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DESCRIPTION JP2015097312

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DESCRIPTION JP2015097312
PROBLEM TO BE SOLVED: To prevent distortion of the output even if the impedance of the filter
circuit connected downstream of the microphone unit is low, and to generate no noise due to the
impedance of the filter circuit, in a band below the cutoff frequency of the filter circuit Also
provides a microphone that can lower the output impedance. A microphone comprising: a
microphone unit; an HOT terminal for balanced output of signals output from the microphone
unit; and a COLD terminal, and an output terminal for outputting the signal to an output circuit.
According to the microphone, a low pass filter is disposed between the microphone unit and the
COLD terminal. [Selected figure] Figure 1
Microphone and microphone device
[0001]
The present invention relates to a microphone.
[0002]
In order to reduce wind noise and vibration noise included in the output of the microphone (in
particular, the condenser microphone), a filter circuit is disposed in front of the output circuit of
the microphone.
Since wind noise and vibration noise are mainly low frequency components (low frequency
components), a high pass filter (low cut filter) is used.
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[0003]
Since the output impedance of the condenser microphone unit is high, in order to lower it, an
impedance converter is disposed at the output side of the condenser microphone unit. An FET
(field effect transistor) is mainly used for this impedance converter. A low pass attenuation high
pass filter is disposed between the subsequent stage of the impedance converter and the output
circuit (see, for example, Patent Document 1).
[0004]
FIG. 9 is a circuit diagram showing a configuration example of a conventional microphone. As
shown in FIG. 9, the microphone 100 includes a microphone unit 1 which is a condenser
microphone unit, an impedance converter 2, a high pass filter 30, and an output amplifier 4.
[0005]
The output of the microphone 100 is a balanced output (balanced output), and its output
terminal is a 3-pin configuration having a HOT terminal 5, a cold terminal 6 and a ground
terminal 7. The positive phase output of the microphone unit 1 is output from the HOT terminal
5, and the negative phase output of the microphone unit 1 is output from the COLD terminal 6.
[0006]
When the high pass filter 30 is viewed from the impedance converter 2, the high pass filter 30
becomes a load for the impedance converter 2. Therefore, it is conceivable to design the input
impedance of the high pass filter 30 low in accordance with the output impedance of the
microphone unit 1 lowered by the impedance converter 2. However, if the input impedance of
the high pass filter 30 is designed to be low, the signal output from the impedance converter 2
becomes a factor of distortion.
[0007]
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Further, since the output impedance of the high pass filter 30 becomes high, a buffer amplifier
by an emitter follower circuit using a transistor is used for the output amplifier 4 disposed at the
subsequent stage of the high pass filter 30. However, in the output amplifier 4, the noise level
due to the high output impedance of the high pass filter 30 is increased. In particular, since the
output impedance below the cutoff frequency of the high pass filter 30 is high, the noise level
below the cutoff frequency is high.
[0008]
The high pass filter 30 includes a capacitor C30 in series with the output of the microphone unit
1 and a resistor R30 in parallel with the output of the microphone unit 1. When the frequency of
the signal output from the microphone unit 1 is low, the impedance by the capacitor C30 is high,
and the signal is not output to the output amplifier 4 side.
[0009]
On the other hand, when the frequency of the signal output from the microphone unit 1
increases, the impedance of the capacitor C30 decreases and a signal is output to the output
amplifier 4 side. As described above, the cut-off frequency is the frequency at which the highpass filter 30 outputs no signal to the output amplifier 4 or the boundary at which the signal is
output.
[0010]
Therefore, when the frequency of the signal output from the microphone unit 1 is higher than
the cut-off frequency, the impedance by the capacitor C30 becomes negligibly small, and the
impedance by the resistor R30 on the microphone unit 1 side viewed from the output amplifier 4
It becomes an output impedance. Here, the higher the impedance by the resistor R30, the higher
the noise level from the microphone unit 1 side. In general, since the impedance of the high-pass
filter 30 due to the resistor R30 is larger than the output impedance of the impedance converter
2, if the high-pass filter 30 is disposed in front of the output amplifier 4, the frequency of the
signal output from the microphone unit 1 is As it becomes higher, the level of noise output from
the output amplifier 4 becomes higher.
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[0011]
Further, the output impedance of the output amplifier 4 is a value obtained by multiplying the
reciprocal of the current amplification factor (h FE) of the transistor used when the output
amplifier 4 is configured by the emitter follower. Therefore, if the output impedance of the
microphone unit 1 is 10 Ω and the h FE of the transistor is 100, the output impedance of the
output amplifier 4 is 1/10 Ω. As described above, when the frequency of the output signal of the
microphone unit 1 becomes higher than the cutoff frequency, the impedance on the microphone
unit 1 side viewed from the output amplifier 4 becomes the value of the resistor R30 that
constitutes the high pass filter 30. Become. Assuming that the resistor R30 is 10 kΩ, the output
impedance of the output amplifier 4 is 1 kΩ.
[0012]
Assuming that the output impedance is 1 kΩ, extraneous noise of about 50 Hz is electrostatically
coupled to the microphone code (not shown) to easily output noise.
[0013]
In order to solve the problems described above, a microphone that does not distort the output
even if the impedance of the circuit connected downstream of the impedance converter 2 is
lowered and does not generate noise due to the impedance of the filter circuit Is desirable.
In addition, it is desirable to use a microphone that can lower the output impedance even in the
band below the cutoff frequency of the filter circuit.
[0014]
JP 2001-238287 A
[0015]
Therefore, an object of the present invention is to provide a microphone capable of obtaining a
high dynamic range without increasing the output impedance due to the frequency of the output
signal while reducing the low frequency component.
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[0016]
According to the present invention, there is provided a microphone unit comprising: a
microphone unit; an HOT terminal for balanced output of signals output from the microphone
unit; and a COLD terminal; and an output terminal for outputting the signal to an output circuit.
The main feature is that a low pass filter is disposed between the microphone unit and the COLD
terminal.
[0017]
According to the present invention, the output impedance does not increase due to the frequency
of the output signal while reducing the low frequency component, and a high dynamic range can
be obtained.
[0018]
It is a circuit diagram showing an embodiment of a microphone concerning the present
invention.
It is a figure which shows the example of the signal waveform in the said microphone,
Comprising: (a) Signal output from HOT terminal, (b) Signal output from COLD terminal, (c)
Signal output from output terminal of mixer circuit, An example of
It is a circuit diagram showing an example of a measurement circuit for measuring a frequency
response of the above-mentioned microphone.
It is a graph which shows the example of the frequency response measured using the said
measurement circuit.
It is a graph which shows the example of measurement of the total harmonic distortion factor
measured using the said measurement circuit. It is a graph which shows the example of
measurement of the noise spectrum measured using the above-mentioned measurement circuit.
It is a circuit diagram which shows the example of the measurement circuit for measuring the
frequency response of the conventional microphone. It is a graph which shows the example of
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the frequency response measured using the said measurement circuit. It is a circuit diagram
showing an example of composition of the conventional microphone.
[0019]
Hereinafter, embodiments of a microphone according to the present invention will be described
with reference to the drawings. FIG. 1 is a circuit diagram showing an example of the
configuration of the microphone 10 according to the present embodiment. As shown in FIG. 1,
the microphone 10 includes a microphone unit 1, an impedance converter 2 disposed
downstream of the microphone unit 1, a low pass filter 3, an output amplifier 4-1 and an output
amplifier 4-2. It has. The microphone unit 1 is, for example, a condenser microphone unit.
[0020]
The microphone 10 is a balanced output (balanced output). Therefore, the output terminal is
composed of three pins including the HOT terminal 5, the COLD terminal 6, and the ground
terminal 7. The impedance conversion 2 and the output amplifier 4-1 are connected in series
between the output end of the microphone unit 1 and the HOT terminal 5, and no filter circuit is
disposed. On the other hand, between the output end of the microphone unit 1 and the COLD
terminal 6, an impedance converter 2, an output amplifier 4-1, a low pass filter 3 and an output
amplifier 4-2 are connected in series in this order. That is, between the microphone unit 1 and
the COLD terminal 6, a filter circuit that reduces high frequency components is disposed.
Therefore, the signal output from the COLD terminal 6 is a signal whose high frequency
component is cut from the signal output from the microphone unit 1.
[0021]
The HOT terminal 5 and the COLD terminal 6 are connected to the input terminal of the mixer
circuit 20 provided in the output circuit. That is, the signals output from the output terminals
(HOT terminal 5 and COLD terminal 6) of the microphone 10 are input to the mixer circuit 20.
The mixer circuit 20 mixes and outputs the input signals. For example, the signals (the signal
output from the HOT terminal 5 and the signal output from the COLD terminal 6) input to the
mixer circuit 20 are subtracted and output from the output terminal 8.
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[0022]
FIG. 2 is a view showing an example of a signal waveform in the microphone 10. FIG. 2A shows
an example of a signal waveform output from the HOT terminal 5 of the microphone 10. FIG. 2B
shows an example of a signal waveform output from the COLD terminal 6 of the microphone 10.
FIG. 2C shows an example of a signal waveform output from the output terminal 8 of the mixer
circuit 20. The horizontal axis in each drawing of FIG. 2 indicates the frequency of the signal, and
the vertical axis indicates the level of the signal.
[0023]
As shown in FIG. 2A, since the signal output from the HOT terminal 5 does not pass through the
filter circuit, the signal level with respect to the frequency is constant. On the other hand, as
shown in FIG. 2B, the signal output from the COLD terminal 6 is a signal passing through the low
pass filter 3. Therefore, although the low band component is output from the signal output from
the COLD terminal 6, the output level is attenuated as the frequency becomes higher, and the
high band component is not output.
[0024]
The mixer circuit 20 included in the output circuit of the microphone 10 subtracts the signal
output from the COLD terminal 6 from the signal output from the HOT terminal 5, for example,
and outputs the result. Therefore, from the output terminal 8 to which the output signal from the
mixer circuit 20 is output, the low frequency component is canceled and not output, and the high
frequency component is output without canceling the signal. The signal output from the output
terminal 8 is as shown in FIG. As described above, in the microphone 10, the low frequency
component of the output signal is cut, and the noise component is attenuated.
[0025]
A phase inverting circuit that inverts the output signal of the output circuit 4-2 may be connected
to the rear stage of the output circuit 4-2, and the mixer circuit 20 may be configured by an
adder. In this case, the positive phase component of the microphone unit 1 is output from the
HOT terminal 5, and the signal having the negative phase component of the microphone unit 1
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and the high frequency component cut out is output from the COLD terminal 6.
[0026]
Therefore, the signal added and synthesized by the mixer circuit 20 and output is the difference
between the positive phase component and the negative phase component. Therefore, as shown
in FIG. 2C, the signal output from the output terminal 8 of the mixer circuit 20 is a signal
obtained by combining the signal output from the HOT terminal 5 and the signal output from the
COLD terminal 6. That is, the low frequency component is canceled and not output, and only the
high frequency component is output.
[0027]
Next, the characteristics of the microphone 10 according to the present embodiment will be
described in comparison with the characteristics of the conventional microphone. The
characteristics shown below exemplify the results measured under the same predetermined
conditions.
[0028]
FIG. 3 is an example of a measurement circuit using the microphone 10. FIG. 4 is a graph
showing an example of measuring the frequency response of the microphone 10 using the
measurement circuit shown in FIG. FIG. 7 is an example of a measurement circuit using a
conventional microphone 100. In FIG. FIG. 8 is a graph showing an example of measuring the
frequency response of the conventional microphone 100 using the measurement circuit shown in
FIG. Each of FIG. 4 and FIG. 8 shows the frequency response when 100 kΩ and 600 Ω are
connected as load resistors to the respective measurement circuits, the horizontal axis showing
the input frequency and the vertical axis showing the level of the output signal.
[0029]
As shown in FIG. 8, in the case of the conventional microphone 100, the level of the output signal
largely changes due to the size of the load resistance. From the difference in output level, the
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output impedance of the microphone 100 at each frequency can be calculated. For example, the
output impedance at a frequency of 1 kHz is 34Ω, but the output impedance at a frequency at
which the output level attenuates by about 3 dB (approximately 150 Hz in FIG. 8) is 56Ω. The
output impedance at a frequency of 50 Hz is 121 Ω. Thus, in the conventional microphone 100,
the output impedance tends to increase when the cutoff frequency of the filter circuit is
exceeded.
[0030]
On the other hand, in the frequency response of the microphone 10 according to the present
embodiment, as shown in FIG. Based on this, when the output impedance of the microphone 10 is
calculated, the output impedance at a frequency of 1 kHz is 48Ω. Also, the output impedance at
a frequency at which the output level attenuates by 3 dB (approximately 90 Hz in FIG. 4) is 35
Ω. The output impedance at a frequency of 50 Hz is 36 Ω.
[0031]
That is, even if the microphone 10 is provided with the low pass filter 3, the output impedance
does not greatly change at frequencies exceeding the cutoff frequency, and the output impedance
is constant regardless of the frequency. Moreover, the output impedance of the microphone 10 is
maintained at a low value. Therefore, according to the microphone 10, it is possible to suppress
an increase in the output impedance due to the frequency of the output signal, and to suppress
the influence of external noise caused by the magnitude of the output impedance.
[0032]
In addition, since the output impedance of the HOT terminal 5 which is a positive phase output is
sufficiently low and the output signal from the microphone unit 1 is input to the low pass filter 3
from here, even if the impedance of the low pass filter 3 is low, Distortion does not occur.
[0033]
Next, the total harmonic distortion (THD) of the microphone 10 will be described.
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FIG. 5 is a graph showing an example of the total harmonic distortion rate of the microphone 10
measured using the measurement circuit shown in FIG. From the total harmonic distortion factor,
it is possible to determine the level of the input signal which is the distortion factor tolerance (1%
distortion) in the output signal.
[0034]
As shown in FIG. 5, in the microphone 10, the input level at which 1% distortion occurs is about
+12 dB, which is very high.
[0035]
The noise spectrum of the microphone 10 will be described.
FIG. 6 is a graph showing an example of measurement of the noise spectrum of the microphone
10. As shown in FIG. 6, the value of the audibility correction (A-weight) of the microphone 10 is
−113 dB.
[0036]
Since the dynamic range is the width of the input level at which 1% of distortion occurs and the
value of audibility correction, the dynamic range of the microphone 10 is about 125 dB (= 113 +
12). As described above, according to the microphone 10, the noise component of the output
signal can be suppressed low and the high dynamic range can be obtained by the simple circuit
configuration.
[0037]
As described above, according to the microphone 10, the output impedance does not increase
due to the frequency of the output signal while reducing the low frequency component, and a
high dynamic range can be obtained.
[0038]
Reference Signs List 1 microphone unit 2 impedance converter 3 low pass filter 4-1 output
amplifier 4-2 output amplifier 5 HOT terminal 6 COLD terminal 7 ground terminal 8 output
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terminal 10 microphone 20 mixer circuit
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