Abstract:
A method for calibrating a focusing error signal of a lightscribe disc includes the following steps. An optical pickup head is moved to a focusing reference surface by utilizing radial voltages to some predetermined measuring positions to find and record the best gain value used to calibrate the asymmetry of the focusing error signal. The best gain fitting curve is formed by curve-fitting, based on the recorded radial voltages and the best gain values. The best gain value is obtained by an interpolation or extrapolation method, and used to calibrate the asymmetry of the focusing error signal of a lightscribe disc.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an optical disc drive, and more particularly, to a method of calibrating focusing error signal asymmetry when a pickup head focuses on an inner ring of a label side of a lightscribe disc. 
         [0003]    2. Description of the Prior Art 
         [0004]    A conventional optical disc has two sides, one being a data side and the other a label side, and a traditional method of labeling the disc is by marking the label side with a pen or a label sticker. A lightscribe label technology has been recently developed, which utilizes a pickup head of an optical disc drive to direct a laser ray to scribe figures or text on the label side of a lightscribe disc, so as to fabricate an artistic and customized optical disc. 
         [0005]    This is shown in  FIG. 1 , which is a diagram of a pickup head reading a label side of a lightscribe disc according to the prior art. The optical disc  10  includes a control feature zone  11 , and  400  spokes  12  arranged at an inner ring of the label side of the optical disc  10 , and distributed on the inner side of the control feature zone  11  with equal angles. By reading and determining a spoke  12 , a relative angle position can be provided for the pickup head  20  to perform scribing. The outer side of the control feature zone  11  provides related data marks for the optical disc  10 , e.g., media ID field, saw-tooth pattern and index mark, etc. Additionally, a data zone and a non-data zone are discriminated by materials with notably different reflection rates. 
         [0006]    Since there are differences between fabricating companies, processes and materials of the optical disc  10 , the pickup head  20  has to read related information recorded in the control feature zone  11  of the optical disc  10  before starting scribing correct figures and text in a label zone  13 , so as to adjust strategy parameters of scribing the optical disc  10  to an optimized state. When reading information, the conventional pickup head  20  utilizes different voltages to form an electromagnetic force, driving a lens  22  supported by a spring line  21 , and beaming a laser ray to the control feature zone  11  of the lightscribe disc  10  rotated by a spindle motor  23 , referring to the luminous flux of a spot  25  received by illuminated parts A, B, C and D in a light detector  24 , calculating (A+C)−(B+D) and utilizing an amplifier  26  to amplify the signal, thereby deriving a focusing error (FE) signal. Utilizing a focusing error signal S-curve going through zero makes the lens  22  focus at the control feature zone  11  for the pickup head  20  to read information in the control feature zone  11  correctly. 
         [0007]    A light path design, fabrication or assembly error may lead to asymmetric illumination between illuminated parts (A+C) and (B+D), leading to focusing error signal asymmetry between the upper part and lower part, and therefore incorrect focusing. Although the prior art can utilize a fixed gain G to calibrate the illuminated parts to make FE=G(A+C)−(B+D) to eliminate the asymmetry of the focusing error signal, the fixed gain G is derived from the center of the pickup head  20  by the lens  22 . When the lens  22  is away from the center of the pickup head  20 , the asymmetry of spot  25  will also change with a variation of the light path and angle of the lens  22 . The fixed gain G is either too large or too small for the other positions of the lens  22 , and therefore the asymmetry of the focusing error signal cannot be alleviated and may even degrade such that the focusing cannot be performed correctly. More particularly, when reading information in the control feature zone  11  of the lightscribe disc  10 , the focusing error signal is not very good; in addition, the non-data mark zone is made with materials of a worse reflection rate, so a smaller part of the asymmetric focusing error signal will shrink rapidly, and the focusing error signal will be lost thereby losing focus, which leads to a longer re-reading and focusing time, and lowers the label scribing efficiency. Therefore, there are still problems for the pickup head of a conventional optical disc drive to enhance symmetry of a focusing error signal. 
       SUMMARY OF THE INVENTION 
       [0008]    One objective of the present invention is to provide a method of calibrating a focusing error signal of a lightscribe disc by calibrating a best gain value of the focusing error signal of a pickup head to improve symmetry of the focusing error signal. 
         [0009]    Another objective of the present invention is to provide a method of calibrating the focusing error signal of a lightscribe disc by fitting a best gain curve to enhance focus servo in the control feature zone of the lightscribe disc to lower a chance of focus failure. 
         [0010]    Yet another objective of the present invention is to provide a method of calibrating focusing error signal of a lightscribe disc by interpolating or extrapolating a best gain curve to derive a best gain value of a symmetric focusing error signal, and reading data in a control feature zone to facilitate scribing a label. 
         [0011]    In order to achieve the aforementioned objectives, the steps of a method according to an exemplary embodiment comprise: setting a focusing reference surface; moving a pickup head to the focusing reference surface; utilizing radial voltages to move a lens to predetermined calibration positions to find and record a best gain value, and calibrating an asymmetry of focusing error signal of each calibration position; fitting a best gain curve according to the recorded radial voltages and the best gain values; and deriving a specific best gain value from the best gain curve according to the radial voltages of the lens to calibrate the asymmetry of the focusing error signal of the lightscribe disc. 
         [0012]    In order to achieve the aforementioned objectives, the steps of a method according to an exemplary embodiment comprise: placing a lightscribe disc in an optical disc drive without rotating the lightscribe disc; moving a pickup head to a control feature zone of the lightscribe disc and keeping it still; utilizing different or equal-difference predetermined radial voltages to move a lens to a calibration position to perform focusing to find a focusing error signal and adjust a level of the focusing error signal; searching for a best gain value, correcting an asymmetry of the focusing error signal, and recording a radial voltage and a best gain value of the calibration position; checking the predetermined calibration positions, and fitting a best gain curve according to the recorded radial voltages and best gain values; and reading a control feature zone, and deriving a specific best gain value from the best gain curve according to a radial voltage of driving a lens to calibrate an asymmetry of the focusing error signal of the lightscribe disc. 
         [0013]    Another exemplary method for calibrating a focusing error signal of a lightscribe disc comprises: placing a lightscribe disc in an optical disc drive without rotating the lightscribe disc; moving an optical pickup head to a control feature zone of the lightscribe disc and keeping it still; utilizing a predetermined radial voltage to move a lens to a calibration position to perform focusing to find a focusing error signal and adjust a level of the focusing error signal; searching for a best gain value, correcting an asymmetry of the focusing error signal, and recording a radial voltage and a best gain value of the calibration position; checking if a predetermined number of calibration positions is reached; when the predetermined number of calibration positions is not reached, fitting a best gain curve according to the recorded radial voltages and best gain values; and reading a control feature zone, and deriving a specific best gain value from the best gain curve according to a radial voltage of driving a lens to calibrate an asymmetry of the focusing error signal of the lightscribe disc. 
         [0014]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a diagram of a pickup head reading a label side of a lightscribe disc according to the prior art. 
           [0016]      FIGS. 2(   a ) to  2 ( d ) are diagrams of a pickup head calibrating a focusing error signal at calibration positions according to an embodiment of the present invention. 
           [0017]      FIG. 3  is a diagram of calibrating a focusing error signal in a control feature zone according to an embodiment of the present invention. 
           [0018]      FIG. 4  is a diagram of a focusing error signal generated during a calibration process according to an embodiment of the present invention. 
           [0019]      FIG. 5  is a diagram of a calibrated best gain curve according to an embodiment of the present invention. 
           [0020]      FIG. 6  is a diagram of a best fitting gain value curve according to an embodiment of the present invention. 
           [0021]      FIG. 7  is a flowchart of a focusing error signal calibration method of a lightscribe disc according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    To achieve the aforementioned goals, the methods adopted and the effects thereof are illustrated as follows with exemplary embodiments in accordance with figures. 
         [0023]    Please refer to  FIGS. 2(   a ) to  2 ( d ), which are diagrams of a pickup head calibrating a focusing error signal at calibration positions according to an embodiment of the present invention. As shown in  FIG. 2(   a ), when calibrating, the pickup head  30  is moved to aim at a focusing reference surface and then stay still, where the focusing reference surface, e.g., a data side of a normal optical disc, is a surface capable of being focused. In this exemplary embodiment, the focusing reference is a control feature zone  32  of the lightscribe disc  31 , and the lightscribe disc  31  is poised still without rotating, such as to prevent the data zone and non-data zone, which have different reflection rates, from passing alternately to interfere with the calibration. The pickup head  30  further utilizes a radial voltage Vt to drive the lens  33  to move along a radial direction T parallel to the label side of the lightscribe disc  31  to a calibration position. Next, the focusing voltage Vf is utilized to drive the lens  33  to move along a focusing direction F vertical to the label side of the lightscribe disc  31 . As shown in  FIG. 2(   b ), at the calibration position, as the focusing voltage Vf increases with time, the lens  33  is driven to move along the focusing direction F such that the focus point of the projecting light from the lens  33  nears and penetrates the control feature zone  21  to perform focusing. As shown in  FIG. 2(   c ), after the lens  33  finishes focusing at the calibration position, the pickup head  30  will generate an asymmetric focusing error S-curve; a positive half period magnitude H of the focusing error signal is not equal to a negative half period magnitude h of the focusing error signal, and, as a level of the focusing error signal is shifted, a bias voltage of the internal circuits should be adjusted to make the focusing error signal take the level L as reference. As shown in  FIG. 2(   d ), a gain G is derived according to the positive period and the negative period (which have different magnitudes) of the focusing error signal, the negative period magnitude h is multiplied with the gain G to calibrate, such that the positive period magnitude H equals the negative period magnitude h multiplying the gain G, i.e., H=h×G, to derive a best gain value G of the calibration position to finish calibration of the calibration position. 
         [0024]    Please refer to  FIG. 3 ,  FIG. 4  and  FIG. 5  simultaneously.  FIG. 3  is a diagram of calibrating a focusing error signal in a control feature zone according to an embodiment of the present invention,  FIG. 4  is a diagram of the focusing error signal generated during the calibration process, and  FIG. 5  is a diagram of the calibrated best gain curve. As shown in  FIG. 3 , in this embodiment, the control feature zone  32  of the lightscribe disc  31  is utilized as the focusing reference surface. A radial width of the control feature zone is 650 μm. In the process of calibration, the pickup head  30  stays still to aim at the control feature zone  32 ; different radial voltages Vt are utilized in the pickup head  30  to drive the lens  33  to multiple distributed calibration positions. In this embodiment, five calibration positions Vt 1 , Vt 2 , Vt 3 , Vt 4  and Vt 5  are exploited to move the lens  33  to five calibration positions which have an interval of 150 μm. Alight examiner  34  is utilized to receive points  35 , which are reflected via the control feature zone  32  from a laser ray, to generate the focusing error signal. 
         [0025]    As shown in  FIG. 4 , referring to the calibration process of the focusing error signal of one single calibration position shown in  FIG. 2 , focusing is performed for each calibration position to derive S-curves of focusing error signals corresponding to each calibration position. Each S-curve has a positive half period magnitude (H 1 , H 2 , H 3 , H 4  and H 5 ) and a negative half period magnitude (h 1 , h 2 , h 3 , h 4  and h 5 ), and the level L is adjusted to make the reference of each S-curve identical. As shown in  FIG. 5 , a best gain value G for each calibration position is searched to adjust the asymmetric focusing error signal of the S-curve of each calibration position, to make the positive half period magnitude equal the negative half period, i.e., H 1 =h 1 ×G 1 , H 2 =h 2 ×G 2 , H 3 =h 3 ×G 3 , H 4 =h 4 ×G 4 , H 5 =h 5 ×G 5 . Radial voltages Vt 1 , Vt 2 , Vt 3 , Vt 4 , Vt 5  and the corresponding best gain values G 1 , G 2 , G 3 , G 4 , G 5  of each calibration position are recorded. 
         [0026]      FIG. 6  is a diagram of a best fitting gain value curve. Utilizing the fact that the recorded radial voltages Vt 1 , Vt 2 , Vt 3 , Vt 4 , Vt 5  of each calibration position are horizontal coordinates, and the corresponding best gain values G 1 , G 2 , G 3 , G 4 , G 5  of each calibration position are vertical coordinates, the best gain curve of the pickup head can be fitted. When the pickup head utilizes an arbitrary radial voltage Vtn to drive the lens to move a distance to perform focus servo, a corresponding best gain value Gn can be derived from interpolation or exploration of the best gain curve according to the radial voltage Vtn, such that a positive period magnitude equals a negative period magnitude of the focusing error signal S-curve after calibration (the focusing error signal S-curve is asymmetric before calibration), to achieve the goal of calibrating the symmetry of the focusing error signal. 
         [0027]    Please refer to  FIG. 7 , which is a flowchart of focusing error signal calibration method of a lightscribe disc according to an embodiment of the present invention. The steps of calibrating symmetry of the focusing error signal with a best gain curve of the present invention are detailed as follows: starting the calibration process in step R 1 , placing a lightscribe disc in an optical disc drive without rotating the lightscribe disc, the lightscribe disc aiming at a pickup head; in step R 2 , moving an optical pickup head to a control feature zone of the lightscribe disc and keeping it still; in step R 3 , utilizing predetermined radial voltages to move a lens to a calibration position, where the predetermined radial voltages can be different voltages or equal-difference voltages for deriving distributed calibration positions; in step R 4 , performing focusing at the calibration positions to find focusing error signal S-curve; in step R 5 , adjusting levels of the focusing error signal S-curve; in step S 6 , searching for a best gain value, correcting an asymmetry of focusing error signal to make a positive half period magnitude equal to a negative half period magnitude, and recording a radial voltage and a best gain value of the calibration position; in step R 7 , checking if a predetermined number of calibration positions are recorded; if the predetermined number of calibration positions is not reached, going back to step R 3 ; if the predetermined number of calibration positions is reached, proceeding to step R 8 . 
         [0028]    In step R 8 , fitting a best gain curve according to the recorded radial voltages and best gain values; in step R 9 , rotating the lightscribe disc and moving the pickup head to read a control feature zone; in step R 10 , deriving a specific best gain value from the best gain curve according to a radial voltage of driving a lens with interpolation or extrapolation; in step R 11 , calibrating an asymmetry of the focusing error signal of the lightscribe disc to make the positive half period magnitude equal to the negative half period magnitude; and finally, in step R 12 , after reading the control feature zone, ending the calibration. 
         [0029]    Therefore, the focusing error signal calibration method for a lightscribe disc of the present invention can fit a best gain curve with interpolation or extrapolation via best gain values derived from calibrating an asymmetry of a focusing error signal, and can also derive a best gain value for a symmetric focusing error signal to improve a symmetry of the focusing error signal and avoid bad focusing characteristics in the control feature zone, enhancing focusing servo of the control feature zone and therefore lowering the opportunity of focusing failure, resulting in reading data in the control feature zone correctly to scribe a label properly. 
         [0030]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.