JP5381256B2 - 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.

  For example, Patent Document 1 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 only a method for suppressing the frequency band that causes howling, and does not estimate the loop gain, and can prevent howling from occurring. It is not a thing.

  Accordingly, an object of the present invention is to provide a howling prevention device that estimates a loop gain and accurately prevents howling.

  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 calculator, and a loop gain estimation unit. The correlation calculator obtains a correlation between the input audio signal and the pseudo noise generated by the noise generation unit. The loop gain estimator estimates a closed loop gain from the correlation peak calculated by the correlation calculator. For example, the sum of correlation values of peak components is estimated as a loop gain. Here, the howling prevention apparatus includes a first suppression unit that suppresses the audio signal based on the estimated loop gain (for example, when the estimated loop gain approaches a predetermined threshold value). Furthermore, the howling prevention apparatus of the present invention includes a howling detection unit that detects howling, and includes a second suppression unit that suppresses an audio signal when howling is detected. The suppression method may be a mode in which the gain is simply lowered, or a mode in which the frequency at which howling is generated by a notch filter or the like may be suppressed. When the howling detection unit does not detect howling, the gain may be restored.

  According to the present invention, it is possible to suppress the occurrence of howling by any chance while estimating the loop gain and preventing howling from occurring.

It is a figure which shows the structure of a howling prevention apparatus. It is a figure explaining the superimposition process of a pseudo noise. It is a figure explaining the structure and function of a correlation calculating part. It is the figure which represented the time-axis characteristic of correlation typically. It is the figure which represented the time-axis characteristic of correlation typically. It is the figure which showed the time-axis characteristic of correlation.

  FIG. 1A 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 performs a process of superimposing pseudo noise on the audio signal collected by the microphone 11 (sound collecting unit) and outputting the signal to the speaker 3 via a subsequent amplification system (not shown). The sound emitted from the speaker 3 returns to the microphone 11 to form a closed loop. The howling prevention apparatus 1 estimates the closed-loop gain by obtaining the correlation between the superimposed pseudo noise and the voice signal that has been fed back. The howling prevention device can prevent howling in advance by suppressing the gain of the audio signal or giving a warning when the estimated loop gain approaches a predetermined threshold value. Furthermore, the howling prevention apparatus 1 of the present embodiment includes a howling detection unit, and when the occurrence of howling is detected, further suppresses the gain of the audio signal and suppresses howling.

  As shown in FIG. 1A, the howling prevention apparatus 1 includes an LPF 12, a front volume 13, a rear volume 14, a superposition unit 15, an M-sequence generator 16, an N-times oversampling unit 17, an HPF 18, and a pseudo noise volume 19. A correlation calculation unit 20, a gain control unit 21, a howling detection unit 22, and a post-stage gain control unit 23.

  The correlation calculation unit 20 includes an HPF 51, a correlation calculator 52, a timer 53, and a loop gain estimation unit 54, as shown in FIG.

  The audio signal picked up by the microphone 11 is input to the LPF 12 and the HPF 51 of the correlation calculation unit 20. With reference to FIG. 2, the pseudo noise superimposing process will be described. 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. 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 front volume 13 (see the lower waveform of the LPF 12 in the figure).

  The front volume 13 outputs the input signal to the rear volume 14 with the gain set by the gain controller 21. The rear-stage volume 14 outputs a signal input with the gain set by the rear-stage gain control unit 23 to the superposition unit 15.

  The M-sequence generator 16 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 it to the N-times oversampling unit 17 (Refer to the lower waveform of the M series generator 16 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) drops to a predetermined level or more (impulse in the acoustic transmission system) so that the loop gain estimation unit 54 described later can perform loop gain estimation processing. Longer than the response convergence time).

  The N-times oversampling unit 17 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 17 in FIG. Represents the time axis). The N-times oversampling unit 17 outputs the oversampled signal to the HPF 18.

  The HPF 18 cuts the low frequency range of the signal input from the N-times oversampling unit 17 (refer to the lower waveform of the HPF 18 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 18 are not essential in the present invention. However, since the sound other than the high frequency of the pseudo noise is cut by the HPF 18, 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 18 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 then outputting it to the amplification system.

  Note that oversampling by the N-times oversampling unit 17 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 18 is input to the pseudo noise volume 19. The pseudo noise volume 19 outputs the output signal of the HPF 18 to the superimposing unit 15 with the gain set by the gain control unit 21. 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 15 superimposes the signal (pseudo noise) output from the HPF 18 on the audio signal output from the rear volume 14 and outputs the signal to the amplification system.

  Next, the configuration and function of the correlation calculation unit 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 16 outputs the same pseudo noise output to the N-times oversampling unit 17 to the correlation calculator 52 (refer to the lower waveform of the M-sequence generator 16 in FIG. Represents the axis). Further, after outputting the pseudo noise, a signal indicating the output timing (timing signal) is transmitted to the timer 53. Upon receiving the timing signal, the timer 53 starts time counting and transmits a timer signal indicating the count time to the loop gain estimation unit 54. Note that the timer 53 is not essential in the present invention.

  The microphone 11 picks up sound including pseudo noise. The audio signal picked up by the microphone 11 is input to the HPF 51 of the correlation calculation unit 20. The HPF 51 cuts the low frequency from the audio signal picked up by the microphone 11 and outputs it to the correlation calculator 52 (refer to the lower waveform of the HPF 51 in the figure, where the waveform represents the frequency axis). The cut-off frequency is determined corresponding to the HPF 18 (for example, 10 kHz).

  The correlation calculator 52 obtains the correlation between the pseudo noise input from the M-sequence generator 16 and the output signal of the HPF 51. Since the M-sequence code has a very high autocorrelation, if the output signal of the HPF 51 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 calculator 52 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 54.

  When the reception timing is input, the loop gain estimation unit 54 refers to the timer signal from the timer 53 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. If the delay time of the closed loop is not measured (there is no timer 53), the output of the reception timing of the correlation calculator 52 is not essential.

  The loop gain estimation unit 54 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 54 first calculates a correlation value of a predetermined level or more from the timing at which the pseudo noise is output, the loop gain estimation unit 54 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 54 calculates a correlation value equal to or higher than a predetermined level, the loop gain estimation unit 54 temporarily stores the correlation value for a predetermined time period t1 in a memory (not shown) and has the highest level in the predetermined time period 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 (such as the code length of pseudo noise), the presence or absence of the HPF 51, and the cutoff frequency.

  When the loop gain estimation unit 54 first calculates a correlation value of a predetermined level or higher and then calculates the correlation value of a predetermined level or higher again after the predetermined time period t1 has elapsed, the loop gain estimation unit 54 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 higher is calculated, the loop gain estimation unit 54 then temporarily stores the correlation value in the predetermined time period t1 in the memory, extracts the highest level correlation value, 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 54 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. As described above, the loop gain estimation unit 54 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 54 extracts all correlation values above a predetermined level until a predetermined time length t2 has elapsed 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.

First, as shown in the first method, the loop gain estimation unit 54 extracts the peak value of the direct wave and obtains the absolute value | a0 |. Then, the loop gain estimation unit 54 obtains 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 53 is essential.)
The absolute value | b0 | is not limited to the value at the timing when the time t3 has passed since the first peak, but when the absolute value of the correlation is the largest after the time t3 has passed and in the vicinity thereof (for example, around several tens of μsec). It is good also as the value of. 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 54 as described above is output to the gain control unit 21. When the estimated loop gain approaches the predetermined threshold th, the gain control unit 21 determines that the possibility of howling is high and instructs to suppress the gain of the front volume 13. Further, the gain control unit 21 may perform a warning (lights the LED, displays a warning on the display, etc.) when the loop gain approaches the threshold value. When warning is given, the user manually adjusts the gain and equalizer.

  Note that only one of the gain suppression processing and the warning processing may be performed, and further warning may be performed while suppressing the gain of the audio signal. Alternatively, a warning may be given first, and then gain suppression processing may be performed.

  Here, the predetermined threshold th differs depending on the loop gain estimation method. The predetermined threshold th may be any value, but a certain margin is set in advance. For example, before the actual use, the user performs an operation to increase or decrease the gain. At this time, when the howling detection unit 22 detects howling, the gain control unit 54 sets the inputted estimated value of the loop gain as the maximum threshold value thmax and sets a certain coefficient α (0 <α ≦ 1). Use th = α × thmax.

  The gain control unit 21 instructs to suppress the gain of the pseudo noise volume 19. However, a gain greater than a predetermined value is held so that the first peak of the pseudo noise correlation can 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.

  A plurality of pseudo noise patterns generated by the M-sequence generator 16 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.

  As described above, the correlation calculation unit 20 estimates the closed loop gain, and performs processing for suppressing the gain of the audio signal and processing for warning when the estimated loop gain approaches a predetermined threshold value. It is possible to prevent howling from occurring.

  Next, gain control of the howling detection unit 22 and the rear volume 14 will be described. The howling detection method of the howling detection unit 22 may be any method. For example, the following method is used.

  The howling detection unit 22 analyzes the frequency of the audio signal and detects the presence or absence of howling. That is, the howling detection unit 22 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 22 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 between a regular sound of a musical instrument or a voice (such as a violin sound) and howling. The presence / absence of a harmonic component is determined, and howling is determined only when there is no harmonic component.

  Information indicating howling occurs and its frequency (referred to as howling occurrence information) is input to the subsequent gain control unit 23. When the howling occurrence information is input from the howling detection unit 22, the rear stage gain control unit 23 sets the gain of the rear stage volume 14 to be suppressed. When the howling detection unit 22 stops detecting howling, the gain is restored (set to 0 dB). When the howling occurrence information is input, the latter-stage gain control unit 23 suppresses the gain in a stepwise manner (for example, by −3 dB every second) and suppresses the gain until no howling is detected. When returning the gain to the original value, the gain may be returned with the same amount of change as at the time of suppression (for example, 3 dB per second), or may be returned more slowly than at the time of suppression (for example, 1 dB per second). . When howling occurrence information is input again while the gain is being returned, the gain is suppressed until no howling occurrence is detected again.

  The howling prevention apparatus of the present embodiment can estimate the loop gain by the correlation calculation unit 20 and prevent the howling from occurring. However, when the disturbance noise increases, the correlation calculation unit 20 There is a possibility that the peak cannot be calculated. In addition, since the correlation of the audio signal is obtained, there is a case where the peak of the correlation cannot be calculated due to a Doppler shift occurring when the microphone moves or the like and the pseudo noise frequency changing. In view of this, when the howling detection unit 22 detects howling, the gain is suppressed by the rear volume 14 so that even if howling occurs, this is immediately suppressed.

  In the present embodiment, an example is shown in which the gain of the audio signal is suppressed by the rear volume 14, but howling may be suppressed by a notch filter or the like that suppresses the frequency detected by the howling detection unit 22. . In this case, the post-stage volume 14 is replaced with a notch filter, and the post-stage gain control unit 23 sets the frequency and gain of the notch filter.

  The howling prevention device may be built in a mixer or a microphone, or may be built in an adapter. In any case, it suffices if it is provided before the amplification system such as an amplifier device.

1-Howling prevention device 3-Speaker 11-Microphone 15-Superimposition unit 20-Correlation calculation unit 22-Howling detection unit 23-Rear stage gain control unit

Claims (2)

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 howling detection unit for detecting the presence or absence of howling;
A first suppression unit that suppresses an audio signal input by the input unit based on a closed-loop gain estimated by the loop gain estimation unit ;
A second suppression unit that suppresses the audio signal input by the input unit when the howling detection unit detects occurrence of howling;
Howling prevention device equipped with. The second suppression unit suppresses the audio signal by suppressing a gain,
The howling prevention apparatus according to claim 1, wherein the gain is restored when the howling detection unit no longer detects howling.

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