Abstract:
A method and apparatus for measuring the crosstalk of an optical disc of an optical disc player adapted to reproduce signals recorded on the disc, in which there are concentrically or spirally formed continuous recorded signal strings or signal recording regions and in which there is provided a positioning aperture at the center of the disc substantially concentric with said recorded signal strings or signal recording regions. A light beam is wobbled at a predetermined amplitude in the direction along the radius of the optical disc. The wobbling period is set so as to be asynchronous with the period of the optical disc rotation. The ratio between the quantity of the traverse signal obtained by the optical pickup and the quantity of strays signals is measured as crosstalk.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method and apparatus for measuring the crosstalk of an optical disc to evaluate the quality of the disc. 
     2. Description of Related Art 
     Among the items for evaluating the quality of optical discs, such as compact discs (trade mark) or mini-discs (trade mark) is a crosstalk which is used to evaluate the degree of signal deterioration by signal leakage from a neighboring track during reproduction of a specified track. 
     The crosstalk is defined as B/A, where A is the signal quantity of a specified track and B is the signal leakage from the neighboring track. 
     Among the methods for measuring the crosstalk of an optical disc, there are hitherto known a reverse tracking method and an off-tracking method. 
     The reverse-tracking method performs routine signal reproduction in a signal area under measurement to find the signal quantity A, and then reproduces signals between two signal strings in the same area under measurement to measure the stray signal quantity B to find the crosstalk B/A. 
     With such reverse tracking method, the playback signals are fairly stable and give relatively high repetition accuracy. 
     However, if the reverse tracking method is to be used, there is required a structure for reproducing signals between two signal strings. This structure is not provided in the usual player. With such reverse tracking method, only the stray signal quantity from a single side of the signal string can be measured, with the result that the measured results differ from the behavior of a real player. 
     On the other hand, the off-track method utilizes the phenomenon that changes in signal quantities from the signal string generated on disengaging a servo circuit used for following a track (the space lying directly above the track) and from the space between the signal strings (between two tracks) appear alternately. Specifically, the off-track method finds the signal quantity A from the signal string lying directly above the signal string and the stray signal quantity B from the signal quantity from the space between signal strings (space between two neighboring tracks) to find the cross-talk B/A. 
     With the off-track method, the measurement state can be regenerated by the function proper to the usual player, whilst the signal leakage from both sides of the signal strings can be evaluated which is similar to that encountered during normal reproduction. 
     However, the off-track method has an inconvenience that, since the difference between the signal quantity from the signal string lying directly above a given track and the signal quantity from the space lying between two neighboring signal tracks, this signal difference signal being called an HF traverse signal, is attributable to the eccentricity occurring at the time of manufacture of the optical disc or the eccentricity when mounting the optical disc on the optical player, which is the eccentricity ascribable to chucking, crosstalk measurement tends to be unstable. 
     Another inconvenience of the off-track method is that, since the trajectory of the light beam of the optical pickup on servo disengagement is at a predetermined distance from the center of rotation, the crosstalk can be measured only at limited portions of the optical disc, and that, should the optical pickup be set on the turntable in a different manner, the path followed by the light beam of the optical pickup is varied to change the measured values, rendering the measured values unstable. 
     Moreover, the off-track method has an inconvenience that, should the amount of eccentricity on mounting the optical disc on the player be small, the chance of the light beam of the optical pickup traversing the signal string (track) of the optical disc decreases, meaning that measurement only at limited portions of the optical disc lying on the light beam trajectory is used to represent the measurement for the entire area of the optical disc. In addition, should there be defects of the optical disc on the light beam trajectory, variations in crosstalk measurement undesirably tend to increase. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method and apparatus for measuring the crosstalk of an optical pickup in which the crosstalk of the optical disc can be measured with stability. 
     In one aspect, the present invention provides a method for measuring the crosstalk of an optical disc in which there are concentrically or spirally formed continuous recorded signal strings or signal recording regions and in which there is provided a positioning aperture at the center of the disc substantially concentrically with the recorded signal strings or signal recording regions. A light beam of an optical pickup of an optical disc player is wobbled at a pre-set amplitude along the radius of the optical disc, with the wobbling period asynchronous with respect to the rotational period of the optical disc. The ratio of the signal quantity of traverse signals obtained by the optical pickup and the signal quantity of the stray signals is measured as a crosstalk. 
     With the measurement method according to the present invention, in which, for measuring the crosstalk of the optical disc, the light beam of the optical pickup is wobbled at a predetermined amplitude in a radial direction of the optical disc, it is possible for the light beam to traverse e.g., 32 to 80 signal strings (tracks) per rotation of the optical disc to provide stable HF traverse signals. Moreover, since the wobbling period is adapted to be asynchronous with the period of rotation of the optical disc, the light beam has its trajectory changed from time to time without following the same trajectory to enable optimum crosstalk measurement over the entire width of the pre-set amplitude along the radius of the optical disc. 
     In another aspect, the present invention provides an apparatus for measuring the crosstalk of an optical disc in which there are concentrically or spirally formed continuous recorded signal strings or signal recording regions and in which there is provided a positioning aperture at the center of the disc substantially concentrically with the recorded signal strings or signal recording regions. The apparatus includes wobbling means for wobbling a light beam of an optical pickup of an optical disc player at a predetermined amplitude and at a period asynchronous with the period of rotation of the optical disc, in a radial direction of the optical disc, and calculating means for calculating the ratio of the signal quantity of traverse signals obtained by the optical pickup and the signal quantity of the stray signals as a crosstalk. 
     With the apparatus for measuring the crosstalk of the optical disc according to the present invention, in which the light beam of the optical pickup is wobbled a predetermined amplitude along the radius of the optical disc in measuring the crosstalk of the optical disc, 32 to 80 signal strings (tracks), for example, can be traversed by the light beam for each revolution of the optical disc, thus producing stable HF traverse signals. In addition, since the wobbling period is asynchronous with the period of disc rotation, the light beam of the optical pickup has its trajectory changed as time elapses, when the optical disc has made 10 complete revolutions, as an example, thus enabling optimum crosstalk measurement over the entire range of the predetermined amplitude along the radius of the optical disc. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view showing an embodiment of a device for measuring the crosstalk of an optical disc embodying the present invention. 
     FIG. 2 is a perspective view showing an illustrative optical disc the crosstalk of which is measured by the measurement device shown in FIG.  1 . 
     FIG. 3 is a schematic view showing an illustrative optical pickup in the measurement device of FIG.  1 . 
     FIG. 4 is a flowchart showing the procedure of the measurement device. 
     FIG. 5 is a schematic view showing the state in which a light beam of the optical pickup traverses a track of an optical disc. 
     FIG. 6 shows HF traverse signals obtained by the optical pickup. 
     FIG. 7 shows exemplary measurement areas of an optical disc the cross-talk of which is measured by the measurement device. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, preferred embodiments of the present invention will be explained in detail. 
     The present invention is applied to, for example, a measurement device  100  configured as shown in FIG.  1 . This measurement device  100  measures the cross-talk of an optical disc  1 , such as a compact disc (CD). 
     The CD  1  is adapted for reproducing the disc at a constant linear velocity. The CD  1  includes concentric tracks, as signal strings or signal recording areas, at a pitch of 1.6 μm, and a positioning aperture  1   a  substantially concentric with respect to the tracks. 
     Referring to FIG. 1, a spindle motor  2  is adapted for rotating the optical disc  1  at a constant linear velocity, along with a turntable  2   a  provided fixedly on its rotary shaft. The spindle motor  2  is adapted for producing a sole PG pulse per each revolution of the optical disc  1  to send the PG pulse to a digital signal processor (DSP)  3  as later explained. 
     As an optical pickup  4  for reproducing signals recorded on the optical disc  1 , such as is shown for example in FIG. 3 is used. The optical disc  4 , shown in FIG. 3, is configured for routing a light beam  4   b  from a laser diode  4   a  through a grating  4   c  to a beam splitter  4   d  and for illuminating the reflected light from the beam splitter  4   d  through an objective lens  4   e  to the optical disc  1 . The optical pickup  4  is also configured for routing the reflected light (detection light) from the optical disc  1  through the objective lens  4   e  and the beam splitter  4   d  to a photodiode  4   f  constituting a detector. 
     The playback signals are sent by the optical pickup  4  to a calculating circuit  5  comprising a CPU. On the other hand, tracking error signals produced by the optical pickup  4  are routed to a tracking servo circuit  6 . 
     When the tracking servo circuit  6  is disengaged, traverse signals traversing a track are obtained as tracking error signals. Output signals of a binary coding circuit  7 , fed with traverse signals, are counted by a counter  8  to permit the amount of eccentricity of the optical disc  1  to be known based on the number of counts per revolution of the optical disc  1 . 
     Specifically, the amount of offset can be obtained by multiplying the track pitch, such as 1.6 μm, with the number of counts per revolution of the optical disc  1  and by halving the resulting product. 
     Taking into account the PG pulse from the spindle motor  2 , a digital signal processor  3  forms wobbling data of a sine wave of a frequency of the order of one-half to one-fourth the number of revolutions of the optical disc, asynchronous with the PG pulses, so that the total amplitude of the wobbling data will be 80 μm, as an example, in consideration of the amount of eccentricity of the optical disc  1 . 
     In this case, since an amount of eccentricity of 70 μm is allowed for an optical disc player in consideration of the amount of eccentricity at the time of manufacture of the optical disc  1  and of the amount of eccentricity of the optical disc  1  occurring at the time of mounting thereof on an optical disc player (the amount of eccentricity produced due to chucking), the wobbling amplitude is set so as to be larger than this allowed amount of eccentricity. 
     In the present embodiment, the number of tracks which the light beam  4   b  of the optical pickup  4  traverses per revolution of the optical disc  4  is selected to be 32 to 80 in order to take the wobbling frequency and amplitude into account. Should the number of tracks traversed per revolution be less than 32 or larger than 80, there is a risk that optimum crosstalk measurement becomes infeasible. 
     Should the number of tracks traversed by a light beam of the optical pickup  4  per revolution of the optical disc  1  be a multiple of “8”, such as, in particular, 64, it is possible to measure the crosstalk of each of eight of sectors obtained on equiangular division of the optical disc  1 . 
     The wobbling data, produced by the digital signal processor (DSP)  3 , is routed via a D/A converter circuit  9 , converting digital signals into analog signals, to a fixed contact  10   c  on the disengaging side of a servo engaging/disengaging switch  10  adapted for switching between engagement and disengagement of the tracking servo. On the other hand, the output signal of a tracking servo circuit  6  is routed to a fixed contact  10   b  on the engaging side of the servo engaging/disengaging switch  10 . A signal obtained at a movable contact  10   a  of the servo engaging/disengaging switch  10  is sent via a tracking drive circuit  11  to the optical pickup  4  to control the objective lens  4   e  and hence the trajectory of the light beam  4   b . A display device  12  demonstrates the calculated results. Otherwise the configuration shown in FIG. 1 is formed similarly to that of a conventional optical disc player. 
     The operation of crosstalk measurement is explained by referring to FIGS. 4 to  7 . In the present embodiment, in measuring the crosstalk of the optical disc  1 , the crosstalks of an inner rim area  1   b , a mid area  1   c  and an outer rim area  1   d  are measured as shown in FIG.  7 . The crosstalks at the inner rim area  1   b , mid area  1   c  and at the outer rim area  1   d  are measured in similar manner. Alternatively, the crosstalk may also be measured over the entire areas of the optical disc  1 . 
     In measuring the crosstalk of the optical disc  1 , the movable contact  10   a  of the servo engaging/disengaging switch  10  is connected to the fixed contact  10   c  on the disengaging side to disengage the tracking servo. The optical disc  1  to be measured then is attached to a turntable  2   a  (step S 1 ). 
     The spindle motor  2  then is actuated to run the optical disc  1  along with the turntable  2   a  in rotation (step S 2 ). A PG pulse per rotation of the optical disc  1  is sent by the spindle motor  2  to the digital signal processor (DSP)  3  (step S 3 ), which then determines the wobbling frequency of the sine wave asynchronous with respect to the PG pulse (step S 4 ). The wobbling frequency is set to one-half to one-fourth of the rotational frequency of the optical disc  1 . 
     The optical pickup  4  then is actuated (step S 5 ), at the same time as the light beam starts to be wobbled with the above-mentioned wobbling frequency (step S 6 ). This takes into account the amount of eccentricity at the time of manufacture of the optical disc  1  and the amount of eccentricity at the time of mounting the optical disc  1  on the turntable  2   a.    
     The amplitude of the wobbling signals is increased (step S 7 ), whilst it is verified by the CPU  5  at step S 8  whether or not the number of times of crossing of the track by the optical disc  1  per revolution of the optical disc  1  is in the range of 32 to 80, for example, 64. The amplitude of the wobbling signals is increased until the number is equal to e.g., 64. 
     In this case, such an HF traverse signal, in which the signal quantity is equal to A each time a track (signal string or signal recording region)  1   t  of the optical disc  1  is traversed by the light beam  4   b  of the optical pickup  4 , with the signal quantity between the tracks  1   t  and  1   t  being a stray signal quantity B, is obtained at the CPU  5 . 
     When the number of times the light beam  4   b  crosses the track  1   t  during each revolution of the optical disc  1  is equal to, for example, 64, the amplitude of the wobbling signal is maintained (step Swollen-out portion  9 ) to measure the crosstalk (step S 10 ). 
     In measuring the crosstalk (step S 10 ), the value B/A is measured for each signal quantity A and each stray signal quantity B of the HF traverse signal shown in FIG.  6  and the values B/A for e.g. ten revolutions of the optical disc  1  are averaged and demonstrated as the crosstalk on the display device  12 . 
     In the present embodiment, in which the light beam  4   b  of the optical pickup  4  is wobbled a pre-set amplitude along the radius of the optical disc  1  for measuring the cross-talk of the optical disc  1 , it is possible for the light beam  4   b  to traverse e.g., 32 to 80, in particular 64, tracks  1   t , during each complete revolution of the optical disc  1 , thus achieving stable HF traverse signals and hence stable cross-talk measurement. 
     Also, in the present embodiment in which the wobbling period of the light beam  4   b  is asynchronous with the period of rotation of the optical disc  1 , the light beam  4   b  of the optical pickup  4  has its trajectory changed from time to time, without passing through the same trajectory, when the optical disc is rotated a plurality of times, such as ten times, so that crosstalk can be measured optimally over a width of a pre-set amplitude along the radius of the optical disc  1 , such as 80 μm. 
     According to the present invention, as described above, the light beam of the optical disc is wobbled a pre-set amplitude in the radial direction of the optical disc for measuring the cross-talk of the optical disc, so that e.g., 32 to 80 tracks, in particular 64 tracks (signals strings or signal recording regions) can be traversed during each complete revolution of the optical disc, with the result that stable HF traverse signals can be obtained to enable stable crosstalk measurement. 
     Also, in the present embodiment in which the wobbling period of the light beam is asynchronous with the period of rotation of the optical disc  1 , the light beam of the optical pickup has its trajectory changed from time to time, without passing through the same trajectory, when the optical disc is rotated a plurality of times, such as ten times, so that crosstalk can be measured optimally over a width of a pre-set amplitude along the radius of the optical disc  1 , such as 80 μm. 
     The present invention is not limited to the above-described embodiments such that a wide variety of different modification can be used without departing from the scope of the invention.