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DESCRIPTION JP2010068391

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DESCRIPTION JP2010068391
The present invention provides a microphone device with improved resistance to electromagnetic
noise. A microphone device (1) according to the present invention includes a condenser
microphone (5) whose capacitance changes in accordance with applied sound pressure, a
charging means for charging the condenser microphone (5), and a discharging means for
discharging the condenser microphone (5). The terminal voltage V of the condenser microphone
5 is compared with a first reference voltage V and a second reference voltage V larger than the
first reference voltage, and the charge means and the discharge means are switched based on the
comparison result. And a comparator 7 that outputs a signal of a cycle corresponding to the
capacitance of the condenser microphone 5, and acquires an audio signal from a change in the
cycle of the output signal from the comparator 7. [Selected figure] Figure 1
Microphone device
[0001]
The present invention relates to a microphone device, and more particularly to a microphone
device using a condenser microphone.
[0002]
Conventionally, a microphone device provided with a condenser microphone is used (for
example, Patent Document 1).
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An exemplary circuit configuration of such a microphone device is schematically shown in FIG. In
the microphone device 20, one electrode of the condenser microphone 22 is connected to the DC
power supply 21, and the other electrode is connected to the input of the amplifier 23. As shown
in the equivalent circuit diagram, the input impedance 24 of the amplifier 23 will be connected in
series with the condenser microphone 22. When sound pressure acts on the condenser
microphone 22, its electrostatic capacitance changes in an oscillating manner, so that an
alternating current component is generated in the voltage V between the electrodes. The
alternating current component is output from the amplifier 23 as an audio signal.
[0003]
By the way, in order to generate an AC component in the inter-electrode voltage V of the
condenser microphone 22 as described above, it is necessary to keep the electric charge stored
in the condenser microphone 22 constant when a sound pressure is applied. . In order to keep
the charge constant, the CR time constant defined by the capacitance of the condenser
microphone 22 and the input impedance 24 of the amplifier 23 is sufficiently larger than the
reciprocal (that is, the period) of the frequency of the sound of interest. There must be.
Therefore, in the microphone device 20, the CR time constant is set large by the input impedance
24 and the capacitance of the condenser microphone 22 using the large amplifier 23 of the input
impedance 24. Therefore, the voltage application line including the input impedance 24 has a
high impedance. Unexamined-Japanese-Patent No. 2004-23202
[0004]
As described above, since the impedance of the voltage application line is set high, the
conventional microphone device 20 has a problem that electromagnetic noise tends to be applied
to the line. Therefore, an object of the present invention is to provide a microphone device with
improved immunity to electromagnetic noise.
[0005]
The present invention relates to a microphone device, and in order to achieve the above object,
the microphone device comprises: a condenser microphone whose capacitance changes in
accordance with an applied sound pressure; and a charging means for charging the condenser
microphone And discharging means for discharging the condenser microphone, and comparing
the terminal voltage of the condenser microphone with a first reference voltage and a second
reference voltage larger than the first reference voltage, based on the comparison result. And a
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2
comparator for switching the charging means and the discharging means to output a signal of a
cycle corresponding to the capacitance of the condenser microphone, and acquiring an audio
signal from a change in the cycle of the output signal from the comparator. It is configured.
[0006]
As one example, the charging means comprises a charging resistor connected in series to the
capacitor microphone, and the discharging means comprises a discharging resistor connected in
parallel to the capacitor microphone.
As another example, the charging means and the discharging means are configured such that at
least one means flows a constant current.
[0007]
The comparator outputs a first level signal when the terminal voltage becomes lower than the
first reference voltage, and outputs a second level signal when the terminal voltage becomes
higher than the second reference voltage. The means comprises a first switch actuated by the
first level signal to form a charging circuit, and the discharge means is a second switch actuated
by the second level signal to form a discharge circuit It may be configured to have In addition, a
hysteresis comparator may be used as the comparator.
[0008]
In the microphone device according to the present invention, a charge / discharge circuit
including a condenser microphone is formed, and an audio signal is acquired by converting a
change in capacitance of the condenser microphone due to a sound pressure into a change in
period of an output signal of the comparator. Can. Since either the charging circuit or the
discharging circuit is always connected to the condenser microphone, the impedance at the input
terminal of the amplifier by the charging circuit or the discharging circuit (combined impedance
of the input impedance of the amplifier and the impedance of the charging circuit or the
discharging circuit) Can be reduced, and the resistance to electromagnetic noise can be improved
as compared with the prior art.
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[0009]
Hereinafter, embodiments of the present invention will be described with reference to the
attached drawings. FIG. 1 schematically shows a circuit diagram of the microphone device
according to the present embodiment. The microphone device 1 according to the present
embodiment includes a charging circuit formed by connecting a DC voltage source 2, a charge
switch 3 and a charging resistor 4 in series, a discharge circuit formed by connecting a discharge
switch 8 and a discharge resistor 9 in series, The charge circuit is connected in series and the
discharge circuit is connected in parallel. The capacitor microphone 5 and the hysteresis
comparator 7 are provided.
[0010]
The hysteresis comparator 7 has a first input terminal (non-inverted input terminal) connected to
the connection point between the charging resistor 4 and the capacitor microphone 5, and a
second input terminal (inverted input terminal) connected to the first reference voltage Vref1. Is
connected to the first reference voltage source 10 that outputs the second reference voltage
source 11, and the third input terminal (inverting input terminal) is connected to the second
reference voltage source 11 that outputs the second reference voltage Vref2 (Vref2> Vref1). It is
done.
[0011]
The comparator 7 compares the terminal voltage VT of the condenser microphone 5 input
through the terminal voltage output line 6 with the first reference voltage Vref1 and the second
reference voltage Vref2, and has a level corresponding to the comparison result. It is configured
to output a signal.
That is, when the terminal voltage VT of the condenser microphone 5 is equal to or lower than
the first reference voltage Vref1, the comparator 7 outputs an H level signal, and the terminal
voltage VT rises to reach the second reference voltage Vref2. Then, the output signal is switched
from H level to L level, and when the terminal voltage VT decreases and reaches the reference
voltage Vref1, the output signal is switched from L level to H level. The output signal of the
comparator 7 is input to these switches 3 and 8 via the output line 12 in order to control the
charge switch 3 and the discharge switch 8 ON / OFF.
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[0012]
In the present embodiment, when the signal of H level is output from the comparator 7, the
charge switch 3 is turned on and the discharge switch 8 is turned off, and when the signal of L
level is output from the comparator, the charge switch 3 is output. And the discharge switch 8 is
turned on.
[0013]
The operation of the microphone device in the present embodiment will be described below.
First, assuming that the terminal voltage VT of the condenser microphone 5 is equal to or lower
than the first reference voltage Vref1 at the start of operation of the microphone device 1, the
comparator 7 charges the signal of H level through the output line 12 Output to switch 8. As a
result, the charge switch 3 is turned on and the discharge switch 8 is turned off. Therefore,
current flows from the DC voltage source 2 to the condenser microphone 5 through the charging
resistor 4 and charges are accumulated, so that the terminal voltage VT of the condenser
microphone 5 rises.
[0014]
When the terminal voltage VT exceeds the reference voltage Vref1 and reaches the second
reference voltage Vref2, the comparator 7 switches the output signal from the H level to the L
level. As a result, the charge switch 3 is turned off and the discharge switch 8 is turned on, so
that the charge is stopped and the charge stored in the capacitor microphone 5 is discharged
through the discharge resistor 9. As a result, the terminal of the capacitor microphone 5 is The
voltage VT drops.
[0015]
When the terminal voltage VT of the condenser microphone 5 decreases to the first reference
voltage Vref1, the comparator 7 switches the output signal from the L level to the H level. As a
result, as described above, the discharge via the discharge resistor 9 is stopped and the charging
of the condenser microphone 5 is started, and the terminal voltage VT rises. Thereafter, the
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comparator 7 repeats the same comparison operation.
[0016]
As described above, in the microphone device 1, the capacitor microphone 5 is charged and
discharged by the CR circuit configured by the charge circuit having the charge resistor 4, the
discharge circuit having the discharge resistor 9, and the capacitor microphone 5. A pulse signal
is output from the comparator 7. The period of this pulse signal is defined by the capacitance of
the condenser microphone 5.
[0017]
FIG. 2 is a waveform diagram showing waveforms of the terminal voltage VT of the condenser
microphone 5 and the output signal of the comparator 7 in a state where no sound pressure is
applied to the microphone 5, that is, in a steady state. A waveform a indicated by a dotted line is
a waveform of the terminal voltage VT of the condenser microphone 5 controlled by the output
of the comparator 7. As apparent from the waveform a, the terminal voltage VT changes in a
curve according to an exponential function. The time t1 is a period in which the charge switch 3
is ON and the discharge switch 8 is OFF, and the time t2 is a period in which the charge switch 3
is OFF and the discharge switch 8 is ON. The time t1 and the time t2 appear alternately on the
time axis.
[0018]
The waveform b represents the output signal waveform of the comparator 7. At time t1, the
waveform b is at H level and at time t2, it is at L level. In the above steady state, the output signal
of the comparator 7 repeats inversion of H level and L level at a constant period as shown by the
waveform b. This cycle is defined by the capacitance of the condenser microphone 5 as described
above. The waveform c will be described later.
[0019]
Next, the case where sound pressure is applied to the condenser microphone will be described.
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When sound pressure is applied to the condenser microphone 5, the capacitance of the
condenser microphone 5 changes, and the time constant in charge and discharge changes
accordingly. Therefore, the time when the terminal voltage VT of the condenser microphone 5
rises from the first reference voltage Vref1 to the second reference voltage Vref2 and the time
when the terminal voltage VT falls from the second reference voltage Vref2 to the first reference
voltage Vref1 respectively change This will result in periodic variations of the output signal of
the comparator 7.
[0020]
FIG. 3 shows the waveforms of the terminal voltage VT of the condenser microphone 5 and the
output signal of the comparator 7 when sound pressure is applied to the condenser microphone
5. The waveform a of the terminal voltage VT of the condenser microphone 5 is displayed only
for the first two cycles for the sake of easy viewing of the drawing, and the latter is omitted. As
shown in the time zone Tm in FIG. 3, when a sound pressure is applied to the condenser
microphone 5, the period of the output signal of the comparator 7 changes due to the change of
the time constant of charge and discharge based on the change of electrostatic capacitance Do.
[0021]
As described above, in the microphone device 1 according to the present embodiment, the period
of the signal output from the comparator 7 changes in accordance with the sound pressure
applied to the condenser microphone 5. That is, the change in the sound pressure appears as the
change in the period of the output signal of the comparator 7. The output signal of the
comparator 7 is input to a digital signal processing unit (not shown). The digital signal processor
detects a change in the period of the input signal, and executes processing to extract an audio
signal based thereon.
[0022]
As described above, since the conventional microphone device is configured to pick up the AC
voltage signal generated between the electrodes of the condenser microphone as it is as an audio
signal, the input impedance of the amplifier is set to a high impedance, and this input impedance
is set. It is necessary to increase the CR time constant defined by the and the condenser
microphone. On the other hand, in the microphone device 1 according to the above-described
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embodiment, a CR charging / discharging circuit is configured by the charging circuit having the
charging resistor 4, the discharging circuit having the discharging resistor 9, and the condenser
microphone 5. An audio signal can be obtained by extracting the capacitance change of the
condenser microphone 5 as a change of the signal cycle, that is, extracting the sound pressure
oscillation of the condenser microphone 5 as a frequency modulation component. Since either
the charging circuit or the discharging circuit is always connected to the condenser microphone,
the impedance at the input terminal of the amplifier by the charging circuit or the discharging
circuit (combined impedance of the input impedance of the amplifier and the impedance of the
charging circuit or discharging circuit) Is smaller than the input impedance of the amplifier and
is less susceptible to external noise. In addition, the output of the comparator 7 is a digital signal,
not an analog output like the amplifier output employed in conventional microphone devices.
Therefore, the microphone device 1 according to the present embodiment also has the advantage
of being less susceptible to external noise than the conventional microphone device.
[0023]
[Other Embodiments] Although one embodiment of the present invention has been described
above, the present invention is not limited to the above-described embodiment, and various
modifications and changes may be made based on the technical idea of the present invention. It
is possible. For example, charging / discharging of the condenser microphone 5 may be
performed by replacing the charging resistor 4 and / or the discharging resistor 9 described
above with a constant current source. The terminal voltage VT of the condenser microphone 5
when the charging resistor 4 and the discharging resistor 9 are respectively replaced by constant
current sources is shown as a waveform c in FIGS. 2 and 3. The waveform c shows the same
period and change in period in FIG. 2 and FIG. 3 as the waveform a. Further, in the embodiment
described above, when the signal of H level is output from the comparator 7, the charge switch 3
is turned on and the discharge switch 8 is turned off, and the signal of L level is output from the
comparator 7 Although the charge switch 3 is turned off and the discharge switch 8 is turned on,
the present invention is not limited to this. When the signal of H level is output from the
comparator 7, the charge switch 3 is turned off and the discharge switch 8 is turned on. When
the signal of L level is output from the comparator 7, the charge switch 3 is turned on. The
discharge switch 8 may be set to be turned off. In this case, if the terminal voltage VT input to
the comparator 7 is lower than the first reference voltage Vref1, an L level signal is input, and if
the input terminal voltage VT is higher than the second reference voltage Vref2, the H level is
output. Signal is output from the comparator 7.
[0024]
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It is a circuit diagram showing composition of a microphone device concerning the present
invention. It is a wave form diagram which shows the waveform of the terminal voltage of a
condenser microphone and the output signal of a comparator in one embodiment of the present
invention. It is another waveform diagram which shows the terminal voltage of a capacitor |
condenser microphone in one Embodiment of this invention, and the waveform of the output
signal of a comparator. It is a circuit diagram which shows the structure of the conventional
microphone apparatus.
Explanation of sign
[0025]
Reference Signs List 1 microphone device 2 direct current voltage source 3 charge switch 4
charge resistor 5 capacitor microphone 7 comparator 8 discharge switch 9 discharge resistor 10
first reference voltage source 11 second reference voltage source
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