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JP2011166433

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DESCRIPTION JP2011166433
The present invention provides a stereo microphone in which a power supply circuit uses as a
noise source a variable angle between the left and right directional axes at which a resistor does
not load the amplifier and is also a noise source. An output circuit of a bidirectional microphone
element (20) includes an inverting amplifier (25) for inverting and outputting a phase, and in
order to obtain one of left and right channel signals, a unidirectional microphone element for
middle The non-inverted output signal of the side microphone element 10 is added to the output
signal of 10, and the output signal of the inverting amplifier 25 is added to the output signal of
the middle microphone element 10 to obtain the other signal of the left and right channels. Add
the inverted output signal of The input resistance to the inverting amplifier 25 and the feedback
resistance can both be divided, and the division ratio of these can be made variable to add the
non-inverted output signal and inverted output signal of the side microphone element 20 to be
added to the output signal of the middle microphone element 10 Switch levels and switch the
angle between the left and right directional axes. [Selected figure] Figure 1
ステレオマイクロホン
[0001]
The present invention relates to a stereo microphone, and more particularly to a technique for
varying the angle between the left and right directional axes of the microphone used for MS
stereo recording, that is, the localization.
[0002]
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1
One of the stereo recording methods is the MS method.
The MS stereo recording system is a system in which recording is performed separately in the
center direction, that is, in the middle (M) direction and in the lateral direction, that is, in the side
(S) direction. Microphones for MS stereo recording are also commercially available. The MS
stereo microphone comprises a unidirectional microphone element for picking up the sound
from the middle direction and a bi-directional microphone element for picking up the sound from
the side direction, and the directional axes of these microphone elements Are arranged
orthogonal to each other.
[0003]
FIG. 3 shows an example of the MS stereo microphone. Reference numeral 10 denotes a
unidirectional microphone for the middle, and 20 denotes a microphone for the bi-directional
side direction sound collection. In this example, condenser type microphone elements are used as
the microphone elements 10 and 20. Each microphone element 10, 20 is incorporated into the
microphone case 40, more precisely within a mesh-like cover 42 which is covered on the upper
half of the microphone case 40. A microphone element 20 is disposed above the microphone
element 10. Each of the microphone elements 10 and 20 is fixed to the microphone case 40 such
that their directivity axes are horizontal and that both directivity axes are at an angle of 90
degrees to each other. A circuit to be described later is incorporated in the microphone case 40,
and a connector 41 for outputting an output signal of the microphone to the outside is provided
at the lower end portion of the microphone case 40.
[0004]
The principle of stereo recording by the MS stereo microphone will be schematically described
with reference to FIG. 8 including the change in localization by the MS stereo recording. In FIGS.
8 (a) and 8 (b), the diagrams at the left end show the directivity of the uni-directional microphone
element for the middle, and draw a cardioid curve as is well known. In FIGS. 8 (a) and 8 (b), the
central view shows the directivity of the bi-directional microphone element for the side. "+" "-"
Described in these directivity curves show the direction of sound pressure. In MS system stereo
recording, the right (R) channel signal is the sum of the output of the middle microphone element
and the output of the side direction sound collection microphone element, and from the middle
microphone element output to the side direction sound collection The left (L) channel signal is
obtained by subtracting the output of the microphone element.
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2
[0005]
The figure on the right end of FIG. 8A shows the right channel signal, and the output of the
middle microphone element and the output of the side microphone element are added 63.4
degrees to the left of the reference axis at the center of the microphone. It becomes
hypercardioid directivity centered on the axis inclined to a certain degree. The figure on the right
end of FIG. 8 (b) shows the left channel signal, and the output of the side microphone element is
subtracted from the output of the middle microphone element so that it is 63.4 to the right of the
reference axis of the microphone center. The hypercardioid directivity is centered on the axis
inclined by degrees. Thus, since it is possible to obtain a collected sound signal in which the
directivity axis is biased to the right and a collected sound signal in which the collected sound
signal is biased to the left, and the directivity of the two collected signals is similar to each other,
A stereo signal can be obtained from the two collected sound signals.
[0006]
In FIG. 8, the directional axes of the right and left channel sound collection signals are
theoretically respectively 63.4 degrees to the right and left from the reference axis at the center
of the microphone. Therefore, the angle between the directional axes of the left and right
channels is about 127 degrees as shown in FIG. 8 (c). The angle of 127 degrees between the
directional axes of the left and right channels can be applied to recording under most conditions,
but the angle can be varied according to preferences or conditions such as the spread of the
recording object. It may be desirable to make the so-called localization variable. In the case of MS
stereo recording, the angle between the directional axes of the left and right channels can be
changed by adjusting the output level of the side microphone element to be added to or
subtracted from the output of the middle microphone element. It is also possible to change the
angle between the left and right directional axes by making the output level of the side
microphone element unchanged and making the output level of the middle microphone element
variable. If the angle between the left and right pointing axes is too large or too small, the stereo
effect is impaired and the localization becomes unclear. Generally, the angle between the left and
right directional axes is 90 degrees at the lower limit and 127 degrees at the upper limit, as
shown in FIG. 8 (c).
[0007]
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3
Next, a conventional example of the MS stereo microphone will be described. In FIG. 4, reference
numeral 10 denotes the unidirectional microphone element 20 as described above, and 20
denotes the bidirectional microphone element. The microphone elements 10 and 20 are both
capacitor type microphone elements, and a polarized voltage is supplied to each of the
microphone elements 10 and 20 from a power supply circuit 22 comprising a DC-DC converter.
The power supply circuit 22 boosts a power supply battery voltage of about 5 V to about ± 100
V and applies the voltage between the diaphragm of the capacitor microphone element and the
fixed electrode opposed thereto. The output signals of the microphone elements 10 and 20 are
respectively amplified by the amplifiers 11 and 21 and output.
[0008]
The microphone output is divided into a left channel and a right channel, and the signals of each
channel have the following circuit configuration in order to perform balanced output in a 3-pin
system. The output end of the amplifier 11 for amplifying the output of the microphone element
10 on the middle side is connected to the pin 2 of the left channel via the amplifier 26 and the
output end of the amplifier 11 is connected to the pin 2 on the right channel via the amplifier 27
It is done. The output end of the amplifier 21 for amplifying the output of the microphone
element 20 on the side side is connected to the third pin of the right channel via an amplifier 29,
and the output end of the amplifier 21 is differential via an input resistor Rs. It is connected to
the inverting input terminal of the inverting amplifier 25 consisting of an amplifier. A feedback
resistor Rf is connected between the output terminal of the inverting amplifier 25 and the
inverting input terminal. The phase difference of the output signal of the inverting amplifier 25
changes according to the ratio between the input resistance Rs and the feedback resistance Rf.
Here, Rs = Rf is set, and the phase difference of the output signal with respect to the input signal
is set to differ by 180 degrees. The output terminal of the inverting amplifier 25 is connected to
pin 3 of the left channel via an amplifier 28. Each of the amplifiers 26 to 29 has an emitter
follower connection.
[0009]
Assuming that the middle output from the amplifier 11 is M and the phase is positive with this as
a reference, M + signals are outputted from the respective pin 2 of the L channel and the R
channel. The second pin is the hot output terminal of the balanced output. On the other hand,
assuming that the side output from the amplifier 21 is S, the phase of the side output S is also the
+ phase, and the signal of S + is output from the third pin of the right channel. The side output S
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4
+ of the amplifier 21 is inverted in phase to S- by passing through the inverting amplifier 25, and
this inverted signal S- is output from the third pin of the left channel through the amplifier 28.
Both left and right channel signals are output as balanced signals from the 3-pin connector, with
pin 1 of each channel grounded and pin 2 the hot side signal pin as described above, The third
pin is a cold side signal pin.
[0010]
As described above, the left channel L balancedly outputs the signals M + and S−, and the right
channel R balanced and output the signals M + and S +. The balanced output of the left channel L
consists of middle output M + on the hot side and + side output S on the cold side, and is oriented
around an axis inclined to the right from the reference axis as shown in FIG. 8 (b) It becomes sex.
The balanced output of the right channel R consists of middle output M + on the hot side + phase
on the cold side and side output S + on the cold side + directivity as shown in FIG. 8A centered on
an axis inclined to the left from the reference axis become. In this way, stereo audio signals can
be obtained.
[0011]
In the MS stereo microphone described above, for example, there may be a case where the sound
collection angle is narrowed from 127 degrees to 90 degrees, for example, as described with
reference to FIG. . FIG. 5 to FIG. 7 show various examples of MS stereo microphones conceived to
make the sound collection angle variable.
[0012]
In FIG. 5, the power supply circuit including the DC-DC converter is divided into the middle side
power supply circuit 221 and the side side power supply circuit 222. For example, the power
supply voltage of the middle side power supply circuit 221 is fixed and the power supply voltage
of the side side power supply circuit 222 is As a variable, the polarization voltage of the side
direction sound collection microphone element 20 is variable. While the output level of the
middle mic element 10 is constant, the output level of the side mic element 20 to be added to or
subtracted from the output of the middle mic element 10 is made variable by changing the
polarization voltage as described above. It can be adjusted. As a result, it is possible to change the
sound collection angle, that is, the angle between the directional axes of the left and right
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channels.
[0013]
However, as in the example shown in FIG. 5, when two power supply circuits comprising DC-DC
converters are provided, these power supply circuits interfere with each other to generate beat
noise. That is, the two power supply circuits 221 and 222 respectively oscillate or switch at
around 1.2 MHz, boost the alternating signal generated by the transformer, and rectify this,
thereby boosting, for example, about CD 5 V to about DC ± 100 V doing. Further, since the
power supply circuits 221 and 222 use self-oscillation circuits in order to reduce the cost, the
oscillation frequencies of the power supply circuits 221 and 222 are unstable and it is difficult to
make both oscillation frequencies coincide. Therefore, if the oscillation frequencies of the
respective power supply circuits are f1 and f2, beat noise of the difference frequency is
generated. In addition, since the two power supply circuits 221 and 222 transmit at a relatively
high frequency as described above, they are easily magnetically coupled and easily interfere with
each other to generate noise. In order to prevent the generation of noise, it is conceivable to use
a crystal oscillator to stabilize the oscillation frequency of each of the power supply circuits 221
and 222, but this is a factor which increases the production cost of the microphone and is not
desirable.
[0014]
FIG. 6 shows another example in which the polarization voltage of the side direction sound
collection microphone element 20 is variable. In this example, the polarization voltage of the side
microphone element 20 is made variable by switching the voltage dividing resistors. Resistors
Rd1, Rd2, Rd3, and Rd4 are connected in series between the + Vp output terminal and the -Vp
output terminal of the power supply circuit 22 that supplies a polarized voltage to the middle
microphone element 10 and the side microphone element 20. The output terminal of the power
supply circuit 22 is connected to the middle microphone element 10 so that the output voltage of
the power supply circuit 22 is directly applied. A switch 31 for selecting one of the + Vp output
end of the power supply circuit 22 and the connection point of the resistors Rd1 and Rd2 is
provided, and a −Vp output end of the power supply circuit 22 and a connection point of the
resistors Rd3 and Rd4 are provided. A switch 32 is provided to select one of them.
[0015]
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6
The switches 31 and 32 operate in conjunction with each other. In one switching mode, the
output voltage of the power supply circuit 22 is directly applied to the side microphone element
20 as a polarization voltage, and in the other switching mode, the resistors Rd1 and Rd2 are
used. , Rd3 and Rd4 are applied as polarization voltages. When the switches 31 and 32 are in the
switching mode shown by the solid line in FIG. 6, the polarization voltage of the side microphone
element 20 is high, and the angle between the directional axes of the left and right channels is
wide as described in the above example. . When the switches 31 and 32 are in the switching
mode shown by the broken line in FIG. 6, the polarization voltage of the side microphone element
20 is low, and the angle between the directivity axes of the left and right channels is narrow as
described in the above example. Become.
[0016]
According to the example shown in FIG. 6, there is a problem that the consumption current of the
DC-DC converter is increased by the flow of current from the DC-DC converter forming the main
body of the power supply circuit 22 to the voltage dividing resistors Rd1, Rd2, Rd3, Rd4. . Since
the DC-DC converter incorporates a rectifier, when the current consumption of the DC-DC
converter increases, the voltage drop due to the rectifier or the like increases and the output
voltage decreases. Therefore, the resistance value of the entire voltage dividing resistor is set to a
high resistance value such as 1 MΩ, for example, so that the current flowing through the voltage
dividing resistor is minimized. However, since current flow through the voltage dividing resistor
can not be avoided, the output power of the DC-DC converter is consumed by the voltage dividing
resistor, and the current required for the signal system is limited.
[0017]
FIG. 7 shows still another example in which the angle (localization) between the directional axes
of the left and right channels is made variable by making the output level of the side microphone
element 20 variable. In this example, voltage dividing resistors Rd5, Rd6, Rd7 and Rd8 and
switches 33 and 34 that operate in conjunction with each other are connected to the output
circuit of the inverting amplifier 25 connected to the signal circuit on the side, The output level is
switched. The voltage dividing resistors Rd5, Rd6, Rd7 and Rd8 are connected in series between
the output end of the amplifier 21 for amplifying the output signal of the side microphone
element 20 and the output end of the inverting amplifier 25. The switch 33 is connected to select
one of the output terminal of the inverting amplifier 25 and the connection point of the resistors
Rd5 and Rd6 and to input it to the amplifier 28. The switch 34 is connected to select one of the
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7
output terminal of the amplifier 21 and the connection point of the resistors Rd7 and Rd8 and to
input it to the amplifier 29.
[0018]
When the interlock switches 33 and 34 are in the switching mode shown by a solid line in FIG. 7,
the output of the inverting amplifier 25 is directly input to the amplifier 28 and the output of the
amplifier 21 is directly input to the amplifier 29. Therefore, the maximum side signals S- and S +
are input to the amplifiers 28 and 29, respectively, and the angles between the directivity axes of
the left and right channels become wide. When the interlock switches 33 and 34 are in the
switching mode shown by the broken line in FIG. 7, the output S- of the inverting amplifier 25 is
divided by the voltage dividing resistor and input to the amplifier 28, and the output S + of the
amplifier 21 is voltage dividing resistor Is divided and input to the amplifier 29. Therefore, the
levels of the side signals S− and S + input to the amplifiers 28 and 29 become low, and the
angles between the directivity axes of the left and right channels become narrow.
[0019]
As described above, also in the example illustrated in FIG. 7, the angles of the directional axes of
the left and right channels can be switched. However, according to this example, when the values
of the voltage dividing resistors Rd5, Rd6, Rd7, Rd8 connected to the side signal output circuit
are low, this voltage dividing resistor becomes a large load of the inverting amplifier 25 and the
output of the inverting amplifier 25 There is a drawback to distorting the signal. Therefore,
increasing the value of the voltage dividing resistors Rd5, Rd6, Rd7, and Rd8 reduces distortion
of the output signal of the inverting amplifier 25, but increases the level of resistance noise
generated from the voltage dividing resistors Rd5, Rd6, Rd7, and Rd8. , The signal to noise ratio
(S / N) is degraded.
[0020]
The stereo microphone of patent document 1 is known as a prior art example of MS system
stereo microphone. There is also known a technique for performing signal processing such as
encoding and decoding of an MS method stereo signal (see, for example, Patent Document 2 and
Patent Document 3). However, the invention described in each of the above patent documents
does not make it possible to switch the angle between the directional axes of the left and right
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8
channels.
[0021]
JP, 2006-174136, A JP, 2008-28574, A JP, 2007-4050, A
[0022]
As an MS-type stereo microphone capable of switching the angle between the directional axes of
the left and right channels, one having a configuration as shown in FIGS. 6 to 8 can be
considered, but according to these examples, as described above The beat noise is generated
from the power supply circuit, the output power of the DC-DC converter that forms the main
body of the power supply circuit is consumed by the voltage dividing resistor, and the current
necessary for the signal system is limited. There are problems such as load and distortion of the
output signal of the inverting amplifier or noise from the voltage dividing resistor.
[0023]
The present invention solves the above-mentioned problems in the MS stereophonic microphone
considered conventionally, that is, the power supply circuit of the MS stereophonic condenser
microphone in which the angle between the left and right directional axes can be switched.
However, it is an object of the present invention to devise a circuit configuration such that the
voltage dividing resistor does not become a load of the amplifier and does not become a noise
source.
[0024]
The present invention comprises a unidirectional microphone element for the middle and a
bidirectional microphone element for the side arranged with the directivity axis orthogonal to the
directivity axis of the microphone element for the middle, and the microphone for the side The
output circuit of the element is provided with an inverting amplifier that inverts and outputs the
phase of the output signal of the side microphone element, and in order to obtain one of the
signals of the left and right channels, the output signal of the middle microphone element Add
the non-inverted output signal and add the inverted output signal of the side microphone
element, which is the output signal of the inverting amplifier, to the output signal of the middle
mic element to obtain the other signal of the left and right channels The input resistance and
feedback resistance of the input and output resistors can both be divided. By making it variable,
the level of the non-inverted output signal and inverted output signal of the side microphone
element to be added to the output signal of the middle microphone element can be switched to
make it possible to switch the angle between the left and right directivity axes. The most major
feature.
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[0025]
When the level ratio of the non-inverted output signal and the inverted output signal of the side
mic element to be added to the output signal of the middle mic element by making the division
ratio of the input resistance to the inverting amplifier and the feedback resistance variable, the
left and right directional axis mutual The angle made by
When the levels of the non-inverted output signal and the inverted output signal are switched to
be low, the angle between the left and right directivity axes narrows.
Switching between the left and right pointing axes is achieved by changing the division ratio of
the input resistance and feedback resistance of the inverting amplifier provided in the signal path
from the side microphone element, which is the signal path of the bidirectional component
Therefore, the distortion of the audio signal converted by the microphone can be reduced
without the amplifier becoming overloaded due to the voltage dividing resistance as in the
examples shown in FIGS. 6 to 8 described above. it can.
Further, the circuit configuration is relatively simple, and the input resistance and the feedback
resistance do not become a noise source.
[0026]
It is a circuit diagram showing an example of a stereo microphone concerning the present
invention.
It is a circuit diagram showing another example of a stereo microphone concerning the present
invention. An example of a mechanical structure of a MS stereo microphone is shown, (a) is a
front sectional view, (b) is a side sectional view. It is a circuit diagram which shows the example
of the stereo microphone considered conventionally. It is a circuit diagram showing another
example of the stereo microphone considered conventionally. It is a circuit diagram which shows
the further another example of the stereo microphone considered conventionally. It is a circuit
diagram which shows the further another example of the stereo microphone considered
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10
conventionally. (A) is a directivity characteristic diagram showing the directivity of one channel,
(b) is a directivity characteristic diagram (c) showing the directivity of the other channel, and FIG.
It is a directional characteristic line figure for demonstrating the variable principle of the angle
which a directional axis mutually makes.
[0027]
Hereinafter, embodiments of a stereo microphone according to the present invention will be
described with reference to the drawings. The mechanical configuration of the stereo
microphone according to the present invention may be the same as the configuration shown in
FIG. 3, so the description of the mechanical configuration will be omitted. Further, in the stereo
microphone according to the present invention, most of the basic circuit configuration as the MS
stereo microphone is the same as the circuit configuration shown in FIG. There is.
[0028]
In FIG. 1, reference numeral 10 denotes a unidirectional microphone element 20, and 20 denotes
a bidirectional microphone element. The microphone elements 10 and 20 are both capacitor type
microphone elements, and a polarized voltage is supplied to each of the microphone elements 10
and 20 from a power supply circuit 22 comprising a DC-DC converter. The power supply circuit
22 boosts the power supply battery voltage of about 5 V to about ± 100 V and applies it
between the diaphragm of the capacitor microphone element and the fixed electrode opposed
thereto. The output signals of the microphone elements 10 and 20 are respectively amplified by
the amplifiers 11 and 21 and output. Each unit 10 and 20 may incorporate an impedance
converter, and the amplifiers 11 and 21 may double as an impedance converter. In any case, the
outputs of the high impedance units 10 and 20 are converted from the amplifiers 11 and 21 to
low impedance and output. The amplifiers 11 and 21 are referred to as first amplifiers for
convenience.
[0029]
The microphone output is divided into a left channel and a right channel, and the signals of each
channel have the following circuit configuration in order to perform balanced output in a 3-pin
system. The output M + of the first amplifier 11 for amplifying the output of the middle
microphone element 10 is output as a hot signal from pin 2 of the left channel through the
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amplifier 26, and the output of the amplifier 11 is also output through the amplifier 27 to the
right channel. It is connected to be output as a hot signal from pin 2. The output terminal of the
first amplifier 21 for amplifying the output of the side microphone element 20 is connected to
the inverting input terminal of an inverting amplifier 25 comprising a differential amplifier via
input resistors Rs1 and Rs2 in series. Between the inverting input terminal and the output
terminal of the inverting amplifier 25, feedback resistors Rf1 and Rf2 connected in series are
connected. The phase of the output signal with respect to the input signal of the inverting
amplifier 25 changes according to the ratio of the input resistance Rs1 + Rs2 and the feedback
resistance Rf1 + Rf2. Here, Rs1 + Rs2 = Rf1 + Rf2 is set, and the phase difference of the output
signal with respect to the input signal is set to be 180 degrees.
[0030]
The non-inverted signal of the side microphone element 20 is output from the amplifier 21 and
this non-inverted signal is output from the third pin of the right channel via the amplifier 29 as a
cold signal S + of the right channel. However, there is a changeover switch 23 between the
amplifier 21 and the amplifier 29. The switch 23 is connected to select one of the output
terminal of the amplifier 21 and the connection point of the input resistors Rs1 and Rs2. .
Further, the output signal of the inverting amplifier 25, that is, the inverted signal S- of the
microphone element 20 on the side is connected so as to be output as a cold signal from the
third pin of the left channel through the amplifier. However, there is a changeover switch 24
between the inverting amplifier 25 and the amplifier 28, and this switch 24 is connected to select
either the output terminal of the inverting amplifier 25 or the connection point of the feedback
resistors Rf1 and Rf2. ing. The two changeover switches 23 and 24 are interlock switches
operating at the same time, and as shown by a solid line in FIG. 1, a switching mode in which the
output end of the amplifier 21 and the output end of the inverting amplifier 25 are selected
Thus, the connection point between the input resistors Rs1 and Rs2 and the connection point
between the feedback resistors Rf1 and Rf2 can be switched to any of the selected modes. Each
of the amplifiers 26 to 29 has an emitter follower connection. Each amplifier 26-29 is referred to
as a second amplifier for convenience.
[0031]
Assuming that the phase of the middle output signal M from the amplifier 11 is M +, an M +
signal is output from each pin 2 of the L channel and the R channel. The second pin is the hot
output terminal of the balanced output. On the other hand, assuming that the side output from
the amplifier 21 is S, the phase of the side output signal S is also the + phase, and the signal of S
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+ is output from the third pin of the right channel. The phase of the side output signal S + of the
amplifier 21 is inverted to S- by passing through the inverting amplifier 25, and the inverted
signal S- is output from the pin 3 of the left channel through the amplifier 28. Both left and right
channel signals are output as balanced signals from the 3-pin connector, with pin 1 being
grounded and pin 2 being the hot side signal pin and pin 3 as described above. Is a signal pin on
the cold side.
[0032]
Now, assuming that the two interlocked switches 23, 24 are in the switching mode shown by the
solid line in FIG. 1, the output S + of the first amplifier 21 is directly input to the second amplifier
29, and the output of the inverting amplifier 25. S- is directly input to the second amplifier 28.
Therefore, in this switching mode, the bi-directional component level output from the side
microphone element 20 has the maximum levels of both the non-inverted signal S + and the
inverted signal S-, and the angle between the directivity axes of the left and right channels is the
largest. become.
[0033]
Next, it is assumed that the two interlocked switches 23, 24 are switched to the switching mode
shown by the broken line in FIG. The output S + of the first amplifier 21 input to the second
amplifier 29 is divided by the input resistors Rs1 and Rs2, and the output S- of the inverting
amplifier 25 input to the second amplifier 28 is the feedback resistor Rf1. It is divided by Rf2.
Therefore, the level of the nondirectional signal S + and the inverted signal S− both of the
bidirectional component level output from the side microphone element 20 become low, and the
angle between the directivity axes of the left and right channels narrows. The values of the input
resistors Rs1 and Rs2 and the values of the feedback resistors Rf1 and Rf2 are set such that the
levels of the absolute values of the non-inverted signal S + and the inverted signal S- coincide
with each other.
[0034]
As described above, in the first embodiment shown in FIG. 1, the input resistances Rs1 and Rs2
and the feedback resistance Rf1 of the inverting amplifier 25 provided in the signal path from
the side microphone element 20 which is the signal path of the bidirectional component. By
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13
making the division ratio of Rf 2 variable, it is possible to switch the angle between the left and
right directional axes. Therefore, the distortion of the audio signal converted by the microphone
can be reduced without the amplifier being overloaded by the voltage dividing resistor. The
circuit configuration is relatively simple. In addition, there is an advantage that the increase in
noise can be suppressed as compared with the previously considered bi-directional level variable
means. That is, although noise is generated in the input resistors Rs1 and Rs2, the noise is small
enough to be ignored. Also, since the feedback resistors Rf1 and Rf2 form a loop for the inverting
amplifier 25, no noise is generated. Therefore, it has become possible to vary the bidirectional
component while suppressing an increase in noise as compared to the MS stereo microphone
having no variable component of the bidirectional component.
[0035]
FIG. 2 shows a second embodiment. This embodiment differs from the embodiment shown in FIG.
1 in that a variable resistor VRs is provided as an input resistor and a variable resistor VRf is
provided as a feedback resistor in order to vary the division ratio of the input resistor and
feedback resistor of the inverting amplifier 25. In other words, instead of the input resistors Rs1
and Rs2, the feedback resistors Rf1 and Rf2 and the changeover switches 23 and 24 in the
embodiment shown in FIG. 1, a variable resistor VRs and a variable resistor VRf are provided. The
variable resistors VRs and VRf operate in conjunction with each other, and operating the variable
resistors continuously changes the resistance value. Due to the change of the resistance value,
the variable resistance is continuously changed so that the level of the absolute value of the noninverted signal S + and the inverted signal S- which are bi-directional components output from
the side microphone element 20 changes continuously. The movable contact of VRs is connected
to the second amplifier 29, and the movable contact of the variable resistor VRf is connected to
the second amplifier. When the variable resistors VRs and VRf are operated in the direction
shown by the solid line in FIG. 2 and the limit position is reached, the levels of the non-inverted
signal S + and the inverted signal S- become maximum, and the angle between the directivity axes
of the left and right channels become. When the variable resistors VRs and VRf are operated in
the direction shown by the broken line in FIG. 2, the levels of the non-inverted signal S + and the
inverted signal S- are divided by the variable resistors VRs and VRf and become lower
continuously at the same level. The angle between the directional axes of the channels is
continuously narrowed.
[0036]
As described above, in the second embodiment shown in FIG. 2, the inverting amplifier 25 is
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connected to the subsequent stage of the first amplifier 21 to obtain the inverted signal of the
side microphone element 20, and the input resistance of the inverting amplifier 25 By changing
the feedback resistance to a variable resistance, the levels of the non-inverted output signal S +
and the inverted output signal S− of the side microphone element 20 to be added to the output
signal of the middle microphone element 10 are switched. We made it possible to change the
angle made by each other. Therefore, also in the second embodiment shown in FIG. 2, the
amplifier is not heavily loaded, and distortion of the audio signal converted by the microphone
can be reduced. Further, the same effect as that of the first embodiment can be obtained in that
the circuit configuration is relatively simple and the input resistance and the feedback resistance
do not become a noise source.
[0037]
The illustrated embodiment shows an example in the case of practicing the present invention,
and any design change can be made without departing from the technical concept described in
the claims. Although the microphone element for the middle and the microphone element for the
side are both described as capacitor type microphone elements, if the microphone element for
the middle is uni-directional and the microphone element for the side is bi-directional, the
microphone element type is arbitrary. It is. Also, the middle microphone element and the side
microphone element may be different from each other.
[0038]
10 middle microphone element 11 first amplifier 20 side microphone element 21 first amplifier
23 switch 24 switch 25 inverting amplifier 26 second amplifier 27 second amplifier 28 second
amplifier 29 second amplifier Rs1, Rs2 Input resistance Rf1, Rf2 feedback resistance
10-05-2019
15
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