparent voltages 3c, 3d, 3i,
 and 3j, directions of the liquid crystal molecules M can be changed freely
 from the horizontal direction to the vertical direction as shown in FIGS.
 10A to 10C.
 At this time, the transparent electrodes 3d and 3i are driven so as to have
 a uniform voltage value. On the other hand, the transparent electrode 3c
 is supplied with the tilt correction control signal Sc.sub.1 for the
 radial tilt correction from the radial tilt controller 22 every pattern
 electrode thereof. Furthermore, the transparent electrode 3j is supplied
 with the tilt correction control signal Sc.sub.2 for the tangential tilt
 correction from the tangential tilt controller 23 every pattern electrode
 thereof.
 Next, the configurations of the transparent electrodes 3c and 3j are now
 described by referring to FIGS. 11A to 11C.
 As shown in FIG. 11A, the transparent electrode 3c is divided into five
 pattern electrodes 60a, 60b, 61a, 61b and 62 arranged so as to be line
 symmetrical. Respective pattern electrodes are mutually insulated. Among
 these pattern electrodes, the pattern electrodes 60a and 60b are driven by
 the same tilt correction control signal Sc.sub.1. The pattern electrodes
 61a and 61b are driven by the same tilt correction control signal
 Sc.sub.1. The tilt correction control signal Sc.sub.1 applied to the
 pattern electrodes 60a and 60b has a polarity opposite to that of the tilt
 correction control signal Sc.sub.1 applied to the pattern electrodes 61a
 and 61b. The reason why the transparent electrode 3c is divided into
 shapes shown in FIG. 11A is that it is desirable to make the shapes of the
 pattern electrodes (i.e., region divisions driven and controlled
 independently) identical to the shapes of the distribution of wavefront
 aberration generated in the radial direction described later. The size of
 the entire transparent electrode 3c is determined so as to make an
 incidence range SP of the optical beam BM onto the transparent electrode
 3c equal to a range as shown in FIG. 11A.
 On the other hand, the transparent electrode 3j is divided into five
 pattern electrodes 64a, 64b, 63a, 63b and 65 arranged so as to be line
 symmetrical as shown in FIG. 11B. Respective pattern electrodes are
 mutually insulated. Among these pattern electrodes, the pattern electrodes
 64a and 64b are driven by the same tilt correction control signal
 Sc.sub.2. The pattern electrodes 63a and 63b are driven by the same tilt
 correction control signal Sc.sub.2. The tilt correction control signal
 Sc.sub.2 applied to the pattern electrodes 64a and 64b has a polarity
 opposite to that of the tilt correction control signal Sc.sub.2 applied to
 the pattern electrodes 63a and 63b. The reason why the transparent
 electrode 3j is divided into shapes shown in FIG. 11B is that it is
 desirable, in the same way as the case of the transparent electrode 3c, to
 make the shapes of the pattern electrodes identical to the shapes of the
 distribution of the wavefront aberration generated in the tangential
 direction described later. The size of the entire transparent electrode 3j
 is determined so as to make an incidence range SP of the optical beam B
 onto the transparent electrode 3j equal to a range as shown in FIG. 11B.
 Next, the principle of correcting the wavefront aberration due to the tilt
 of the optical disc 1 conducted by the liquid crystal panel 3, and the
 factors determining the shapes of the above described respective pattern
 electrodes will be described by referring to FIGS. 11 to 13. Hereafter,
 the case where the wavefront aberration in the radial direction is
 corrected (i.e., the case where the wavefront aberration is corrected by
 applying the tilt correction control signal Sc.sub.1 to the transparent
 electrode 3c) will be described.
 First of all, the wavefront aberration in the pupil face of the objective
 lens 2 is represented as W (r, .O slashed.), where (r, .O slashed.) is
 polar coordinates of the pupil face.
 Now, if the optical disc 1 is inclined with respect to the axis of the
 optical beam BM (i.e., a tilt has occurred), then wavefront aberration
 (mainly the coma aberration) occurs as described above, and it becomes
 impossible to narrow down the optical beam BM by using the objective lens
 2. In this case, wavefront aberration represented by a following
 expression (1) occupies a major portion of the wavefront aberration Wtlt
 (r, .O slashed.) caused by the tilt angle.
EQU Wtlt (r, .O slashed.).apprxeq..omega..sub.31.times.r.sup.3.times.cos .O
 slashed.+.omega..sub.11.times.r.times.cos .O slashed. (1)
 wherein .omega..sub.31 and .omega..sub.11 represent constants given by the
 tilt angle of the optical disc 1, the thickness of the substrate, the
 refractive index of the substrate and the numerical aperture (NA) of the
 objective lens 2, .omega..sub.31 represents the coma aberration,
 .omega..sub.11 represents the aberration caused by a movement of an image
 point. A result of calculation of the wavefront aberration distribution on
 the pupil face using this expression (1) corresponds to a wavefront
 aberration distribution shown in FIG. 12 (i.e., a wavefront aberration
 distribution caused by the tilt angle of the radial direction) described
 later.
 Representing the standard deviation of the wavefront aberration W (r, .O
 slashed.) on the pupil face by Wrms, the standard deviation Wrm is
 represented by a following expression (2).
EQU Wrms={.intg..intg.(W (r, .O slashed.)-Wo.sup.2)r dr d.O
 slashed./.pi.}.sup.1/2 (2)
 wherein Wo in the expression (2) is the average value of W (r, .O slashed.)
 on the pupil face. This Wrms is used for the evaluation of the wavefront
 aberration. By reducing this Wrms, a favorable reproduction less
 influenced by the wavefront aberration can be conducted.
 As evident from the expression (2), the wavefront aberration can be
 corrected by making the value of W (r, .O slashed.) small. Thus, in order
 to correct the Wtlt (r, .O slashed.) caused by the inclination of the
 optical disc 1 to its radial direction, assuming that the refractive index
 of a region of the liquid crystal 3g corresponding to a certain pattern
 electrode is changed by .DELTA.n by controlling the tilt correction
 control signal Sc.sub.1 applied to respective pattern electrodes on the
 transparent electrode 3c of the liquid crystal panel 3, by this change of
 the refractive index, it is possible to give an optical path difference
 .DELTA.n.times.d (where d is the thickness of the liquid crystal 3g) to
 the optical beam BM passing through a region corresponding to that pattern
 electrode.
 Assuming that the optical path length given by the liquid crystal 3g is Wlc
 (r, .O slashed.), the wavefront aberration W (r, .O slashed.) in the
 radial direction on the pupil face of the objective lens 2 when the liquid
 crystal panel 3 is disposed is represented by a following expression (3).
EQU W (r, .O slashed.)=Wtlt (r, .O slashed.)+Wlc (r, .O slashed.) (3)
 As evident form the expression (3), the wavefront aberration W (r, .O
 slashed.) caused by the tilt in the radial direction of the optical disc 1
 can be canceled by satisfying the following expression.
EQU W (r, .O slashed.)=Wtlt (r, .O slashed.)+Wlc (r, .O slashed.)=0
 In other words, it is found that it is necessary to give to the optical
 beam BM a wavefront aberration having a polarity opposite to the wavefront
 aberration Wtlt (r, .O slashed.) caused by the tilt in the radial
 direction of the optical disc 1, i.e., a wavefront aberration Wlc (r, .O
 slashed.) satisfying a following expression by using the liquid crystal
 3g.
EQU Wlc (r, .O slashed.)=-Wtlt (r, .O slashed.)
 Therefore, in order to give the wavefront aberration Wlc (r, .O slashed.)
 of the opposite polarity to the wavefront aberration Wtlt (r, .O slashed.)
 caused by the tilt angle of the optical disc 1, by using the liquid
 crystal 3g, respective pattern electrodes may me equipped so as to divide
 the liquid crystal 3g in association with the wavefront aberration
 distribution caused by the tilt angle in the radial direction of the
 optical disc 1 shown in FIG. 12, and the voltages applied to regions
 corresponding to the respective pattern electrodes may be controlled so as
 to give the wavefront aberration of opposite polarity to the wavefront
 aberration caused by the tilt in the radial direction to the wavefront
 aberration caused by the tilt in the radial direction.
 FIG. 12 shows the wavefront aberration distribution of the radial direction
 seen on the pupil face of the objective lens 2. To be more concrete, FIG.
 12 shows the wavefront aberration distribution in an optimum image point
 of the optical spot, within the range of a maximum region of the incident
 optical beam BM, in the case where the information recording face of the
 optical disc 1 is inclined by +1.degree. in the radial direction.
 Centering around a region A having a range of -25 nm to +25 nm, values of
 the wavefront aberration are represented by boundary lines of regions A to
 K each having a range value of 50 nm. In FIG. 12, X2--X2 is an axis
 corresponding to the direction of inclination of the optical disc 1 (i.e.,
 corresponding to the radial direction). In FIG. 13, the wavefront
 aberration distribution is represented by the distribution characteristic
 on the X2--X2 axis.
 The distribution itself of the wavefront aberration does not depend upon
 the magnitude of the tilt in the radial direction, but has a constant
 distribution shape. The wavefront aberration amount changes depending upon
 the magnitude of the tilt. This point will now be described by referring
 to FIG. 13. The peak value of a curve shown in FIG. 13 becomes higher as
 the tilt becomes large, and becomes lower as the tilt becomes small.
 Paying attention to the distribution of the wavefront aberration, the shape
 of division of the transparent electrode 3c is made similar to the
 wavefront aberration distribution of FIG. 12 in the liquid crystal panel
 of the present embodiment. The region of the liquid crystal 3g
 corresponding to each pattern electrode gives a phase difference to the
 optical beam BM so as to cancel the wavefront aberration Wtlt (r, .O
 slashed.) which is occurring. The influence of the wavefront aberration
 Wtlt (r, .O slashed.) caused by the tilt in the radial direction is thus
 reduced to such a range as not to affect the reproduction. In other words,
 the voltage control is effected by using the tilt correction control
 signal Sc.sub.1 every division region of the liquid crystal 3g (every
 division region corresponding to each pattern electrode). As a result, the
 direction of the liquid crystal molecules M is changed, and the refractive
 index of each division region is changed. Thereby, by giving a phase
 difference to the optical beam BM, the wavefront aberration Wtlt (r, .O
 slashed.)caused when the disc 1 is inclined in the radial direction is
 corrected.
 As heretofore described, each of the pattern electrodes shown in FIG. 11A
 has a shape which is set on the basis of the wavefront aberration
 distribution in the case where the recording face of the optical disc 1 is
 inclined in the radial direction by +1.degree. (as shown in FIG. 12). The
 transparent electrode 3c has five pattern electrodes corresponding to the
 case where the wavefront aberration is approximated by five values.
 Incidentally, the region corresponding to the pattern electrode 62 is a
 region containing the region having the value of the wavefront aberration
 equivalent to 0. The region of the liquid crystal 3g corresponding to the
 pattern electrode 61b and the region of the liquid crystal 3g
 corresponding to the pattern electrode 60b have symmetrical shapes, and
 the values of the phase differences given to the transmitted optical beam
 BM by these regions have opposite polarities. Furthermore, the region of
 the liquid crystal 3g corresponding to the pattern electrode 60a and the
 region of the liquid crystal 3g corresponding to the pattern electrode 61a
 have symmetrical shapes, and the values of the phase differences given to
 the transmitted optical beam B by these regions have opposite polarities.
 In the foregoing description referring to FIGS. 10 to 13, the case where
 the wavefront aberration of the optical disc 1 caused in the radial
 direction is corrected has been described. In the case where the wavefront
 aberration caused in the tangential direction of the optical disc 1 is
 corrected, by applying the contents of the shapes of the pattern
 electrodes of the transparent electrode 3c and so on with rotating them by
 an angle of 90.degree., it corresponds to the case where the wavefront
 aberration of the tangential direction is corrected by using the
 transparent electrode 3j. Therefore, the shapes of the pattern electrodes
 64a, 64b, 63a, 63b and 65 in the transparent electrode 3j are also made
 into the shape similar to the wavefront aberration distribution having a
 symmetry axis parallel to the tangential direction (i.e., the wavefront
 aberration distribution in the case where the X2--X2 axis in FIG. 12 is
 set as the tangential direction).
 The pattern electrodes 64a, 64b, 63a, 63b and 65 are driven by the tilt
 correction control signal Sc.sub.2, and correct the wavefront aberration
 caused by the tilt in the tangential direction.
 As described above, according to the operation of the information
 reproducing apparatus S of the first embodiment, the tilt is detected on
 the basis of the central detected signal Scent and the like obtained by
 the irradiation of the optical beam B. For detecting the tilt, therefore,
 it is not necessary to emit another optical beam (such as an optical beam
 exclusive for the tilt direction as in the conventional technique) other
 than the optical beam B. The configuration for correcting the aberration
 can thus be simplified.
 Furthermore, since a mechanical operating portion is not required for the
 tilt detection, the reliability of the aberration correcting apparatus is
 improved and the size can be reduced.
 Therefore, the aberration caused by the tilt of the optical axis of the
 optical beam BM can be corrected accurately by a simple and small-sized
 configuration.
 Further, since the radial tilt is detected by using the inner detected
 signal Sin or the outer detected signal Sout from an adjacent track and
 the tangential tilt is detected by using the central detected signal
 Scent, the tilts in the respective directions can be accurately detected
 and the wavefront aberration can be corrected.
 Furthermore, since the inner crosstalk amount and the outer crosstalk
 amount are detected, and the wavefront aberration caused in the radial
 direction is corrected by making the difference between the inner
 crosstalk amount and the outer crosstalk amount equal to zero, the
 wavefront aberration can be corrected accurately with a simple
 configuration.
 Furthermore, since the front crosstalk amount and the rear crosstalk amount
 are detected, and the wavefront aberration caused in the tangential
 direction is corrected by making the difference between the front
 crosstalk amount and the rear crosstalk amount equal to zero, the
 wavefront aberration can be corrected accurately with a simple
 configuration.
 Furthermore, since the wavefront aberration is corrected by using the
 liquid crystal panel 3 disposed on the optical path of the optical beam B,
 the wavefront aberration can be corrected with a simple configuration.
 Furthermore, since the wavefront aberration is corrected by applying a
 voltage based upon the tilt to the liquid crystal 3g or 3m in the liquid
 crystal panel 3 and thereby giving a phase difference to the optical beam
 B, the wavefront aberration of the optical beam B can be corrected
 efficiently.
 Furthermore, since the transparent electrode in the liquid crystal panel 3
 includes a plurality of pattern electrodes having shapes corresponding to
 the distribution of the wavefront aberration caused in the optical beam B,
 and the radial tilt controller 22 or the tangential tilt controller 23
 applies a voltage individually to each pattern electrode to correct the
 wavefront aberration, the wavefront can be corrected efficiently.
 Furthermore, since the optical beam B is focused onto the optical disc 1 by
 the objective lens 2 and the record information is reproduced by the
 demodulation unit 12, the wavefront aberration caused by the inclination
 of the optical axis can be corrected and the information can be reproduced
 accurately with a simple configuration.
 (II) Second Embodiment
 Next, a second embodiment which is another embodiment of the present
 invention will now be described by referring to FIGS. 14 to 16.
 In the above described first embodiment, the configuration having the
 multiplier 44, 45, 54 or 55 in the crosstalk amount detection units 30,
 31, 33 or 34 has been described. However, the second embodiment has such a
 configuration that each crosstalk amount is detected without using
 multipliers involving the complicated configurations in the circuit
 formation.
 First of all, the configuration of the crosstalk amount detection unit in
 the second embodiment is described by referring to FIG. 14. In the second
 embodiment, the configuration other than the respective crosstalk amount
 detection units is the same as that of the information reproducing
 apparatus S of the first embodiment, and consequently the detailed
 description thereof are omitted. Furthermore, the information reproducing
 apparatus of the second embodiment also has the four crosstalk amount
 detection units conducting the same operation in the same way as the
 information reproducing apparatus S of the first embodiment. In the
 ensuing description, however, a crosstalk amount detection unit 30' for
 detecting the inner crosstalk amount in the second embodiment will be
 described as a representative.
 As shown in FIG. 14, the crosstalk amount detection unit 30' includes: an
 absolute value circuit 70; a comparator 71; a zero-cross detection circuit
 72; an extraction circuit 73; a polarity selection circuit 74; an AND
 circuit 77; a polarity detection circuit 78; and three delay circuits 79A
 to 79C.
 The polarity selection circuit 74 includes: an inverter 75; and a
 change-over unit 76.
 The operation will now be described by referring to FIGS. 14 and 15.
 In the crosstalk amount detection units such as 30 of the information
 recording and reproducing apparatus S of the above described first
 embodiment, the reference signal Sr.sub.1 is subtracted from the inputted
 central detected signal Scent and a resultant difference is multiplied by
 the inner detected signal Sin, for example. In the crosstalk amount
 detection unit 30' of the second embodiment, only sample values of the
 central detected signal Scent located near zero-cross points are
 extracted, and the crosstalk amount is detected according to the extracted
 sample values. This is equivalent to the case where the reference signal
 Sr.sub.1 in the crosstalk amount detection unit 30 of the information
 recording and reproducing apparatus S of the first embodiment is not a
 signal having a periodic waveform but a signal having only fixed values of
 zero.
 The inner detected signal Sin inputted to the crosstalk amount detection
 unit 30' is delayed in the delay circuit 79A by one sampling period in the
 inner detected signal Sin, and is then outputted to the polarity detection
 circuit 78. In the polarity detection circuit 78, the polarity of the
 inner detected signal Sin is detected. In the absolute value circuit 70,
 the absolute value of the inner detected signal Sin is detected and is
 outputted as an absolute value signal Sab.
 Then, in the comparator 71, each sample value contained in the absolute
 value signal Sab is compared with a value of a threshold signal Sth
 inputted thereto beforehand. Only each of sample values in the absolute
 value signal Sab having an absolute value greater than the value of the
 threshold signal Sth is outputted as a compared signal Scm (as shown in a
 fourth uppermost stage of FIG. 15). The reason why the comparator 71
 compares the value of the threshold signal Sth with each sample value
 contained in the absolute value signal Sab and extracts only sample values
 each having a value larger than the value of threshold signal Sth in this
 way is that the sample values each having a value close to the zero level
 and smaller than the value of the threshold signal Sth among the sample
 values contained in the inner detected signal Sin can be handled as the
 sample values which cannot give the crosstalk to the central detected
 signal Scent.
 On the other hand, the central detected signal Scent inputted to the
 crosstalk amount detection unit 30' is delayed in the delay circuit 79C by
 one sampling period of the central detected signal Scent, and outputted to
 the extraction circuit 73. In addition, the central detected signal Scent
 is inputted to the zero-cross detection circuit 72. The zero-cross
 detection circuit 72 generates a zero-cross signal Szr having a preset
 predetermined pulse width containing the timing of the zero-cross point of
 the central detected signal Scent (as shown in a third uppermost stage of
 FIG. 15).
 Then, the above described compared signal Scm is delayed in the delay
 circuit 79B by one sampling period of the inner detected signal Sin. A
 resultant delayed signal and the zero-cross signal Szr are inputted to the
 AND circuit 77. As a logical product of them, a logical product signal Sen
 (as shown in a fifth uppermost stage of FIG. 15) is outputted. This
 logical product signal Sen detects such timing that the inner detected
 signal Sin is large enough to exert an influence of crosstalk on the
 central detected signal Scent and the central detected signal Scent is
 located near a zero-cross point. At such timing, the logical product
 signal Sen becomes "HIGH".
 Then, the logical product signal Sen is inputted to an enable terminal of
 the extraction circuit 73. Only sample values of the central detected
 signal Scent inputted to the extraction circuit 73 at such timing that the
 logical product signal Sen is "HIGH" are inputted to the polarity
 selection circuit 74 as an extracted signal Spu (as shown in a third
 lowermost stage of FIG. 15). This extracted signal Spu indicates the
 crosstalk amount contained in the sample values of the central detected
 signal Scent located near zero-cross points (which is the sum of the outer
 crosstalk amount and the inner crosstalk amount caused by the influence of
 the sample values having such a magnitude as to be capable of providing
 the central detected signal Scent with crosstalk, among the sample values
 contained in the inner detected signal Sin).
 Then, in the polarity selection circuit 74, the inputted extracted signal
 Spu is inputted to one of terminals of the switch 76 as it is, and is
 inputted to the inverter 75. Thereafter, the inverter 75 inverts the
 polarity of the inputted extracted signal Spu, and outputs a resultant
 signal to the other of the terminals of the switch 76 as an inverted
 extracted signal Spur.
 On the other hand, the switch 76 is supplied with a polarity signal Sch (as
 shown in a second lowermost stage of FIG. 15) obtained as a result of
 judging the polarity of the inner detected signal Sin in the polarity
 detection circuit 78.
 Then, on the basis of the polarity signal Sch, the switch 76 selects the
 extracted signal Spu when the polarity of the inner detected signal Sin is
 positive, and selects the inverted extracted signal Spur when the polarity
 of the inner detected signal Sin is negative. The selected signal is
 outputted as an inner crosstalk signal Scti'.
 By the operation of the polarity selection circuit 74, a process similar to
 the process conducted by the multiplier 44 in the information reproducing
 apparatus S of the first embodiment is conducted on the extracted signal
 Spu. The inner crosstalk signal Scti' indicating the magnitude of the
 inner crosstalk is thus outputted. The timing adjustment of the whole in
 the operation heretofore described is executed by the delay circuits 79A
 to 79C.
 Then, on the basis of the outer crosstalk signal generated by processing
 similar to that of the crosstalk amount detection unit 30', and the above
 described inner crosstalk signal Scti', the tilt amount in the radial
 direction is detected by the operation similar to that of the information
 reproducing apparatus S of the first embodiment. The wavefront aberration
 caused by the tilt in the radial direction is thus corrected.
 As for the tangential direction as well, the front crosstalk amount and the
 rear crosstalk amount are respectively detected by two crosstalk amount
 detection units each having a configuration similar to that of the
 crosstalk amount detection unit 30'. On the basis of the detected front
 crosstalk amount and rear crosstalk amount, the tilt amount in the
 tangential direction is detected by the operation similar to that of the
 information reproducing apparatus S of the first embodiment. The wavefront
 aberration caused by the tilt in the tangential direction is thus
 corrected.
 Next, the configuration and operation of the zero-cross detection circuit
 72 in the above described crosstalk amount detection unit 30' is described
 by referring to FIG. 16.
 As shown in FIG. 16A, the zero-cross detection circuit 72 includes: an
 absolute value circuit 80; a comparator 81; delay circuits 82, 83, 85 and
 86; an exclusive OR circuit 84; and AND circuits 87 and 88.
 The operation is now described by referring to FIG. 16B.
 The central detected signal Scent inputted to the zero-cross detection
 circuit 72 is inputted as it is to the absolute value circuit 80, one
 terminal of the exclusive OR circuit 84 and the delay circuit 82.
 Then, in the delay circuit 82, the central detected signal Scent is delayed
 by one sampling period (i.e., by one sampling period of the sampling
 frequency of the A/D converter 26). Furthermore, the central detected
 signal Scent is delayed by the same time in the delay circuit 83 as well,
 and is inputted to the other terminal of the exclusive OR circuit 84. In
 other words, a sample value represented by a character D.sub.1 in FIG. 16
 B and a sample value represented by a character D.sub.3 are simultaneously
 inputted to the exclusive OR circuit 84. The exclusive OR circuit 84
 outputs an exclusive OR signal Ss.sub.2 which becomes "HIGH" only when the
 sign of the sample value indicated by the character D.sub.1 is different
 from the sign of the sample value indicated by the character D.sub.3. The
 exclusive OR signal Ss.sub.2 is inputted to one terminal of the AND
 circuit 88.
 As for the central detected signal Scent inputted to the absolute value
 circuit 80, the absolute value thereof is detected in the absolute value
 circuit 80 and is outputted as an absolute value signal Sab'.
 Then, each sample value contained in the absolute value signal Sab' is
 compared in the comparator 81 with a value of a threshold signal Sth'
 inputted beforehand. A comparison signal Sd.sub.5 is thus outputted. When
 a sample value in the absolute value signal Sab' having an absolute value
 larger than the value of the threshold signal Sth' is inputted to the
 comparator 81, the comparison signal Sd.sub.5 becomes "HIGH." When a
 sample value in the absolute value signal Sab' having an absolute value
 smaller than the value of the threshold signal Sth' is inputted to the
 comparator 81, the comparison signal Sd.sub.5 becomes "LOW." The
 comparison signal Sd.sub.5 is outputted to a first terminal of the AND
 circuit 87 having three input terminals.
 On the other hand, the comparison signal Sd.sub.5 is inputted to the delay
 circuit 85 as well, is delayed therein by one sampling period of the
 sampling frequency of the A/D converter 26, and is outputted as a delayed
 comparison signal Sd.sub.4. This delayed comparison signal Sd.sub.4 is
 inputted to the subsequent delay circuit 86 as it is. In addition, the
 delayed comparison signal Sd.sub.4 is inverted in "HIGH"/"LOW" and is
 inputted to a second terminal of the AND circuit 84.
 Next, the delay circuit 86 further delays the delayed comparison signal
 Sd.sub.4 by one sampling period of the sampling frequency of the A/D
 converter 26, and outputs a resultant signal to a third terminal of the
 AND circuit 87 as a delayed comparison signal Sd.sub.3.
 The AND circuit 87 generates a logical product signal Ss.sub.1. Only when
 all of three signals simultaneously inputted are "HIGH," the logical
 product signal Ss.sub.1 becomes "HIGH." The logical product signal
 Ss.sub.1 is inputted to the other terminal of the AND circuit 88.
 Here, the comparison signal Sd.sub.5 corresponds to the sample value
 indicated by the character D.sub.3 in FIG. 16B, the delayed comparison
 signal Sd.sub.4 corresponds to the sample value indicated by a character
 D.sub.2 in FIG. 16B, and the delayed comparison signal Sd.sub.3
 corresponds to the sample value indicated by the character D.sub.1 in FIG.
 16B. The delayed comparison signal Sd.sub.4 is inputted to the AND circuit
 87 after being inverted. As a result, the logical product signal Ss.sub.1
 inputted to the other terminal of the AND circuit 88 becomes "HIGH" only
 when both of the absolute value of the sample value indicated by the
 character D.sub.3 and the absolute value of the sample value indicated by
 the character D.sub.1 are larger than the value of the threshold signal
 Sth' and the absolute value of the sample value indicated by the character
 D.sub.2 is smaller than the value of the threshold signal Sth'.
 On the other hand, the one terminal of the AND circuit 88 is supplied with
 the exclusive OR signal Ss.sub.2 which becomes "HIGH" only when the sample
 value indicated by the character D.sub.1 and the sample value indicated by
 the character D.sub.3 have different signs. As a result, the zero-cross
 signal Szr which is the output signal of the AND circuit 88 becomes
 "HIGH," when a sample value, which has an absolute value smaller than the
 value of the threshold signal Sth' and which is a sample value indicated
 by the character D.sub.2 having a sign different from that of both of the
 sample values adjacent thereto (i.e., zero-cross sample value), is
 inputted to the extraction circuit 73.
 Strictly speaking, the zero-cross signal Szr becomes "HIGH" when the sample
 value indicated by the character D.sub.3 is outputted from the zero-cross
 detection circuit 72. However, the central detected signal Scent inputted
 to the extraction circuit 73 has been delayed in the delay circuit 79C by
 one sampling period. As a result, the logical product signal Sen, which
 becomes "HIGH" when the sample value of the central detected signal Scent
 indicated by the character D.sub.2 is inputted to the extraction circuit
 73, is inputted to the enable terminal of the extraction circuit 73.
 According to the information reproducing apparatus including the crosstalk
 amount detection unit 30' of the second embodiment, multipliers each
 having a complicated configuration become unnecessary as heretofore
 described. Effects similar to those of the information reproducing
 apparatus S can be obtained with a simple configuration.
 In the above described configuration of the crosstalk amount detection unit
 30', the delay circuits 79A to 79C are needed in the case where the
 configuration shown in FIG. 16A is used as the zero-cross detection
 circuit 72. If a configuration which does not include the delay circuits
 (e.g., such a configuration that only sample values each located near the
 zero level and having a value which does not exceed a predetermined
 threshold value are extracted out of the sample values of the central
 detected signal Scent and are outputted as the zero-cross signal Szr) is
 used as the zero-cross detection circuit 72, the delay circuits 79a to 79C
 become unnecessary.
 (III) Third Embodiment
 A third embodiment which is another embodiment of the present invention
 will now be described by referring to FIGS. 17 and 18.
 In the above described first and second embodiments, the radial tilt
 detector 20 generates the difference signal Ssr representing the tilt
 amount of the radial direction by subtracting the outer crosstalk signal
 Scto from the inner crosstalk signal Scti, and the tangential tilt
 detector 21 generates the difference signal Sst representing the tilt
 amount of the tangential direction by subtracting the rear crosstalk
 signal Sctr from the front crosstalk signal Sctf.
 In a radial tilt detector and a tangential tilt detector of the third
 embodiment replacing the radial tilt detector 20 and the tangential tilt
 detector 21, a crosstalk canceler (a so-called CTC) for removing the
 crosstalk from the central detected signal Scent is constituted. By
 regarding a tap coefficient indicating the cancel amount in the CTC as a
 parameter indicating the crosstalk amount, the crosstalk amount is
 detected.
 First of all, the principle of the third embodiment will now be described
 by referring to the case where the radial tilt is detected.
 The crosstalk amount on the central detected signal Scent is represented by
 a following expression.
EQU Scent=Sr.sub.1 +a.times.Sin+b.times.Sout
 wherein Sr.sub.1 is the above described reference signal, i.e., an ideal
 central detected signal which does not contain the crosstalk, a is a
 crosstalk coefficient from the inner detected signal Sin, and b is a
 crosstalk coefficient from the outer detected signal Sout.
 On the other hand, assuming that a tap coefficient for removing the inner
 crosstalk is Cin and a tap coefficient for removing the outer crosstalk is
 Cout, the central detected signal Scent' after the crosstalk has been
 removed is expression by a following expression.
 Scent'=Scent-Cin.times.Sin-Cout.times.Sout =Sr.sub.1
 +(a-Cin).times.Sin+(b-Cout).times.Sout
 If the crosstalk remains in the central detected signal Scent', a
 coefficient control unit for controlling the tap coefficient in the third
 embodiment controls the tap coefficient so as to cancel the remaining
 crosstalk. (Details of the coefficient control unit will be described
 later.) The central detected signal Scent' finally becomes a signal
 containing no crosstalk. In other words, the relationship between the
 inner crosstalk and the coefficient a and the relationship between the
 outer crosstalk and the coefficient b are respectively expressed by
 following expressions.
EQU a-Cin=0 and b-Cout=0.
 Therefore,
 a=Cin and b=Cout.
 Eventually, the respective tap coefficients a and b represent respective
 crosstalk amounts.
 Next, the radial tilt detector and the tangential tilt detector of the
 third embodiment for detecting the tilt amounts on the basis of the above
 described principle will now be described by referring to FIGS. 17 and 18.
 In FIGS. 17 and 18, the same constitutional elements as those in the first
 embodiment carry the same reference numerals, and the detailed
 explanations thereof are omitted. Furthermore, in the third embodiment,
 configuration other then the radial tilt detector and the tangential tilt
 detector described later is the same as that of the information
 reproducing apparatus S of the first embodiment, and consequently the
 detailed explanations thereof are omitted.
 First of all, the radial tilt detector of the third embodiment is described
 by referring to FIG. 17.
 As shown in FIG. 17, a radial tilt detector 20' of the third embodiment
 includes: coefficient control units 90 and 91; filters 94 and 95 which are
 digital transversal filters each having the number of taps equivalent to
 "1"; a subtracter 96 serving as one example of a subtraction device; and a
 subtracter 32 similar to that of the radial tilt detector 20 of the first
 embodiment.
 The coefficient control unit 90 includes: the crosstalk amount detection
 unit 30 of the first embodiment; and an integrator 92.
 The coefficient control unit 91 includes: the crosstalk amount detection
 unit 31 of the first embodiment; and an integrator 93.
 The operation is now described.
 The inner detected signal Sin inputted to the radial tilt detector 20' is
 outputted to the filter 94, and is also outputted to a multiplier 44 in
 the crosstalk amount detection unit 30.
 On the other hand, from the central detected signal Scent inputted to the
 crosstalk amount detector 20', a filter signal Sfti representing the inner
 crosstalk amount supplied from the filter 94 and a filter signal Sfto
 representing the outer crosstalk amount supplied from the filter 95 are
 subtracted in the subtracter 96. Thereafter, the result is outputted to a
 subtracter 42 in the crosstalk amount detection unit 30 and a subtracter
 43 in the crosstalk amount detection unit 31 as a central detected signal
 Scent' with the reduced crosstalk.
 Then, by using the inner detected signal Sin and the central detected
 signal Scent' which are being inputted, the crosstalk amount detection
 unit 30 generates the inner crosstalk signal Scti representing the inner
 crosstalk amount by conducting an operation similar to that of the first
 embodiment, and outputs the inner crosstalk signal Scti to the integrator
 92.
 After that, by integrating and averaging the inner crosstalk signal Scti,
 the integrator 92 generates a tap control signal Scin (corresponding to
 the above described tap coefficient Cin) for controlling the tap
 coefficient of the filter 94, and outputs the tap control signal Scin to
 the filter 94 and the subtracter 32.
 Finally, the filter 94 generates the above described filter signal Sfti on
 the basis of the tap control signal Scin, and outputs the filter signal
 Sfti to the subtracter 96.
 On the other hand, the basis of the central detected signal Scent' and the
 outer detected signal Sout respectively inputted, the coefficient control
 unit 91 generates a tap control signal Scout (corresponding to the above
 described tap coefficient Cout) for controlling the tap coefficient of the
 filter 95 by conducting an operation similar to that of the coefficient
 control unit 90, and outputs the tap control signal Scout to the filter 95
 and the subtracter 32.
 The filter 95 generates the above described filter signal Sfto on the basis
 of the tap control signal Scout, and outputs the filter signal Sfto to the
 subtracter 96.
 In the subtracter 96, the filter signal Sfti and the filter signal Sf to
 which are being inputted are subtracted from the central detected signal
 Scent. A central detected signal Scent' with the crosstalk amount reduced
 is newly generated. In the generation of the central detected signal
 Scent', the inner crosstalk and the outer crosstalk contained in the
 central detected signal Scent are not removed at once by the filter signal
 Sfti and the filter signal Sfto, but they are gradually reduced by
 repetition of the operation of a closed loop including the coefficient
 control units 90 and 91 and the filters 94 and 95. Finally, the central
 detected signal Scent' which does not contain respective crosstalks is
 generated.
 On the other hand, on the basis of the above described principle, the
 subtracter 32 subtracts the tap control signal Scout from the tap control
 signal Scin, thereby generates a difference signal Ssr' representing the
 crosstalk amount of the radial direction, and outputs the difference
 signal Ssr' to the radial tilt controller 22.
 Next, the tangential tilt detector of the third embodiment will now be
 described by referring to FIG. 18.
 As shown in FIG. 18, the tangential tilt detector 21' includes: coefficient
 control units 97 and 98; filters 99 and 100 which are digital transversal
 filters each having the number of taps equivalent to 1; and a subtracter
 101. The tangential tilt detector 21' further includes: a subtracter 35;
 and delay circuits 36 and 37 in the same way as the tangential tilt
 detector 21 of the first embodiment.
 The coefficient control unit 97 includes: the crosstalk amount detection
 unit 34 of the first embodiment; and an integrator 102.
 The coefficient control unit 98 includes: the crosstalk amount detection
 unit 33 of the first embodiment; and an integrator 103.
 The operation is now described.
 The central detected signal Scent inputted to the tangential tilt detector
 21' is outputted to the delay circuit 36, and the filter 100, and is also
 outputted to a multiplier 54 in the crosstalk amount detection unit 33.
 From the delayed signal Sd.sub.1 outputted from the delay circuit 36, a
 filter signal Sftf representing the front crosstalk amount supplied from
 the filter 100 and a filter signal Sftr representing the rear crosstalk
 amount supplied from the filter 99 are subtracted in the subtracter 101.
 Thereafter, the result is outputted to a subtracter 52 in the crosstalk
 amount detection unit 33 and a subtracter 53 in the crosstalk amount
 detection unit 34 as a delayed signal Sd.sub.1 ' with reduced crosstalk.
 By using the central detected signal Scent and the delayed signal Sd.sub.1
 ' which are being inputted, the crosstalk amount detection unit 33
 generates the front crosstalk signal Sctf representing the front crosstalk
 amount by conducting an operation similar to that of the first embodiment,
 and outputs the front crosstalk signal Sctf to the integrator 103.
 After that, by integrating and averaging the front crosstalk signal Sctf,
 the integrator 103 generates a tap control signal Scf for controlling the
 tap coefficient of the filter 100, and outputs the tap control signal Scf
 to the filter 100 and the subtracter 35.
 Finally, the filter 100 generates the above described filter signal Sftf on
 the basis of the tap control signal Scf, and outputs the filter signal
 Sftf to the subtracter 101.
 On the other hand, on the basis of the delayed signal Sd.sub.1 ' and the
 delayed signal Sd.sub.2 (obtained by further delaying the delayed signal
 Sd.sub.1 in the delay circuit 37) respectively inputted, the coefficient
 control unit 97 generates a tap control signal Scr for controlling the tap
 coefficient of the filter 99 by conducting an operation similar to that of
 the coefficient control unit 98, and outputs the tap control signal Scr to
 the filter 99 and the subtracter 35.
 Finally, the filter 99 generates the above described filter signal Sftr on
 the basis of the tap control signal Scr, and outputs the filter signal
 Sftr to the subtracter 101.
 Then, in the subtracter 101, the filter signal Sftr and the filter signal
 Sftf which are being inputted are subtracted from the delayed signal
 Sd.sub.1. A delayed signal Sd.sub.1 ' with the crosstalk amount reduced is
 newly generated.
 On the other hand, on the basis of the above described principle, the
 subtracter 35 subtracts the tap control signal Scr from the tap control
 signal Scf, thereby generates a difference signal Sst' representing the
 crosstalk amount in the tangential direction, and outputs the difference
 signal Sst' to the tangential tilt controller 23.
 After the above described operation of the radial tilt detector 20' and the
 tangential tilt detector 21', the tilt correction control signal Sc.sub.1
 and the tilt correction control signal Sc.sub.2 are generated in the
 radial tilt controller 22 and the tangential tilt controller 23,
 respectively. By the tilt correction control signal Sc.sub.1 and the tilt
 correction control signal Sc.sub.2, the liquid crystal panel 3 is driven
 to correct the wavefront aberration.
 By the operation of the radial tilt detector 20' and the tangential tilt
 detector 21' of the third embodiment as well, effects similar to those of
 the information reproducing apparatus S of the first embodiment can be
 obtained as heretofore described.
 In the above described third embodiment, the crosstalk amount is detected
 by four single-tap filters. Alternatively, tilts in each direction may be
 detected by using filters having two or more taps. In this case, for
 example, the sum of the tap control signals of filters concerning the
 inner crosstalk in the radial direction may be calculated and used as the
 tap control signal representing the inner crosstalk amount, and the sum of
 the tap control signals of filters concerning the outer crosstalk may be
 calculated and used as the tap control signal representing the outer
 crosstalk amount.
 In the foregoing description of the third embodiment, the radial tilt
 detector 20' or the tangential tilt detector 21' are used instead of the
 radial tilt detector 20 or the tangential tilt detector 21 shown in FIG.
 2. Instead of outputting the central detected signal Scent outputted from
 the A/D converter 26 as it is to the demodulation unit 12 as shown in FIG.
 2, however, it is also possible to connect the radial tilt detector 20'
 shown in FIG. 17 and the tangential tilt detector 21' shown in FIG. 18 in
 series, input the central detected signal Scent' generated in the radial
 tilt detector 20' to the tangential tilt detector 21' as the central
 detected signal Scent in the tangential tilt detector 21', output a
 delayed signal Sd.sub.1 ' obtained as a result thereof to the demodulation
 unit 12, and thereby reproduce the information. In this case, the tilt
 amount of the radial direction is detected and the inner crosstalk and the
 outer crosstalk are removed from the central detected signal Scent in the
 radial tilt detector 20'. Subsequently, in the tangential tilt detector
 21', the tilt amount of the tangential direction is detected, the front
 crosstalk and the rear crosstalk are removed from the central detected
 signal Scent', and the delayed signal Sd.sub.1 ' is outputted. On the
 basis thereof, the information is reproduced.
 (IV) Fourth Embodiment
 A fourth embodiment which is another embodiment of the present invention
 will now be described by referring to FIGS. 19 to 21.
 First of all, the principle of the fourth embodiment is now described. When
 a tilt occurs in, for example, the radial direction, the optical spot
 spreads in the radial direction as shown in FIG. 3A or 3C. As compared
 with the case where there is no tilt, therefore, the total amount of the
 crosstalk (i.e., the sum of the inner crosstalk amount and the outer
 crosstalk amount) increases. In other words, the total amount of crosstalk
 becomes the minimum when there is no tilt.
 In the tangential direction as well, there is no tilt when the sum of the
 front crosstalk amount and the rear crosstalk amount becomes the minimum,
 in the same way.
 In the fourth embodiment, therefore, the sum of the inner crosstalk amount
 and the outer crosstalk amount and the sum of the front crosstalk amount
 and the rear crosstalk amount are separately calculated respectively for
 the radial direction and the tangential direction. The liquid crystal
 panel 3 is driven so as to minimize them respectively. The wavefront
 aberration caused by the tilt in each direction is thus corrected.
 Next, an information reproducing apparatus in the fourth embodiment will
 now be described by referring to FIGS. 19 to 21.
 First of all, the entire configuration of a signal process unit in the
 information reproducing apparatus of the fourth embodiment is described by
 referring to FIG. 19. In the information reproducing apparatus of the
 fourth embodiment, the configuration other than the signal process unit is
 the same as that of the information reproducing apparatus S of the first
 embodiment. Consequently, the same constitutional elements as those of the
 first embodiment carry the same reference numerals and the detailed
 descriptions thereof are omitted.
 As shown in FIG. 19, a signal process unit 11" of the fourth embodiment is
 obtained from the signal process unit 11 of the first embodiment by
 replacing the subtracters 32 and 35 respectively with adders 104 and 105,
 and replacing the radial tilt controller 22 and the tangential tilt
 controller 23 respectively with a radial tilt controller 106 and a
 tangential tilt controller 107. The remaining configuration is the same as
 that of the signal process unit 11 of the first embodiment.
 Next, the entire operation of the signal process unit 11" is described.
 As for the tilt in the radial direction, the crosstalk amount detection
 unit 30 and the crosstalk amount detection unit 31 of the first embodiment
 generates the inner crosstalk signal Scti and the outer crosstalk signal
 Scto and supply them to the adder 104 on the basis of the inner detected
 signal Sin, the outer detected signal Sout, and the central detected
 signal Scent inputted to the radial tilt detector 20" in the signal
 process unit 11".
 Then, the adder 104 adds the inner crosstalk signal Scti and the outer
 crosstalk signal Scto together and generates a sum signal Srsum
 representing the sum of the inner crosstalk amount and the outer crosstalk
 amount.
 Thereafter, the radial tilt controller 106 generates a tilt correction
 control signal Sc.sub.1 ' so as to minimize the sum signal Srsum, drives
 the liquid crystal panel 3 therewith, and corrects the wavefront
 aberration in the radial direction.
 As for the tilt of the tangential direction, the delay circuits 36 and 37,
 and the crosstalk amount detection unit 33 and the crosstalk amount
 detection unit 34 of the first embodiment generates the front crosstalk
 signal Sctf and the rear crosstalk signal Sctr and supply them to the
 adder 105 on the basis of the central detected signal Scent inputted to
 the radial tilt detector 21" in the signal process unit 11".
 The adder 105 adds the front crosstalk signal Sctf and the rear crosstalk
 signal Sctr together and generates a sum signal Stsum representing the sum
 of the front crosstalk amount and the rear crosstalk amount.
 Thereafter, the radial tilt controller 106 generates a tilt correction
 control signal Sc.sub.2 ' so as to minimize the sum signal Stsum, drives
 the liquid crystal panel 3 therewith, and corrects the wavefront
 aberration in the tangential direction.
 The configuration of the radial tilt controller 106 and the tangential tilt
 controller 107 will now be described by referring to FIG. 20. The radial
 tilt controller 106 and the tangential tilt controller 107 have the same
 basic configuration. In the ensuing description, therefore, the radial
 tilt controller 106 will be described as a representative.
 As shown in FIG. 20A, the radial tilt controller 106 includes: a
 synchronous detection circuit 110; an integrator 111; an adder 113; an
 oscillator 112; and a driver 114.
 As shown in FIG. 20C, the synchronous detection circuit 110 includes: a
 band pass filter 115; an inverter 116; a switch 117; and a comparator 118.
 The entire operation of the radial tilt controller 106 is now described by
 referring to FIG. 20A.
 In the synchronous detection circuit 110, the sum signal Srsum inputted to
 the radial tilt controller 106 is subjected to a synchronous detection as
 described later on the basis of a wobbling signal Sosc (having such an
 amplitude and a period as not to affect the correction of the wavefront
 aberration conducted by the liquid crystal panel 3) supplied from the
 oscillator 112. A detected signal Sk representing the tilt amount of the
 radial direction is thus generated.
 The detected signal Sk is averaged in the integrator 111. An averaged
 signal Sk' having a fixed level is thus generated.
 Thereafter, the wobbling signal Sosc and the averaged signal Sk' are added
 together in the adder 113. As a result, the averaged signal Sk' is wobbled
 by the wobbling signal Sosc. A resultant signal is inputted to the driver
 114 as a superposed averaged signal Sx.
 The driver 114 generates a tilt correction control signal Sc.sub.1 ' and
 outputs it to the liquid crystal panel 3 in order to drive the liquid
 crystal panel 3 in such a direction as to reduce the absolute value of the
 averaged signal Sk' contained in the superposed averaged signal Sx and
 thereby correct the wavefront aberration.
 Next, a detailed operation of the synchronous detection circuit 110 will
 now be described by referring to FIG. 20A, FIG. 20C and FIG. 21.
 First of all, the case where the tilt in the radial direction is positive
 is described.
 The comparator 118 judges the polarity of the wobbling signal Sosc (as
 shown in the right uppermost stage of FIG. 21 A), generates a decision
 signal Scmp alternating in polarity according to the polarity change of
 the wobbling signal Sosc (as shown in a third uppermost right stage of
 FIG. 21A), and outputs the decision signal Scmp to the switch 117.
 On the other hand, the band pass filter 115 removes the noise contained in
 the sum signal Srsum (since the tilt is positive, its center level is also
 shifted in the positive direction) and outputs a band pass signal Sbpf.
 The waveform of the band pass signal Sbpf will now be described by
 referring to the left part of FIG. 21A (which shows the relation between
 the sum of the inner crosstalk amount and the outer crosstalk amount and
 the tilt). Since a tilt currently occurs in a positive direction, the
 superposed averaged signal Sx for driving the driver 114 also has its
 center (the averaged signal Sk') shifted in the positive direction.
 Therefore, the center level of the sum signal Srsum obtained by the
 irradiation of the optical beam B corrected in the wavefront aberration on
 the basis of the tilt correction control signal Sc.sub.1 ' which is
 generated on the basis of the superposed averaged signal Sx deviated in
 the positive direction is also deviated in the positive direction. As a
 result, the band pass signal Sbpf also has a waveform as shown in a left
 part and a right part of FIG. 21A.
 The band pass signal Sbpf having such a waveform is inputted to one
 terminal of the switch 117. The band pass signal Sbpf inverted in polarity
 by the inverter 116 is inputted to the other terminal of the switch 117.
 The switch 17 is changed over by the decision signal Scmp corresponding to
 the polarity of the wobbling signal Sosc. By doing so, the waveform of the
 detected signal Sk becomes as shown in the lowermost right stage of FIG.
 21A. Therefore, the averaged signal Sk' which is its average value also
 becomes positive.
 Therefore, the driver 114 generates and outputs the tilt correction control
 signal Sc.sub.1 ' so as to move the average value (i.e., the level of the
 averaged signal Sk') of the superposed averaged signal Sx, which is
 obtained by superposing the wobbling signal Sosc onto the averaged signal
 Sk', into the negative direction. As a result, the tilt occurring in the
 positive direction is gradually decreased.
 Next, the case where the tilt in the radial direction is negative is
 described.
 The comparator 118 judges the polarity of the wobbling signal Sosc (as
 shown in the right uppermost stage of FIG. 21 C), generates the decision
 signal Scmp alternating in polarity according to the polarity change of
 the wobbling signal Sosc (as shown in a third uppermost right stage of
 FIG. 21C), and outputs the decision signal Scmp to the switch 117.
 On the other hand, the band pass filter 115 removes the noise contained in
 the sum signal Srsum (since the tilt is negative, its center level is also
 shifted in the negative direction) and outputs the band pass signal Sbpf.
 The waveform of the band pass signal Sbpf will now be described by
 referring to the left part of FIG. 21C. Since a tilt currently occurs in a
 negative direction, the superposed averaged signal Sx for driving the
 driver 114 also has its center shifted in the negative direction.
 Therefore, the center level of the sum signal Srsum obtained by the
 irradiation of the optical beam B corrected in the wavefront aberration on
 the basis of the tilt correction control signal Sc.sub.1 ' is also
 deviated in the negative direction. As a result, the band pass signal Sbpf
 also has a waveform as shown in a left part and a right part of FIG. 21C.
 Thus, by changing over the switch 17 by the decision signal Scmp
 corresponding to the polarity of the wobbling signal Sosc, the waveform of
 the detected signal Sk becomes as shown in the lowermost right stage of
 FIG. 21C. Therefore, the averaged signal Sk' which is its average value
 also becomes negative.
 Therefore, the driver 114 generates and outputs the tilt correction control
 signal Sc.sub.1 ' so as to move the average value (i.e., the level of the
 averaged signal Sk') of the superposed averaged signal Sx, which is
 obtained by superposing the wobbling signal Sosc onto the averaged signal
 Sk', into the positive direction. As a result, the tilt occurring in the
 negative direction is gradually decreased.
 Finally, the case where the tilt in the radial direction does not
 substantially exist is described.
 The comparator 118 judges the polarity of the wobbling signal Sosc (as
 shown in the right uppermost stage of FIG. 21 B), generates the decision
 signal Scmp (as shown in a third uppermost right stage of FIG. 21B), and
 outputs the decision signal Scmp to the switch 117.
 On the other hand, the band pass filter 115 removes the noise contained in
 the sum signal Srsum (since the tilt does not substantially exist, its
 center level is also at a zero level) and outputs the band pass signal
 Sbpf.
 The waveform of the band pass signal Sbpf will now be described by
 referring to the left part of FIG. 21B. Since a tilt does not
 substantially exist currently, the superposed averaged signal Sx for
 driving the driver 114 also has its center at the zero level. Therefore,
 the center level of the sum signal Srsum obtained by the irradiation of
 the optical beam B corrected in the wavefront aberration on the basis of
 the tilt correction control signal Sc.sub.1 ' is also at the zero level.
 As a result, the band pass signal Sbpf also has a waveform as shown in a
 left part and a right part of FIG. 21B.
 Thus, by changing over the switch 17 by the decision signal Scmp, the
 waveform of the detected signal Sk becomes as shown in the lowermost right
 stage of FIG. 21B. Therefore, the averaged signal Sk' which is its average
 value also becomes substantially zero.
 Therefore, the driver 114 generates and outputs the tilt correction control
 signal Sc.sub.1 ' so as to drive the liquid crystal panel 3 without giving
 any correction with respect to the light beam B on the basis of the
 superposed averaged signal Sx, which is obtained by superposing the
 wobbling signal Sosc onto the averaged signal Sk'. As a result, the
 correction for the wavefront aberration is not performed since the liquid
 crystal panel 3 does not give the phase difference to the light beam 3.
 By the operations described above, by driving the liquid crystal panel 3 by
 use of the tilt correction controlling signal Sc.sub.1 ' based on the
 superimposed averaged signal Sx, whose center level is changed in
 correspondence with the tilt amount being generated, the wavefront
 aberration due to the tilt in the radial direction can be corrected.
 Incidentally, in the tangential tilt controller 107, by the same operation
 as that of the above described radial tilt controller 106 on the basis of
 the sum signal Stsum, the liquid crystal panel 3 is driven by use of the
 tilt correction controlling signal Sc2 based on the superimposed averaged
 signal Sx whose center level is changed in correspondence with the tilt
 amount generated in the tangential direction. Hence, the wavefront
 aberration due to the tilt in the tangential direction is corrected.
 As described above, according to the operation of the information
 reproducing apparatus of the fourth embodiment, since the tilt is detected
 on the basis of the center detected signal Scent obtained by the
 irradiation of the light beam B, it is not necessary to emit an exclusive
 light beam for the tilt detection besides the light beam B, and it is
 possible to simplify the structure for the aberration correcting
 apparatus.
 Further, since the mechanical driving portion is not necessary for the
 aberration correction, the reliability as the aberration correcting
 apparatus is certainly improved and the reduction in size is prompted.
 Therefore, by use of the structure which is simplified and reduced in size,
 it is possible to exactly correct the aberration due to the tilt of the
 optical axis of the light beam.
 It is also possible to correct the wavefront aberration by exactly
 detecting the tilts in the respective directions as the radial tilt is
 detected by use of the inner detected signal Sin or the outer detected
 signal Sout from the adjacent track and the tangential tilt is detected by
 use of the center detected signal Scent.
 Moreover, since the inner crosstalk amount and the outer crosstalk amount
 are detected and the wavefront aberration generated in the radial
 direction is corrected so as to minimize the sum of these crosstalk
 amounts, the wavefront aberration can be exactly corrected by means of a
 simple structure.
 Since the wavefront aberration is corrected by use of the liquid crystal
 panel 3 disposed on the optical path of the light beam B, the wavefront
 aberration can be corrected by means of a simple structure.
 Since the wavefront aberration is corrected by giving the phase difference
 to the light beam B by applying the voltage based on the tilt to the
 liquid crystal panel 3g or 3m of the liquid crystal panel 3, it is
 possible to correct the wavefront aberration of the light beam B
 efficiently.
 Since the transparent electrode of the liquid crystal panel 3 are
 constructed so as to include a plurality of pattern electrodes having the
 shapes corresponding to the distribution of the wavefront aberration
 generated in the light beam B, and since the radial tilt controller 106 or
 the tangential tilt controller 107 corrects the wavefront aberration by
 applying the voltage separately to each of the pattern electrodes, it is
 possible to correct the wavefront aberration efficiently.
 Since the light beam B is collected on the optical disc 1 by the objective
 lens 2 and the record information is reproduced by the demodulation unit
 12, it is possible to accurately reproduce the record information with
 correcting the wavefront aberration due to the inclination of the optical
 axis by means of the simple structure.
 Incidentally, in the above described fourth embodiment, the crosstalk
 amount detection unit 30' of the second embodiment may be employed, in
 place of the crosstalk mount detection units 30, 31, 33 or 34.
 Further, in the above described fourth embodiment, it is possible to
 employ, in place of the radial tilt detection unit 20" or tangential tilt
 detection unit 21", such a configuration that the subtracter 32 is
 replaced by the adder 104 in the radial tilt detection unit 20' of the
 third embodiment or the subtracter 35 is replaced by the adder 105 in the
 tangential tilt detection unit 21' of the third embodiment.
 In addition, the synchronous detection circuit 110 may have such a
 configuration of having the band pass filter 115 and the multiplier 119
 multiplying the band pass signal Sbpf with the wobbling signal Sosc, other
 than the configuration shown in FIG. 20C, so as to realize the above
 described operation.
 (V) Fifth Embodiment
 A fifth embodiment which is another embodiment of the present invention
 will now be described by referring to FIGS. 22A to 22C.
 The fifth embodiment is to perform the operations of the radial tilt
 controller 106 or the tangential tilt controller 107 in the above
 described fourth embodiment, by means of the software.
 In the information reproducing apparatus of the fifth embodiment, the
 configuration other than the radial tilt controller or the tangential tilt
 controller is the same as that of the information reproducing apparatus of
 the fourth embodiment. Consequently, the same constitutional elements as
 those of the fourth embodiment carry the same reference numerals and the
 detailed descriptions thereof are omitted. In addition, the radial tilt
 controller and the tangential tilt controller have the same basic
 configuration. In the ensuing description, therefore, the radial tilt
 controller will be described as a representative.
 As shown in FIG. 22A, the radial tilt controller 106' in the information
 reproducing apparatus of the fifth embodiment includes: a controller 120
 such as a CPU; and a driver 12. Within the controller 120, a ROM (Read
 Only Memory) is included which stores in advance a software program
 corresponding to a flow chart indicating an operation described later
 (FIG. 22B).
 Next, the operation of the radial tilt controller 106' will be explained
 with referring to FIG. 22B and 22C.
 In the controller 120 to which the sum signal Srsum is being inputted, at
 first, the control signal Scc outputted from the controller 120 to drive
 the driver 121 is increased by a minute amount (Step S1). The driver 121
 is driven by this control signal Scc, so that the tilt correction
 controlling signal Sc1" to drive the liquid crystal panel 3 in the radial
 direction is generated.
 Then, the liquid crystal panel 3 is actually driven by the tilt correction
 controlling signal Sc1", so that the crosstalk signal Sct+ is detected,
 which is the change of the sum signal Srsum resulting from the correction
 of the wavefront aberration of the light beam B (Step S2).
 Then, the control signal Scc is decreased by a minute amount (whose
 absolute value is the same as that increased at the Step S1) from a
 condition before the Step S1 (Step S3). The driver 121 is driven by this
 control signal Scc, so that the tilt correction controlling signal Sc1" is
 generated.
 Then, the liquid crystal panel 3 is actually driven by the tilt correction
 controlling signal Sc1", so that the crosstalk signal Sct- is detected,
 which is the change of the sum signal Srsum resulting from the correction
 of the wavefront aberration of the light beam B (Step S4).
 Then, the crosstalk amount Sct+ and the crosstalk amount Sct- which are
 detected in the above manner are compared with each other (Step S5). If
 the crosstalk amount Sct+ is larger than the crosstalk amount Sct-
 (Sct+&gt;Sct-), since the tilt in the radial direction is generated in the
 positive direction (refer to a first quadrant and a fourth quadrant in
 FIG. 22C), the control signal Scc to drive the liquid crystal panel 3 is
 decreased so as to correct the wavefront aberration due to it into the
 negative direction (Step S6). Then, the tilt correction controlling signal
 Sc1" corresponding to the driver 121 is generated.
 On the other hand, at the Step S5, if the crosstalk amount Sct+ is smaller
 than the crosstalk amount Sct- (Sct+&lt;Sct-), since the tilt in the radial
 direction is generated in the negative direction (refer to a second
 quadrant and a third quadrant in FIG. 22C), the control signal Scc to
 drive the liquid crystal panel 3 is increased so as to correct the
 wavefront aberration due to it into the positive direction (Step S7).
 Then, the tilt correction controlling signal Sc1" corresponding to the
 driver 121 is generated.
 Further, at the Step S5, if the crosstalk amount Sct+ is equal to the
 crosstalk amount Sct- (Sct+=Sct-), since the tilt in the radial direction
 is not substantially generated (refer to an axis of the sum signal Srsum
 in FIG. 22C), the change of the control signal Scc is not necessary.
 Accordingly, the driver 121 is controlled to generate the tilt correction
 controlling signal Sc1" as it presently is.
 Then, when the changing control of the controlling signal Scc is ended, it
 is judged whether or not the electric power source of the information
 reproducing apparatus of the fifth embodiment is turned off (Step S8). If
 it is turned off (Step S8; YES), the process is ended as it is. If it is
 not turned off (Step S8; NO), the operation flow returns to the Step S1,
 so as to repeat the above described tilt correction.
 As described above, according to the information reproducing apparatus of
 the fifth embodiment, since the wavefront aberration is corrected by means
 of the software, the same effect as that of the fourth embodiment can be
 attained by a simple configuration.
 (VI) Modified Embodiment
 Next, a modified embodiment of the present invention is explained by
 referring to FIG. 23.
 In the above explained each embodiment, the wavefront aberration due to the
 tilt in each direction is corrected by use of the liquid crystal panel 3
 (refer to FIG. 9 etc.,). Other than that, it is possible as the modified
 embodiment to constitute such that an inclination mechanism 203 is driven
 as one example of an inclination device by a tilt correction controlling
 signal Sc' (i.e., a radial tilt correction controlling signal Sc1'
 generated by a radial tilt controller 22' on the basis of the difference
 signal Ssr and a tangential tilt correction controlling signal Sc2'
 generated by a tangential tilt controller 23' on the basis of the
 difference signal Sst), so as to remove the tilt of the light beam B
 itself as shown in FIG. 23. The inclination mechanism 203 may be
 constructed by a known actuator to change the direction of the objective
 lens 2, the semiconductor laser 7 or the like, such as an electromagnetic
 actuator. The inclination mechanism 203 may be commonly used as a whole or
 one portion of the focusing servo actuator, the tracking servo actuator
 and so on.
 According to this modified embodiment, it is not necessary to emit an
 exclusive light beam for the tilt detection besides the light beam B, and
 it is possible to simplify the structure for the aberration correcting
 apparatus.
 The invention may be embodied in other specific forms without departing
 from the spirit or essential characteristics thereof. The present
 embodiments are 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 and range of equivalency of the claims are
 therefore intended to be embraced therein.
 The entire disclosure of Japanese Patent Application No.09-270778 filed on
 Oct. 3, 1997 including the specification, claims, drawings and summary is
 incorporated herein by reference in its entirety.