Abstract:
The present invention discloses a method for determining disc type. Perform a focus servo process by emitting a light beam with a specific wavelength and a focus error signal is generated. According to the overshooting value after the focus error signal forming an S curve, the disc type is determined.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method for determining disc type, and in particular to a method for determining disc type with a focus error signal.  
           [0003]    2. Description of the Related Art  
           [0004]    To quickly read information stored on disc, optical storage devices must determine disc type quickly and precisely to allow the pickup head to read data on the disc.  
           [0005]    Conventional methods for this are disclosed in Taiwan patent No. 397969 “Disc discrimination device and disc discrimination method”. Protection layer thickness on a CD is about 1.2 mm, and on a DVD is about 0.6 mm. Conventionally, disc type is determined according to the thickness difference therebetween.  
           [0006]    As shown in FIGS. 1 a  and  1   b,  the conventional method emits a light beam toward the disc from a light source  100 . An optical sensor  120  receives the light beam reflected from the disc in segments  121  and  122 . Distance between surface of the protection layer and the recording layer is calculated.  
           [0007]    As shown in FIG. 1 a,  a CD  110  reflects the light beam to the optical sensor  120  such that segment  122  receives more light intensity than segment  121 . By comparing voltage difference from segments  121  and  122 , a thickness for protection layer  110  is obtained.  
           [0008]    As shown in FIG. 1 b,  a DVD  115 , with protection layer thinner than that of the CD  110 , reflects the light beam to the optical sensor  120  such that segment  121  receives more light intensity than segment  122 . By comparing voltage difference transmitted from segments  121  and  122 , a thickness for protection layer  115  is obtained. Thus, the conventional method determines disc type.  
           [0009]    The conventional method, however, requires additional optical sensors, increasing costs, taking more time, and complicating process.  
         SUMMARY OF THE INVENTION  
         [0010]    Thus, there is a need for a simpler and more economical method for determining disc type.  
           [0011]    The method for determining disc type comprises performing a focus servo process by emitting a light beam with a specific wavelength to the disc for generating a focus error signal. According to an overshooting value of the focus error signal after the focus error signal forming an S curve, disc type is determined. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:  
         [0013]    [0013]FIG. 1 a  is a schematic diagram of a conventional method for determining disc type with a CD in an optical drive;  
         [0014]    [0014]FIG. 1 b  is a schematic diagram of a conventional method for determining disc type with a DVD in an optical drive;  
         [0015]    [0015]FIG. 2 shows the reading structure of a pickup head;  
         [0016]    [0016]FIG. 3 a  shows the reflecting light path that focus point is on the recording layer;  
         [0017]    [0017]FIG. 3 b  shows the reflecting light path that focus point is in the rear of the recording layer;  
         [0018]    [0018]FIG. 4 a  shows a light spot projected on an optical sensor when focus point is in the rear of the recording layer;  
         [0019]    [0019]FIG. 4 b  shows a light spot projected on an optical sensor when the pickup head arrives the focus position;  
         [0020]    [0020]FIG. 4 c  shows a light spot projected on an optical sensor when the focus point is in front of the recording layer;  
         [0021]    [0021]FIG. 5 shows a focus error signal as a pickup head detects a CD with a CD laser;  
         [0022]    [0022]FIG. 6 shows a focus error signal as a pickup head detects a DVD with a DVD laser;  
         [0023]    [0023]FIG. 7 shows a focus error signal as a pickup head detects a DVD with a CD laser;  
         [0024]    [0024]FIG. 8 is a flow chart of the steps of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    Firstly, a focus servo process is described as follows.  
         [0026]    [0026]FIG. 2 shows the reading structure of a pickup head, comprising a semiconductor laser  200 , a collimator lens  205 , a beam splitter  210 , a quarter-wave plate  220 , a condenser lens  225 , a lens  240 , and an optical sensor  250 . The semiconductor laser  200  emits a light beam through the collimator lens  205 , the beam splitter  210 , the quarter-wave plate  220 , and the condenser lens  225 , to a disc  230 . The disc  230  reflects the light beam from the disc  230  through the condenser lens  225 , the quarter-wave plate  220 , the beam splitter  210 , and a lens  240  to an optical sensor  250 .  
         [0027]    When a disc is loaded into an optical storage device, the pickup head moves near and then away the disc for detecting the precise position of the recording layer. At the precise position, the focus point is on the recording layer. The procedure of finding the precise position of pickup head is called the focus servo process. A signal for optical storage device to determine whether the pickup head is at the precise position or not is called the focus error signal. The focus error signal is described follows.  
         [0028]    [0028]FIGS. 3 a  and  3   b  show the reflecting light path that focus point is on the recording layer and not on the recording layer. As shown in FIG. 3 a,  when the pickup head is at the precise position, the focus point of the light beam  330  is on recording layer  320  of the disc and the reflected beam forms a spot on an optical sensor  310  through a lens  314  and  312 . The spot formed on the optical sensor has equal lengths in x-axis direction and Y-axis direction. As shown in FIG. 4 b,  when the pickup head is at the focus position, the spot is a circular light spot (equal length in X and Y direction) on detection segments  401 ,  402 ,  403 , and  404  of the optical sensor  310 . The x-axis detection segments  402 ,  404  receive the same light intensity as the y-axis detection segments  401  and  403 . Then, a difference amplifier subtracts voltage signals of the detection segments  401  and  403  from voltage signals of the detection segments  402  and  404 , and produces the focus error signal value of zero.  
         [0029]    As shown in FIG. 3 b,  when the focus point of the incident beam  330  is near the recording layer  320  and is in the rear of the recording layer  320  of the disc, the spot formed on the optical sensor has not equal lengths in x-axis direction and Y-axis direction. As shown in FIG. 4 a,  when the pickup head is too close to the disc  320 , the focus point of the incident beam  330  is near the recording layer  320  and is in the rear of the recording layer  320  of the disc. The formed spot is an oval light spot (longer length in Y direction and shorter length in X direction) on detection segments  401 ,  402 ,  403 , and  404  of the optical sensor  310 . The x-axis detection segments  402 ,  404  receive less light intensity than the y-axis detection segments  401 ,  403 . The difference amplifier subtracts the voltage signals of the detection segments  401  and  403  from the voltage signals of the detection segments  402  and  404 , producing a negative focus error signal value.  
         [0030]    As shown in FIG. 4 c,  when the focus point of the incident beam  330  is near the recording layer  320  and is in front of the recording layer  320  of the disc. The formed spot is an oval light spot (longer length in X direction and shorter length in Y direction) on detection segments  401 ,  402 ,  403 , and  404  of the optical sensor  310 . The x-axis detection segments  404 ,  402  receive more light intensity than the y-axis detection segments  401 ,  403 . Thus, the difference amplifier produces a positive focus error signal value after subtracting the voltage signal of the detection segments  401  and  403  from the voltage signal of the detection segments  402  and  404 .  
         [0031]    According to FIGS. 4 c,    4   b,  and  4   a,  when the focus servo process is activated, the pickup head moves from an initial position toward the disc, the value of focus error signal will change from positive to negative When the pickup head is at the initial position (far from the disc), the reflected beam i s weak, and the focus error signal closes to zero. As the pickup head closes to the disc and the focus point is in front of t he recording layer, the focus error signal strengthens, with a positive peak value  511 . As the pickup head continues to near the disc, the value of the focus error signal decreases, reaching zero when the pickup head is in the precise position. As the pickup head continues to near the disc and the focus point is in the rear of the disc, the focus err or signal decreases, first to a negative peak value, then to zero value. The waveform of focus error signal formed by moving the pickup head close to the disc is called S curve.  
         [0032]    As shown in FIG. 5, when the focus servo process uses a CD laser (wavelength 780 nm) to detect a CD, the S curve of the focus error signal first reaches positive peak value  511  and then decreases to a negative peak value. After the S curve of the focus error signal formed and the signal again rising to the zero cross point, the overshooting value  512  of the focus error signal is lower.  
         [0033]    As shown in FIG. 6, when the focus servo process uses a DVD laser (wavelength 650 nm) to detect a DVD, the overshooting value after the focus error signal forming the S curve is higher.  
         [0034]    If use the CD laser to detect the DVD, the overshooting value after the focus error signal forming the S curve is the highest. As shown in FIG. 7, when the focus servo process uses a CD laser (wavelength 780 nm) to detect a DVD, the overshooting value  532  after the focus error signal forming the S curve is the highest.  
         [0035]    Thus, at the start up procedure of an optical storage device, the pickup head performs the focus servo process with a CD laser. If the focus error signal has an obvious overshooting value after the focus error signal forming the S curve, the optical storage device determines the loaded disc is a DVD. If there is no obvious overshooting value after the focus error signal forming the S curve, the optical storage device determines the loaded disc is a CD.  
         [0036]    [0036]FIG. 8 shows the determination process. When a disc is loaded into the optical storage device (step S 1 ), the pickup head performs the focus servo process with a CD laser (step S 2 ). Then, the optical storage device determines the disc type according to the overshooting value after the focus error signal forming the S curve. Finally, the optical storage device uses an appropriate laser (CD laser or DVD laser) to read the determined disc.  
         [0037]    While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art) Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.