Abstract:
To identify the type of a loaded disc, 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 the amplitude of the high frequency signal to be superimposed on the drive signal, the optical disc device being configured to read data from an optical disc loaded therein by a signal outputted from the detector, wherein: the controller causes the light source to irradiate the loaded optical disc with light in a condition in which a high frequency signal different from a high frequency signal for data reading is superimposed on the drive signal; and the controller identifies a type of the loaded optical disc based on the return light from the loaded optical disc.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese patent application JP 2009-294151 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 disk device, and in particular, relates to a method of identifying the type of an optical disc loaded therein. 
         [0003]    Optical disc is generally classified into CD (Compact Disc), DVD (Digital Versatile Disc), and BD (Blu-ray Disc). Furthermore, there are a plurality of types, such as ROM (read only), -R (writable), and -RE (rewritable), in each kind of disc; playing conditions are different depending on the type of disc. For this reason, an optical disk device is required to identify the type of a disc loaded therein. 
         [0004]    Conventionally, optical disc devices identify the type of disc through any one of the following methods to perform playing sequences differing depending on the type of disc: 
         [0005]    (1) a method of reading management information recorded in a Burst Cutting Area (BCA) to identify the type of disc by the read management information; 
         [0006]    (2) a method of identifying the type of disc by Differential Phase Detection (DPD) amplitude; and 
         [0007]    (3) a method of identifying the type of disc by RF amplitude. 
         [0008]    For example, JP2007-18581A discloses a technique related to this invention. 
         [0009]    As described above, there are various methods for an optical disc device to identify the type of a disc loaded therein. The above-described method (1) enables reliable disc identification because the disc standard stipulates that the type of disc must be recorded in management information (a BCA). Actually, however, there are discs without BCAs. Furthermore, if a BCA is unclean or scratched, the BCA data might be unable to be read from the disc. 
         [0010]    DPD is used for tracking adjustment, but tracking adjustment can be made through other methods. Therefore, in an optical disc device which does not employ the DPD, the method (2) cannot be used. 
         [0011]    The method using RF signal amplitude utilizes the phenomenon that the intensity of a signal obtained from the return light from a data area, in which data is written, differs depending on the disc, namely, that ROM type discs have greater reflectance and RF amplitude is likely to be larger because of their structure, while -R and -RE type discs have smaller reflectance. However, the reflectance of disc considerably varies among discs, so the identification using reflectance has been less reliable. 
         [0012]    For these reasons, the type of a loaded disc sometimes cannot be identified exactly even through the above-described three methods. A method for exactly identifying the type of disc has been desired. 
         [0013]    An object of this invention is to provide a method of identifying the type of a loaded disc, particularly whether the disc is a read only (ROM) disc or a writable (-R or -RE) disc. 
       SUMMARY OF THE INVENTION 
       [0014]    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 the amplitude of the high frequency signal to be superimposed on the drive signal, the optical disc device being configured to read data from an optical disc loaded therein by a signal outputted from the detector. The controller causes the light source to irradiate the loaded optical disc with light in a condition in which a high frequency signal different from a high frequency signal for data reading is superimposed on the drive signal. The controller identifies a type of the loaded optical disc based on the return light from the loaded optical disc detected by the detector. 
         [0015]    According to an aspect of this invention, a loaded disc can be identified as either a read only disc or a writable disc. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The present invention can be appreciated by the description which follows in conjunction with the following figures, wherein: 
           [0017]      FIG. 1  is a block diagram illustrating a configuration of an optical disc device according to an embodiment of this invention; 
           [0018]      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; 
           [0019]      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; 
           [0020]      FIG. 4  is an explanatory diagram illustrating measured asymmetry outputted from the photoelectric conversion element according to an embodiment of this invention; 
           [0021]      FIGS. 5A and 5B  are flowcharts of a disc identification procedure  1  in the optical disc device according to an embodiment of this invention; 
           [0022]      FIG. 6  is a flowchart of an optical disc identification procedure  2  according to an embodiment of this invention; 
           [0023]      FIG. 7  is a flowchart of a first modified example of the optical disc identification procedure  2  according to an embodiment of this invention; and 
           [0024]      FIG. 8  is a flowchart of a second modified example of the optical disc identification procedure according to an embodiment of this invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0025]      FIG. 1  is a block diagram illustrating a configuration of an optical disc device  100  according to an embodiment of this invention. 
         [0026]    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 . 
         [0027]    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 . 
         [0028]    The spindle motor  102  rotates and drives an optical disc  101  loaded in the optical disc device  100 . 
         [0029]    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. 
         [0030]    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. 
         [0031]    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. It should be noted that a part of the return light enters the laser beam source  108 . 
         [0032]    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. 
         [0033]    The monitor diode  121  is for detecting laser power for APC (Automatic Power Control). The signal bandwidth for the monitor diode  121  may be sufficiently low compared with the high-frequency wave which is superimposed on the laser beam in playing. The monitor diode  121  output signal  122  detected by the monitor diode  121  is sent to the laser power controller  123 . 
         [0034]    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 . 
         [0035]    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). 
         [0036]    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 . It should be noted that the optical disc identification module  112  may be constituted by a program to be executed by the system controller  114 . 
         [0037]    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. 
         [0038]    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 . 
         [0039]    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. 
         [0040]    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 . 
         [0041]      FIG. 2  is a block diagram illustrating a configuration of the laser driver  113  and the laser power controller  123 . 
         [0042]    The laser power controller  123  comprises a playing power target value generator  131  and a subtractor  132 . 
         [0043]    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 . 
         [0044]    The laser driver  113  comprises an amplifier  133 , a high-frequency signal generator  134 , a switch  136 , and an adder  137 . 
         [0045]    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. 
         [0046]    The amplifier  133  amplifies a received difference value  124  and sends the amplified value to the adder  137 . 
         [0047]    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 . 
         [0048]    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 . 
         [0049]    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. 
         [0050]    The switch  136  controls on and off of the high-frequency signal generator  134 . 
         [0051]    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. 
         [0052]    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. 
         [0053]    Therefore, the amplitude of superimposed high-frequency signal should be adjusted to moderate amplitude. 
         [0054]    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 . 
         [0055]      FIG. 3  illustrates a principle that the photoelectric converter  107  generates an asymmetric output. 
         [0056]    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). 
         [0057]    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 by 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. 
         [0058]    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. 
         [0059]    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. 
         [0060]    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 ). 
         [0061]    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. 
         [0062]    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 . 
         [0063]    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. 
         [0064]    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 . 
         [0065]    The asymmetry of the RF signal can be calculated by Beta expressed in the following Expression (1): 
         [0000]    
       
         
           
             
               
                 
                   Beta 
                   = 
                   
                     
                       ( 
                       
                         B 
                         + 
                         A 
                       
                       ) 
                     
                     
                       ( 
                       
                         B 
                         - 
                         A 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0066]    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. 
         [0067]    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 
                   ) 
                 
               
             
           
         
       
     
         [0068]    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. 
         [0069]    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. 
         [0070]    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. 
         [0071]      FIGS. 5A and 5B  are flowcharts of a disc identification procedure in the optical disc device  100  in the embodiment of this invention. The procedure disclosed in  FIGS. 5A and 5B  is executed by the optical disc identification module  112 . 
         [0072]    To identify an optical disc  101  loaded in the optical disc device  100 , the procedure first moves the optical pickup  120  to a position for disc identification (for example, the innermost circumference) ( 201 ), and turns on the laser beam source  108  to emit a laser beam with a preset initial wavelength and a preset initial amount of light ( 202 ). 
         [0073]    Next, an offset voltage to be applied to the photoelectric converter  107  is adjusted based on the amount of the return light received by the photoelectric converter  107  ( 203 ). 
         [0074]    Then, the objective lens  103  is moved by the actuator, starts a focus sweep ( 204 ), and a focus error signal (FE) and an intensity signal (PE) are obtained ( 205 ). 
         [0075]    The position in thickness where the focus error signal is compared with the intensity signal have been obtained with the specifications of discs to locate the recording layer of the loaded disc, and a type of the loaded disc  101  is identified as either a CD, a DVD, or a BD to end the general identification of the disc ( 206 ). 
         [0076]    Thereafter, the laser beam source is turned on to emit a laser beam with a wavelength for the general type of disc and in the amount of light preset for the type of disc ( 211 ), and the offset voltage to be applied to the photoelectric converter  107  is adjusted based on the amount of the return light received by the photoelectric converter  107  ( 212 ). 
         [0077]    After that, the objective lens  103  is moved by the actuator, a focus error signal (FE) and an intensity signal (PE) are obtained, and the amplitudes of the obtained focus error signal (FE) and intensity signal (PE) are adjusted to match them to their identification thresholds by adjusting the gain with which the signal processor  110  processes the signals ( 213 ). 
         [0078]    Next, the optical pickup  120  is moved to a position for focusing ( 214 ) and focuses on an arbitrary layer ( 215 ). Then, the focus jumps onto the layer for disc identification ( 216 ). The layer of the destination of the focus jump is usually the L 0  layer on which the management information (a BCA in a BD, for example) is recorded. 
         [0079]    The management information (BCA) recorded on the destination layer (the L 0  layer) ( 217 ) is read and it determines whether the management information has been read properly and whether the management information contains the type of disc ( 218 ). 
         [0080]    If the read management information includes information about the type of disc, the type of the loaded disc is identified, the disc identification procedure is terminated, and a procedure of playing the disc is started. 
         [0081]    On the other hand, if the type of disc has not been read, namely, if the management information is not read properly, or if the read management information does not include the type of disc, a disc identification procedure  2  is prepared to execute. 
         [0082]    Specifically, it starts a tracking servo to prepare for properly reading data recorded on the disc ( 219 ). It should be noted that the tracking servo may be off in measurement of the asymmetry. Then, the disc identification procedure  2  is started ( FIGS. 6 and 7 ) ( 220 ). 
         [0083]      FIG. 6  is a flowchart of an optical disc identification procedure  2  in the embodiment of this invention; the procedure is executed by the optical disc identification module  112 . 
         [0084]    First, 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 on which the high-frequency signal is superimposed ( 231 ). The HF-amp 1  is preferably an amplitude of a high-frequency signal which is not for data reading. 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. 
         [0085]    Next, the photoelectric converter  107  receives the return light from the optical disc  101  and outputs an RF signal ( 232 ). 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  ( 233 ). 
         [0086]    The optical disc identification module  112  compares the value of Asymmetry (Asym 1 ) of the RF signal outputted from the photoelectric converter  107  with a predetermined threshold to identify the type of the loaded optical disc ( 234 ). The threshold is preferably a value with a given margin added to the maximum value which is acceptable to the specification as asymmetry. 
         [0087]    In the identification, if the value of Asymmetry is greater than the predetermined threshold, the loaded optical disc is defined as a read only ROM type disc ( 235 ), and the optical disc identification procedure is terminated, and the procedure returns to the optical disc identification procedure  1  ( 236 ). On the other hand, if the value of Asymmetry is the predetermined threshold value or less, the type of the loaded disc cannot be identified. Therefore, the procedure returns to the optical disc identification procedure  1  and the type of optical disc will be identified by another sequence ( 237 ). 
         [0088]    As described above, according to the optical disc identification procedure  2  illustrated in  FIG. 6 , one measurement of Asymmetry after changing the amplitude of the high frequency signal enables the identification of the type of disc, providing a speedy identification of the type of disc. 
         [0089]      FIG. 7  is a flowchart of a first modified example of the optical disc identification procedure  2  in the embodiment of this invention. 
         [0090]    First, the optical disc identification module  112  sets the 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 ( 241 ). This HF-amp 2  is preferably a normal amplitude of the high frequency signal which has been predetermined for the general type of optical disc. This setting of the HF-amp 2  to meet the optimum read condition provides a waveform suitable for identification. 
         [0091]    Next, the photoelectric converter  107  receives the return light from the optical disc  101  and outputs an RF signal ( 242 ). 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  ( 243 ). 
         [0092]    The optical disc identification module  112  sets an amplitude of the high frequency signal HF-amp 3  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 ( 244 ). The HF-amp 3  is preferably an amplitude of the high-frequency signal which is not for data reading. 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. 
         [0093]    Next, the photoelectric converter  107  receives the return light from the optical disc  101  and outputs an RF signal ( 245 ). 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  ( 246 ). 
         [0094]    With respect to the order of the measurements, the measurement of Asym 3  may precede the measurement of Asym 2 . 
         [0095]    The optical disc identification module  112  obtains the difference (ΔAsym) between the value of Asymmetries Asym 2  and Asym 3  of the RF signal obtained under different conditions of high frequency superimposition ( 247 ). Then the obtained difference of Asymmetries (i.e. a variation of Asymmetries) is compared with a predetermined threshold to identify the type of the loaded optical disc ( 248 ). This threshold is preferably set at the value obtained by deducting a predetermined margin from a value approximate to the maximum value or the maximum value of the asymmetry acceptable to the specification. 
         [0096]    In the identification, if the variation of the asymmetry is more than the predetermined threshold, the optical disc identification module  112  identifies the loaded optical disc as a ROM type disc ( 249 ), and terminates the optical disc identification procedure, and the procedure returns to the optical disc identification procedure  1  ( 250 ). On the other hand, if the variation of Asymmetry is less than or equal to the predetermined threshold, the type of the loaded optical disc cannot be identified. Therefore, the procedure returns to the optical disc identification procedure  1  and the type of optical disc will be identified by another sequence ( 251 ). 
         [0097]    As described above, according to the first modified example illustrated in  FIG. 7 , the type of disc is identified by the variation of Asymmetry caused by changing the amplitude of the high frequency signal, so the type of the loaded disc can be identified even if the amplitude is zero or in the Asymmetry ranges of the specification (under the threshold at  234 ). 
         [0098]      FIG. 8  is a flowchart of a second modified example of the optical disc identification procedure in the embodiment of this invention. 
         [0099]    The second modified example is an optical disc identification procedure of the combination of the procedure illustrated in  FIG. 6  and the first modified example illustrated in  FIG. 7 . 
         [0100]    Namely, first, the optical disc identification procedure illustrated in  FIG. 6  ( 231  to  235 ) is executed. If the type of the loaded optical disc cannot be identified, the steps  241  to  243  and  247  to  251  in the modified example illustrated in  FIG. 7  are executed. Since the steps  244  to  246  of the modified example illustrated in  FIG. 7  are the same as the steps  231  to  233  in the optical disc identification procedure illustrated in  FIG. 6 , it is not necessary to measure the asymmetry without the high frequency superimposed in steps  244  to  246 ; a simplified procedure is achieved. 
         [0101]    Although the procedures illustrated in  FIGS. 5A ,  5 B,  6 ,  7 , and  8  have been described to be executed by the optical disc identification module  112 , they may be executed by the system controller  114 . 
         [0102]    The procedures described above use the value of Asymmetry to identify the type of disc, but may use another value (Beta) indicating asymmetry. Otherwise, instead of the asymmetry of the RF output, the amplitude of the RF output may be used to identify the type of disc. This is because, if the asymmetry is found, the amplitude of the RF output will be smaller. 
         [0103]    As described above, according to the embodiment of this invention, the symmetry of the RF signal outputted from the photoelectric converter  107  is measured with change of the amplitude of the high frequency signal to be superimposed on the laser beam emitted by the laser beam source  108 , or the amplitude of the high frequency signal superimposed on the laser drive signal. Since the type of disc is identified by variation of symmetry of the RF signal caused by changing the amplitude of the high frequency signal, the disc identification is achieved with high reliability. 
         [0104]    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.