BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a tracking servo technique in an optical information recording and/or reproducing apparatus. 
     2. Description of the Related Art 
     In an optical information recording and/or reproducing apparatus, a tacking servo is performed so that a light beam used for recording and/or reproducing the information can follow or trace an information track on an optical disc. There are various tracking servo methods. Among them, from a view point of improving a usage efficiency of the light beam emitted from a semiconductor laser, a “one beam tracking method” using only one light beam is advantageous as compared with other methods using a plurality of light beams. 
     As the one beam tracking method, a so-called (radial) push pull method is well known. The push pull method is a method of detecting a difference between two outputs of two-divided light detector, which is divided in a radial direction of the optical disc (which is referred to as a “radial direction”), as a tracking error signal, to thereby detect a drift of the light beam position with respect to the information track on the optical disc. 
     However, in the above-mentioned push pull method, there is a problem that a drift or shift is generated in a target value of the tracking servo control due to a shift between the objective lens position of an optical pickup and the optical axis of the light detector (which is referred to as a “lens shift” hereinafter). 
     In more detail, the optical pickup has such a structure that an actuator is movably mounted on a slider, and that the objective lens is movably mounted on the actuator. Here, if the disc is eccentric or if the slider does not smoothly move in the radial direction, the actuator performs a fine adjustment of tracking by shifting only the objective lens. 
     In case that the actuator moves the objective lens in this way, the relative position of the objective lens with respect to the optical axis of the light detector (i.e., the division line of the two-divided light detector) also moves. As a result, the light spot position with respect to the light detector is changed, so that the drift is generated in the target value of the tracking servo. 
     This phenomenon is explained with referring to FIG.  8 . In case that there is no lens shift, the value of the tracking error signal becomes “0” under a condition that the light spot is positioned on a central line of the information track. Therefore, the target value of the tracking servo becomes an original point O. However, if there is the lens shift, since the tracking error signal includes an offset component due to the lens shift, the target value of the tracking servo becomes a point Os shown in FIG. 8, so that the drift of the target value is generated. This phenomenon is the more significantly observed as the track pitch of the optical disc is the narrow. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a tracking control apparatus, which can reduce an influence of the lens shift, by using the one beam tracking method. 
     The above object of the present invention can be achieved by a first tracking control apparatus for a disc having a physical factor changing a shape or a reflection coefficient in a tangential direction of a record track of the disc. The first tracking control apparatus is provided with: a detecting device having a first detector, a second detector disposed adjacent to the first detector in a radial direction of the disc, a third detector disposed adjacent to the second detector in the tangential direction and a fourth detector disposed adjacent to the first detector in the tangential direction and adjacent to the third detector in the radial direction; a first amplifying device for amplifying an output signal of the first detector and an output signal of the fourth detector by a first gain; a second amplifying device for amplifying an output signal of the second detector and an output signal of the third detector by a second gain; a first adding device for calculating a first sum signal, which is a sum of the amplified output signal of the first detector and the amplified output signal of the fourth detector; a second adding device for calculating a second sum signal, which is a sum of the amplified output signal of the second detector and the amplified output signal of the third detector; a first subtracting device for calculating a difference between the first sum signal and the second sum signal and outputting the difference as a tracking error signal; a second subtracting device for calculating a first difference signal, which is a difference between the output signal of the first detector and the output signal of the fourth detector; a third subtracting device for calculating a second difference signal, which is a difference between the output signal of the second detector and the output signal of the third detector; and an amplitude comparing device for comparing amplitudes of the first difference signal and the second difference signal with each other, and determining the first gain and the second gain on the basis of a result of comparison. 
     According to the first tracking control apparatus of the present invention, the tracking error signal is generated by calculating the difference between the first sum signal, which is the sum of the output signals of the first and fourth detectors, and the second sum signal, which is the sum of the output signals of the second and third detectors. By comparing the amplitudes of the first difference signal, which is the difference between the output signals of the first and fourth detectors, and the second difference signal, which is the difference between the output signals of the second and third detectors, and, on the basis of the result of the comparison, the first gain for the first sum signal and the gain for the second sum signal are determined. Here, since the difference between the amplitude of the first difference signal and the amplitude of the second difference signal indicates the lens shift component, it is possible to obtain the tracking error signal, in which the influence of the lens shift is removed, by adjusting the gains for the first and second sum signals on the basis of this difference indicating the lens shift component. 
     The above object of the present invention can be also achieved by a second tracking control apparatus for a disc having a physical factor changing a shape or a reflection coefficient in a tangential direction of a record track of the disc. The second tracking control apparatus is provided with: a detecting device having a first detector, a second detector disposed adjacent to the first detector in a radial direction of the disc, a third detector disposed adjacent to the second detector in the tangential direction and a fourth detector disposed adjacent to the first detector in the tangential direction and adjacent to the third detector in the radial direction; a first amplifying device for amplifying a first sum signal, which is a sum of an output signal of the first detector and an output signal of the fourth detector, by a first gain; a second amplifying device for amplifying a second sum signal, which is a sum of an output signal of the second detector and an output signal of the third detector, by a second gain; a first subtracting device for calculating a difference between the first sum signal and the second sum signal and outputting the difference as a tracking error signal; a second subtracting device for calculating a first difference signal, which is a difference between the output signal of the first detector and the output signal of the fourth detector; a third subtracting device for calculating a second difference signal, which is a difference between the output signal of the second detector and the output signal of the third detector; and an amplitude comparing device for comparing amplitudes of the first difference signal and the second difference signal with each other, and determining the first gain and the second gain on the basis of a result of comparison. 
     According to the second tracking control apparatus of the present invention, the tracking error signal is generated by calculating the difference between the first sum signal, which is the sum of the output signals of the first and fourth detectors, and the second sum signal, which is the sum of the output signals of the second and third detectors. By comparing the amplitudes of the first difference signal, which is the difference between the output signals of the first and fourth detectors, and the second difference signal, which is the difference between the output signals of the second and third detectors, and, on the basis of the result of the comparison, the first gain for the first sum signal and the gain for the second sum signal are determined. Here, since the difference between the amplitude of the first difference signal and the amplitude of the second difference signal indicates the lens shift component, it is possible to obtain the tracking error signal, in which the influence of the lens shift is removed, by adjusting the gains for the first and second sum signals on the basis of this difference indicating the lens shift component. 
     In one aspect of the first or second tracking control apparatus of the present invention, the amplitude comparing device determines the first gain and the second gain so as to make the amplitude of the first difference signal and the amplitude of the second difference signal equal to each other. 
     According to this aspect, it is possible to remove the lens shift component in the tracking error signal, by this determining process of the amplitude comparing device. 
     In another aspect of the first or second tracking control apparatus of the present invention, the apparatus is further provided with: a first holding device for holding the first difference signal and supplying the held first difference signal to the amplitude comparing device; and a second holding device for holding the second difference signal and supplying the held second difference signal to the amplitude comparing device. 
     According to this aspect, it is possible to continuously monitor the amplitudes of the first and second difference signals. 
     In another aspect of the first or second tracking control apparatus of the present invention, the apparatus is further provided with a driving device for moving a light beam in the radial direction on the disc on the basis of the tracking error signal. 
     According to this aspect, it is possible to perform an accurate tracking control, which is not influenced by the lens shift. 
     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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a tracking control circuit as one embodiment of the present invention; 
     FIG. 2A is a schematic plan view of land and groove tracks on an optical disc with a reproduction beam spot for explaining a tangential push pull signal in the embodiment; 
     FIG. 2B is a front view of a four divided light detector in one state, which obtains the tangential push pull signal in the embodiment; 
     FIG. 2C is a front view of the four divided light detector in another state, which obtains the tangential push pull signal in the embodiment; 
     FIG. 3A is a graph showing one example of a waveform of the tangential push pull signal in the embodiment; 
     FIG. 3B is a graph showing another example of the waveform of the tangential push pull signal in the embodiment; 
     FIG. 4 is a block diagram showing a tracking control circuit as another embodiment of the present invention; 
     FIG. 5A is one diagram showing a comparison result of the drifts of target values of tracking error signals in the present embodiment and the conventional push pull method; 
     FIG. 5B is another diagram showing a comparison result of the drifts of target values of tracking error signals in the present embodiment and the conventional push pull method; 
     FIG. 6A is a schematic plan view of one example of the land and groove track structure on the optical disc, to which the tracking control apparatus of the present embodiment can be applied; 
     FIG. 6B is a schematic plan view of another example of the land and groove track structure on the optical disc, to which the tracking control apparatus of the present embodiment can be applied; 
     FIG. 6C is a schematic plan view of another example of the land and groove track structure on the optical disc, to which the tracking control apparatus of the present embodiment can be applied; 
     FIG. 7A is a schematic plan view of another example of the land and groove track structure on the optical disc, to which the tracking control apparatus of the present embodiment can be applied; 
     FIG. 7B is a schematic plan view of another example of the land and groove track structure on the optical disc, to which the tracking control apparatus of the present embodiment can be applied; 
     FIG. 7C is a schematic plan view of another example of the land and groove track structure on the optical disc, to which the tracking control apparatus of the present embodiment can be applied; 
     FIG. 7D is a schematic plan view of another example of the land and groove track structure on the optical disc, to which the tracking control apparatus of the present embodiment can be applied; and 
     FIG. 8 is a graph showing a problem in the conventional push pull method. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is adapted to detect a lens shift amount by obtaining a tangential push pull signal by using a four divided light detector and then correct a tracking error signal so as to cancel the drift or offset of the target value of the tracking servo due to the lens shift, on the basis of the detected lens shift amount. 
     At first, the tangential push pull signal is explained. The tangential push pull signal is a signal obtained on an optical disc which has a portion where a shape or reflection coefficient changes in a tangential direction. By a four divided light detector  5  shown in FIGS. 2B and 2C, a first tangential push pull signal TP 1  and a second tangential push pull signal TP 2  can be obtained respectively as follows. 
       TP   1 =(output signal of a detector  5   a )−(output signal of a detector  5   d ) 
     
       
           TP   2 =(output signal of a detector  5   b )−(output signal of a detector  5   c ) 
       
     
     By inverting the order of adding and subtracting, the first tangential push pull signal TP 1  and the second tangential push pull signal TP 2  may be obtained respectively as follows. 
     
       
           TP   1 =(output signal of the detector  5   d )−(output signal of the detector  5   a ) 
       
     
     
       
           TP   2 =(output signal of the detector  5   c )−(output signal of the detector  5   b ) 
       
     
     The value of the tangential push pull signal becomes “0”, at a portion where the shape or reflection coefficient does not change in the tangential direction on the optical disc. However, when the light spot passes through a portion where the shape or reflection coefficient changes in the tangential direction, the value of the tangential push pull signal becomes a value other than “0” depending upon the shape or reflection coefficient. For example, as shown in FIG. 2A, in an optical disc having a mirror portion at one portion of a groove track, when the light spot passes through this mirror portion, the tangential push pull signals TP 1  and TP 2  exhibit the waveforms as shown in FIGS. 3A and 3B. Here, if there is no lens shift for the light spot as shown in FIG. 2C, the amplitude of the tangential push pull signals TP 1  and TP 2  are equal to each other as shown in FIG.  3 A. On the other hand, if the center of the light spot is shifted from the center of the four divided detector  5  due to the lens shift as shown in FIG. 2B, difference between the amplitudes of the tangential push pull signals TP 1  and TP 2  is generated as shown in FIG.  3 B. The respective amplitudes of the tangential push pull signals TP 1  and TP 2  change approximately in proportional to the lens shift amount. 
     Therefore, by monitoring those two tangential push pull signals TP 1  and TP 2 , it is judged that there is no lens shift if the monitored amplitudes of those two are equal to each other, while it is judged that there is the lens shift if the monitored amplitudes of those two are different from each other. Further, by checking which amplitude is the greater between those two, it is possible to recognize the direction of the lens shift. Thus, as controlling the gain of the detection signal in the detector  5  so that the amplitudes of those two tangential push pull signals are equal to each other, it is possible to remove the influence of the lens shift in the tracking error signal. 
     More concretely, with referring to the detector  5  shown in FIG. 2B, the tracking error signal TE is obtained as following. 
     
       
           TE =α×(output of the detector  5   a +output of detector  5   d )−β×(output of the detector  5   b +output of detector  5   c ) 
       
     
     wherein α and β represent the gains of the amplifiers for amplifying the outputs of the detectors respectively. Those gains α and β are determined so as to satisfy a following equation by using the tangential push pull signals TP 1  and TP 2 . 
     
       
         α× TP   1 =β× TP   2   
       
     
     By this, it is possible to remove the drift or offset component in the tracking error signal due to the lens shift. 
     The tangential push pull signal can be obtained by using a pit (e.g., a synchronization pattern) which exists at a certain cycle in case of the optical disc exclusive for reproduction. In case of an optical disc capable of recording, the tangential push pull signal can be obtained by using the mirror portion provided on the groove track in advance. The types of the optical discs from which the tangential push pull signal can be obtained in the present embodiment will be described later. 
     Next, the embodiments of the present invention will be now explained with referring to the drawings. 
     FIG. 1 shows a structure of a tracking control circuit of one embodiment of the present invention. In FIG. 1, a tracking control circuit  100  has a four divided detector  5 . The four divided detector  5  has four detectors  5   a  to  5   d , on which a light spot SP is formed as the light beam reflected from the optical disc is irradiated thereonto. 
     The outputs of the detectors  5   a  and  5   d  are inputted to an amplifier  6   a , while the outputs of the detectors  5   b  and  5   c  are inputted to an amplifier  6   b . The amplifier  6   a  amplifies the outputs of the detectors  5   a  and  5   d  by a gain (α) corresponding to a gain control signal  12   a  while the amplifier  6   b  amplifies the outputs of the detectors  5   b  and  5   c  by a gain (β) corresponding to a gain control signal  12   b . The output signals of the amplifier  6   a  are inputted to a subtracter  7   a  and an adder  9   a  while the output signals of the amplifier  6   b  are inputted to a subtracter  7   b  and an adder  9   b . The adder  9   a  generates a sum signal of the output signals of the detectors  5   a  and  5   d  while the adder  9   b  generates a sum signal of the output signals of the detectors  5   b  and  5   c . A subtracter  10  generates a difference between those sum signals and outputs it as a tracking error signal. 
     This tracking error signal is transmitted to a tracking servo circuit  101 , which controls an actuator  102  for actuating an optical pickup, by sending a servo control signal to the actuator  102 . In the optical pickup, the actuator  102  is movably mounted on a slider  103 , and that the objective lens (not illustrated) is movably mounted on the actuator  102 . Here, if the disc is eccentric or if the slider does not smoothly move in the radial direction, the actuator  102  performs a fine adjustment of tracking by shifting only the objective lens, under the control of the tracking servo circuit  101 . The objective lens and the four divided detector  5  as well as a light source (e.g., a semiconductor laser) etc., are mounted in the optical pickup. All the constitutional elements of the tracking control circuit  100  shown in FIG. 1 may be equipped in the optical pickup. Alternatively, the tracking control circuit  100  may be partially equipped in the optical pickup (e.g., only the four divided detector  5  may be equipped, or only the four divided detector  5  and the amplifiers  6   a  and  6   b  may be equipped in the optical pickup while other constitutional elements are equipped in a processing circuit in the information recording and/or reproducing apparatus). 
     On the other hand, the subtracter  7   a  generates a difference between the outputs signals of the detectors  5   a  and  5   d , and supplies it as a tangential push pull signal TP 1  to a sample hold circuit  8   a . In the same manner, the subtracter  7   b  generates a difference between the outputs signals of the detectors  5   b  and  5   c , and supplies it as a tangential push pull signal TP 2  to a sample hold circuit  8   b . The sample hold circuits  8   a  and  8   b  hold continuously the tangential push pull signals TP 1  and TP 2  respectively and send them to an amplitude comparator  4 . The amplitude comparator  4  determines the gains α and β respectively for the amplifiers  6   a  and  6   b  so as to make the values of the tangential push pull signals TP 1  and TP 2  equal to each other, and supply them as the gain control signals  12   a  and  12   b  respectively to the amplifier  6   a  and  6   b.    
     By those, the drift or offset component in the tracking error signal due to the lens shift is removed. As a result, the subtracter  10  outputs the tracking error signal which is not influenced by the lens shift. 
     Incidentally, in the above explained example, the tangential push pull signals are continuously held. Instead, in case that the position where the tangential push pull signal can be obtained (e.g., the mirror portion in the above explained example) or the cycle when the tangential push pull signal appears is known, it is possible to predict the timing of the tangential push pull signal and supply the tangential push pull signal to the amplitude comparator  4  only at the predicted timing when the tangential push pull signal is supposed to be obtained. 
     Also, in the circuit shown in FIG. 1, one amplitude  6   a  is used as a first amplifying device, to which the output signals of the detectors  5   a  and  5   d  are inputted. Instead, as the first amplifying device, one amplifier for amplifying the output signal of the detector  5   a  and another amplifier for amplifying the output of the detector  5   d  may be provided, such that the gains of those two amplifiers are equally set to the first gain (α). The same thing can be said for the relationship between the amplifier  6   b  functioning as the second amplifying device and the detectors  5   b  and  5   c.    
     FIG. 4 shows a structure of a tracking control circuit of another embodiment of the present invention. Although the operating process executed by the circuit shown in FIG. 4 is basically equivalent to that executed by the circuit shown in FIG. 1, the detailed circuit structure of those are different from each other. In FIG. 4, the same constitutional elements as those in FIG. 1 carry the same reference numerals and the explanations thereof are omitted. 
     In the circuit shown in FIG. 4, a tracking control circuit  200  is constructed as follows. Namely, amplifiers  14   a  and  14   b  are disposed at a previous stage of the subtracter  10 , and low pass filters  11   a  and  11   b  are inserted at previous stages of the amplifiers  14   a  and  14   b  respectively. Those low pass filters  11   a  and  11   b  are filters to extract only the tracking error signal components, and which may be added to the circuit shown in FIG.  1 . 
     FIGS. 5A and 5B show a comparison result between the present embodiment and the conventional push pull method. The conditions are assumed here as follows. 
     light beam wavelength λ=410 nm 
     track pitch=0.4 μm 
     groove width=0.2 μm 
     groove depth=λ/8 
     It is also assumed that a short mirror portion is disposed on the groove track (as shown in FIG.  2 A). 
     FIG. 5A shows a waveform of the tracking error signal in case that there is a lens shift of 4%. According to the conventional push pull method, a point where the value of the tracking error signal becomes “0” is shifted from the track center, while this point coincides with the track center according to the present embodiment. 
     FIG. 5B shows the drift or offset amount of the target value of the tracking servo due to the lens shift. According to the conventional push pull method, the drift or offset amount of the target value increases in proportional to the increase of the lens shift, while the drift or offset of the target value is substantially kept to be “0” regardless of the increase of the lens shift according to the present embodiment. 
     Next, the optical disc to which the tracking control apparatus of the present embodiment can be applied is explained. As aforementioned, the tangential push pull signal appears at a portion where the shape or reflection coefficient changes in the tangential direction of the disc, and at other portions, it becomes “0” value. Therefore, the present embodiment can be applied to an optical disc which has such a physical characteristic that the shape or reflection coefficient changes in the tangential direction of the disc. 
     From this point of view, examples of the optical discs are shown in FIG. 6A to FIG. 7D, to which the present invention can be applied. 
     FIG. 6A shows an optical disc, on which a pre-pit PP is formed on the groove track for the purpose of an address detection. FIG. 6B shows an optical disc on which a mirror portion MR is disposed on the groove track as a record track. FIG. 6C shows an optical disc on which the groove track is wobbled such that wobbling portion WB may be partial or continuous. 
     Each of FIG. 7A to FIG. 7D shows an optical disc on which a pre-pit PP is formed on a land track. Among those, FIG. 7C shows an example that the pre-pit PP on the land track contacts with the groove track and that the length of the pre-pit PP is certainly long. FIG. 7D shows an example that information is recorded on the land track. 
     Those examples shown in FIG. 6A to FIG. 7D are just examples, and the present invention can be applied to other types of optical discs as long as they have the aforementioned physical characteristic. 
     The tracking control apparatus of the present invention as described above in detail can be applied to an information recording apparatus, an information reproducing apparatus and an information recording and reproducing apparatus for an optical disc. 
     As described above in detail, according to the present invention, the lens shift amount is detected by monitoring the tangential push pull signal, and the gains of the tracking error signal are controlled so as to remove the influence of the lens shift. Thus, even in case that the lens shift certainly exists, it is possible to prevent the target value of the tracking servo from being drifted or shifted, and thereby it is possible to realize a reliable tracking control suitable for the disc especially having a narrow pitch. 
     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.11-268184 filed on Sep. 22, 1999 including the specification, claims, drawings and summary is incorporated herein by reference in its entirety.
A disc has a physical factor changing a shape or a reflection coefficient in a tangential direction of a record track. A tracking control apparatus for the disc is provided with a detecting device having a first detector, a second detector disposed adjacent to the first detector in a radial direction of the disc, a third detector disposed adjacent to the second detector in the tangential direction and a fourth detector disposed adjacent to the first detector in the tangential direction and adjacent to the third detector in the radial direction; a first amplifying device for amplifying output signals of the first and fourth detectors by a first gain; a second amplifying device for amplifying output signals of the second and third detectors by a second gain; a first adding device for calculating a first sum signal, which is a sum of the amplified output signals of the first and fourth detectors; a second adding device for calculating a second sum signal, which is a sum of the amplified output signals of the second and third detectors; and a first subtracting device for calculating a difference between the first an second sum signals and outputting the difference as a tracking error signal. The tracking control apparatus is also provided with an amplitude comparing device for determining the first gain and the second gain on the basis of differences between the output signals.