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JP2002218595

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DESCRIPTION JP2002218595
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
acoustic detection device, and more particularly to an acoustic detection device capable of
optically detecting a vibration displacement of a diaphragm which is vibrated by an external
acoustic signal, such as an optical microphone device.
[0002]
2. Description of the Related Art FIG. 7 shows the configuration of an optical microphone device
disclosed in JP-A-11-355893 as an example of a conventional sound detection device. A laser
beam is emitted from a semiconductor laser 9 provided opposite to the diaphragm 1 which
vibrates by sound, and the emitted light is irradiated to the diaphragm 1 through the objective
lens 2. Reflected light from the diaphragm 1 is detected by the diaphragm displacement
detection photodiode 6 through the objective lens 2 so that detection of vibration displacement is
performed. Further, the amount of laser emission from the semiconductor laser 9 is monitored by
a monitor photodiode 7 provided in the vicinity of the diaphragm displacement detection
photodiode 6, and an electrical signal from this monitor photodiode 7 is a laser APC (emission
stabilization circuit). The laser light is returned to the semiconductor laser 9 through 8 to
perform control such that the amount of laser light emission becomes constant.
[0003]
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1
The semiconductor laser 9 and the photodiodes 6 and 7 are incorporated in a housing 10, and
the objective lens 2 is attached to the housing 10. In the acoustic detection apparatus having
such a structure, the amount of movement (displacement) due to the vibration of the diaphragm
1 is detected as a focus error signal (FES signal) as in the optical pickup. Depending on the
configuration of the optical element incorporated in the housing 10, various methods such as the
Foucault knife edge method and the astigmatism method may be used.
[0004]
FIG. 8 shows a circuit for extracting displacement signals of the reflection surface of the
diaphragm 1. It is shown that two diaphragm displacement detection photodiodes 6 are prepared
at different positions, and the difference signals from these photodiodes 6a and 6b are extracted
through the differential amplifier 13 and extracted as the displacement signal 14 of the
diaphragm reflection surface. ing. When taking a difference signal between the photoelectric
conversion output A from the photodiode 6a and the photoelectric conversion output B from the
photodiode 6b as described above and using it as a displacement signal, the Foucault knife edge
method is used. When the astigmatism method is used, four detectors (photodiodes) of a, b, c,
and d are arranged in a row by two each and arranged in a square, and their diagonal sum
signals (a + c) and (b + d) are These differential signals are taken out using the differential
amplifier 13. The method of generating the difference signal is the same as the Foucault knife
edge method.
[0005]
As described above, the reflected light from the diaphragm 1 is guided to focus servo detection
means such as the diaphragm displacement detection photodiode 6 and the movement of the
diaphragm 1 is taken out as a focus error detection signal, whereby an output with good linearity
can be obtained. In this case, a stable detection output can be obtained by setting the vibration
range of the diaphragm 1 within the linear range of the focus error detection signal. In such a
conventional device, there is no restriction on the vibration of the diaphragm 1, and an acoustic
output with good linearity can be obtained from the point where the amplitude of the diaphragm
1 can be detected without contact.
[0006]
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2
However, in order to stably extract the displacement signal of the diaphragm in such a
conventional acoustic detection device, the monitor photodiode 7 as shown in FIG. In order to
feed back the light emission monitor signal obtained from the monitor photodiode 7 and stabilize
the light emission of the semiconductor laser 9, the light emission stabilization circuit 8 is
required. Therefore, there is a problem that the internal structure of the detection unit case 10 is
complicated and the drive circuit for the semiconductor laser 9 such as the light emission
stabilization circuit 8 must be provided outside. The present invention does not require a monitor
photodiode in the detection unit housing, and drives the semiconductor laser by providing a
constant current circuit having a simple configuration instead of the APC circuit, thereby
reducing the number of parts significantly and inexpensively. It aims at providing an acoustic
detection device.
[0007]
SUMMARY OF THE INVENTION An acoustic detection apparatus according to the present
invention is a light emitting and receiving device in which a vibrating plate vibrating by sound
and a vibrating plate disposed opposite to the vibrating plate are disposed on the same plane. An
objective lens disposed on an optical path between the element substrate and the diaphragm and
the light emitting / receiving element substrate, and adjusting an optimum focal position of light
emitted from the light emitting element and / or reflected light from the diaphragm; And an
actuator for minutely moving the objective lens on the optical path, wherein light reflected from
the diaphragm is received by the light receiving element and extracted as an electric signal to
detect vibration displacement of the diaphragm due to sound. In the detection device, the
difference signal is divided by the sum signal of the electric signals from the two light receiving
elements, and the signal is separated out from the signal in the audible band of the displacement
signal and the displacement signal extracting means which is normalized and extracted as the
displacement signal. Taken out as a detection signal of the vibration displacement Te, is provided
with a displacement signal separating means for supplying a servo control signal for said
actuator to separate the audible band following signals of the displacement signal.
[0008]
DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 6 is a view showing the configuration of
a head portion of an optical microphone device as an example of an acoustic detection device
according to the present invention.
The acoustic diaphragm 1 is attached to one end of a housing 12 having an opening, and the
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other end is attached to the detection unit housing 10. A light receiving / emitting element
substrate 4 is disposed in the detection unit housing 10 so as to be opposed to the diaphragm 1,
and the light emitting element 4a and the light receiving element 4b are disposed on the same
surface.
[0009]
Radiation light from the light emitting element 4a disposed at the center of the light emitting /
receiving element substrate 4 is configured to be condensed on the diaphragm 1 via the objective
lens 2 disposed on the optical path. A light transmitting sound pressure blocking plate 5 is
provided between the objective lens 2 and the diaphragm 1 so as to close one end of the
detection unit housing 10, and the blocking plate 5 makes the objective lens by external sound. 2
is configured not to vibrate. Reflected light from the diaphragm 1 is diffracted or refracted by the
reflected light beam dividing element 3 through the objective lens 2 and condensed on the light
receiving element 4 b of the light emitting / receiving element substrate 4.
[0010]
The objective lens 2 is provided with a uniaxial actuator 11 for an objective lens, and by driving
the actuator 11, the objective lens 2 is minutely moved on the optical path, and adjustment is
made so as to condense emitted light or reflected light at the optimum focal position. Is done. An
actuator control target value signal for adjusting the objective lens 2 to the optimum focus
position is applied to the actuator 11 by a circuit (not shown).
[0011]
In the present invention, the electric signal from the light receiving element 4b is supplied as a
servo control signal to the actuator 11 by performing predetermined processing. FIG. 1 is a
circuit diagram showing the configuration of a control circuit used in the acoustic detection
device of the present invention. By employing such a circuit configuration, it is possible to omit
the monitor photodiode 7 and the APC circuit 8 which are required in the conventional device
shown in FIG. In the present invention, two photodiodes are prepared as the light receiving
element 4b. The minute current portion of the difference output “A−B” 21 and the sum
output “A + B” 22 of the two photodiodes are enlarged via the logarithmic amplifiers 23 and
24 and supplied to the input of the differential amplifier 25.
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[0012]
Unlike the optical disk, the acoustic diaphragm 1 has no unevenness and can use a portion where
reflection is stable. Therefore, all the signal light detected from the signal detection photodiode is
the laser emission itself of the light emitting element 4a. At the output of the differential
amplifier 25, a signal obtained by dividing the difference signal by the sum signal of the electric
signals from the two light receiving elements 4b is obtained. By taking the output of the
differential amplifier 25 through the antilog amplifier 26, the normalized displacement signal can
be taken out. The extracted displacement signal is separated by a high pass filter (HPF) 27 into a
signal of 20 Hz or more in the audible band, and is taken out as an audio output 400 through a
microphone amplifier 28. The audio output 400 is a detection signal of vibration displacement.
Further, a signal below the audible band of the normalized displacement signal, which is the
output of the inverse logarithmic amplifier 26, is separated by the low pass filter (LPF) 29 and
taken out as the servo control signal 200.
[0013]
The servo control signal 200 extracted in this manner and the actuator control target value
signal 100 generated by a circuit not shown in advance are input to the servo circuit 30, and an
actuator control signal 300 is obtained as an output signal thereof. By driving and controlling the
actuator 11 using the actuator control signal 300, the position of the objective lens 2 can be
controlled to be an optimum distance for detection of the diaphragm 1.
[0014]
FIG. 2 shows the open loop characteristics of the servo circuit 30. As shown in FIG. As shown in
FIG. 2, the servo gain characteristic is uniform from zero to a constant frequency, and then
gradually decreases and changes so that the gain becomes zero at the cutoff frequency fc. In the
present embodiment, the cutoff frequency fc at which the gain is 0 is set to 20 Hz, but this may
be arbitrarily set by changing the filter coefficients of the high pass filter 27 and the low pass
filter 29 in conjunction. Can.
[0015]
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FIG. 3 is a block diagram showing the configuration of the servo circuit 30. As shown in FIG. The
servo circuit 30 comprises an error signal generation circuit 32, a compensation circuit 34, a
drive circuit 36 and a feedback circuit 38. An error signal 500 is obtained by inputting the
actuator control target value signal 100 and the servo control signal 200 output from the
feedback circuit 38 to the error signal generation circuit 32, and the error signal 500 is used as
the compensation circuit 34 and the drive circuit 36. The actuator control signal 300 can be
obtained by taking it out.
[0016]
FIG. 4 is a diagram showing a specific circuit configuration of the compensation circuit 34 and
the drive circuit 36 shown in FIG. The configurations themselves of the servo circuit 30, the
compensation circuit 34 and the drive circuit 36 shown in FIGS. 3 and 4 are well known, and are
often used when performing servo control using a focus error signal with an optical pickup or
the like. Therefore, the description of the details of the specific circuit configuration of FIG. 4 is
omitted. The portion indicated by A in the compensation circuit 34 of FIG. 4 indicates a
differentiating circuit, and the phase of the error signal 500 is advanced by this differentiating
circuit. Further, a portion B indicates an integration circuit, and the integration circuit delays the
phase of the error signal 500. FIG. 5 is a diagram for explaining phase compensation of the error
signal 500. Phase delay compensation is performed by an integration circuit between frequencies
f1 and f2, and phase lead compensation is performed by a differentiation circuit between
frequencies f3 and f4. It is shown that.
[0017]
As described above, according to the present invention, the difference signal is divided by the
sum signal of the electric signals from the two light receiving elements, this is normalized and
taken out as the displacement signal, and in the audible band of the displacement signal. The
signal is separated and extracted as a detection signal of vibration displacement, and the signal
below the audible band of the displacement signal is separated and supplied as a servo control
signal to the actuator, so that the fluctuation of the light emission is eliminated to displace the
diaphragm. The signal can be extracted stably. In addition, there is an advantage that the circuit
configuration can be simplified because the monitor photodiode and the APC circuit, which are
conventionally required, are not required. Furthermore, since the signal below the audible band
of the displacement signal is separated and used as a servo control signal for the actuator, the
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signal outside the audible band can be extracted with a good S / N ratio.
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