Abstract:
A method for determining runout disc is disclosed. The method comprises: focusing on a focal point on a disc; driving the disc to spin the disc; generating a crossover signal according to a track of the disc crossing the focal point; and determining that the disc is a runout disc when the frequency of the crossover signal exceeds a pre-determine value.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to runout detection for optical discs, and in particular, to methods for detecting a runout disc in an optical apparatus. 
     2. Description of the Related Art 
     During the manufacturing stage of an optical disc, a concentric circling track is formatted for data storage, whereby data are written along the track. When a reading operation is performed, the track is followed confirming that the written data are locatable and accessible. Generally, distances between adjacent tracks, the track pitch, vary with different optical disc standards. For a compact disc (CD), the average track pitch is 1.6 μm. Alternatively, a digital versatile disc (DVD) has an average track pitch of 0.74 μm. 
     When manufacturing an optical disc, a motor may dig a central hole on the optical disc to facilitate installation. Theoretically, the central hole should be exactly in the center of the concentric circle. In practice, errors are unavoidable. The optical disc is made of polycarbonate, modeled and shaped under certain thermal conditions. Errors of deviation from the center of the central hole of the optical disc may range from 10 μm to 100 μm. The deviation is also referred to as an excentricity or a runout, which may cause track seeking failure where data stored on the optical disc are consequently inaccessible. It is therefore desirable to determine whether an optical disc has runout before reading data from it. 
     BRIEF SUMMARY OF THE INVENTION 
     An exemplary embodiment of a runout detection method is provided. Firstly, a focal point is formed on an optical disc, and the optical disc is driven to spin. While the optical disc spins, a crossover signal is generated when a track crosses the focal point. The optical disc is deemed as a runout disc when the frequency of the crossover signal exceeds a predetermined value. 
     In an alternative embodiment of a runout detection method, the optical disc is first driven to spin. Next, a focal point is locked onto the optical disc, such that a reflection signal is received from the focal point. A crossover frequency of occurrences when a track crosses the focal point is determined. Thereby, the optical disc is deemed as a runout disc when the crossover frequency exceeds a predetermined value. A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The 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 an embodiment of an optical disc driver according to the invention; 
         FIG. 2  is a waveform diagram of the crossover signal according to the invention; and 
         FIG. 3  is a flowchart of a runout detection method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
       FIG. 1  shows an embodiment of an optical disc driver according to the invention. The optical disc driver comprises a motor  11 , a focus generator  13  and a sensor  16 . The motor  11  comprises a central axis  111  for installation of an optical disc  12  by its central hole  121 , such that the optical disc  12  is driven to spin. When the motor  11  spins the optical disc  12 , the focus generator  13  generates a focal point  14  on the optical disc  12 . The focal point  14  is located within ranges of data containing blocks on the optical disc  12  so that data on the track can be readable. A reflection signal  15  is reflected from the optical disc  12  to the sensor  16 . The reflection signal  15  is typically an RP digital signal comprising stored information on the optical disc  12 . When the track crosses the focal point  14 , a crossover signal is induced as a part of the reflection signal  15 . 
     If the optical disc  12  is a runout disc, which means there is an offset between the central hole  121  and the center of the concentric circle, the optical disc  12  spins elliptically. Since the focal point  14  is locked onto a static position, the track may repeatedly cross the focal point  14  while the optical disc  12  is elliptically spinning, and the crossover signal is thereby obtained by a sensor  16  along with the reflection signal  15 . 
       FIG. 2  is a waveform diagram of the crossover signal according to the invention. When the track crosses the focal point  14 , a crossover signal  21  is generated. A higher crossover signal  21  frequency indicates tight waveforms, and conversely, a lower frequency indicates loose waveforms. Thus, the density of the waveform is proportional to the crossover signal  21  frequency, which can be a basis for detecting a runout disc. 
     Experiments show that when the crossover signal  21  frequency lies between 15 to 30 KHz, the optical disc  12  is deemed a normal disc. When a frequency of 30 KHz or even up to 50 KHz is detected, the optical disc  12  is confirmed to be a runout disc. The crossover signal  21  frequency can be used to effectively detect a runout disc. 
     In the embodiment of the invention, the crossover signal  21  is estimated and generated every 8.3 ms to render the waveform in  FIG. 2 ; however, it is not limited in the invention. 
       FIG. 3  is a flowchart of a runout detection method according to the invention. Firstly, in step S 1 , the motor  11  drives the optical disc  12  to spin. In step S 2 , the position of focal point  14  is locked onto a static position while the optical disc  12  is spinning. A crossover signal is generated each time the data track crosses the focal point  14 . In step S 3 , a sensor  16  is used to capture the crossover signal, and analysis thereof is performed in step S 4 . In step S 5 , it is determined whether the frequency of the crossover signal exceeds 30 KHz. If so, step S 51  is processed, in which the optical disc  12  is deemed as a runout disc. Otherwise, the optical disc  12  is deemed as a normal disc in step S 52 . In an alternative embodiment, the crossover signal is generated when a track crosses the focal point, and a means for receiving the crossover signal is provided to receive the crossover signal before further procedures. Specifically, the reflected digital signals are collected from the focal point as the crossover signal when the track crosses the focal point. Frequency of the crossover signal is counted. The occurrence when the frequency falls within a predetermined range is also counted. The optical disc is deemed as a runout disc when the occurrence is less than a predetermined value. 
     In summary, the embodiments of the invention introduce an effective method to determine whether an optical disc is a runout disc, and accordingly, a particular process can be further performed to avoid data access failures. The application for the invention is submitted due to high industrial value. 
     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. 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.