Patent Abstract:
The present invention is a method and system to provide adaptive control in an optical storage medium. The method comprises providing a beam of light, reflecting the beam of light off an optical disk and detecting the reflected beam. A value of the reflected beam is compared with a predetermined value, and an output signal is generated if the value of the reflected beam is greater than the predetermined value. A timing signal having a timing interval is generated and a control signal is generated if the output signal occurs over the timing interval. One of a current servo signal and a predetermined servo signal is provided in response to the control signal and a position of the light beam is controlled based on one of the current servo signal and the predetermined servo signal.

Full Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates in general to optical disk storage systems and more particularly, to a method and apparatus for providing adaptive control of track servo.  
           [0003]    2. Description of the Related Art  
           [0004]    In recent years, optical disk devices have been used to record or reproduce large amounts of data. Optical disks are storage mediums from which data is read and to which data is written by laser. Each optical disk can store a large amount of data, typically in the order of 6 gigabytes. Such optical disk devices are under active technical developments for achieving higher recording density.  
           [0005]    Current rewritable optical disks include spiral-shaped groove tracks having concave and convex portions formed on the surface of a disk substrate. On the surface of the substrate, a thin film that includes a recording material as a component is attached. During fabrication of the disks, concave and convex portions (pits) are often formed on the recording surface, simultaneously with the formation of guide grooves for tracking control, so as to record address information of each sector.  
           [0006]    Each track of the optical disk is irradiated with a light beam having a predetermined recording power, so as to form recording marks on the recording thin film. The portions irradiated with the light beam (i.e., the recording marks) have different optical characteristics (reflection characteristics) from the other portions of the recording thin film. Thus, the recorded information can be reproduced or read by irradiating the track with a predetermined reproduction power and detecting light reflected from the recording film.  
           [0007]    Accurate reading of information recorded on such optical disks may be impaired due to the existence of defects such as fingerprints, black dots, scratches and interruptions. When such defects are encountered during the read process, the optical head may be directed to move to a different track. In particular, defects are detected by monitoring a change in the beam strength signal reflected off the disk. Although the read information corrupted by defects resulting from scratches and black dots are irrecoverable, defects resulting from fingerprints render noisy but meaningful data. If the defect is not properly processed and handled, the resulting tracking error signal will be erroneous. Upon reading the erroneous tracking error signal, the servo control will direct the optical head to move off the current track. As a result, system performance is unnecessarily compromised.  
           [0008]    Accordingly, there is a need in the technology to overcome the aforementioned problems.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    The present invention is a method and system to provide adaptive control in an optical storage medium. The method comprises providing a beam of light, reflecting the beam of light off an optical disk and detecting the reflected beam. A value of the reflected beam is compared with a predetermined value, and an output signal is generated if the value of the reflected beam is greater than the predetermined value. A timing signal having a timing interval is generated and a control signal is generated if the output signal occurs over the timing interval. One of a current servo signal and a predetermined servo signal is provided in response to the control signal and a position of the light beam is controlled based on one of the current servo signal and the predetermined servo signal.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 illustrates one embodiment of an optical disk apparatus provided in accordance with the principles of the invention.  
         [0011]    [0011]FIG. 2 illustrates one embodiment of the comparator circuit  36  and the controller circuit  32  of FIG. 1.  
         [0012]    FIGS.  3 A-D illustrate one embodiment of the timing diagram implemented in the process of the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]    The present invention is an apparatus and methods for providing adaptive control of track servo in an optical disk system.  
         [0014]    Referring to the drawings-more particularly by reference numbers, FIG. 1 illustrates one embodiment of an optical disk apparatus  10 . The optical disk apparatus  10  includes an optical disk  12  that is rotated by a spin motor  14 . An optical pickup  16  scans the tracks on the rotating disk  12  with a laser beam. The optical pickup  16  comprises an optical system including a laser  18  that provides a light source, and an objective lens  22 . The laser  18  is driven by a laser driver  20  to emit a laser beam. The laser beam is incident on the objective lens  22  via optical elements (not shown) such as a collimator lens and a beam splitter. The laser beam  22   a  is focused on the recording surface of the optical disk  12  by the objective lens  22  to form a small spot on the recording surface.  
         [0015]    The light reflected from the optical disk  12  propagates back to the objective lens  22  and is separated from the incident laser beam by the beam splitter. The reflected light beam is detected by the photodetector  24 . This photodetector  24  converts this reflected light beam into electric signals.  
         [0016]    The electric signal is then provided to a preamplifier and conditioning circuit  26 , which amplifies and conditions the electric signal. Based on the received electric signal, the preamplifier and conditioning circuit  26  generates a plurality of signals, including a track error signal  28   a,  a focus error signal  28   b  and a beam strength signal  28   c.  The beam strength signal  28   c  is a signal generated from either the main or the side beams of the reflected light beam, or a combination of both the main and side beams, and it represents the disc reflection of the beam spot as the optical head moves across the disc surface. The tracking error signal represents the tracking servo quality based on the reflected light beam. It is understood that additional signals may be provided by the circuit  26 .  
         [0017]    One aspect of the invention involves using the beam strength signal  28   c  to detect defects and to distinguish the defects detected. Defects may result from (but are not limited to) black dots, interruptions, scratches and fingerprints. A black dot is a media defect characterized by loss of reflectivity of the incident laser beam. Such a defect is typically identified when reflectivity is less than 80% of typical reflected beam strength. An interruption is a media defect that results in reflectivity that is higher than typical reflected beam strength. Such a defect is typically identified when reflectivity is greater than 80% than typical reflected beam strength. Scratches on the disc surface result in signal characteristics that are similar to that obtained due to a black dot, and is treated in the same way. Defects from fingerprints generally weaken the reflected beam, and makes it noisier. However, the resulting reflected beam and tracking signal are still usable. Defects from fingerprints typically result in reflectivity that is 33% less than typical reflected beam strength.  
         [0018]    In one embodiment of the invention, defects resulting in loss of reflectivity at or below a first predetermined level are distinguished from defects resulting in loss of reflectivity at or above a second predetermined level. It is to be understood that defects resulting in loss of reflectivity at or below a first predetermined level may also be similarly distinguished from defects resulting in loss of reflectivity at or above a third predetermined level.  
         [0019]    In another embodiment, to ascertain the existence of the defects, a timer is triggered upon the initial detection of the defect. If the beam strength is generally consistent over the duration of a predetermined interval, the defect is considered to be a true defect, as opposed to random noise or spikes. Upon such confirmation, the system of the invention proceeds to take action.  
         [0020]    The signals  28   a,    28   b  and  28   c  are converted to digital signals by an analog-to-digital converter (ADC)  30 . The ADC  30  provides the tracking error signal  28   a  and the focus error signal  28   b  to a controller circuit  32 , while the digitized beam strength signal  34  is provided to a comparator circuit  36 . The comparator circuit  36  generates digital defect level signals, based on the beam strength  34  signal. The controller circuit  32  generates a servo output signal  40 , which is converted back to an analog signal via digital-to-analog circuit  42 . The analog servo output signal  40  is provided to a voice coil motor (VCM) driver  44 , which supplies a drive current to the tracking actuator  46  in accordance with the servo output signal to drive the tracking actuator. The tracking actuator  46  moves the objective lens in the radial direction of the optical disk  12 , so that the beam spot is positioned in the radial direction of the disk so as to track the pit stream. In other words, tracking control is performed.  
         [0021]    [0021]FIG. 2 illustrates one embodiment of the comparator circuit  36  and controller circuit  32  of FIG. 1. As shown, the comparator circuit comprises three comparators  36   a,    36   b  and  36   c.  It is understood that the comparator circuit  36  may include fewer or a greater number of comparators, as determined by need or design. In one embodiment, the comparator circuit  36  comprises at least one comparator. Each comparator  36   a,    36   b  and  36   c  has two input terminals, one of which is coupled to receive the digitized beam strength signal  34  from ADC  30 . The other input terminal of each comparator  36   a,    36   b  and  36   c  is coupled to receive a reference signal, such as Ref  1 , Ref  2  and Ref 3 . The output of each comparator  36   a,    36   b  and  36   c  is coupled to a defect controller  50  in the controller circuit  32 . For example, Ref  1  may be a reference signal level for data having defects due to a fingerprint; Ref  2  may be a reference level for data having defects due to black dots or scratches; and Ref  3  may be a reference level for data having defects due to interruptions. If the digitized beam strength  34  signal is greater than Ref  1 , Ref  2  or Ref  3 , the respective comparator  36   a,    36   b  or  36   c  will generate an output signal.  
         [0022]    The controller circuit  32  receives the output signal(s) from the comparators  36   a,    36   b,    36   c  and determines if it should direct the VCM driver  44  to continue reading or to hold the optical pickup  16  at a previous track error level. For example, if only data with fingerprint defects are detected, the controller circuit  32  will direct the VCM driver  44  to continue reading data. If data with black dot or interruption or scratches defects are detected, the controller circuit  32  will direct the VCM driver  44  to hold the optical pickup  16  at the track error level prior to encountering the defect.  
         [0023]    In one embodiment, the controller circuit  32  comprises a defect controller  50 , a switch  52  and a direct current (D.C.) Hold circuit  54 . The controller circuit  32  may provide either the servo input signal  38  or a predetermined signal from D.C. Hold circuit  54  as the servo output signal  40 . In one embodiment, the D.C. Hold circuit  54  provides a D.C. level that is substantially the same signal level as a servo input signal immediately prior to encountering a defect. If the beam strength signal  28   c  is within a range that is considered typical or normal, the servo input signal  38  is provided as the servo output signal  40 . In one embodiment, the defect controller  50  issues a control signal to direct the switch  52  to move to position P 1 , where the servo input signal  38  is provided as the servo output signal  40 . However, if the beam strength  28   c  falls below a minimum level or above a maximum level, the defect controller  50  issues a control signal to direct the switch to move to P 2 , where a predetermined D.C. level is provided as the servo output signal  40 . Details of this process are discussed in the following sections.  
         [0024]    In operation, each comparator  36   a,    36   b  and  36   c  receives the digitized beam strength signal  34  and compares it to a respective reference signal level, Ref  1 , Ref  2  and Ref 3 , as provided by reference signal circuits  52   a,    52   b  and  52   c.  In one embodiment, if the beam strength signal  34  is higher than the absolute value of the reference signal level Ref  1 , Ref  2  or Ref  3 , the respective comparator  36   a,    36   b  and  36   c  will generate an output signal.  
         [0025]    FIGS.  3 A-D illustrate one embodiment of the timing diagram implemented in the process of the invention. FIG. 3A illustrates one embodiment of the beam strength of the reflected beam off the disk  12  (FIG. 1) from laser beam  22 . The beam strength of a typical signal is represented by r. FIG. 3A illustrates one embodiment of a reflected beam rA that results from a loss of reflectivity, and a reflected beam rB that results from over reflectivity. In one embodiment, a first predetermined level, Ref  1 , r 1 , is established to monitor defects resulting from a loss of reflectivity of at least 33% of the typical reflected beam strength, r. In a second embodiment, a second predetermined level, Ref  2 , r 2 , is established to monitor defects resulting from a loss of reflectivity of at least 80% of the typical reflected beam strength, r. In a third embodiment, a third predetermined level, Ref.  3 , r 3 , is established to monitor defects resulting from an increased reflectivity of at least 80% of the typical reflected beam strength, r. It is to be understood that the first, second and third predetermined levels may be established at any level that the user determines or as required. In addition, a greater number of predetermined levels may also be established. With reference to FIG. 2, each comparator  36   a,    36   b  and  36   c  receives the digitized beam strength signal  34  and compares it to a respective reference signal level, Ref  1 , Ref  2  and Ref 3 , as provided by reference signal circuits  52   a,    52   b  and  52   c.  In one embodiment, if the beam strength signal  34  falls below a first predetermined level, such as is higher than r 1 , the comparator  36   a  will generate an output signal, Defect Level  1  (see FIG. 3B). If the beam strength signal  34  falls below a second predetermined level, r 2 , the comparator  36   b  will generate an output signal, Defect Level  2  (see FIG. 3B). In a further embodiment, if the beam strength signal  34  is greater than a third predetermined level, r 3 , the comparator  36   c  will generate an output signal, Defect Level  3  (see FIG. 3B). In one embodiment, the output signal Defect Level  3  is provided as an inversion of the output signal Defect Level  2 , so as to distinguish between the two signals.  
         [0026]    Each of the output signals, Defect Level  1 ,  2  and  3  are provided to the Defect Controller  50  within the controller circuit  32 . In response to each output signal, generates one-shot timer having pre-defined timing interval. In one embodiment, the one-shot timer T 1  is triggered by the positive or rising edge of the Defect Level  1  signal, while the one-shot timer T 2  (having a duration of t 2 ) is generated by the negative or trailing edge of the output signal Defect Level  2 . The one-shot timer T 3  (having a duration of t 3 ) is triggered by the positive or rising edge of the Defect Level  3  signal. In one embodiment, t 1 , t 2  and t 3  are of the same duration. In a second embodiment, t 1 , t 2  and t 3  are 45 microseconds.  
         [0027]    As shown in FIG. 3D, an output signal, Defect  1  is generated by Defect Controller  50 , if the Defect Level  1  signal is present. At the rising or leading edge of Defect Level  1 , the defect controller  50  issues a first control signal to direct the switch  52  to position P 2 . The signal Defect  1  indicates that a defect resulting from a loss of reflectivity at a first predetermined level (such as that from a fingerprint) has been detected. If no other defects are detected during t 1 , the defect controller  50  issues a second control signal to direct the switch  52  to move back to position P 1 , so that the controller circuit  32  will continue to receive the servo input signal in an unaltered fashion. This is because defects arising from fingerprints, while noisy, are still of a sufficiently good quality to be of use.  
         [0028]    However, if, during the interval t 1 , the signal beam strength falls to that of a second predetermined level (see FIG. 3B, Defect Level  2 ; and FIG. 3C, T 2 ), the defect controller  50  will generate an output signal Defect  2 . In one embodiment, the generation of Defect  2  indicates that a non-fingerprint defect has been encountered, and that signal recovery is required. In this situation, the defect controller  50  will continue to direct the switch  52  to remain in position P 2 , so that the servo input signal is held at a predetermined level as set by D.C. Hold circuit  54  for the duration of dt 2 . In one embodiment, the duration dt 2  has a time interval that begins from the time that Defect Level  2  is generated and ends when the one-shot timer T 2  times out. This time duration is typically needed to allow for the track error signal to recover from the defect. Once timing dt 2  times out, the defect controller  50  will issue a control signal to direct the switch  52  to return back to position P 1 .  
         [0029]    Similarly, an output signal Defect Level  3 , representative of a third type of defect, may be generated by comparator  36   c  when a defect having a reflectivity that is greater than Ref  3  is encountered. If the third defect is encountered during t 1 , the defect controller  50  may generate a control signal to direct the switch  52  to remain in position P 2 , so that the servo input signal is held at a predetermined level as set by D.C. Hold circuit  54  for the duration of dt 3 . In one embodiment, the duration dt 3  has a time interval that begins from the time that Defect Level  3  is generated and ends when the one-shot timer T 3  times out. This time duration is typically needed to allow for the track error signal to recover from the defect. Once timing dt 3  times out, the defect controller  50  will issue a control signal to direct the switch  52  to return back to position P 1 .  
         [0030]    By implementing the invention, signals that have been marginally corrupted due to defects such as fingerprints, but are would otherwise provide meaningful data, are processed in a normal manner. Signals that have been corrupted and are irrecoverable due to defects such as black dots, scratches and interruptions are not processed. Instead, the servo output signal based on such corrupted read signals are switched to a predetermined level (typically a previously uncorrupted signal level), until normal read signal levels are received.  
         [0031]    Through the implementation of the invention, servo tracking in an optical disk apparatus may be provided with greater accuracy. As a result, system performance is enhanced.  
         [0032]    While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art.

Technology Classification (CPC): 6