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BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
stereo microphone device, and more particularly to a device in which its vibration noise is
reduced. [Technical Background of the Invention] As is well known, a cassette tape recorder is
small, lightweight and suitable for carrying, and is a two-way stereo type that kills headphones
and enjoys playing tapes while walking, for example. In recent years, smaller ones have appeared
than compact tape cassettes. On the other hand, in this type of cassette tape recorder, a
microphone built-in type in which a microphone for sound recording is built in the cabinet, and a
microphone external type in which a separate microphone unit is connected to the cabinet There
is. Among them, the external microphone type is provided with a microphone unit in which a
microphone is housed in a case and a plug electrically connected to the microphone is protruded
from the case separately from the cabinet, and the plug of this microphone unit Can be fitted into
a microphone jack provided in the cabinet. By the way, generally, in a tape recorder, since a
motor for tube running is built in the cabinet, vibration due to the rotation of the motor occurs
when the tape is running. As a result, the microphone vibrates and vibration noise is mixed in the
output signal. And this problem is particularly serious in the above-mentioned small cassette tape
recorder. Therefore, it is usually necessary to take anti-vibration measures for the microphone. In
order to prevent vibration, conventionally, the microphone is attached to the cabinet via a buffer
material, so that the vibration is not transmitted or the cabinet itself is not vibrated, and its mass
is increased to the microphone. A microphone for vibration noise cancellation is installed. Among
them, the means using a microphone for vibration noise cancellation installs the vibration noise
canceling microphone in the opposite direction to the microphone for signal, and mixes the
output signals from both microphones in the opposite phase to obtain vibration noise. It is
intended to offset the components, and is described in detail in, for example, "Broadcasting
Technology" magazine, published in February, 1977, P, 1801 'microphone vibration noise and its
reduction method. [Problems of Background Art] However, the conventional vibration isolation
means as described above causes the following problems.
First of all, the means of attaching the microphone to the cabinet via the buffer material violates
the miniaturization which requires a lot of space if the microphone is covered with the buffer
material rather than the vibration is transmitted to the microphone at all. Also, even if external
microphones are of the type, because the cassette tape recorder itself is small, it can not be
configured to shut off the vibration with only dust or dirt that can not increase the shape of the
microphone unit. It is a thing. Furthermore, increasing the mass of the cabinet itself is, of course,
an undesirable measure as it goes against the quantification. Also, in the case of means using a
microphone for vibration noise cancellation, since one vibration noise cancellation microphone
must be installed for one signal microphone, in the case of stereo type, two vibration noise
cancellation microphones are used. It is necessary, complicated and economically
disadvantageous. SUMMARY OF THE INVENTION The present invention has been made in
consideration of the above circumstances, and it is an object of the present invention to provide a
very good stereo microphone device which can prevent the influence of vibration noise with a
simple configuration and is suitable for miniaturization. To aim. [Summary of the invention] That
is, in the stereo microphone device according to the present invention, the microphone support
body to which the first and second microphone bodies are attached in opposite directions to each
other at both ends is attached to the mounting base via the vibration isolation mechanism. In the
attached stereo microphone device, the output terminal of the first microphone body is
connected to the common connection point through the first resistor, and the reference potential
is connected in series via the first capacitor and the second resistor. It is connected to a point,
and the output terminal of the second microphone body is connected to the common connection
point via a third resistor, and connected to the reference potential point via a second capacitor
and a fourth resistor in series. Between the common connection point and the reference potential
point, connecting a third capacitor and a fifth resistance by printing a bias voltage of a constant
level with respect to the reference potential point. Di, it is possible to prevent the effects of
vibration noise with a simple structure, but was L Rinishi that can suitably miniaturized.
invention will be described in detail with reference to the drawings. FIG. 4 shows a microphone
unit used in a cassette tape recorder of the external type. That is, in the figure, 11 is a case
formed in a flat T-shape, and a bottom portion not visible in the figure is opened.
Further, nets 12 and 13 are provided at both ends of the case 11. The bottom opening of the
case 11 is closed by fitting a mounting substrate 14 formed in a substantially T-shape. Here, on
the mounting substrate 14, for example, a sponge or the like is formed in a substantially ring
shape, and nine cushioning members A straight cylindrical microphone support 17 is
inserted into the buffer members At both ends of the microphone support 17,
microphones 18.19 for L and R channels are fitted in opposite directions. Each of the
microphones 113 and 19 has single directivity. The microphones 1 / J and 19 are connected to
the printed wiring board 22 installed on the mounting substrate 14 through the lead wires
20.21. Further, a plug 23 fitted to a microphone jack provided in a cabinet of a cassette tape
recorder (not shown) is attached to the printed wiring board 22. For this reason, in a state where
the microphone unit is assembled, the output signal from each of the microphones 18.19 is
supplied to the amplification circuit or the like provided in the cabinet by fitting the plan 23 to
the above-mentioned jacks for microphones To be served. Here, the printed wiring board 22 has
a circuit as shown in FIG. That is, one of the leads' 820121 extended from the microphone 18.19
is commonly grounded together with the microphone support 17. The other of the lead wires 20
extended from the microphone 18 is connected to the common connection point A via the
resistor R □, and is grounded via the capacitor C □ and the resistor R2 in series. Furthermore,
the other of the lead wires 2 extended from the microphone 19 is connected to the common
connection point A via the resistor R1, and is grounded via the capacitor C2 and the resistor R4
in series. There is. As a matter of fact, the common connection point is grounded via a capacitor
C1 ', and is grounded in series via a resistor R8 and a constant voltage source 24 of the
illustrated polarity. Each of the microphones 18 and 19 includes a drain electrode and a source
electrode S of a field effect transistor (hereinafter referred to as FET) 26 connected to the electret
condenser microphone 25 as shown in FIG. In the configuration as described above, its operation
will be described below.
However, in this case, since it is stereo, R, = R, R, = J, C, = C, and the input sound pressure of each
of the microphones 18.19 is represented by the e-) G input voltage of the FET 26, As V L I V R.
Then, the output voltage V at point a in FIG. Is expressed by the following equation. Here, since
the crosstalk component of the output signal from both microphones 18.19 is expressed by the
term of () in the above equation (1), the crosstalk is generated when (jvcsRl) is sufficiently large,
that is, when the frequency is high. It can be seen that the crosstalk increases when (jcocs R1) is
small, ie when the frequency is low. Then, assuming that R,> R, R2> R1, the above va is expressed
by the following equation. Then, in the above equation (2), jωC1Rt ′ ′ jωC, R in the frequency
domain ωC8R5. If this is the case, the frequency response of the right term in () of equation (2)
is Becomes flat in the frequency domain where That is, according to the
configuration as in the above-described embodiment, vibration noise in the low band can be
canceled each other by the increase of crosstalk. In addition, since vibration noise in the high
region can be considerably absorbed by the buffer member 15.16, crosstalk can be reduced and
separation can be improved. FIG. 3 shows a modification of the above embodiment, in which the
switch 29 is interposed between the common connection point A and the capacitor C1. In this
case, if the switch 29 is turned off, the denominator of the term on the right side of () of the
equation (2) becomes, so that it is possible to cut off the low range component by ω <R2C 1 For
example, it is effective to reduce wind noise and the like. The present invention is not limited to
the above embodiment, and various modifications can be made without departing from the scope
of the invention. According to the present invention, as described in detail above, according to
the present invention, the influence of vibration noise can be prevented with a simple
configuration, and a very good stereo microphone device suitable for miniaturization is provided.
can do.
Brief description of the drawings
FIG. 1 is a circuit diagram showing an embodiment of the stereo microphone device according to
the present invention, FIG. 2 is a circuit diagram showing the details of the microphone of the
embodiment, and FIG. WJ 3 is a circuit showing a modification of the embodiment. FIG. 4 is an
exploded perspective view showing a microphone unit to which the present invention is applied.
JJ ... Case% 12.13 ... Net, 14 ... Mounting board, 15.16 ... Buffer member, 17 ... Microphone
support, IFI, 19 ... Microphone. 20.21 ··· Lead wire, 22 ··· Printed wiring board, 23 · · · · · · · · · · · · · ·
· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 2.・
Output terminal, 29 ... switch. Applicant agent Patent attorney Atsushi Takee Fig. 1 Fig. 2 Fig. 3