Abstract:
A method for detecting a typical defect area on a disc track includes the following steps. Firstly, a source signal is provided. Then, first and second signals are generated according to the source signal. The first and second signals are held at the peak level of the source signal and respectively decreased at first and second drop rates. Then, first and second threshold values are subtracted from the first and second signals to generate first and second slice signals, respectively. Afterwards, the source signal is compared with either the first slice signal or the second slice signal. When the first slice signal is larger than the source signal, a typical defect signal is changed from a first level to a second level. Whereas, the typical defect signal is changed from the second level to the first level when the second slice signal is smaller than the source signal.

Description:
[0001]    This application is a divisional application of co-pending application Ser. No. 12/270,361, filed on Nov. 13, 2008, the contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a device and a method for detecting a defect on a disc track, and more particularly to a defect-detecting device and a defect-detecting method for used in an optical disc drive. 
       BACKGROUND OF THE INVENTION 
       [0003]    Referring to  FIG. 1 , a conventional servo control system of an optical disc drive is shown. An optical disc  110  having a center hole is placed on a turn table  122 . The turn table  122  is driven to rotate by a spindle motor  120  such that the optical disc  110  is rotated with the turn table  122 . An actuator  180  outputs a driving force for an optical pickup head (PUH)  100  of the optical disc drive in a tracking (radial) direction or a focusing direction. 
         [0004]    When an electronic signal is generated in response to the optical signal reflected from the optical disc  110  and received by the optical pickup head  100 , a pre-amplifier  150  process the electronic signal into a series of servo error signals. The servo error signals include for example a radio-frequency signal RF, a sub-beam added signal (SBAD), a wobble signal, a focusing error signal FE, a tracking error signal TE and the like. 
         [0005]    According to these servo error signals, the controller  170  generates a series of servo control signals to the actuator  180 . Generally, the servo control signals include a tracking control signal and a focusing control signal. In addition, the actuator  180  includes at least a tracking coil and a focusing coil. According to the tracking control signal, the tracking coil generates a radial driving force for actuating a trace shift of the optical pickup head  100  in the tracking direction. According to the focusing control signal, the focusing coil generates a focusing-direction driving force for actuating a shift of the optical pickup head  100  in the focusing direction. Under the control of the radial driving force and the focusing-direction driving force, the optical pickup head  100  is located at the proper focusing position and onto the desired track. 
         [0006]    As known, due to production failure or scratch, some defects are readily formed on the surface of the optical disc  110 . Since data within the defective area of the optical disc  110  can not be read by the optical pickup head  100 , the servo error signals outputted from the pre-amplifier  150  would become beyond expectation. The fluctuation of the servo error signals would further result in the unexpected servo control signals by the controller  170  and thus the actuator  180  may generate improper driving forces. Under this circumstance, the servo control system of the optical disc drive is unstable, which may cause the optical pickup head  100  to have focusing fail or slip track. Eventually, these defects could result in reading or writing errors. 
         [0007]      FIG. 2  is a timing waveform diagram illustrating a relationship between a sub-beam added signal, a low-pass signal a defect signal. In the drawings, the term SBAD indicates a sub-beam added signal. The sub-beam added signal SBAD is filtered by a low-pass filter (not shown) to generate a low-pass signal (SBAD Low-pass ). Please refer to  FIGS. 1 and 2 . In a case that the optical pickup head  100  is moved along a track having a bright defect area with strong reflection, the sub-beam added signal SBAD rises up at a faster rate but the low-pass signal SBAD Low-pass  rises up at a slower rate. If the difference between the sub-beam added signal SBAD and the low-pass signal SBAD Low-pass  is larger than a first threshold value (M 1 ), the defect signal (DETECT) outputted from the controller  170  is changed from a first level (e.g. a low level) to a second level (e.g. a high level). Until the difference between the sub-beam added signal SBAD and the low-pass signal SBAD Lowpass  is smaller than the first threshold value (M 1 ), the defect signal returns to the first level. On the contrary, in a case that the optical pickup head  100  is moved along a track having a typical defect area (e.g. a scratch), the sub-beam added signal SBAD drops down at a faster rate but the low-pass signal SBAD Low-pass  drops down at a slower rate. If the difference between the sub-beam added signal SBAD and the low-pass signal SBAD Low-pass  is smaller than a second threshold value (−M 2 ), the defect signal (DETECT) outputted from the controller  170  is changed from the first level to the second level. Until the difference between the sub-beam added signal SBAD and the low-pass signal SBAD Low-pass  is larger than the second threshold value (−M 2 ), the defect signal returns to the first level. According to the defect signal (DETECT), the driving force generated from the actuator  180  is adjusted in order to protect the optical disc drive from being subject to focusing fail or slip track. 
         [0008]      FIG. 3  schematically illustrates a defect-detecting circuit disclosed in description of U.S. Pat. No. 6,882,611. As shown in  FIG. 3 , the defect-detecting circuit includes a low-pass filter  24 , a subtractor  28 , two comparators  22   a  and  22   b , and an OR gate  26 . A negative input end of the comparator  22   a  is coupled to a preset positive threshold (DFTH_P), which represents the first threshold value for detecting the bright defect. A positive end of the comparator  22   b  is coupled to a preset negative threshold (DFTH_N), which represents the second threshold value for detecting the typical defect. 
         [0009]    The sub-beam added signal SBAD is filtered by the low-pass filter  24  to generate a low-pass signal (SBAD_LPF). The subtractor  28  subtracts the SBAD_LPF from the original SBAD and transmits the result to the two comparators  22   a  and  22   b . If the result is larger than the first threshold value, the area is determined as a bright defect area. Whereas, if the result is smaller than the second threshold value, the area is determined as a typical defect area. 
         [0010]      FIG. 4  is a timing waveform diagram illustrating related signals processed in the defect-detecting circuit of  FIG. 3 , in which a relatively long defect is present. In a case that the optical pickup head is moved along a track having a relatively long defect area (one kind of typical defect area), the sub-beam added signal SBAD drops down at a faster rate but the low-pass signal SBAD_LPF drops down at a slower rate. At the time spot T 1 , since the difference between the sub-beam added signal SBAD and the low-pass signal SBAD_LPF is smaller than the second threshold value (DFTH_N), the defect signal (DETECT) is changed from the low level to the high level. Meanwhile, the detected area is determined as a typical defect area. 
         [0011]    However, the above defect-detecting circuit usually generates inaccurate defect signals during the optical pickup head moves along the track having the long defect area. As the low-pass signal SBAD_LPF continuously drops down, the difference between the sub-beam added signal SBAD and the low-pass signal SBAD_LPF becomes larger than the second threshold value (DFTH_N) at the time spot T 2 . Meanwhile, the defect signal (DETECT) is changed from the high level to the low level and thus the defect signal (DETECT) is erroneously ended at the time spot T 2 . Under this circumstance, the defect-detecting circuit discriminates that no defect area is present. 
         [0012]    At the time spot T 3 , since the difference between the sub-beam added signal SBAD and the low-pass signal SBAD_LPF is larger than the first threshold value (DFTH_P), the defect signal (DETECT) is changed from the low level to the high level. Meanwhile, the detected area is determined as a typical defect area. Until the difference between the sub-beam added signal SBAD and the low-pass signal SBAD_LPF is smaller than the first threshold value (DFTH_P) at the time spot T 4 , the defect signal (DETECT) is changed from the high level to the low level. In other words, an erroneous defect signal is generated from the time point T 3  to T 4 . 
         [0013]    As previously described, an erroneous defect signal is generated if a long defect is present. In a case that a long typical defect area is present (as shown in  FIG. 4 ), two pulses sequentially including a typical defect area and a bright defect area occur. Moreover, in a case that a long bright defect is present, two pulses sequentially including a bright defect area and a typical defect area occur. 
         [0014]    For avoiding generation of the erroneous defect signal, U.S. Pat. No. 6,882,611 disclosed a defect-detecting circuit and a defect-detecting method, and the contents thereof are hereby incorporated by reference. As shown in  FIG. 5 , the defect-detecting circuit includes a switch  51 , a capacitor  30 , a low-pass filter  24 , a subtractor  28 , an OR gate  26  and two comparators  22   a  and  22   b . A negative input end of the comparator  22   a  is coupled to a preset positive threshold (DFTH_P), which indicates the first threshold value for detecting the bright defect. A positive end of the comparator  22   b  is coupled to a preset negative threshold (DFTH_N), which indicates the second threshold value for detecting the typical defect. The sub-beam added signal SBAD is filtered by the low-pass filter  24  to generate a low-pass signal (SBAD_LPF). The subtractor  28  subtracts the SBAD_LPF from the original SBAD and couples to the two comparators  22   a  and  22   b . The switch S 1  controls whether the SBAD is sent to the low-pass filter  24  and is controlled by the defect signal. 
         [0015]      FIG. 6  is a timing waveform diagram illustrating related signals processed in the defect-detecting circuit of  FIG. 5 , in which a relatively long defect is present. In a case that the optical pickup head is moved along a track having a relatively long defect (one kind of typical defect), the sub-beam added signal SBAD drops down at a faster rate but the low-pass signal SBAD_LPF drops down at a slower rate. At the time spot T 5 , since the difference between the sub-beam added signal SBAD and the low-pass signal SBAD_LPF is smaller than the second threshold value (DFTH_N), the defect signal (DETECT) is changed from the low level to the high level and the detected area is determined as a typical defect area. Once the defect signal (DETECT) is changed from the low level to the high level, the switch S 1  is opened such that the SBAD signal can no longer flow through the low-pass filter  24  and the value of the low-pass signal (SBAD_LPF) is held by the capacitor  30 . 
         [0016]    Until the difference between the sub-beam added signal SBAD and the low-pass signal SBAD_LPF is greater than the second threshold value (DFTH_N) at the time spot T 6 , the defect signal (DETECT) is changed from the high level to the low level. Meanwhile, the switch S 1  is closed and the SBAD signal can flow through the low-pass filter  24 . 
         [0017]    In accordance with the defect-detecting circuit and the defect-detecting method shown in  FIGS. 5 and 6 , the area corresponding to the defect signal (DETECT) from the time spot T 5  to T 6  is determined as a typical defect area. Therefore, the long typical defect can be detected by using the second threshold value (DFTH_N) as a reference level. Similarly, the defect-detecting circuit can be used to detect the long bright defect by using the first threshold value (DFTH_P) as a reference level. 
         [0018]    The defect-detecting circuit shown in  FIG. 5  is usually implemented by an analog circuit. Since the analog circuit occupies much layout space of the integrated chip and fails to withstand process deviation, the application of this defect-detecting circuit is restrained. Moreover, it is difficult to precisely control the timing of the defect signal changing from the first level to the second level. That is, it is not flexible to adjust the time spots T 5  and T 6 . 
         [0019]    For solving the above-mentioned drawbacks, digitalized defect-detecting circuit and method are disclosed in description of US publication No. 20040145987, and the contents thereof are hereby incorporated by reference. As shown in  FIG. 7 , the defect-detecting circuit disclosed in US publication No. 20040145987 includes an A/D converter  200 , a maximum value detector  202 , a minimum value detector  204 , a first comparator  206 , a second comparator  208  and an OR gate  209 . A first threshold value TH 1  and a second threshold value TH 2  are received by the first comparator  206  and the second comparator  208 , respectively. 
         [0020]    In the digitalized defect-detecting circuit, a radio frequency (RF) signal is converted by the A/D converter  200  into a digital format. The maximum value detector  202  detects an upper value of the digital RF signal, while the minimum value detector  204  detects a bottom value of the digital RF signal. The first comparator  206  compares the upper value of the digital RF with the first threshold value TH 1 . A logic value “0” is outputted if the upper value of the digital RF signal is larger than the first threshold value TH 1 , while a logic value “1” is outputted if the upper value of the digital RF signal is smaller than the first threshold TH 1 . The second comparator  208  compares the bottom value of the digital RF with the second threshold value TH 2 . A logic value “0” is outputted if the bottom value of the digital RF signal is smaller than the second threshold TH 2 , while a logic value “1” is outputted if the bottom value of the digital RF signal is larger than the second threshold value TH 2 . The output ends of the first comparator  206  and the second comparator  208  are coupled to the OR gate  209 . According to the comparing results at the first comparator  206  and the second comparator  208 , the OR gate  209  outputs a defect signal. 
         [0021]    In the digitalized defect-detecting circuit of  FIG. 7 , asymmetry amount of the RF signal is calculated and used as an index for determining asymmetry degree of the RF signal. Since either the first threshold value TH 1  or the second threshold value TH 2  is used to determine one kind of defect (the typical defect or the bright defect), it is also difficult to precisely control the timing of the defect signal changing from the first level to the second level. 
         [0022]    Therefore, there is a need of providing improved defect-detecting circuit and method so as to obviate the drawbacks encountered from the prior art. 
       SUMMARY OF THE INVENTION 
       [0023]    The present invention provides a defect-detecting circuit for detecting defects on a disc track by using two threshold values (slice signals) to determine the typical defect and the bright defect, thereby precisely controlling the timing of the defect signal. 
         [0024]    The present invention provides a defect-detecting circuit having reduced layout space of the integrated chip. 
         [0025]    The present invention also relates to a defect-detecting method for detecting defects on a disc track. 
         [0026]    In accordance with an aspect of the present invention, there is provided a defect-detecting device for detecting a typical defect area and a bright defect area on a disc track. The defect-detecting device includes a typical defect-detecting circuit and a bright defect-detecting circuit. The typical defect-detecting circuit includes a first peak hold unit, a first subtractor, a second peak hold unit, a second subtractor, a first multiplexer and a first comparator. The first peak hold unit receives a source signal and holds the peak level of the source signal, thereby outputting a first signal at a first drop rate. The first subtractor is electrically connected to the first peak hold unit for subtracting a first threshold value from the first signal, thereby generating a first slice signal. The second peak hold unit receives the source signal and holds the peak level of the source signal, thereby outputting a second signal at a second drop rate. The second subtractor is electrically connected to the second peak hold unit for subtracting a second threshold value from the second signal, thereby generating a second slice signal. The first multiplexer is electrically connected to the first subtractor and the second subtractor, and has a first input end to receive the first slice signal, a second input end to receive the second slice signal, an output end and a select end. The first comparator has a first end receiving the source signal and a second end coupled to the output end of the first multiplexer for comparing the source signal with either the first slice signal or the second slice signal, thereby generating a typical defect signal. The first slice signal and the second slice signal are selectively outputted from the first multiplexer according to the typical defect signal inputted into the select end of the first multiplexer. The bright defect-detecting circuit includes a first bottom hold unit, a first adder, a second bottom hold unit, a second adder, a second multiplexer and a second comparator. The first bottom hold unit receives a source signal and holds the bottom level of the source signal, thereby outputting a third signal at a first rise rate. The first adder is electrically connected to the first bottom hold unit for adding a third threshold value and the third signal, thereby generating a third slice signal. The second bottom hold unit receives a source signal and holds the bottom level of the source signal, thereby outputting a fourth signal at a second rise rate. The second adder is electrically connected to the second bottom hold unit for adding a fourth threshold value and the fourth signal, thereby generating a fourth slice signal. The second multiplexer is electrically connected to the first adder and the second adder, and has a first input end to receive the third slice signal, a second input end to receive the fourth slice signal, an output end and a select end. The second comparator having a first end receiving the source signal and a second end coupled to the output end of the second multiplexer for comparing the source signal with either the third slice signal or the fourth slice signal, thereby generating a bright defect signal. The third slice signal and the fourth slice signal are selectively outputted from the second multiplexer according to the bright defect signal inputted into the select end of the second multiplexer. When a typical defect area is detected by the defect-detecting device, the bright defect-detecting circuit is disenabled but the typical defect-detecting circuit is enabled to detect the typical defect signal. Whereas, when a bright defect area is detected by the defect-detecting device, the typical defect-detecting circuit is disenabled but the bright defect-detecting circuit is enabled to detect the bright defect signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The above contents of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
           [0028]      FIG. 1  (prior art) is a functional block diagram illustrating a conventional servo control system; 
           [0029]      FIG. 2  (prior art) is a timing waveform diagram illustrating a relationship between a sub-beam added signal, a low-pass signal a defect signal; 
           [0030]      FIG. 3  (prior art) schematically illustrates a defect-detecting circuit according to prior art; 
           [0031]      FIG. 4  (prior art) is a timing waveform diagram illustrating related signals processed in the defect-detecting circuit of  FIG. 3 , in which a relatively long defect is present; 
           [0032]      FIG. 5  (prior art) schematically illustrates another defect-detecting circuit according to prior art a defect-detecting circuit; 
           [0033]      FIG. 6  (prior art) is a timing waveform diagram illustrating related signals processed in the defect-detecting circuit of  FIG. 5 , in which a relatively long defect is present; 
           [0034]      FIG. 7  (prior art) schematically illustrates a digitalized defect-detecting circuit according to prior art; 
           [0035]      FIG. 8  schematically illustrates a typical defect-detecting circuit for detecting typical defects according to a first preferred embodiment of the present invention; 
           [0036]      FIGS. 9A ,  9 B and  9 C are timing waveform diagrams illustrating related signals processed in the typical defect-detecting circuit of  FIG. 8 ; 
           [0037]      FIG. 10  schematically illustrates a bright defect-detecting circuit for detecting bright defects according to a second preferred embodiment of the present invention; and 
           [0038]      FIGS. 11A ,  11 B and  11 C are timing waveform diagrams illustrating related signals processed in the bright defect-detecting circuit of  FIG. 10 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0039]    The present invention will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of preferred embodiments of this invention are presented herein for purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed. The schematic drawings, not to scale, are employed to illustrate the specific features of the present invention. 
         [0040]    Referring to  FIG. 8 , a typical defect-detecting circuit for detecting typical defects according to a first preferred embodiment of the present invention is illustrated. The typical defect-detecting circuit of  FIG. 8  principally includes a first peak hold unit  210 , a first subtractor  215 , a second peak hold unit  220 , a second subtractor  225 , a multiplexer  230  and a comparator  240 . A source signal, e.g. a sub-beam added signal SBAD, a RF envelope signal or a RF top envelope signal, is inputted into a negative input end of the comparator  240 , the first peak hold unit  210  and the second peak hold unit  220 . A first signal and a second signal are outputted from the first peak hold unit  210  and the second peak hold unit  220 , respectively. The first subtractor  215  subtracts a first threshold value TH 1  from the first signal and transmits a first slice signal to the low-level input end L of the multiplexer  230 . The second subtractor  225  subtracts a second threshold value TH 2  from the second signal and then transmits a second slice signal to the high-level input end H of the multiplexer  230 . The output end of the multiplexer  230  is coupled to a positive input end of the comparator  240  so as to output a typical defect signal. The typical defect signal is also connected to the select end S of the multiplexer  230 . 
         [0041]    The first peak hold unit  210  holds the peak level of the source signal, and outputs the first signal at a first drop rate. Similarly, the second peak hold unit  220  holds the peak level of the source signal, and outputs the second signal at a second drop rate. By adjusting the first drop rate and the second drop rate respectively according to the digitalized circuitry design cooperating with threshold values (or slice signals), the typical defect signal would be accurately detected regardless of the length thereof. 
         [0042]      FIGS. 9A ,  9 B and  9 C are timing waveform diagrams illustrating related signals processed in the typical defect-detecting circuit of  FIG. 8 . As shown in  FIG. 9A , the source signal (I) is a sub-beam added signal SBAD. The first signal (II) is outputted from the first peak hold unit  210  and decreased at a faster drop rate. The second signal (III) is outputted from the second peak hold unit  220  and decreased at a slower drop rate. As shown in  FIG. 9B , the first subtractor  215  generates a first slice signal (IV) by subtracting the first threshold value TH 1  from the first signal. The second subtractor  225  generates the second slice signal (V) by subtracting the second threshold value TH 2  from the second signal. The typical defect signal generated from the typical defect-detecting circuit is shown in  FIG. 9C . 
         [0043]    As can be seen from  FIGS. 9A ,  9 B and  9 C, the sub-beam added signal (I) is gradually decreased when a typical defect area is formed on a disc track. At the same time, the first signal (II) outputted from the first peak hold unit  210  is decreased at the first drop rate. By subtracting the first threshold value TH 1  from the first signal, the first subtractor  215  generates the first slice signal (IV). In addition, the second signal (III) outputted from the second peak hold unit  220  is decreased at the second drop rate. By subtracting the second threshold value TH 2  from the second signal, the second subtractor  225  generates the second slice signal (V). Since the typical defect signal inputted into the select end S of the multiplexer  230  is at the low-level state, the multiplexer  230  outputs the first slice signal (IV) to the positive input end of the comparator  240 . The sub-beam added signal (I) is also inputted into the negative input end of the comparator  240 . 
         [0044]    At the time spot T 1 , since the first slice signal (IV) is larger than the sub-beam added signal (I), the typical defect signal outputted from the comparator  240  is at a high-level state, which indicates the outset of a typical defect area on a disc track. Since the typical defect signal inputted into the select end S of the multiplexer  230  is at the high-level state, the multiplexer  230  outputs the second slice signal (V) to the positive input end of the comparator  240 . The sub-beam added signal (I) is also inputted into the negative input end of the comparator  240 . 
         [0045]    At the time spot T 2 , since the second slice signal (V) is smaller than the sub-beam added signal (I), the typical defect signal outputted from the comparator  240  is at a low-level state, which indicates conclusion of the typical defect area. Since the typical defect signal inputted into the select end S of the multiplexer  230  is at the low-level state, the multiplexer  230  outputs the first slice signal (IV) to the positive input end of the comparator  240 . The sub-beam added signal (I) is also inputted into the negative input end of the comparator  240 . 
         [0046]    By the typical defect-detecting method and circuit described above, the area corresponding to the defect signal from the time spot T 3  to T 4  and the area corresponding to the defect signal from the time spot T 5  to T 6  are determined as typical defect areas. Accordingly, the typical defect areas corresponding to the typical defect signal from T 1  to T 2 , from T 3  to T 4  and from T 5  to T 6  are accurately detected regardless of the length thereof. 
         [0047]    Referring to  FIG. 10 , a bright defect-detecting circuit for detecting bright defects according to a second preferred embodiment of the present invention is illustrated. The bright defect-detecting circuit of  FIG. 10  principally includes a first bottom hold unit  250 , a first adder  255 , a second bottom hold unit  260 , a second adder  265 , a multiplexer  270  and a comparator  280 . A source signal, e.g. a sub-beam added signal SBAD, a RF envelope signal or a RF bottom envelope signal, is inputted into a positive input end of the comparator  280  and received by the first bottom hold unit  250  and the second bottom hold unit  260 . A third signal and a fourth signal are outputted from the first bottom hold unit  250  and the second bottom hold unit  260 , respectively. The first adder  255  adds a third threshold value TH 3  and the third signal and transmits a third slice signal to the low-level input end L of the multiplexer  270 . The second adder  265  adds a fourth threshold value TH 4  and the fourth signal and transmits a fourth slice signal to the high-level input end H of the multiplexer  270 . The output end of the multiplexer  270  is coupled to a negative input end of the comparator  280  so as to output a bright defect signal. The bright defect signal is also connected to the select end S of the multiplexer  270 . 
         [0048]    The first bottom hold unit  250  holds the bottom level of the source signal, and outputs the third signal at a first rise rate. Similarly, the second bottom hold unit  260  holds the bottom level of the source signal, and outputs the second signal at a second rise rate. By adjusting the first rise rate and the second rise rate according to the digitalized circuitry design and cooperating with two threshold values (or slice signals), the bright defect signal would be accurately detected regardless of the length thereof. 
         [0049]      FIGS. 11A ,  11 B and  11 C are timing waveform diagrams illustrating related signals processed in the bright defect-detecting circuit of  FIG. 10 . As shown in  FIG. 11A , the source signal (I) is a sub-beam added signal SBAD. The third signal (III) is outputted from the first bottom hold unit  250  and increased at a faster rise rate. The fourth signal (IV) is outputted from the second bottom hold unit  260  and increased at a slower rise rate. As shown in  FIG. 11B , the first adder  255  generates a third slice signal (IV) by adding the third threshold value TH 3  and the third signal. The second adder  265  generates the fourth slice signal (V) by adding the fourth threshold value TH 4  and the fourth signal. The bright defect signal generated from the bright defect-detecting circuit is shown in  FIG. 11C . 
         [0050]    As can be seen from  FIGS. 11A ,  11 B and  11 C, the sub-beam added signal (I) is gradually increased when a bright defect area is formed on a disc track. At the same time, the third signal (II) is increased at the first rise rate. By adding the third threshold value TH 3  and the third signal, the first adder  255  generates the third slice signal (IV). In addition, the fourth signal (III) is increased at the second rise rate. By adding the fourth threshold value TH 4  and the fourth signal, the second adder  265  generates the fourth slice signal (V). Since the bright defect signal inputted into the select end S of the multiplexer  270  is at the low-level state, the multiplexer  270  outputs the third slice signal (IV) to the negative input end of the comparator  280 . The sub-beam added signal (I) is also inputted into the positive input end of the comparator  280 . 
         [0051]    At the time spot T 1 , since the sub-beam added signal (I) is larger than the third slice signal (IV), the bright defect signal outputted from the comparator  280  is at a high-level state, which indicates the outset of a bright defect area on a disc track. Since the defect signal inputted into the select end S of the multiplexer  270  is at the high-level state, the multiplexer  270  outputs the fourth slice signal (V) to the negative input end of the comparator  280 . The sub-beam added signal (I) is also inputted into the positive input end of the comparator  280 . 
         [0052]    At the time spot T 2 , since the sub-beam added signal (I) is smaller than the fourth slice signal (V), the bright defect signal outputted from the comparator  280  is at a low-level state, which indicates conclusion of the typical defect area. Since the bright defect signal inputted into the select end S of the multiplexer  270  is at the low-level state, the multiplexer  270  outputs the third slice signal (IV) to the negative input end of the comparator  240 . The sub-beam added signal (I) is also inputted into the positive input end of the comparator  240 . 
         [0053]    By the bright defect-detecting method and circuit described above, the area corresponding to the defect signal from the time spot T 3  to T 4  is also determined as a bright defect area. Accordingly, the bright defect areas corresponding to the bright defect signal from T 1  to T 2  and from T 3  to T 4  are accurately detected regardless of the length thereof. 
         [0054]    Moreover, the functions of the typical defect-detecting circuit and the bright defect-detecting circuit are combined into the defect-detecting device of the present invention. In a case that a typical defect area is detected by the defect-detecting device, the typical defect-detecting circuit needs to be enabled to detect the typical defect signal but the bright defect-detecting circuit needs to be disenabled in order to prevent from erroneously generating the bright defect signal. Until conclusion of detecting the typical defect area, the bright defect-detecting circuit is enabled. On the contrary, in a case that a bright defect area is detected by the defect-detecting device, the bright defect-detecting circuit needs to be enabled to detect the bright defect signal but the typical defect-detecting circuit needs to be disenabled in order to prevent from erroneously generating the typical defect signal. Until conclusion of detecting the typical defect area, the typical defect-detecting circuit is enabled. 
         [0055]    The typical defect-detecting circuit and the bright defect-detecting circuit of the present invention can be implemented by digitalized circuitry. For example, the sub-beam added signal SBAD is used as the source signal after converted into a digital format by an A/D converter. The first peak hold unit  210 , the first subtractor  215 , the second peak hold unit  220 , the second subtractor  225 , the multiplexer  230  and the comparator  240  included in the typical defect-detecting circuit can be implemented by digitalized circuitry. Likewise, the first bottom hold unit  250 , the first adder  255 , the second bottom hold unit  260 , the second adder  265 , the multiplexer  270  and the comparator  280  can also be implemented by digitalized circuitry. As a consequence, the defect-detecting device of the present invention has reduced layout space of the integrated chip and is more cost-effective when compared with the prior art. 
         [0056]    While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not to be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.