Abstract:
A method for correcting disc type determination is disclosed. The method detects a tracking error signal, and verifies accuracy of a disc type determination according thereto.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method for correcting disc type determination, and in particular to a method that corrects erroneous disc type determination immediately. 
   2. Description of the Related Art 
   To quickly read information stored on a disc, optical storage devices must determine disc type quickly and precisely to allow the pickup head to commence reading. 
   When a disc is inserted into an optical storage device, focusing servo adjusts the focus position of a pickup head and determines the disc type preliminary. Before the operation of ‘track on’ is executed by the tracking servo, a tracking error signal is detected and thereafter normalized. The normalization multiplies the tracking error signal by an appropriate gain to produce a normalized tracking error signal to be processed in subsequent calculation. After the normalization, the optical storage device begins to read the disc with a CD laser beam for CD or a DVD laser beam for DVD. Conventionally, the optical storage device discovers errors in disc type determination when the pickup head cannot read the information stored in the disc. Thereafter the optical storage devices will try different setting modules to make sure the errors (i.e. trial and error method). 
   However, this conventional method discovers errors belatedly, wasting much time in correcting the mistake. 
   SUMMARY OF THE INVENTION 
   Thus, there is a need for a more effective method for correcting disc type determination. The method of the present invention detects a tracking error signal and accuracy of the disc type determination according to the amplitude of the tracking error signal. 
   The present invention detects erroneous disc type determination immediately and corrects the disc determination in an effective way. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
       FIG. 1  shows the tracking error signal produced by detection of a CD by a CD laser beam; 
       FIG. 2  shows the tracking error signal produced by detection of a DVD by a DVD laser beam; 
       FIG. 3  shows the track error signal produced by detection of a DVD by a CD laser beam; 
       FIG. 4  shows the tracking error signal produced by detection of a CD by a DVD laser beam; 
       FIG. 5  is a schematic diagram of a tracking error detection system; 
       FIG. 6   a  shows CD laser beams precisely incident on a track of a CD; 
       FIG. 6   b  shows CD laser beams imprecisely incident on a track of a CD; 
       FIG. 6   c  shows CD laser beams imprecisely incident on a track of a CD; 
       FIG. 7   a ,  7   b  and  7   c  show a process of detecting a DVD with a CD laser beam; 
       FIG. 8  is a flowchart of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   When a disc is inserted into an optical storage device, the optical storage device performs focusing and tracking. In the focusing process, a pickup head nears the disk, projects laser beams onto the disk, and receives reflected laser beams from the disk. Thus, the disk type can be identified by the focused locations of the laser beams on the disk. Then, before the tracking process (i.e. track on), the optical storage device detects and multiplies a tracking error signal with a gain to normalize the tracking error signal. 
     FIG. 5  shows a tracking error signal detection system. A light beam  600  is split into three beams  601 ,  602  and  603  by a diffractometer  514 . The three beams  601 ,  602  and  603  are reflected by a reflector  512 , then pass through a condenser lens  511 , and reach the disc  520 . The disc  520  reflects the three beams  601 ,  602  and  603  through the condenser lens  511 , are reflected by the reflector  512 , split by the beam splitter  515 , to contact the optical sensors  531 ,  532 , and  533  respectively. As shown in  FIG. 6   a , when the beams  601 ,  602  and  603  are irradiated on the disc, the beams  601  and  602  are for tracking (i.e. detected as the tracking error signal); while the beam  603  is for focusing and reproducing. Because the beam reflected from a track of the disc is weaker than that from a blank area thereof, when the beams  601 ,  602  and  603  are precisely incident on a track  610 , the beams  601  and  602  contact the track  610  with the same contact area. Thus, optical sensors  531  and  532  receive the same amount of light from the reflected beams  601  and  602 . The voltage signals output from the optical sensors  531  and  532  are the same. A differential amplifier  540  subtracts the voltage signal from the optical sensor  531  by the voltage signal from the optical sensor  532  and produces a tracking error signal with amplitude of zero. As shown in  FIGS. 6   b  and  6   c , when the beams  601  and  602  are imprecisely incident on the track  610 , the beams  601  and  602  contact the track  610  with different contact areas. The optical sensors  531  and  532  receive unequal amounts of light and output different voltage signals. The differential amplifier  540  subtracts the voltage signal of the optical sensor  531  from the voltage signal of the optical sensor  532 , producing a tracking error signal with positive or negative amplitude. 
   When the disc type determination in the focusing process is correct, the pickup head detects the disc with its corresponding light beam. For example, as shown in  FIG. 1 , when the pickup head detects a CD with a CD laser beam, the differential amplifier produces a sinusoidal wave signal. While as shown in  FIG. 2 , when the pickup head detects a DVD with a DVD laser beam, the differential amplifier produces a triangular wave signal. 
   When the disc type determination in the focusing process is incorrect, the differential amplifier produces an abnormal tracking error signal. As shown in  FIG. 3 , when the pickup head detects a CD with a DVD laser beam, the amplitude of the tracking error signal is approximate to zero. As shown in  FIG. 4 , when the pickup head detects a DVD with a CD laser beam, the tracking error signal has a smaller amplitude than that produced when the pickup head detects a DVD with a DVD laser beam. 
     FIGS. 1 ,  2 ,  3  and  4  mentioned above share the same scale along the horizontal and vertical axes. The amplitude of the tracking error signal in  FIG. 1  is about 1.47V. The amplitude of the tracking error signal in  FIG. 2  is about 1.31V. The amplitude of the tracking error signal in  FIG. 3  is about 0.53V. The amplitude of the tracking error signal in  FIG. 4  is about 0.64V. Thus, the above values can take as reference values to confirm the accuracy of the disc type determination. For an example, the amplitude of the tracking error signal is approximate to zero when the pickup head detects a DVD with a CD laser beam as shown in  FIG. 3  is due to the diameter of a CD laser beam (about 0.8 μm) exceeding a track pitch of a DVD. Thus, as shown in  FIGS. 7   a ,  7   b  and  7   c , when the track  620  slips, the beams  601  and  602  have the same contact area contacting the tracks  620  at any time. When the optical storage device performs tracking, the beams  601  and  602  have the same reflective brightness, the optical sensor  531  and  532  receive the same amount of light, and the differential amplifier produces tracking error signal with almost zero amplitude. That is, the amplitude of the tracking error signal is smaller than a regular threshold value, the inserted disc can be recognized as a DVD. Before the servo system entering the tracking process, the optical storage device usually normalizes the tracking error signal to correct the beforehand variation (to support various discs) and also for subsequent calculation. The normalization steps are low-pass filtering of the tracking error signal and multiplying by a gain. The time for normalization process should be limited to a certain extent. 
   If the amplitude of the tracking error signal is quite small, the low-pass filtered signal is also. Thus comparing the low-pass filtered signal with a reference threshold, the optical storage device can identifies whether there is an erroneous determination. In other words, if the low-pass filtered signal is less than the reference threshold, the disc type determination is incorrect. 
   As well, comparing a real gain produced in the normalization with an upper threshold gain, the optical storage device also identifies whether there has been an erroneous determination. If the real gain value exceeds the upper threshold gain too much, the disc type determination is incorrect. 
   The tracking error signal with tiny amplitude has to multiply the gain repeatedly when normalization. Thus if the normalization time is too long, the optical storage device also determines whether there has been an erroneous disc type determination. 
     FIG. 7  is a flowchart of the present invention. The optical storage device detects the tracking error signal (S 1 ), low-pass filters the tracking error signal, detects the accuracy of the low-pass filtered signal, and identifies whether there has been an erroneous disc type determination. 
   In conclusion, the present invention provides effective identification of erroneous disc type determination immediately and corrects the mistake. 
   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.