Abstract:
An AR measuring apparatus receives the reflection light of an optical beam irradiated on an optical recording medium having a recording surface on which pre-pits are repeatedly formed between tracks by means of a first light receiving surface and a second light receiving surface, obtains a push-pull signal based on the difference between light receiving signals from the first and second light receiving surfaces, collects sample data by sampling the push-pull signals for a predetermined period of time, repeats the sampling for the predetermined period of time by a plurality of times, determines whether or not each data value corresponding to a pre-pit position of the sample data by the plurality of times is obtained by a predetermined number of times or greater, and when data values obtained by the predetermined number of times or greater are determined, calculates as an aperture ratio, a ratio between the minimum value and the maximum value of values corresponding to a pre-pit component of the data values obtained by the predetermined number of times or greater.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an aperture ratio (AR) measuring apparatus for an optical recording medium having a recording surface on which pre-pits carrying information regarding the information recording tracks are repeatedly formed between such information recording tracks.  
           [0003]    2. Description of the Related Background Art  
           [0004]    Recently, CD-R, CD-RW, DVD-R, DVD-RW, DVD-RAM, etc. have been known as optical recording discs on which information data can be written. In addition, information recording/playing apparatuses are in practical use which can record and reproduce information data on such a recording disc.  
           [0005]    [0005]FIG. 1 is a schematic view of the area configuration of a DVD-RW as such a recording disc.  
           [0006]    As shown in FIG. 1, a DVD-RW has a data configuration comprising the following areas from the inner edge to the outer edge: PCA (Power Calibration Area), RMA (Recording Management Area), lead-in area, data area, and lead-out area. The PCA is an area on which data is written as a test to determine the recording power of laser beam. The RMA is an area on which management information regarding recording is written. A part of the lead-in area contains an embossed area. The embossed area contains phase pits formed on the disc beforehand. Information regarding copy prevention is written on the embossed area.  
           [0007]    [0007]FIG. 2 shows a part of the recording surface of a disc on which data can be recorded.  
           [0008]    As shown in FIG. 2, on a substrate  101 , convex groove tracks  103  on which information pits Pt for carrying information data are formed and concave land tracks  102  are alternately formed, spirally or concentrically. Furthermore, a plurality of LPPs (land pre-pits)  104  are formed between the adjacent groove tracks  103 . The LPPs  104  are formed beforehand on the land tracks  102  to indicate the recording timing and the address of information data for a disc recorder which records such information data. The groove tracks  103  are formed as grooves from the side of the substrate  101  and the LPPs  104  are formed as pits from the side of the substrate  101 .  
           [0009]    In a disc player for playing an optical disc having such LPPs, an LPP detection circuit is equipped. The LPP detection circuit includes a binarization circuit. The LPP detection circuit receives a reflection beam from the optical disc by means of an optical detector divided into two sections in the track tangential direction of a pickup, and obtains the differential signal of an output signal of the optical detector, that is, a radial push-pull signal PP. The push-pull signal PP has a waveform as shown in FIG. 3. An LPP component projects from the push-pull signal PP. A pre-pit detection signal PP D  showing LPP detection is generated by comparing the level of the push-pull signal PP with a threshold value.  
           [0010]    As shown in FIG. 4, the pre-pit detection signal PP D  exhibits a pulse-shaped level change at each pickup read position corresponding to the LPP. The pre-pit detection signal PP D  contains a synchronous pulse P SYNC  at the beginning of each period T as shown in FIG. 4. The synchronous pulse P SYNC  is followed by two pre-data pulses at a predetermined interval to indicate data such as address, etc. These pre-data pulses do not always exist at each period. As shown in FIG. 4, the third pulse from the synchronous pulse P SYNC  is a pre-data pulse P D  carrying a sector address. When information is recorded on an optical disc, such information is recorded by detecting the address on the optical disc based on this pre-pit detection signal PP D .  
           [0011]    In the manufacture of an optical disc containing LPPs, the LPPs of the manufactured optical disc must conform to a prescribed LPP standard. An AR (Aperture Ratio) is measured to determine whether an optical disc conforms to said LPP standard. For AR measurement, the LPP component of the push-pull signal PP (the third LPP position from the synchronous LPP) is repeatedly sampled for a predetermined period of time. By this sampling, the overlapped waveform of the push-pull signal PP at the third LPP position, that is, an AR waveform, is indicated on a display unit such as an oscilloscope as shown in FIG. 5. It is necessary that the maximum value APmax and the minimum value APmin of the peak values from the maximum value WOmax of the groove track component of the indicated push-pull signal PP should be detected, and the AR value is calculated by the equation AR=APmin/APmax in order to confirm that such AR value is greater than a specified value. A greater AR value means that the binarization range becomes wider and pre-pit detection accuracy becomes higher.  
           [0012]    If there is foreign matter such as dirt stuck to a disc, the foreign matter may introduce noise into the push-pull signal PP of the LPP component in AR measurement. Moreover, if such noise is generated at such a level that the minimum value APmin of the peak value of the push-pull signal PP decreases, the calculated AR value may not reach the prescribed value even though the disc conforms to the standard. Therefore, it is necessary to decrease the influence of foreign matter as much as possible in order to correctly calculate the AR value.  
         SUMMARY OF THE INVENTION  
         [0013]    An object of the present invention is to provide an AR measuring apparatus for an optical recording medium having pre-pits which can detect an AR value properly by eliminating the influence of foreign matter stuck to the disc surface as much as possible.  
           [0014]    According to the present invention, there is provided an aperture ratio measuring apparatus for an optical recording medium having a recording surface provided with pre-pits which are repeatedly formed between tracks and carries information related to the tracks, comprising: an optical detector having a light receiving surface divided into a first and second light receiving faces in the tangential direction of the track, for receiving the reflected light of a light beam radiated onto the recording surface on the first and second light receiving faces to output first and second light detection signals corresponding to respective amounts of the received light on the first and second light receiving faces; a subtractor for calculating the difference between the first and second light detection signals output from the optical detector to generate a push-pull signal; a collection device for collecting sample data by sampling the push-pull signal for a predetermined period of time and for repeating the sampling operation for the predetermined period of time by a plurality of times; a frequency determination device for determining whether each data value corresponding to a pre-pit position of the sample data by the plurality of times collected by the collection device is obtained by a predetermined number of times or greater; and an aperture ratio calculator, when data values obtained by the predetermined number of times or greater are determined by the frequency determination device, for calculating as an aperture ratio, a ratio between the minimum value and the maximum value of values corresponding to a pre-pit component of the data values obtained by the predetermined number of times or greater.  
           [0015]    According to the present invention, there is provided an aperture ratio measuring method for an optical recording medium having a recording surface provided with pre-pits which are repeatedly formed between tracks and carries information related to the tracks, comprising: an optical detection step for receiving the reflected light of a light beam radiated onto the recording surface, on a light receiving surface having first and second light receiving faces divided in the tangent direction of the track, to output first and second light detection signals corresponding to respective amounts of the received light on the first and second light receiving faces; a subtraction step for calculating the difference between the first and second light detection signals to generate a push-pull signal; a collection step for collecting sample data by sampling the push-pull signal for a predetermined period of time and for repeating the sampling operation for the predetermined period of time by a plurality of times; a frequency determination step for determining whether each data value corresponding to a pre-pit position of the sample data by the plurality of times collected in the collection step is obtained by a predetermined number of times or greater; and an aperture ratio calculation step, when data values obtained by the predetermined number of times or greater are determined in the frequency determination step, for calculating as an aperture ratio, a ratio between the minimum value and the maximum value of values corresponding to a pre-pit component of the data values obtained by the predetermined number of times or greater.  
           [0016]    According to the present invention, there is provided a computer data signal representing a series of instructing which cause a computer to perform steps to execute a measuring process in an aperture ratio measuring apparatus for an optical recording medium having a recording surface provided with pre-pits which are repeatedly formed between tracks and carries information related to the tracks, the steps comprising: an optical detection step for receiving the reflected light of a light beam radiated onto the recording surface, on a light receiving surface having first and second light receiving faces divided in the tangent direction of the track, to output first and second light detection signals corresponding to respective amounts of the received light on the first and second light receiving faces; a subtraction step for calculating the difference between the first and second light detection signals to generate a push-pull signal; a collection step for collecting sample data by sampling the push-pull signal for a predetermined period of time and for repeating the sampling operation for the predetermined period of time by a plurality of times; a frequency determination step for determining whether each data value corresponding to a pre-pit position of the sample data by the plurality of times collected in the collection step is obtained by a predetermined number of times or greater; and an aperture ratio calculation step, when data values obtained by the predetermined number of times or greater are determined in the frequency determination step, for calculating as an aperture ratio, a ratio between the minimum value and the maximum value of values corresponding to a pre-pit component of the data values obtained by the predetermined number of times or greater. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 shows the layout structure of each area of a DVD-RW;  
         [0018]    [0018]FIG. 2 shows the configuration of the recording surface of a DVD-RW;  
         [0019]    [0019]FIG. 3 shows the waveform of a radial push-pull signal containing an LPP component;  
         [0020]    [0020]FIG. 4 shows the waveform of a pre-pit detection signal;  
         [0021]    [0021]FIG. 5 shows an AR waveform;  
         [0022]    [0022]FIG. 6 is a block diagram of an AR measuring apparatus according to the present invention;  
         [0023]    [0023]FIG. 7 is a block diagram of the configurations of a head amplifier and a pre-pit detection circuit in the apparatus in FIG. 6;  
         [0024]    [0024]FIG. 8 is a block diagram of the schematic configuration of an oscilloscope in the apparatus in FIG. 6;  
         [0025]    [0025]FIG. 9 is a flowchart of an inspection procedure for an optical disc;  
         [0026]    [0026]FIG. 10 is a flowchart illustrating an AR calculation operation by means of a CPU in FIG. 7;  
         [0027]    [0027]FIG. 11 is a flowchart illustrating the continued procedure of the AR calculation operation in FIG. 10;  
         [0028]    [0028]FIG. 12 is a flowchart illustrating the continued procedure of the AR calculation operation in FIG. 11;  
         [0029]    [0029]FIGS. 13A to  13 C show a data value setting operation of a data signal in the AR calculation operation; and  
         [0030]    [0030]FIGS. 14A to  14 B show averaging and rounding operations of data value in the AR calculation operation. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]    The embodiments of the present invention will be described in detail below with reference to the accompanying drawings.  
         [0032]    [0032]FIG. 6 shows an AR measuring apparatus according to the present invention. This AR measuring apparatus comprises a writing/reading head  2  which can write and read information on an optical disc  1  to be inspected. The writing/reading head  2  is equipped with a recording beam light generator (not shown) for recording information data on a writable or rewritable optical disc  1  having a recording surface as shown in FIG. 2, a reading beam light generator (not shown) for reading recorded information (including information data) from the optical disc  1 , and an optical detector divided into four sections ( 20  in FIG. 7).  
         [0033]    It is not necessary to provide a recording beam light generator and a reading beam light generator separately. One light beam generator may be used if it generates a recording light beam when recording is performed and a reading light beam when reading is performed.  
         [0034]    The reading beam light generator radiates a reading beam light onto the optical disc  1 , which is rotatively driven rotated by a spindle motor  9 , so as to make an information reading spot formed on the recording surface thereof. As shown in FIG. 7, the four-section optical detector  20  comprises a photoelectric transfer element having light-receiving surfaces  20   a  to  20   d  divided into four by the direction in line with the tangent of the information recording track (groove track  103 ) of the optical disc  1  and the direction orthogonal to the tangent of the recording track. The photoelectric transfer element receives the reflection light of the information reading spot reflected from the optical disc  1  by each of the four light-receiving surfaces  20   a  to  20   d , converts each of the four received reflection lights into separate electric signals, and outputs the signals as light receiving signals Ra to Rd.  
         [0035]    A servo controller  4  generates a focus error signal, a tracking error signal, and a slider drive signal based on these light receiving signals Ra to Rd. The focus error signal is supplied to a focusing actuator (not shown) which is mounted on the writing/reading head  2 . The focusing actuator adjusts the focus of the information reading spot based on the focus error signal. The tracking error signal is supplied to a tracking actuator (not shown) mounted on the writing/reading head  2 . The tracking actuator adjusts, in the radial direction of the disc, the position where the information reading spot is formed based on the tracking error signal. The slider drive signal is supplied to a slider  10 . The slider  10  moves the writing/reading head  2  in the radial direction of the disc at a speed corresponding to the slider drive signal.  
         [0036]    The light receiving signals Ra to Rd are supplied to a head amplifier  25  having adders  21  to  23  and a subtracter  24 . The adder  21  adds the light receiving signals Ra and Rd, and the adder  22  adds the light receiving signals Rb and Rc. That is, the adder  21  adds the light receiving signals Ra and Rd obtained from the respective light-receiving surfaces  20   a  and  20   d  of the four-section optical detector  20 , and outputs an added light receiving signal R a+d . The adder  22  adds the light receiving signals Rb and Rc obtained from the respective light-receiving surfaces  20   b  and  20   c  of the four-section optical detector  20 , and outputs an added light receiving signal R b+c .  
         [0037]    The adder  23  adds the output signals R a+d  and R b+c  from adder  21  and adder  22  respectively. The output signal from the adder  23  is a reading signal, that is, an RF signal, and is supplied to an information data reproduction circuit  30 . The information data reproduction circuit  30  binarizes the reading signal, then sequentially performs a demodulating process, error correcting process, and various information decoding processes so as to reproduce information data (video data, audio data, computer data) recorded on the optical disc  1 , and outputs these data.  
         [0038]    The subtracter  24  subtracts the output signal R b+c  of the adder  22  from the output signal R a+d  of the adder  21 . The output signal of the subtracter  24  becomes a signal that indicates the frequency of wobbling of the groove track  103 , and is supplied to a spindle servo unit  26  of the spindle motor  9 . The spindle servo unit  26  rotatively drives the spindle motor  9  so that the frequency obtained from the output signal of the subtracter  24  corresponds to a predetermined rotational speed. As the configuration of the spindle servo unit  26  has already been disclosed in the Japanese Patent Laid-Open Publication No. Hei 10-283638, a description of it is omitted here.  
         [0039]    A pre-pit detection circuit  5  detects a land pre-pit (LPP)  104  formed on a land track (pre-pit track)  102  of the optical disc  1  as shown in FIG. 2 based on each of the output signals of the adders  21 ,  22 , then supplies a pre-pit detection signal PP D  to a recording processing circuit  7 .  
         [0040]    The recording processing circuit  7  recognizes the current position where the writing/reading head  2  is recording, that is, the position on the groove track  103  based on the pre-pit detection signal PP D , and supplies a control signal to the servo controller  4  for making the writing/reading head  2  skip tracks from this recording position to the desired recording position. The recording processing circuit  7  also generates a record modulating data signal by performing the desired record modulating process on the information data to be recorded (information data to be inspected), and supplies the signal to the writing/reading head  2 . A recording beam light generator mounted on the writing/reading head  2  generates a recording beam light corresponding to the record modulating data signal, and radiates this beam light onto the groove track  103  of the optical disc  1 . At this time, heat is conveyed to the area on the groove track  103  where the recording beam is radiated, and an information pit is formed thereon.  
         [0041]    The configuration of the recording processing circuit  7  has also been disclosed in the Japanese Patent Laid-Open Publication No. Hei 10-283638, so a detailed description of it is omitted here.  
         [0042]    The pre-pit detection circuit  5  comprises an amplifier  31  for amplifying the output signal R a+d  of the adder  21 , an amplifier  32  for amplifying the output signal R b+c  of the adder  22 , a subtracter  33  for subtracting the output signal of the amplifier  32  from the output signal of the amplifier  31  and outputting the result as a radial push-pull signal (group wobble signal) PP, and a binarization circuit  34  for binarizing the output push-pull signal PP of the subtracter  33  by a threshold value TH so as to generate a pre-pit detection signal PP D , as shown in FIG. 7. The gain G 1  of the amplifier  31  and the gain G 2  of the amplifier  32  are set to be G 1 =G 2 .  
         [0043]    The push-pull signal PP output from the subtracter  33  is supplied to an oscilloscope  61 . The oscilloscope  61  samples the push-pull signal PP and indicates, for example, the part corresponding to the LPP in the push-pull signal PP.  
         [0044]    The oscilloscope  61  is connected to a personal computer  62 . The personal computer  62  calculates the threshold value TH by using the level data of the push-pull signal PP stored in the internal memory of the oscilloscope  61  (for example, sample memory  93  to be described). No specific configuration of the personal computer  62  is shown here, but it contains a CPU  65  and an internal memory  66 . A D/A converter  63  is connected to the output port of the personal computer  62 . The D/A converter  63  converts the slice level calculated by the personal computer  62  into an analog signal. The output signal of the D/A converter  63  is supplied to the binarization circuit  34  as a threshold value signal for binarization.  
         [0045]    Connection of the personal computer  62  to the oscilloscope  61  and the D/A converter  63  is based on an interface standard such as GPIB, 10BASE-T, or RS-232C.  
         [0046]    The signal output from the binarization circuit  34  (pre-pit detection signal PP D ) is supplied to an error rate detection circuit (not shown). An error rate corresponding to the supplied signal is detected there.  
         [0047]    The oscilloscope  61  may be configured, for example, as shown in FIG. 8. That is, the oscilloscope  61  comprises an A/D converter  91 , a control circuit  92 , a sample memory  93 , a display memory  94 , X and Y drivers  95  and  96 , a display panel  97 , an operation unit  98 , and an interface  99 . The A/D converter  91  converts an input analog signal into a digital signal. The control circuit  92  sequentially writes sample data of the digital signal obtained by the A/D converter  91  into the sample memory  93 , and fetches the data to display by reading data from the sample memory  93  and writing the data into the display memory  94 . The X and Y drivers  95  and  96  drive the display panel  97  according to the data written in the display memory  94  so as to display the waveform of the input analog signal on the display panel  97 . The interface  99  is a circuit based on an interface standard such as GPIB, 10BASE-T, or RS-232C for connecting the personal computer  62 . The interface  99  transfers data written in the sample memory  93  to the personal computer  62  through the control circuit  92 . The interface  99  relays and supplies commands from the personal computer  62  to the control circuit  92 .  
         [0048]    Inspection of a manufactured optical disc by means of such an AR measuring apparatus is performed in the order shown in FIG. 9. That is, a disc to be inspected is randomly selected from a single lot (step S 1 ). Information data for inspection is recorded on the disc to be inspected (step S 2 ). A push-pull signal PP is generated as described above by reading from the recorded disc to be inspected by the writing/reading head  2 , and an AR value is measured (step S 3 ). Whether or not the measured AR value is more than a specified value is determined (step S 4 ). If the AR value is more than the specified value, all the optical discs of the lot pass the inspection for LPP (step S 5 ). If the AR value is below the specified value, all the optical discs of the lot fail the inspection (step S 6 ).  
         [0049]    During measurement of the AR in step S 3 , the push-pull signal PP output from the subtracter  33  is supplied to the oscilloscope  61 . At the oscilloscope  61 , the push-pull signal PP is sampled by the A/D converter  91  according to a clock with high frequency. The control circuit  92  stores the sampling data in the memory  93  sequentially. The control circuit  92  detects a negative peak value which is not less than a predetermined value of sampling data supplied from the A/D converter  91 . Once the peak value is detected, the sampling data is stored in the memory  93  as trigger for a predetermined period of time. This peak value corresponds to the synchronous pulse P SYNC  in FIG. 4. The writing into the memory  93  for a predetermined period of time is repeated n times. The control circuit  92  reads the data stored in the memory  93 , supplies it to the display memory  94 , and displays a waveform on the display panel  97 . The read timing is, for example, set in response to a command from the operation unit  98 . An overlapped waveform of a push-pull signal PP containing the third LPP component of the object to be inspected, that is, an AR waveform, is displayed. In this display, the time axis is adjusted so that the peak value position of the push-pull signal PP becomes the center line of display (time axial center line). The possible value for sampling data to have is 0 to M, depending on the resolution of the A/D converter  91 .  
         [0050]    The personal computer  62  can read and fetch sample data stored in the sample memory  93  by commanding the control circuit  92  of the oscilloscope  61  through the interface  99 .  
         [0051]    The CPU  65  of the personal computer  62  calculates an AR value by the AR calculation operation as shown in FIGS.  10  to  12 .  
         [0052]    During the AR calculation operation, the CPU  65  sets the number of times N for measuring the AR waveform and the number of times n for fetching data (step S 11 ). The numbers of times N and n may be set by input operation through the user&#39;s keyboard (not shown) or may be set previously.  
         [0053]    The personal computer  62  initializes M+1 (M is a positive integer) data signals Y[0] to Y[M] formed in the internal memory  66  to  0  (step S 12 ), and sets a variable j to 0 (step S 13 ). The personal computer  62  also initializes M+1 data signals X[0] to X[M] formed in the internal memory  66  to  0  (step S 14 ), and sets a variable i to 0 (step S 15 ). The variable j is the present number of times for measuring the AR waveform and the variable i is the present number of times for fetching data.  
         [0054]    The CPU  65  reads sampling data positioned on the display center line of the oscilloscope  61  as data D from the sample memory  93  (step S 16 ), and stores data X[D] as 1 (step S 17 ). The data signal number of the data X[D] is the value of the data D. The CPU  65  then adds 1 to the variable i (step S 18 ), and determines whether or not the variable i has reached n, the number of times for fetching data (step S 19 ). If i&lt;n, control returns to step S 16  and reads sampling data positioned on the remaining display center line as the data D.  
         [0055]    If i=n, n pieces of data have been read, the CPU  65  adds each value of data signals X[0] to X[M] to the corresponding data signals Y[0] to Y[M] (step S 20 ). That is, the following operation is performed; Y[0]=Y[0]+X[0], Y[1]=Y[1]+X[1], . . . , Y[M−1]=Y[M−1]+X[M−1], Y[M]=Y[M]+X[M]. The CPU  65  adds 1 to variable j (step S 21 ), and determines whether or not variable j was reached N, the number of times for measuring AR (step S 22 ). If j&lt;N, control returns to step S 14  and repeats the operation of measuring the AR waveform in steps S 14 -S 21 . During this repeated operation, reading from different tracks of the disc to be inspected is performed.  
         [0056]    If j=N, as shown in FIG. 11, the average value of each data value of data signals Y[0] to Y[M] is calculated (step S 23 ). That is, the following operation is performed; Y[0]=Y[0]/N, Y[1]=Y[1]/N, . . . , Y[M−1]=Y[M−1]/N, Y[M]=Y[M]/N. Each averaged data value of data signals Y[0] to Y[M] is rounded to the nearest whole number (step S 24 ). As each data value of data signals Y[0] to Y[M] is averaged and the average value is rounded, any data having grave contingency such as noise is eliminated.  
         [0057]    After step S 24  is executed, the CPU  65  of the personal computer  62  initializes M+1 data signals Z[0] to Z[M] formed in the internal memory  66  to  0  (step S 25 ), and sets a variable f and a variable g to 0 (step S 26 ). The data signals Z[0] to Z[M] correspond to the data signals Y[0] to Y[M]. The CPU  65  reads a data signal Y[f] from the internal memory  66  (step S 27 ), and determines whether or not the variable g is an even number (step S 28 ). If the variable g is an even number, whether or not the data Y[f] is 1 is determined (step S 29 ). If the variable g is an odd number, whether or not the data signal Y[f] is 0 is determined (step S 30 ). If it is determined that Y[f]=1 in step S 29  or that Y[f]=0 in step S 30 , a data signal Z[g] is equalized to f (step S 31 ). Next, the variable g is added to 1 (step S 32 ) and the variable f is added to 1 (step S 33 ). Then whether or not the variable f has reached the fixed value M is determined (step S 34 ). If f&lt;M, the control by the CPU  65  returns to step S 27  and reads the next data signal Y[f] from the internal memory  66 .  
         [0058]    If it is determined that Y[f]=0 in step S 29  or that Y[f]=1 in step S 30 , the control by the CPU  65  jumps to step S 33 . By the operations in steps S 25  to S 34 , the value of f when the data signal Y[f] changes from 0 to 1 or from 1 to 0 is sequentially written into the data signal Z[g].  
         [0059]    If f=M in step S 34 , as shown in FIG. 12, the value of the variable g after subtracting 1 is set as K (step S 35 ). The value of K is the value of g when a value is finally set for the data signal Z[g] at step S 31 .  
         [0060]    The CPU  65  then sets a variable L to 0 and sets the variable g to 1 (step S 36 ). Next, whether or not Z[g +1] Z[g] is larger than the variable L is determined (step S 37 ). Z[g+1]−Z[g] means Z [even number]−Z [odd number] and indicates the length of an area where 0 continues. If Z[g+1]−Z[g]&gt;L, the variable L is set as Z[g+1]−Z[g], and the maximum value WOmax of a wobbling group component is set as Z[g], and the minimum value LPmin of a LPP component is set as Z[g+1] (step S 38 ). After the variable g is added to 2 (step S 39 ), whether or not the variable g has reached K is determined (step S 40 ). If g&lt;K, the control by the CPU  65  returns to step S 37  and whether or not Z[g+1]−Z[g]&gt;L is determined using the new Z (even number) and Z (odd number) obtained in step S 39 .  
         [0061]    If Z[g+1]−Z[g]≦L in step S 37 , the control jumps to step S 39 .  
         [0062]    If g=K in step S 40 , the maximum value LPmax of the LPP component is set as Z[g] (step S 41 ). APmin is calculated by LPmin−WOmax, and APmax is calculated by LPmax−WOmax (step S 42 ). An AR value is then calculated by APmin/APmax (step S 43 ). The CPU  65  displays the AR value calculated in step S 43  on a display (not shown). Furthermore, whether or not the AR value is more than a specified value may be displayed as the result of step S 4 .  
         [0063]    [0063]FIG. 13A shows an AR waveform displayed on the display panel  97  of the oscilloscope  61 . In FIG. 13A, a line indicated by a reference code A on the display panel  97  is the display center line. The frequency of each data positioned on the display the center line A becomes as shown in FIG. 13B when displayed as a histogram. In the histogram display, a reference code WG corresponds to the wobbling group component, and a reference code LP corresponds to the LPP component. Other frequencies can be an orphan accidentally caused by the wobbling group part or LPP, or can be noise. The parts displayed as histograms are stored as X[D]=1 in the internal memory  66 , as shown in FIG. 13C, regardless of the height of the frequency. Other parts are stored as 0 as their initial value is.  
         [0064]    As the AR waveform measurement is repeated N times, the case where N=3 gives the result signals X[0] to X[M] by the first measurement, the result signals X[0] to X[M] by the second measurement, and the result signals X[0] to X[M] by the third measurement, as shown in FIG. 14A. The average value of these results is obtained in step S 23  and the average value is rounded to the nearest whole number in step S 24 . Consequently, the results which are common among the first to third measurements remain as data signals Y[0] to Y[M], as shown in FIG. 14B. The rounding operation eliminates accidental orphans such as noise. The case of FIG. 14B contains an orphan having a value isolated from the range of fluctuation of the wobbling group component of the push-pull signal PP and the range of fluctuation of the LPP component of the push-pull signal. As the length of an area where 0 continues becomes maximum at the area formed between the maximum value of the wobbling group component WOmax, and the orphan value, the orphan data value becomes the minimum value of the range of fluctuation of the LPP component LPmin. Therefore, APmin is calculated by LPmin−WOmax, and APmax is calculated by LPmax−WOmax. The AR value is calculated by APmin/APmax.  
         [0065]    The above-mentioned embodiment employs an oscilloscope. However, the present invention is not limited to the use of an oscilloscope. Any display unit of a configuration that is equipped with a memory that can store sampled data obtained by sampling a push-pull signal PP with high frequency can be used. It is not necessary for the display unit to indicate an AR waveform.  
         [0066]    In the case of the above-mentioned embodiment, a rounding operation is performed in step S 24 . However, any method which can detect the fact that each data value at the point of time of the pre-pit position of the sample data by plural sampling occurred more than a predetermined frequency (preferably, a frequency more than half the above-mentioned plural times) can be used.  
         [0067]    As described above, according to the present invention, it is possible to calculate the AR value of a disc to be inspected accurately, eliminating the irregular influences of dirt or scratches on the disc as much as possible. Therefore, the accuracy of the disc inspection can be improved, the possibility of rejecting a defect-free disc as defective can be avoided.  
         [0068]    This application is based on Japanese Patent Application No. 2001-47778 which is hereby incorporated by reference.