JPH08223684A - Howling detection device - Google Patents

Abstract

PURPOSE: To quickly detect the howling in a simple constitution of a howling detection device by extracting the necessary frequency through a filter means, detecting an envelope, and also detecting that the amplitude of the extracted frequency continuously increases. CONSTITUTION: A BPF(band-pass-filter) 2 extracts only the signal components of extremely narrow frequency bands among those input sound signals. A peak holding circuit 3 holds the peak value of the input signal for a period of a certain time window Wn that is set by a timing generator 9 and then outputs the peak value to a comparator 4. At the same time, the peal value, i.e., the output of the circuit 3 acquired in the preceding time window Wn-1 is supplied to the comparator 4 via a delay circuit 5 and a weighting arithmetic circuit 6. The result of comparison of the comparator 4 is sent to a counter 7 and a counter check member 8 is confirmed. Thereby, it is decided whether an envelope rather increases and then decided whether the howling occurs or not.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a howling detection device suitable for application to an acoustic feedback system and the like.

[0002]

2. Description of the Related Art In various audio signal transmission systems such as a so-called acoustic amplification system from a microphone to a speaker, howling of the audio signal may occur due to spatial feedback from the speaker to the microphone or circuit feedback. Usually, it is an important subject in various audio equipments to appropriately suppress the howling. Various methods can be considered to suppress howling, such as reducing the gain of the signal when howling is detected.In any case, in order to suppress howling by various methods, it is necessary to accurately determine that howling has occurred in the signal. Detection is a prerequisite.

[0003]

As a howling detection method that has been conventionally studied, a howling is analyzed by analyzing a spectrum on a frequency axis of a signal and, for example, a spectrum level at a specific frequency is remarkably increased. It is determined that it has occurred. However, with such a detection method,
There is a problem in that processing such as fast Fourier transform is required, the procedure and method until analysis become complicated, the scale of the detection circuit or detection system becomes large, and the cost also increases. For this reason, it cannot be easily mounted on various audio equipment. Since analysis is performed in a more complicated procedure, it takes time to detect, and there is a difficulty in quick howling suppression operation.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an object thereof is to provide a howling detection device having a simple structure and capable of performing detection rapidly.

Therefore, as a howling detection device, a filter means for extracting a signal component of a required frequency band from an input signal, an envelope detection means for detecting an envelope of an output signal of the filter means, and an increasing tendency of the detected envelope. And a howling determining means for determining howling has occurred when an envelope increasing tendency continued for a predetermined time or longer is detected by the tendency determining means.

In particular, the envelope detecting means is configured to detect and output an extreme value of the output signal of the filter means for each detection period of a predetermined time length. Further, the tendency determination means compares the extreme value in the current detection period by the envelope detection means with a value obtained by adding a predetermined coefficient to the extreme value in the previous detection period to determine whether the envelope has an increasing tendency. It is configured so that it can be discriminated.

Further, the howling discriminating means, when the tendency for envelope increase is detected by the tendency discriminating means,
Even if a temporary decrease tendency is detected within a certain period of time, it is configured not to judge that the continuous increase tendency is interrupted.

[0008]

By extracting the required frequency with the filter means and performing envelope detection, it is possible to easily and immediately detect that the amplitude of the signal of that frequency has continuously increased.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First and second embodiments will be described below as embodiments of the howling detection apparatus of the present invention, and then the howling detection apparatus is applied to a power amplifier device having an audio signal transmission system from a microphone to a speaker. An example will be described.

[First Embodiment] First, FIG. 2 shows an output waveform of a typical microphone when howling occurs at a certain frequency. The output of the microphone mentioned here is a signal input to the howling detection apparatus of this embodiment. As shown in FIG. 2, the signal at the time of howling has a sinusoidal waveform whose amplitude increases monotonically.

Therefore, howling can be detected by limiting the signal by the bandpass filter and confirming the continuous increase in the amplitude of the frequency signal extracted by the bandpass filter. That is, the howling detection apparatus of the present embodiment performs howling detection by extracting the frequency band in which howling is desired to be detected with a bandpass filter and confirming the continuous increase of the envelope of the signal of that frequency.

FIG. 1 is a block diagram of a first embodiment as a howling detection device. An audio signal from a microphone or the like is input from the terminal 1 and first supplied to the bandpass filter 2. Only a signal component in a very narrow frequency band of the audio signal input by the bandpass filter 2 is extracted and supplied to the peak hold circuit 3. Therefore, the input signal of the peak hold circuit 3 is a signal of a specific frequency that is almost a sine wave. The pass frequency of the band pass filter 2 is set according to the frequency in the audio signal band for which howling is desired to be detected.

The peak hold circuit 3 holds and outputs the peak value of the input signal within a certain time window W _n set by the timing generator 9. Time window W
_n is set according to the frequency passing through the bandpass filter 2, and for example, the time length W _L of the time window W _n is set.
Is approximately one cycle time of the frequency. Figure 2
As described above, the time window W _n corresponding to almost one cycle of the signal is set.

The peak value for each time window W _n output from the peak hold circuit 3 is supplied to the comparator 4. The output of the peak hold circuit 3 is supplied as a signal to be compared with the output of the peak hold circuit 3 by the comparator 4 via the delay circuit 5 and the weighting calculation circuit 6. That is, this is a signal obtained by multiplying the peak value in the previous time window W _(n-1) by a certain coefficient.

The comparison operation of the comparator 4 is performed at the control timing by the timing setting circuit 9. That is, it is performed at the timing when the current peak value and the previous peak value are supplied to the comparator. The comparison result of the comparator 4 is sent to the counter 7. Then, the counting state of the counter 7 is confirmed by the counter check unit 8 to detect whether or not howling is occurring. When the howling occurs, the counter check unit 8 outputs the detection signal HD from the terminal 10.

The operation of the howling detection apparatus of this embodiment will be described with reference to the flow chart of FIG. It should be noted that this flowchart only shows the flow of operation in each part of the howling detection apparatus until howling detection. The operation of the howling detection device is such that when an audio signal from a microphone is input (F
101), which is band-limited by the bandpass filter 2 (F102). As a detection operation for a signal that has passed through the bandpass filter 2, first, the variable n is set to 1, and the count value C of the counter 7 is reset to zero (F10
3). Note that the variable n is not a variable that is actually processed on the circuit, but is used as a variable indicating the passage of time for convenience of explanation of the operation in FIG.

In step F104, the timing generator 9
Causes the peak hold circuit 3 to execute the peak detection operation of the input signal in the time window Wn which is, for example, W _L time. This peak detection operation is repeatedly executed for each time window Wn.

The peak value PK (Wn), which is the output of the peak hold circuit 3, is supplied to the comparator 4. At this time, the comparator 4 detected the peak value by the peak hold circuit 3 in the previous time window Wn-1. The peak value PK (Wn-1) is delayed by the delay circuit 5, further multiplied by the coefficient UP in the weighting calculation unit 6, and supplied. Therefore, the delay time of the delay circuit 5 is 1 of the center frequency passing through the bandpass filter 2.
It is set to a period of time, that is, approximately W _L time. The peak value PK of this time in the comparator 4 is set as step F105.
(Wn) and the previous peak value PK (Wn−1) × coefficient UP are compared.

As a comparison result, the current peak value PK (Wn)>
When the previous peak value PK (Wn-1) x UP is obtained, the count value C of the counter 7 is incremented (F10
6). On the other hand, the current peak value PK (Wn) is the previous peak value P
If the level is equal to or less than K (Wn-1) × coefficient UP, the count value C of the counter 7 is reset to zero (F107).

As the timing generator 9, step F1
At 08, the operation of the peak hold circuit 3 in the next time window Wn is executed as shown by the increment of the variable n and the return to step F104. However, the counter check unit 8 monitors the count value C, and the count value C
Is greater than the predetermined value TH, it is determined that howling has occurred, and the detection signal HD is output from the terminal 10 (F109, F110).

That is, in this howling detection apparatus, the counter 7 is counted when the amplitude level of the signal of the frequency to be detected increases, and the counting operation is reset when the amplitude level decreases. Therefore,
Only when the certain frequency component extracted by the bandpass filter 2 continuously increases, the count value C of the counter 7 exceeds a certain predetermined value TH, that is, the count value is counted only when howling occurs. The value C> the predetermined value TH, and howling can be detected.

Since most of the howling detection device can be configured by an analog circuit, A / D conversion of a voice signal,
Since there is no delay due to D / A conversion, etc., and complicated analysis processing using fast Fourier transform etc. is not necessary, it is excellent in signal followability and can detect howling quickly,
Therefore, howling detection at a relatively high frequency can be performed well.

Moreover, since it has a simple circuit structure and can be realized by a small-scale circuit, it is suitable for mounting on various devices and for use in an audio system. In this embodiment, howling detection can be performed only for a specific frequency extracted by the bandpass filter 2, and howling detection for the entire audio signal band in an actual acoustic device or the like can be performed using this howling detection device. How to do this will be described later as an application example.

[Second Embodiment] FIG. 4 is a block diagram showing a second embodiment of a howling detection apparatus. This embodiment differs from the first embodiment in that a release counter 11 is provided, and the other parts are the same as in the first embodiment.

During the howling detection operation, even if the howling waveform is actually input to the howling detection device, it is determined that the peak holding circuit 3 causes a detection error and the continuous increase in amplitude is interrupted. May be done. As the waveform during howling, the amplitude may be momentarily decreased as shown in FIG. 5, for example. In such a case, howling may not be detected even though howling is occurring. The second embodiment is one in which howling can be favorably detected even in such cases.

The release counter 11 shown in FIG. 4 counts or resets according to the output result of the comparator 4, and the reset of the counter 7 is executed based on the count value of the release counter 11. Become.

The operation of the howling detection apparatus will be described with reference to FIG. In FIG. 6, steps F201 and F202 are the same as steps F101 and F102 of FIG. 3, and when an audio signal from a microphone or the like is input, it is band-limited by the band pass filter 2. As a detection operation for the signal passed through the band pass filter 2, first, the variable n is set to 1, and the count value C of the counter 7 and the count value CX of the release counter 11 are reset to zero (F203). Figure 3
Similarly to the above, the variable n is not a variable processed on an actual circuit but is used for convenience as a variable indicating the passage of time.

In step F204, the timing generator 9
Causes the peak hold circuit 3 to execute the peak detection operation of the input signal in the time window Wn which is, for example, W _L time. Further, the peak value P which is the output of the peak hold circuit 3
K (Wn) is supplied to the comparator 4, and at this time, the comparator 4
The peak value PK (Wn-1) detected by the peak hold circuit 3 in the previous time window Wn-1 is delayed by the delay circuit 5, further multiplied by the coefficient UP in the weighting calculation unit 6, and supplied. . Then, in step F205, the current peak value PK (Wn) and the previous peak value PK (Wn) in the comparator 4 are set.
-1) × coefficient UP comparison processing.

As a comparison result, the current peak value PK (Wn)>
When the previous peak value PK (Wn-1) x UP is obtained, the count value C of the counter 7 is incremented (F21
0). Also, in this case, the count value CX of the release counter 11 is reset (F211). On the other hand, the current peak value PK (Wn) is the previous peak value PK (Wn-1) × coefficient U
If the level is equal to or lower than P, first, the count value CX of the release counter 11 is incremented (F206).
Then, in step F208, the count value CX of the release counter 11 is checked (F208), and only when the count value CX exceeds a predetermined value RL, the count value C of the counter 7 is reset to zero (F209).

As the timing generator 9, step F2
The operation of the peak hold circuit 3 in the next time window Wn is executed, as shown at 12 by incrementing the variable n and returning to step F204. However, as in the first embodiment, the counter check unit 8 monitors the count value C. If the count value C becomes larger than the predetermined value TH, it is determined that howling has occurred, and the detection signal H from the terminal 10 is detected.
D will be output (F213, F214).

That is, also in the second embodiment, the counter 7 is counted when the amplitude level of the signal of the frequency to be detected increases, as in the first embodiment.
When the increasing tendency of the amplitude level continues until the count value C of the counter 7 exceeds a certain predetermined value TH, howling is determined to occur. here,
Even if the peak hold circuit 3 is erroneously detected during the howling or the howling waveform shows a momentary decrease in the amplitude, the count value CX of the release counter 11 is a predetermined value R.
As long as it has not reached L, the counter 7 is not reset, so it is not immediately determined that the continuous increase tendency has been interrupted. In other words, even if a momentary decrease in amplitude is detected a certain number of times (RL or less) when howling occurs,
In such a case, it can be prevented that the howling is mistakenly recognized.

[Application Example] An example in which the howling detection apparatus configured as in the first and second embodiments described above is applied to a howling suppressing operation in a power amplifier system will be described with reference to FIGS. 7 and 8. To do. In FIG. 7, the audio signal picked up by the microphone 20 and converted into an electric signal is amplified by the microphone amplifier 21 and then supplied to the part as the howling detection device described in the above embodiment. In this case, the bandpass filter 22-1
And howling detector 23-1, bandpass filter 22-2
The howling detection section 23-2, ... The bandpass filter 22-n and the howling detection section 23-n respectively constitute the howling detection apparatus described as the embodiment.

The audio signal from the microphone amplifier 21 is supplied to the bandpass filters 22-1, 22-2, ... 22-n,
Only predetermined band components are extracted. A bandpass filter 2 serving as a component (corresponding to the bandpass filter 2 in FIG. 1 or 4) of each howling detection device.
2-1, 22-2, ... 22-n are, for example, 1/3 each
It has a narrow band pass characteristic of about octave, and the entire band as an audio signal transmitted by the device is divided into about 1/3 octaves.

Each bandpass filter 22-1, 22-2, ...
The audio signals of each band extracted by 22-n are supplied to the howling detectors 23-1, 23-2, ... 23-n, respectively. Each howling detector 23-1, 23-2, ... 2
The 3-n has a configuration after the band pass filter 2 of FIG. 1 or FIG. 4 so as to detect whether or not howling has occurred for each input narrow band signal. Further, in the case of FIG. 7, the input audio signal is directly input to each of the subsequent phase shift units 24-1, 24-2, ... 24-n.
It also has a signal path to supply to. Howling detector 2
23-1, 23-2, ..., 23-n detect the detection signal HD when the occurrence of howling is detected for the input audio signal, and shift the detection signal HD to the phase shift units 24-1, 24-2 ,. will be supplied to n.

Phase shift units 24-1, 24-2, ... 24-n
When the detection signal HD is supplied, performs the necessary amount of phase shift processing on the input audio signal. While the detection signal HD is not supplied, the input signal is output without any processing. Each phase shift unit 2
Outputs from 4-n, 24-2, ..., 24-n are supplied to the mixer 25, mixed, amplified by the power amplifier 26, and output as sound from the speaker 27.

In other words, in this system, the howling detectors 23-1, 23-2, ... In each frequency band divided by the bandpass filters 22-1, 22-2 ,.
Howling detection is individually performed at 23-n. Then, the phase shift unit 24 corresponding to the frequency at which howling occurs
Phase shift processing is performed only at -X. The phase-shifted audio signal is mixed with another signal from the phase shift unit 5 which has not been subjected to the phase shift processing in the mixer 25, and is supplied to the speaker 27 via the power amplifier 26.

In howling, a loop transfer function including an acoustic system from a microphone to a speaker has a gain of 1 or more,
And because it occurs at a frequency with a phase characteristic of 0 °,
In this way, by changing the phase condition of the signal at the frequency at which the howling occurs, the phase characteristic becomes zero.
The condition of ° is eliminated, and howling can be suppressed.

Further, in the case of this system, the phase is changed only at the specific frequency at which the howling occurs, the spectrum of the audio signal output from the speaker 27 does not change due to the phase shift, and further human beings. Of the output sound from the speaker 27, the sound quality of the output sound from the speaker 27 hardly deteriorates in terms of sound quality because the auditory sense of the sound is much less sensitive to the change of the phase than the change of the amplitude and the change of the frequency characteristic. Will be things.

In such a system, the number of band divisions n by the bandpass filter 3, that is, the number of mounted howling detection devices according to the embodiment, should be set in accordance with the device in which the howling suppression device is adopted. In the case of a device having a narrow audio signal band such as a telephone, the number is, for example, about 5 to 10, and in the case of being applied to a device such as a hi-fi audio device which requires high sound quality and a high band, it is about 20 to 30. Good. In any case, as described above, each howling detection device can be realized as a small device with a simple circuit configuration, and therefore the system as a whole does not become so large.

The howling detector 23 and the phase shifter 24 may be arranged in parallel on the signal path. Further, the phase shift unit 24 and the howling detection unit 23 may be arranged in series in this order, and the phase shift operation control for howling occurrence may be performed in a feedback manner.

By the way, in a device in which the frequency band in which the howling is likely to occur is almost fixed, the bandpass filter 22 and the howling detecting section 23, that is, the howling detecting device of the embodiment may be provided only as one unit. That is, from the supplied audio signal, the bandpass filter 2
The band in which the howling occurrence probability is high is extracted by 2, and howling is suppressed if necessary, and then mixed with the signal component of the other band of the original input signal.

FIG. 8 shows another application example. However, the same parts as those in FIG. In this application example, howling detection is performed for each band by a plurality of howling detection devices as in FIG. 7. That is, band division is performed by a plurality of band pass filters 22-1, 22-2, ... 22-n, and howling detection units 23-1, 23-2 ,. Howling is detected by n. If it is detected, howling suppression processing is performed for that band, but in this example, the gain variable unit 28- is used instead of the phase shift unit of FIG.
28-n are provided.

As the howling suppressing operation, each of the gain varying units 28-1, 28-2, ... 28-n has a corresponding howling detecting unit 23-1, 23-2 ,. When the detection signal HD is supplied from n, a necessary amount of gain reduction processing is performed on the input audio signal. Variable gain part 2
28-n, 8-1, 28-2, ... 28-n, are output as they are without performing any processing on the input signal while the detection signal HD is not supplied. Also in the case of this example, a howling suppression system using the howling detection apparatus of the embodiment is realized. In this case, although the howling detection unit 23 and the gain variable unit 28 are arranged in parallel on the signal path of each band, the howling detection unit 23 and the gain variable unit 28 are serially arranged in this order or the reverse order. You may arrange.

[Other Applications and Modifications of Howling Detection Apparatus] Various applications and modifications of the howling detection apparatus of the present invention will be briefly described below. First, the howling detection apparatus of the present invention can be applied as a part of a howling suppression apparatus in various devices other than the power amplifier system shown as the application example. For example, the present invention can be well applied to, for example, telephones and other communication devices, various audio devices, so-called electric musical instruments that convert acoustic signals into electrical signals and transmit the electrical signals.

In the case of performing feedback control such as picking up an acoustic signal output from a speaker or picking up vibration of a diaphragm and correcting output characteristics accordingly (so-called acoustic feedback, motional feedback). Can be applied as an oscillation detection unit as a device for avoiding oscillation. Further, when a device such as an earphone, a headphone, or a hearing aid has a feedback system such as an active noise canceller, it can be applied as an oscillation detection unit in a protection circuit for avoiding oscillation.

Further, a howling detection device of the present invention and FIG.
A single howling suppression device may be configured by combining the phase shifter 24 of FIG. In this case, for example, in a system in which a microphone part and a speaker part are connected by a cord and used, by interposing a howling suppression device, it is possible to exert an effect as a single howling suppression device in various usage opportunities. Become. The present invention can also be applied to the case of detecting howling and signal oscillation for the purpose other than howling suppression.

Further, various modified examples of the circuit structure as the howling detection apparatus can be considered. It is also conceivable that the bandpass filter 2 of FIG. 1 or FIG. 4 can change the pass band, or can be provided with a specific learning / adaptive function so as to follow the frequency at which howling occurs. Further, the bandpass filter 2 may not necessarily have a bandpass form depending on the type of signal to be handled, and may be a circuit that limits a certain signal component.

Further, all the circuit configurations shown in FIGS. 1 and 4 may be configured as a digital signal processing circuit, or the calculation part may be processed by a digital signal processor or a microcomputer. In such a case, it is possible to change the weighting coefficient UP according to the environment. In the case of digitization, the signal followability is lowered to some extent due to the delay time in A / D conversion and D / A conversion and the convenience of the sampling frequency. However, when digitized, more accurate and flexible control becomes possible.

[0049]

As described above, in the howling detection apparatus of the present invention, it is easy to continuously increase the amplitude of the signal of the frequency by extracting the required frequency with the filter means and performing the envelope detection. Moreover, it can be detected immediately. That is, there is no need for complicated processing such as frequency analysis for howling detection, which can be realized by a simple circuit, and a detection operation with good followability can be obtained. This also has the advantage that downsizing of the device, cost reduction, and flexible adaptation to various devices are realized. Further, since the present invention can be easily configured as an analog processing circuit, it is possible to deal with the case where howling occurs in a relatively high frequency band, and it is possible to make the followability excellent.

[Brief description of drawings]

FIG. 1 is a block diagram of a first embodiment of a howling detection apparatus of the present invention.

FIG. 2 is an explanatory diagram of an audio signal waveform during howling.

FIG. 3 is a flowchart of the operation of the howling detection apparatus according to the first embodiment.

FIG. 4 is a block diagram of a second embodiment of a howling detection device of the present invention.

FIG. 5 is an explanatory diagram of an audio signal waveform during howling.

FIG. 6 is a flowchart of the operation of the howling detection apparatus according to the second embodiment.

FIG. 7 is a block diagram of an application example of a howling detection apparatus of the present invention.

FIG. 8 is a block diagram of an application example of the howling detection apparatus of the present invention.

[Explanation of symbols]

 1, 10 terminals 2, 22-1 to 22-n band pass filter 3 peak hold circuit 4 comparator 5 delay circuit 6 weighting operation unit 7 counter 8 counter check unit 9 timing generator 11 release counter 23-1 to 23-n Howling detection unit 24-1 to 24-n Phase shift unit 28-1 to 28-n Gain variable unit 25 Mixer

Claims (3)

[Claims] 1. A filter means for extracting a signal component of a required frequency band from an input signal, an envelope detecting means for detecting an envelope of an output signal of the filter means, and an increasing tendency of the envelope detected by the envelope detecting means. And a howling determination means for determining howling has occurred when an envelope increasing tendency that continues for a predetermined time or more is detected by the tendency determination means. Howling detection device. 2. The envelope detecting means is configured to detect and output an extreme value for each detection period of a predetermined time length with respect to the output signal of the filter means, and the tendency determining means is configured to output the envelope. By comparing the extreme value in the current detection period by the detection means with the value obtained by adding a predetermined coefficient to the extreme value in the previous detection period, it is possible to determine whether or not the envelope tends to increase. The howling detection apparatus according to claim 1, wherein the howling detection apparatus is configured. 3. The howling determining means interrupts the continuous increasing tendency even if a temporary decreasing tendency is detected within a certain time while the envelope increasing tendency is detected by the tendency determining means. The howling detection apparatus according to claim 1, wherein the howling detection apparatus is configured so as not to judge that the howling detection is performed.