JP5515538B2 - Howling prevention device - Google Patents

Description

  The present invention relates to a howling prevention device for preventing howling.

  Conventionally, various techniques for preventing howling have been proposed in loudspeaking systems such as lectures and concerts. A general howling suppression method is to attenuate a frequency band causing howling with a filter when the occurrence of howling is detected. The frequency and gain to be attenuated are often manually set by a PA engineer.

  Japanese Patent Application Laid-Open No. 2004-228561 describes a method of detecting the presence or absence of howling from the frequency characteristics of an input signal and calculating filter characteristics for suppressing howling.

JP-A-6-327088

  However, the conventional howling suppression method such as the device of Patent Document 1 is merely a method for suppressing the frequency band in which howling occurs, and does not estimate the loop gain, and can prevent the howling from occurring. is not. Although it is conceivable to determine the filter characteristics in an environment where the howling is most likely to occur, there is a problem that the signal is suppressed even in an environment where no howling originally occurs and the sound quality is greatly deteriorated.

  On the other hand, it may be possible for the PA engineer to manually set the filter characteristics in response to environmental changes, but each time a skilled skill such as selecting the frequency and gain to be suppressed is necessary, which is troublesome. There is.

  Therefore, an object of the present invention is to provide a howling prevention device that accurately prevents howling according to the environment of an acoustic space.

  The howling prevention apparatus according to the present invention includes an input unit that inputs an audio signal, a noise generation unit that generates pseudo noise, a superimposition unit that superimposes pseudo noise on the audio signal, a correlation calculation unit, a loop gain estimation unit, and a loop gain estimation unit. A suppression unit that suppresses an audio signal with a suppression degree according to an estimated value that is an estimated gain value, a howling detection unit that detects presence / absence of howling, and a control unit that controls the suppression unit are provided. The correlation calculation unit obtains a correlation between the input audio signal and the pseudo noise generated by the noise generation unit. The loop gain estimation unit estimates a closed-loop gain from the correlation peak calculated by the correlation calculation unit. For example, the sum of correlation values of peak components is estimated as a loop gain.

  Here, the control unit strongly sets the characteristic of the degree of suppression according to the estimated value when the howling detection unit detects howling. For example, it is set to a value lower by 3 dB, for example. Thereafter, since the voice signal is suppressed according to the estimated value with the newly set characteristic, howling can be prevented accurately.

  The howling prevention device includes a distance measuring unit that measures the distance between the speaker and the microphone, and an operation unit that receives an instruction to initially set the characteristics. When the control unit receives an initial setting instruction from the operation unit, the control unit measures a limit suppression degree (limit gain) and performs a measurement process of inputting an estimated value and a distance at that time. The control unit performs measurement processing at a plurality of points (for example, three points). The plurality of points to be measured include at least one position having different angular directions. And a control part calculates a characteristic based on the limit gain, estimated value, and distance which were measured in the measurement process of each point. As a result, it is possible to set the characteristics in consideration of changes in the angular direction.

  Note that the control unit may change the characteristics set in the initial setting process according to the distance of the distance measuring means during normal use. For example, the limit gain is changed as the distance is shorter.

  Further, when the howling detection unit detects the occurrence of howling, the control unit stores the estimated value at that time. This estimated value is taken as the maximum estimated value. Then, the control unit instructs the suppression unit to suppress howling, and stores the suppression degree when howling is suppressed. When the howling detection unit detects howling occurrence frequency and the suppression unit has an equalizer, the frequency and gain to be suppressed are stored as the degree of suppression. The control unit performs the storage process as described above.

  In addition, the control unit reads the estimated value and the suppression degree stored in the storage process, and executes a suppression process for controlling the suppression degree of the suppression unit according to the estimated value calculated by the loop gain estimation unit. For example, when increasing the degree of equalizer application (amount of gain reduction) in proportion to the estimated value, the gain is minimized at the maximum estimated value, and the estimated value (suppressed) from the proportional coefficient to the maximum gain (gain = 1). This is the start estimate.) Then, the degree of application of the equalizer is set stronger as the estimated value increases from the suppression start estimated value.

  When the control unit detects howling at a frequency different from the already stored frequency, the control unit newly stores frequency characteristics for suppressing the frequency, and performs suppression in a plurality of frequency bands in the suppression process. Set.

  As described above, the howling prevention apparatus of the present invention stores the estimated loop gain when howling occurs and the degree of suppression thereof, and automatically sets the frequency and gain to be suppressed according to the estimated loop gain. Thus, it is possible to prevent the occurrence of howling according to the environmental change of the acoustic space without requiring the skill.

  According to the present invention, since the frequency and gain to be suppressed are automatically set according to the estimated loop gain, it is possible to accurately prevent howling according to the environmental change of the acoustic space without requiring skilled skills. it can.

It is the block diagram which showed the structure of the howling prevention apparatus. It is the block diagram which showed the structure and process content of the pseudo noise superimposition part. It is the block diagram which showed the structure and processing content of the calculating part. It is the figure which showed the time-axis characteristic of correlation. It is the figure which showed the time-axis characteristic of correlation. It is the figure which showed the time-axis characteristic of correlation. It is a figure explaining a variable equalizer. It is the figure which showed the change aspect of the threshold value. It is a figure which shows the method of initial setting. It is a figure which shows the method of initial setting. It is the figure which showed the example which updates the set gain characteristic. It is the figure which showed the setting aspect of a threshold value and an equalizer curve. It is a flowchart which shows the gain characteristic change operation at the time of normal use. It is the flowchart which showed the operation | movement of a memory | storage process.

  FIG. 1 is a block diagram showing the configuration of the howling prevention apparatus of the present invention. In the description of this embodiment, unless otherwise specified, all audio signals are digital signals, and the configurations of A / D conversion and D / A conversion are omitted.

  The howling prevention apparatus 1 includes a calculation unit 5 that inputs an audio signal collected by a microphone 11 (sound collection unit), a pseudo noise superimposing unit 7, an operation unit 8, and a howling detection unit 9. The pseudo noise superimposing unit 7 superimposes pseudo noise on the audio signal picked up by the microphone 11.

  The audio signal on which the pseudo noise is superimposed by the pseudo noise superimposing unit 7 is amplified by a subsequent amplification system (amplifier) and emitted from the speaker 3. The sound emitted from the speaker 3 returns to the microphone 11 to form a closed loop. As described above, the howling prevention device 1 is built in a loudspeaker system, and is built in, for example, a mixer or a microphone.

  The howling prevention apparatus 1 estimates the closed-loop gain in the arithmetic unit 5. The howling prevention apparatus 1 suppresses an audio signal with an equalizer characteristic corresponding to the estimated loop gain.

  As shown in FIG. 1, the howling prevention apparatus 1 includes an LPF 12, a variable equalizer 13, a superposition unit 14, an M-sequence generator 15, an N-times oversampling unit 16, an HPF 17, a pseudo noise volume 18, an HPF 19, and a correlation calculation unit 20. A timer 21, a loop gain estimation unit 22, a gain control unit 23, and a storage unit 24.

  The calculation unit 5 includes an M-sequence generator 15, an HPF 19, a correlation calculation unit 20, a timer 21, and a loop gain estimation unit 22. The pseudo noise superimposing unit 7 includes a variable equalizer 13, a superimposing unit 14, an M sequence generator 15, an N-times oversampling unit 16, an HPF 17, a pseudo noise volume 18, a gain control unit 23, and a storage unit 24.

  The audio signal picked up by the microphone 11 is input to the LPF 12 of the pseudo noise superimposing unit 7 and the HPF 19 of the calculating unit 5. The configuration and function of the pseudo noise superimposing unit 7 will be described with reference to FIG. The lower column of each component shows the waveform of the signal output by each component.

  An audio signal picked up by the microphone 11 is input to the LPF 12 of the pseudo noise superimposing unit 7. Note that the waveform of the signal shown in the lower column of the microphone 11 is an example, and signals having various waveforms are actually input to the LPF 12.

  The LPF 12 cuts the high frequency from the collected audio signal and outputs it to the variable equalizer 13 (refer to the lower waveform of the LPF 12 in the figure).

  The variable equalizer 13 corresponds to the suppression unit of the present invention, suppresses the output signal of the LPF 12 of the microphone 11 with the frequency characteristic set by the gain control unit 23, and outputs it to the superimposition unit 14. The degree of suppression of the variable equalizer 13 is set by the detection result of the howling detection unit 9. Details will be described later.

  The M-sequence generator 15 corresponds to the noise generation unit of the present invention, periodically generates a signal with high autocorrelation such as a PN code (M-sequence) as pseudo noise, and outputs the signal to the N-times oversampling unit 16 (Refer to the lower waveform of the M-sequence generator 15 in the figure, where the waveform in the lowermost column represents the time axis). In addition, you may use other random numbers, such as not only M series but Gold series.

  Note that the pseudo-noise output period is the time until the component of the reflected wave (indirect wave) decreases to a predetermined level or more (impulse in the acoustic transmission system) so that the loop gain estimation unit 22 described later can perform loop gain estimation processing. Longer than the response convergence time).

  The N-times oversampling unit 16 oversamples the pseudo noise. For example, 16 times oversampling is performed, the code period of each bit of the PN code is expanded, and the pseudo noise length is 16 times (refer to the lower waveform of the N-times oversampling unit 16 in FIG. Represents the time axis). The N-times oversampling unit 16 outputs the oversampled signal to the HPF 17.

  The HPF 17 cuts the low frequency range of the signal input from the N-times oversampling unit 16 (refer to the lower waveform of the HPF 17 in the figure, where the waveform in the lowermost column represents the time axis). The cutoff frequency is set to 10 kHz, for example.

  Note that LPF 12 and HPF 17 are not essential in the present invention. However, since the HPF 17 cuts the sound other than the high frequency range of the pseudo noise, even if the sound is emitted from the speaker 3, there is no sense of incongruity in hearing (noise is difficult to hear). Further, the LPF 12 prevents the high-frequency pseudo noise once input to the microphone from being output to the amplification system again, and the pseudo noise loop phenomenon can be suppressed. When the LPF 12 and the HPF 17 are omitted, the pseudo noise loop phenomenon may be suppressed by subtracting the pseudo noise component from the audio signal picked up by the microphone 11 and outputting it to the amplification system.

  Note that oversampling by the N-times oversampling unit 16 is not essential in the present invention. However, by performing oversampling, the temporal redundancy of pseudo noise increases, and the accuracy of correlation calculation can be improved. Actually, the presence or absence of oversampling may be set according to the required accuracy and the code length of the pseudo noise.

  The signal output from the HPF 17 is input to the pseudo noise volume 18. The pseudo noise volume 18 outputs the output signal of the HPF 17 to the superposition unit 14 with the gain set by the gain control unit 23. The level of the pseudo noise may be a weak level that does not cause a sense of incongruity, but a level that can detect the peak value of the pseudo noise correlation is secured.

  The superimposing unit 14 superimposes the signal (pseudo noise) output from the HPF 17 on the audio signal output from the variable equalizer 13 and outputs the signal to the amplification system.

  Next, the configuration and function of the calculation unit 5 will be described with reference to FIG. The lower column of each component shows the waveform of the signal output by each component. The M-sequence generator 15 outputs the same pseudo noise as that output to the N-times oversampling unit 16 to the correlation calculation unit 20 (refer to the lower waveform of the M-sequence generator 15 in FIG. Represents the axis). Further, after outputting the pseudo noise, the M series generator 15 transmits a signal (timing signal) indicating the output timing to the timer 21. When receiving the timing signal, the timer 21 starts time counting and transmits a timer signal indicating the count time to the loop gain estimating unit 22. Note that the timer 21 is not essential in the present invention.

  The microphone 11 picks up sound including pseudo noise. An audio signal picked up by the microphone 11 is input to the HPF 19 of the calculation unit 5. The HPF 19 cuts the low frequency from the audio signal picked up by the microphone 11 and outputs it to the correlation calculation unit 20 (refer to the lower waveform of the HPF 19 in the figure, where the waveform represents the frequency axis). The cut-off frequency is determined corresponding to the HPF 17 (for example, 10 kHz).

  The correlation calculation unit 20 obtains the correlation between the pseudo noise input from the M-sequence generator 15 and the output signal of the HPF 19. Since the M-sequence code has a very high autocorrelation, if the output signal of the HPF 19 includes the same M-sequence pseudo noise, the level of the correlation value is as shown in the waveform of FIG. Get higher. The correlation calculation unit 20 outputs the timing (reception timing) at which the high level correlation value is calculated and the correlation value at that time to the loop gain estimation unit 22.

  When the reception timing is input, the loop gain estimation unit 22 refers to the timer signal from the timer 21 and obtains a time difference from the timing at which the pseudo noise is output to the reception timing. This time difference corresponds to the delay time of the closed loop. Note that the output of the reception timing of the correlation calculation unit 20 is not essential unless the delay time of the closed loop is measured (when the timer 21 is not provided).

  The loop gain estimation unit 22 performs processing for estimating the loop gain. Although various modes can be considered as a method for estimating the loop gain, for example, it is performed in the following modes.

  First, the first estimation method will be described with reference to FIG. FIG. 4 is a diagram schematically showing the time axis characteristic of the correlation.

  When the loop gain estimation unit 22 first calculates a correlation value of a predetermined level or more from the timing when the pseudo noise is output, the loop gain estimation unit 22 regards the correlation value in the first calculated time zone as a direct wave and obtains a peak component of the direct wave. . That is, when the loop gain estimator 22 calculates a correlation value equal to or higher than a predetermined level, the loop gain estimation unit 22 then temporarily stores the correlation value in a predetermined time zone t1 in a memory (not shown), and the highest level in the predetermined time zone t1 A correlation value is extracted and set as a peak value a0. The predetermined level is set according to the level of stationary noise. The predetermined time period t1 for extracting the peak value is set according to the accuracy of correlation value calculation (code length of pseudo noise, etc.), the presence or absence of the HPF 19, and the cutoff frequency.

  When the loop gain estimation unit 22 first calculates a correlation value of a predetermined level or higher and then calculates the correlation value of the predetermined level or higher again after the predetermined time period t1 has elapsed, the loop gain estimation unit 22 converts the correlation value as a reflected wave. The peak component of the reflected wave is determined. Similarly to the above, when the correlation value of the predetermined level or more is calculated, the loop gain estimation unit 22 then temporarily stores the correlation value of the predetermined time zone t1 in the memory, extracts the correlation value of the highest level, and the peak value a1 And Similarly, the peak value (a1, a2,...) Of the reflected wave is extracted for a predetermined time length t2. The predetermined time length t2 referred to here corresponds to a pseudo-noise output period. When the reverberation time in the room is known to some extent, the time t2 may be set in advance or may be manually input by the user.

  Then, the loop gain estimation unit 22 obtains absolute values (| a1 |, | a2 |,...) Of the peak values of the extracted direct wave and reflected wave, and estimates the loop gain from the sum of the absolute values. Thus, the loop gain estimation unit 22 performs the process of estimating the loop gain from the feedback component of the direct wave and the reflected component of the reflected wave that affects howling, and thus can estimate the loop gain with high accuracy. The first estimation method assumes that it is often the peak component that affects howling, and performs loop gain estimation based on the sum of correlation values of the peak components of the direct wave and the reflected wave.

  Next, the second method will be described with reference to FIG. FIG. 5 is a diagram schematically showing the time axis characteristic of the correlation.

  The loop gain estimator 22 extracts all correlation values of a predetermined level or higher until a predetermined time length t2 elapses from the timing at which the pseudo noise is output, and calculates the sum of these absolute values (determines an integral value). The predetermined level in this case is also set according to the level of stationary noise. The predetermined time length t2 also corresponds to the pseudo-noise output period.

  As described above, the second method performs the loop gain estimation with high accuracy by summing all the components of the direct wave and the indirect wave.

  Next, the third method will be described with reference to FIG. FIG. 6A is a diagram showing the time-axis characteristic (absolute value) of the correlation, and FIG. 6B schematically shows the time-axis characteristic.

  The loop gain estimation unit 22 first extracts the peak value of the direct wave and obtains the absolute value | a0 | as shown in the first method. Then, the loop gain estimation unit 22 acquires the absolute value | b0 | of the correlation when the time t3 further elapses from the peak. The time t3 is obtained as the time (closed loop delay time) from the timing at which the pseudo noise is output to the timing at which the correlation peak is first calculated. (In this method, the timer 21 is indispensable.) The absolute value | b0 | is not limited to the value of the timing at which the time t3 has elapsed from the first peak, but after the time t3 has elapsed and in the vicinity thereof (for example, It may be a value when the absolute value of the correlation is the largest at around several tens of μsec. Then, the loop gain estimation unit 22 estimates a ratio (| b0 | / | a0 |) between the absolute value | a0 | and the absolute value | b0 | as a loop gain.

  In the third method, after first extracting the peak component of the direct wave, the waveform around the timing when the time t3 has passed is determined as a direct wave in which the pseudo noise output from the speaker 3 is looped again, and the loop The gain is estimated.

  As a modification of the third method, the peak component of the direct wave extracted first may be estimated as the loop gain. Since the direct wave component has a large influence on the occurrence of howling, the loop gain can be easily estimated.

  In addition, since the output period of the pseudo noise is set to be longer than the convergence time of the impulse response in the acoustic transmission system in each of the first method, the second method, and the third method, after outputting the pseudo noise, Next, until the pseudo noise is output, dummy noise may be output to eliminate the silent section. By always outputting a noise sound, the pseudo noise becomes inconspicuous and there is no sense of incongruity in hearing.

  The loop gain estimated by the loop gain estimation unit 22 as described above is output to the gain control unit 23. The gain control unit 23 sets the degree of suppression of the variable equalizer 13 according to the estimated loop gain (hereinafter referred to as an estimated value), and performs the suppression process. Hereinafter, an example of setting the frequency characteristic of the variable equalizer 13 will be described as an example of setting the degree of suppression. FIG. 7A is a block diagram showing a detailed configuration of the variable equalizer 13.

  As shown in FIG. 2A, the variable equalizer 13 includes a gain adjuster 51, an adder 52, an equalizer (EQ) 53, an adder 54, and a gain adjuster 55. The gain adjuster 51 outputs an input signal with a gain Ga set by the gain control unit 23. Adder 52 subtracts the output signal of gain adjuster 51 from the input signal and outputs the result to EQ 53. The EQ 53 is, for example, a notch filter, and the equalizer curve (the center frequency of the notch filter) is determined by the howling occurrence frequency detected by the howling detection unit 9.

The adder 54 adds the output signal of the gain adjuster 51 and the output signal of the EQ 53 and outputs the result to the gain adjuster 55. The gain adjuster 55 is set by the gain controller 23 and adjusts the gain of the entire frequency band. The gain adjuster 55 is fixed during normal use, and is mainly used for gain adjustment during initial setting described later. The gain Gout of the output signal of the adder 54 is as follows:
Gout = Ga + (1-Ga) × Geq = Geq + (1-Geq) × Ga
It is represented by The gain controller 23 sets the degree of EQ 53, that is, the frequency characteristic of the equalizer, by setting the gain Ga. Here, the gain controller 23 sets the gain Ga according to the estimated value input from the loop gain estimator 22. The characteristics of the gain Ga are stored in the storage unit 24, and the gain control unit 23 reads out and sets the characteristics of the gain Ga from the storage unit 24.

  For example, as shown in FIG. 8B, when the estimated value is larger than the predetermined threshold th1, the characteristics such that the gain Ga decreases in proportion to the increase in the estimated value, or as shown in FIG. As shown in FIG. 8B, when the estimated value exceeds the predetermined threshold th2 (th2> th1), the characteristic that the gain reduction degree of the gain Ga further increases or the estimated value is A characteristic that the gain Ga is fixed when the threshold value th2 is exceeded is stored in the storage unit 24 in advance. Note that the increase in the estimated value and the decrease in the gain Ga do not have to be in a proportional relationship. For example, a mode in which the degree of decrease in the gain Ga is gradually reduced according to the increase in the estimated value may be employed.

  For the gain characteristics (threshold values th1 and th2, and slope (proportional coefficient)), default values stored in the storage unit 24 may be set as they are, but initial settings are made according to the installation environment. May be.

  When performing the initial setting, the user operates the operation unit 8 to instruct to initialize each value. Then, the gain control unit 23 increases the loop gain by gradually increasing the gain of the gain adjuster 55 from the minimum value while keeping the gain Ga at the maximum (Ga = 1.0), thereby generating howling. Alternatively, the gain of the gain adjuster 55 is fixed, and the user brings the microphone 11 close to the speaker 3 to generate howling.

  When detecting howling, the howling detection unit 9 outputs information indicating howling occurs (howling occurrence information) to the gain control unit 23. When the howling occurrence information is input from the howling detection unit 9, the gain control unit 23 stores the estimated value at that time in the storage unit 24. This estimated value is set as a maximum estimated value and is set as a threshold value th2. Thereafter, the gain control unit 23 decreases the gain Ga, and when the howling detection unit 9 detects that the howling is suppressed, the value of the gain Ga at that time is stored as a limit gain in association with the maximum estimated value. Stored in the unit 24. Then, if the relationship between the estimated value and the gain Ga is a proportional relationship, the suppression start estimated value (threshold value th1) is an estimated value when the gain Ga becomes 1, so the estimated value and the gain Ga It is calculated by the correspondence. The proportionality coefficient may be an arbitrary value, but can also be obtained by moving the microphone 11 and measuring another limit gain at a position where the distance from the speaker is different. For example, when the user moves the microphone 11 and operates the operation unit 8 to instruct to perform the second measurement, the gain control unit 23 gradually increases the gain Ga and generates howling again. The gain control unit 23 sets the value of the gain Ga immediately before the howling occurs as the second limit gain. Alternatively, the microphone 11 is brought close to the speaker 3 to generate howling, and the gain control unit 23 decreases the gain Ga, and the value of the gain Ga when suppressing howling is set as a limit gain. Then, a proportionality coefficient is obtained from the two measured limit gains and estimated values. Note that the threshold value th1 is actually preferably a value slightly lower than the value obtained by calculation with a certain margin. That is, the threshold value obtained by calculation is set to th1 ′, and a certain coefficient α (0 <α ≦ 1) is used to set th1 = α × th1 ′. As described above, the gain characteristics shown in FIGS. 7B, 8A, and 8B are initially set.

  The initial setting method as described above is a setting method when it is assumed that the estimated value changes when the distance between the speaker and the microphone changes, and the limit gain changes in proportion to the change of the estimated value. That is, this is a method when it is assumed that the relative angle between the speaker and the microphone does not change, the distance between the speaker and the microphone is increased, the actual loop gain is decreased, and the limit gain is increased.

  On the other hand, as shown in FIG. 9A, when the relative angle between the speaker and the microphone actually changes even when the distance between the speaker and the microphone is constant (when the microphone moves in the angular direction). However, it is considered that the estimated value changes and the limit gain changes proportionally. That is, since the sound emitted from the speaker has directivity, it is considered that the loop gain decreases and the limit gain increases even when the microphone moves away from the sound output axis of the speaker. However, the directivity of the voice is sharp in the high range (for example, 10 kHz or more) and dull in the mid-low range. Therefore, when the microphone moves in the angular direction, the loop gain does not decrease in the middle and low range as the high range, and there is a difference in the change of the loop gain in the high and middle and low ranges. Therefore, when the estimated value is calculated using pseudo noise only in the high frequency range, it is considered that the change in the limit gain with respect to the change in the estimated value in the angular direction is smaller than the change in the distance direction. Therefore, a method for initially setting the gain characteristics can be considered as follows.

  In the initial setting method of the gain characteristic in this example, as shown in FIG. 9A, measurement processing is performed at three points with different microphone positions. In the example of FIG. 2A, for ease of explanation, two points (points A and B) on the sound output axis of the speaker, and two points (points A and C) having a constant distance are used. In the example shown in Fig. 4, the point C may be any position as long as the position is not on the sound output axis of the speaker.

  First, when the user issues an initial setting instruction using the operation unit 8 and instructs to start measurement of the first point (A point), the gain control unit 23 sets the gain Ga to the maximum (Ga = 1.0). In this state, the gain of the gain adjuster 55 is gradually increased from the minimum value to increase the loop gain, thereby generating howling. Alternatively, the gain Ga is fixed, and the user brings the microphone 11 close to the speaker 3 to generate howling.

When the howling occurrence information is input from the howling detection unit 9, the gain control unit 23 decreases the gain Ga. When the howling detection unit 9 detects that the howling is suppressed, the gain control unit 23 sets the gain Ga at that time to the limit gain G. _A is stored in the storage unit 24 as _A. Further, the estimated value X _{A at} that time and the distance r _A between the speaker 3 and the microphone 11 are obtained and stored in the storage unit 24. The distance between the speaker 3 and the microphone 11 is obtained from the closed loop delay time and the sound speed using the timer 21. Before determining the distance, the speaker 3 and the microphone 11 are contacted in advance (distance zero) to determine the delay time, and the delay time at the distance zero is set as a delay time other than the delay due to the acoustic space (in-device delay time), The difference between the measured delay time and the in-device delay time may be used as the closed loop delay time.

In the initial setting in this example, the above measurement process is also performed at the remaining points B and C. That is, after the measurement at point A, the user uses the operation unit 8 to instruct the start of measurement at the second point (referred to as point B). Similarly to the above, the gain control unit 23 obtains the limit gain G B at the point _B , the estimated value X _{B at} that time, and the distance r _B between the speaker 3 and the microphone 11 and stores them in the storage unit 24. Thereafter, the user uses the operation unit 8 to instruct the start of measurement of the third point (C point), and the gain control unit 23 determines the limit gain G _{C at} the C point, the estimated value X _{C at} that time, the speaker 3. And the distance r _C between the microphone 11 and the microphone 11 (the value of r _A may be used as it is) is stored in the storage unit 24.

  The relationship between the limit gain, the estimated value, and the distance at each position measured as described above is as shown in the graphs of FIGS. 9B and 9C.

  Since the points A and B are on the sound output axis of the speaker 3, the change in the estimated value due to the change in distance has the same characteristics in the high range and the middle and low range, and the relationship between the estimated value and the limit gain Is considered to have the steepest slope (this slope is a). On the other hand, the point A and the point C are the same distance from the speaker 3 and only moved in the angular direction, and therefore the loop gain in the mid-low range does not change as the high range estimated value changes. The limit gain is considered to have the most gradual slope (this slope is b).

  Therefore, howling occurs below the gain values on the straight line (Ga = aX + a0) connecting the points A and B and the straight line connecting the points A and C (Ga = bX + b0) shown in FIG. It does not occur, and howling occurs when the gain value on each straight line is exceeded. Therefore, as shown in FIG. 4D, if the minimum limit gain value is set as the gain characteristic in each estimated value, the gain characteristic corresponding to both the change in the distance direction and the change in the angle direction is set. be able to. The upper limit gain Gmax may be a maximum value (Ga = 1) or may be manually set by the user using the operation unit 8.

  However, the value of the intercept b0 of the straight line connecting the points A and C is changed in accordance with the estimated value and distance measured during normal use, as shown in FIG. That is, the intercept b0 is obtained from the relationship between the distance and the estimated value (X = (XB−XA / rB−rA) r + X0) and the relationship b0 = Ga−bX shown in FIG. It can be expressed as a function of the distance r. Therefore, the intercept b0 is changed by the estimated value X and the distance r measured during normal use. As a result, the gain characteristic is set so that the intercept b0 becomes smaller and the limit gain becomes smaller as the distance becomes shorter.

  In the above setting method, an example in which there is one speaker is shown. However, when there are a plurality of speakers, the same measurement is performed for each speaker. As a result of the measurement, the gain control unit 23 sets the gain characteristic obtained by the speaker having the largest estimated value, or sets the gain characteristic obtained by the speaker having the closest distance.

As shown in FIG. 10A, when the points other than the two points (C point) on the sound output axis of the speaker are set at positions different from the points A and B, as shown in FIG. 10B. Then, C ′ point obtained by plotting the distance value measured at point C on the straight line connecting points A and B in the relationship between the distance and the estimated value is obtained, and the estimated value X _{C ′} at that time is obtained. Then, as shown in FIG. 10C, the estimated value X _{C ′} is substituted on the straight line connecting the points A and B in the relationship between the estimated value distance and the gain, and the limit gain G _{C ′} is obtained. As a result, two points (point C and point C ′) having the same distance and moved in the angular direction can be determined, and gain characteristics can be set.

  The gain control unit 23 changes the gain Ga according to the estimated value (and distance) input from the loop gain estimation unit 22 during normal use according to the characteristics of the gain Ga set as described above.

  When the gain Ga is changed, the frequency characteristic of the variable equalizer 13 shows a characteristic as shown in FIG. As shown in FIG. 6C, when the gain Ga is maximum (= 1.0), Gout = Ga, and EQ 53 does not contribute at all, and gain 1 (flat characteristics) at all frequencies. When the gain Ga is set to the minimum (for example, 0), Gout = Geq, and the frequency characteristic of the EQ 53 becomes the frequency characteristic of the variable equalizer 13 as it is. When the gain Ga is changed between 0 and 1, the frequency characteristic of the EQ 53 changes to a flat characteristic. That is, the gain control unit 23 changes the gain Ga according to the estimated value, thereby changing the equalizer characteristic of the variable equalizer 13.

  As shown in FIGS. 11A and 11B, when howling occurs again in the gain Ga characteristic that has been set once, the entire characteristic is set to a value that is lower by a predetermined value (for example, 3 dB). Alternatively, the gain Ga may be decreased until the howling is suppressed, and the set gain characteristic may be updated, or the gain of the gain adjuster 55 may be decreased so as to decrease the gain uniformly in all frequency bands. May be.

  Next, an equalizer curve (filter coefficient) setting method for EQ53 will be described. The characteristic of EQ 53 is also set by the detection result of the howling detection unit 9. Here, an example in which the EQ 53 functions as a notch filter that lowers the gain of a predetermined frequency will be described. As described above, when the howling detection unit 9 detects that howling has occurred, the howling detection unit 9 detects the frequency at which howling has occurred, and outputs the howling occurrence information including frequency information. The gain control unit 23 inputs howling occurrence information from the howling detection unit 9, and sets the center frequency F1 of the EQ 53 according to the frequency at which howling occurs. When howling occurs at a plurality of frequencies, a plurality of center frequencies are set (see FIG. 12C). In order to suppress a plurality of frequencies, a plurality of variable equalizers 13 are provided, and the respective gains Ga and center frequencies are set. The set center frequency of EQ 53 is stored in the storage unit 24. The bandwidth (Q value) is arbitrary. The gain control unit 23 reads the center frequency of the EQ 53 determined as described above from the storage unit 24, decreases the gain Ga in accordance with the increase of the estimated value, and controls the degree of application of the equalizer.

  The detection method of the howling detection unit 9 may be any method, for example, as follows. That is, the howling detection unit 9 converts (FFT) the signal input from the microphone 11 into a signal in the frequency domain, and holds the signal after FFT for a plurality of frames. Then, when the signal of each frequency component is at a predetermined level or higher and continues for a predetermined time or longer, it is determined that howling has occurred at that frequency. Note that the howling detection unit 9 detects a frequency component that is equal to or higher than a predetermined level and continues for a predetermined time or more in order to distinguish a regular sound of a musical instrument or a voice (such as a violin sound) from howling. The presence / absence of a harmonic component is determined, and howling is determined only when there is no harmonic component.

  A summary of the threshold value and equalizer curve setting (storage process) and the suppression process for controlling the gain according to the input of the estimated value will be described with reference to FIG.

  As shown in FIG. 12A, when no howling has occurred, EQ53 has a gain of 1 (flat characteristic) at all frequencies. Here, when howling occurrence information is input, the center frequency F1 of EQ53 is set as the howling occurrence frequency, as shown in FIG. In this case, the equalizer curve indicated by the one-dot broken line in FIG. The estimated value at this time is set as the maximum estimated value (when the gain characteristic of FIG. 8A is set, it becomes the threshold value th2). The gain control unit 23 stores the maximum estimated value and the center frequency F1 in the storage unit 24.

  Then, the gain control unit 23 reads a predetermined gain characteristic (for example, the characteristic shown in FIG. 8A or FIG. 9D) stored in the storage unit 24. Alternatively, the gain control unit 23 may decrease the gain Ga until howling is suppressed, and store the gain value when howling is suppressed in the storage unit 24. As a result, the frequency characteristic indicated by the solid line in FIG. 12B is set as the overall frequency characteristic of the variable equalizer 13. Thereafter, the gain controller 23 changes the gain of the gain adjuster 51 according to the estimated value input from the loop gain estimator 22 and controls the degree of application of the equalizer.

  Even if such suppression processing is performed, the occurrence of howling may be detected again. In this case, the gain control unit 23 performs the following process. First, when howling occurrence information indicating that howling has occurred at a frequency different from the frequency detected in the past is input, as shown in FIG. The frequency F2 is set. At this time, the center frequency F1 and gain value of EQ53 already stored in the storage unit 24 are fixed. In this case, as a frequency characteristic of the entire equalizer, a characteristic indicated by a one-dot broken line in FIG. Then, the gain control unit 23 reduces the gain of the variable equalizer in which the center frequency F2 is set until the gain characteristic similar to the above or howling is suppressed, and stores the gain value when the howling is suppressed. 24.

  Note that the estimated value when a new howling occurs may be different from the above-described maximum estimated value or may be the same. Also, the suppression start estimated value (threshold value th1) may be common or different in each stage of the variable equalizer 13. If they are different, the maximum estimated value and the suppression start estimated value are stored in the storage unit 24.

  Thereafter, the gain control unit 23 controls the degree of application of the equalizer in a plurality of bands in accordance with the estimated value input from the loop gain estimation unit 22.

  Further, as shown in FIG. 4D, when howling occurs at the same frequency (for example, frequency F1) as the already set frequency, the gain at the frequency F1 is further reduced. That is, as shown in FIG. 11A or FIG. 11B, the gain Ga is set to a value lower by a predetermined value (for example, 3 dB). Alternatively, the gain Ga is changed to a lower value until howling is suppressed.

  As shown in FIG. 11A or 11B, when the gain Ga is changed to a low value, the threshold th1 that is the estimated suppression start value is also changed to a lower value. Further, in FIG. 11B, the threshold value th2 may be left as it is, and the entire gain Ga may be lowered (characteristic of the broken line), or the gain Ga at the threshold value th2 before the change may be maintained. The threshold value th2 may be reduced.

  If howling occurs at an estimated value greater than the threshold th2, the gain adjuster 55 is set to suppress the gain in the entire frequency band, or the estimated value is the threshold as shown in FIG. When the characteristic that the gain Ga is fixed when it exceeds th2 is set, the threshold value th2 is changed to a large value.

  Next, FIG. 13 is a flowchart showing a gain characteristic changing operation during normal use. The gain control unit 23 starts this operation when the howling occurrence information is input from the howling detection unit. First, the gain control unit detects a frequency included in howling occurrence information (s51).

  When the howling occurrence is detected at a frequency different from the frequency detected in the past or when the howling occurrence is first detected (s52 → Yes), the gain control unit 23 sets the center frequency of the EQ53 (s53). Then, the gain control unit 23 sets the characteristics of the gain Ga (s54). For example, the gain characteristics as shown in FIGS. 8A and 9D are set. The characteristics of the set gain Ga are stored in the storage unit 24. The gain characteristic at this time may be determined by a storage process shown in FIG.

  On the other hand, in the processing of s52, if the frequency is the same as the frequency detected in the past, the processing of s53 and s54 is passed and the gain characteristic is changed (s55). For example, the gain Ga is changed to a characteristic that reduces the gain Ga as a whole by 3 dB (see FIG. 11A or FIG. 11B). Thereafter, the gain control unit 23 reads the characteristics of the gain Ga stored in the storage unit 24 in the storage process from the storage unit 24, and the gain adjuster 51 of the variable equalizer 13 according to the estimated value input from the loop gain estimation unit 22. Adjust the gain.

  Next, FIG. 14 is a flowchart showing the operation of the storage process showing another aspect of the gain Ga characteristic setting method. The gain control unit 23 starts this operation when the howling occurrence information is input from the howling detection unit. First, the gain controller 23 stores the currently input estimated value as the maximum estimated value (s11). If the maximum estimated value is already stored in the storage unit 24, this process is ignored. However, when the estimated value currently input is larger than the maximum estimated value that is already stored, the estimated value that is currently input is updated. Thereafter, the gain control unit 23 detects the frequency included in the howling occurrence information (s12).

  Here, when the howling occurrence is detected at a frequency different from the frequency detected in the past or when the howling occurrence is first detected (s13 → Yes), the gain control unit 23 sets the center frequency of the EQ53 (s14). . If it is the same frequency as the frequency detected in the past, the process of s14 passes.

  Then, the gain control unit 23 decreases the gain of the gain adjuster 51 of the variable equalizer 13 (s15). The gain reduction amount in one step may be any value, for example, -3 dB. Thereafter, the gain control unit 23 determines whether howling has been suppressed (howling occurrence information is no longer input from the howling detection unit 9) (s16). If howling is not suppressed, the gain of the gain adjuster 51 is lowered again (s16 → s15). When the howling is suppressed, the gain control unit 23 stores the gain value at that time in the storage unit 24 (s17). Then, a suppression start estimated value is calculated from the maximum estimated value stored in s11 and the gain value stored in s17, and stored in the storage unit 24 (s18). If the gain value and the threshold value th1 are already stored in the storage unit 24, these values are updated.

  Thereafter, the gain control unit 23 reads the threshold value th1, the maximum estimated value, and the gain value stored in the storage unit 24 in the storage process from the storage unit 24, and according to the estimated value input from the loop gain estimation unit 22 The gain of the gain adjuster 51 of the variable equalizer 13 is adjusted. That is, the gain control unit performs a suppression process for controlling the degree of application of the equalizer according to the estimated value at that time. When howling occurrence information is input in the suppression process, the storage process is performed again.

  As described above, the howling prevention apparatus can prevent the howling from occurring by estimating the closed loop gain and suppressing the audio signal with the equalizer characteristic corresponding to the estimated loop gain. In addition, the howling prevention device 1 does not require skilled skills because the equalizer characteristics are automatically set based on the estimated loop gain, and can accurately prevent the occurrence of howling according to the environmental change of the acoustic space. it can.

  In FIG. 1, the gain control unit 23 instructs to suppress the gain of the pseudo noise volume 18. However, a gain equal to or higher than a predetermined value is held so that the first peak of the pseudo noise correlation can always be detected. For this predetermined value, a value measured in advance in a laboratory or the like may be used, or a test is performed before actual use in the installation environment to obtain a limit gain that can calculate a correlation peak, and a certain margin is obtained. You may set the value which saw.

  Note that a plurality of pseudo noise patterns generated by the M-sequence generator 15 may be prepared, and these patterns may be switched. For example, by switching the pseudo noise pattern for each microphone (for each input channel), even when multiple microphones are used at the same time, the correlation can be calculated with high accuracy without interfering with each other's pseudo noise. Can do. Since the loop gain of the closed loop can be estimated individually for each microphone, howling can be suitably prevented even when a plurality of microphones are used simultaneously.

  In particular, when a Gold sequence is used as the pseudo-noise, it is possible to generate many types of code sequences by switching the tap position of the code generation circuit (shift register), so that it is compatible with a large-scale PA system. be able to.

  In the above-described embodiment, the variable equalizer 13 includes an equalizer, and the equalizer characteristic is set according to the loop gain estimated by the gain control unit 23. However, the configuration includes only a gain adjustment and is suppressed. It may be an aspect of controlling the gain in the entire frequency band as the degree.

1-mixer 3-speaker 5-calculation unit 7-pseudo-noise superimposing unit 8-operation unit 9-howling detection unit 11-microphone

Claims (6)

An input unit for inputting an audio signal picked up by the sound pickup unit ;
A noise generator that generates pseudo noise;
A superimposing unit that superimposes the pseudo noise on the audio signal input by the input unit and outputs it to an amplification system;
A correlation calculator for obtaining a correlation between the audio signal input by the input unit and the pseudo noise generated by the noise generation unit;
A loop gain estimator for estimating a closed-loop gain from the correlation value calculated by the correlation calculator;
A suppression unit that suppresses the audio signal input by the input unit with a suppression degree according to an estimated value that is a gain value estimated by the loop gain estimation unit;
A howling detection unit for detecting the presence or absence of howling;
When the howling detection unit detects howling, a control unit that strongly sets a degree of suppression according to the estimated value;
Equipped with a,
The howling detection unit detects a frequency at which howling occurs,
The suppression unit includes an equalizer that suppresses the audio signal;
The degree of suppression is information indicating the frequency characteristics of the equalizer,
The said control part is the howling prevention apparatus which sets to the said suppression part and controls the frequency characteristic of the equalizer of the said suppression part so that the frequency which the said howling detection part detected may be suppressed . An input unit for inputting an audio signal picked up by the sound pickup unit ;
A noise generator that generates pseudo noise;
A superimposing unit that superimposes the pseudo noise on the audio signal input by the input unit and outputs it to an amplification system;
A correlation calculator for obtaining a correlation between the audio signal input by the input unit and the pseudo noise generated by the noise generation unit;
A loop gain estimator for estimating a gain of a closed loop based on a peak of a correlation value calculated by the correlation calculator;
A suppression unit that suppresses the audio signal input by the input unit with a suppression degree according to an estimated value that is a gain value estimated by the loop gain estimation unit;
A howling detection unit for detecting the presence or absence of howling;
When the howling detection unit detects howling, a control unit that strongly sets a degree of suppression according to the estimated value;
Howling prevention device equipped with. 3. The howling prevention apparatus according to claim 1, wherein when the occurrence of howling is detected at a frequency different from a howling that has occurred in the past, the control unit sets the suppression unit so as to further suppress the frequency. Distance measuring means for measuring the distance between the speaker and the microphone;
An operation unit that receives an instruction to initially set the characteristics of the degree of suppression with respect to the estimated value,
When the control unit receives an instruction for the initial setting through the operation unit,
While measuring the degree of suppression of the limit that can suppress howling by changing the degree of suppression of the suppression unit, the estimated value at that time, and the measurement process to input the distance is executed at a plurality of points,
The howling prevention apparatus in any one of Claims 1-3 which perform the initial setting process which calculates the said characteristic based on the suppression degree of the said limit measured in several measurement processes, an estimated value, and distance. The howling prevention apparatus according to claim 4 , wherein the control unit changes the characteristic set in the initial setting process according to a distance of the distance measuring unit. When the howling detection unit detects howling, the control unit stores an estimated value at that time, and sets the suppression unit to suppress the audio signal and suppress howling, and the howling detection unit When it is detected that howling is suppressed, a storage process for storing the degree of suppression of the suppression unit when howling is suppressed is executed.
It reads the estimated value and the suppression level stored in the storage process, in accordance with the estimated value being input from the loop gain estimation unit, to any one of claims 1 to 5 for controlling the suppression level to be set to the suppressing unit The howling prevention apparatus of description.

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