hereinafter , preferred embodiments of the present invention will be described with reference to the accompanying drawings . fig1 is a block diagram showing a construction of a defect detection device of embodiment 1 of the present invention . the defect detection device of fig1 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 . fig2 is a graph showing the waveforms of signals used in the defect detection device of fig1 . 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 . 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 . 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 fig2 ). 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 . 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 . 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 . 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 . 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 . 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 . 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 . the operation of the defect detection device of fig1 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 . 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 . 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 . 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 fig2 ). next , the operation of the defect detection device of fig1 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 fig2 . 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 . to avoid the occurrence described above , as shown in fig2 , 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 . 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 . fig3 is a block diagram showing a construction of a defect detection device of embodiment 2 of the present invention . the defect detection device of fig3 is different from the defect detection device of fig1 in that the switch 104 is omitted and that an integration circuit 260 is provided in place of the integration circuit 160 . the defect detection device of fig3 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 . fig4 is a graph showing the waveforms of signals used in the defect detection device of fig3 . referring to fig3 , 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 ”. 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 . 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 . as a result , as shown in fig4 , 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 . 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 . fig5 is a block diagram showing a construction of a defect detection device of embodiment 3 of the present invention . the defect detection device of fig5 is different from the defect detection device of fig3 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 . the defect detection device of fig5 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 . fig6 is a graph showing the waveforms of signals used in the defect detection device of fig5 . referring to fig5 , 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 fig6 ). 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 . 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 ”. 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 . 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 fig6 , 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 . 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 . 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 . as a result , as shown in fig6 , 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 . 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 . fig7 is a block diagram showing a construction of a defect detection device of embodiment 4 of the present invention . the defect detection device of fig7 is different from the defect detection device of fig5 in that a high - speed envelope detection circuit 440 is provided in place of the high - speed envelope detection circuit 340 . the defect detection device of fig7 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 . fig8 is a graph showing the waveforms of signals used in the defect detection device of fig7 . 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 . the high - speed envelope detection circuit 440 is different from the high - speed envelope detection circuit 140 in fig1 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 . 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 ”. 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 fig8 , 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 . as a result , as shown in fig8 , 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 . 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 . 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 . 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 . 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 . 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 . 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 .