JP2002171590A - Stereophonic microphone adopting ms system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To relieve the effect of a time difference of output signals of both mid and side microphones onto a synthetic directive characteristic. SOLUTION: The stereophonic microphone is provided with a mid microphone 21 with a single directivity whose maximum sensitivity direction is directed to a front direction (0 deg.) and a side microphone 22 with a bidirectivity whose maximum sensitivity direction is directed to left and right directions (±90 deg.) with respect to the front direction. The microphones 21, 22 are placed apart by a prescribe distance on a major axis in the front direction in a way that the microphone 21 comes closer to the front more than the microphone 22. A phase shift circuit 25 provides a time delay equal to a relative time difference between output signals EM, ES from the microphone 21, 22 within a range of a sound wave incident angle of 0 deg.-90 deg. to the output signal EM of the microphone 21 so as to reduce the time difference between the output signals EM, ES. An output signal EM' from the phase shift circuit 25 is added to/subtracted from the output signal ES from the microphone 22 to respectively obtain a left channel output signal EL/a right channel output signal ER.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MS stereo microphone in which a mid microphone and a side microphone are arranged at a distance from each other. More specifically, an MS stereo system in which a time delay is applied to a microphone output signal that advances in time to reduce the time difference between the output signals of both microphones, thereby reducing the influence on the combined directional characteristics. It relates to a microphone.

[0002]

2. Description of the Related Art Conventionally, for a stereo microphone of the MS system, for business use such as broadcasting, a mid (Mid) microphone and a side (Side) microphone are arranged vertically so that there is no time difference between both microphones at least in a horizontal plane. Is considered.

[0003]

However, for consumer use,
Since the shape is cylindrical for convenience of handling, the mid microphone and the side microphone are arranged slightly apart in front and rear.

[0004] Therefore, the output signal of the mid microphone and the output signal of the side microphone are added and subtracted to obtain a left (L) channel output signal and a right (R) channel output signal, respectively.
In the MS system for obtaining the channel output signal, the output signals of both microphones have different time differences depending on the incident direction of the sound wave due to the above-mentioned distance, and the output signals of both channels are combined because of the phase interference. Has a drawback that the directivity characteristics of the image change in comparison with the ideal case with no time difference. This change is generally significant at frequencies above a few kHz.

An object of the present invention is to provide a stereo microphone of the MS system 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 combined directional characteristics.

[0006]

According to the present invention, there is provided a mid microphone having a single directivity in which the direction of maximum sensitivity is directed in the front direction (0 °), and a direction in which the maximum sensitivity direction is shifted in the right and left directions (± 90 °), and a side microphone having bidirectional directivity (90 °) is located at a predetermined distance on the main axis in the front direction. One of the arranged microphones is a first microphone, the other microphone is a second microphone, and the incident angle of the sound wave is 0 ° to 90 °.
A signal obtained by giving a time delay equal to the relative time difference between the output signals of the first and second microphones in a predetermined direction within the range of
Is added to and subtracted from the microphone output signal to obtain a left channel output signal and a right channel output signal, respectively.

In the present invention, the mid microphone and the side microphone are moved back and forth (on the main axis in the front direction).
The microphones are spaced apart from each other, and the output signals of these two microphones have different time differences depending on the incident direction of the sound wave.
However, in the present invention, among the output signals of the mid microphone and the side microphone, the angle of incidence of the sound wave is 0 °
A time delay equal to the relative time difference between the output signals of both microphones in a predetermined direction within a range of 90 ° is provided. As a result, the time difference between the output signals of the two microphones is reduced, the influence on the combined directional characteristics is reduced, and the left channel output signal and the right channel output signal can be obtained favorably.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION (A) In an MS stereo microphone, an ideal case in which a mid (Mid) microphone and a side (Side) microphone are at the same point will be described.

In the configuration of the stereo microphone of the MS system, the mid microphone is unidirectional, and its maximum sensitivity direction is arranged facing the front (0 °). Therefore,
If the incident angle of the plane wave is θ and the front direction is 0 ° in the horizontal plane and the counterclockwise direction is set, the output voltage (output signal)
E _M is expressed by equation (1), where E _O is the output voltage in the direction of maximum sensitivity, and has a heart-shaped directional characteristic shown in FIG. This is the directional characteristic in the horizontal plane, but becomes the same in all the planes including the main axes in the 0 ° and 180 ° directions. E _M = E _O (0.5 + 0.5 cos θ) (1)

Next, the side microphones are generally bidirectional, and are arranged with their maximum sensitivity directions at 90 ° and 270 ° (−90 °) with respect to the 0 ° direction. In this case, the output voltage (output signal) E _S is equal to the output voltage in the direction of maximum sensitivity E _{O which} is the same as that of the mid microphone.
The directional characteristic is represented by Expression (2) and is indicated by a broken line in FIG. This directional characteristic is the same not only in the horizontal plane but also in all planes. E _S = E _O sin θ (2)

In the stereo microphone of the MS system, the output signal of the left (L) channel is obtained by adding the output voltages E _M and E _S of both the mid and side microphones.
The combined output voltage E _L is expressed by equation (3). E _L = E _M + E _S = E _O (0.5 + 0.5 cos θ + sin θ) (3)

[0012] Then, the output signal of the right (R) channel, mid, output voltage E _M side both microphones, obtained by subtracting E _S, the combined output voltage E _R is
It is expressed by equation (4). E _R = E _M -E _S = E _O (0.5 + 0.5 cos θ−sin θ) (4)

Here, by transforming equations (3) and (4), they can be expressed as equations (5) and (6), respectively.

(Equation 1)

Accordingly, E _L has the maximum sensitivity in the direction where θ is 63.5 °, and has a combined directional characteristic as shown by the solid line in FIG. In addition, E _R has a maximum sensitivity in the direction of 63.5 ° taken in the clockwise direction, which is opposite to the case where θ is −63.5 °, that is, in the case of E _L. Becomes

In the stereo microphone of the MS system, as described above, E _L and E _R obtained by adding and subtracting the output signals of both the mid and side microphones.
Is used as a stereo signal. In this case, if both microphones are ideally located at the same point, the same combined directional characteristics can be obtained over the entire band.

(B) An explanation will be given of a case where the mid microphone and the side microphone are not at the same point in the stereo microphone of the MS system. When the mid microphone and the side microphone are arranged in front and behind in a horizontal plane as shown in FIG. 3, the two microphones have a distance difference. Therefore, both microphones are examined combined directivity characteristics when you have spaced apart by d _O back and forth in a horizontal plane.

As shown in FIG. 3, and θ the direction of arrival of a plane wave in a counterclockwise direction, the maximum sensitivity direction in the rear of the mid microphone disposed toward the 0 ° direction, the maximum sensitivity direction apart by d _O 90 Position the side microphones oriented in the ° and 270 ° (-90 °) directions. In such an arrangement, the relative distance between the mid and side microphones in the horizontal plane depends on the incident direction (θ) of the sound wave, and is represented by d _O cos θ. Therefore, the relative time difference between the two microphones is d _O cos θ / c (c: sound speed).
d _O cos θ / c. The output voltage E _M of the unidirectional mid microphone is expressed by equation (7) as in the case of (A). E _M = E _O (0.5 + 0.5 cos θ) (7)

On the other hand, the output voltage E _S of the bidirectional side microphone is expressed by equation (8) since the phase angle with respect to the mid microphone is ωd _O cos θ / c.

(Equation 2)

Therefore, the composite output voltage E _L of the left (L) channel is expressed as in equation (9), and the composite output voltage E _R of the right (R) channel is expressed as in equation (10).

(Equation 3)

[0020] Here, E _L, combined directivity characteristic of E _R is maximum sensitivity direction is different only because it is both the same, or less E _L
To consider. The coefficient portion of the sinθ term in equation (9) can be expressed as in equation (11). This (1
By substituting equation (1) into equation (9) and rearranging, it is expressed as equation (12).

[0021]

(Equation 4)

Then, taking the absolute value of the equation (12), d_O
= 2.7 cm, combined directional characteristics at some frequencies
FIG. 4 shows the characteristics. Shown by broken lines in the figure
Characteristics, the mid and side microphones are at the same point
This is an ideal combined directional characteristic. These two characteristics
By comparison, at frequencies below about 2 kHz, d _O
The impact of the project is expected to be small. However, more than 2kHz
The combined directional characteristic at the frequency of_OBig impact by
Receive.

(C) A method and an embodiment for reducing the influence of d _O on the combined directional characteristics will be described. (A) Reduction Method First, the influence of d _O on the combined directional characteristics differs depending on the incident angle θ of the sound wave. That is, for a sound wave in which θ is in the first half of the microphone, that is, in the range of 90 ° to 0 ° to 270 ° (-90 °), the output signal of the side microphone has a time delay with respect to the output signal of the mid microphone. Occurs. However, θ is the latter half of the microphone, that is, 90
For sound waves in the range of ° to 180 ° to 270 ° (-90 °), the output signal of the mid microphone has a time delay with respect to the output signal of the side microphone.

However, when θ is 90 ° -180 ° -270 °
As can be seen from FIG. 4, the combined directional characteristics in the range up to have relatively little change from the ideal combined directional characteristics.
This is for the following reasons.

That is, when θ = 180 °, the time difference between the output signals of both microphones becomes maximum, but the sensitivity is zero. When θ is 90 ° and 270 °, the mid microphone has half the sensitivity of the side microphone, but the time difference is zero. 90 ° to 180 ° excluding these two angles, 27
In the range of 0 ° to 180 °, the time difference is 90 ° → 18
0 °, 270 ° → 180 °.
As is clear from FIG. 7, the sensitivity of the mid microphone is 90 ° → 180 °
This is because the influence on the combined directional characteristics due to the phase interference based on the time difference decreases because the angle decreases rapidly from 0 ° to 180 °.

Therefore, in order to reduce the influence of d _O on the combined directional characteristics, 90 ° to 0 ° to 270 ° (−90 °) which is also an important angle range for a stereo microphone.
I think that measures should be taken for the range of °). In addition,
An angle that has a particularly large effect within this range of angles is in the range of 10 ° to 70 ° as shown in FIG. In other words, in this angle range, the side microphone has a time delay with respect to the mid microphone, and as a countermeasure, a time delay is given to the output signal of the mid microphone, so that the output signal relative to the side microphone is delayed. What is necessary is just to synthesize | combine by reducing time difference.

(B) Time delay circuit A circuit capable of obtaining a constant delay time over a wide frequency range can be easily realized by using digital technology. It is impossible at present at this point. Further, as shown in FIGS. 5A and 5B, a phase shift circuit using an operational amplifier is known, but there is a problem in terms of an SN ratio when used in a microphone.

Therefore, the phase shift circuit 10 having the configuration shown in FIG. 6 is practical. The phase circuit 10 has a primary low-pass filter (LPF) 11 to which an input signal is input and a primary high-pass filter (HPF) 12 to which an input signal is input. The output signal is extracted by subtraction in the differential circuit 13. This phase circuit 10 has a flat amplitude-frequency characteristic,
Only the phase angle has a phase characteristic of a lag phase that changes with frequency.

That is, when the amplification of the phase shift circuit 10 is set to 1, the input-output transfer function _TF of the phase circuit 10 is expressed by the equation (13), and the phase frequency characteristic and the phase frequency characteristic as shown in FIG. Become.

(Equation 5)

In this case, | _TF | = 1, so that the amplitude frequency characteristic is flat, and the phase angle φ is a lag phase represented by the equation (14). It is ^{_{φ = -2tan -1 ωC O R O}} ··· (14) the phase angle phi, in the frequency range of ωC _O R _O «1 φ = -
2ωC _O R _O, and the becomes proportional to the frequency. Therefore, the frequency of .omega.C _O R _O << 1 obtained delay time constant value consisting -2C _O R _O. At a high frequency, φ is −
It becomes a constant value of π (−180 °).

Since the transfer function T _F has a flat amplitude-frequency characteristic and only the phase angle φ changes with frequency, it can be expressed by an exponential function as shown in equation (15). T _F = e ^{j φ} (15)

(C) Embodiment FIG. 8 shows the configuration of an MS stereo microphone 20 according to an embodiment. This stereo microphone 20 has a unidirectional mid microphone 21 whose maximum sensitivity direction is directed to the front direction (0 °), and whose maximum sensitivity direction is directed to left and right directions (± 90 °) with respect to the front direction. And a side microphone 22 having bidirectionality. Although not shown, the mid microphone 21 and the side microphone 22 are positioned at a predetermined distance d on the main axis in the front direction.
They are located apart by _O (see FIG. 3). Further, the mid microphone 21 is arranged on the front side of the side microphone 22.

Further, the stereo microphone 20, an amplifier 23 for obtaining the output signal E _M mid microphone 21, an amplifier 24 for obtaining an output signal E _S side microphone 22, the output signal E _M mid microphone 21 A phase shift circuit 25 as a 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 which the incident angle of the sound wave is in the range of 0 ° to 90 °. Have. Here, the phase shift circuit 25 has the same configuration as the phase shift circuit 10 shown in FIG. 6 described above.

The stereo microphone 20 adds the output signal E _M 'of the phase shift circuit 25 and the output signal E _S of the side microphone 22 and outputs a left (L) channel output signal E _L.
, An output terminal 27 for outputting the left channel output signal E _L , and an output signal E _M ′ of the phase shift circuit 25.
It has a subtracter 28 to obtain a more side output signal E _S subtraction to right (R) channel output signals E _R of the microphone 22, and an output terminal 29 for outputting the right channel output signal E _R.

Consider the stereo microphone 20 shown in FIG. Output voltage E _M of mid microphone 21
Is (1) in the same manner as described in the above (A) and (B).
6) It is expressed by the equation. E _M = E _O (0.5 + 0.5 cos θ) (16)

Also, the output voltage E _{S of the} side microphone
Is also expressed by equation (17), as described in the above section (B).

(Equation 6)

[0037] Here, E _L, combined directivity characteristic of E _R is maximum sensitivity direction is different only because it is both the same, or less E _L
To consider. The output voltage E _M of the mid microphone 21 is given a time delay by the phase shift circuit 25, and the output signal E _M ′ of the phase shift circuit 25 and the output signal E _S of the mid microphone 22 are added to generate a combined output voltage E _L. And the transfer function T _F of the phase shift circuit 25 is obtained by the above (14).
Since it is expressed by the equation, E _L / E _O is expressed by the equation (18).

[0038]

(Equation 7)

In the equation (18), (ωd _O cosθ /
The phase angle c + φ) is a relative phase angle between the output signal E _M of the mid microphone 21 and the output signal E _S of the side microphone 22.

Accordingly, since φ is the delay phase represented by the equation (14), the equation (19) is satisfied.
₁ (f = f ₁ ) and θ = θ ₁ , e ⁰ = 1, and E
_L / E ₀ matches the above equation (3), and is the same as when both the mid and side microphones are at the same point. ^{_{ωd O cosθ / c-2tan -1}} ωC O R O = 0 ··· (19)

At frequencies and angles of incidence other than f ₁ and θ ₁ , compared to the case where the phase shift circuit 25 is not inserted, (ω
Since the relative phase angle (docos θ / c + φ) is reduced by φ, a combined directional characteristic close to the combined directional characteristic when both the mid and side microphones are at the same point can be obtained.

Next, a concrete example will be described. As described above, when d _O = 2.7 cm, the influence of d _O on the combined directional characteristics is a frequency of 2 kHz or more, and θ ₁ may be considered to be in the range of 10 ° to 70 °. Therefore, the above f _{1 is set} to 4 kHz, and θ ₁ = 35 °
As a Request C _O R _O satisfying the expression (19).

From the equation (19), the equation representing C _O R _O is obtained as shown in the equation (20), where d _O = 2.7 cm and f ₁ =
4 kHz, the θ ₁ = 35 °, the C _O R _O ≒ 0.04ms.

(Equation 8)

[0044] Therefore, ωC _O R _O = 1, that is, determine the crossover frequency of the low-pass filter and a high-pass filter, is about 4 kHz. FIG. 7 shows the phase frequency characteristics of the phase shift circuit 25.

Here, when the equation (18) is modified to show a real part and an imaginary part, the equation (21) is obtained.

(Equation 9)

Then, the absolute value of the equation (21) is calculated using a phase shift circuit 25 having a phase frequency characteristic as shown in FIG.
When the combined directional characteristics at the same frequency as in FIG. 4 are obtained, the result is as shown in FIG. Similar to FIG. 4, the combined directional characteristics indicated by broken lines are ideal combined directional characteristics when both the mid and side microphones are at the same point. According to FIG. 9, up to about 5 kHz, the combined directional characteristics are fairly close to the ideal combined directional characteristics indicated by the broken line, and theoretically sufficient effects are obtained.

Next, the results of trial production of the stereo microphone 20 having the configuration shown in FIG. 8 and measurement of its combined directional characteristics will be described. FIG. 10 shows the output voltage directivity frequency characteristics of the unidirectional mid microphone 21 used. FIG. 11 shows the bidirectional side microphone 2 used.
2 shows the output voltage directivity frequency characteristic. These two microphones 21 and 22 were arranged with a center interval d _O of 2.7 cm in the front direction (0 °). Then, the output signal of the mid microphone 21 and the output signal of the side microphone 22 through the phase shift circuit 25 having the phase characteristic shown in FIG.
The combined directional characteristics were measured. FIG. 12 shows the measurement results. According to FIG. 12, the directional characteristics close to the theoretical characteristics shown in FIG. 9 are obtained up to about 5 kHz, and it can be determined that good results are obtained.

In the above-described embodiment, the phase shift circuit 25 (same configuration as the phase shift circuit 10 in FIG. 6) is used alone to give a time delay to the output signal E _M of the mid microphone 21. , But this phase shift circuit 25
May be used in cascade connection. As a result, the time delay assigned to one phase shift circuit can be reduced,
Therefore it is possible because it can reduce the .omega.C _O R _O in one phase shifting circuit, widening the frequency range can be obtained combined directivity characteristic close to the ideal synthesis directional pattern.

In the above embodiment, the mid
The microphone 21 is more positive than the side microphone 22
Although the one arranged on the surface side is shown, the present invention
Microphone 22 is more frontal than mid microphone 21
The same applies to those arranged on the side.
In that case, the output signal E of the mid microphone 21 _M
Instead of adding a time delay to the
2 output signal E_SWill be given a time delay.

[0050]

According to the present invention, in a MS stereo microphone in which a mid microphone and a side microphone are arranged at a distance from each other, a time delay is given to an output signal of a microphone which is temporally advanced. , The time difference between the output signals of both microphones is reduced, the influence on the combined directional characteristics can be reduced, and the output signals of the left and right channels can be obtained favorably.

[Brief description of the drawings]

FIG. 1 is a diagram showing the directional characteristics of both a mid and side microphone.

FIG. 2 is a diagram illustrating combined directional characteristics of output signals of both left and right channels.

[3] Mid, side two microphones is a diagram for explaining a case in which apart d _O.

FIG. 4 is a combined directional characteristic of each frequency (d _O = 2.7 cm).
FIG.

FIG. 5 is a diagram illustrating an example of a phase shift circuit using an operational amplifier.

FIG. 6 is a diagram illustrating a phase shift circuit using a primary LPF and an HPF.

FIG. 7 is a diagram illustrating a phase shift frequency characteristic of a phase shift circuit.

FIG. 8 is a block diagram illustrating a configuration of an MS stereo microphone as an embodiment.

FIG. 9 shows a combined directional characteristic (d _O = 2.7 cm, θ ₁ = 35 °, f ₁ = 4k) of each frequency when a phase shift circuit is used.
(Hz).

FIG. 10 is a diagram illustrating output voltage directivity frequency characteristics of a mid microphone (unidirectional).

FIG. 11 is a diagram showing output voltage directivity frequency characteristics of a side microphone (bidirectional).

FIG. 12 is a diagram showing a measurement result of a combined directional characteristic of a prototype stereo microphone.

[Explanation of symbols]

 Reference Signs List 10 phase shift circuit 11 low pass filter (LPF) 12 high pass filter (HPF) 13 differential circuit 21 mid microphone 22 side microphone 25 phase shift circuit 26 adder 27, 29 output terminal 28 subtracter

Claims (3)

[Claims] 1. A mid-microphone having a single directivity whose maximum sensitivity direction is directed in the front direction (0 °), and a mid microphone having the maximum sensitivity direction directed in the right and left directions (± 90 °) with respect to the front direction. In an MS stereo microphone in which a side microphone having directivity is arranged at a predetermined distance on the main axis in the front direction, one of the mid microphone and the side microphone arranged on the front side First microphone
And the other microphone is a second microphone, and a relative time difference between output signals of the first microphone and the second microphone in a predetermined direction in which an incident angle of a sound wave is in a range of 0 ° to 90 °. And a signal obtained by adding a time delay equal to the output signal of the first microphone and the output signal of the second microphone to the left and right channel output signals, respectively. A stereo microphone of the MS system, characterized in that: 2. A time delaying means for giving the time delay to an output signal of the first microphone includes a primary low-pass filter to which an input signal is input, and a primary high-pass filter to which the input signal is input. 2. The stereo microphone according to claim 1, wherein a phase shift circuit is used to extract an output signal by subtracting an output signal of the primary high-pass filter from an output signal of the primary low-pass filter. 3. The stereo microphone according to claim 2, wherein a plurality of the phase shift circuits are connected in cascade as the time delay means.