Patent Publication Number: US-2010124154-A1

Title: Signal processing devices and signal processing methods

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
     This application claims the benefit of U.S. Provisional Application No. 61/115,963, filed on Nov. 19, 2008 and incorporated herein by reference. 
     BACKGROUND  
     The present invention relates to reading information from an optical storage medium, and more particularly, to signal processing devices and related signal processing methods for dealing with a defect signal associated with defective areas on an optical storage medium (e.g., an optical disc). 
     Optical storage media, such as read-only, recordable, or rewritable optical discs, have become popular data carriers nowadays. In general, the stored data are reproduced from reading a recording layer (i.e., a reflective layer) of an optical storage medium through directing a laser beam with a proper power onto the recording layer and then detecting signals reflected from the recording layer. To protect the recording layer, a protective layer made of, for example, polycarbonate is generally formed on the recording layer. Therefore, the laser beam emitted from a laser diode has to pass through the protective layer before arriving at the recording layer; similarly, the laser beam reflected from the recording layer has to pass through the protective layer before being detected by an optical pickup head. Therefore, the signal quality of the reflected laser beam detected by the optical pickup head is actually affected by the protective layer. However, the optical storage medium, such as an optical disc, might have defective areas due to scratch, dirt, or fingerprint on a surface of the protective layer. 
     Regarding the current high-density optical disc drive (e.g., a Blu-ray disc drive), it is more difficult to do the servo control due to smaller track pitch. Particularly, when there are defective areas on an optical disc, the servo control mechanism, including a focus control loop and a tracking control loop, usually applies inappropriate servo control effort around the beginning position and end position of each defective area, which degrades the data reading performance of the optical disc greatly.  FIG. 1  is a waveform diagram of a defect signal S 1 , a servo output signal (e.g., a tracking servo output TRO or focus servo output FOO) S 2 , and a radio-frequency (RF) signal S 3  which are generated when an optical pickup head of an optical disc drive accesses an optical disc with a defective area formed thereon. In a conventional optical disc drive, a protection means is employed to hold the servo control setting when the defect signal S 1  indicates that the optical pickup head is accessing the defective area. In general, the defect signal S 1  is generated to detect defective areas on the optical disc in a real-time manner, and has a transition from a first logic level (e.g., ‘0’) to a second logic level (e.g., ‘1’) to indicate the beginning point of a detected defective area, and another transition from the second logic level (e.g., ‘1’) to the first logic level (e.g., ‘0’) to indicate the end point of the detected defective area in an ideal case. However, there may be a delay between the timing when the defect signal S 1  has a rising edge (i.e., a transition from the first logic level to the second logic level to indicate the beginning point of a defective area) and the timing when the optical pickup head is located at the actual beginning point of the defective area. Due to different reflection characteristics between a normal area and a defective area on the optical disc, the servo control mechanism, as one can see, applies a first servo control effort FOO 1 /TRO 1  before the defect signal S 1  has a rising edge at time T 1 , and applies a second servo control effort FOO 2 /TRO 2  after the defect signal S 1  has a falling edge at time T 2 . As one can see, when the transition of the defect signal S 1  for indicating the beginning point of a detected defective area is generated too late, the amount of the first servo control effort FOO 1 /TRO 1  would become larger to seriously shift the focus point/tracking point of the laser beam from a correct position. As a result, when the optical pickup head leaves the defective area, the second servo control effort FOO 2 /TRO 2  would be larger to make the erroneously shifted focus point/tracking point moved to the correct position, which causes serious distortion in the RF signal S 3  and might result in reading failure of the normal area immediately following the defective area. 
     Therefore, how to avoid or mitigate the signal quality degradation caused by applying inappropriate servo control effort due to defective areas formed on the optical disc becomes an important issue to be resolved. 
     SUMMARY OF THE INVENTION  
     In accordance with embodiments of the present invention, exemplary signal processing devices and signal processing methods for dealing with a defect signal associated with defective areas on an optical storage medium (e.g., an optical disc) are proposed. 
     According to a first aspect of the present invention, a signal processing device is provided. The signal processing device includes a processing circuit and a signal generating circuit. The processing circuit is for determining a position of at least one defective area on an optical storage medium according to a defect signal, and accordingly recording defect position information of the at least one defect. The signal generating circuit is coupled to the processing circuit, and implemented for generating an output signal according to at least the recorded defect position information of the at least one defective area. 
     According to a second aspect of the present invention, a signal processing device is provided. The signal processing device includes a processing circuit and a signal generating circuit. The processing circuit is implemented for recording defect information of at least one defective area on an optical storage medium according to a defect signal derived during a first full rotation of the optical storage medium. The signal generating circuit is coupled to the processing circuit, and implemented for generating an adjusted defect signal by adjusting the defect signal derived during a second full rotation of the optical storage medium, which follows the first full rotation of the optical storage medium, according to the recorded defect information of the at least one defective area. 
     According to a third aspect of the present invention, a signal processing device is provided. The signal processing device includes a processing circuit and a signal generating circuit. The processing circuit is implemented for detecting a starting point of a signal portion, which is indicative of a corresponding defective area on an optical storage medium and included in a defect signal, and when the starting point of the signal portion is detected, estimating an amount of latest servo control effort applied before the starting point of the signal portion. The signal generating circuit is coupled to the processing circuit, and implemented for controlling a servo control circuit to compensate for the amount of latest servo control effort applied before the starting point of the signal portion. 
     According to a fourth aspect of the present invention, a signal processing method is provided. The signal processing method includes: determining a position of at least one defective area on an optical storage medium according to a defect signal; recording defect position information of the at least one defective area; and generating an output signal according to at least the recorded defect position information of the at least one defective area. 
     According to a fifth aspect of the present invention, a signal processing method is provided. The signal processing method includes: recording defect information of at least one defective area on an optical storage medium according to a defect signal derived during a first full rotation of the optical storage medium; and generating an adjusted defect signal by adjusting the defect signal derived during a second full rotation of the optical storage medium, which follows the first full rotation of the optical storage medium, according to the recorded defect information of the at least one defective area. 
     According to a sixth aspect of the present invention, a signal processing method is provided. The signal processing method includes: detecting a starting point of a signal portion, which is indicative of a corresponding defective area on an optical storage medium and included in a defect signal, and when the starting point of the signal portion is detected, estimating an amount of latest servo control effort applied before the starting point of the signal portion; and controlling a servo control circuit to compensate for the amount of latest servo control effort applied before the starting point of the signal portion. 
     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  
         FIG. 1  is a waveform diagram of a defect signal, a servo output signal and a radio-frequency signal which are generated when an optical pickup head of an optical disc drive accesses an optical disc with a defective area formed thereon. 
         FIG. 2  is a block diagram of a generalized signal processing device according to an exemplary embodiment of the present invention. 
         FIG. 3  is a block diagram illustrating one detailed implementation of the signal processing device shown in  FIG. 2 . 
         FIG. 4  is a waveform diagram of a defect signal derived during a first full rotation of an optical storage medium, a defect signal derived during a second full rotation of the optical storage medium, a specific signal, and an adjusted defect signal. 
         FIG. 5  is a diagram illustrating the relation between the position on an optical storage medium and a counter value generated by a counter shown in  FIG. 3 . 
         FIG. 6  is a block diagram illustrating an optical disc drive with the signal processing device shown in  FIG. 3 . 
         FIG. 7  is a block diagram illustrating an optical disc drive with a feedforward control mechanism implemented therein. 
         FIG. 8  is a waveform diagram of a defect signal and a servo output signal (e.g., a tracking servo output or focus servo output). 
     
    
    
     DETAILED DESCRIPTION  
     Certain terms are used throughout the description and following claims to refer to particular components. As one skilled in the art will appreciate, manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following description and in the claims, the terms “include” and “comprise” are used in an open-ended fashion, and thus should be interpreted to mean “include, but not limited to . . . ”. Also, the term “couple” is intended to mean either an indirect or direct electrical connection. Accordingly, if one device is coupled to another device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections. 
       FIG. 2  is a block diagram of a generalized signal processing device according to an exemplary embodiment of the present invention. The signal processing device  200  includes, but is not limited to, a processing circuit  202  and a signal generating circuit  204 . The processing circuit  202  is implemented for determining a position of at least one defective area on an optical storage medium (e.g., an optical disc) according to a defect signal S 1 , and accordingly recording defect position information DATA_P of the at least one defective area. The signal generating circuit  204  is coupled to the processing circuit  202 , and implemented for generating an output signal S_OUT according to at least the recorded defect position information DATA_P of the at least one defective area. As a frequency generator (FG) signal generated in response to a spindle rotation has a predetermined number of FG pulses per one full rotation of the optical storage medium, address information can be obtained from a wobble signal derived from a wobble track on the optical storage medium or a data signal (e.g., eight-to-fourteen modulation data) derived from a data track on the optical storage medium, and a clock signal with a predetermined clock frequency can be employed to count an absolute time elapsed after an optical storage apparatus starts rotating the optical storage medium, the processing circuit  202  therefore can obtain absolute positions of defective areas found in each full rotation of the optical storage medium through referring to the frequency generator (FG) signal, the wobble signal, the data signal, or the absolute time. 
     By way of example, not a limitation, the output signal S_OUT in one exemplary implementation can be used to serve as a servo protection signal for preventing the servo control mechanism from applying inappropriate servo control effort before an optical pickup head enters the defective area on the optical storage medium. For example, the signal generating circuit  204  generates the output signal S_OUT by adjusting the original defect signal S 1  according to the recorded defect position information DATA_P obtained by the processing circuit  202 . However, it should be noted that using the output signal S_OUT to act as a servo protection signal is for illustrative purposes only. Any application using a signal generated according to recorded defect position information DATA_P of defective area(s) on an optical storage medium falls within the scope of the present invention. 
     Please refer to  FIG. 3 , which is a block diagram illustrating one exemplary implementation of the signal processing device shown in  FIG. 2 . In this exemplary implementation, the signal processing device  300  includes a processing circuit  302  configured for recording defect information of at least one defective area on an optical storage medium (e.g., an optical disc) according to a defect signal S 1  derived during a first full rotation of the optical storage medium, and a signal generating circuit  304  configured for generating an adjusted defect signal (i.e., an output signal S_OUT) by adjusting the defect signal S 1  derived during a second full rotation of the optical storage medium, which follows the first full rotation of the optical storage medium, according to the recorded defect information of the at least one defective area. More specifically, the processing circuit  302  in this exemplary implementation determines a position of the at least one defective area on the optical storage medium according to the defect signal S 1  derived during the first full rotation of the optical storage medium, and then records defect position information of the at least one defective area as the defect information of the at least one defective area. The defect position information will be referenced for adjusting the defect signal S 1  derived during the following second full rotation of the optical storage medium. In this exemplary implementation, the signal generating circuit  304  generates the adjusted defect signal (i.e., the output signal S_OUT) by advancing a starting point of a signal portion, which is indicative of a corresponding defective area on the optical storage medium and included in the defect signal S 1  derived during the second full rotation of the optical storage medium, according to the recorded defect information of the at least one defective area. Further description directed to operations of the signal processing device  300  shown in  FIG. 3  is detailed as follows. 
     Please refer to  FIG. 3  in conjunction with  FIG. 4  and  FIG. 5 .  FIG. 4  is a waveform diagram of the defect signal S 1  derived during the first full rotation of the optical storage medium, the defect signal S 1  derived during the second full rotation of the optical storage medium, a specific signal S 1 ′, and the adjusted defect signal (i.e., the output signal S_OUT).  FIG. 5  is a diagram illustrating the relation between the position on the optical storage medium  502  and the counter value CNT generated by the counter  318 . As shown in  FIG. 3 , the processing circuit  302  includes a comparing unit  312  and a defect position information recording unit  314 . The comparing unit  312  is coupled to the defect position information recording unit  314 , and implemented for comparing a width of a specific signal portion included in the defect signal S 1  with a predetermined threshold value PDEF_TH, where the specific signal portion is indicative of a corresponding defective area on the optical storage medium. With regard to the defect position information recording unit  314 , when the comparing unit  312  detects that the width of the specific signal portion substantially reaches the predetermined threshold value PDEF_TH, the defect position information recording unit  314  records defect position information of the corresponding defective area according to a position of the corresponding defective area on the optical storage medium. In this exemplary implementation, the operations of the comparing unit  312  and the defect position information recording unit  314  are simply based on a counter output which is derived from counting, for example, FG pulses included in an FG signal generated in response to a spindle rotation. As shown in  FIG. 3 , the defect position information recording unit  314  includes a storage  316  and a counter  318 . As the FG signal has a predetermined number of FG pulses per one full rotation of the optical storage medium, the counter  318  in this exemplary embodiment may be configured to count the FG pulses to achieve the objective of counting each full rotation of the optical storage medium to generate a counter value CNT indicative of a corresponding position on the optical storage medium where the optical pickup head is located. However, it should be noted that counting the FG pulses merely serves as one possible implementation, and is not meant to be taken as a limitation to the scope of the present invention. Any implementation using a counter to generate a counter value for indicating a position on the optical storage medium during one full rotation of the optical storage medium obeys the spirit of the present invention. 
     The storage  316  records the defect position information of a defective area by storing the counter value CNT corresponding to the defective area. Please refer to  FIG. 5 , which is a diagram illustrating the relation between the position on the optical storage medium  502  and the counter value CNT generated by the counter  318 . In one full rotation of the optical storage medium  502 , the counter  318  is reset to an initial value (e.g., 0) to the counter value CNT, and then increases the counter value CNT gradually. Please note that the counter  318  would be reset after each full rotation of the optical storage medium  502 . Assume that the optical storage medium  502  is rotated in a counter-clockwise direction. Therefore, the optical pickup head moves alone the track  504  on the optical storage medium  502  in a clockwise direction. Provided that each full rotation of the optical storage medium  502  begins at the same absolute position determined according to the FG signal generated in response to the spindle rotation, the counter  318  is reset (CNT=0) when the optical pickup head is located at the position P 0  of the track  504 , the counter  318  has the counter value CNT equal to N when the optical pickup head is located at the position P 1  of the track  504 , the counter  318  has the counter value CNT equal to 2*N when the optical pickup head is located at the position P 2  of the track  504 , and the counter  318  has the counter value CNT equal to 3*N when the optical pickup head is located at the position P 3  of the track  504 . 
     As can be seen from  FIG. 5 , there are two defective areas Defect_ 1  and Defect_ 2  on the optical storage medium  502 . When the optical pickup head reads the track  504  during a first full rotation of the optical storage medium  502 , the optical pickup head enters the defective area Defect_ 1  and the defective area Defect_ 2  in order. Therefore, as shown in  FIG. 4 , the defect signal S 1  generated by any conventional means has one signal portion SP_ 1 , which is indicative of the corresponding defective area Defect_ 1  on the optical storage medium  502  and included in the defect signal S 1  derived during the first full rotation of the optical storage medium  502 , and another signal portion SP_ 2 , which is indicative of the corresponding defective area Defect_ 2  on the optical storage medium  502  and included in the defect signal S 1  derived during the first full rotation of the optical storage medium  502 . The rising edge of the signal portion SP_ 1  corresponds to a start point of the defective area Defect_ 1  along the track  504  where the optical pickup head moves, and the falling edge of the signal portion SP_ 1  corresponds to an end point of the defective area Defect_ 1  along the track  504  where the optical pickup head moves. The counter value CNT corresponding to the rising edge of the signal portion SP_ 1  is denoted by C 0 , and the counter value CNT corresponding to the falling edge of the signal portion SP_ 1  is denoted by C 1  (C 1 &gt;C 0 ). The comparing unit  312  can easily determine whether the width of the signal portion SP_ 1  substantially reaches the predetermined threshold value PDEF_TH by the counter values C 0  and C 1 . For example, the comparing unit  312  calculates a difference value between the counter values C 0  and C 1 , and then compares the difference value (i.e., C 1 −C 0 ) with the predetermined threshold value PDEF_TH. As the difference value (C 1 −C 0 ) exceeds the predetermined threshold value PDEF_TH, the defect position information recording unit  314  records defect position information of the corresponding defective area Defect_ 1  according to a position of the corresponding defective area Defect_ 1  on the optical storage medium  502 . For example, the counter value C 0  indicative of the position of the corresponding defective area Defect_ 1  on the optical storage medium  502  is stored into the storage  316 . 
     Regarding the other defective area Defect_ 2  on the optical storage medium  502 , the counter value CNT corresponding to the rising edge of the signal portion SP_ 2  is denoted by C 2 , and the counter value CNT corresponding to the falling edge of the signal portion SP_ 2  is denoted by C 3 . Similarly, the comparing unit  312  calculates a difference value between the counter values C 3  and C 2 , and then compares the difference value (i.e., C 3 −C 2 ) with the predetermined threshold value PDEF_TH. As the difference value (C 3 −C 2 ) is smaller than the predetermined threshold value PDEF_TH, the defect position information recording unit  314  does not record defect position information of the corresponding defective area Defect_ 2 . In other words, the counter value C 3  indicative of the position of the corresponding defective area Defect_ 2  on the optical storage medium  502  is not stored into the storage  316 . 
     Due to product cost consideration, the storage  316  implemented for recording defect position information of defective area(s) generally has a limited capacity. Therefore, the comparing unit  312  is used to identify any defective area with a significant effect upon the track where the optical pickup head is accessing, and only the counter value corresponding to the qualified defective area is allowed to be recorded in the storage  316 . In this way, the comparing unit  312  stores counter values each corresponding to a rising edge of a specific defect signal portion with a signal width substantially reaching the predetermined threshold value PDEF_TH into the storage  316  until the storage space allocated in the storage  316  for recording counter values during one full rotation of the optical storage medium is full or one full rotation of the optical storage medium is completed. However, the comparing unit  312  can be omitted in an alternative design. Therefore, counter values each corresponding to a corresponding defective area are successively stored into the storage  316  until the storage space allocated in the storage  316  for recording counter values during one full rotation of the optical storage medium is full or one full rotation of the optical storage medium is completed. This also falls within the scope of the present invention. 
     The counter values stored in the storage  316  will be referenced by the signal generating circuit  304  for generating the output signal S_OUT. As shown in  FIG. 3 , the signal generating circuit  304  includes an adjusting unit  322 , a comparing unit  324 , and a signal generating unit  326 , where the signal generating unit  326  has a signal generator  328  and an OR gate  330 . The adjusting unit  322  is implemented for adjusting each stored counter value corresponding to a specific defective area (e.g., the counter value C 0  corresponding to the defective area Defect_ 1 ) by a first adjustment value A 1  and a second adjustment value A 2 . In this way, a first adjusted counter value CNT_Adv and a second adjusted counter value CNT_Ext are generated, respectively. The comparing unit  324  is coupled to the counter  318  and the adjusting unit  322 , and implemented for comparing the counter value CNT currently counted by the counter  318  with the first adjusted counter value CNT_Adv and the second adjusted counter value CNT_Ext. For example, when the counter value CNT currently counted by the counter  318  is equal to the first adjusted counter value CNT_Adv, the comparing unit  324  generates a first indication signal D 1  to notify the signal generating unit  326 , and when the counter value CNT currently counted by the counter  318  is equal to the second adjusted counter value CNT_Ext, the comparing unit  324  generates a second indication signal D 2  to notify the signal generating unit  326 . The signal generating unit  326  is coupled to the comparing unit  324 , and implemented for generating a specific signal S 1 ′ which has a level transition when the counter value CNT currently counted by the counter  318  substantially reaches either of the first adjusted counter value CNT_Adv and the second adjusted counter value CNT_Ext, and outputting the output signal S_OUT according to at least the specific signal S 1 ′. More specifically, the signal generating unit  326  makes the generated specific signal S 1 ′ have a level transition when notified by either of the first indication signal D 1  and the second indication signal D 2 . 
     In this exemplary implementation, the adjusting unit  322  subtracts the first adjustment value Al from the stored counter value corresponding to a defective area to generate the first adjusted counter value CNT_Adv, and adds the second adjustment value A 2  to the stored counter value corresponding to the defective area to generate the second adjusted counter value CNT_Ext. Taking the aforementioned counter value C 0  recorded in the storage  316  during the first full rotation of the optical storage medium  502  for example, the corresponding first adjusted counter value CNT_Adv would be set by C 0 −A 1 , and the corresponding second adjusted counter value CNT_Ext would be set by C 0 +A 2  during the second full rotation of the optical storage medium  502 . The comparing unit  324  therefore compares the counter value CNT currently counted by the counter  318  during the second full rotation of the optical storage medium  502  with the first adjusted counter value CNT_Adv (i.e., C 0 −A 1 ) and the second adjusted counter value CNT_Ext (i.e., C 0 +A 2 ), respectively. The signal generator  328  in the signal generating unit  326  makes the specific signal S 1 ′ have a level transition from a first logic level (e.g., ‘0’) to a second logic level (e.g., ‘1’) when the first indication signal D 1  indicates that the counter value CNT currently counted by the counter  318  substantially reaches the first adjusted counter value CNT_Adv, and makes the specific signal S 1 ′ have a level transition from the second logic level (e.g., ‘1’) to the first logic level (e.g., ‘0’) when the second indication signal D 2  indicates that the counter value CNT currently counted by the counter  318  substantially reaches the second adjusted counter value CNT_Ext. The exemplary specific signal S 1 ′ generated from the signal generator  328  is shown in  FIG. 4 . A signal portion SP_ 3  with a high logic level is generated in the specific signal S 1 ′. 
     The OR gate  330  in the signal generating unit  326  generates the output signal S_OUT by performing an OR logic operation upon the specific signal S 1 ′ and the defect signal S 1 . The output signal S_OUT therefore can be used to replace the defect signal S 1  which may act as a servo protection signal referenced to prevent the servo control mechanism from applying inappropriate servo control effort when the optical pickup head enters a defective area on an optical storage medium. That is, the output signal S_OUT can act as an adjusted defect signal in such an exemplary implementation. As can be seen from  FIG. 4 , the signal generating circuit  326  generates the adjusted defect signal (i.e., the output signal S_OUT) by advancing a starting point of a signal portion SP_ 1 ′, which is indicative of a corresponding defective area on the optical storage medium and included in the defect signal S 1  derived during the second full rotation of the optical storage medium, by the first adjustment value A 1 . In other words, when the output signal S_OUT is employed to act as a servo protection signal, the servo protection applied to the servo control mechanism for holding the servo control settings is enabled in advance, which effectively blocks the servo control mechanism from applying the aforementioned inappropriate servo control effect (e.g., FOO 1 /TRO 1  shown in  FIG. 1 ). In this way, the servo control effort can be controlled appropriately before the starting point of a defective area (i.e., before the optical pickup head starts entering the defective area). Thus, the actual focus point and/or tracking point of the laser beam emitted from the optical pickup head would not be seriously shifted from the correct position at the end point of the defective area (i.e., when the optical pickup head just leaves the defective area). As the signal distortion of the RF signal, as shown in  FIG. 1 , can be avoided or alleviated, data reading performance of the normal area immediately following the defective area can be enhanced greatly. 
     In above exemplary implementation, the output signal S_OUT is generated by the OR gate  330  according to the specific signal S 1 ′ generated in response to the count value (e.g., C 0 ) recorded during the first full rotation of the optical storage medium and the defect signal S 1  derived during the second full rotation of the optical storage medium. As the track pitch is quite small for a high-density optical disc drive (e.g., a Blu-ray disc drive), the waveform of the defect signal S 1  derived during the second full rotation of the optical storage medium is almost identical to that of the defect signal S 1  derived during the first full rotation of the optical storage medium. However, it is possible that the rising edge of the signal portion SP_ 1 ′ is not aligned with that of the signal portion SP_ 1  due to certain factors, such as unstable spindle rotation. In a case where the rising edge of the signal portion SP_ 1 ′ lags behind that of the signal portion SP_ 1 , and the signal generator  328  is configured to make the specific signal S 1 ′ have a level transition from the second logic level (e.g., ‘1’) to the first logic level (e.g., ‘0’) when the counter value CNT currently counted by the counter  318  substantially reaches the stored counter value (e.g., C 0 ), the falling edge of the signal portion SP_ 3  would lead the rising edge of the signal portion SP_ 1 ′. As a result, due to the OR logic operation performed by the OR gate  330 , the resultant output signal S_OUT will not have a consistent high logic level during an interval between the falling edge of the signal portion SP_ 3  and the rising edge of the signal portion SP_ 1 ′. If the output signal S_OUT is used to serve as the aforementioned servo protection signal, the servo protection is erroneously disabled in a short period within an interval between the falling edge of the signal portion SP_ 3  and the rising edge of the signal portion SP_ 1 ′. To avoid such a problem, the second adjusted counter value CNT_Ext is preferably set to guarantee that the falling edge of the signal portion SP_ 3  falls behind the rising edge of the signal portion SP_ 1 ′. However, if the output signal S_OUT is used by a specific application rather than the aforementioned servo protection or the above-mentioned problem is not significant under certain operational conditions, the hardware associated with the second adjusted counter value CNT_Ext may be omitted. That is, in an alternative design, the signal generator  328  is implemented to make the specific signal S 1 ′ have a level transition from the first logic level (e.g., ‘0’) to the second logic level (e.g., ‘1’) when the counter value CNT currently counted by the counter  318  substantially reaches the first adjusted counter value CNT_Adv, and then make the specific signal S 1 ′ have a level transition from the second logic level (e.g., ‘1’) to the first logic level (e.g., ‘0’) when the counter value CNT currently counted by the counter  318  substantially reaches the stored counter value (e.g., C 0 ). This also falls within the scope of the present invention. 
     The signal processing device  300  can be disposed in an optical disc drive to provide the output signal S_OUT acting as the servo protection signal referenced to prevent the servo control mechanism from applying inappropriate servo control effort due to defective areas on the optical storage medium. For clarity, please refer to  FIG. 6 , which shows an optical disc drive  600  having the signal processing device  300  implemented therein. The optical disc drive  600  includes, but is not limited to, a spindle motor  602 , an optical pickup head  604 , a defect detection circuit  606 , the signal processing device  300 , a servo control circuit  610 , and a driver  612 . The spindle motor  602  is for rotating the optical storage medium (e.g., an optical disc)  502  at a target rotational speed, where a frequency generator signal FG is generated in response to the spindle rotation of the spindle motor  602 . The optical pickup head  604  is for emitting a laser beam to access the optical storage medium  502 . The defect detection circuit  606  is for generating the defect signal S 1  according to signals generated from the optical pickup head  604 ; however, this is for illustrative purposes only. Actually, the defect signal S 1  fed into the signal processing device  300  can be derived by any conventional means. As the present invention does not focus on the defect detection, further description directed to generating the defect signal S 1  is omitted here for brevity. The driver  612  for generating driving signals to control movement of the lens included in the optical pickup head  604  according to the servo output signal, including a tracking servo output TRO and/or a focus servo output FOO. Due to the adjusted defect signal (i.e., the output signal S_OUT), the servo control circuit  610  is therefore protected from applying inappropriate servo control effort to the optical pickup head  604 . As mentioned above, the exemplary signal processing device  300  determines the beginning and end of one full rotation of the optical storage medium  502  according to the frequency generator signal FG to identify a current position of the optical pickup head  604  on the optical storage medium  502 ; however, this is for illustrative purposes only. In an alternative design, the current position of the optical pickup head  604  on the optical storage medium  502  can also be obtained by a wobble signal derived from a wobble track on the optical storage medium  502  or a data signal (e.g., eight-to-fourteen modulation data) derived from a data track on the optical storage medium  502 . 
     Briefly summarized, during a current full rotation of an optical storage medium, defect information (e.g., counter values) of defective areas on the optical storage medium is recorded, and an adjusted defect signal is generated by adjusting the defect signal according to defect information (e.g., counter values) of defective areas that is recorded during a previous full rotation of the optical storage medium. 
     As described in above paragraphs, the inappropriate servo control effort is eliminated or mitigated with the help of the adjusted defect signal (e.g., the output signal S_OUT). In another exemplary embodiment of the present invention, a feedforward control mechanism applied to the servo control is proposed. Please refer to  FIG. 7 , which is a block diagram illustrating an optical disc drive  700  with a feedforward control mechanism implemented therein. The optical disc drive  700  includes a spindle motor  702  for rotating an optical storage medium (e.g., an optical disc)  701  at a target rotational speed, an optical pickup head  704  for emitting a laser beam to access the optical storage medium  701 , a defect detection circuit  706  for generating a defect signal S 1 , a signal processing device  708  having a processing circuit  714  and a signal generating circuit  716 , a driver  712  for generating driving signals to control the lens in the optical pickup head  704  according to a servo output signal, including a tracking servo output TRO and/or a focus servo output FOO, and a servo control circuit  710  for generating the servo output signal. In this exemplary embodiment, the processing circuit  714  is implemented for detecting a starting point of a signal portion, which is indicative of a corresponding defective area on the optical storage medium  701  and included in the defect signal S 1 , and estimating an amount of latest servo control effort applied before the starting point of the detected signal portion.  FIG. 8  is a waveform diagram of a defect signal S 1  and a servo output signal (e.g., a tracking servo output TRO or focus servo output FOO) S 2 . At time T 1 , the processing circuit  714  detects a starting point (e.g., a rising edge) of a signal portion SP, which is indicative of a corresponding defective area on the optical storage medium  701  and included in the defect signal S 1 , and estimates the amount of latest servo control effort FOO 1 /TRO 1  (i.e., an inappropriate servo control effort) applied before the starting point of the detected signal portion SP. Next, the signal generating circuit  716 , which is coupled to the processing circuit  714 , generates a control signal S_CTRL to control the servo control circuit  710  to compensate for the amount of latest servo control effort FOO 1 /TRO 1  applied before the starting point of the detected signal portion SP. As shown in  FIG. 8 , in accordance with the control signal S_CTRL generated due to the estimated amount of latest servo control effort FOO 1  /TRO 1 , the servo control circuit  710  applies an inverse servo control effort FOO 1 ′/TRO 1 ′ after the starting point of the detected signal portion SP. That is, the inverse servo control effort FOO 1 ′ /TRO 1 ′ is applied when the optical pickup head  704  is currently accessing a defective area indicated by the corresponding signal portion SP. In this way, the inappropriate servo control effort FOO 1 /TRO 1  can be cancelled or mitigated by the inverse servo control effort FOO 1 ′/TRO 1 ′. 
     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. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.