JP4938594B2 - Howling suppression device - Google Patents

Description

  The present invention relates to a howling suppression device.

In an acoustic device having a microphone or the like and a speaker or the like, an audio signal output from the speaker circulates and is input to the microphone to form a feedback loop. For this reason, howling may occur depending on the installation environment of the acoustic device, the set volume of the speaker, the positional relationship between the microphone and the speaker, and the like. In order to suppress this howling, a howling suppression device is generally used that detects a frequency band in which howling occurs and suppresses the level of an audio signal in that frequency band (see, for example, Patent Document 1 and Patent Document 2). ). FIG. 7 shows a general howling suppression apparatus 300. The audio signal input to the microphone 310 is amplified by the microphone amplifier 320 and is divided into a plurality of frequency bands by the dividing unit 400. The detection unit 410 detects howling of the audio signal by comparing the level of the audio signal for each frequency band with a threshold of a predetermined level. Based on the detection result of the detection unit 410, the control unit 420 sequentially sets filter coefficients for suppressing howling in the plurality of notch filters (NF) 430-1 to 430-n. Therefore, the notch filter 430-n outputs an audio signal in which howling is suppressed. Then, the audio signal in which howling is suppressed is amplified by the power amplifier 330 and output from the speaker 340.
Japanese Patent Laid-Open No. 7-143034 JP 2004-274122 A

  By the way, when the threshold level for detecting howling in the above-described howling suppression apparatus 300 is set high, the probability of erroneously detecting speech or music from a sound source as howling is generally low, but howling is not performed. Since the detection time until detection becomes long, the time when it feels uncomfortable in hearing becomes long. On the other hand, when the threshold level is set low, the howling detection time can be shortened, but the probability of erroneously detecting howling increases, contrary to the case of the high threshold described above. Therefore, in the howling suppression apparatus that compares the level of the audio signal and the threshold level, there is a problem that it is difficult to reduce the probability of erroneous detection of howling while shortening the howling detection time.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a howling suppression apparatus that can shorten the detection time of howling and reduce the probability of erroneous detection of howling.

In order to achieve the above object, a howling suppression apparatus of the present invention includes a detection unit that detects howling of an input audio signal by comparing a level of the input audio signal with a threshold of a predetermined level. In addition, a plurality of filters that sequentially apply and output the audio signal to the output and a detection result from the detection unit suppress the howling detected by the detection unit for the plurality of filters. A setting unit that sets a filter coefficient for performing a threshold value, and a threshold value changing unit that raises the threshold value of the predetermined level at a predetermined timing, and at the predetermined timing, at least once howling is performed by the detection unit. It is the timing after it is detected.

  It is possible to provide a howling suppression device that can shorten the detection time of howling and reduce the probability of erroneous detection of howling.

  From the description of this specification and the accompanying drawings, at least the following matters will become apparent.

  FIG. 1 is a diagram showing an embodiment of an acoustic device to which the present invention is applied. The acoustic device 1 shown in FIG. 1 is a device that amplifies an audio signal input from a microphone or the like (not shown), suppresses howling, and outputs it to a speaker or the like (not shown). The howling suppression device 20 is configured.

  The microphone amplifier 10 is a circuit that amplifies an analog audio signal from a microphone (not shown) to a level that can be processed by the howling suppression device 20, and a non-inverting amplifier circuit using an operational amplifier, for example, can be used.

  The power amplifier 11 is a circuit that amplifies the audio signal output from the howling suppression device 20 to a level at which a speaker or the like can be driven.

  The howling suppression device 20 is a device that suppresses howling of an audio signal output from the microphone amplifier 10 and outputs it to the power amplifier 11, and includes an AD converter (ADC) 30 and notch filters (NF) 40-1 to 40-n. , A DA converter (DAC) 50, coefficient memories 60-1 to 60-n, a dividing unit 70, a detecting unit 71, a coefficient calculating unit 72, a control unit 73, and a memory 74. As the howling suppression device 20 of the present embodiment, for example, a DSP (Digital Signal Processor) including an AD converter and a DA converter can be employed.

  The AD converter 30 converts the analog audio signal output from the microphone amplifier 10 into digital.

  The notch filters 40-1 to 40-n are n filters for suppressing howling, and the frequency characteristics are determined based on the filter coefficients set in the coefficient memories 60-1 to 60-n. . The notch filters 40-1 to 40-n in the present embodiment include a coefficient fixed filter that does not change the filter coefficient when the filter coefficient is set in the coefficient memory, and a coefficient variation filter that can change the set filter coefficient. Suppose that

  The DA converter 50 converts the digital audio signal output from the notch filter 40-n to analog and outputs the analog signal to the power amplifier 11.

  The coefficient memories 60-1 to 60-n are n memories in which filter coefficients for the notch filters 40-1 to 40-n are set, respectively, and addresses A1 to An are allocated thereto. The address values are increased in the order of addresses A1 to An, and the frequency characteristics of the notch filters 40-1 to 40-n are flat in the coefficient memories 60-1 to 60-n as an initial state. It is assumed that a proper filter coefficient is set.

  The dividing unit 70 divides the audio signal digitally converted by the AD converter 30 into a plurality of frequency bands by, for example, FFT (Fast Fourier Transform).

  The detection unit 71 compares the peak level of the audio signal for each frequency band divided by the dividing unit 70 with a predetermined level threshold, and the peak level of the audio signal exceeds the predetermined level threshold It is determined that howling has occurred, and howling is detected. In the present embodiment, the detection unit 71 sets a threshold value by referring to the threshold value storage unit 86 in the memory 74.

  The coefficient calculation unit 72 calculates the center frequency, the Q value, and the like of the notch filters 40-1 to 40-n in order to suppress howling detected by the detection unit 71.

 The control unit 73 includes a setting unit 80 and a threshold value changing unit 81. By referring to the detection result of the detection unit 71, the calculation result of the coefficient calculation unit 72, and various data stored in the memory 74, The howling suppression device 20 is controlled in an integrated manner.

  The setting unit 80 sets filter coefficients for suppressing howling in the coefficient memories 60-1 to 60-n based on the detection result of the detection unit 71, and writes various data in a writable area of the memory 74. The setting unit 80 of this embodiment is provided with pointers for addressing the coefficient memories 60-1 to 60-n, and the filter coefficient is set in the coefficient memory at the address specified by the pointer. . In addition, the designation of the addresses of the coefficient memories 60-1 to 60-n is performed by, for example, a decoder (not shown) that decodes a pointer value provided in the setting unit 80. In the present embodiment, in the initial state, the pointer value is 0 (zero), and when the detection unit 71 detects howling, the setting unit 80 increments the pointer value by one.

  The threshold value changing unit 81 sets the threshold value storage unit 86 of the memory 74 so that the threshold value of the detection unit 71 increases based on the value of the determination flag of the determination flag storage unit 87 provided in the memory 74. To change. In the present embodiment, the threshold before the howling suppression apparatus is activated and the level rises is set as the initial threshold.

  The memory 74 stores various data necessary for controlling the howling suppression apparatus 20 of the present embodiment, and has an area writable by the setting unit 80. FIG. 2 is a diagram illustrating a part of the storage unit included in the memory 74. The memory 74 of the present embodiment includes a ROM (Read Only Memory) and a RAM (Random Access Memory), a coefficient table 85 is provided in the ROM storage area, and a threshold storage unit 86 is provided in the RAM storage area. A determination flag storage unit 87 and a management flag storage unit 88 are provided.

  The coefficient table 85 is a table in which filter coefficients corresponding to the center frequency, Q value, and the like of the notch filters 40-1 to 40-n calculated by the coefficient calculation unit 72 are stored. The filter coefficients stored in the coefficient table 85 are read based on the calculation result calculated by the coefficient calculation unit 72 and set in the coefficient memories 60-1 to 60-n. It is assumed that filter coefficients are written in advance in the coefficient table 85 of the present embodiment.

  The threshold value storage unit 86 stores a threshold value as a threshold value control signal for controlling the threshold value in the detection unit 71. In the present embodiment, when the howling suppression device 20 is activated, the setting unit 80 refers to, for example, an external ROM (not shown) and sets an initial threshold value in the threshold value storage unit 86. The threshold value stored in the threshold value storage unit 86 is changed by the threshold value changing unit 81 at a predetermined timing. In the present embodiment, the threshold value control signal is the threshold value itself, but the threshold value control signal is not limited to the threshold value and may be any signal that can control the threshold value. For example, the threshold value storage unit 86 selects an initial threshold value, a threshold value higher than the initial threshold value, and any of these two threshold values as a threshold value control signal. A selection signal may be stored. In this case, the threshold value in the detection unit 71 can be changed by changing the selection signal stored in the threshold value storage unit 86.

  The determination flag storage unit 87 stores a value of a determination flag indicating whether or not the number of howling detections is equal to or greater than n, which is the number of notch filters. In this embodiment, the number of howling detections is the number of notch filters. When the number is less than the number, the value of the determination flag is “0”, and when the number of howling detections is equal to or greater than the number of notch filters, the value of the determination flag is “1”. Therefore, when the filter coefficient is set in all the coefficient memories of the notch filter, the value of the determination flag becomes “1”.

  The management flag storage unit 88 includes storage memories 90-1 to 90-n corresponding to the coefficient memories 60-1 to 60-n, respectively, and the notch filter 40 is included in each of the storage memories 90-1 to 90-n. A management flag is stored that manages whether −1 to 40-n is a coefficient fixed filter or a coefficient variation filter. In this embodiment, the storage memory with the management flag value “1” is a coefficient fixed filter, and the storage memory with the management flag value “0” is a coefficient variation filter. As an initial state, the value of the management flag can be stored as either “0” or “1”. Further, when the howling suppression device 20 is activated, the setting unit 80 refers to, for example, an external ROM (not shown) and sets the value of the management flag in the management flag storage unit 88.

  Here, the pointer updating operation in the setting unit 80 when the notch filters 40-1 and 40-2 are coefficient fixed filters and the notch filters 40-3 to 40-n are coefficient fluctuation filters will be described with reference to FIG. explain. First, when the number of howling detections is equal to or less than the number of notch filters, the pointer value is incremented when howling is detected, and coefficient memories 60-1 to 60-n based on the pointer value are designated. Therefore, regardless of the coefficient fixed filter and the coefficient variation filter, filter coefficients for suppressing howling are sequentially set in the coefficient memories 60-1 to 60-n. Next, a case where the number of howling detections exceeds the number of notch filters will be described. Since the notch filters 40-1 and 40-2 are coefficient fixed filters, the setting unit 80 refers to the value of the management flag in the management flag storage unit 88 and sets the coefficient memories 60-1 and 60-2 as pointers. Exclude from specification. Therefore, after the number of howling detections exceeds the number of notch filters, the coefficient memories 60-3 to 60-n of the coefficient variation filter are repeatedly specified based on the pointer values.

== Filter coefficient setting processing for suppressing howling ==
Next, a filter coefficient setting process for suppressing howling in the howling suppression apparatus 20 will be described with reference to flowcharts illustrating an example of the filter coefficient setting process illustrated in FIGS. 4 and 5. In the following description, the number of notch filters is eight, that is, n = 8, and two notch filters 40-1 and 40-2 are coefficient fixed filters and notch filters 40-3 to 40-8. One is a coefficient variation filter.

  When the howling suppression device 20 is activated, the setting unit 80 sets an initial threshold value in the threshold value storage unit 86, sets the value of the determination flag to “0”, and stores the values in the storage memories 90-1 and 90-2. The management flag value is set to “1”, and the management flag values in the storage memories 90-3 to 90-8 are set to “0” (S101). Then, the dividing unit 70 performs FFT processing on the audio signal input to the howling suppression apparatus 20 and divides the audio signal into a plurality of frequency bands (S102). The detection unit 71 compares the level of the audio signal for each frequency band divided by the division unit 70 with the level of the initial threshold value, and determines whether howling has been detected (S103). If howling is not detected (S103: NO), the audio signal input to the howling suppression device 20 is again subjected to FFT processing in the dividing unit 70 (S102). On the other hand, when howling is detected (S103: YES), the setting unit 80 increments the value of the pointer (S104). In the present embodiment, the setting unit 80 refers to the value of the pointer and selects a process based on the value of the pointer.

<First Operation of Howling Suppression Device 20>
In the first operation, after the pointer value is incremented (S104), the pointer value is 2 or less (S105: YES), that is, the number of howling detections is less than the number of coefficient fixed filters. explain. First, the coefficient calculation unit 72 calculates a center frequency, a Q value, and the like for suppressing howling (S106). When the value of the pointer is 1 (S107: 1), the setting unit 80 reads out the filter coefficient corresponding to the calculation result of the coefficient calculation unit 72 from the coefficient table 85, and stores it in the coefficient memory 60-1 for the notch filter 40-1. Set (S108). When the pointer value is 2 (S107: 2), the filter coefficient for suppressing howling is set in the coefficient memory 60-2, similarly to the case where the pointer value is 1 (S109). After the filter coefficients are set, the FFT process is executed again (S102).

<Second Operation of Howling Suppression Device 20>
In the second operation, the case where the pointer value is incremented (S104) and the pointer value is not 2 or less (S105: NO), that is, the number of howling detections exceeds the number of coefficient fixed filters will be described. . First, the coefficient calculation unit 72 calculates a center frequency, a Q value, and the like for suppressing howling (S110). Then, the setting unit 80 refers to the value of the determination flag stored in the determination flag storage unit 87 (S111). When the value of the determination flag is “1” (S111: 1), that is, when the filter coefficient is set in all the coefficient memories of the eight notch filters, the setting unit 80 performs processing based on the pointer value. Selected (S115). When the value of the determination flag is “0” (S111: 0), it is determined whether the value of the pointer is 8 (S112). If the pointer value is not 8 (S112: NO), a process based on the pointer value is selected (S115). When the value of the pointer is 8 (S112: YES), the setting unit 80 changes the value of the determination flag from “0” to “1” (S113). Further, the threshold value changing unit 81 refers to the value of the determination flag in the determination flag storage unit 87 and increases the threshold value in the threshold value storage unit 86. (S114). Then, a process based on the pointer value is selected (S115). In step S115, a process corresponding to the pointer value is selected.

  When the pointer value is 3 (S115: 3), first, the setting unit 80 initializes the coefficient memory 60-4, which is a coefficient memory having an address one greater than the coefficient memory 60-3 designated by the pointer value. (S116). In the present embodiment, the coefficient memory is initialized by setting the coefficient memory to the same state as the initial state. In the present embodiment, the initialization of the coefficient memory is a process for repeatedly setting the filter coefficient in the coefficient variation filter. Then, the setting unit 80 sets a filter coefficient for suppressing howling in the coefficient memory 60-3 for the notch filter 40-3 (S117). After the filter coefficient is set in the coefficient memory 60-3, the audio signal input to the howling suppression apparatus 20 is again subjected to the FFT process in the dividing unit 70 (S102). When the value of the pointer is 4 (S115: 4), the same processing as in the case where the value of the pointer is 3 is performed. Therefore, the coefficient memory 60-5 is initialized (S118), and the filter coefficient is set in the coefficient memory 60-4 (S119). Then, the FFT process is performed again (S102). In the present embodiment, even when the pointer value is 5 to 7 (not shown), the same processing as that when the pointer value is 3 or 4 is performed.

  When the value of the pointer is 8 (S115: 8), that is, when the value of the pointer is the same as the number of notch filters, the setting unit 80 refers to the management flag storage unit 88 in order to suppress further howling detected. Then, the coefficient memory 60-3 having the smallest address among the coefficient memories of the coefficient variation filter is initialized (S120). Then, the filter coefficient is set in the coefficient memory 60-8 for the notch filter 40-8 (S121). In order to designate the coefficient memory 60-3 initialized in step 120, the setting unit 80 changes the value of the pointer from 8 to 2 (S122), and returns to the FFT processing (S102). Thereafter, when howling is detected (S103), the pointer is incremented (S104). As a result, the value of the pointer becomes 3. Therefore, in the second operation described above, any one of the coefficient memories 60-3 to 60-8 is designated by the setting unit 80 based on the value of the pointer.

  FIG. 6 is a diagram for explaining howling detection in the detection unit 71. FIG. 6 is an example when the FFT processing (S102) is first performed after the howling suppression apparatus 20 is activated. As described above, the detection unit 71 compares the level of the audio signal for each frequency band with the level of the initial threshold value (S103). Here, in FIG. 6, since the level of the audio signal in the frequency bands F1, F2, F3, and F4 is higher than the initial threshold level, the audio signal in the frequency bands F1 to F4 is detected as howling. Therefore, filter coefficients for suppressing the detected howling are set in the coefficient memories 60-1 to 60-4 (S108, S109, S117, S119). Further, howling is detected (not shown), and when the pointer value is equal to eight as the number of notch filters, the threshold level increases as shown in FIG. 6 (S114), and the above processing is continued. The As described above, since the threshold level increases, for example, compared with the case where the threshold value after the threshold level is increased is set after the start-up, in the frequency bands F2 and F3. Since howling can be detected early in time, howling detection time can be shortened. Furthermore, since the threshold level is increased when howling is detected eight times, for example, the probability of erroneous detection can be reduced as compared with a case where the threshold value is not increased.

  In the howling suppression apparatus 20 of the present embodiment having the above-described configuration, when activated, the detection unit 71 is set with a low initial threshold value, and the threshold level increases at a predetermined timing. Has been. In general, howling that occurs is specific howling that occurs according to the installation environment of the acoustic device 1, and uncertain howling that occurs depending on the positional relationship between the microphone and the speaker of the acoustic device 1, the set volume of the speaker, and the like. It is roughly divided into Inherent howling has a higher probability of occurrence and tends to occur earlier in time than uncertain howling. Therefore, in the howling suppression apparatus 20 of the present embodiment, there is a high possibility that inherent howling that generally occurs earlier in time is detected with a low level threshold, and howling detection time can be shortened. It is. Further, since the threshold level is raised at a predetermined timing after the howling suppression device 20 is activated, it is possible to reduce the probability of erroneous detection of howling after the threshold value is raised.

  Moreover, the howling suppression apparatus 20 of this embodiment raises a threshold value at the timing when 8 times howling is detected. In this way, by raising the threshold value after detecting howling at least once, it is possible to prevent the threshold value from rising at a timing when howling has not yet occurred.

  Moreover, the howling suppression apparatus 20 of this embodiment raises the threshold value at the timing when 8 times howling, which is the number of notch filters, is detected. For the howling detected exceeding the number of notch filters, the filter coefficient for suppressing the detected howling for any one of the coefficient memories 60-3 to 60-8 designated by the pointer. Is set. Therefore, the howling suppression apparatus 20 of the present embodiment can suppress howling detected exceeding the number of notch filters.

  Moreover, the howling suppression apparatus 20 of this embodiment raises a threshold value at the timing when 8 times howling which is the number of notch filters is detected, and uses the notch filters 40-1 and 40-2 as coefficient fixed filters. Therefore, a filter coefficient for suppressing howling that occurs earlier in time is first set in the coefficient fixed filter. The howling suppression apparatus 20 of the present embodiment does not change the filter coefficient set in the coefficient fixed filter when suppressing howling detected exceeding the number of notch filters. As described above, howling that occurs earlier in time is more likely to be inherent howling, and the coefficient fixed filter is more likely to suppress inherent howling that has a higher probability of occurring. Therefore, it is possible to reduce the occurrence probability of howling.

  In addition, the said Example is for making an understanding of this invention easy, and is not for limiting and interpreting this invention. The present invention can be changed and improved without departing from the gist thereof, and the present invention includes equivalents thereof.

  For example, in the present embodiment, the notch filter has been described as including a coefficient variation filter and a coefficient fixed filter. However, the notch filter may include only a coefficient fixed filter. In this case, the timing for raising the threshold is assumed to be the number of times equal to the number of notch filters after howling is detected at least once. As a result, it is possible to prevent the threshold value from rising at a timing at which howling has not yet occurred, and furthermore, at least one notch filter can suppress howling detected by a high threshold value, thereby detecting howling. It is possible to shorten the time and reduce the probability of false detection of howling.

  In the present embodiment, for example, the threshold level is increased by detecting howling. However, the threshold value changing unit 81 refers to the output of a timer circuit (not shown) that counts for a certain period of time. By doing so, the threshold level may be increased after a predetermined time has elapsed.

It is a figure showing an embodiment of an acoustic device to which the present invention is applied. It is a figure which shows an example of the memory | storage part which a memory has. It is a figure which shows an example of the process which designates a coefficient memory. It is a flowchart which shows an example of the filter coefficient setting process for suppressing howling. It is a flowchart which shows an example of the filter coefficient setting process for suppressing howling. It is a figure for demonstrating howling detection in the detection part. It is a figure which shows an example of a general howling suppression apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Sound apparatus 10 Microphone amplifier 11 Power amplifier 20 Howling suppression apparatus 30 AD converter (ADC)
40-1 to 40-n Notch filter (NF)
50 DA converter (DAC)
60-1 to 60-n Coefficient memory 70 Dividing unit 71 Detection unit 72 Coefficient calculating unit 73 Control unit 74 Memory 80 Setting unit 81 Threshold changing unit 85 Coefficient table 86 Threshold storage unit 87 Judgment flag storage unit 88 Management flag Memory

Claims (4)

A detection unit for detecting howling of the input audio signal by comparing a level of the input audio signal with a threshold of a predetermined level;
A plurality of filters that sequentially perform output on the audio signal and output;
Based on the detection result from the detection unit, a setting unit for setting a filter coefficient for suppressing the howling detected by the detection unit for the plurality of filters;
A threshold value changing unit for increasing the threshold value of the predetermined level at a predetermined timing;
Equipped with a,
The predetermined timing is
It is the timing after howling is detected at least once by the detection unit,
A howling suppression device characterized by the above. The predetermined timing is
Within a period from howling is detected at least once by the detection unit to howling is detected the same number of times as the number of the plurality of filters ,
The howling suppression apparatus of Claim 1 characterized by these. The setting unit
A filter for suppressing the howling detected by the detection unit with respect to a filter in which a filter coefficient for suppressing howling is not set among the plurality of filters based on a detection result from the detection unit. When a coefficient is set and a filter coefficient for suppressing howling is set for all of the plurality of filters, the howling detected by the detection unit is suppressed for any one of the plurality of filters. Set the filter coefficient to
The predetermined timing is
It is a timing after howling is detected the same number of times as the number of the plurality of filters in the detection unit,
Howling suppression device according to claim 1, wherein the. The setting unit
When a filter coefficient for suppressing howling is set in all of the plurality of filters, for one of the plurality of filters other than a part of the filters in which the filter coefficient is set earlier in time, Setting a filter coefficient for suppressing the howling detected by the detection unit;
The howling suppression apparatus of Claim 3 characterized by these.

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