reference will now be made in detail to the present preferred embodiments of the present invention , examples of which are illustrated in the accompanying drawings , wherein like reference numerals refer to like elements throughout . fig2 a shows the structure of an octal photodetector 202 . fig2 b shows the structure of an apparatus for adding light receiving signals generated by the octal photodetector to each other and subtracting some light receiving signals from the other light receiving signals fig2 b shows an adder 204 for adding light receiving signals generated by right outside light receiving devices a 1 and d 1 of the octal photodetector 202 to light receiving signals generated by left outside light receiving devices b 1 and c 1 of the octal photodetector 202 and a subtracter 206 for subtracting the light receiving signals generated by the left outside light receiving devices b 1 and c 1 of the octal photodetector 202 from the light receiving signals generated by the right outside light receiving devices a 1 and d 1 of the octal photodetector 202 . the octal photodetector 202 consists of eight light receiving devices a 1 , a 2 , b 1 , b 2 , c 1 , c 2 , d 1 , and d 2 , which are divided along radial and tangential directions of a disc . the inside light receiving devices a 2 , b 2 , c 2 , and d 2 are divided to be smaller than the outside light receiving devices a 1 , b 1 , c 1 , and d 1 . each respective light receiving device a 1 , a 2 , b 1 , b 2 , c 1 , c 2 , d 1 , and d 2 , generates a light receiving signal corresponding to the intensity of an optical spot 210 overlapping the light receiving surface . the adder 204 sums the light receiving signals generated by the four outside light receiving devices a 1 , b 1 , c 1 , and d 1 of octal photodetector 202 and provides the result as a sum signal sum . the subtracter 206 subtracts the sum of the light receiving signals generated by the left outside two light receiving devices b 1 and c 1 of the octal photodetector 202 from the sum of the light receiving signals generated by the right outside two light receiving devices a 1 and d 1 of the octal photodetector 202 and provides the subtraction result as a difference signal diff . fig3 a through 3c show the waveforms of a track error signal ( fig3 a ), a signal ( fig3 b ) generated by the subtracter 206 shown in fig2 b , and a signal ( fig3 c ) generated by the adder 204 shown in fig2 b , when an optical wavelength is 400 nm , a numerical aperture ( na ) of an object lens is 0 . 6 , and a track pitch is 0 . 37 μm . as shown in fig3 b , while the envelope of the signal generated by the subtracter 206 is clear , the envelope of the signal ( fig3 c ) generated by the adder 204 is unclear . therefore , in the method of detecting the track cross signal according to the present invention , the track cross signal is detected from the envelope of the rf signal generated by subtracting the signal generated from the right light receiving devices of the photodetector from the signal generated by the left light receiving device of the photodetector . fig4 a through 4f show waveforms for schematically illustrating the method of detecting the track cross signal according to the present invention . the waveforms shown in fig4 a through 4f are generated by a disc , in which a track is formed so that a pit and a mirror alternate with each other in the radial direction of the disc . another signal is generated by a disc , in which a track is formed so that a land and a groove alternate with each other . the method of generating the track cross signal according to the present invention will now be described in detail with reference to fig4 a through 4f . 1 ) the rf signal rf 0 shown in fig4 a is obtained by subtracting the signal generated by the right light receiving devices of the photodetector from the signal generated by the left light receiving devices of the photodetector . in the signal shown in fig4 a , the upper envelope is the level of a mirror signal and the lower envelope is the levels of the pit and the mirror . change in the level of a mirror signal is caused by change in the reflection factor of a disc . the reflection factor of a disc partially changes according to positions on the disc . the level of the mirror signal changes , as shown in fig4 a , due to the change in reflection factor . in the lower envelope , a trough corresponds to a track center , that is , the pit and a crest corresponds to a mirror . the reason why the crest does not coincide with the mirror level is that a signal level deteriorates due to crosstalk between adjacent tracks . in the case where a distance between the crests is short , the pit is positioned in an adjacent track on a locus along which the optical spot moves . in the case where the distance between the crests is long , the pit is not positioned on the adjacent track . the reason why the upper envelope overlaps the lower envelope in the rf signal is that the real locus of the optical spot does not transect the track at an angle of 90 degrees but transects the track at a very small angle . although an optical pick - up moves in the radial direction of a disc , since the disc rotates during a search operation , a high frequency component determined by the pit formed in the track is generated . the high frequency component is the rf signal . as shown in fig4 a , parts , from which the rf signal is omitted due to dust and scratch , exist . 2 ) the signal rf 1 shown in fig4 b is obtained by removing a direct current ( dc ) component by performing ac coupling using a capacitor in order to facilitate detection of envelopes . 3 ) the peak signal and the bottom signal shown in fig4 c and 4d are obtained by performing peak hold and bottom hold on the rf 1 signal . 4 ) the difference signal rf 2 shown in fig4 e is obtained by subtracting the bottom signal from the peak signal . 5 ) the track cross signal shown in fig4 f is obtained by binarizing the difference signal rf 2 by a predetermined threshold value th . here , the threshold value th is determined by the average of the peak hold signal shown in fig4 c and the bottom hold signal shown in fig4 d . the threshold value th is described to be uniform in fig4 e . however , the threshold value th changes due to changes in the level of the peak hold signal and the level of the bottom hold signal . when the track cross signal shown in fig4 f is compared with the track cross signal shown in fig4 a , it is noted that the track cross signal shown in fig4 f is a pulse signal , which is at a low level in the trough of the lower envelope shown in fig4 a and is at a high level in the crest of the lower envelope shown in fig4 a . therefore , it is possible to know the number of moved tracks by counting the number of pulses of the track cross signal shown in fig4 f . since the signal rf 0 is obtained by subtracting the light receiving signal generated by the right light receiving devices in the radial direction of the photodetector from the light receiving signal generated by the left light receiving devices in the radial direction of the photodetector in the present invention , it is possible to obtain the track cross signal with a quarter photodetector divided in the radial and tangential directions of the disc , a quarter photodetector divided in the radial direction of the disc , or an octal photodetector divided in the radial and tangential directions of the disc , as described with reference to the apparatus for detecting the track cross signal according to the present invention shown in fig5 through 9 . fig5 is a block diagram showing the structure of the apparatus for detecting the track cross signal according to the present invention . the apparatus shown in fig5 includes a photodetector 502 , a current / voltage converter 504 , a radial subtracter 506 , and a track cross signal generator 520 . the track cross signal generator 520 includes a capacitor 508 , a peak hold circuit 510 , a bottom hold circuit 512 , a subtracter 514 , an average value circuit 516 , and a waveform shaping circuit 518 . the capacitor 508 performs ac coupling on the signal rf 0 generated by the radial subtracter 506 and generates the signal rf 1 shown in fig4 b . the peak hold circuit 510 and the bottom hold circuit 512 hold the peak value and the bottom value , respectively , of the signal rf 1 generated by the capacitor 508 , and generate the peak hold signal and the bottom hold signal shown in fig4 c and 4d . the subtracter 514 subtracts the bottom hold signal generated by the bottom hold circuit 512 from the peak hold signal generated by the peak hold circuit 510 and obtains the difference signal rf 2 shown in fig4 e . the average value circuit 516 generates the threshold value th for binarization by the average value of the peak hold signal generated by the peak hold circuit 510 and the bottom hold signal generated by the bottom hold circuit 512 . the waveform shaping circuit 518 binarizes the difference signal rf 2 generated by the subtracter 514 by the threshold value th generated by the average value circuit 516 and obtains the track cross signal shown in fig4 f . since the signal rf 0 is obtained by subtracting the light receiving signal generated by the right light receiving device in the radial direction of the photodetector from the light receiving signal generated by the left light receiving device in the radial direction of the photodetector , in the apparatus for detecting the track cross signal according to the present invention , the signal rf 0 can be obtained by the quarter photodetector divided in the radial and tangential directions of the disc , the quarter photodetector divided in the radial direction of the disc , or the octal photodetector divided in the radial and tangential directions of the disc . the structure of a radial adder changes according to the kind of photodetector . various photodetectors and corresponding adders are described with reference to fig6 a through 9c . fig6 a and 6b show the structure of an apparatus for obtaining the signal rf 0 using a quarter photodetector . fig6 a shows a quarter photodetector 602 . fig6 b shows the structure of a radial subtracter 608 for obtaining the signal rf 0 using the light receiving signals generated by the quarter photodetector 602 . the radial subtracter 608 shown in fig6 b subtracts the sum of the light receiving signals generated by the light receiving devices b 1 and c 1 , located on the left in the radial direction of the quarter photodetector 602 , from the sum of the light receiving signals generated by the light receiving devices at and d 1 , located on the right in the radial direction of the quarter photodetector 602 . reference numeral 610 denotes an optical spot . fig7 a and 7b show the structure of an apparatus for obtaining the signal rf 0 by another type of quarter photodetector . fig7 a shows a photodetector 702 . fig7 b shows the structure of a radial subtracter 708 for obtaining the signal rf 0 using the light receiving signals generated by the photodetector 702 . the quarter photodetector 702 shown in fig7 a is divided into four sections in the radial direction so that inside light receiving devices are narrower than outside light receiving devices . it is possible to disperse the influence of crosstalk with the quarter light photodetector 702 since the inside light receiving devices can intensively detect the main lobe of the optical spot reflected from the disc and the outside light receiving devices can intensively detect the side lobe of the optical spot . the radial subtracter 708 shown in fig7 b subtracts the sum of the light receiving signals generated by the light receiving devices b 1 and b 2 , located on the left in the radial direction of the quarter photodetector 602 , from the sum of the light receiving signals generated by the right light receiving devices a 1 and a 2 , located on the right in the radial direction of the quarter photodetector 602 . in taking the sum of the light receiving signals generated by the right light receiving devices a 1 and a 2 or the sum of the light receiving signals generated by the left light receiving devices b 1 and b 2 after multiplying the light receiving signals generated by the inside light receiving devices a 2 and b 2 by predetermined coefficients with amplifiers 704 and 706 , the multiplication results are added together by adders 703 and 705 . here , the coefficient k of the light receiving signals generated by the inside light receiving devices is equal to 1 , greater than 1 or smaller than 1 . as an alternate to setting a coefficient of 1 , amplifiers 704 and 706 may be removed and the light receiving signals generated by the light receiving devices a 2 and b 2 may be connected directly to the adders 703 and 705 respectively . when the coefficient k is greater than 1 , the light receiving signals generated by the inside light receiving devices a 2 and b 2 of the quarter photodetector 702 contribute more to the addition result . when the coefficient k is much smaller than 1 , the light receiving signals generated by the outside light receiving devices a 1 and b 1 of the quarter photodetector 702 contribute more to the addition result . fig8 a and 8b show another structure of the apparatus for obtaining the signal rf 0 by the quarter photodetector shown in fig7 a . fig8 a shows a quarter photodetector 802 . fig8 b shows the structure of a radial subtracter 808 for obtaining the signal rf 0 using the light receiving signals generated by a quarter photodetector 802 . the quarter photodetector 802 shown in fig8 a is the same as the quarter photodetector 702 shown in fig7 a . the radial subtracter 808 shown in fig8 b subtracts the light receiving signal generated by the inside light receiving device b 2 of the quarter photodetector 802 , from the light receiving signal generated by the inside light receiving device a 2 of the quarter photodetector 802 . the radial subtracter 808 shown in fig8 b generates the signal rf 0 , which is less affected by crosstalk , since the light receiving devices inside the quarter photodetector 802 strongly detect the main lobe of the optical spot reflected from the disc . fig9 a and 9b show the structure of the apparatus for obtaining the signal rf 0 by an octal photodetector . fig9 a shows an octal photodetector 902 . fig9 b shows the structure of a radial subtracter 908 for obtaining the signal rf 0 using the light receiving signals generated by the octal photodetector 902 . the octal photodetector 902 shown in fig9 a divides the photodetector into eight sections in the radial and tangential directions so that the inside light receiving devices , a 2 , b 2 , c 2 , d 2 , are narrower than the outside light receiving devices , a 1 , b 1 , c 1 , d 1 . it is possible to reduce the influence of crosstalk with the octal light receiving device 902 since the inside light receiving devices can strongly detect the main lobe of the optical spot reflected from the disc and the outside light receiving devices can strongly detect the side lobe of the optical spot reflected from the disc . the radial subtracter 908 shown in fig9 b subtracts the sum of the light receiving signals generated by the light receiving devices b 1 and c 1 , located on the left and the outside in the radial direction of the octal photodetector 902 , from the sum of the light receiving signals generated by the light receiving devices a 1 and d 1 located on the right and the outside in the radial direction of the octal photodetector 902 . an alternate embodiment of the apparatus shown in fig9 b is shown in fig9 c . in the embodiment shown in fig9 c , each left inside light receiving signal , b 2 , c 2 , is multiplied by a coefficient k at amplifiers 904 b and 904 c , respectively , and the multiplication results are added to left outside light receiving signals b 1 and c 1 by adder 903 b . similarly , each right inside light receiving signal , a 2 , d 2 , is multiplied by a coefficient k at amplifiers 904 a and 904 b , respectively , and the multiplication results are added to right outside light receiving signals a 1 and d 1 by adder 903 a . left added signal , ( b 1 + c 1 + kc 2 + kb 2 ) is subtracted from right added signal ( a 1 + d 1 + ka 2 + kd 2 ) at amplifier 908 to obtain the signal rf 0 . when the coefficient k is much larger than 1 , the light receiving signals generated by the inside light receiving devices a 2 , b 2 , c 2 and d 2 of the octal photodetector 902 contribute more to the addition result . when the coefficient is much smaller than 1 , the light receiving signals generated by the outside light receiving devices a 1 , b 1 , c 1 and d 1 contribute more to the addition result . fig1 is a block diagram showing the structure of a search apparatus , to which the track cross signal detecting apparatus according to the present invention is applied . the apparatus shown in fig1 includes an optical pick - up 1002 , a current / voltage converter 1004 , a radial subtracter 1006 , a track cross signal generator 1008 , a servo error detector 1010 , a servo controller 1012 , and an optical pick - up driver 1014 . here , the track cross signal generator 1008 corresponds to the track cross signal generator 520 shown in fig5 the optical pickup 1002 includes one of the photodetectors shown in fig6 a , 7 a , and 9 a and the radial subtracter 1006 corresponds to an appropriate one of the radial subtracters shown in fig6 b , 7 b , 8 b , 9 b and 9 c as identified above . the servo controller 1012 transfers the optical pick - up 1002 to a target track using the track cross signal generated by the track cross signal generator 520 and the . servo error signal generated by the servo error detector 1010 , in performing a track search operation . as mentioned above , in the track cross signal detecting method according to the present invention , the track cross signal is generated using an rf signal obtained by subtracting some light receiving signals generated by some light receiving devices divided into sections in the radial direction from other light receiving signals generated by other light receiving devices divided into sections in the radial direction , in the photodetector . when the track pitch is smaller than the size of the optical spot , it is possible to generate the track cross signal which is less affected by crosstalk caused by an adjacent track . although a few preferred embodiments of the present invention have been shown and described , it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention , the scope of which is defined in the claims and their equivalents .