Abstract:
A multimode filter in an optical storage device is used for filtering an error signal and outputting a frequency signal. The multimode filter comprises a CLV and CAV mode filter and a switch. The CLV mode filter is used for filtering the error signal and extracting a narrow bandwidth signal, whereas the CAV mode filter is used for filtering the error signal and extracting a wide bandwidth signal. The switch is used for selecting either the CLV mode filter or the CAV mode filter according to the recording mode of the optical storage device.

Description:
REFERENCE TO RELATED APPLICATION  
         [0001]    The present application claims priority from Taiwan Application No.090108828, entitled “Multimode filter,” filed on Apr. 12, 2001.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to a multimode filter, and more particularly to a multimode filter in an optical storage device.  
           [0004]    2. Description of the Related Art  
           [0005]    Optical storage devices are widely utilized in many fields. CD-ROMs have become basic devices on personal computers (PCs). With the popularity of CD-Rs and CD-RWs, it is more convenient for users to record data or music. Conventionally, the recording mode of optical disk is constant linear velocity (CLV). As a result, recordable optical devices, for example CD-R, CD-RW, DVD-R, DVD-RW, DVD+RW or DVD-RAM, are recorded with CLV mode. It does not matter whether the position of the pick-up head of the optical device is at the outer or inner side of the optical disk, the speed of recording data into the disk is always kept constant. When the optical storage device operates with high rotational speed, the speed of data recording into the disk is limited by the rotational speed of the spindle motor. Such bottleneck can be resolved by using constant angular velocity (CAV) recording mode. For example, the rotational speed of the inner side of the disk is 8000 rpm when the optical storage device records data with 16× CLV mode. If the recording speed of the optical storage device is changed to 16× CAV mode at the inner side of the disk, the rotational speed at the outer side of the disk becomes 40×.  
           [0006]    To label the location of the data in the disk, the tracks of the disk are divided into many sectors units. These sector units include minutes, seconds and blocks. A latest development in labeling is to form the edges of the tracks with a shape like a wave profile. As a result, when the disk is rotated, the reflected laser beam by the edges of the tracks with a shape like a wave profile forms a wobble signal. By using the frequency modulation (FM) the absolute time in pre-groove (ATIP) is encoded in the wobble signal. Thus, when the optical storage device receives the reflected signals from the reflected laser beam, it extracts the wobble signal from the reflected signals and decodes the wobble signal to attain the ATIP, thereby the position of the pick-up head is known.  
           [0007]    [0007]FIG. 1A is the profile of a recordable disk. Disk  100  is a recordable disk, for example CD-R, CD-RW, DVD-R, DVD-RW, DVD+RW or DVD-RAM. The disk  100  is grooved with tracks  130  for data storage. FIG. 1B shows the detailed structure of tracks  130 . As shown in FIG. 1B, the edges  135  of the tracks  130  are formed with a shape like a continuous wave profile. The position of the pick-up head is known by extracting the wobble signal and decoding the ATIP. Thus, extracting the wobble signal is critical for identifying the position of the pick-up head. FIG. 1C shows the relationship of a photo-detector (PD) and the tracks  130 . The PD includes a sensor  171 , a sensor  173 , a sensor  175 , and a sensor  177 . The spot area  150  of the laser is shown by dotted line. The sensors  171 ,  173 ,  175 ,  177  can sense the laser simultaneously. When the optical storage device is using a CLV mode for recording, the frequency of the wobble signal is 22.05 KHz±1 KHz, where the center frequency is fixed to 22.05 KHz. Since the ATIP is encoded in the wobble signal, the ATIP can be known by extracting the wobble signal. A common way to extract the wobble signal is using the main beam push pull signal, i.e. the tracking error signal. If the values sensed by the sensors  171 ,  173 ,  175 ,  177  are A, B, C, and D respectively, then the main beam push pull signal is (A+D)−(B+C). By filtering the main beam push pull signal with a CLV mode filter of center frequency of 22.05 KHz, a narrow bandwidth wobble signal can be extracted.  
           [0008]    Turning now to FIG. 2, it shows a device for extracting a wobble signal when the optical storage device is using CLV mode. By using a CLV mode filer  270  to filter a main beam push pull signal  250 , the wobble signal  255  is extracted. Since the center frequency of the CLV mode filter  270  is 22.05 KHz when the speed of the optical storage device is 1×, the frequency of the narrow bandwidth wobble signal  255  extracted from the CLV mode filter  270  is also 22.05 KHz.  
           [0009]    When the speed of the optical storage device with CLV mode is 1×, since the frequency of the wobble signal is fixed to 22.05 KHz±1 KHz, the wobble signal can be extracted by a filter with a fixed center frequency. However, when the optical storage device is using CAV mode, the frequency of the wobble signal increases with the linear velocity. The frequency of the wobble signal increases from 22.05 KHz to 22.05×2.505 KHz. In other words, the filter with fixed center frequency is not suitable for CAV mode. Since the frequency of the wobble signal is changed with the radial position of the pick-up head for CAV mode, a method is proposed for adjusting the center frequency of the filter dynamically to extract the wobble signal. However, the method can only be realized when the frequency of the wobble signal is known. The moving speed of the pick-up head is quite high when it is searching data; it is difficult to know the exact frequency of the wobble signal at the moment. Thus, the idea of dynamically adjusting the center frequency of a filter is not realized yet and becomes a bottleneck of developing CAV technology.  
         SUMMARY OF THE INVENTION  
         [0010]    It is therefore an object of the present invention to provide a multimode filter to enhance the access speed of an optical storage device with CLV or CAV mode.  
           [0011]    According to the object of the invention, a preferred embodiment is described as follows:  
           [0012]    A multimode filter comprises a CLV mode filter, a CAV mode filter, and a switch. The center frequency of the CLV mode filter is fixed, and the CLV mode filter is used to extract a wobble signal for an optical storage device with CLV mode. The CAV mode filter comprises a high pass filter and a low pass filter. The CAV mode filter has a wide range of the operational frequency bandwidth and is used to extract a wobble signal for an optical storage device with CAV mode. By using the CLV mode filter, an error signal is filtered and a narrow bandwidth wobble signal is extracted. By using the CAV mode filter, an error signal is filtered and a wide bandwidth wobble signal is extracted. The switch is permitted to select either the CLV mode filter or the CAV mode filter according to the recording mode of the optical storage device.  
           [0013]    Other objects and advantages of the present invention will become apparent from the following descriptions, taken in connection with the accompanying drawings, wherein, by way of illustration and example, an embodiment of the present invention is disclosed. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0014]    The following detailed description, given by way of examples and not intended to limit the invention to the embodiments described herein, will be best understood in conjunction with the accompanying drawings, in which:  
         [0015]    [0015]FIG. 1A is a diagram showing the profile of a recordable disk;  
         [0016]    [0016]FIG. 1B is a diagram showing the detail structure of tracks;  
         [0017]    [0017]FIG. 1C is a diagram showing the relationship of a photo-detector and the tracks  130 ;  
         [0018]    [0018]FIG. 2 is a diagram showing device for extracting a wobble signal when the optical storage device is using CLV mode;  
         [0019]    [0019]FIG. 3 is a block diagram of a multimode filter according to a preferred embodiment of the invention.  
         [0020]    [0020]FIG. 4 is a block diagram of the CAV mode filter in FIG. 3.  
         [0021]    [0021]FIG. 5 is a diagram showing the distribution of servo error signals, radio frequency signals, and bandwidth of a CAV mode filter. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    Detailed descriptions of the preferred embodiment are provided herein. It is to be understood, however, that the present invention may be embodied in various forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching one skilled in the art to employ the present invention in virtually any appropriately detailed system, structure or manner.  
         [0023]    The idea of the present invention is using a CAV mode filter with wide bandwidth to extract a wobble signal. The bandwidth of the CAV mode filter can cover the frequency range of the wobble signal. Thus, no matter the pick-up head is located at the inner or outer side of a disk, the frequency range of the wobble signal is within the bandwidth of the CAV filter, so that the wobble signal can be extracted efficiently.  
         [0024]    Turning now to FIG. 3, it shows a block diagram of a multimode filter according to a preferred embodiment of the invention. The multimode filter  300  comprises a CLV mode filter  270 , a CAV mode filter  370 , and a switch  390 . Since the center frequency of the CLV mode filter  270  is 22.05 KHz, the CLV mode filter  270  is used to filter a main beam push pull signal  250  and extract a narrow bandwidth wobble signal  255  when the speed of the optical storage device is 1× using CLV mode. Since the operational frequency bandwidth of the CAV mode filter  370  is 22.05˜55 KHz, the CAV mode filter  370  is used to filter the main beam push pull signal  250  and extract a wide bandwidth wobble signal  355  when the optical storage device is using CAV mode. In other words, if the optical storage device is operated in CLV mode, a switch  390  is then connected to the CLV mode filter  270  to output the narrow bandwidth wobble signal  255 ; if the optical storage device is operated in CAV mode, a switch  390  is then connected to the CAV mode filter  370  to output the wide bandwidth wobble signal  355 . Thus, the optical storage device can change the operational frequency range of the multimode filter  300  according to the recording mode.  
         [0025]    Referring now to FIG. 4, it shows the block diagram of the CAV mode filter in FIG. 3. As shown in FIG. 4, the CAV mode filter  370  comprises a high pass filter  410  and a low pass filter  430 . The high pass filter  410  filters the low frequency content of the main beam push-pull signal  250  and generates an intermediate signal  450 , then feeds the intermediate signal  450  into the low pass filter  430  to filter the high frequency content and extracts the wide bandwidth wobble signal  355 .  
         [0026]    Further, FIG. 5 shows the Bode plot of servo error signals, radio frequency signals, and bandwidth of CAV mode filter. The servo error signals  510  are used for compensating the axial wobble and radial runout when the disk is rotated. Since the axial wobble and radial runout are both physical phenomena and usually results in low frequency error signals. The frequency of the servo error signals  510  are low, for example lower than 3 KHz, and the frequency does not change with the speed of the optical storage device. On the other hand, with the frequency response  530  of the CAV mode filter  370 , the low corner frequency f L  can be attained and represented as M×22.05 KHz and the high corner frequency can be attained and represented as f H  is M×55 KHz, wherein the M is the CAV speed of the optical storage device. Thereby, the CAV mode filter  370  can extract the wide bandwidth wobble signal  355  by adjusting the bandwidth of the CAV mode filter  370  according to the CLV speed of the optical storage device. The data recorded on the disk is RF (Radio Frequency) signal recorded on the tracks  130 . There are different length pits and lands formed on the tracks according to the data. The reflection rate of pits is different from lands, it is noted that the data is recorded with the variations of the reflection lights. After transforming the reflection lights, the RF signals  570  are attained. Since the frequency of the RF signals  570  are related to the rotational speed of the disk, the frequency of the RF signals  570  changes with the speed of the optical storage device. That is, the frequency of the RF signals  570  ranges from N×196 KHz to N×720 KHz, wherein the N is the CLV speed of the optical storage device.  
         [0027]    As shown in FIG. 5, the operational frequency domain  530  ranges from M×22.05 KHz to M×55 KHz. Relatively, the frequency of the servo error signals  510  is low (lower than 3 KHz). Thus, the influence of the servo error signals  510  on the wobble signals  355  extracted from the CAV mode filter  370  is limited. Take the second order CAV mode filter  370  for example, the capability of noise depression over the servo error signals is −40 dB×log (22.05/3)=−34 dB. When the speed of the optical storage device increases, the frequency response  530  of the CAV mode filter  370  is enhanced, however the frequency of the servo error signals  510  does not change. In this way, capability of noise depression over the servo error signals is optimized. Thus, the influence of the servo error signals on wobble signals almost can be neglected when the speed of the optical storage device is high. Further, with the lowest frequency of the RF signals f RFL , N×196 KHz and a high corner frequency of the CAV mode filter  370  f H  is M×55 KHz, the capability of noise depression of the CAV mode filter  370  over the RF signals can be attained as −40 dB×log (196/55)=−22 dB. The resulted the capability of noise depression indicates that the RF signals do not interfere with the wobble signal. Moreover, when the speed of the optical storage device increases, the frequencies of the CAV mode filter and the RF signals are enhanced synchronously. The capability of noise depression of the CAV mode filter  370  is not affected by the difference speed used by the optical storage device.  
         [0028]    It is disclosed above that the multimode filter of the present invention can be used for non-wobble signal depression, such as servo error signals or RF signals. In addition, the center frequency can be adjusted dynamically without outside frequency. Therefore, the invention is easy to implement, renders high performance and furthermore it meets the design requirement of the new optical storage device.  
         [0029]    While the invention has been described with reference to various illustrative embodiments, the description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to those skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as may fall within the scope of the invention defined by the following claims and their equivalents.