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DESCRIPTION JP2002171590

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DESCRIPTION JP2002171590
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
MS stereophonic microphone in which a mid microphone and a side microphone are spaced
apart from each other. Specifically, a MS stereo that reduces the influence on combined
directional characteristics by providing a time delay to the output signals of microphones
advancing in time and reducing the time difference between the output signals of both
microphones. It relates to a microphone.
[0002]
2. Description of the Related Art Conventionally, as for stereo microphones of the MS system,
mid microphones and side microphones are vertically disposed in commercial applications such
as for broadcasting, and there is no time difference between both microphones at least in a
horizontal plane. Is considered.
[0003]
However, in the case of consumer products, since the shape is cylindrical for the convenience of
handling, the mid microphones and the side microphones are disposed slightly apart in the frontrear direction.
[0004]
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1
Therefore, in the MS scheme in which the output signal of the mid microphone and the output
signal of the side microphone are added and subtracted to obtain the left (L) channel output
signal and the right (R) channel output signal, respectively, Therefore, the output signals of both
microphones have different time differences depending on the incident direction of the sound
wave, and the directional characteristics of the combined output signals of both channels change
compared to the ideal case without time difference due to phase interference due to it There is a
drawback to
This change is generally significant at frequencies above a few kHz.
[0005]
An object of the present invention is to provide an MS type stereo microphone in which the time
difference between the output signals of the mid microphone and the side microphone is reduced
to reduce the influence on the synthesized directivity characteristic.
[0006]
SUMMARY OF THE INVENTION According to the present invention, there is provided a midmicrophone having a single directivity in which the direction of maximum sensitivity is directed
to the front direction (0.degree.), And the direction of maximum sensitivity to left and right In the
stereo microphone of the MS system, in which a side microphone having a directivity toward 90
°) is disposed at a predetermined distance on the main axis in the front direction, one of the mid
microphone and the side microphone is One of the disposed microphones is a first microphone,
and the other microphone is a second microphone, and the incident angle of the sound wave is in
a predetermined direction within the range of 0 ° to 90 ° of the first and second microphones.
The signal obtained by applying to the output signal of the first microphone a time delay equal to
the relative time difference of the output signal and the output of the second microphone The
signals are added and subtracted to obtain a left channel output signal and a right channel output
signal, respectively.
[0007]
In the present invention, the mid microphones and the side microphones are disposed back and
forth (on the main axis in the front direction), and the output signals of both microphones have
different time differences depending on the incident direction of the sound wave.
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However, in the present invention, among the output signals of the mid microphone and the side
microphone, the output signal of the microphone advancing in time, the output of both
microphones in a predetermined direction within the range of 0 ° to 90 ° of the sound wave
incident angle A time delay equal to the relative time difference of the signals is applied.
As a result, the time difference between the output signals of both microphones is reduced, the
influence on the combined directivity characteristic is reduced, and it is possible to obtain the left
channel output signal and the right channel output signal well.
[0008]
DESCRIPTION OF THE PREFERRED EMBODIMENTS (A) In an MS stereo microphone, an ideal
case will be described in which a mid microphone and a side microphone are at the same point.
[0009]
In the configuration of the MS-type stereo microphone, the mid microphone is uni-directional,
and the maximum sensitivity direction is directed to the front (0 °).
Therefore, assuming that the incident angle of the plane wave is θ and the front direction is 0 °
in the horizontal plane, the output voltage (output signal) EM is expressed by the equation (1)
using the output voltage in the maximum sensitivity direction as EO. It becomes a heart-shaped
directivity characteristic shown in FIG.
This is a directional characteristic in the horizontal plane, but the same directional characteristic
in all planes including the principal axis in the direction of 0 ° and 180 °. EM = EO (0.5 + 0.5
cos θ) (1)
[0010]
Next, the side microphones are generally bi-directional, and their maximum sensitivity is oriented
at 90 ° and 270 ° (−90 °) with respect to the 0 ° direction. The output voltage (output
signal) ES in this case is expressed by the equation (2), assuming that the output voltage in the
direction of maximum sensitivity is the same as that of the mid microphone, and has a directivity
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characteristic shown by a broken line in FIG. This directivity characteristic is the same not only in
the horizontal plane but also in all planes. ES = Eosinθ (2)
[0011]
In the MS stereo microphone, an output signal of the left (L) channel is obtained by adding the
output voltages EM and ES of both the mid and side microphones, and the combined output
voltage EL is expressed by the equation (3) . EL = EM + ES = EO (0.5 + 0.5 cos θ + sin θ) (3)
[0012]
Next, the output signal of the right (R) channel is obtained by subtracting the output voltages EM
and ES of both the mid and side microphones, and the combined output voltage ER is expressed
by equation (4). ER = EM-ES = EO (0.5 + 0.5 cos θ-sin θ) (4)
[0013]
Here, when equations (3) and (4) are modified, they can be expressed as equations (5) and (6),
respectively.
[0014]
Therefore, EL has a synthetic directional characteristic as shown by a solid line in FIG. 2 which
has the maximum sensitivity in the direction of θ of 63.5 °.
In addition, ER has a maximum sensitivity in the direction of 63.5 ° taken in the clockwise
direction contrary to the case of θ of -63.5 °, that is, EL, and has a combined directional
characteristic as shown by a broken line in FIG. .
[0015]
As described above, in the MS stereo microphone, EL and ER obtained by adding and subtracting
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the output signals of the mid and side microphones are used as stereo signals. In this case, in the
ideal case where both microphones are at the same point, the same combined directional
characteristic can be obtained over the entire band.
[0016]
(B) In the MS stereo microphone, the case where the mid microphone and the side microphone
are not at the same point will be described. When the mid microphones and the side
microphones are disposed back and forth in the horizontal plane as shown in FIG. 3, both
microphones have a distance difference. Therefore, we will consider the combined directional
characteristics in the case where both microphones are disposed apart from each other in the
horizontal plane by dO.
[0017]
As shown in FIG. 3, the direction of arrival of the plane wave is counterclockwise and θ is the
maximum sensitivity direction, and the maximum sensitivity direction is 90 ° and 270 ° behind
the mid microphone arranged by dO behind the mid microphone. Arrange the side microphones
in the direction of (-90 °). In such an arrangement, the relative distance in the horizontal plane
of both the mid and side microphones differs depending on the incident direction (θ) of the
sound wave, and is expressed by dO cos θ. Therefore, the relative time difference between the
two microphones is dO cos θ / c (c: sound velocity), and when the angular frequency of the sine
wave is represented by ω as phase angle, it is ω d O cos θ / c. The output voltage EM of the
unidirectional mid microphone is expressed by equation (7) as in the case of (A). EM = EO (0.5 +
0.5 cos θ) (7)
[0018]
On the other hand, the output voltage ES of the bi-directional side microphone is expressed by
equation (8) because the phase angle with respect to the mid microphone is ω d O cos θ / c.
[0019]
Therefore, the combined output voltage EL of the left (L) channel is represented by equation (9),
and the combined output voltage ER of the right (R) channel is represented by equation (10).
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[0020]
Here, since the synthetic directional characteristics of EL and ER are the same except that the
maximum sensitivity direction is different, EL will be examined below.
The coefficient part of the sin θ term in equation (9) can be expressed as equation (11).
Substituting the equation (11) into the equation (9) and arranging it, the equation (12) is
obtained.
[0022]
Then, taking the absolute value of the equation (12) and determining the composite directivity
characteristics at several frequencies with dO = 2.7 cm, the result is as shown in FIG. The
characteristic shown by the broken line in the figure is an ideal composite directivity
characteristic that the mid and side microphones are at the same point. From a comparison of
these two characteristics, it is considered that the effect of dO is small at frequencies below
approximately 2 kHz. However, the combined directivity at a frequency of 2 kHz or more is
greatly affected by dO.
[0023]
(C) A method, an embodiment and the like for reducing the influence of dO on composite
directional characteristics will be described. (A) Method to reduce First of all, the influence of the
dO on the combined directivity characteristic differs depending on the incident angle θ of the
sound wave. That is, for a sound wave in which θ is in the front half of the microphone, that is,
in the range of 90 ° to 0 ° to 270 ° (−90 °), the output signal of the side microphone lags
behind the output signal of the mid microphone. It will occur. However, for sound waves in which
θ is in the rear half of the microphone, that is, in the range of 90 ° to 180 ° to 270 ° (−90
°), the output signal of the mid microphone is delayed with respect to the output signal of the
side microphone. It will occur.
[0024]
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However, as is clear from FIG. 4, the synthetic directivity in the range of 90 ° to 180 ° to 270
° has a relatively small change from the ideal synthetic directivity. This is due to the following
reasons.
[0025]
That is, at θ = 180 °, the time difference between the output signals of both microphones is
maximum but the sensitivity is zero. Also, at θ of 90 ° and 270 °, the mid microphone has a
sensitivity of half that of the side microphone, but the time difference is zero. In the range of
angles of 90 ° to 180 ° and 270 ° to 180 ° excluding these two angles, the time difference
increases toward 90 ° → 180 °, 270 ° → 180 °, as apparent from FIG. Because the
sensitivity of the mid microphones decreases sharply toward 90 ° → 180 ° and 270 ° → 180
° compared to the sensitivity of the side microphones, the influence on the combined directivity
characteristic by the phase interference based on the time difference decreases It is.
[0026]
Therefore, in order to reduce the influence of dO on the composite directional characteristics, it
is preferable to take measures in the range of 90 ° to 0 ° to 270 ° (-90 °) which is also a
range of angles important as a stereo microphone. In addition, also in the range of this angle, the
angle with a big influence is the range of 10 degrees-70 degrees, as FIG. 4 shows. That is, since
the side microphones cause a time delay with respect to the mid microphone in the range of this
angle, as a countermeasure, a time delay is given to the output signal of the mid microphone, and
relative to the output signal of the side microphone It may be synthesized by reducing the time
difference.
[0027]
(B) Time delay circuit A circuit which can obtain a constant delay time over a wide frequency
range is easy to use by digital technology, but to incorporate it in a microphone, it is necessary in
terms of power consumption and size. It is impossible at present. Also, as shown in FIGS. 5A and
5B, a phase shift circuit using an operational amplifier is known, but there is a problem in the SN
ratio in using it for a microphone.
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[0028]
Therefore, the phase shift circuit 10 configured as shown in FIG. 6 is practical. The phase circuit
10 includes a first-order low-pass filter (LPF) 11 to which an input signal is input, and a firstorder high-pass filter (HPF) 12 to which an input signal is input. An output signal is taken out by
subtraction in the differential circuit 13. In this phase circuit 10, the amplitude frequency
characteristic is flat, and only the phase angle becomes the phase characteristic of the delay
phase which changes with frequency.
[0029]
That is, when the amplification degree of the phase shift circuit 10 is set to 1, the transfer
function TF between the input and the output of the phase circuit 10 is expressed by equation
(13), and the phase frequency characteristic as shown in FIG.
[0030]
In this case, | TF | = 1, so that the amplitude frequency characteristic is flat and the phase angle
φ is the delay phase represented by equation (14).
φ = −2 tan −1 ω CORO (14) The phase angle φ becomes φ = −2 ω CORO in the frequency
range of ω CORO << 1 and is proportional to the frequency. Therefore, at the frequency of
ωCORO << 1, a delay time of a constant value of −2CORO can be obtained. Also, at high
frequencies, φ is a constant value of −π (−180 °).
[0031]
Further, since the transfer function TF has a flat amplitude frequency characteristic and only the
phase angle φ changes with frequency, the transfer function TF can be expressed by an equation
(15) as an exponential function. TF = ejφ (15)
[0032]
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(C) Embodiment FIG. 8 shows the configuration of a stereo microphone 20 of the MS system
according to the embodiment. The stereo microphone 20 is a mid microphone 21 having a single
directivity with the maximum sensitivity direction directed to the front direction (0 °) and the
maximum sensitivity direction directed to the left and right directions (± 90 °) with respect to
the front direction. And a side microphone 22 having both directivity. Although not shown, the
mid microphones 21 and the side microphones 22 are disposed on the main axis in the front
direction at a predetermined distance d0 (see FIG. 3). Also, the mid microphone 21 is disposed on
the front side of the side microphone 22.
[0033]
In addition, the stereo microphone 20 includes an amplifier 23 for obtaining an output signal EM
of the mid microphone 21, an amplifier 24 for obtaining an output signal ES of the side
microphone 22, and an incident angle of a sound wave to the output signal EM of the mid
microphone 21. And a phase shift circuit 25 as time delay means for giving a time delay equal to
the relative time difference between the output signals of the mid microphone 21 and the side
microphone 22 in a predetermined direction in the range of 0 ° to 90 °. Here, the phase shift
circuit 25 has a configuration similar to that of the phase shift circuit 10 shown in FIG. 6
described above.
[0034]
The stereo microphone 20 adds the output signal EM 'of the phase shift circuit 25 and the output
signal ES of the side microphone 22 to obtain the left (L) channel output signal EL, and the left
channel output signal EL. And a subtractor 28 for obtaining the right (R) channel output signal
ER by subtracting the output signal ES of the side microphone 22 from the output signal EM ′
of the phase shift circuit 25 and its right channel output signal ER And an output terminal 29 for
outputting the
[0035]
Consider the stereo microphone 20 shown in FIG.
The output voltage EM of the mid microphone 21 is expressed by the equation (16) as described
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in the above (A) and (B). EM = EO (0.5 + 0.5 cos θ) (16)
[0036]
Further, the output voltage ES of the side microphone is also expressed by equation (17), as
described in the paragraph (B).
[0037]
Here, since the synthetic directional characteristics of EL and ER are the same except that the
maximum sensitivity direction is different, the EL will be examined below.
A phase delay circuit 25 applies a time delay to the output voltage EM of the mid microphone 21
and adds the output signal EM 'of the phase shift circuit 25 and the output signal ES of the mid
microphone 22 to obtain a combined output voltage EL. Since the transfer function TF of the
phase shift circuit 25 is expressed by equation (14) described above, EL / EO is expressed by
equation (18).
[0039]
In this equation (18), the phase angle (ωdO cos θ / c + φ) is the relative phase angle between
the output signal EM of the mid microphone 21 and the output signal ES of the side microphone
22.
[0040]
Therefore, since φ is a delay phase represented by equation (14), equation (19) holds. For ω =
ω1 (f = f1), θ = θ1, e0 = 1 and EL / E0 is the above-mentioned (3), which is the same as when
the mid and side microphones are at the same point.
ω d O cos θ / c − 2 tan −1 ω CORO = 0 (19)
[0041]
Also, at frequencies and incident angles other than f1 and θ1, the relative phase angle (ω docos
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θ / c + φ) decreases by φ as compared to the case where the phase shift circuit 25 is not
inserted, so both the mid and side microphones Synthetic directional characteristics close to
synthetic directional characteristics at the same point can be obtained.
[0042]
Next, theoretical examples will be described.
As described above, assuming that dO = 2.7 cm, the influence of the dO on the synthetic
directivity is at a frequency of 2 kHz or more, and the angle θ1 may be in the range of 10 ° to
70 °. Therefore, CORO satisfying the equation (19) is obtained by setting the above f1 to 4 kHz
and setting θ1 = 35 °.
[0043]
If the equation representing CORO is determined from the equation (19), then the equation (20)
is obtained, and in the case of dO = 2.7 cm, f1 = 4 kHz, θ1 = 35 °, CORO ≒ 0.04 ms.
[0044]
Therefore, ωCORO = 1, ie, the crossover frequency of the low pass filter and the high pass filter
is approximately 4 kHz.
The phase frequency characteristics of the phase shift circuit 25 are as shown in FIG.
[0045]
Here, when the equation (18) is modified and represented by the real part and the imaginary
part, it becomes as the equation (21).
[0046]
Then, using the phase shift circuit 25 having phase frequency characteristics as shown in FIG. 7,
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the absolute value of equation (21) is obtained, and the combined directional characteristics at
the same frequency as in FIG. It will be.
Similar to FIG. 4, the synthesized directivity shown by the broken line is an ideal synthesized
directivity when both the mid and side microphones are at the same point. According to FIG. 9,
up to about 5 kHz, synthetic directional characteristics quite similar to the ideal synthetic
directional characteristics shown by the broken line are shown, and theoretically sufficient
effects for practical use are obtained.
[0047]
Next, a stereo microphone 20 having the configuration shown in FIG. 8 is produced on a trial
basis, and the result of measuring the synthesized directional characteristics will be described.
FIG. 10 shows the output voltage directivity frequency characteristics of the used uni-directional
mid microphone 21. As shown in FIG. FIG. 11 shows the output voltage directivity frequency
characteristics of the used bidirectional side microphone 22. As shown in FIG. The two
microphones 21 and 22 were disposed at a center distance dO of 2.7 cm in the front direction (0
°). Then, the output signal of the mid microphone 21 passing through the phase shift circuit 25
having the phase characteristic shown in FIG. 7 and the output signal of the side microphone 22
are added to obtain a left (L) channel combined output, and the combined directional
characteristics Was measured. FIG. 12 shows the measurement results. According to FIG. 12, the
directivity characteristic close to the theoretical characteristic shown in FIG. 9 is obtained up to
about 5 kHz, and it can be judged that a good result is obtained.
[0048]
In the above-described embodiment, it has been described that the phase shift circuit 25 (having
the same configuration as the phase shift circuit 10 in FIG. 6) alone is used to give a time delay to
the output signal EM of the mid microphone 21. A plurality of phase shift circuits 25 may be
connected in cascade. As a result, the time delay of one phase shift circuit can be reduced, and
hence ωCORO in one phase shift circuit can be reduced, so that it is possible to obtain a
frequency range in which combined directional characteristics close to ideal combined
directional characteristics can be obtained. It can be spread.
[0049]
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In the above embodiment, the mid microphone 21 is disposed on the front side with respect to
the side microphone 22. However, in the present invention, the side microphone 22 is disposed
on the front side with respect to the mid microphone 21. The same applies to things. In that case,
instead of giving a time delay to the output signal EM of the mid microphone 21, a time delay
will be given to the output signal ES of the side microphone 22.
[0050]
According to the present invention, in the MS type stereo microphone in which the mid
microphone and the side microphone are disposed separately from each other, time delay is
added to the output signal of the microphone advancing in time. The time difference between the
output signals of both microphones can be reduced, the influence on the combined directivity
characteristic can be reduced, and the output signals of the left channel and the right channel
can be obtained well.
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