Abstract:
A defect detection circuit and method for detecting disk defects in an optical disc data reproduction system allows for correct detection of defects not only during operation in a constant linear velocity mode but also during operation in a constant angular velocity mode using a channel bit clock signal as a reference signal. The circuit includes a comparator for comparing an RF signal with a first compare voltage having a predetermined voltage level and a second compare voltage having a voltage level lower than the first compare voltage, and for generating a first comparison signal activated in response to the RF signal having a level higher than that of the first compare voltage and a second comparison signal activated in response to the RF signal having a voltage level lower than that of the second compare voltage. A defect detector receives the first and the second comparison signals and a channel bit clock signal, and generates a defect detecting signal when both the first and the second comparison signals are deactivated for a period of time less than nT in duration (here, n is a positive integer and T is a period of a channel bit clock signal) and when the first or the second comparison signal is deactivated for a period of time longer than the maximum pit length+mT where m is an integer larger than 0.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to data reproduction in an optical disk system, such as a compact disc player (CDP), or a digital versatile disc (DVD) player, and more particularly, to a defect detection circuit and a method for detecting defects during data reproduction in an optical disk system. The circuit and method prevent erroneous data from being generated that would otherwise arise from a missing or irregularly generated radio frequency (RF) signal due to disc surface scratches or disk defects generated during fabrication. 
     2. Description of the Related Art 
     A defect detection circuit in an optical disk data reproduction system according to conventional technology is described as follows with reference to the attached drawings. 
     FIG. 1 is a schematic block diagram of a conventional defect detection circuit in an optical disk data reproduction system, which includes first and second comparators  11  and  13 , first and second delayers  15  and  17 , and an AND gate  19 . 
     FIGS. 2A through 2F are input and output waveforms of the respective elements of the circuit shown in FIG.  1 . FIG. 2A shows an RF signal exhibiting a defect. FIG. 2B shows the resulting output waveform of the first comparator  11 . FIG. 2C shows the resulting output waveform of the second comparator  13 . FIG. 2D shows the output waveform of the first delayer  15 , and FIG. 2E shows the output waveform of the second delayer  17 . FIG. 2F shows the resulting output waveform of the AND gate  19 . 
     Referring to FIGS. 1 and 2, the first comparator  11  receives the RF signal of FIG. 2A via a positive input terminal, and compares it to a first compare voltage Vthp received via a negative input terminal. The first comparator  11  generates an output signal at a ‘high’ logic level when the voltage level of the RF signal is higher than that of the first compare voltage Vthp, and generates an output signal at a ‘low’ logic level when the voltage level of the RF signal is lower than that of the first compare voltage Vthp, as shown in FIG.  2 B. The second comparator  13  likewise receives a second compare voltage Vthn via a positive input terminal, and compares it to the RF signal received via a negative input terminal. The second comparator  13  generates an output signal at a ‘high’ logic level when the voltage level of the RF signal is lower than that of the second compare voltage Vthn, and generates an output signal at a ‘low’ logic level when the voltage level of the RF signal is higher than the second compare voltage Vthn. 
     The first delayer  15  receives the output of the first comparator  11  as an input signal, and when the input signal is at a ‘high’ logic level, the first delayer  15  delays the input signal for a predetermined delay time Td provided by a microprocessor (not shown) to maintain the ‘high’ logic level during the time duration Td, such that the first delayer generates the signal shown in FIG.  2 D. The second delayer  17  receives the output of the second comparator  13  as an input signal, and when the input signal is at a ‘high’ logic level, the second delayer  17  delays the input signal for a predetermined delay time Td, to maintain the ‘high’ logic level during the time duration Td such that the second delayer generates the signal shown in FIG.  2 E. 
     As a result, the first and second delayers  15  and  17  continuously generate a signal at a ‘high’ logic level when the first and second comparators  11  and  13  generate a signal at a ‘high’ logic level within the delay time Td provided by the microprocessor (not shown), even though the outputs of the first and second comparators  11  and  13  return to a ‘low’ logic level, namely, when the RF signal is normally generated. However, when the ‘high’ logic level is not generated by the first and second comparators  11  and  13  following the delay time Td, namely, when an abnormal RF signal is generated due to defect, the first and second delayers  15  and  17  generate an output signal at a ‘low’ logic level indicating that there is a defect. 
     The AND gate  19  receives the output signals of the first and second delayers  15  and  17 , performs an AND operation on the output signals, and generates the defect detection signal shown in FIG. 2F at an output terminal OUT. 
     The conventional defect detection circuit described above detects defects corresponding to a certain delay time Td generated by the microprocessor (not shown), regardless of a channel bit clock signal BCK commonly used as a reference signal when data recorded on the disc is reproduced. In the conventional defect detection circuit, the delay time Td with respect to the channel bit clock signal BCK, is equal for the inner and outer circumferences of the disc operating in a constant linear velocity (CLV) mode in which the same channel bit clock signal BCK is used during reproduction of data stored at the inner and outer circumferences. Accordingly, no errors are generated during reproduction. 
     However, a correct defect detection signal may not be generated during operation in a constant angular velocity (CAV) mode in which different channel bit clock signals BCK are used in the inner and outer circumferences of the disc. Namely, in CAV mode, since channel bit clock signals BCK having different frequencies are used for the inner and outer circumferences, a certain delay time Td is sensed differently for the inner and outer circumferences. 
     For example, assume the frequency of the channel bit clock signal BCK for reading data recorded in a central track of the disc to be 4.32 MHz in CAV mode. In this case, to read data recorded at the inner most circumference, a channel bit clock signal BCK of 2.16 MHz is used, which is half of 4.32 MHz. In the outer most circumference, the data is read by a channel bit clock signal BCK of 6.48 MHz, which is 1.5 times 4.32 MHz. Therefore, since the frequency of the channel bit clock signal BCK becomes larger in the outer circumference, a certain delay time Td generated in the microprocessor becomes shorter in the inner circumference, as compared to the outer circumference. 
     As a result, in a system operating in CAV mode, the conventional defect detecting circuit generates a certain delay time Td in the microprocessor. However, the system senses that the delay times Td having different lengths are generated during reproduction of data stored in the inner and outer circumferences of the disc. Accordingly, a correct defect detection signal is not generated. 
     SUMMARY OF THE INVENTION 
     To solve the above problems, it is an object of the present invention to provide a defect detecting circuit for an optical disc data reproduction system which correctly detects defects existing in an RF signal since delay time is controlled according to a varying channel bit clock signal in the optical disc reproducing system. 
     It is another object of the present invention to provide a defect detecting method performed in the defect detecting circuit of the optical disc reproducing system. 
     In a first aspect, the apparatus of the present invention is directed to a defect detection circuit in an optical disk data reproduction system including an optical pick up unit for generating an analog RF signal, for detecting defects in the RF signal. A comparator compares the RF signal voltage level with a first compare voltage having a predetermined voltage level and with a second compare voltage having a predetermined voltage level lower than the first compare voltage. In response to the RF signal voltage level being greater than that of the first compare voltage, a first comparison signal is generated. In response to the RF signal voltage level being less than that of the second compare voltage, a second comparison signal is generated. A defect detector receives the first and second comparison signals along with a channel bit clock signal. A defect detecting signal is generated when both the first and second comparison signals are deactivated for a time period that is greater in length than a first time duration. Alternatively, the defect detecting signal is generated when the first or the second comparison signal is deactivated for a time period that is greater in length than a second time duration. 
     In a preferred embodiment, the first time duration is an integer multiple of the period of the channel bit clock signal. The first time duration preferably corresponds to a predetermined minimum pit length. The second time duration preferably comprises a predetermined maximum pit length in addition to an integer multiple of the period of the channel bit clock. 
     In a second aspect, the present invention comprises a defect detection circuit in an optical disk data reproduction system including an optical pick up unit for generating an analog RF signal, the defect detection circuit for detecting defects in the RF signal. A comparator compares the RF signal voltage level with a first compare voltage having a predetermined voltage level and with a second compare voltage having a predetermined voltage level lower than the first compare voltage. In response to the RF signal voltage level being greater than that of the first compare voltage, a first comparison signal is generated. In response to the RF signal voltage level being less than that of the second compare voltage, a second comparison signal is generated. A first defect detector receives the first and second comparison signals and a channel bit clock signal and in response to both the first and second comparison signals being deactivated over a time period greater in length than a first time duration, a first defect detecting signal is generated. A second defect detector receives a first comparison signal and the channel bit clock signal and in response to the first comparison signal being deactivated over a time period longer than a second time duration, a second defect detecting signal is generated. A third defect detector receives the second comparison signal and the channel bit clock signal and in response to the second comparison signal being deactivated over a time period longer than the second time duration, a third defect detecting signal is generated. A first logic combining circuit performs an OR operation on the first, second, and third defect detecting signals and generate the OR operation result as the defect detecting signal. 
     In a preferred embodiment, the comparator comprises a first comparator for generating the first comparison signal activated in response to the RF signal having a level higher than the first compare voltage, and a second comparator for generating the second comparison signal activated in response to the RF signal having a level lower than the second compare voltage. The first defect detector preferably comprises first OR-operator for performing an OR operation on the first comparison signal and the second comparison signal and generating a third comparison signal for responding to the activation of the first comparison signal or the second comparison signal. A signal generator receives the third comparison signal and the channel bit clock signal and generates the first defect detecting signal which is activated when the third comparison signal is deactivated for a time period which is longer than the time period of a predetermined pit length. 
     The signal generator preferably comprises a second OR-operator for performing an OR operation on the channel bit clock signal and the first defect detecting signal. A first counter has a reset state which is deactivated in response to activation of the third comparison signal. The counter initiates counting of the output signal of the second OR-operator in response to the deactivation of the third comparison signal. The counter further generates the first defect detecting signal activated when the count value equals the first time duration. The second defect detector preferably comprises a third OR-operator for performing an OR operation on the second defect detecting signal and the channel bit clock signal. A second counter has a reset state which is deactivated in response to activation of the first comparison signal. The counter initiates counting of the output signal of the third OR operator in response to the deactivation of the first comparison signal. The counter further generates the second defect detecting signal activated when the count value equals the second time duration. The third defect detector preferably comprises a fourth OR-operator for performing an OR operation on the third defect detecting signal and the channel bit clock signal. A third counter has a reset state which is deactivated in response to activation of the second comparison signal. The counter initiates counting of the output signal of the fourth OR-operator in response to the deactivation of the second comparison signal. A third defect detecting signal is activated when the count value equals the second time duration. 
     The defect detecting circuit and method make it possible to correctly detect defects not only in a constant linear velocity mode but also in a constant angular velocity mode in which the frequency of the channel bit clock signals used in the inner circumference and the outer circumference of the disc are different by detecting various defects existing in the disc using the channel bit clock signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     FIG. 1 is a schematic block diagram of a conventional defect detecting circuit in an optical disc data reproduction system. 
     FIGS. 2A-2F are input and output waveforms of the respective elements of the apparatus of FIG.  1 . 
     FIG. 3 is a schematic block diagram of a defect detecting circuit in an optical disc reproducing system, in accordance with the present invention. 
     FIGS. 4A-4H are output waveform diagrams of the respective units of the circuit of FIG.  3 . 
     FIGS. 5A-5D are waveform diagrams for explaining how to set the counter values of the first through third counters of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinafter, the structure and the operation of a defect detecting circuit of an optical disc data reproduction system according to the present invention will be described as follows with reference to the attached drawings. For purposes of the present specification, the active state of the respective signals is assumed to be a ‘high’ logic level. The inactive state is assumed to be a ‘low’ logic level. 
     FIG. 3 is a schematic block diagram of a defect detection circuit for an optical disc data reproduction system, in accordance with the present invention. The primary components of the defect detecting circuit include a comparator  61  and a defect detector  51 . 
     The comparator  61  includes first and second comparators  21  and  23 . The defect detector  51  includes a first defect detector  41  including a first OR gate  25 , a second OR gate  27 , and a first counter  35 , a second defect detector  43  including a third OR gate  29  and a second counter  37 , a third defect detector  45  including a fourth OR gate  31  and a third counter  39 , and a logic combining unit  33 . 
     The structure and operation of the defect detecting circuit is now described as follows with reference to FIG.  3 . The first comparator  21  receives an RF signal via a positive input terminal and a first compare voltage Vthp via a negative input terminal, compares the voltage level of the RF signal with that of the first compare voltage, and outputs the comparison result to the first OR gate  25  and the reset terminal R of the second counter  37  as a first comparison signal CS 1 . The second comparator  23  receives a second compare voltage Vthn via a positive input terminal and the RF signal via a negative input terminal, compares the voltage level of the second compare voltage with that of the RF signal, and outputs the comparison result to the first OR gate  25  and the reset terminal R of the third counter  39  as a second comparison signal CS 2 . 
     The first OR gate  25  of the first defect detector  41  receives the first comparison signal CS 1  and the second comparison signal CS 2 , so as to perform an OR operation, and outputs the OR operation result to the reset terminal R of the first counter  35  as a third comparison signal CS 3 . The second OR gate  27  performs an OR operation on a first defect detecting signal DET 1  generated from the first counter  35  and the channel bit clock signal BCK used as a reference signal when the data recorded on the disc is restored and outputs the OR operation result to the clock terminal CK of the first counter  35 . The first counter  35  is reset to a ‘low’ logic level when the third comparison signal CS 3  input to the reset terminal R is at a ‘high’ level, and counts the output signal of the second OR gate  27  input to the clock terminal CK when the third comparison signal CS 3  is at a ‘low’ logic level. At this time, the first counter  35  continuously outputs the first defect detecting signal at a ‘low’ logic level until the channel bit clock signal BCK corresponding to the minimum pit length, for example 3T (here, T is a period of a channel bit clock signal), is counted. When the first counter  35  counts the minimum pit length 3T, the first defect detecting signal DET 1  is generated at a ‘high’ logic level, indicating that defects exist in the RF signal. This signal DET 1  is provided to the logic combining unit  33  and the second OR gate  27 . A signal generator can therefore be realized as the second OR gate  27  and the first counter  35 . 
     The third OR gate  29  of the second defect detector  43  performs an OR operation on a second defect detecting signal DET 2  and the channel bit clock signal BCK and outputs the OR operation result to the clock terminal CK of the second counter  37 . The second counter  37  is reset to a ‘low’ logic level when the first comparison signal CS 1  input via the reset terminal R is at a ‘high’ logic level, and counts the output signal of the third OR gate  29  input via the clock terminal CK when the first comparison signal CS 1  is at a ‘low’ logic level. At this time, the second counter  37  generates the second defect detecting signal DET 2  at a ‘low’ logic level unit 1  the channel bit clock signal BCK corresponding to the maximum pit length (11T or 14T)+mT (Hereinafter, a case in which the maximum pit length is 14T, and m is 2, is taken as an example.) is counted, namely, when a value of 16T is counted. When the second counter  37  counts to a value of 16T, the second defect detecting signal is generated at a ‘high’ logic level, indicating that the defects exist in the RF signal. The signal DET 2  is provided to the logic combining unit  33  and the third OR gate  29 . 
     The fourth OR gate  31  of the third defect detector  45  performs an OR operation on the third defect detecting signal DET 3  output from the third counter  39  and the channel bit clock signal BCK, and outputs the OR operation result to the clock terminal CK of the third counter  39 . The third counter  39  is reset when the second comparison signal CS 2  input via the reset terminal R is at a ‘high’ logic level and counts the output signal of the fourth OR gate  29  input via the clock terminal CK when the second comparison signal CS 2  is at a ‘low’ logic level. At this time, the third counter  39  continuously generates the third defect detecting signal of a ‘low’ logic level until the channel bit clock signal BCK corresponding to the maximum pit length (11T or 14T)+mT (Hereinafter, a case in which the maximum pit length is 14T, and m is 2, is taken as an example.) is counted, namely, when 16T is counted. When the third counter  39  counts 16T, the third defect detecting signal is generated at a ‘high’ logic level, indicating that the defects exist in the RF signal. This signal DET 3  is provided to the logic combining unit  33  and the fourth OR gate  31 . 
     The logic combining unit  33  performs an OR operation on the first, second, and third defect detecting signals DET 1 , DET 2 , DET 3 , respectively generated by the first, second, and third defect detectors  41 ,  43 , and  45  and outputs the OR operation result to the output terminal OUT as a final defect detection signal. The logic combining unit  33  is preferably realized by an OR gate. 
     FIGS. 4A through 4H are output waveform diagrams of the respective units of the circuit of FIG.  3 . FIG. 4A shows an RF signal. FIG. 4B shows the first comparison signal CS 1  generated by the first comparator  21 . FIG. 4C shows the second comparison signal CS 2  generated by the second comparator  23 . FIG. 4D shows the third comparison signal CS 3  generated by the first OR gate  25 . FIG. 4E shows the first defect detecting signal DET 1 . FIG. 4F shows the second defect detecting signal DET 2 . FIG. 4G shows the third defect detecting signal DET 3 . FIG. 4H shows the defect detecting signal output from the logic combining unit  33 . 
     FIGS. 5A through 5D arc waveform diagrams for explaining how to set the counter values of the first through third counters  35 ,  37 ,  39  shown in FIG.  3 . FIG. 5A shows the RF signal. FIG. 5B shows the first comparison signal CS 1  which is the output of the first comparator  21 . FIG. 5C shows the second comparison signal CS 2  which is the output of the second comparator  23 . FIG. 5D shows the third comparison signal CS 3  which is the output of the first OR gate  25 . 
     The operation of the defect detecting circuit of the optical disc data reproduction system according to the present invention is now described in detail with reference to FIGS. 3,  4 , and  5 . 
     The first comparator  21  receives the RF signal shown in FIG. 4A at the positive input terminal and the first compare voltage Vthp at the negative input terminal and generates the first comparison signal CS 1  having a ‘high’ logic level during times at which the level of the RF signal is greater than that of the first comparison voltage Vthp, and a ‘low’ logic level during times at which the level of the RF signal is less than that of the first compare voltage Vthp as shown in FIG.  4 B. The second comparator  23  receives the RF signal shown in FIG. 4A at the negative input terminal and the second compare voltage Vthn at the positive input terminal and generates the second comparison signal CS 2  having a ‘low’ logic level during times at which the level of the RF signal is greater than that of the second compare voltage Vthn and a ‘high’ logic level during times at which the level of the RF signal is less than that of the second compare voltage Vthn as shown in FIG.  4 C. The first OR gate  25  performs an OR operation on the first and second comparison signals CS 1  and CS 2  shown in FIGS. 4B and 4C and outputs the third comparison signal CS 3  shown in FIG. 4D to the reset terminal R of the first counter  35 . 
     The second OR gate  27  of the first defect detector  41  performs an OR operation on the first defect detecting signal DET 1  output from the first counter  35  and the channel bit clock signal BCK, and outputs the OR-operated signal to the clock terminal CK of the first counter  35 . At this time, when the third comparison signal CS 3  at a ‘high’ logic level is input via the reset terminal R of the first counter  35 , a first defect detecting signal DET 1  at a ‘low’ logic level is generated. When the output of the third comparison signal CS 3  is at a ‘low’ logic level, counting of the channel bit clock signal BCK input via the clock terminal CK begins. Here, since the second OR gate  27  outputs the channel bit clock signal BCK when the first counter  35  generates the defect detecting signal DET 1  at a ‘low’ logic level, the channel bit clock signal BCK is provided via the clock terminal CK of the first counter  35 . 
     At this time, the first counter  35  continuously outputs the first defect detection signal DET 1  at a ‘low’ logic level when the third comparison signal CS 3  at a ‘high’ logic level is input via the reset terminal R before the channel bit clock signal BCK is counted to a value corresponding to the minimum pit length 3T. When the first counter  35  counts the channel bit clock signal BCK to a value corresponding to the minimum pit length 3T, when the third comparison signal CS 3  at a ‘high’ logic level is not provided, the first counter  35  generates the first defect detecting signal DET 1  of a ‘high’ logic level, indicating that a defect exists in the RF signal, as shown in FIG.  4 E. 
     The reason for setting the first defect detecting signal at a ‘high’ logic level when the first counter  35  counts to a value of 3T is now described with reference to FIG.  5 . Referring to FIG. 5, in the case of a normal RF signal shown in FIG. 5A, the time taken for the RF signal to fall from the level of the first compare voltage Vthp to the second compare voltage Vthn, or the time taken for the RF signal to rise from the level of the second compare voltage Vthn to the first compare voltage Vthp is smaller than 3T. Namely, the ‘low’ logic level section  61  of the third comparison signal CS 3  shown in FIG. 5D is smaller than 3T, which is the minimum pit length. Therefore, when the third comparison signal CS 3  falls to the ‘low’ logic level and does not rise to the ‘high’ logic level after the lapse of 3T, this means that a defect exists in the RF signal. Therefore, the first defect detecting signal DET 1  at ‘high’ logic level is generated. 
     The second counter  37  of the second defect detector  43  receives the first comparison signal CS 1  via the reset terminal R, is reset when the first comparison signal CS 1  is at a ‘high’ logic level, and outputs the second defect detecting signal DET 2  at a ‘low’ logic level. When the first comparison signal CS 1  transitions to ‘low’ logic level, the second counter  37  is not in a reset state and begins to count the channel bit clock signal provided from the third OR gate  29  to the clock terminal CK. At this time, the third OR gate  29  outputs the channel bit clock BCK when counter  37  generates the defect detecting signal DET 2  at a ‘low’ logic level. 
     When a first comparison signal CS 1  at a ‘high’ logic level is provided before the second counter  37  counts a channel bit clock signal corresponding to a time period of 16T, the second counter  37  continuously outputs the second defect detecting signal DET 2  at a ‘low’ logic level. When a first comparison signal CS 1  at a ‘high’ logic level is not provided, when the second counter  37  counts the channel bit clock signal to a value corresponding to 16T, the second counter  37  generates a second defect detecting signal DET 2  at a ‘high’ logic level, indicating that a defect exists in the RF signal, as shown in FIG.  4 F. 
     The reason why the second defect detecting signal DET 2  is generated at a ‘high’ logic level when the second counter  37  counts to a value corresponding to 16T is described with reference to FIG.  5 . As shown in FIG. 5A, when the RF signal having the maximum pit length 14T is generated, the first comparison signal CS 1  transitions to a ‘low’ logic level during section  63  for a longer time period than 14T (14T+mT) as shown in FIG.  5 B. The length of the section  63  is defined as 16T(m=2), which can be defined as a predetermined value due to system characteristics. It is therefore determined that a normal RF signal is generated, as the first comparison signal CS 1  at a ‘high’ logic level is not generated before the second counter  37  counts to a value of 16T. Accordingly, a second defect detecting signal DET 2  at a ‘low’ logic level is generated. However, when the first comparison signal CS 1  at a ‘high’ logic level is not generated after counting to a value of 16T, this means that a defect exists in the RF signal. Therefore a second defect detecting signal DET 2  at ‘high’ logic level is generated. 
     The third counter  39  of the third defect detector  45  receives the second comparison signal CS 2  via the reset terminal R, is reset when the second comparison signal CS 2  is at a ‘high’ logic level, and upon being reset, outputs a signal at a ‘low’ logic level. When the second comparison signal CS 2  is at a ‘low’ logic level, the third counter  39  is not in a reset state and begins to count the channel bit clock signal provided by the fourth OR gate  31  to the clock terminal CK (here, the fourth OR gate  31  outputs the channel bit clock signal BCK when the third counter  39  generates the reset signal). 
     At this time, when the second comparison signal CS 2  at a ‘high’ logic level is provided before the third counter  39  counts to a value corresponding to 16T, the third counter  39  outputs a third defect detecting signal DET 3  at a ‘low’ logic level. When the third counter  39  counts to a value corresponding to 16T when the second comparison signal CS 2  at a ‘high’ logic level is not provided, the third counter  39  generates the third defect detecting signal DET 3  at a ‘high’ logic level indicating that a defect exists in the RF signal. Here, the reason why the third defect detecting signal DET 3  is generated at a ‘high’ logic level when the third counter  39  counts to a value corresponding to 16T is similar to the reason described above with respect to the second defect detector  43 . 
     The logic combining unit  33  in the form of a fifth OR gate receives the first, second, and third defect detecting signals DET 1 , DET 2 , and DET 3  shown respectively in FIGS. 4E through 4F, performs an OR operation on the first, second, and third defect detecting signals, and outputs the OR operation result to the output terminal OUT as a final defect detecting signal. Namely, the defect detecting signal shown in FIG. 4H is generated for the above example having a ‘low’ logic level during a time period in which the normal RF signal is generated and having a ‘high’ logic level during a time period in which the normal RF signal is not generated due to the defect. 
     The defect detecting circuit of the optical disc data reproduction system according to the present invention detects first, second, and third defect detecting signals using the channel bit clock signal BCK. In this manner, it is possible to correctly detect the defect by counting the channel bit clock signal BCK of 3T and 16T, not only for the case of constant linear velocity in which the channel bit clock signals BCK used in the inner circumference and the outer circumference of the disc are the same, but also in the case of constant angular velocity in which the channel bit clock signals BCK used in the inner circumference and the outer circumference of the disc are different. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.