Abstract:
The defect detection device includes: an amplification section for amplifying a reflection signal corresponding to the intensity of light reflected from an optical disc according to a control signal indicating recording or playback; an envelope detection section for outputting an envelope of the amplified signal; a first pulse generation section for outputting a pulse when the level of the control signal changes; an integration section for integrating the envelope; a differential signal generation section for receiving the envelope as a first input signal and the integrated results as a second input signal and outputting a differential signal corresponding to the difference between these signals; and a comparison section for comparing the differential signal with a predetermined value and outputting the results as a defect detection signal. The second input signal is changed so as to reduce the possibility that the defect detection signal may indicate the presence of a defect over the duration of the pulse.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a defect detection device used for an optical disc apparatus and the like for detecting a defect (point where normal write/read operation fails) on an optical disc.  
         [0002]     In recent years, in computer systems in which the information amount to be handled has substantially increased, large-capacity, high-speed optical disc apparatuses permitting random access operation have come into widespread use as recording/playback apparatuses for information data. These apparatuses use optical discs such as CD-R (compact disc recordable), CD-RW (CD rewritable), DVD-R/RW (digital versatile disc recordable/rewritable) and DVD-RAM (DVD random access memory), for example, as recording media.  
         [0003]     A defect detection device used for an optical disc apparatus as described above generally detects a defect on an optical disc by converging a light beam on the optical disc and detecting a change in the envelope of a signal corresponding to the intensity of light reflected from the optical disc, and outputs a defect detection signal indicating the presence/absence of a defect. The defect detection signal may be used as a signal for holding a preceding value in a servo circuit that controls tracking and focusing servo for the optical disc, or used to obtain an extraction signal for determining a recording-prohibited region of the optical disc using a CPU incorporated in the optical disc apparatus for various controls.  
         [0004]      FIG. 9  is a block diagram showing a construction of a conventional defect detection device.  FIG. 10  is a graph showing the waveforms of signals used in the defect detection device of  FIG. 9 . The operation of the defect detection device of  FIG. 9  is as follows.  
         [0005]     A light beam is converged on an optical disc, and a reflection signal AS corresponding to the intensity of light reflected from the optical disc is input into a variable gain amplifier  902 . The variable gain amplifier  902  amplifies the reflection signal AS with a gain corresponding to a recording gate signal WTGT indicating that it is during recording of data into the disc or during playback of data from the disc, to obtain a predetermined amplitude, and outputs the resultant signal to a high-speed envelope detection circuit  940 . By this use of the variable gain amplifier  902 , it is possible to prevent the difference in the level of light reflected from the disc between during recording and during playback from being detected as a change in envelope.  
         [0006]     The high-speed envelope detection circuit  940  detects the envelope of the input signal, and outputs the results to a differential circuit  906  and an integration circuit  960 . The integration circuit  960  integrates the output of the high-speed envelope detection circuit  940 , and outputs the results to the differential circuit  906 . For example, assume that the envelope of the reflection signal AS abruptly changes due to a defect present on an optical disc. An envelope signal EM output from the envelope detection circuit  940 , of which the time constant is small, has a waveform following the abrupt change of the envelope, as shown in  FIG. 10 . On the contrary, an output signal IS of the integration circuit  960  has a smoothly changing waveform, in spite of the abrupt change of the reflection signal AS, as shown in  FIG. 10 .  
         [0007]     The differential circuit  906  outputs a differential signal DF corresponding to the difference between the envelope signal EM and the output signal IS of the integration circuit  960  to a comparator  908 . The comparator  908  digitizes the differential signal DF using a signal SD output from a D/A converter  912  as a slice level, to generate and output a defect detection signal DD (see Japanese Laid-Open Patent Publication No. 2003-196853, for example).  
         [0008]     However, the device of  FIG. 9  fails to eliminate the level difference in reflected light between during recording and during playback due to a variation in the setting of the gain of the variable gain amplifier  902  and other reasons. Therefore, at a shift of the operation for an optical disc from recording to playback or from playback to recording, a level difference arises in the output AP of the variable gain amplifier  902  as shown in  FIG. 10 .  
         [0009]     In particular, when the operation for an optical disc shifts from recording to playback, while the envelope signal EM output from the envelope detection circuit  940  follows the level difference of the reflection signal AS, it takes time for the signal IS, obtained by integrating the envelope signal EM, to follow the level difference. The differential signal DF is therefore higher than the signal SD, and thus a false defect detection signal DD is output over a considerable time period (pulse FS in  FIG. 10 ). As a result, highly stable playback is not achieved. When the operation for an optical disc shifts from playback to recording, correct defect detection is unattainable over a considerable time period although a false defect detection signal DD will not be output.  
       SUMMARY OF THE INVENTION  
       [0010]     An object of the present invention is providing a defect detection device capable of detecting a defect on an optical disc further correctly for achievement of highly stable recording and playback.  
         [0011]     Specifically, the defect detection device of the present invention includes: an amplification section for amplifying a reflection signal corresponding to the intensity of light reflected from an optical disc irradiated with a light beam, with a gain corresponding to a control signal indicating which is performed, recording or playback, for the optical disc, and outputting the amplified signal; an envelope detection section for obtaining an envelope of the output of the amplification section and outputting the obtained envelope; a first pulse generation section for outputting a pulse of a predetermined length when the level of the control signal changes; an integration section for integrating the output of the envelope detection section and outputting the integrated results; a differential signal generation section for receiving the output of the envelope detection section as a first input signal and the output of the integration section as a second input signal, generating a differential signal corresponding to a difference between the first input signal and the second input signal, and outputting the generated differential signal; and a comparison section for comparing the output of the differential signal generation section with a predetermined value, and outputting the results as a defect detection signal indicating the presence/absence of a defect, wherein the second input signal of the differential signal generation section is changed so as to reduce the possibility that the defect detection signal may indicate the presence of a defect during the time period for which the first pulse generation section outputs a pulse.  
         [0012]     According to the invention described above, the possibility that the defect detection signal may indicate the presence of a defect is reduced during the time period for which the first pulse generation section outputs a pulse. This makes it possible to reduce the frequency at which a false defect detection signal indicating the presence of a defect is output although there is actually no defect immediately after switching of the operation for an optical disc to recording or playback.  
         [0013]     The defect detection device described above preferably further includes a switch for selecting the signal output from the envelope detection section during the time period for which the first pulse generation section outputs a pulse and the signal output from the integration section during the remaining time period, and outputting the selected signal as the second input signal of the differential signal generation section.  
         [0014]     According to the invention described above, the first and second input signals of the differential signal generation section are equal to each other during the time period for which the first pulse generation section outputs a pulse. This prevents a false defect detection signal from being output.  
         [0015]     In the defect detection device described above, the integration section preferably reduces its time constant during the time period for which the first pulse generation section outputs a pulse.  
         [0016]     According to the invention described above, the second input signal of the differential signal generation section swiftly follows the first input signal of the differential signal generation section. This makes it possible to reduce the frequency at which a false defect detection signal is output.  
         [0017]     Preferably, the defect detection device described above further includes a second pulse generation section for outputting a pulse of a predetermined length once the pulse output from the first pulse generation section terminates, wherein the integration section has a first capacitor, one end of the first capacitor being grounded and the other end serving as the output of the integration section, and the integration section supplies a predetermined voltage to the first capacitor, the predetermined voltage having a value on the side of a level of the signal output from the envelope detection section when a defect is present with respect to a level of the signal output from the envelope detection section when no defect is present, during the time period for which the first pulse generation section outputs a pulse, and reduces the time constant of the integration section during the time period for which the second pulse generation section outputs a pulse.  
         [0018]     According to the invention described above, it is possible to prevent a false defect detection signal from being output during the time period for which the first pulse generation section outputs a pulse. In addition, the time constant of the integration section is reduced during the time period for which the second pulse generation section outputs a pulse. This makes it possible to restart the defect detection soon after the termination of the time period during which the first pulse generation section outputs a pulse.  
         [0019]     Preferably, the envelope detection section has a second capacitor, one end of the second capacitor being grounded and the other end serving as the output of the envelope detection section, and the envelope detection section supplies the predetermined voltage to the second capacitor during the time period for which the first pulse generation section outputs a pulse.  
         [0020]     According to the invention described above, the same voltage as that supplied to the first capacitor of the integration section is supplied to the second capacitor of the envelope detection section during the time period for which the first pulse generation section outputs a pulse. Therefore, the differential signal generation section does not output a signal opposite in polarity to that output during defect detection. Since a narrow dynamic range is enough for the output of the differential signal generation section, the sensitivity of the defect detection can be enhanced.  
         [0021]     Preferably, the integration section has a first switch for supplying the predetermined voltage to the first capacitor, the envelope detection section has a second switch for supplying the predetermined voltage to the second capacitor, and the envelope detection section is constructed so that the product of the capacitance of the first capacitor and the ON resistance of the first switch and the product of the capacitance of the second capacitor and the ON resistance of the second switch are substantially equal to each other.  
         [0022]     According to the invention described above, the times required for the voltages at the first and second capacitors to reach a predetermined voltage during the time period for which the first pulse generation section outputs a pulse can be made roughly equal to each other. This ensures a narrow dynamic range for the output of the differential signal generation section.  
         [0023]     As described above, according to the present invention, the possibility of outputting a false defect detection signal can be reduced, and thus a defect on an optical disc can be detected further correctly. As a result, highly stable recording and playback can be achieved. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]      FIG. 1  is a block diagram showing a construction of a defect detection device of Embodiment 1 of the present invention.  
         [0025]      FIG. 2  is a graph showing the waveforms of signals used in the defect detection device of  FIG. 1 .  
         [0026]      FIG. 3  is a block diagram showing a construction of a defect detection device of Embodiment 2 of the present invention.  
         [0027]      FIG. 4  is a graph showing the waveforms of signals used in the defect detection device of  FIG.3 .  
         [0028]      FIG. 5  is a block diagram showing a construction of a defect detection device of Embodiment 3 of the present invention.  
         [0029]      FIG. 6  is a graph showing the waveforms of signals used in the defect detection device of  FIG. 5 .  
         [0030]      FIG. 7  is a block diagram showing a construction of a defect detection device of Embodiment 4 of the present invention.  
         [0031]      FIG. 8  is a graph showing the waveforms of signals used in the defect detection device of  FIG. 7 .  
         [0032]      FIG. 9  is a block diagram showing a construction of a conventional defect detection device.  
         [0033]      FIG. 10  is a graph showing the waveforms of signals used in the defect detection device of  FIG. 9 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]     Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.  
         [heading-0035]     (Embodiment 1)  
         [0036]      FIG. 1  is a block diagram showing a construction of a defect detection device of Embodiment 1 of the present invention. The defect detection device of  FIG. 1  includes a variable gain amplifier  102  as the amplification section, a switch  104 , a differential circuit  106  as the differential signal generation section, a comparator  108  as the comparison section, a D/A converter  112 , an edge detection circuit  122 , a monostable multivibrator circuit  124  as the first pulse generation section, a high-speed envelope detection circuit  140  as the envelope detection section, and an integration circuit  160  as the integration section.  
         [0037]      FIG. 2  is a graph showing the waveforms of signals used in the defect detection device of  FIG. 1 .  
         [0038]     A light beam is converged on an optical disc and reflected from the optical disc. A plurality of light receiving elements (not shown) receive the reflected light and output electric signals corresponding to the intensity of the received light. The signals output from the plurality of light receiving elements are added together, and the resultant signal of the full addition is input into the variable gain amplifier  102  as a reflection signal AS.  
         [0039]     The variable gain amplifier  102  also receives a recording gate signal WTGT. The recording gate signal WTGT, which is a signal indicating which operation, recording or playback, is being performed for the optical disc, is in a high logical state (“H”) during recording and in a low logical state (“L”) during playback, for example.  
         [0040]     During recording, the light beam emitted to the optical disc is modulated so that the power changes from 15 mW at maximum to 0.5 mW at minimum, for example. During playback, the light beam is emitted with a comparatively small constant power. As the power of the light beam is greater and brighter, the level of the reflection signal AS is higher. Accordingly, a large difference arises in the level of the reflection signal AS between during playback and during recording (see  FIG. 2 ). To minimize this difference, the variable gain amplifier  102  changes the gain with the recording gate signal WTGT. Specifically, the variable gain amplifier  102  amplifies the reflection signal AS with a small gain during recording and a large gain during playback, and outputs the resultant amplifier output signal AP to the high-speed envelope detection circuit  140 .  
         [0041]     The high-speed envelope detection circuit  140 , which may be a general detection circuit, obtains the upper (bright-side) envelope of the amplifier output signal AP. The high-speed envelope detection circuit  140  includes an amplifier  142 , current sources  144  and  148  and a capacitor  146 . One end of the capacitor  146  is grounded, and the other end thereof serves as the output of the high-speed envelope detection circuit  140 . In the high-speed envelope detection circuit  140 , the current source  144  charges the capacitor  146  with a current of a magnitude corresponding to the amplifier output signal AP, and the current source  148  discharges the capacitor  146  with a current of a set magnitude. The high-speed envelope detection circuit  140  outputs the voltage at the capacitor  146  to the switch  104 , the negative input terminal of the differential circuit  106 , and the integration circuit  160 , as the envelope signal EM.  
         [0042]     The integration circuit  160  includes a resistance  162  and a capacitor  164 . One end of the resistance  162  receives the envelope signal EM, and the other end thereof is connected to the switch  104  and also to one end of the capacitor  164 . The other end of the capacitor  164  is grounded. Thus, the integration circuit  160  outputs a signal obtained by integrating the envelope signal EM to the switch  104 .  
         [0043]     The edge detection circuit  122  outputs a pulse to the monostable multivibrator circuit  124  every time the level of the recording gate signal WTGT changes. Once receiving a pulse from the edge detection circuit  122 , the monostable multivibrator circuit  124  generates a pulse of being “H” for a time t 1  of a predetermined length, and outputs this pulse to the switch  104  as a signal MM 1 .  
         [0044]     The switch  104  selects the output of the integration circuit  160  when the signal MM 1  is “L” and selects the envelope signal EM when it is “H”, and outputs the selected signal to the positive input terminal of the differential circuit  106  as a signal IS.  
         [0045]     The differential circuit  106  calculates the difference between the envelope signal EM as the input signal from the high-speed envelope detection circuit  140  and the signal IS as the input signal from the switch  104 , and outputs the results to the comparator  108  as a differential signal DF.  
         [0046]     The D/A converter  112 , having a previously input digital value, converts this value to a voltage and outputs the voltage to the comparator  108  as a threshold SD. The comparator  108  compares the differential signal DF with the threshold SD, and outputs a signal of “H” when the differential signal DF is greater than the threshold SD and a signal of “L” when it is smaller than the threshold SD, as a defect detection signal DD. An arbitrary value may be given to the D/A converter  112 , so that the threshold SD for the defect detection can be set freely.  
         [0047]     The operation of the defect detection device of  FIG. 1  observed when a defect is detected during playback, for example, will be described. Note that substantially the same operation will also be observed during recording. Assuming that some time has passed from a start of playback, the output signal MM 1  of the monostable multivibrator circuit  124  is “L”, and thus the switch  104  selects the output of the integration circuit  160 .  
         [0048]     When a defect is present on an optical disc, light reflected from the defect point of the optical disc is weak, and therefore the levels of the reflection signal AS and the amplifier output signal AP normally drop. The high-speed envelope detection circuit  140  follows the amplifier output signal AP of which the level drops due to the defect, and outputs the envelope signal EM having substantially the same level as the signal AP.  
         [0049]     The integration circuit  160 , of which the time constant is longer than that of the high-speed envelope detection circuit  160 , does not follow the level drop of the envelope signal EM due to the defect. In other words, the output IS of the switch  104  that has selected the output of the integration circuit  160  little changes its level over the time period during which the level of the envelope signal EM is low due to the defect.  
         [0050]     Accordingly, if a defect is present, a great change occurs in the differential signal DF output from the differential circuit  106  that calculates the difference between the output IS of the switch  104  and the envelope signal EM. The comparator  108  then outputs a pulse TS 1  indicating detection of a defect, as the defect detection signal DD (see  FIG. 2 ).  
         [0051]     Next, the operation of the defect detection device of  FIG. 1  observed when the operation for the optical disc is switched from recording to playback or from playback to recording, that is, the power of the light beam and the state of the optical disc apparatus are changed accordingly will be described. If the gain of the variable gain amplifier  102  is inappropriate due to a variation in the setting of the gain and the like, a difference may arise in the level of the envelope (upper envelope) of the amplifier output signal AP between during recording and during playback, as shown in  FIG. 2 .  
         [0052]     If the level of the upper envelope of the amplifier output signal AP drops when the operation for the optical disc switches from recording to playback or from playback to recording, this will present substantially the same state as that observed when a defect is detected. Accordingly, the comparator  108  will output a pulse (false defect signal FS 1 ) as the defect detection signal DD.  
         [0053]     To avoid the occurrence described above, as shown in  FIG. 2 , the output signal MM 1  of the monostable multivibrator circuit  124  is put in “H” for a time t 1  after a change of the level of the recording gate signal WTGT, causing the switch  104  to select the envelope signal EM. This results in that the two input signals of the differential circuit  106  are the same signal and thus the differential signal DF output from the differential circuit  106  is zero. With the differential signal DF being kept smaller than the threshold SD, the defect detection signal DD output from the comparator  106  is kept “L”. No false defect signal FS 1  is thus generated. After the lapse of the time t 1 , the false defect signal FS 1  will be generated. However, by extending the time t 1  sufficiently, generation of the false defect signal FS 1  can be prevented.  
         [0054]     As described above, in the defect detection device of Embodiment 1, generation of a false defect signal is prevented during the time period for which the monostable multivibrator circuit  124  outputs a pulse. Therefore, the possibility of generating a false defect signal can be reduced.  
         [heading-0055]     (Embodiment 2)  
         [0056]      FIG. 3  is a block diagram showing a construction of a defect detection device of Embodiment 2 of the present invention. The defect detection device of  FIG. 3  is different from the defect detection device of  FIG. 1  in that the switch  104  is omitted and that an integration circuit  260  is provided in place of the integration circuit  160 .  
         [0057]     The defect detection device of  FIG. 3  also includes a variable gain amplifier  202 , a differential circuit  206 , a comparator  208 , a D/A converter  212 , an edge detection circuit  222 , a monostable multivibrator circuit  224  and a high-speed envelope detection circuit  240 , which are respectively substantially the same in construction as the variable gain amplifier  102 , the differential circuit  106 , the comparator  108 , the D/A converter  112 , the edge detection circuit  122 , the monostable multivibrator circuit  124  and the high-speed envelope detection circuit  140 . Description of these components is therefore omitted here.  FIG. 4  is a graph showing the waveforms of signals used in the defect detection device of  FIG. 3 .  
         [0058]     Referring to  FIG. 3 , the integration circuit  260  includes a resistance  262 , a capacitor  264  and a switch  266 . One end of the resistance  262  receives the envelope signal EM, and the other end thereof is connected to the positive input terminal of the differential circuit  206  and also to one end of the capacitor  264 . The other end of the capacitor  264  is grounded. Thus, the integration circuit  260  outputs a signal obtained by integrating the envelope signal EM to the differential circuit  206  as the signal IS. The switch  266  is placed between both ends of the resistance  262 . By operating the switch  266 , therefore, the time constant of the integration circuit  260  can be switched. More specifically, the switch  266  is ON when the output signal MM 1  of the monostable multivibrator circuit  224  is “H” and OFF when it is “L”.  
         [0059]     The differential circuit  206  calculates the difference between the envelope signal EM as the input signal from the high-speed envelope detection. circuit  240  and the signal IS as the input signal from the integration circuit  260 , and outputs the results to the comparator  208  as the differential signal DF.  
         [0060]     The level of the recording gate signal WTGT changes with switching of the operation for an optical disc from recording to playback or from playback to recording. The monostable multivibrator circuit  224  puts its output signal MM 1  in “H” for a time t 1  after a change of the level of the recording gate signal WTGT. This causes the switch  266  to be ON and thus the time constant of the integration circuit  260  to be made small.  
         [0061]     As a result, as shown in  FIG. 4 , the output signal IS of the integration circuit  260  changes fast to be close to the envelope signal EM. This suppresses the differential signal DF output from the differential circuit  206  from becoming so large, and thus widely shortens the time period during which a false defect signal FS 2  is generated. If the time constant of the integration circuit  260  can be made sufficiently small, generation of the false defect signal FS 2  itself can be prevented.  
         [0062]     As described above, according to the defect detection device of Embodiment 2, the possibility of generation of a false defect signal can be reduced even immediately after switching of the operation for an optical disc from recording to playback or from playback to recording, by reducing the time constant of the integration circuit  260 .  
         [heading-0063]     (Embodiment 3)  
         [0064]      FIG. 5  is a block diagram showing a construction of a defect detection device of Embodiment 3 of the present invention. The defect detection device of  FIG. 5  is different from the defect detection device of  FIG. 3  in that a second monostable multivibrator circuit  326  as the second pulse generation section is additionally provided and that an integration circuit  360  is provided in place of the integration circuit  260 .  
         [0065]     The defect detection device of  FIG. 5  also includes a variable gain amplifier  302 , a differential circuit  306 , a comparator  308 , a D/A converter  312 , an edge detection circuit  322 , a first monostable multivibrator circuit  324  and a high-speed envelope detection circuit  340 , which are respectively substantially the same in construction as the variable gain amplifier  202 , the differential circuit  206 , the comparator  208 , the D/A converter  212 , the edge detection circuit  222 , the monostable multivibrator circuit  224  and the high-speed envelope detection circuit  240 . Description of these components is therefore omitted here.  FIG. 6  is a graph showing the waveforms of signals used in the defect detection device of  FIG. 5 .  
         [0066]     Referring to  FIG. 5 , the first monostable multivibrator circuit  324  outputs its output signal MM 1  to the second monostable multivibrator circuit  326  and the integration circuit  360 . The second monostable multivibrator circuit  326  outputs its output signal MM 2  to the integration circuit  360 . The second monostable multivibrator circuit  326  generates and outputs a pulse of being “H” for a time t 2  of a predetermined length once the pulse output from the first monostable multivibrator circuit  324  terminates, that is, once the level of the signal MM 1  changes from “H” to “L” (see  FIG. 6 ).  
         [0067]     The integration circuit  360  includes a resistance  362 , a capacitor  364  and switches  366 ,  368  and  372 . One end of the resistance  362  receives the envelope signal EM, and the other end thereof is connected to the positive input terminal of the differential circuit  306  and also to one end of the capacitor  364  via the switch  368 . The other end of the capacitor  364  is grounded. Thus, the integration circuit  360  outputs a signal obtained by integrating the envelope signal EM to the differential circuit  306  as the signal IS.  
         [0068]     The switch  368  selects the capacitor  364  when the signal MM 1  is “L” and a reference voltage VREF when it is “H”, to connect the selected one to the positive input terminal of the differential circuit  306 . The switch  372  is ON only when the signal MM 1  is “H”, to supply the reference voltage VREF to the capacitor  364 . The switch  366  is placed between both ends of the resistance  362 . By operating the switch  366 , therefore, the time constant of the integration circuit  360  is switched. More specifically, the switch  366  is ON when the output signal MM 2  of the second monostable multivibrator circuit  326  is “H” and OFF when it is “L”.  
         [0069]     The differential circuit  306  calculates the difference between the envelope signal EM as the input signal from the high-speed envelope detection circuit  340  and the signal IS as the input signal from the integration circuit  360 , and outputs the results to the comparator  308  as the differential signal DF.  
         [0070]     The level of the recording gate signal WTGT changes with switching of the operation for an optical disc from recording to playback or from playback to recording. As shown in  FIG. 6 , the first monostable multivibrator circuit  324  puts its output signal MM 1  in “H” for a time t 1  after a change of the level of the recording gate signal WTGT. With the signal MM 1  being “H”, the switch  368  selects the reference voltage VREF, and thus the reference voltage VREF is output from the integration circuit  360  as the signal IS. Also, the switch  372  is made ON to supply the reference voltage VREF to the capacitor  364 , so that the voltage at the capacitor  364  is initialized to the reference voltage VREF.  
         [0071]     The reference voltage VREF is a voltage having a value on the side of the level of the envelope signal EM observed when a defect is present with respect to the level of the envelope signal EM observed when no defect is present. Herein, the reference voltage VREF is set at a value lower than the voltage of the envelope signal EM.  
         [0072]     The second monostable multivibrator circuit  326  puts its output signal MM 2  in “H” for a time t 2  after termination of a pulse output from the first monostable multivibrator circuit  324 . With the signal MM 2  being “H”, the switch  366  is made ON to reduce the time constant of the integration circuit  360 .  
         [0073]     As a result, as shown in  FIG. 6 , the output signal IS of the integration circuit  360  is fixed to a low voltage during the time period for which the first monostable multivibrator circuit  324  outputs a pulse, and thereafter rapidly goes closer to the envelope signal EM during the time period for which the second monostable multivibrator circuit  326  outputs a pulse. In other words, during the time t 1 +t 2 , the differential signal DF output from the differential circuit  306  changes its level in the direction opposite to that in which it changes when a defect is detected (that is, toward a negative voltage). Accordingly, the comparator  308  is prevented from generating a false defect signal.  
         [0074]     As described above, according to the defect detection device of Embodiment 3, generation of a false defect signal is prevented even immediately after switching of the operation for an optical disc from recording to playback or from playback to recording, by fixing the output signal IS of the integration circuit  360  at a low level and thereafter reducing the time constant of the integration circuit  360 . Thus, highly reliable defect detection free from false detection can be achieved.  
         [heading-0075]     (Embodiment 4)  
         [0076]      FIG. 7  is a block diagram showing a construction of a defect detection device of Embodiment 4 of the present invention. The defect detection device of  FIG. 7  is different from the defect detection device of  FIG. 5  in that a high-speed envelope detection circuit  440  is provided in place of the high-speed envelope detection circuit  340 .  
         [0077]     The defect detection device of  FIG. 7  also includes a variable gain amplifier  402 , a differential circuit  406 , a comparator  408 , a D/A converter  412 , an edge detection circuit  422 , a first monostable multivibrator circuit  424 , a second monostable multivibrator circuit  426  and an integration circuit  460 , which are respectively substantially the same in construction as the variable gain amplifier  302 , the differential circuit  306 , the comparator  308 , the D/A converter  312 , the edge detection circuit  322 , the first monostable multivibrator circuit  324 , the second monostable multivibrator circuit  326  and the integration circuit  360 . Description of these components is therefore omitted here.  FIG. 8  is a graph showing the waveforms of signals used in the defect detection device of  FIG. 7 .  
         [0078]     The integration circuit  460  includes a resistance  462 , a capacitor (first capacitor)  464  and switches  466 ,  468  and  472 , which are respectively substantially the same in construction as the resistance  362 , the capacitor  364  and the switches  366 ,  368  and  372 . The switch  472  constitutes a first switch.  
         [0079]     The high-speed envelope detection circuit  440  is different from the high-speed envelope detection circuit  140  in  FIG. 1  in that a switch (second switch)  452  is newly provided. The high-speed envelope detection circuit  440  also includes an amplifier  442 , current sources  444  and  448 , and a capacitor (second capacitor)  446 , which are respectively substantially the same in construction as the amplifier  142 , the current sources  144  and  148 , and the capacitor  146 .  
         [0080]     The switch  452  is operable to supply the reference voltage VREF to the output of the high-speed envelope detection circuit  440 , that is, to the capacitor  446  only when the level of the output signal MM 1  of the first monostable multivibrator circuit  424  is “H”.  
         [0081]     The level of the recording gate signal WTGT changes with switching of the operation for an optical disc from recording to playback or from playback to recording. As shown in  FIG. 8 , the first monostable multivibrator circuit  424  puts its output signal MM 1  in “H” for a time t 1  after a change of the level of the recording gate signal WTGT. With the signal MM 1  being “H”, the switch  452  is made ON, allowing the voltage at the capacitor  446 , that is, the envelope signal EM output from the high-speed envelope detection circuit  440  to be initialized to the reference voltage VREF. Thereafter, as the switch  452  is turned OFF, the envelope signal EM rapidly resumes its original value.  
         [0082]     As a result, as shown in  FIG. 8 , the envelope signal EM is substantially equal in level to the output signal IS of the integration circuit  460  during the time period (time t 1 +t 2 ) for which the first and second monostable multivibrator circuit  424  and  426  output their pulses. Accordingly, the output of the differential circuit  406  is substantially zero, and thus the comparator  408  is prevented from generating a false defect signal.  
         [0083]     Also, during the above time period, the differential circuit  406  does not output a signal opposite in polarity to that output when a defect is detected (signal having a negative value). Accordingly, the dynamic range of the output is prevented from becoming wide, and thus in the D/A converter  412  that outputs the threshold SD to the comparator  408 , the quantization step size can be made small without changing the resolution (the number of bits of the input digital value). Since the threshold SD can be set more appropriately, the sensitivity of the defect detection can be enhanced.  
         [0084]     The switches  452  and  472  have a resistance (ON resistance) even when they are ON. Therefore, the voltages at the capacitors  446  and  464  gradually decrease during the initialization of the capacitors with a pulse of the signal MM 1 . Since the voltages at the capacitors  446  and  464  are given to the differential circuit  406 , they should desirably be equal to each other at least until termination of a pulse of the signal MM 1 .  
         [0085]     In view of the above, the high-speed envelope detection circuit  440  and the integration circuit  460  may be constructed so that the product of the capacitance of the capacitor  446  and the ON resistance of the switch  452  and the product of the capacitance of the capacitor  464  and the ON resistance of the switch  472  are substantially equal to each other.  
         [0086]     As described above, according to the defect detection device of Embodiment 4, generation of a false defect signal is prevented even immediately after switching of the operation for an optical disc from recording to playback or from playback to recording, by fixing the envelope signal EM output from the high-speed envelope detection circuit  440  and the output signal IS of the integration circuit  460  at a low level and thereafter reducing the time constant of the integration circuit  460 . Thus, highly reliable defect detection free from false detection can be achieved. In addition, the sensitivity of the defect detection can be enhanced.  
         [0087]     As described above, the defect detection device of the present invention can detect a defect on an optical disc further correctly, and is useful as a defect detection device and the like used in an optical disc apparatus and the like.  
         [0088]     While the present invention has been described in preferred embodiments, it will be apparent to those skilled in the art that the disclosed invention may be modified in numerous ways and may assume many embodiments other than that specifically set out and described above. Accordingly, it is intended by the appended claims to cover all modifications of the invention which fall within the true spirit and scope of the invention.