JP3336729B2 - Sound field control device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sound field control device for suppressing howling and coloration in an acoustic feedback system in which a microphone and a speaker are arranged in an acoustic space to acoustically form a feedback loop. In this method, howling and coloration can be easily suppressed.

[0002]

2. Description of the Related Art When a microphone and a speaker are located in the same acoustic space, such as in a hall or a lecture hall, sound emitted from the speaker is acoustically fed back to the microphone to form an acoustic feedback system (Acoustic Feedback Loop). However, there is a problem that a sharp peak appears on the frequency axis to cause coloration in terms of audibility and howling occurs at the peak position. In addition, in order to adjust (extend) the reverberation time of a hall or the like, a so-called electroacoustic reverberation support device in which a microphone and a speaker are arranged in an acoustic space to constitute an acoustic feedback system also causes coloration and howling. There's a problem.

As a method of suppressing coloration and howling in an acoustic feedback system, conventionally, a closed loop transfer function G (ω) of the acoustic feedback system is measured and its inverse transfer function G (ω) is measured.
^-1 (ω) has been obtained, and a filter having the characteristic has been inserted into a loop. However, the response of the closed-loop transfer function G (ω) is generally very long, and it has been difficult to find the inverse transfer function. For this reason, the following method (a) or (b) has actually been adopted.

(A) The peak and dip are smoothed manually while monitoring the amplitude characteristic in a closed loop (normal equalizing).

(B) The peak and dip of the minimum phase element are smoothed manually while simultaneously monitoring the amplitude characteristic and the phase characteristic in a closed loop (parametric equalization).

[0006]

The methods (a) and (b) require experience and require a very long time for adjustment.

An object of the present invention is to solve the above-mentioned problems in the prior art and to provide a sound field control device capable of easily suppressing howling and coloration.

[0008]

According to the first aspect of the present invention,
In an acoustic feedback system in which microphones and speaker means are arranged in an acoustic space and acoustically form a feedback loop , the open-loop transfer function of the feedback loop is represented by the product of its all-pass element and its minimum phase element.
When the filter means is provided, a filter means having substantially the opposite characteristic of the minimum phase element is arranged in the feedback loop .

According to a second aspect of the present invention, there is provided an acoustic feedback system in which a microphone and a loudspeaker are arranged in an acoustic space to acoustically form a feedback loop. Let H _min ⁻¹ (ω) be the inverse characteristic of the minimum phase element, H _ap (ω) be the all-pass element, and A be the constant part of the open-loop transfer function, be approximately −H _min ⁻¹ (ω) / (1 ). −A · H _ap (ω)).

According to a third aspect of the present invention, in an acoustic feedback system in which a microphone and a loudspeaker are arranged in an acoustic space to acoustically form a feedback loop, an open loop transmission for measuring an open loop transmission characteristic of the loop is provided. From the characteristic measuring means and the measured open-loop transfer characteristic, the inverse characteristic of the minimum phase element of the open-loop transfer function of the loop is calculated as H
_min ⁻¹ (ω), H _ap (ω) for the all-pass element, and A for the constant part of the open-loop transfer function, and approximately H _min ⁻¹ (ω) or −H _min ⁻¹ (ω) / (1− A · H _ap (ω)) calculating means for obtaining an FIR filter coefficient substantially corresponding to the characteristic of the signal, and the FIR filter coefficient arranged in the feedback loop and convolving the obtained FIR filter coefficient with the signal in the loop. And FIR filter means.

[0011]

According to the first aspect of the present invention, by inserting a filter having substantially the reverse characteristic of the minimum phase element of the open loop transfer function into the loop, unstable elements caused by the open loop transfer function can be eliminated. It can be removed from the closed loop.
As a result, the time response characteristic can be shortened, the peak factor of the pole of the closed-loop transfer function can be smoothed (the unevenness of the peak points can be reduced), the howling margin can be expanded, and howling can be effectively suppressed. can do.

According to the second aspect of the present invention, a filter having a characteristic of substantially -H _min ^-1 (ω) / (1-A · H _ap (ω)) is inserted into the loop. The time response can be further shortened, and the peaks themselves can disappear or be reduced to very low levels. Therefore, the howling margin is further expanded and the coloration is reduced.

In the first and second aspects of the present invention, the filter characteristics of the filter inserted into the loop can be obtained from the open-loop transfer characteristic measured with the acoustic feedback system in an open-loop state. It can be easily obtained as compared with the case where it is obtained from the closed loop transfer characteristic.

According to the third aspect of the present invention, the measurement of the open loop transfer characteristic, the calculation of the FIR filter coefficient based on the measurement result, and the actual sound field by the FIR filter means using the obtained FIR filter coefficient A series of control processes can be performed, and the inventions according to claims 1 and 2 can be easily realized.

[0015]

【Example】

(Embodiment 1) FIG. 1 shows an embodiment of the present invention. A microphone 12 is provided in an acoustic space 10 such as a hall.
And a PA (Public Address) speaker 14 are provided. The microphone 12 picks up a voice of a person or a musical instrument. The collected sound is amplified by the microphone amplifier 16, the filter characteristic is given by the filter unit 18 (for example, an impulse response corresponding to the filter characteristic is convolved with the input signal), and the amplified sound is amplified by the power amplifier 20 and the speaker is amplified. It is uttered from 14. The sound uttered from the speaker 14 is returned to the microphone amplifier 16 and forms an acoustic feedback system 22. The filter means 18 is set to have substantially the inverse characteristic of the minimum phase element of the open loop transfer function of the acoustic feedback system.

Here, a comparison is made of the closed-loop transfer characteristics between the case where the filter means 18 is inserted in the feedback loop and the case where the filter means 18 is not inserted. FIG. 2A shows the filter unit 1.
8 schematically shows the acoustic feedback system of FIG. 1 when 8 is not inserted. (B) is a block diagram of the same. Here, A: a constant part of the open-loop transfer function H (ω): a frequency function part of the open-loop transfer function (in this specification, only this is referred to as an open-loop transfer function.) | A · H ( ω) | <1. According to FIG. 2B, the closed loop transfer function G
(Ω) is expressed as follows.

G (ω) = A · H (ω) + (A · H)
(Ω)) ² + (A · H (ω)) ³ + ... = A · H (ω) / (1−A · H (ω))

FIG. 3 shows a simulation result of the impulse response characteristics and the frequency characteristics of the closed loop transfer function G (ω). Here, the case where the amplification factor A is changed every 3 dB is shown. According to this, the impulse response does not converge to zero at -11.5 dB or more, and a number of peak points occur in the frequency characteristics, and the levels of these peak points are not uniform. Due to the instability of these responses and the unevenness of the frequency peaks, the howling margin of this system is narrow, and the utterance level from the speaker 14 cannot be increased much.

On the other hand, FIG. 4A schematically shows the acoustic feedback system 22 shown in FIG. (B) is a block diagram of the same. Where H
_min (omega): open-loop transfer function of minimum phase component ^{_{H min -1 (ω): H}} min (ω) inverse characteristic of ^{_{(H min -1 (ω) =}} 1 / H min (ω)) | A · H (Ω) | <1. According to FIG. 4B, the closed-loop transfer function G
(Ω) is expressed as follows.

G (ω) = A · H (ω) · H _min ⁻¹ (ω) + (A · H (ω) · H _min ⁻¹ (ω) ) ² + (A · H (ω) · H _min ^-1 (ω) ) ³ +... = A · H (ω) · H _min ^-1 (ω) / (1−A · H (ω) · H _min ^-1 (ω) ) = A · H _ap (Ω) / (1−A · H _ap (ω)) where H _ap (ω) is an all-pass element (all-pass filter). In other words, a function whose amplitude is constant regardless of frequency and changes with frequency by the phase angle

FIG. 5 shows simulation results of the impulse response characteristics and frequency characteristics of the closed loop transfer function G (ω). Here, the case where the amplification factor A is changed every 3 dB is shown. According to this, in the impulse response, the response converges to zero even at -2.5 dB, and many peak points are generated, but the levels of the peak points are averaged. That is, the inverse characteristic H _min ^-1 (ω) of the minimum phase element
By incorporating the filter means 18 having the following, the unstable element caused by the open loop transfer function can be removed from the closed loop transfer function, whereby the time response can be shortened and the peak factor of the closed loop transfer function can be smoothed. . Therefore, the howling margin is increased, and the utterance level from the speaker 14 can be increased.

Next, based on the measurement of the open loop transfer characteristic H (ω), the inverse filter characteristic H
An example of a procedure for obtaining _min ⁻¹ (ω) will be described. Open loop transfer function H (ω) and its minimum phase element H
_The following relationship holds between _min (ω) and its all-pass element H _ap (ω).

H (ω) = H _min (ω) × H _ap (ω) The arrangement of the poles and zeros of H (ω), H _min (ω), and H _ap (ω) is shown on a complex plane. It looks like 6. That is, the minimum phase element H _min (ω) is the open loop transfer function H
The position of the zero point in the unstable region of (ω) on the real axis iδ is shifted from the − side to the + side with the imaginary axis ω as the symmetry axis. In addition, the all-pass element H _ap (ω) arranges a zero point at a position of a zero point in an unstable region of the open-loop transfer function H (ω), and positions the zero point on the real axis iδ with respect to the imaginary axis ω. The pole is arranged at a position shifted from the negative side to the positive side as an axis.

Further, H (ω), H _min (ω), H
_The following relationship holds for _ap (ω).

| H (ω) | = | H _min (ω) | × | _Hap (ω) || _Hap (ω) | = 1 (that is, _Hap (ω) has only phase characteristics) H _min (ω) | = | H (ω) | ∴ | H _min ⁻¹ (ω) | = 1 / | H (ω) | Also, the phase characteristic θ (ω) of the open loop transfer function H (ω) is It is expressed by the following equation.

Θ (ω) = θ _min (ω) + θ _ap (ω) + θ
_lin (ω) where θ _min (ω): phase characteristic of minimum phase element θ _ap (ω): phase characteristic of all-pass element θ _lin (ω): phase characteristic of linear element

FIG. 7 shows an example of a procedure for obtaining the inverse characteristic H _min ^-1 (ω) of the minimum phase element of the open loop transfer function H (ω) and setting the filter characteristic of the filter means 18 in FIG. That is, first, (1) the open-loop transfer characteristic H
(Ω) is measured, and based on this, (2) the phase characteristic θ _min (ω) of the minimum phase element is obtained, and (3) the inverse characteristic H _min ⁻¹ (ω) of the minimum phase element. Then, (4) an FIR filter coefficient corresponding to the obtained inverse characteristic H _min ^-1 (ω) is obtained, and (5) this FIR filter coefficient is set in the filter means 18. The specific contents of each step will be described.

(1) Measurement of open-loop transfer characteristic H (ω) The acoustic feedback system 22 is opened, an impulse signal is emitted from the speaker 14 and collected by the microphone 16, and the impulse response is Fourier-transformed. An open loop transfer characteristic H (ω) is obtained. This open loop transfer characteristic H
(Ω), the frequency amplitude characteristic | H (ω) | and the frequency phase characteristic θ (ω) are uniquely determined.

(2) Phase characteristic θ of minimum phase element
8, 9 the calculation procedure of the _min (omega) phase characteristic theta _min minimum phase element calculating the (omega). Each step will be described.

Phase characteristic θ (ω) The measured phase characteristic θ (ω) is represented by θ (ω) = θ _min (ω) + θ _ap (ω) + θ _lin (ω) as described above. Unwrapping process The measured phase characteristics θ (ω) are connected to convert them into a series. Removal of linear phase θ _lin (ω) (wavefront delay) The linear phase θ _lin (ω) is approximated as θ _lin (ω) = aω from the gradient (= a) of the point (1,2) obtained by the above process. Therefore, the linear phase θ _lin (ω) is subtracted from the phase characteristic θ (ω) and removed. Linear phase θ
_{When lin} (ω) is removed, a characteristic of θ _min (ω) + θ _ap (ω) is obtained.

Group delay characteristic calculation θ _min (ω) + θ _ap (ω) = ψ (ω), and ψ (ω)
Is differentiated to calculate a group delay characteristic −dψ (ω) / dω. Operation to eliminate zeros in unstable region By taking the absolute value | −dψ (ω) / dω | of the group delay characteristic, an element without any delay (that is, an element having no zero in the unstable region) produce. This corresponds to an operation of generating H _min (ω) from H (ω) in FIG. That is, by this operation, an element H _min (ω) having no delay causing howling is extracted.

Absolute value | −d 遅 延 (ω) of group delay characteristic obtained in integration step
/ Dω |

[0033]

(Equation 1) Ask for. Linear phase removal Linear phase θ _lin (ω) 'of the integral value obtained in the process
Since is approximated as the slope (= b) than θ _lin (ω) '= bω point (3,4), the linear phase theta _lin integrated values
(Ω) 'is subtracted. Subtracted value

[0034]

(Equation 2) Is obtained as the phase characteristic θ _lin (ω) of the minimum phase element.

(3) Calculation of Inverse Characteristics H _min ^-1 (ω) of Minimum Phase Element Obtaining H _min ^-1 (ω) means that the real part H _min ^-1 (ω) is obtained.
(Ω) Re and the imaginary part H _min ^-1 (ω) Im are determined. These are determined as follows.

H _min ⁻¹ (ω) Re = | H _min ⁻¹ (ω) | cos (−θ _min (ω)) = (1 / | H _min (ω) |) cos (−θ _min (ω) ) H _min ⁻¹ (ω) Im = | H _min ⁻¹ (ω) | sin (−θ _min (ω)) = (1 / | H (ω) |) sin (−θ _min (ω))

(4) Calculation of FIR Filter Coefficient Inverse characteristic H of the minimum phase characteristic obtained as described above.
_The impulse response is obtained by performing an inverse Fourier transform on _min ⁻¹ (ω) , and this is obtained as an FIR filter coefficient. (5) Filter coefficient setting The obtained FIR filter coefficient is set in the FIR filter as the filter means 18. Then, when the loop is closed, the closed-loop transfer characteristic of FIG. 5 is obtained.

(Embodiment 2) FIG. 10 shows an embodiment of the second aspect of the present invention. In an acoustic space 10 such as a hall, a microphone 12 and a PA speaker 14 are arranged. The microphone 12 picks up a voice of a person or a musical instrument. The picked-up sound is amplified by the microphone amplifier 16, and a filter characteristic is given by the filter unit 24 (for example, an impulse response corresponding to the filter characteristic is convolved with the input signal), and the power amplifier 20.
And is amplified from the speaker 14. The sound uttered from the speaker 14 is returned to the microphone amplifier 16 to form an acoustic feedback system 26. Filter means 24
Is set to a characteristic of approximately −H _min ⁻¹ (ω) / (1−A · H _ap (ω)).

FIG. 11A shows the acoustic feedback system 26 shown in FIG.
Is schematically represented. (B) is a block diagram of the same. According to FIG. 11B, the closed-loop transfer function G (ω) is expressed as follows.

G (ω) = A · H (ω) · [−H _min ⁻¹ (ω) / (1−A · H _ap (ω))] + ｛A · H (ω) · [−H _min ^{-1 (ω) / (1-} A · H ap (ω)) ^{]} 2 + {A · H (} ω) · ^{_{[-H min -1 (ω) / (}} 1-A · H ap (ω)) ]｝ ³ + ... = A · H (ω) · [−H _min ⁻¹ (ω) / (1-A · _Hap (ω))] / [1-A · H (ω) · (−H _min ⁻¹ (ω) ) / (1−A · H _ap (ω))] = − A · H _ap (ω)

FIG. 12 shows simulation results of the impulse response characteristics and frequency characteristics of the closed loop transfer function G (ω). Here, the case where the amplification factor A is changed every 3 dB is shown. According to this, a flat frequency characteristic in which the time response waveform is extremely short and the peak disappears is obtained. Therefore, the howling margin is maximized and the coloration is eliminated.

(Embodiment 3) FIG. 13 shows an embodiment of the third aspect of the present invention. In an acoustic space 10 such as a hall, a microphone 12 and a PA speaker 14 are arranged. The sound picked up by the microphone 12 is amplified by the microphone amplifier 16 and then A / D
The signal is converted into a digital signal by the converter 28. This digital signal is input to the input terminal 1 of the switch means 32 when measuring the closed loop transfer characteristic. Also, during actual use (performance, lecture, etc.) after the filter characteristics have been set,
It is input to an FIR filter 30 as a filter means. The FIR filter 30 has a filter characteristic H _min ⁻¹ (ω) or −H stored in the coefficient memory 34.
_The input signal is convolved with coefficient information of the impulse response corresponding to _min ⁻¹ (ω) / (1−A · H _ap (ω)) to give a filter characteristic to the input signal.

The measurement signal generating means 36 generates, for example, an impulse signal as a measurement signal when measuring the open-loop transfer characteristic. This measurement signal is input to the input terminal 3 of the switch means 32. The switch means 32 has two output terminals a and b, and guides a signal input to the input terminals 1, 2, 3 to one of the outputs a and b. Switching control means 38
Performs switching of the switch means 32 based on the instruction operation of the operator. That is, when the measurement of the open loop transfer characteristic is instructed, the input terminal 3 is connected to the output terminal a, and the input terminal 1 is connected to the output terminal b. When the operator performs a measurement start instruction operation in this state, a measurement signal is output from the output terminal a, converted into an analog signal by the D / A converter 40, amplified by the power amplifier 20, and emitted from the speaker 14. The microphone 12 picks up the sound, converts the sound into a digital signal by the A / D converter 28, and outputs the digital signal to the open loop transfer characteristic measuring means 4 through the switch means 32.
Enter 2

The open-loop transfer characteristic measuring means 42 obtains an open-loop transfer characteristic A · H (ω) by performing a Fourier transform on the collected impulse response h (t). The obtained open-loop transfer characteristic A · H (ω) is input to the calculating means 44.
Here, since the constant part A is known in advance as the gain of the electric system, the frequency amplitude characteristic | H (ω) | and the frequency phase characteristic θ (ω) can be uniquely understood from the transfer characteristic H (ω). The calculating means 44 includes the filter characteristic calculating means 4
In step 6, the filter characteristic H _min ^-1 (ω) or -H is obtained from the open loop transfer characteristic H (ω) by the above-described method or the like.
_min ⁻¹ (ω) / (1−A · H _ap (ω)) is calculated (which one is calculated depends on an arbitrary selection operation by the operator). The convolution coefficient calculation means 48 obtains a corresponding impulse response by performing an inverse Fourier transform on the calculated filter characteristic, and stores the coefficient value (delay time and gain) of the impulse response in the coefficient memory 34.

When the setting of the filter characteristics is completed as described above, the switch means 32 switches the connection. That is, the input terminal 2 is connected to the output terminal a, and the other input / output terminals are open. As a result, the acoustic feedback system 50 is configured, and enters a standby state. Therefore, if a performance, a lecture, or the like is performed using the microphone 12 in this state,
In the FIR filter 30, the FIR filter coefficient is convolved with the collected signal of the microphone 12, and
A filter characteristic is given to suppress howling (when the filter characteristic is set to H _min ^-1 (ω) ) or suppress howling and coloration (when the filter characteristic is set to -H _min ^-1 (ω) / (1- _{A.H ap} (.omega.))) Can be carried out in the state where it is set.

The whole or part of a series of operations from the start of measurement of the open-loop transfer characteristic to the setting of the filter characteristic in the coefficient memory 34 can be automatically programmed and sequentially programmed.

In each of the above embodiments, the frequency characteristic excluding the peak component has been described as being flattened as a whole. However, in addition to smoothing the peak factor or reducing the peak itself, an arbitrary frequency characteristic can be obtained. It can also be provided.

The contents described in the above embodiment are based on an exact solution filter obtained in accordance with a logical expression. Such a filter is used for changing the sound field such as movement of a person in the sound field and temperature change. Is very critical to For this reason, in practice, it is effective to use a filter from which fine peaks and dips are removed within a range where the characteristics of the filter are not changed, in order to perform a stable operation.

For example, it is assumed that the frequency amplitude characteristics of the exact solution filters H _min (ω) and H _min ^-1 (ω) obtained according to the theoretical formula are those shown in FIGS. 14 (a) and 14 (b). In this characteristic, fine peaks and dips occur. These fine peaks and dips can be removed, for example, as follows.

First, H _min (ω) is subjected to an inverse Fourier transform, and the time response characteristic h of the minimum phase element shown in FIG.
_{Find min} (t). When the obtained h _min (t) is multiplied by a coefficient that attenuates with time as a time window as shown in FIG. 15B, the time of the minimum phase element whose time response is shortened as shown in FIG. The response waveform h _min (t) ′ is obtained. The frequency amplitude characteristic H _min (ω) ′ and the inverse characteristic H _min ⁻¹ (ω) ′ of the time response waveform h _min (t) ′ are as shown in FIGS. 15D and 15E, respectively. 1
4 (a) and 4 (b), the finer peaks and dips are eliminated (however, the characteristics of the characteristics shown in FIGS. 14 (a) and 14 (b) remain). Therefore, by using H _min (ω) ′ and H _min ⁻¹ (ω) ′ obtained in this manner, a stable operation against a change in the sound field such as a movement of a person in the sound field or a change in temperature can be obtained. Can be.

More specifically, for example, in the configuration of FIG. 13, the filter characteristic calculating means 46 calculates the minimum phase element H _min (ω) from the input open-loop transfer characteristic H (ω).
Is obtained by inverse Fourier transform to obtain the time response h _min (t) ′ of the minimum phase element shown in FIG.
By multiplying the desired time window as shown in FIG.
H _min (t) ′ of (c) is obtained. And h
_min (t) ′ is Fourier transformed to obtain H _min (ω) ′,
From this, the filter characteristic H _min ⁻¹ (ω) ′ is obtained.

In the above-described embodiment, the present invention is used for the purpose of suppressing howling and coloration caused by an acoustic feedback system which is inevitably generated in an acoustic space. However, the reverberation time of a hall or the like is adjusted (extended). )
The present invention is similarly applied to a so-called electroacoustic reverberation support device in which a microphone and a speaker are arranged in an acoustic space to form an acoustic feedback system and other systems having various acoustic feedback systems. Can be prevented.

[0053]

As described above, according to the first aspect of the present invention, a filter having substantially the inverse characteristic of the minimum phase element of the open-loop transfer function is inserted into the loop, so that the open-loop transfer function is reduced. Can be removed from the closed loop. As a result, the time response waveform of the system can be shortened, the level irregularities at the peak points of the frequency characteristics can be reduced, the howling margin can be expanded, and howling can be effectively suppressed.

According to the second aspect of the present invention, a filter having a characteristic of approximately −H _min ⁻¹ (ω) / (1−A · H _ap (ω)) is inserted into the loop. The time response waveform of the system can be shortened, and the peak itself can be eliminated or reduced to a very low level. Therefore, the howling margin can be expanded further,
Coloration is also reduced.

The filter characteristics of the filter inserted into the loop in the first and second aspects of the present invention can be obtained from the open-loop transfer characteristic measured with the acoustic feedback system in an open-loop state. It can be easily obtained as compared with the case where it is obtained from the closed loop transfer characteristic.

According to the third aspect of the present invention, the measurement of the open loop transfer characteristic, the calculation of the FIR filter coefficient based on the measurement result, and the control of the sound field by the FIR filter means using the obtained FIR filter coefficient are performed. A series of processes can be performed, and the inventions according to claims 1 and 2 can be easily realized.

[Brief description of the drawings]

FIG. 1 is a system configuration diagram showing an embodiment of the invention described in claim 1;

FIG. 2 shows a filter means 1 in the sound field control device of FIG.
It is the figure which represented the structure which removed 8 typically, and the block diagram which shows the system.

3 is a diagram showing an impulse response waveform and a frequency characteristic at the time of a closed loop in the sound field control device of FIG. 2;

FIG. 4 is a diagram schematically showing the configuration of FIG. 1 and a block diagram showing its system.

5 is a diagram illustrating an impulse response waveform and a frequency characteristic in a closed loop in the sound field control device of FIG. 1;

FIG. 6: H of H (ω) = H _min (ω) × H _ap (ω)
FIG. 4 is a diagram showing the arrangement of poles and zeros of (ω), H _min (ω), and H _ap (ω) on a complex plane.

FIG. 7 is a flowchart illustrating an example of a procedure from measurement of an open-loop transfer characteristic to setting of a filter characteristic.

FIG. 8 is a flowchart illustrating an example (first half) of a calculation procedure of a phase characteristic θ _min (ω) of a minimum phase element.

FIG. 9 is a flowchart illustrating an example (second half) of a calculation procedure of the phase characteristic θ _min (ω) of the minimum phase element.

FIG. 10 is a system configuration diagram showing one embodiment of the invention described in claim 2;

FIG. 11 is a diagram schematically showing the configuration of FIG. 10 and a block diagram showing its system.

12 is a diagram illustrating an impulse response waveform and a frequency characteristic in a closed loop in the sound field control device in FIG.

FIG. 13 is a system configuration diagram showing one embodiment of the invention described in claim 3.

FIG. 14: Exact solution H obtained according to the theoretical formula
_min (ω) and frequency amplitude characteristics of H _min ⁻¹ (ω) .

FIG. 15 is a diagram showing an example of an operation for removing peaks and dips from the characteristics of FIG.

[Explanation of symbols]

 Reference Signs List 10 acoustic space 12 microphone 14 speaker 18, 24 filter means 22, 26, 50 acoustic feedback system 30 FIR filter means 42 open loop transfer characteristic measuring means 44 arithmetic means

(56) References JP-A-1-240099 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04R 3/04 101

Claims (3)

(57) [Claims] 1. A microphone and speaker unit disposed within the acoustic space, the acoustic feedback loop to form an acoustically feedback loop, open-loop transfer function of the feedback loop in its entirety
When expressed by the product of the bandpass element and its minimum phase element,
A sound field control apparatus comprising a feedback loop and filter means having substantially opposite characteristics of the minimum phase element. 2. An acoustic feedback system in which a microphone and a loudspeaker are arranged in an acoustic space to acoustically form a feedback loop, wherein an inverse of a minimum phase element of an open loop transfer function of the loop is provided in the feedback loop. Assuming that the characteristic is H _min ^-1 (ω), the all-pass element is H _ap (ω), and the constant part of the open-loop transfer function is A, −H _min ^-1 (ω) / (1−A · H _ap ( ω)) A sound field control device in which filter means having the following characteristics are arranged. 3. An acoustic feedback system in which a microphone and a loudspeaker are arranged in an acoustic space to acoustically form a feedback loop, wherein an open-loop transfer characteristic measuring means for measuring an open-loop transfer characteristic of the loop is provided. From the measured open-loop transfer characteristic, the inverse characteristic of the minimum phase element of the open-loop transfer function of the loop is expressed as H
_min ⁻¹ (ω), H _ap (ω) for the all-pass element, and A for the constant part of the open-loop transfer function, and approximately H _min ⁻¹ (ω) or −H _min ⁻¹ (ω) / (1− A · H _ap (ω)) calculating means for obtaining an FIR filter coefficient substantially corresponding to the characteristic of the signal, and arranged in the feedback loop to convolve the obtained FIR filter coefficient with the signal in the loop. A sound field control device comprising FIR filter means.