Abstract:
The present invention uses a stationary medium and a stationary optical pickup unit. The stationary optical pickup unit has a laser, which sends a beam of light through an objective lens, which focuses the beam on a point on the track of the stationary medium. The beam is then reflected toward a re-directing surface (e.g., prism) which diverts it to a photodiode array. Instead of the entire optical pickup moving to follow the track as in conventional systems, only the objective lens inside of the optical pickup moves. The position of the objective lens determines where on the information track the laser will reach. Depending upon where the laser beam reaches, the reflected light beam received by the photodetector array changes. This, in turn, affects the amount of light sensed by each photodetector in the array, thereby causing the output of each of the four photodetectors to change each time the objective lens moves. To implement the invention, the output of the photodetector array is manipulated wherein the manipulated signal corresponds to the signal that would be received if the photodetector were actually following the track directly above the laser as in prior art systems. In this manner, a signal can be used in a tracking servo that corresponds to movement within a conventional media player when actually the pickup and the medium are stationary.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to media players, and more specifically to recording and playback tracking control on a media player. 
     2. Background of the Invention 
     Media players include devices commonly referred to as Compact Disc (CD) players, Digital Video Disc (DVD) players, Video Laser Disc (LD) players, and others. Media players typically operate by embedding data into a disc and/or reading the data from the disc with a laser beam. The disc usually spins and an optical pickup that houses the laser is moved in a radial direction about the spinning disc allowing the laser beam to be positioned above any area on the disc where information needs to be read from or recorded to. 
     It is desirable for more information to be embedded onto the disc because more information allows the user to watch a longer movie, listen to more songs, or make use of more functionality on the same type of disc. More information can be embedded into the disc by packing the information more closely together and rotating the disc faster while moving the optical pickup more precisely. This creates problems because the disc has a relatively large mass, so to spin it at a very high speed while still steadying the disc enough to be read by the laser is very difficult. Moreover, to move the pickup radially with precision is difficult as well. Before further discussing this problem, an overview of media players is provided. 
     Media Players 
     An optical media player can be either a device or system that is capable of retrieving information stored in an optically recordable medium, such as an optical disc, or a device or system that is capable of both recording information to and retrieving information from an aptically recordable medium. Examples of devices that are capable of retrieving information from an optical disc include CD players, LD players, DVD players, and compact disc read-only-memory (CD-Rom) drives. Examples of devices that are capable of both recording information to an optical disc and retrieving information from an optical disc include recordable mini-disc (MD) players, magneto-optical (MO) disc drives and compact disc recordable (CD-R) drives. 
     Information is generally stored by an optical disc in the form of concentric or spiral tracks sometimes referred to as information tracks. In the case where information is already stored by an optical disc, the information tracks contain regions of optical contrast that represent the stored information. In the case of an unrecorded or blank optical disc containing pre-formatted tracks for recording information, a track that will become an information track may or may not have regions of optical contrast. The area located between two information tracks on an optical disc is sometimes referred to as a non-information track. 
     When an optical storage device is in its normal mode of operation, (i.e. retrieving information from or recording information to an optical disc), the storage device rotates the disc while using a light beam to retrieve information from or record information to the disc. As the optical disc rotates, the light beam radially traverses the disc. While the light beam traverses the optical disc, a tracking servo loop in the optical disc storage device keeps the beam of light centered on the information track, or the track that will become the information track in the case of recording information to a disc. 
     Tracking Servo 
     An optical disc tracking servo is a closed loop system that allows a light beam to remain centered on an optical disc information track during normal operation of an optical disc storage device. The tracking servo readjusts the radial position of the light beam by sensing when the light beam drifts off the center of the information track. The tracking servo senses when the light beam is not centered on an information track by measuring the intensity of light reflected by the surface of the optical disc. 
     Generally, the intensity of light reflected by the surface of an optical disc is the least when it is reflected by the center of an information track. Using this principle, a tracking servo generally senses the intensity of light reflected at one or both edges of an information track to detect when a light beam is drifting off center and to determine in which direction the light beam is drifting. Therefore, a tracking servo system that is in a closed loop mode of operation senses when the light beam floats off the center of the information track by detecting changes in the intensity of light reflected at one or both edges of an information track and moves the beam back into a position where the intensity of reflected light is optimal for center tracking. 
     In the case where a tracking servo measures the intensity of light reflected at both edges of an information track, the intensity of reflected light that is optimal for center tracking occurs when the intensity of light reflected at both edges of an information track is the same. The same principle holds true for both one and three beam optical disc storage devices. In the case where a tracking servo measures the intensity of light reflected at one edge of an information track, the intensity of reflected light that is optimal for center tracking is based on some calibrated value. The latter method is less favored due to difficulties associated with calibrating an appropriate centering value. 
     High Density Mediums 
     As technology advances, it is desirable to store more information into the medium being recorded to or read from. One manner in which to embed more data in a medium that is comparable in size and compatible with current mediums (i.e., DVDs and CDs) is to create a track that is more tightly spiraled. In this way the disc will have more surface with which to embed the information to be used. This creates problems itself because then the radial distance between each successive track is reduced. This means the laser must be moved radially in a more precise manner. This is difficult and the tracking servo used to accomplish the precise radial movements becomes prohibitively expensive. 
     Moreover, it would be desirable to cause the laser to traverse the medium quicker. In this way more data can be processed by the system faster, which results in a more satisfying experience for the user. One way to accomplish this using current techniques would be to spin the disc faster. This however creates problems of its own because the inertia of the disc increases as it spins faster, which further exacerbates the problems that already exist in steadying a relatively massive disc as it spins. And to run a tracking servo with a faster spinning disc becomes exponentially more difficult. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of the invention is to be able to very quickly record or follow an information track that can be much denser than is typically found in the prior art. The invention uses a stationary medium and a stationary optical pickup unit. The stationary optical pickup unit has a laser, which sends a beam of light through an objective lens. The objective lens focuses the beam on a point on the track of the stationary medium. The beam is then reflected toward a re-directing surface (e.g., a prism) which diverts it to a photodiode array. Instead of the entire optical pickup moving to follow the track as in conventional systems like CD players and DVD players, only the objective lens inside of the optical pickup moves. 
     The position of the objective lens determines where on the information track the laser will reach. Depending upon where the laser beam reaches, the reflected light beam received by the photodetector array changes. This, in turn, affects the amount of light sensed by each photodetector in the array, thereby causing the output of each of the photodetectors to change each time the objective lens moves. 
     To implement the invention, the output of the photodetector array is manipulated wherein the manipulated signal corresponds to the signal that would be received if the photodetector were actually following the track directly above the laser as in prior art systems. In this manner, a signal can be used in a tracking servo that corresponds to movement within a conventional media player when actually the pickup and the medium are stationary. This alleviates the prior art problems associated with steadying a rotating disc while trying to maximize its speed of rotation and moving an optical pickup radially while trying to fine tune the movement to work with a tightly spiraled track or a concentric track with a very close spacing between tracks, since the objective lens can be moved much more quickly, precisely, and with a near zero inertia. 
     In one embodiment, the invention comprises a stationary optical pickup unit, a tracking servo, a photodetector signal manipulator, and an angular position driver. The stationary optical pickup unit has a laser, which sends a beam of light through an objective lens, which focuses the beam on a point on the track of the stationary medium. The beam is then reflected toward a re-directing surface (e.g., prism) which diverts it to a photodiode array. The objective lens has three degrees of freedom, which comprise an x-axis, a y-axis, and a z-axis. The z-axis is used for focus, while the x-axis and the y-axis are used for tracking and cause the objective lens to move in a pattern to follow the track. (i.e., if the track is a spiral pattern, then the objective lens is caused to move in a spiral.) 
     One embodiment uses a four photodetector array. In operation, the output of the array is sent to the signal manipulator where two of the four signals pass through a first amplifier (one signal is differential with respect to the other signal) and the other two of the four signals pass through a second amplifier (again one signal is differential with respect to the other). The result is two signals that are passed through the tracking servo block where they undergo phase gain compensation, which provides for tracking servo stability. The two signals after phase gain compensation are sent to two separate amplifiers. Each of the amplifiers also receives either a sine or cosine signal that come as output from the angular position driver. The sine and cosine signals are generated by a radial ramp in conjunction with an oscillator capable of producing a regular and sinusoidal signal. 
     After the amplifiers each combine the sine/cosine drive signal with the signals that underwent phase gain compensation, the resulting two signals loop back to the stationary optical pickup unit. The loopback signals provide inputs to the pickup unit (for instance, at x and y lens driver coils) which cause the objective lens to move in a direction about an x and y axis and the process repeats at the next position on the media track. 
     It should be noted that the photodetector outputs may undergo phase gain compensation before undergoing manipulation rather than after. It should also be noted that a spiral track is used throughout this document for the purpose of example only and the present invention applies to other information track patterns as well. Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more fully understood by reference to the following drawings, which are for illustrative purposes only: 
         FIG. 1  is a diagram of a prior art photodetector array associated with an information track. 
         FIG. 2  is a functional diagram of a micro optical tracking control device according to an embodiment of the present invention. 
         FIG. 3  is a functional diagram showing a photodetector signal manipulator according to an embodiment of the present invention. 
         FIG. 4  is a functional diagram showing the angular position driver according to an embodiment of the present invention. 
         FIG. 5  is a diagram of a prior art optical pickup unit. 
         FIG. 6  shows an arrangement of a stationary optical pickup unit and a stationary disc according to an embodiment of the present invention. 
         FIG. 7  is a schematic perspective view of an optical pickup unit having three degrees of freedom according to an embodiment of the present invention. 
         FIG. 8  is a functional diagram of a micro optical recording and playback tracking control apparatus according to an embodiment of the present invention. 
         FIG. 9  is a functional diagram showing how an embodiment of the present invention operates upon a three dimensional medium. 
         FIG. 10  is a functional diagram showing how an embodiment of the present invention operates upon a medium having multiple information tracks. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to a method and apparatus for recording and playback tracking control that is capable of functioning on a micro optical level. The term micro optical refers to the ability to record to or play from an information track that has data embedded the track that is much more closely spaced than is currently available. This is possible because the present invention utilizes an optical pickup that is capable of guiding a laser beam in a much more precise and a much faster manner than is currently possible. 
     Referring more specifically to the drawings,  FIG. 1  shows an example of a photodetector array associated with an information track that is implemented in the prior art. Information track  100  has a spiral pattern. A spiral track is used throughout this document for the purpose of example only. The present invention is equally applicable to other information track layouts as well, including concentric, substantially circular, and other patterns. Photodetector array  110  is in position  1 , when it is in the 12 o&#39;clock position. In that position photodetectors A and B are above the track  100 , while photodetectors C and D are below the track  100 . At position  2  (the 3 o&#39;clock position) photodetectors A and B are right of the track while photodetectors C and D are left of the track (note that this is the case because the photodetector array  110  has undergone a ninety degree rotation between positions  1  and  2 ). At position  3  (the 6 o&#39;clock position) photodetectors D and C are above the track, while photodetectors B and A are below the track. At position  4  (the 9 o&#39;clock position) photodetectors D and C are right of the track, while photodetectors B and A are left of the track. 
       FIG. 1  is used to illustrate the point that as the photodetector array moves about the track, the position of the photodetectors changes, and hence the signals they receive changes. A prior art tracking servo may be implemented by rotating a photodetector array about a track as shown in  FIG. 1 . 
     Micro Optical Tracking Control 
     Unlike  FIG. 1 , the photodetector array remains stationary according to the present invention and the outputs of the photodetectors are manipulated to achieve equivalent signals to those illustrated in  FIG. 1 .  FIG. 2  is a functional block diagram of a micro optical tracking apparatus in accordance with the present invention. The micro optical apparatus includes a stationary optical pickup  300 . Stationary optical pickup  300  comprises a light source  310  (e.g., a laser), an objective lens  320 , a photodetector array  330 , and a redirecting surface  340  (e.g., a prism). Light  350  is transmitted via light source  310  and it passes through redirecting surface  340 , then through objective lens  320 , before contacting an information track  360 . 
     Information track  360  reflects the light through redirecting surface  340  where it is guided to photodetector array  330 . Depending on where the laser contacted information track  360  determines the initial unmodified output  335  of photodetectors  330 . This initial unmodified output changes in a regular sinusoidal fashion similar to that shown in  FIG. 1 . This unmodified output is capable of being manipulated by the present invention to simulate the same results as shown in  FIG. 1 . To accomplish this task, the unmodified outputs of the photodetectors provide input to signal manipulator  370  where they are transformed to signals equivalent to those shown in  FIG. 1 . 
     Next, the manipulated signals are sent to a tracking servo  380 . Tracking servo  380  applies phase gain compensation and those signals are combined with a generated sine and cosine signal from an angular position driver  390 . Angular position driver  390  is used to modify the signals in such a manner that when they loop back to stationary optical pickup  300 , driver mechanisms  395  (such as x and y lens driver coils) in the pickup receive input that cause them to move the objective lens about an x and y axis, so that it is in position to guide the laser to the next location on the track and the process repeats. 
     Photodetector Signal Manipulator 
     The purpose of the photodetector signal manipulator  370  is to modify the output of the photodetector array  330  wherein the manipulated output  375  corresponds to the signal that occurs when the pickup follows the track. In this manner, a signal can be used in a tracking servo that corresponds to movement within a conventional media player when actually the pickup and the medium are stationary. This alleviates the prior art problems associated with steadying a rotating disc while trying to maximize its speed of rotation and moving an optical pickup radially while trying to fine tune the movement to work with a tightly spiraled track, or other dense track such as a concentric track having multiple tracks that are substantially circular with very small spacings between the tracks. 
       FIG. 3  is a functional diagram showing the photodetector signal manipulator according to an embodiment of the present invention. A stationary optical pickup  400  provides four unmodified photodetector output signals labeled  410   a ,  410   b ,  410   c , and  410   d . It should be noted that any number of photodetectors may be used with the present invention. The unmodified outputs  410   a - d  are obtained by a light source in pickup  400  directing a laser onto an information track and having the reflection of the laser diverted into the photodetectors. The photodetectors sense the strength of the reflection of this light and the unmodified outputs  410   a - d  are values representing the strength of these reflections. 
     The signals  410   a  and  410   d  are combined in a first modification path  420  in photodetector signal manipulator  401 , while signals  410   b  and  410   c  are combined in a second modification path  430 . In the first modification path  420 , signal  410   a  is passed to a positive amplifier  440  while signal  410   d  is passed to a negative amplifier  441 . The two signals then are passed to a summing circuit  442  and passed through another amplifier  443 . This results in a first modified output signal  450 . 
     The signals  410   b  and  410   c  are combined in a second modification path  430 . In the second modification path  430 , signal  410   b  is passed to a positive amplifier  460  while signal  410   c  is passed to a negative amplifier  461 . The two signals then are passed to a summing circuit  462  and passed through another amplifier  463 . This results in a second modified output signal  470 . 
     The first and second modified output signals  450  and  470  are passed through a tracking servo  480  and are combined with POSsine and POScosine signals  491  and  492  from the angular position driver  493  to produce output signals  495  and  496  before looping back to pickup  400  at x and y lens driver coils  498  and  499 . 
     Angular Position Driver 
     The purpose of the angular position driver is to modify the tracking signals in such a way that the position of the objective lens in the pickup can be changed so that it will direct the laser to inscribe a track at a rate required to recover recorded information. For example, in the case where the track is a spiral, the angular position driver continually modifies the tracking signal so that the spiral shape of the information track is accounted for. 
       FIG. 4  is a diagram showing the angular position driver according to an embodiment of the present invention. Block  500  is the stationary optical pickup. For the purpose of this embodiment, block  500  provides four unmodified photodetector output signals labeled  510   a ,  510   b ,  510   c , and  510   d . The signals  510   a  through  510   d  are sent to a signal manipulator  520 , such as that described in  FIG. 3 . The outputs of signal manipulator  520  are first and second modified signals  530  and  531 , which are passed to tracking servo block  535 . Amplifiers  540  and  541  receive first and second modified signals  530  and  531  as well as generated POSsine and POScosine signals  551  and  552  from angular position driver  550 . 
     Angular position driver  550  in this embodiment includes a radial ramp  560  that sends a ramp signal to a summing circuit  558  which is combined with an offset  559 . The offset  559  is used to start the spiral and corresponds to the inner radius of the spiral information track. A sine wave oscillator  553  generates sine signal  554  and cosine signal  555 . A first multiplier  556  receives sine signal  554  and combines it with the output of the summing circuit  558 . A second multiplier  557  receives cosine signal  555  and also combines it with the output of the summing circuit  558 . The result is the generated POSsine and POScosine signals  551  and  552  respectively. 
     It should be noted that the above embodiment of the angular position driver applies to a spiral track. In the case of a track with concentric circles, the angular position driver might simply be an oscillator capable of producing a regular and sinusoidal signal. The output of amplifiers  540  and  541  are x and y drive signals  595  and  596  respectively, which loop back to pickup  500  and connect to x and y lens driver coils  597  and  598  that are configured to move objective lens  599  to the appropriate next position. 
     Embodiment of a Stationary Optical Pickup 
     The stationary optical pickup according to the present invention has an objective lens that has three degrees of freedom. The lens moves in the x and y directions to track the information found on the medium it is shining the laser upon. It also moves in the z direction for focus. Unlike conventional pickups, however, the entire pickup unit itself does not move for tracking and focus. Instead, the entire pickup is only moved incrementally in the x, y, or z direction to follow another spiral information track. 
     An example of a prior art pickup moved for tracking and focus is shown in  FIG. 5 . The entire base  610  of the pickup  600  is moved in a radial direction  620  with respect to track  630 . In addition, track  630  rotates in direction  650 , so by a combination of rotation in direction  650  and movement by pickup  600  in radial direction  620  the entire information track is covered. 
     In the present invention, the pickup does not move at all when it follows a single information track, nor does the information track. This arrangement is shown in  FIG. 6 . Stationary optical pickup  700  has an objective lens  710  that moves with three degrees of freedom. Information track  720  is stationary as well. Laser  730  is configured to move in the direction of arrow  740  solely by the movement of objective lens  710  so that it can cover the entire information track. 
     The present invention is designed to work with any optical pickup apparatus that has three degrees of freedom. Such an apparatus is well known to those skilled in the art. For illustration, one embodiment of an optical pickup having three degrees of freedom is described below in connection with  FIG. 7 . As shown in  FIG. 7 , a three-axis actuator for an optical pickup has a base  31 . A lens holder  33  having an objective lens  32  is mounted on the base  31 . Magnets  34 A,  34 B,  34 C, and  34 D are installed at the base  31  around the lens holder  33 . A focus coil  35  is wound around the lens holder  33 . A plurality of tracking coils  36 A- 36 H are attached to the focus coil  35 , and a plurality of suspension members  37  for supporting the lens holder  33  such that the lens holder  33  is movable above the base  31 . 
     The lens holder  33  is moved along the X and Y-axes by the interaction between the magnets  34 A- 34 D and a current flowing through the tracking coils  36 A- 36 H. That is, each of the one portions of the first tracking coils  36 A and  36 B and third tracking coils  36 E and  36 F face the first and third magnets  34 A and  34 C, respectively, and move the lens holder  33  in the X-axis direction. Each of the one portions of the second tracking coils  36 C and  36 D and fourth tracking coils  36 G and  36 G face the second and fourth magnets  34 B and  34 D, respectively, and move the lens holder  33  in the Y-axis direction. 
     For example, currents flow in the same direction through the respective portions of the first tracking coils  36 A and  36 B, which face the first magnet  34 A. If a current is applied counterclockwise to the tracking coil  36 A, the current flows downward through the portion of the tracking coil  36 A, which faces the first magnet  34 A, i.e., the left portion. Here, the current is applied clockwise to the tracking coil  36 B and thus the current flows downward through the portion of the tracking coil  36 B, which faces the first magnet  34 A, i.e., the right portion. 
     The current is applied to the second, third, and fourth tracking coils  36 C and  36 D,  36 E and  36 F, and  36 G and  36 H in the same manner as to the first tracking coils  36 A and  36   b . Therefore, since the directions of currents flowing through coil portions facing the magnets  34 A- 34 D are identical, force is applied to the lens holder  33  in one direction. Each of the suspension members  37  is formed of elastic material so as to serve as an elastically biasing member, which is capable of stretching. One end of each suspension member  37  is fixed to the lens holder  33  and the other end thereof is fixed to support members installed on the base  31 . The focus coil is wound around the lens holder  33 . Accordingly, the objective lens  32  is moved upward and downward along a focusing direction (i.e., along a Z-axis) by the interaction between a current applied to the focus coil  35  and the magnets  34 A- 34 D. The above three-axis moving actuator for an optical pickup moves the objective lens in the tangential direction of a disc track as well as in the tracking and focusing directions. Accordingly, focusing, tracking, and errors of the tangential direction can be corrected, which enables the actuator to be widely used as an optical pickup for high-density discs. 
     Embodiment of a Micro Optical Recording and Playback Tracking Control Apparatus 
     One embodiment of a micro optical recording and playback tracking control apparatus is shown in  FIG. 8 . The apparatus is used to read information track  1000  of medium  1005 . To this end, stationary pickup unit  1010  has a laser  1015 , which sends a beam of light  1020  through an objective lens  1025 , which focuses the beam on a point  1030  on the track of the stationary medium  1005 . The beam is then reflected toward a re-directing surface  1040  (e.g., prism) which diverts it to a photodiode array  1045 . 
     The output of the photodiode array  1045  comprises unmodified photodetector output signals  1046   a  through  1046   d , which are sent to a signal manipulation block  1050 . Signals  1046   a  and  1046   d  are sent to a first modification path, while signals  1046   d  and  1046   c  are sent to a second modification path. In the first modification path, signal  1046   a  is passed to a positive amplifier  1055  while signal  1046   d  is passed to a negative amplifier  1056 . The two signals then are passed to a summing circuit  1057  and passed through another amplifier  1058 . This results in a first modified output signal  1059 . 
     The signals  1046   b  and  1046   c  are sent to a second modification path. In the second modification path, signal  1046   b  is passed to a positive amplifier  1060  while signal  1046   c  is passed to a negative amplifier  1061 . The two signals then are passed to a summing circuit  1063  and passed through another amplifier  1064 . This results in a second modified output signal  1069 . 
     The first and second modified output signals  1059  and  1069  are passed to a tracking servo  1070 . In the tracking servo signal  1059  undergoes a phase gain compensation process known to those skilled in the art by passing through phase  1075  and then through gain  1076 . Likewise, signal  1069  passes through phase  1077  and through gain  1078 . The results are passed to first and second amplifiers  1080  and  1081 . In the angular position driver  1085  a POSsine and a POScosine signal  1088  and  1089  that are sent to amplifiers  1080  and  1081  respectively that received the signals that underwent phase gain compensation. Angular position driver outputs the POSsine signal  1088  and the POScosine signal  1089  in any manner known to those skilled in the art. For instance it can use a sine generator to produce the POSsine signal  1088  and it can use the same sine generator coupled with a phase shifter to output the POScosine signal  1089 . 
     The output of amplifiers  1080  and  1081  result in x and y driver signals  1085  and  1086 , which loop back to pickup unit  1010  and provide input to the unit so that the objective lens  1025  can be moved to the next appropriate position, wherein the laser is guided to the next position on the information track. The movement of the lens is achieved in any manner well known to those skilled in the art. For instance, it may be accomplished by connecting x and y lens driver coils in the pickup  1010  to move the lens, or in the manner described in connection with  FIG. 3 . 
     Alternate Media Types 
     The present invention works well with any suitable medium including three-dimensional mediums like holographs and two dimensional mediums having multiple information tracks in the x and y directions.  FIG. 9  provides an example of how an embodiment of the present invention can be adapted to operate upon a three dimensional medium. Medium  1100  includes multiple information tracks  1110   a  through  1110   d . Each information track  1110  is at a different level. Track  1110   a  is at level  1 . Track  1110   b  is at level  2 . Track  1110   c  is at level  3 . Track  1110   d  is at level  4 . The player according to the present invention includes the stationary optical pickup  1120 , the signal manipulator  1130 , the tracking servo  1140 , and the angular position drive block  1150 . 
     Within the optical pickup  1120 , an objective lens  1160  has three degrees of freedom. The z direction  1170  corresponds to a vertical direction, wherein the objective lens may be focused in the z direction to any particular information track, but it can also be cause to move to a different level. For instance, position  1  of the lens  1160  causes the lens to focus on level  1110   a . Position  2  of the lens  1160  causes the lens to focus on level  1110   b . Position  3  of the lens  1160  causes the lens to focus on level  1110   c . Position  4  of the lens  1160  causes the lens to focus on level  1110   d.    
       FIG. 10  provides an example of how an embodiment of the present invention can be adapted to operate upon a medium having multiple information tracks in the x and y directions. Medium  1200  includes multiple information tracks  1210   a  through  1210   d . Each information track  1210  is at a different position with respect to an x, y grid. Track  1210   a  is at position  1 . Track  1210   b  is at position  2 . Track  1210   c  is at position  3 . Track  1210   d  is at position  4 . The player according to the present invention includes the stationary optical pickup  1220 , the signal manipulator  1230 , the tracking servo  1240 , and the angular position drive block  1250 . 
     Within the optical pickup  1220 , an objective lens  1260  has three degrees of freedom. The objective lens may be moved in the x and y directions to any particular information track. For instance, position  1  of the lens  1260  causes the lens to read track  1210   a . Position  2  of the lens  1260  causes the lens to read track  1210   b . Position  3  of the lens  1260  causes the lens to read track  1210   c . Position  4  of the lens  1260  causes the lens to read track  1210   d    
     The operation of the present invention is the same as that described in various embodiments of the present invention, except the lens operates on each level in this manner. Although the description above contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Thus the scope of this invention should be determined by the appended claims and their legal equivalents.