Abstract:
An optical recording/reproducing apparatus for determining a direction in which an optical pick-up moves, and detecting a track cross signal. The track cross signal is useful as a basis of determining tracking pull-in after searching a track. In the present track cross signal detecting method, the track cross signal is obtained by binarizing the envelope of an RF signal RF 0  obtained by subtracting some light receiving signals generated by some light receiving devices in the radial direction from other light receiving signals generated by other light receiving devices in the radial direction. The light receiving signals originate from a photodetector, divided into two or more sections in the radial direction. The track cross signal detecting method is especially useful for generating the track cross signal when the track pitch is smaller than the size of an optical spot, the signal being less affected by crosstalk caused by an adjacent track.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Application No. 99-39331, filed Sep. 14, 1999, in the Korean Patent Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical recording/reproducing apparatus, and more particularly, to a method of determining a direction in which an optical pick-up moves, and detecting a track cross signal used as a basis of determining tracking pull-in after searching a track. 
     2. Description of the Related Art 
     Track searching of an optical recording/reproducing apparatus means searching for a target track by moving an optical pick-up in a radial direction of a disc. In track searching, the number of moved tracks must be counted in order to determine whether the optical pick-up arrived at the target track. In order to count the number of moved tracks, a track cross signal is necessary. The track cross signal is a pulse generated when the optical pick-up transects a track. That is, it is possible to know the number of moved tracks by counting the pulses generated by the track cross signal. Also, it is necessary to compensate for the number of tracks moved by the eccentricity of a disc. That is, the number of tracks must be increased in a direction, where the influence of the eccentricity increases, but reduced in a direction, where the influence of the eccentricity is reduced. It is determined whether to increase or to reduce the number of tracks using the fact that the phase of a track error signal is inverse to the phase of a track cross signal. Also, the track cross signal is necessary to determine the point of time of the tracking pull-in, after the optical pick-up reaches the target track. 
     Since the track cross signal is necessary to determine the amount of movement of a track, compensation for the influence of the eccentricity, and the point of time of the tracking pull-in after searching the track, it is important to obtain a correct track cross signal. 
     In a conventional method, the track cross signal is detected by the envelope of a sum signal generated by a quarter photodetector. However, in a high density optical disc such as an HD-DVD, the width of a track with respect to the size of an optical spot is much smaller than in a conventional CD/DVD. Accordingly, crosstalk caused by an adjacent track is mixed with an RF signal. Therefore, it is not easy to detect the envelope of the sum signal. 
     FIGS. 1A through 1C show changes in a track error signal and an RF signal according to a track width. FIGS. 1A,  1 B, and  1 C show the track error signals and RF signals when the optical wavelength is 400 nm, the numerical aperture (NA) of an object lens is 0.6, and track pitches are 0.74 μm (in the case of a DVD), 0.46 μm, and 0.37 μm, respectively. 
     As shown in FIGS. 1A through 1C, it is difficult to detect the envelope of the RF signal as the track pitch becomes narrower with respect to a uniform optical spot. This is because crosstalk caused by an adjacent track increases due to a narrow track pitch. 
     Therefore, it is difficult to detect the track cross signal as the track pitch becomes narrower with respect to the optical spot. This means that it is not easy to detect a track in a high density optical disc. 
     SUMMARY OF THE INVENTION 
     To solve the above problems, it is an object of the present invention to provide an improved method of detecting a track cross signal for a disc having high density narrow tracks. 
     It is another object of the present invention to provide an apparatus for detecting a track cross signal, which is suitable for the above method. 
     Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention. 
     Accordingly, to achieve the first object, there is provided a method of detecting a track cross signal, in which a track cross signal is obtained by binarizing the envelope of an RF signal RF 0  obtained by subtracting some light receiving signals generated by some light receiving devices in the radial direction from other light receiving signals generated by other light receiving devices in the radial direction, in a photodetector divided into two sections in the radial direction. 
     Here, a track cross signal detection signal can be obtained by a quarter photodetector divided in the radial and tangential direction of a disc, a quarter photodetector divided in the radial direction of a disc, or an octal photodetector divided in radial and tangential directions of a disc. 
     To achieve the second object, there is provided an apparatus for detecting a track cross signal, comprising a radial subtracter for obtaining an RF signal RF 0  by subtracting some light receiving signals generated by some light receiving devices in the radial direction from other light receiving signals generated by other light receiving devices in the radial direction, in a photodetector divided into two in the radial direction and a track cross signal generator for obtaining a track cross signal by binarizing the envelope of the RF signal RF 0  with the radial subtracter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings in which: 
     FIGS. 1A through 1C show changes in a track error signal and an RF signal according to a track width according to a conventional method; 
     FIGS. 2A and 2B show the structure of an octal photodetector and the structure of an apparatus for adding light receiving signals generated by the octal photodetector to each other or subtracting some light receiving signals from the other light receiving signals, respectively; 
     FIGS. 3A,  3 B and  3 C show the waveforms of a track error signal, a signal generated by the subtracter shown in FIG. 2, and a signal generated by the adder shown in FIG. 2, respectively; 
     FIGS. 4A through 4F show waveforms for schematically illustrating a method of detecting a track cross signal according to the present invention; 
     FIG. 5 is a block diagram showing the structure of an apparatus for detecting a track cross signal according to the present invention; 
     FIGS. 6A and 6B show the structure of an apparatus for obtaining an RF 0  signal by a quarter photodetector according to the present invention; 
     FIGS. 7A and 7B show the structure of an apparatus for obtaining the RF 0  signal by another quarter photodetector according to the present invention; 
     FIGS. 8A and 8B show another structure of the apparatus for obtaining the RF 0  signal by the quarter photodetector shown in FIG. 7A according to the present invention; 
     FIGS. 9A and 9B show the structure of an apparatus for obtaining the RF 0  signal by an octal photodetector according to the present invention; 
     FIG. 9C shows an alternate construction of a subtractor for obtaining the RF 0  signal using the octal photodetector shown in FIG. 9A; and 
     FIG. 10 is a block diagram showing the structure of a searching apparatus, to which the apparatus for detecting the track cross signal according to the present invention is applied. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. 
     FIG. 2A shows the structure of an octal photodetector  202 . FIG. 2B 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 FIG. 2B 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. 
     FIGS. 3A through 3C show the waveforms of a track error signal (FIG.  3 A), a signal (FIG. 3B) generated by the subtracter  206  shown in FIG. 2B, and a signal (FIG. 3C) generated by the adder  204  shown in FIG. 2B, 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 FIG. 3B, while the envelope of the signal generated by the subtracter  206  is clear, the envelope of the signal (FIG. 3C) 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. 
     FIGS. 4A through 4F show waveforms for schematically illustrating the method of detecting the track cross signal according to the present invention. The waveforms shown in FIGS. 4A 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 FIGS. 4A through 4F. 
     1) The RF signal RF 0  shown in FIG. 4A 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 FIG. 4A, 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 FIG. 4A, 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 FIG. 4A, parts, from which the RF signal is omitted due to dust and scratch, exist. 
     2) The signal RF 1  shown in FIG. 4B 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 FIGS. 4C and 4D are obtained by performing peak hold and bottom hold on the RF 1  signal. 
     4) The difference signal RF 2  shown in FIG. 4E is obtained by subtracting the bottom signal from the peak signal. 
     5) The track cross signal shown in FIG. 4F 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 FIG.  4 C and the bottom hold signal shown in FIG.  4 D. The threshold value TH is described to be uniform in FIG.  4 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 FIG. 4F is compared with the track cross signal shown in FIG. 4A, it is noted that the track cross signal shown in FIG. 4F is a pulse signal, which is at a low level in the trough of the lower envelope shown in FIG.  4 A and is at a high level in the crest of the lower envelope shown in FIG.  4 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 FIG.  4 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 FIGS. 5 through 9. 
     FIG. 5 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 FIG. 5 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 FIG.  4 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 FIGS. 4C 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 FIG.  4 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 FIG.  4 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 FIGS. 6A through 9C. 
     FIGS. 6A and 6B show the structure of an apparatus for obtaining the signal RF 0  using a quarter photodetector. FIG. 6A shows a quarter photodetector  602 . FIG. 6B 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 FIG. 6B 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. 
     FIGS. 7A and 7B show the structure of an apparatus for obtaining the signal RF 0  by another type of quarter photodetector. FIG. 7A shows a photodetector  702 . FIG. 7B 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 FIG. 7A 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 FIG. 7B 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. 
     FIGS. 8A and 8B show another structure of the apparatus for obtaining the signal RF 0  by the quarter photodetector shown in FIG.  7 A. FIG. 8A shows a quarter photodetector  802 . FIG. 8B 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 FIG. 8A is the same as the quarter photodetector  702  shown in FIG.  7 A. 
     The radial subtracter  808  shown in FIG. 8B 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 FIG. 8B 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. 
     FIGS. 9A and 9B show the structure of the apparatus for obtaining the signal RF 0  by an octal photodetector. FIG. 9A shows an octal photodetector  902 . FIG. 9B 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 FIG. 9A 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 FIG. 9B 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 FIG. 9B is shown in FIG.  9 C. In the embodiment shown in FIG. 9C, 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. 
     FIG. 10 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 FIG. 10 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 FIG. 5, the optical pickup  1002  includes one of the photodetectors shown in FIGS. 6A,  7 A, and  9 A and the radial subtracter  1006  corresponds to an appropriate one of the radial subtracters shown in FIGS. 6B,  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.