Patent Publication Number: US-2005117503-A1

Title: Data reading device and pre-pit detection circuit

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a data reading device for reading data from an optical data recording medium, such as a digital versatile disk-recordable (DVD-R) or a digital versatile disk-rewritable (DVD-RW), and to a pre-pit detection circuit and a pre-pit detection method for use with the optical data reading device.  
      2. Description of the Related Art  
      Groove tracks serving as recording tracks, and land tracks serving as guide tracks are formed on an optical data recording medium which can record data at high recording density, such as a DVD-R or DVD-RW. Information data are recorded on each of the groove tracks by forming a recording mark therein. Pre-pits serving as phase pits which hold pre-information are formed in the land tracks in advance.  
      An optical recording-and-reproducing system for use with such an optical data recording medium is configured to include an optical pickup device which effects writing and reading of data from and to the optical data recording medium by a laser unit, a pre-pit detection circuit for detecting pre-pits provided in the optical data recording medium, a servo controller for generating a focusing error signal, a tracking error signal, a slider drive signal, and the like, on the basis of light-receiving signals from the optical pickup device, and an information data reproducing device which binarizes a read signal obtained in the optical pickup device, and subsequently performs processing of demodulation, error correction, and various types of data decoding operations, thereby reproducing information data recorded in the optical data recording medium (see, e.g., JP-A-2003-16673).  
     SUMMARY OF THE INVENTION  
      The optical recording-and-reproducing system disclosed in JP-A-2003-16673 is constituted so as to compensate for degradation in the performance of detecting wobbling or land pre-pits caused by an error in assembly of an optical system, a change in positional relationship of the optical system caused by secular changes or temperature changes, or eccentricity of an optical data recording medium (optical disk) by controlling the amount of current passing through an actuator, thereby mechanically correcting the position of an objective lens.  
      The constitution of JP-A-2003-16673 is configured such that an error in assembly of an optical system, or a change in positional relationship of the optical system is compensated for by mechanically driving the objective lens. Therefore, the constitution entails a problem of mechanical driving of the objective lens practically failing to keep pace with the fluctuation, to thus fail to remove eccentricity-dependent components which fluctuate at an extremely high frequency.  
      An example one of the problems to be solved by the present invention is difficulty encountered in effecting mechanical driving of an objective lens to remove eccentricity-dependent components superposed on land pre-pit (hereinafter referred to simply as “LPP”) components in terms of frequency, as well as in removing the eccentricity-dependent components.  
      According to an aspect of the present invention, a data reading device which reads data recorded on a data recording medium on which pre-pits are formed in advance, the data reading device includes a laser light source that emits a light beam, an objective lens that converges the light beam, thereby forms a light spot on the data recording medium, an actuator that drives the objective lens, a light-receiving unit having a first light-receiving region and a second light-receiving region which are split along a division line corresponding to a direction of recording tracks and which receive the light beam reflected from the data recording medium, an amplitude control unit that adjusts an amplitude of an output signal output from at least one of the first light-receiving region and the second light-receiving region, a computing unit that computes the output signal adjusted by the amplitude control unit, thereby generates a push-pull signal, a pre-pit detection unit that detects a pre-pit signal on basis of the push-pull signal, and an extraction unit that extracts an eccentricity component of the data recording medium. Preferably, the amplitude control unit adjusts the amplitude of the output signal on basis of the eccentricity component.  
      According to another aspect of the present invention, a pre-pit detection circuit that detects a pre-pit formed on a data recording medium, includes an amplitude control unit that adjusts an amplitude of an output signal output from at least one of a first light-receiving region and a second light-receiving region which receive a light beam reflected on the data recording medium, a computing unit that computes the output signal adjusted by the amplitude control unit, thereby generates a push-pull signal, and a pre-pit detection unit that detects a pre-pit signal on basis of the push-pull signal. Preferably, the amplitude control unit adjusts amplitudes of a first output signal and a second output signal on basis of an eccentricity component of the data recording medium.  
      According to yet another aspect of the present invention, a pre-pit detection method for detecting a pre-pit formed on a data recording medium, includes extracting an eccentricity component of the data recording medium, compensating for an amplitude of an output signal output from at least one of a first light receiving region and a second light receiving region on basis of the eccentricity component of the data recording medium, subjecting the output signal whose amplitude has been compensated to logical operation, thereby generating a push-pull signal, and detecting the pre-pit on basis of the push-pull signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a diagram showing one portion of a recording face of an optical disk;  
       FIG. 2  is a schematic diagram showing a data recording-and-reproducing apparatus;  
       FIGS. 3A and 3B  are schematic diagrams showing a pickup device;  
       FIG. 4  is a schematic diagram showing a first embodiment of a pre-pit signal detector according to the invention;  
       FIG. 5  is a diagram showing a waveform of a push-pull signal;  
       FIG. 6  is a diagram showing an AR waveform;  
       FIG. 7  is a diagram showing a disk eccentricity dependency of AR characteristics;  
       FIGS. 8A and 8B  show push-pull signals where the disk has small eccentricity;  
       FIGS. 9A and 9B  show push-pull signals where the disk has large eccentricity;  
       FIG. 10  is a diagram showing a pickup device including a lens sensor;  
       FIG. 11  is a schematic diagram showing a second embodiment of a pre-pit signal detector according to the invention;  
       FIG. 12  is a schematic diagram showing a third embodiment of a pre-pit signal detector according to the invention;  
       FIG. 13  is a graph showing a relationship between signals Rad and Rbc (AR detection circuit ratio (A+D)/(B+C)) and an AR value; and  
       FIG. 14  is a schematic diagram showing a fourth embodiment of a pre-pit signal detector according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Hereinafter, embodiments of a data reading device and a pre-pit detection circuit according to the invention will be described by reference to the drawings.  
     FIRST EMBODIMENT  
      First, the data reading device according to the invention will be described by reference to FIGS.  1  to  3 B. In the following description, a data recording-and-reproducing apparatus which is capable of recording and reproducing data is adopted as an example of the data reading device.  
       FIG. 1  is a partially enlarged diagram showing one portion of a recording face of an optical disk according to the invention.  FIG. 2  is a schematic diagram showing a data recording-and-reproducing apparatus according to the invention.  FIGS. 3A and 3B  are schematic diagrams showing a configuration of a pickup device of the data recording-and-reproducing apparatus.  
      The data recording-and-reproducing apparatus of the embodiment is a data reading device which detects land pre-pits accurately at the time of reading of data after the data have been recorded on a data recording medium, such as a DVD-R, or a DVD-RW, in which land pre-pits are provided in advance. In the following description, a DVD-R is adopted as an example of an optical disk serving as the data recording medium for use with the data recording-and-reproducing apparatus.  
      In  FIG. 1 , the optical disk  1  is a dye-coated type DVD-R provided with a dye layer  5  and of a write-once type. The optical disk  1  includes grooved tracks  2  serving as recording tracks, and land tracks  3  serving as guide tracks—which guide a light beam B for reproduction and recording. Furthermore, the optical disk  1  includes a protective layer  7  for protecting the groove tracks  2  and the land tracks  3 , and a metal-deposited face  6  for reflecting the light beam B when reproducing recorded data.  
      A pre-pit  4  for holding pre-information is formed in the land track  3  on the optical disk  1  in advance of shipment. The groove track  2  is wobbled at a frequency corresponding to a rated rotational velocity of the optical disk  1 . Record control information on the basis of wobbling of the groove track  2  is recorded in advance of shipment of the optical disk  1 , as is the case of the pre-pit  4 .  
      When recording information other than the above-mentioned pre-information is to be recorded on the optical disk  1 , the pre-information is obtained by sampling a wobbling frequency of the groove track  2  and detecting the pre-pits  4  so as to control rotation of the optical disk to a predetermined rotational velocity. On the basis of the thus-obtained pre-information, the optimum output for the light beam B, or the like, is set, address information on the optical disk  1 , or the like, is obtained, and recording information is recorded at a record position corresponding to the address information.  
      Here, when the recording information is to be recorded, a pit is formed by radiation of the light beam B, and the recording information is recorded while being controlled in such a manner that the center of the pit coincides with the center of the groove track  2 . At this time, a light spot SP is set to such a size that not only is the light beam B emitted on the groove track  2 , but also a portion of the light beam B is emitted on the land track  3 , as shown in  FIG. 1 .  
      Here, by a radial push-pull method making use of a part of reflected light from the light spot SP emitted to the land track  3 , the pre-pits  4  are detected, whereby pre-information corresponding thereto is obtained. The “radial push-pull method” means a push-pull method making use of a light-receiving element split along a line parallel to the direction in which the light beam travels over the optical disk  1 . A wobble signal is detected as pre-information from the groove track  2  with use of the reflected light of the light spot SP which has been emitted onto the groove track  2 , whereby a clock signal for rotation control is obtained.  
      Next, a general configuration and operations of the data recording-and-reproducing apparatus which incorporates a pre-pit detector of the embodiment will be described by reference to FIGS.  2  to  3 B.  
       FIG. 2  is a block diagram showing a general configuration of the data recording-and-reproducing apparatus.  FIGS. 3A and 3B  are diagrams showing a configuration of a pickup device of the data recording-and-reproducing apparatus.  
      As shown in  FIG. 2 , the data recording-and-reproducing apparatus  100  is configured from a pickup device  10 , a reproduction amplifier  11 , a decoder  12 , a CPU  13 , an encoder  14 , a power control circuit  15 , a laser drive circuit  16 , a pre-pit signal decoder  18 , a pre-pit signal detector  19 , phase comparators  21  and  23 , a wobble signal extractor  22 , a reference clock generator  24 , a spindle driver  25 , a spindle motor  26 , a low pass filter (LPF)  28 , a voltage controlled oscillator (VCO)  29 , and a servo controller  30 . The pre-pit signal detector  19  corresponds to the pre-pit detection circuit according to the invention. Digital data Srr to be recorded are input into the thus-configured data recording-and-reproducing apparatus  100  from an external host computer via an interface  17 .  
      The pickup device  10  emits a laser beam onto a data recording surface of the optical disk  1 , which is the target of the data recording and reproduction, on the basis of a laser drive signal Sd 1 , there by effecting data writing onto the optical disk  1  and data reading from the optical disk  1 . The pickup device  10  detects a signal corresponding to the pre-pit  4  and the groove track  2  by use of the reflected light of the light beam B according to the radial push-pull method. When recording, the pickup device  10  records the digital data Srr to be recorded, and detects digital data—which have been already recorded—through use of the reflected light of the light beam B.  
      More specifically, as shown in  FIG. 3A , the pickup device  10  is configured from a semiconductor laser  111 , a collimator lens  112 , a beam splitter  113 , a deflecting prism  114 , an objective lens  115 , an actuator  116 , a detection lens  117 , and a quadrant photodetector  120 .  
      The semiconductor laser  111  emits the light beam B under a state where the optical disk  1  is rotatably driven by the spindle motor  26 . The light beam B emitted from the semiconductor laser  111  is converted into parallel light through the collimator lens  112 , and enters the beam splitter  113 . The light beam B which has passed through the beam splitter  113  is deflected through the deflection prism  114 , and enters the objective lens  115 . Thereafter, the light beam B is condensed on the recording surface of the optical disk  1  by the objective lens  115 , thereby forming a light spot (see  FIG. 1 ). The light beam B reflected on the optical disk  1  is returned to parallel light by the objective lens  115 , is deflected through the deflection prism  114 , and thereafter enters the beam splitter  113 . The light beam B is changed by 90° in its orientation by the beam splitter  113 , subsequently propagates through the detection lens  117 , and is received by the quadrant photo detector  120 .  
      The quadrant photodetector  120  is a photo detector of a rectangular shape which outputs an electric signal commensurate with intensity of a received signal. The quadrant photodetector  120  is divided into four equal light-receiving regions A, B, C, and D which respectively correspond to regions of the optical disk  1  along the radial direction and the tangential direction (the direction along which the grooves are formed). More specifically, the quadrant photodetector  120  is divided in half by a first division line provided along the tangential direction of the outer periphery of the optical disk  1 , that is, a division line corresponding to the direction of the tracks, into two groups of light receiving regions A, D and light receiving regions B, C, and further divided in half by a second division line corresponding to the radial direction of the optical disk  1  into two groups of light receiving regions A, B and light receiving regions C, D, to thereby divided into four regions. A light beam received by the receiving regions A, B, C, and D of the quadrant photodetector  120  is converted into light-receiving signals Ra, Rb, Rc, and Rd commensurate with the amount of the received light. The light-receiving signals Ra, Rb, Rc, and Rd are transmitted to the reproduction amplifier  11  as a pickup detection signal Sdt.  
      The reproduction amplifier  11  amplifies the pickup signal detection signal Sdt output from the pickup device  10 , and outputs a pre-information signal Spp corresponding to the pre-pit  4  and the wobble signal of the groove track  2 , and outputs an amplification signal Sp corresponding to the digital data which have been already been recorded.  
      The decoder  12  performs 8-16 demodulation and de-interleave processing on the amplification signal Sp, thereby decoding the amplification signal Sp, and outputs a demodulated signal Sdm to the CPU  13 .  
      The pre-pit signal detector  19  outputs, to the pre-pit signal decoder  18  and the phase comparator  23 , a pulse signal serving as a pit detection signal Spd on the basis of the pre-information signal Spp. The pre-pit signal detector  19  constitutes a principal portion of the invention, and the details thereof will be described later.  
      The phase comparator  23 , the LPF  28 , and the VCO  29  integrally constitute a PLL circuit. The PLL circuit outputs to the encoder  14  and the pre-pit signal detector  19  a recording clock signal Scr which is synchronized with a phase of the pre-pit detection signal Spd, which has been input to the PLL circuit.  
      The wobble signal extractor  22  includes a band-pass filter (BPF) which extracts a wobble signal component from the pre-information signal Spp, and a comparator which compares the thus-extracted wobble signal component with a predetermined reference value. The wobble signal extractor  22  outputs a pulse signal only during a period when the amplification level of the wobble signal component exceeds the reference value. More specifically, the wobble signal extractor  22  slices the wobble signal component in a pulse train form, thereby outputting the signal as an extracted wobble signal Swb to the comparator  21 .  
      The phase comparator  21  compares a phase of the thus-input extracted wobble signal Swb and that of a reference clock signal Sref which includes a reference frequency component of the rotational velocity of the optical disk  1  supplied from the reference clock generator  24 , thereby obtaining a difference signal. The phase comparator  21  supplies the thus-obtained difference signal to the spindle motor  26  via the spindle driver  25  as a rotation control signal. As a result, spindle servo control is exercised against the spindle motor  26 , whereby the optical disk  1  rotates at a rotational velocity determined on the basis of a frequency and a phase of the reference clock signal Sref.  
      Under control by the CPU  13 , the interface  17  performs interface processing on the digital data Srr—which has been transmitted from the host computer—so that the digital data Srr are acquired by the data recording-and-reproducing apparatus  100 , and outputs the thus-processed digital data Srr to the encoder  14  via the CPU  13 .  
      The encoder  14  performs an unillustrated ECC generation processing, 8-16 modulation, and scrambling on the digital data Srr with use of the recording clock signal Scr from the VCO  29 , thereby generating a modulating signal Sre, and outputs the modulating signal Sre to the power control circuit  15  and the pre-pit signal detector  19 .  
      The power control circuit  15  performs write strategy processing on the modulating signal Sre on the basis of the recording clock signal Scr so that recording pits are formed in a good shape on the optical disk  1 , and outputs a record signal Sd for use with driving the laser diode (unillustrated) in the pickup device  10 . The laser drive circuit  16  outputs a laser drive signal Sdl, on the basis of the record signal Sd, for actually driving the laser diode and radiating the light beam B.  
      On the basis of a pre-information decoded signal Spj output from the pre-pit signal decoder  18  on the basis of the pre-pit detection signal Spd, the CPU  13  acquires pre-information, and controls recording operation for recording the digital data Srr onto the optical disk  1  at a position corresponding to the address information contained in the thus-acquired pre-information. Furthermore, on the basis of the demodulated signals Sdm, the CPU  13  outputs a reproduction signal corresponding to the previously-recorded digital data by way of the interface  17  to the outside, and controls the data recording-and-reproducing apparatus  100  in its entirety. Furthermore, the CPU  13  generates a status signal Srp which indicates whether the data recording-and-reproducing apparatus  100  is under a recording status or reproducing status, and outputs the status signal Srp to the pre-pit signal detector  19 .  
      The servo controller  30  is a circuit which generates the focusing error signal and the tracking error signal respectively on the basis of the pickup detection signal Sdt, that is, the light-receiving signals Ra, Rb, Rc, and Rd. The focusing error signal is a signal which drives the objective lens  115  for correcting a focal point of the light beam B, thereby correcting a relative distance between the objective lens  115  and the optical disk  1 . The tracking error signal is a signal which drives the objective lens  15  for adjusting in the radial direction of the disk a position where the data reading spot of the light beam B is formed. The focusing error signal and the tracking error signal are respectively supplied to the actuator  116 . The actuator  116  drives the objective lens  115  on the basis of the focusing error signal and the tracking error signal.  
      Next, the pre-pit signal detector  19  of the embodiment will be described by reference to  FIG. 4 .  
       FIG. 4  is a block diagram of the pre-pit detection circuit for use with the data recording-and-reproducing apparatus of the embodiment.  
      The pre-pit signal detector  19  is a pre-pit detection circuit for detecting the pre-pit detection signal Spd. The pre-pit detection signal Spd is a signal component corresponding to the pre-pit  4  formed on the optical disk  1  in advance on the basis of the light-receiving signals Ra, Rb, Rc, and Rd which have been generated by the quadrant light-receiving element  120 .  
      The pre-pit signal detector  19  is disposed subsequent to adders  121 ,  122  provided in the reproduction amplifier  11 , as shown in  FIG. 4 . The pre-pit signal detector  19  includes buffer amplifiers  51 ,  52  for effecting impedance matching, a gain control circuit  41  for adjusting gain of the amplifier  31 , a gain control circuit  42  for adjusting gain of the amplifier  32 , a subtracter  33  for subtracting an output signal of the amplifier  32  from an output signal from the amplifier  32 , thereby outputting the result as a push-pull signal P, and a binarization circuit  34  for binarizing the push-pull signal P at a threshold value TH, thereby outputting the result from the subtracter  33 .  
      The pre-information signal Spp is input into the pre-pit signal detector  19  from the reproduction amplifier  11 . The reproduction amplifier  11  adds the light-receiving signals Ra and Rd obtained in the light-receiving regions A and D of the quadrant light-receiving element  120 , and generates an addition signal Rad by the adder  121  in the reproduction amplifier  11 , adds the light-receiving signals Rb and Rc obtained in the light-receiving regions B and C of the quadrant light receiving element  120 , and generates an addition signal Rbc by the adder  122  in the reproduction amplifier  11 , and the addition signals Rad and Rbc are transmitted to the pre-pit signal detector  19  as the pre-information signal Spp.  
      In the pre-pit signal detector  19 , the addition signal Rad within the pre-information signal Spp is subjected to impedance matching by the buffer amplifier  51 , and thereafter transmitted to the amplifier  31 . The addition signal Rbc within the pre-information signal Spp is subjected to impedance matching by the buffer amplifier  52 , and thereafter transmitted to the amplifier  32 . A gain value adjusted by the gain control circuit  41  is input to the amplifier  31 , and again of the addition signal Rad is adjusted in accordance with the thus-input gain value. In a similar manner, a gain value adjusted by the gain control circuit  42  is input to the amplifier  32 , and a gain of the addition signal Rbc is adjusted in accordance with the thus-input gain value. The gain adjustment will be described in greater detail later.  
      The addition signals Rad and Rbc respectively adjusted with use of predetermined gain values are subjected to subtraction by the subtracter  33 , whereby the push-pull signal P is generated. Here, the thus-generated push-pull signal P is of a waveform in which a pulse component is superposed on a substantially sinusoidal waveform. In the push-pull signal P, the substantially sinusoidal waveform is the signal component which corresponds to a shape of the groove, and the pulse component protruding from the sinusoidal is the LPP component which corresponds to the pre-pit  4 . The binarization circuit  34  slices out the LPP component at the threshold value TH which has been controlled so as to detect the LPP component of the push-pull component P, thereby generating a pre-pit detection signal PPd.  
      Meanwhile, with regard to detection of the LPP component, criteria which must be satisfied by the optical disk  1  include an aperture ratio (hereinafter, referred to as “AR”). The AR is defined from the maximum peak value APmax and the minimum peak value APmin in the maximum value WOmax of a groove track component in the push-pull signal P, as follows: 
 
 AR (%)= AP min/ AP max×100   (1) 
 
      With regard to detection of LPP, a large AR value means that the range where binarization is available is wide, and that accuracy in pre-pit detection is increased. The AR value is required to be greater than 15% after recording into the information tracks of the optical disk  1 , however, generally, noise components, or the like, are easily embedded after recording of data in the information tracks, thereby reducing the AR value.  
       FIG. 7  is a graph showing disk-eccentricity-dependency of the AR characteristics, in which eccentricity of the optical disk  1  and AR characteristics, which are post-recording LPP characteristics, are shown.  FIG. 7  shows that the smaller the disk eccentricity, the better the AR characteristics tend to be, and when the disk eccentricity is increased, the AR characteristics lower. The reason why the AR characteristics decrease with increase in the eccentricity, of the disk is assumed to be as follows. When the optical disk  1  has some eccentricity, the objective lens  115  is deviated laterally to the direction of the tracks, in accordance with the eccentricity, and the motion of the objective lens  115  is directly reflected in a shift in intensity distribution of the addition signals Rad, Rbc on the quadrant photodetector  20 .  
       FIGS. 8A  to  9 B show the above-mentioned shifts in intensity distribution along with actual electric signals.  FIGS. 8A and 8B  show cases where the eccentricity of the disk is small.  FIGS. 9A and 9B  show cases where the eccentricity of the disk is large. The drawings show that push-pull components (A+D) (that is, the addition signal Rad) and the push-pull components (B+C) (that is, the addition signal Rbc) vary corresponding to a tracking error (hereinafter referred to as “TE”) signal residual component (eccentricity information), and the greater the eccentricity of the disk, the greater the changes in amplitude of the push-pull components (A+D) and (B+C).  
      Furthermore, since the total quantity of light entering the quadrant photodetector  120  is constant, the change in amplitude of the push-pull components (A+D) varies inversely with that of the push-pull components (B+C). More specifically, when the amplitude of (A+D) increases, that of (B+C) decreases, and when the amplitude of (A+D) decreases, that of (B+C) increases. As a result, an amplitude of the push-pull signal P in its entirety, which is a difference between the addition signals Rad and Rbc increases, whereby the AR value tends to fall easily.  
      According to the embodiment, the amplifiers  31  and  32  adjust amplitudes of the addition signals Rad and Rbc so as to render the difference between the addition signals Rad and Rbc uniform.  
      More specifically, a signal dependent on the eccentricity component, that is, a signal which is proportional to the deviation of the objective lens  115 —which is disposed in the pickup device  10  (hereinafter referred to as “disk-eccentricity-dependent component signal”)—is input to the pre-pit signal detector  19  via an unillustrated signal line. By controlling the amplitudes of the addition signals Rad, Rbc corresponding to the magnitude of the disk-eccentricity-dependent component signal, the difference between the addition signals Rad and Rbc is decreased.  
      Here, the disk-eccentricity-dependent component signal is, for instance, a tracking error residual component or an actuator drive current, or the like. The tracking error residual component is obtained by extracting, by a band-pass filter, only a frequency component which depends on the eccentricity component, from the tracking error signal generated by the servo controller  30 . The actuator drive current is a current supplied to the actuator  116  in accordance with the tracking error signal. In the embodiment, for instance, as shown in  FIG. 3A , a current measurement circuit  116   a  is provided for measurement of the actuator drive current supplied to the actuator  116 . The actuator drive current measured by the current measurement circuit  116   a  is output to a phase compensation circuit  44  as a disk-eccentricity-dependent component signal.  
      More specifically, the disk-eccentricity-dependent component signal input to the pre-pit signal detector  19  is subjected to phase compensation by the phase compensation circuit  44 , and supplied to the gain control circuits  41  and  42 . Here, an inverting amplifier  43  for reversing the sign (positive or negative) of an input value is disposed between the gain control circuit  41  and the phase compensation circuit  44 . The gain control circuits  41 ,  42  respectively supply, to the amplifiers  31 ,  32 , predetermined gain values corresponding to signs and magnitudes of the disk-eccentricity-dependent component signals, which have been respectively input to the circuits  41 ,  42 . The amplifiers  31 ,  32  adjust the gains of the addition signals Rad, Rbc in accordance with the gain values and control so that the difference in amplitude between the addition signals Rad and Rbc is decreased.  
      As described above, in the pre-pit signal detector  19  of the embodiment, the addition signals Rad, Rbc obtained from the quadrant photodetector  20  are amplified on the basis of the deviation of the objective lens with use of the respective gain values by the amplifiers  31 ,  32 , thereby being controlled so that the difference in amplitude between the addition signals Rad and Rbc is decreased. As a result, the pre-pit signal detector  19  compensates for noise of the push-pull signal P caused by eccentricity of the optical disk  1 , and prevents degradation of the push-pull signal P caused by the same. Therefore, even when the optical disk  1  has some eccentricity, AR characteristics are improved by preventing degradation of the push-pull signal P along with the eccentricity-dependent-component which depends on the deviation amount of the objective lens. As described above, the embodiment provides a pre-pit detection circuit and a data reading device which enables, even when the disk has large eccentricity, acquisition of a good push-pull signal P—as if it were obtained from a disk whose eccentricity is close to zero—merely by addition of a simple circuit configuration, whereby, even during data reading after data recording, pre-pits can be detected accurately.  
      As described above, the data recording-and-reproducing apparatus  100 , which is a data reading device of the embodiment, is an apparatus which reads information recorded on an optical disk  1  serving as a data recording medium on which pre-pits have been formed in advance. The data recording-and-reproducing apparatus  100  has the semiconductor laser  111  serving as a laser light source for radiating a light beam, the objective lens  115  for converting the light beam, thereby forming a light spot on the data recording medium, the actuator  116  for driving the objective lens  115 , and the quadrant light-receiving element  120  which is a light-receiving unit having first light-receiving regions A, D and second light-receiving regions B, C which have been respectively divided by division lines corresponding to the direction of the tracks of the optical disk  1 , and which receive light beam reflected on the optical disk  1 .  
      Furthermore, the data recording-and-reproducing apparatus  100  has the amplifier  31  serving as a first amplitude control unit, the amplifier  32  serving as a second amplitude control unit, the subtracter  33  serving as a computing unit, the binarization circuit  34  serving as a pre-pit detection unit, and an extraction unit. The amplifier  31  controls the amplitude of the addition signal Rad, which is a first output signal output from the first light-receiving regions A, D. The amplifier  32  controls the amplitude of the addition signal Rbc, which is a second output signal output from the second light-receiving regions B, C. The subtracter  33  performs subtraction of the addition signal Rad controlled by the amplifier  131  and the addition signal Rbc controlled by the amplifier  32 , thereby generating the push-pull signal P. The binarization circuit  34  detects the pre-pit signal PPd on the basis of the push-pull signal P. The extraction unit (e.g., the servo controller  30 ) extracts an eccentricity component of the optical disk  1 . In the data recording-and-reproducing apparatus  100 , the amplifiers  31  and  32  adjust amplitudes of the addition signals Rad, Rbc on the basis of the eccentricity component of the optical disk  1 .  
      Therefore, the embodiment enables control of the difference between the addition signals Rad and Rbc so as to be decreased by adjusting the amplitudes of the addition signals Rad, Rbc on the basis of the eccentricity component of the optical disk  1 . Accordingly, even when the optical disk  1  has some eccentricity, the push-pull signal P is prevented from degrading in accordance with the eccentricity-dependent component depending on the deviation of the objective lens, whereby the AR characteristics are improved. As described above, the embodiment provides a pre-pit detection circuit and a data reading device which enable, even when the disk has large eccentricity, acquisition of a good push-pull signal P—as if it were obtained from a disk whose eccentricity is close to zero—merely by addition of a simple circuit configuration, whereby, even during data reading after data recording, pre-pits can be detected accurately.  
      In the embodiment, the tracking error residual component and actuator drive current are employed as a signal to be input into the phase compensation circuit  44  serving as a signal proportional to the deviation of the objective lens, however, the embodiment is not limited thereto. For instance, as shown in  FIG. 10 , the embodiment may adopt a lens sensor  118  which directly detects the deviation of the objective lens, and use a detection output from the lens sensor  118  as a signal proportional to the deviation amount of the objective lens  115 .  
     SECOND EMBODIMENT  
       FIG. 11  is a schematic diagram showing a second embodiment of the pre-pit signal detector  19  according to the invention.  
      The pre-pit signal detector  19  of the embodiment is obtained by replacing the pre-pit detector  19  of an open-loop servo type shown in  FIG. 4  with that of a closed-servo loop type. In the following, repeated descriptions of elements identical with those of the first embodiment are omitted.  
      The pre-pit signal detector  19  of the embodiment includes a band-pass filter  45  connected subsequent to the subtracter  33 , and a level detection circuit  46  connected subsequent to the band-pass filter  45 .  
      The band-pass filter  45  allows to pass only signals which depend on eccentricity of the disk (e.g., signals ranging from 0 to 100 Hz) among the push-pull signals P output from the subtracter  33 . In other words, the band-pass filter  45  generates the disk-eccentricity-dependent component signal corresponding to the amount of eccentricity of the disk  1  on the basis of the push-pull signal P.  
      The level detection circuit  46  detects a level of the disk-eccentricity-dependent component signal output from the band-pass filter  45 , and outputs the level value of the thus-detected disk-eccentricity-dependent component signal to the phase compensation circuit  44 .  
      In other words, the embodiment is configured as follows: a signal proportional to the deviation of the objective lens is not input from the outside into the phase compensation circuit  44 , but only an eccentricity component is extracted from the push-pull signal P, whereby the disk-eccentricity-dependent component signal, which is proportional to the deviation of the objective lens, is fed back.  
      The phase compensation circuit  44  performs phase compensation processing on the level value of the disk-eccentricity-dependent component signal supplied from the level detection circuit  46 , and thereafter supplies the level value of the disk-eccentricity-dependent component signal to the gain control circuit  42  and the inverting amplifier  43 . The gain control circuit  42  determines a gain value corresponding to the level value, and supplies the thus-determined gain value to the amplifier  32 . The inverting amplifier  43  reverses the sign (positive and negative) of the thus-supplied level value, and supplies the resultant signal to the amplifier  33 . The amplifier  33  determines a gain value corresponding to the thus-supplied level value, and supplies the gain value to the amplifier  31 .  
      Also according to the embodiment, a good push-pull signal P-as if it were obtained from a disk whose eccentricity is close to zero—an be obtained even when the disk has large amount of eccentricity, this is achieved by changing gains in accordance with magnitude of the disk-eccentricity-dependent component signal which changes in accordance with the amount of the disk eccentricity.  
      As described above, the pre-pit signal detector  19  of the embodiment performs feedback control upon extraction of only an eccentricity component (DC to 100 Hz) . Therefore, a good push-pull signal P—as if it were obtained from a disk whose eccentricity is close to zero—an be obtained, even with a disk having large eccentricity.  
      Meanwhile, the embodiment may be configured such that the cut-off frequency of the band-pass filter (BPF) is changed in accordance with the rotational velocity of the disk.  
     THIRD EMBODIMENT  
       FIG. 12  is a schematic diagram showing a third embodiment of the pre-pit signal detector  19  according to the invention. The pre-pit signal detector  19  of the embodiment is substantially identical in configuration with that of the second embodiment shown in  FIG. 11 , however, it differs in that an output signal from the inverting amplifier  43  is supplied to the gain control circuit  41  via a balance adjustment circuit  47 , which is a balance adjustment unit. In the following, repeated descriptions of elements identical with those of the first and second embodiments are omitted.  
      The balance adjustment circuit  47  is a circuit which adjusts the level value supplied to the gain control circuit  41  so as to obtain the best AR characteristics. Generally, on an assumption that absolute values of gains supplied to the amplifiers  31  and  32  are identical, AR is expected to be improved when the amplitude ratio between the addition signals Rad and Rbc, that is, a gain balance between the amplifiers  31  and  32 , is close to zero. However, in actual, as shown in  FIG. 11 , there are cases where AR is improved when the ratio of the output signal Rad from the adder  121  and the output signal Rc from the adder  122  deviates from  1 . In the example shown in  FIG. 11 , the AR value reaches the maximum value when (A+D)/(B+C)=1.05, this indicates that the AR characteristics, which are the post-recording LPP characteristics, are better when the balance between (A+D) and (B+C) is approximately 1:1.05, rather than 1:1.  
      The balance adjustment circuit  47  increases or decreases the level value supplied from the inverting amplifier  43  so as to maintain the gain balance of the gain values supplied to the amplifiers  31  and  32 , thereby maintaining the amplitude ratio between the addition signals Rad and Rbc at a predetermined value.  
      The pre-pit signal detector  19  of the embodiment enables optimization of the AR value by adjustment of gain balance between the amplifier  31  and  32  so that the AR characteristics become the maximum, thereby widening a range where binarization is applicable and increasing accuracy in detection of a pre-pit.  
     FOURTH EMBODIMENT  
       FIG. 14  is a schematic diagram showing a fourth embodiment of the pre-pit signal detector  19  according to the invention.  
      The pre-pit signal detector  19  of the embodiment is substantially identical in configuration with that of the third embodiment shown in  FIG. 13 , however, it differs in that characteristics of the push-pull signal P are measured by a push-pull signal characteristics measurement circuit  48 , and the balance adjustment circuit  47  is controlled so as to cancel the noise component of the push-pull signal P.  
      Meanwhile, the characteristics measured by the push-pull signal characteristics measurement circuit  48  include the AR characteristics, an error ratio of the push-pull signal P (a ratio of the number of the actually-detected push-pull signals P to the number of push-pull signals expected to be detected), and the like.  
      That is, the pre-pit signal detector  19  of the embodiment is configured such that the push-pull signal characteristics measurement circuit  48  measures the AR characteristics and the error ratio of the push-pull signal P on the basis of the push-pull signal P, and the gain balance of the adders  31 ,  32  is adjusted so as to optimize the gain balance between the amplifiers  31 ,  32 , thereby maximizing the AR characteristics in accordance with the AR characteristics, the error ratio, and the like.  
      As described above, according to the embodiment, the gain balance between the amplifiers  31 ,  32  is fed back so as to optimize the gain balance. Accordingly, the AR characteristics, which are the characteristics of the post-recording LPP, can be increased, and the eccentricity of the disk can be made equal to zero under any circumstances, thereby bringing the gain balance into a state where the best AR characteristics are achieved.