Abstract:
To determine the optimum amplitude of high frequency signal for the loaded disc with a simple configuration to achieve stable playing performance, there is provided an optical disc device comprising: a light source driven by a drive signal with a high frequency signal superimposed; a detector for detecting return light emitted from the light source; and a controller for controlling amplitude of the high frequency signal to be superimposed on the drive signal for the light source; the optical disc device being configured to read data on an optical disc loaded therein by a signal outputted from the detector, wherein the controller determines the amplitude of the high frequency signal to be superimposed on the drive signal based on asymmetry of the signal outputted from the detector.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese patent application JP 2009-294190 filed on Dec. 25, 2009, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to an optical disc device, and in particular, relates to an optical disc device that can optimally adjust the amplitude of a high frequency signal to be superimposed on a drive signal of a laser beam source to reduce laser noise. 
         [0003]    An optical disc device shines a laser beam emitted from a laser beam source onto an optical disc to play data recorded on the optical disc with laser light reflected on the surface of the optical disc. Because of the structure of the optical disc device, a part of the return light from the optical disc enters the laser beam source. The return light may remarkably increase noise in the laser beam. Hence, the optical disc device superimposes a high frequency signal onto a drive signal of the laser beam source so that the laser beam source emits an intermittent beam modulated with the high frequency signal, not a continuous beam, and the timing of the emission of laser beam is staggered from the timing of the entrance of return light to suppress variation of the intensity of the laser beam (for example, refer to JP2008-112578A and JP2009-110602A). 
         [0004]    As described above, superimposition of high frequency in playing an optical disc provides stable playing performance. However, when the amplitude of the high frequency signal to be superimposed on the laser beam source is smaller and the laser beam is less modulated, the laser noise might not be sufficiently reduced even though a high frequency is superimposed. For example, variation of characteristics of laser diodes or variation of characteristics of optical systems may cause insufficient modulation. If the laser noise is not sufficiently reduced by these causes, data recorded on the optical disc may not be able to be played. 
         [0005]    On the other hand, when an optical disc is played with a high frequency signal having excessive amplitude superimposed, the data recorded on the optical disc might be destroyed. Therefore, it has been desired that the amplitude of the high frequency signal to be superimposed be smaller to suppress the peak power of the laser beam. 
         [0006]    For this reason, the amplitude of the high frequency signal is required to be adjusted to achieve an optimum modulation. 
         [0007]    Conventionally, a method of determining the amount of high frequency signal to be superimposed based on the error rate of data read from an optical disc has been proposed. To measure the error rate, it is disadvantageously necessary to play the data from the optical disc (namely, to perform processes up to decoding). Before adjusting the amount of high frequency signal to be superimposed, adjustments of tracking and others are required, taking much time until determination of the amount of high frequency signal to be superimposed. 
         [0008]    In addition, it is necessary to play data in a certain zone continuously for calculating the error rate; it took time to read the data for calculation of the error rate. Accordingly, a method for quick determination of the amount of high frequency signal to be superimposed has been desired. 
         [0009]    In the meanwhile, a method of determining the amount of high frequency signal to be superimposed based on the amount of laser noise has been proposed. To measure the amount of laser noise, however, a dedicated detector which can detect a high frequency signal; the configuration of the optical disc device becomes complicated to increase costs. 
         [0010]    An object of this invention is quick determination of the optimum amplitude of high frequency signal for the loaded disc with a simple configuration to achieve stable playing performance. 
       SUMMARY OF THE INVENTION 
       [0011]    A representative aspect of this invention is as follows. That is, there is provided an optical disc device comprising: a light source driven by a drive signal with a high frequency signal superimposed; a detector for detecting return light emitted from the light source; and a controller for controlling amplitude of the high frequency signal to be superimposed on the drive signal for the light source; the optical disc device being configured to read data on an optical disc loaded therein by a signal outputted from the detector, wherein the controller determines the amplitude of the high frequency signal to be superimposed on the drive signal based on asymmetry of the signal outputted from the detector. 
         [0012]    According to an aspect of this invention, the optimum amplitude of high frequency signal for a loaded disc can be determined with a simple configuration. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The present invention can be appreciated by the description which follows in conjunction with the following figures, wherein: 
           [0014]      FIG. 1  is a block diagram illustrating a configuration of an optical disc device according to an embodiment of this invention; 
           [0015]      FIG. 2  is a block diagram illustrating a configuration of a laser driver and a laser power controller according to an embodiment of this invention; 
           [0016]      FIG. 3  is an explanatory diagram illustrating a principle that a photoelectric conversion element generates an asymmetric output according to an embodiment of this invention; 
           [0017]      FIG. 4  is an explanatory diagram illustrating measured asymmetry outputted from the photoelectric conversion element according to an embodiment of this invention; 
           [0018]      FIG. 5  is a flowchart of an HF adjustment procedure in the optical disc device according to an first embodiment of this invention; 
           [0019]      FIG. 6  is a flowchart of an HF adjustment procedure in the optical disc device according to a second embodiment of this invention; and 
           [0020]      FIG. 7  is an explanatory diagram illustrating the example of measurement of asymmetry with changing an amplitude of high frequency signal according to a HF adjustment procedure of the second embodiment of this invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0021]      FIG. 1  is a block diagram illustrating a configuration of an optical disc device  100  according to an embodiment of this invention. 
         [0022]    The optical disc device  100  in this embodiment is coupled to a host computer  150  and outputs data read from a loaded optical disc  101  (for example, a Blu-ray Disc) to the host computer  150 . The optical disc device  100  may have a function of recording data received from the host computer  150  onto a writable optical disc  101 . 
         [0023]    The optical disc device in this embodiment comprises a spindle motor  102 , an I/V converter  109 , a signal processor  110 , a demodulator  111 , an optical-disc identification module  112 , a laser driver  113 , a system controller  114 , a memory  115 , a data bus  116 , an optical pickup  120 , and a laser power controller  123 . 
         [0024]    The spindle motor  102  rotates and drives an optical disc  101  loaded in the optical disc device  100 . 
         [0025]    The optical pickup  120  comprises an objective lens  103 , a splitter  104 , a collimating lens  105 , a collecting lens  106 , a photoelectric converter  107 , a laser beam source  108 , and a monitor diode  121 . When read data from an optical disc  101 , the optical pickup  120  irradiates the optical disc  101  with a weak laser beam, reads data recorded on the optical disc  101  using the reflection of the laser beam (return light), and outputs an RF signal corresponding to the reflection. 
         [0026]    The laser beam source  108  is a semiconductor laser that generates a desired intensity of laser beam for recording and playing, and emits a laser beam having a wavelength specified for each type of disc to be loaded. The laser beam emitted from the laser beam source  108  impinges on a specific radial position of the recording surface of the optical disc  101  through the collimating lens  105  and the objective lens  103 . The objective lens  103  is driven by an actuator to adjust the laser beam to be focused on the disc surface. 
         [0027]    In recording data onto the optical disc  101 , the laser beam source  108  irradiates the optical disc  101  with a more intensive laser beam than in playing data therefrom. By thermal phase-change at the spot where the laser beam has impinged on the optical disc  101 , a recording pit is formed on the recording layer. The recording pit changes the reflectance of the recording layer to record data. 
         [0028]    The laser light reflected on the recording surface of the optical disc  101  is split by the splitter  104 , collected by the collecting lens  106 , and introduced to the photoelectric converter  107 . The photoelectric converter  107  converts the received reflected light into an electric signal (an RF signal) and outputs the RF signal corresponding to the return light. It should be noted that a part of the return light enters the laser beam source  108 . 
         [0029]    The monitor diode  121  is for detecting laser power for APC (Automatic Power Control). The signal bandwidth for the monitor diode may be sufficiently low compared with the high-frequency wave which is superimposed on the laser beam in playing. The monitor diode output signal  122  detected by the monitor diode  121  is sent to the laser power controller  123 . 
         [0030]    The I/V converter  109  converts a current signal outputted from the photoelectric converter  107  into a voltage signal (an RF signal) and amplifies it. The signal processor  110  is a DSP (Digital Signal Processor), which converts the RF signal outputted from the I/V converter  109  into digital data. The signal processor  110  outputs an optical disc identifier signal differing depending on the structure of the optical disc, a focus error signal for adjusting the focal point of a laser beam, and a tracking error signal for following a track of the optical disc  101 . 
         [0031]    The demodulator  111  demodulates the digital data outputted from the signal processor  110  in accordance with the format specified for each type of optical disc, performs error detection and error correction on it, and then temporarily stores the demodulated data in the memory  115  (buffer). 
         [0032]    The optical disc identification module  112  identifies the type of the loaded optical disc  101  with reference to the optical disc identifier signal outputted from the signal processor  110 . The identification result on the optical disc  101  outputted from the optical disc identification  112  is sent to the system controller  114  via the data bus  116 . The system controller  114  controls modules to be in optimum conditions (playing conditions and writing conditions) for the identified optical disc based on the identification result of the optical disc. 
         [0033]    The laser driver  113  outputs a laser drive signal  117  for driving the laser beam source  108  in the optical pickup  120 . The laser power controller  123  sets laser power target values in playing and writing in accordance with the type of the optical disc identified by the optical disc identification module  112 . 
         [0034]    The system controller  114  comprises a microprocessor for controlling operations of the optical disc device  100  and a memory. The memory in the system controller  114  stores a program to be executed and data necessary for executing the program. The system controller  114  further comprises an interface for controlling sending and receiving of data and commands between the optical disc device  100  and the host computer  150  coupled thereto. The system controller  114  controls reading of data temporarily stored in the memory  115  and writing of data to the memory  115 . The system controller  114  interprets a command received from the host computer  150  and processes the received command. 
         [0035]    The memory  115  includes a buffer area in which data read from the optical disc  101  is temporarily stored. The data bus  116  connects modules in the optical disc device  100 . 
         [0036]      FIG. 2  is a block diagram illustrating a configuration of the laser driver  113  and the laser power controller  123 . 
         [0037]    The laser power controller  123  comprises a playing power target value generator  131  and a subtractor  132 . 
         [0038]    First, in accordance with the type of the optical disc  101  identified by the optical disc identification module  112 , the system controller  114  sets a target value of the average laser power for playing the optical disc on a playing power target value generator  131 . The subtractor  132  calculates the difference value  124  between the target value set on the playing power target value generator  131  and the monitor diode&#39;s output value  122 . The calculated difference value  124  is sent to the laser driver  113 . 
         [0039]    The laser driver  113  comprises an amplifier  133 , a high-frequency signal generator  134 , a switch  136 , and an adder  137 . 
         [0040]    The laser driver  113  controls the intensity of the laser beam to be outputted from the laser beam source  108  with the difference value  124  calculated by the laser power controller  123 . This control compensates for a temperature change around the laser beam source  108  and a change in the I/L (current to luminance) ratio caused by degradation over time to achieve stable control of the intensity of the laser beam. 
         [0041]    The amplifier  133  amplifies a received difference value  124  and sends the amplified value to the adder  137 . 
         [0042]    The high frequency signal generator  134  comprises a variable gain amplifier  135 , an amplitude controller  138 , and a frequency controller  139 , and generates a high-frequency signal to be superimposed onto the laser drive signal  117 . 
         [0043]    The amplitude and the frequency of the high-frequency signal to be outputted from the high-frequency signal generator  134 , or the amplitude and the frequency of the high-frequency signal to be superimposed onto the playing laser power, can be determined by the system controller  114 . Specifically, the amplitude controller  138  controls the amplitude of the high-frequency signal to be superimposed with the value determined by the optical disc identification module  112  or the system controller  114 . The frequency controller  139  controls the frequency of the high-frequency signal to be superimposed with the value determined by the optical disc identification module  112  or the system controller  114 . 
         [0044]    The variable gain amplifier  135  amplifies an amplitude signal outputted from the amplitude controller  138  and the high-frequency signal outputted from the frequency controller  139  with the gain controlled by the difference value  124  to generate a high-frequency signal having a desired amplitude. 
         [0045]    The switch  136  controls on and off of the high-frequency signal generator  134 . 
         [0046]    When the switch  136  is off, the high-frequency signal generator  134  does not output a high-frequency signal, so a high-frequency signal is not superimposed onto the laser output. In this state, the return light, which is light reflected on the surface of the optical disc  101  and incoming into the laser beam source  108 , cause interference with resonance cavity of a laser diode included in the laser beam source  108 . Thus, the laser beam is emitted with an amplitude tremor. Hence, the signal quality (S/N ratio) is degraded. 
         [0047]    On the other hand, in the state in which the amplitude of superimposed high-frequency signal is excessive, an erroneous data is recorded on the optical disc  101  and a recording layer of the optical disc degrades in relation of the peak power of the laser beam in which high-frequency signal is superimposed. 
         [0048]    Therefore, the amplitude of superimposed high-frequency signal should be adjusted to moderate amplitude. 
         [0049]    The adder  137  adds the output of the amplifier  133  and the output of the high-frequency signal generator  134 . The output of the adder  137  is outputted as a laser drive current  117  from the laser driver  113 . 
         [0050]      FIG. 3  illustrates a principle that the photoelectric converter  107  generates an asymmetric output. 
         [0051]    An optical disc, particularly a ROM-type disc e.g. a BD-ROM, has pits  301  formed by indenting a substrate made of synthetic resin (for example, polycarbonate); the amount of the return light  302  changes around a pit. The photoelectric converter  107  detects the change of the return light to read data from the optical disc. Accordingly, the RF signal  303  outputted from the photoelectric converter  107  varies with the amount of reflection  302  (return light). 
         [0052]    In general, the reflectance of an optical disc is higher at a space and the amount of the return light is higher. In contrast, the reflectance is lower at a mark and the amount of the return light is lower. Accordingly, the level of the RF output  303  is higher at a space and lower at a mark. The return light from the optical disc  101  is split by the splitter  104  so as not to return to the laser beam source  108 . Actually, however, a part of the return light passes through the splitter  104  and enters the laser beam source  108  as return light. When the laser light enters the laser beam source  108 , a change in laser output called scoop noise occurs. When a laser beam scans a space, the amount of return light is greater, so that the intensity of the laser beam emitted from the laser beam source  108  diminishes and the RF signal outputted from the photoelectric converter  107  becomes weaker. On the other hand, when a laser beam scans a mark, the amount of return light is smaller, so that the intensity of the laser beam emitted from the laser beam source  108  increases and the RF signal outputted from the photoelectric converter  107  becomes stronger. In this way, the RF signal varies depending on the position of a mark recorded on a disc. 
         [0053]    As a result, the level of the RF signal outputted from the photoelectric converter  107  is higher at a space and lower at a mark as indicated by a waveform  304 , which is denoted by a dashed line. Namely, the waveform of the RF output is asymmetric between the positive side and the negative side. This is because oscillation within the laser changes by return light to cause noise, so that the laser output changes. 
         [0054]    To avoid the RF output from getting asymmetric in this way, the laser beam emitted by the laser beam source  108  is preferably an intermittent beam modulated with a high-frequency signal, instead of a continuous beam. The intermittent beam can suppress laser noise by reducing the interference between the laser beam and the return light, so that symmetry in the RF output can be maintained. 
         [0055]    The inventors of this invention have found through measurement that the symmetry of the RF signal outputted from the photoelectric converter  107  differs depending on the type of optical disc between when a high-frequency signal is superimposed on the laser beam emitted by the laser beam source  108  and when a high-frequency signal is not superimposed. In particular, ROM-type discs and R-type discs showed remarkably different symmetries (refer to  FIG. 4 ). 
         [0056]    For example, in a BD-ROM disc, the state in continuous radiation (HF OFF) is compared with the state in normal radiation (HF ON). Comparing beta when the HF has been set at a minimum value (HF=0), namely beta in the state of DC playing under the continuous radiation, with beta when the HF has been set at a value for normal playing (HF=30), the value increases by three times. In contrast, in a case of BD-R discs, the values change a little comparing beta when the HF has been set at the minimum value with beta when the HF has been set at 30. Furthermore, when the HF has been set at zero, the beta of the BD-ROM disc is greater than the beta of the BD-R discs. 
         [0057]    In the above description, when HF=0, a high frequency signal is not superimposed on the laser drive signal; and when HF=30, it is a normal condition for playing a Blu-ray Disc (a normal data reading state) with a high frequency signal superimposed on the laser drive signal. The data reading state is a state with high frequency wave superimposed, for example, to play management information or user data recorded on the optical disc  101 . 
         [0058]    The amount of high frequency signal to be superimposed in this normal playing condition is predetermined depending on the type of disc and the number of layers of the disc; the amount of superimposition is set in accordance with the identified type of the disc. In this connection, the amount of the high frequency signal to be superimposed may be determined after adjusting the predetermined initial value depending on the variation of characteristics of the disc. Adjusting the high frequency signal to meet the normal conditions suitable for playing a disc achieves reduction in laser noise and less error rate (SER: Symbol Error Rate) in playing a disc. 
         [0059]    Utilizing the characteristic that the asymmetry of the RF signal changes with the setting of the HF, the type of an optical disc can be identified by measuring the asymmetry of the RF signal outputted from the photoelectric converter  107  with change of the amplitude of the high frequency signal superimposed on the laser beam outputted by the laser beam source  108 . 
         [0060]    The asymmetry of the RF signal can be calculated by Beta expressed in the following Expression (1): 
         [0000]    
       
         
           
             
               
                 
                   Beta 
                   = 
                   
                     
                       ( 
                       
                         B 
                         + 
                         A 
                       
                       ) 
                     
                     
                       ( 
                       
                         B 
                         - 
                         A 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0061]    In the Expression (1), A represents an amplitude between a positive peak of the RF signal and the zero level and B represents an amplitude between a negative peak of the RF signal and the zero level. In other words, a value of Beta indicates the extent of unbalance between the positive side amplitude and the negative side amplitude with respect to the total amplitude and is expressed by percent. Although the value of Beta is used as an index in writing data, it may be used in playing data. 
         [0062]    The asymmetry of the RF signal can also be calculated by a value of Asymmetry expressed in the following Expression (2): 
         [0000]    
       
         
           
             
               
                 
                   Asymmetry 
                   = 
                   
                     
                       
                         ( 
                         
                           
                             I 
                             
                               8 
                                
                               H 
                             
                           
                           + 
                           
                             I 
                             
                               8 
                                
                               L 
                             
                           
                         
                         ) 
                       
                       - 
                       
                         ( 
                         
                           
                             I 
                             
                               2 
                                
                               H 
                             
                           
                           + 
                           
                             I 
                             
                               2 
                                
                               L 
                             
                           
                         
                         ) 
                       
                     
                     
                       2 
                       × 
                       
                         I 
                         
                           8 
                            
                           
                               
                           
                            
                           PP 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0063]    In the Expression (2), I 8H  represents a positive peak voltage of the RF output waveform when reading a 8T mark; I 8L  represents a negative peak voltage of the RF output waveform when reading the 8T mark. I 8PP  represents a peak-to-peak voltage of the RF power waveform when reading the 8T mark and can be expressed by I 8H −I 8L . I 2H  represents a positive peak voltage of the RF power waveform when reading a 2T mark and I 2L  represents a negative peak voltage of the RF power waveform when reading the 2T mark. 
         [0064]    Accordingly, the value of Asymmetry is the ratio of the difference between the center of the signal level when reading the longest 8T mark, (I 8H +I 8L )/2, and the center of the signal level when reading the shortest 2T mark, (I 2H +I 2L )/2, to the peak-to-peak voltage when reading the longest 8T mark, and is expressed by percent. 
         [0065]    The value of Asymmetry and the value of Beta are calculated and outputted by the signal processor  110 . The outputted Asymmetry (or Beta) is sent to the optical disc identification module  112  and used in optical disc identification, which will be described later. 
         [0066]      FIG. 5  is a flowchart of an HF adjustment procedure in the optical disc device  100  of a first embodiment of this invention; the procedure is executed by the system controller  114 . 
         [0067]    First, the optical disc device  100  identifies the type of the loaded optical disc  101 , reads management information, adjusts laser power, and adjusts the focus. Then, the focus jumps onto the L 0  layer. It is sufficient that tracking adjustment be performed after the HF adjustment illustrated in  FIG. 5  (in other words, tracking may be off during the HF adjustment), but it may be performed before the HF adjustment. In these operations, a laser beam with a predetermined initial amount of high frequency signal superimposed is emitted. 
         [0068]    Next, the system controller  114  sets an amplitude of high frequency signal, HFamp 1 , on the high frequency signal generator circuit  134  with tracking either on or off, and irradiates the optical disc  101  with a laser beam in which a high frequency signal is superimposed ( 201 ). The HFamp 1  is preferably the amplitude for normal data reading, which is predetermined depending on the type of optical disc. The HFamp 1  set at the optimum read condition provides a waveform suitable for determining the HF. 
         [0069]    Next, the photoelectric converter  107  receives the return light from the optical disc  101  and outputs an RF signal ( 202 ). The signal processor  110  calculates the Asymmetry (Asym 2 ) from the RF signal outputted from the photoelectric converter  107  and sends the calculated Asymmetry to the optical disc identification module  112  ( 203 ). 
         [0070]    The optical disc identification module  112  sets an amplitude of the high frequency signal HF-amp 2  on the high-frequency signal generator  134  and irradiates the optical disc  101  with a laser beam in which the high frequency signal is superimposed ( 204 ). The HF-amp 2  is preferably an amplitude of the high-frequency signal which is not for playing. For example, it may be a minimum amplitude settable to the optical disc device  100  (for example, zero), which is different from the value in normal data reading. This is because, if the amplitude of the superimposed high-frequency signal is zero, the asymmetry of the RF signal outputted from the photoelectric converter  107  will be greatest. 
         [0071]    Next, the photoelectric converter  107  receives the return light from the optical disc  101  and outputs an RF signal ( 205 ). The signal processor  110  calculates the value of Asymmetry (Asym 3 ) from the RF signal outputted by the photoelectric converter  107  and sends the calculated value of Asymmetry to the optical disc identification module  112  ( 206 ). 
         [0072]    The system controller  114  obtains the difference (ΔAsym) between Asymmetries Asym 1  and Asym 2  of RF signals obtained in different conditions of high frequency superposition ( 207 ) and compares the obtained difference of Asymmetries (i.e. a variation of Asymmetries) with a predetermined threshold (Asym_sub) ( 208 ). 
         [0073]    If the variation of the asymmetry is equal to or more than the predetermined threshold, the amplitude of high frequency signal is not in condition suitable for playing the optical disc  101 , so the system controller  114  adds a predetermined value (ΔHFamp) to the HFamp 1  ( 210 ), adds the predetermined value (ΔHFamp) to the HFamp 2  ( 211 ), and the procedure returns to the step  201  to measure the Asymmetry of the RF signal with a new amplitude of high frequency signal. It should be noted that the system controller  114  may change the amplitude of high frequency signal constantly during a series of measurement, but not at every set of measurement. 
         [0074]    On the other hand, if the variation of Asymmetry is smaller than the predetermined threshold, the amplitude of high frequency signal is in condition suitable for playing the optical disc  101 , so the system controller  114  determines the average between the HFamp 1  and the HFamp 2  to be the amount of high frequency signal to be superimposed to play the optical disc ( 209 ). The amount of high frequency signal to be superimposed need not be the average between the HFamp 1  and the HFamp 2  as long as it is either the HFamp 1  or the HFamp 2 , otherwise, any value between HFamp 1  and HFamp 2 . 
         [0075]    Although the HF adjustment in the first embodiment determines the amount of high frequency signal to be superimposed using Asymmetry, it may use the above-described Beta. 
         [0076]    Otherwise, other properties in playing data may be used (for example, error rate) to determine the definitive amount of high frequency signal to be superimposed. 
         [0077]    Next, a modified example of the HF adjustment procedure of the first embodiment will be described. 
         [0078]    The HF adjustment procedure in the first embodiment increases the amplitude of high frequency signal starting from the initial value in the optimum reading condition to find the optimum value for the amplitude of high frequency signal as summarized in the following (1). In addition, following modifications (2) and (3) are suggested. 
         [0079]    (1) The initial value of the amplitude of high frequency signal: in condition where an optical disc can be played (for example, initially-set normal data reading condition) 
         [0080]    Change of the amplitude of high frequency signal: to be increased from the initial value 
         [0081]    (2) The initial value of the amplitude of high frequency signal: in condition where an optical disc can be played (for example, initially-set normal data reading condition) 
         [0082]    Change of the amplitude of high frequency signal: to be decreased from the initial value 
         [0083]    (3) The initial value of the amplitude of high frequency signal: minimum (a high frequency signal is not superimposed.) 
         [0084]    Change of the amplitude of high frequency signal: to be increased from the initial value 
         [0085]    In the meanwhile, using a plurality of methods in combination, the smallest value of the optimum values obtained by the plurality of methods may be determined to be the amount of high frequency signal to be superimposed. This is because it is preferable that the peak value of the laser power (namely, the amplitude of the high frequency signal to be superimposed) be smaller for the tolerance against impingement of laser beam in playing. 
         [0086]    Otherwise, the HF adjustment in the first embodiment described above may be performed in a recovery procedure after the amount of high frequency signal to be superimposed is determined based on an error rate at the initial setting. In this example, at least one of the procedures (1) to (3) may be performed in the recovery procedure. In this regard, the recovery procedure includes a process of resetting the playing condition when data cannot be read because playing condition has not been adjusted appropriately (for example high SER) in initialization. 
         [0087]      FIG. 6  is a flowchart of an HF adjustment procedure in the optical disc device  100  of a second embodiment of this invention; the procedure is executed by the system controller  114 . 
         [0088]    The HF adjustment of the second embodiment determines the amount of high frequency signal to be superimposed based on the value of Asymmetry measured in a single HF set value instead of the difference of Asymmetries between two HF set values, unlike the HF adjustment of the first embodiment described above. 
         [0089]    First, like the HF adjustment of the first embodiment, the optical disc device  100  identifies the type of the loaded optical disc  101 , reads management information, adjusts laser power, and adjusts the focus. Then, the focus jumps to the L 0  layer. 
         [0090]    Next, the optical disc identification module  112  sets the amplitude of the high frequency signal HF-amp  1  on the high frequency signal generator  134  and irradiates the optical disc  101  with a laser beam in which the high-frequency signal is superimposed ( 221 ). The HF-amp 1  is preferably a minimum amplitude settable to the optical disc device  100  (for example, zero), which is different from the value in normal data reading. This is because, if the amplitude of the superimposed high-frequency signal is zero, the asymmetry of the RF signal outputted from the photoelectric converter  107  will be greatest. 
         [0091]    After that, the photoelectric converter  107  receives the return light from the optical disc  101  and outputs an RF signal ( 222 ). The signal processor  110  calculates the value of Asymmetry from the RF signal outputted by the photoelectric converter  107  and sends the calculated value of Asymmetry to the optical disc identification module  112  ( 223 ). 
         [0092]    The system controller  114  compares the value of Asymmetry (Asym 1 ) of the RF signal outputted from the photoelectric converter  107  with a predetermined threshold (Asym_th) ( 224 ). The threshold is preferably the maximum value (or with a given margin added to the maximum value) which is acceptable to the specification as asymmetry. 
         [0093]    If the Asymmetry is equal to or more than a predetermined threshold, the amplitude of high frequency signal is out of specification and is not in condition suitable for playing the optical disc  101 , so the system controller  114  adds a predetermined value (ΔHFamp) to the amplitude of high frequency signal ( 226 ). Then the procedure returns to the step  222 , and obtains the RF signal outputted from the photoelectric converter element  107 . It should be noted that the amplitude of high frequency signal may be constantly changed during a series of measurement, but not at every measurement. 
         [0094]    On the other hand, if the Asymmetry is less than the predetermined threshold, the amplitude of high frequency signal is within the specification and is in condition for playing the optical disc  101 . So, the system controller  114  determines the current amplitude of high frequency signal to be the amount of high frequency signal to be superimposed for playing the optical disc ( 225 ). 
         [0095]      FIG. 7  is a drawing for illustrating the example where the amplitude of high frequency signal is constantly changed in the HF adjustment of the second embodiment of this invention. 
         [0096]    As the amplitude of high frequency signal  701  is increased, the Asymmetry  702  decreases. A first measurement of Asymmetry is performed with the amplitude of high frequency signal at the initial value (in the optimum condition for reading which has been set initially) ( 711 ). 
         [0097]    Thereafter, as the amplitude of high frequency signal is increased, the measurement of Asymmetry is repeated until the Asymmetry becomes less than a predetermined threshold ( 712 ,  713 ). The amplitude of high frequency signal at which the Asymmetry has become smaller than the threshold (the specification value of Asymmetry)  703  is determined to be the amount of high frequency signal to be superimposed for playing the optical disc ( 714 ). 
         [0098]    As described above, according to the embodiments of this invention, the optimum amplitude of high frequency signal for a loaded disc can be obtained with a simple configuration. Therefore, the peak power of a laser beam can be reduced and its average power can be increased without extra high frequency signal superimposed. 
         [0099]    Furthermore, according to the embodiments of this invention, the amount of high frequency signal to be superimposed can be adjusted before playing data from an optical disc (namely, before the process up to decoding). 
         [0100]    In particular, if the amplitude of high frequency signal is decreased from the initially set value for the optimum reading condition like in the modification (2) of the first embodiment or if it is increased from the minimum value like in the modification (3), a smaller amount of high frequency signal to be superimposed can be determined. 
         [0101]    While the present invention has been described in detail and pictorially in the accompanying drawings, the present invention is not limited to such detail but covers various obvious modifications and equivalent arrangements, which fall within the purview of the appended claims.