JP3227068B2 - Howling prevention 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 device for preventing howling in an acoustic device.

[0002]

2. Description of the Related Art In an audio device or the like that amplifies an audio signal input from a microphone and outputs the amplified signal from a speaker, howling may occur when the microphone is moved closer to the speaker or the output level from the speaker is increased. This is because a feedback loop is formed in a system including the sound field and the acoustic device, and the sound output from the speaker is repeatedly input to the microphone and amplified to be in an oscillation state.

[0003] Many proposals have been made in the past to prevent this howling. For example, there is a proposal to provide a high-cut filter in an acoustic circuit to prevent howling. Since the high-frequency component is cut, the sound quality is impaired, so that the use thereof is limited. In addition, there is a drawback that howling due to a signal component having a cut-off frequency or less is not effective. Although there is a method using a low cut filter instead of a high cut filter, there is a similar drawback.

There is also a method in which an AD converter, a delay circuit, and a DA converter are provided in an amplifier system, the delay time of the delay circuit is temporally changed according to the output signal of the oscillator, and the digital processing is performed. . However, this method has a drawback that the sound quality is deteriorated because the signal is delayed to prevent howling. Further, a method has been proposed in which a frequency band in which howling occurs is detected first, and a filter characteristic is set based on the detection result to lower the signal level in the howling band. This method first detects the howling band, so if the howling band changes,
For example, it is not so effective when the position of the microphone moves, when a person enters or exits the venue and the acoustic conditions change, or when the temperature constantly changes.

There is also a method for preventing howling by modulating the audio signal in the howling band to such an extent that the modulation is not perceptible to the sense of hearing. This method is effective even when the howling band changes, but originally, since the energy is dispersed by shifting the frequency to prevent howling, a great prevention effect cannot be expected by this method alone.

[0006]

SUMMARY OF THE INVENTION In view of the above circumstances, an object of the present invention is to provide a howling prevention apparatus for effectively preventing howling without deteriorating sound quality.

[0007]

According to the present invention, there is provided a howling prevention apparatus for analyzing a frequency component of an input signal, a notch filter capable of changing a signal level, and a variable notch filter for reducing a signal level. A modulation means for modulating the frequency component, and a frequency band having a high probability of howling is determined based on the frequency component analysis result obtained by the analyzer, and a band of a notch filter is set so as to reduce a signal level of the band having a high probability. Control means for controlling the modulation means to be in an on state and an off state, respectively, in accordance with whether or not the above processing is effective.

Here, the analyzer detects the peak value of the signal level of each of a plurality of bands of the input signal, and the control means controls howling based on the peak value obtained by the analyzer. Judgment of the likelihood of the signal, setting the band of the notch filter so as to lower the signal level of the band with a high probability, and setting the modulating means to the ON state and the OFF state, respectively, depending on whether the processing of the analyzer is valid or not. It is preferable that the temperature be controlled to

[0009]

According to the howling prevention apparatus of the present invention, the analyzer first analyzes the frequency component of the input signal. Based on the analysis result, the control means determines a frequency band in which the probability of howling is high, and sets the operating characteristic of the notch filter so as to reduce the signal level in a band in which the probability of howling is high. At the same time, the control means controls the modulation means to be in an off state while the processing of the analyzer is valid, and controls the modulation means to be in an on state when the processing of the analyzer becomes unnecessary. The reason for such control is that if the input signal modulated by the modulation means is analyzed, the howling point cannot be detected accurately.

The notch filter whose band has been set by the control means lowers the signal level in a band where the probability of howling is high, thereby preventing howling. Further, the modulating means, which is turned on by the control means, modulates the audio signal to reduce howling. By using the notch filter and the modulating means together in this way, the amount of reduction in the signal level of the notch filter can be reduced and the degree of modulation in the modulating means can be reduced, so that howling can be effectively performed without deteriorating sound quality. Can be prevented.

[0011]

Embodiments of the present invention will be described below. FIG. 1 is a functional block diagram of a first embodiment of a howling prevention device according to the present invention. In the first embodiment, when detecting a point with a high probability of howling, this howling prevention device is provided in the acoustic space, and the volume level of the signal path from input to output is gradually increased. Detection shall be performed in the state. By gradually increasing the volume level, howling occurs sequentially from a howling point having a high probability, and a signal having a high probability is detected by detecting a signal of the howling.

Further, the control for increasing the volume level little by little may be performed automatically or manually. In the following description, it is assumed that the processing is automatically performed. In FIG. 1, the control means 50 includes an operation element (various switches, encoders) and a memory (R
AM, ROM, etc.), a display function (LCD, LED, their driver, etc.), an I / F for communication with an external device, etc. First, a filter setting mode is operated by operating a filter setting switch therein. Set to. In such a state, an analog signal input to the input terminal 10 from a microphone or the like is first converted into a digital signal by the A / D converter 20 and supplied to the analyzer 30 and the notch filter 40.

The analyzer 30 divides an input signal into a plurality of bands, detects a signal level of each band, detects a peak value of a signal level of each band, holds the peak value, and holds the peak value. The value is supplied to the control means 50.
The control means 50 determines the probability of howling based on the peak value from the analyzer 30, sets the operating condition of the filter so as to lower the signal level in a band having a high probability of howling, and sends the information to the filter 40. Supply. In this case, when the difference between the peak value of the band having a high probability of howling and the peak value of the surrounding band is large, the control means 50 greatly reduces the signal level and narrows the width of Q, and when the difference is small, the signal level becomes small. Is controlled to be slightly lowered and the width of Q is widened.

The notch filter 40 includes ten state variable filters (State) having the same Q and gain.
Variable Filter)
The operating conditions of the ten filters are sequentially set based on the information received from the control unit 50, and the band of the whole notch filter 40 is set. The notch filter 40 whose band has been set in this way filters the signal supplied from the A / D converter to lower the signal level of the band having a high probability of howling by the set level, and modulates the signal. Pass to 60.

When the setting of the notch filter 40 is completed as described above, the filter setting switch is operated to release the filter setting mode, and the mode is set to a mode for operating as a howling prevention device. In addition, while the processing of the analyzer 30 is valid, that is, while the filter setting switch is set to the filter setting mode by operating the filter setting switch and the analyzer 30 detects the analysis result used to determine the probability of howling, The control means 50 controls the modulation means 60 to an off state, and when the analysis result of the analyzer 30 becomes unnecessary, that is, when the filter setting mode is released by operating the filter setting switch, the modulation means 60 is turned on. Control.

As described above, the control means 50 controls the modulation means 6
The reason why the control is switched from 0 to the off state or the on state is that if the input signal is modulated by the modulating means 60, the howling point cannot be accurately analyzed. When the control unit 50 controls the modulation unit 60 to be turned on, the modulation unit 60 modulates the signal from the notch filter 40 and then supplies the signal to the D / A converter 70.

In this embodiment, the modulation means 60 employs the FM modulation method. First, the signal is modulated by moving the readout point by about 0.1 Hz by the LFO.
The pitch of the musical tone is slightly distorted to reduce howling. However, mixing with the original signal is not performed. When the modulating means 60 is controlled to be in the off state by the control means 50, the modulating means 60 performs D
The signal is supplied to the / A converter 70.

The D / A converter 70 converts a digital signal into an analog signal and outputs it to an output terminal 80. FIG.
FIG. 3 is a functional block diagram of an analyzer according to the first embodiment of the present invention. As shown in FIG. 2, the digital signal from the analyzer input terminal 31
There are 90 circuits for performing signal processing for each of the 0 bands. Each circuit includes a band-pass filter (BPF) 32 that passes only a signal of a predetermined band, a level detector 33 that detects a signal level of each divided band, and a peak value of a signal level of each band. A peak value detector 34 for detecting the peak value, a peak holder 35 for holding the peak value,
An analyzer output terminal 36 is provided. The peak value held in the peak holder 35 is supplied from the analyzer output terminal 36 to the control means 50.

The BPF 32 and the level detector 33 all have the same Q and gain, are set so that the center frequencies of the respective bands are arranged on average over the audible range, and the input signals have the same bandwidth. It is divided into 90 bands. In the present embodiment, a second-order IIR filter is used as the BPF 32, and the bandwidth is set to 1/9 octave band. The peak holder 35 is cleared at the time of initial setting. Then, for each sample, the absolute value of the output of the BPF 32 is compared with the value of the peak holder 35, and the larger value is stored in the peak holder 35. Each time the value of the peak holder 35 is read by the control means 50, the peak holder 3
5 is cleared.

FIG. 3 is a functional block diagram of a second embodiment in which the howling prevention apparatus of the present invention is applied to a stereo sound apparatus. As shown in FIG. 3, the anti-howling device has input terminals 10 for the L and R channels, respectively.
L, 10R, A / D converters 20L, 20R, notch filters 40L, 40R, modulating means 60L, 60R, D
/ A converters 70L, 70R, output terminals 80L, 80
R, analyzer 30 common to both channels, control means 5
It consists of 0. Then, the L ·
If there is a possibility of howling in any of the R channels, the control means 50 simultaneously sets the bands of the notch filters 40L and 40R of both channels so as to lower the signal level of that band.

The control means 50 includes modulation means 60L,
The ON / OFF control of the 60R is performed simultaneously for both the left and right channels. In FIG. 3, the analyzer 30 is common to both the L and R channels, but a dedicated analyzer may be provided for each channel. The reason why the analyzer is used in common for both channels in the present embodiment is to improve the utilization efficiency by sharing the analyzer by time-sharing processing as described below when the probability of howling is determined by the analyzer.
In other words, to determine the level of probability of howling,
Since it is sufficient to measure the signal level of several waves in each band, the analyzer can be used in a time division manner. For example, in the case of the stereo mode, the measurement of the L channel and the measurement of the R channel may be performed alternately at every minimum time (about 0.1 second) necessary for the determination.

In each of the above embodiments, after A / D conversion,
Although an example has been described in which howling prevention processing is performed and D / A conversion is performed last and then output, digital signals may be transmitted and received as they are, or signal processing may be performed as analog signals from the beginning. In each of the above embodiments, the number of bands of the analyzer 30 is set to 90 and the bandwidth is set to 1/9 octave band. However, these values are necessary and sufficient to prevent howling under the conditions of use of the acoustic device. I just need.

In each of the above embodiments, ten state variable filters are used for the notch filter 40. However, the type of the filter may be other than this, and the number of filters may prevent howling in the acoustic device. The number may be any number necessary and sufficient to perform. Further, in each of the above embodiments, the example in which the FM modulation method is used as the modulation means has been described, but the AM modulation method may be used. For example, if the original sound X is F
Hilbert transform is performed by an IR filter to obtain a waveform Y shifted by 90 degrees, and XS + YC is obtained using sine waves S and C shifted by 90 degrees in phase from each other by several Hz. May be obtained.

Further, in the analyzer of each of the above embodiments, the entire band is divided into 90 bands. However, the analyzer can be made compact by dividing the entire band into several groups and processing. it can. That is, for example, 90 bands are 30 low bands, 30 mid bands,
Divided into three groups of 30 high frequency bands, and the frequency ranges are 20 Hz to 200 Hz, 200 Hz to 2 kHz, and 2
kHz to 20 kHz, and peak values are detected for each of the three groups by time division processing. In this way, the detection time of the middle band can be reduced to about 1/10 of the low band, and the detection time of the high band can be reduced to about 1/100 of the low band. However, if this method is adopted, the parameters for setting the notch filter must be transferred from the analyzer to the notch filter.However, the size of the analyzer is greatly reduced in exchange for a slight increase in the parameter transfer time. can do.

Further, by separately providing a high-resolution analyzer, the size of the analyzer can be further reduced. That is, first, the probability of howling is determined by a 1/9 octave analyzer, and then only the band having a high probability of howling can be measured with high accuracy by a high-resolution analyzer having a BPF with a higher Q. .

In addition, as described above, while ensuring the same howling prevention effect by making the bandwidth narrower,
It is possible to reduce discomfort in hearing and improve the sound quality of the acoustic device. Note that this high-resolution analyzer may be provided as separate hardware, or the same hardware may be used also as a high-resolution analyzer, and time-division processing may be performed to transfer coefficients for high-resolution measurement to function as a high-resolution analyzer. Is also good. According to this method, the scale of the analyzer can be reduced as compared with the case where the entire band is measured by an analyzer having a high Q BPF.

In the above description, the modulating means 60, 60
Although it is described that the L and 60R always perform the modulation, it is also possible to provide a modulation on / off switch in the control means 50 and turn on / off the modulation as needed. In that case, the anti-howling device of the present invention operates when modulation is on.

[0028]

As described above, according to the howling prevention apparatus of the present invention, the analyzer analyzes the frequency component of the input signal, and the control means determines a band having a high probability of howling based on the analysis result. By reducing the signal level in the band where the probability of howling is high to prevent howling, and further modulating the audio signal by the modulation means to prevent howling from occurring, the howling prevention effect due to the notch filter and the howling prevention effect due to modulation. Work synergistically to effectively prevent howling without deteriorating sound quality.

[Brief description of the drawings]

FIG. 1 is a functional block diagram of a first embodiment of a howling prevention device according to the present invention.

FIG. 2 is a functional block diagram of an analyzer according to the first embodiment of the present invention.

FIG. 3 is a functional block diagram of a second embodiment of a howling prevention device according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Input terminal 20 A / D converter 30 Analyzer 31 Analyzer input terminal 31 32 BPF 33 Level detector 34 Peak value detector 35 Peak holder 36 Analyzer output terminal 40 Notch filter 50 Control means 60 Modulation means 70 D / A converter 80 Output terminal

Continuation of the front page (56) References JP-A-5-137192 (JP, A) JP-A-54-5363 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04R 3 / 02

Claims (1)

(57) [Claims] An analyzer for analyzing a frequency component of an input signal, a notch filter capable of changing a band for lowering a signal level, a modulating means for FM-modulating an input signal, and a frequency component analysis result obtained by the analyzer A frequency band in which the probability of howling is high is determined based on the frequency band of the notch filter so as to lower the signal level of the band in which the probability is high, and the modulation means is turned off when the processing of the analyzer is valid. And a control means for controlling the modulation means to be in an on state when the processing of the analyzer is invalid.