Sound amplifying apparatus with automatic howl-suppressing function

Abstract: A sound signal picked-up by a microphone is processed in a howl suppresser including a digital filter. A frequency analyzer performs frequency analysis of the picked-up sound signal. A howl detector detects a howl contained in the sound signal from a result of frequency analysis by the frequency analyzer. An operation parts calculates coefficients to be set to the digital filter to suppress the howl according to a detection result by the howl detector, and a control part sets the calculated coefficients to the digital filter. The howl detector judges that a maximum peak power level among power levels of the sound signal in a frequency region analyzed by the frequency analyzer is a howl component when a ratio of the maximum peak power level to a mean power level of the sound signal is larger than a predetermined threshold level, preferably for a predetermined threshold time. Also, a threshold control means is provided for controlling the threshold level and/or the threshold time so as to enhance accuracy of howl detection.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sound amplifying apparatus for amplifying sound or voice picked-up by a microphone and delivering amplified sound or voice through speaker, and more particularly to a sound amplifying apparatus having howl-suppressing capability. 2. Description of the Prior Art At a lecture or the like using electric acoustic appliances such as microphone and speaker, howl often occurs when the lecturer moves or if the condition in the hall changes. The "howl" is undesirable prolonged sound produced because of acoustic feedback. In case howling occurs, the acoustic adjuster ("mixer", hereafter) either lowers the sound signal level in the frequency hand in which the howl would be occurring by means of a graphic equalizer, or lowers the entire output level. When the howl is suppressed or the position of the lecturer is changed, i.e., when the condition of sound pickup varies, the mixer returns the characteristic of the graphic equalizer or the entire level to the original characteristic or level. Every time howl occurs, the mixer repeats this action to suppress the howl. In such constitution, however, when howl occurs, the mixer must always lower the frequency of the graphic equalizer, so that it takes labor to suppress the howl. Also, since the frequency band for lowering the graphic equalizer cannot be instantly and accurately known, and it takes time to suppress the howl. Arrangements are known in the prior art which can detect and filter resonating feedback frequencies. One such arrangement is that described in WO 91/20134, wherein audio signals are digitized and an FFT is conducted on samples of the digitized signals to produce corresponding frequency spectrums. These spectrums are analyzed, such as by determining one or more peak frequency magnitudes which are 33 dB greater than harmonics or subharmonics of the frequency in a plurality of several successive spectrums, to detect resonating feedback frequencies. The offending frequency is then filtered in the time domain, either in the digitized form or analog form, to eliminate the feedback. SUMMARY OF THE INVENTION It is a primary object of the invention to present a sound amplifying apparatus capable of suppressing howl automatically and accurately, even in the conditions of relatively large background noise level such as air-conditioning noise and murmur of voices. To achieve this object, the present invention provides a sound amplifying apparatus comprising: a microphone for picking up a sound to obtain a sound signal; an analog-to-digital converter for converting the sound signal from the microphone to a digital sound signal; howl suppressing means including a digital filter for processing the digital sound signal; a digital-to-analog converter for converting a processed digital sound signal from the howl processing means to a processed analog signal; an amplifying means for amplifying the processed analog sound signal to obtain an amplified sound signal; a speaker responsive to the amplified sound signal for generating an amplified sound; frequency analyzing means for frequency analyzing the digital sound signal from the analog-to-digital converter in real time; howl detecting means for detecting a howl contained in the sound signal from a result of frequency analysis by the frequency analyzing means; operation means for calculating coefficients to be set to the digital filter to suppress the howl according to a detection result by the howl detecting means; and control means for setting the calculated coefficients to the digital filter, wherein the howl detecting means detects a maximum peak power level among power levels of the sound signal in a frequency region analyzed by the frequency analyzing means; and characterised in that the howl detecting means calculates an adjusted mean power level of the sound signal by omitting first to m-th largest peak power levels from all power levels in the frequency region, where m is a predetermined integer larger than 0, and calculating a mean value of the remaining power levels, calculates a ratio of the maximum peak power level to the adjusted mean power level, and judges that the maximum power level is a howl component when the ratio is larger than a predetermined threshold level. Preferably, the howl detecting means may judge the maximum peak power level as a howl component when the ratio of the maximum peak power level to the mean power level is larger than the predetermined threshold level for a predetermined threshold time. In another aspect of the invention, the sound amplifying apparatus may include a threshold control means for controlling the threshold level and the threshold time. The threshold control means may be responsive to the result of frequency analysis by the frequency analyzing means for changing the threshold level depending on a frequency band in which the frequency of the maximum peak power level is located or depending on a frequency characteristic of a background noise contained in the sound signal or depending on a frequency characteristic of the sound signal. The apparatus may further comprise a voice judging means responsive to the result of frequency analysis by the frequency analyzing means for judging whether the picked-up sound is a voice or not, and the threshold control means may be responsive to a judging result by the voice judging means for changing the threshold level when the picked-up sound is a voice. The apparatus may further comprise a frequency characteristic measuring means for measuring a frequency characteristic of a room in which the microphone and speaker are located from a position of the speaker to a position of the microphone, and the threshold control means may be responsive to a measuring result by the frequency characteristic measuring means for changing the threshold level depending on the frequency characteristic of the room. The apparatus may further comprise a echo measuring means for measuring an echo time in a room in which the microphone and speaker are located, and the threshold control means may be responsive to a measuring result by the echo time measuring means for changing the threshold time depending on the echo time. The above and other features and advantages of the invention will be more apparent from the following description with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a block diagram of a sound amplifying apparatus in an embodiment of the invention. Fig. 2 is a flow diagram showing a method for detecting howl by the howl detecting part in the embodiment of Fig. 1. Fig. 3 is a comparative diagram of a method of an embodiment of the invention and a conventional method, in which (a) shows a howl waveform, and (b) shows comparison of ratio of maximum peak level and mean power level. Fig. 4 shows a result of detection of howl in an embodiment of the invention, in which (a) shows a howl waveform, (b) shows ratio of peak level and mean power level, and (c) shows peak frequency. Fig. 5 shows an input signal waveform in an embodiment of the invention and its FFT frequency characteristic diagram, in which (a) shows the input signal waveform, and (b) shows a frequency analyzed waveform. Fig. 6 shows a result of detecting howl in another embodiment of the invention, in which (a) shows a howl waveform, (b) shows changes of peak level with time, and (c) shows changes of peak frequency with time. Fig. 7 is a block diagram of a sound amplifying apparatus in another embodiment of the invention. Fig. 8 is an explanatory diagram for calculating the threshold time in the embodiment of Fig. 7, in which (a) shows setting example of threshold level, and (b) shows setting example of threshold time. Fig. 9 is a block diagram of a sound amplifying apparatus in still another embodiment of the invention. Fig. 10 is a diagram showing a changing method of threshold level in the embodiment of Fig. 9, in which (a) shows sound frequency characteristic, and (b) shows method of changing threshold. Fig. 11 is a block diagram of a sound amplifying apparatus in still another embodiment of the invention. Fig. 12 is an explanatory diagram of threshold time calculation in the embodiment of Fig. 11. Fig. 13 is a block diagram of a sound amplifying apparatus in still another embodiment of the invention. Fig. 14 is an explanatory diagram of threshold level calculation in the embodiment of Fig. 13. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 is a block diagram of a sound amplifying appara-tus in an embodiment of the invention. In Fig. 1, numeral 1 denotes a microphone for picking up sound, 16 is a microphone amplifier, 2 is an A/D (analog/digital) converter for converting the picked-up sound into a digital sound signal, 3 is a D/A (digital/analog) converter for converting a digital sound signal into an analog sound signal, 4 is an amplifier for amplifying the output signal of the D/A converter, 5 is a speaker for reproducing sound from the signal amplified by the amplifier 4, 6 is a howl suppressing part for lowering the signal level at the howling frequency by applying a notch filter processing to the digital sound signal from the A/D converter 2, 7 is a frequency analyzing part for transforming the signal from the A/D converter 2 into a frequency region by fast Fourier transform (FFT) or by using plural band-pass filters, 10 is a howl detecting part for detecting howl on the basis of the frequency analysis result of the frequency analyzing part 7, 8 is an operating part for calculating coefficients of a digital filter included in the howl suppressing part, and 9 is a control part for setting the coefficients of the digital filter obtained in the operating part 8 to the digital filter in the howl suppressing part. In the thus composed sound amplifying apparatus, its operation is described below. Sound, such as a performance by a performer, is picked up by the microphone 1, processed through the A/D converter 2, howl suppressing part 6 and D/A converter 3, amplified in the amplifier 4, and is reproduced through the speaker 5. Usually, in this state, in order that howl may not occur, the gain of the amplifier and microphone amplifier are adjusted appropriately. However, when the performer moves or the direction of the microphone 1 is changed, the loop gain in the sound pickup and reproducing system increases. When the loop gain exceeds 1, howl is produced. The sound signal from the microphone 1 is converted into a digital sound signal in the A/D converter 2. The digital sound signal is fed into the howl suppressing part 6 and the frequency analyzing part 7. The digital sound signal from the A/D converter 2 is converted into components in the frequency region, or power spectrums, in the frequency analyzing part 7 by FFT processing. Next is shown a method of detecting howl in the howl detecting part 10. Fig. 2 is a flow chart of processing in the howling detecting part 10. First, the largest level of the power levels in the frequency region is searched (step 201). Then the mean value of the power levels in the frequency region is calculated by the method expressed as formula (1) shown below. To determine the mean value, the largest three power levels in the frequency region are removed (the number of the largest power levels to be removed may be changed according to the interval of frequency to be analyzed such that the number is smaller when the frequency interval is wider and larger when the frequency interval is narrower), and all of the remaining power levels are added (step 202). The added result is divided by the number of added power levels to obtain the mean value (step 203). where P1 frequency of the largest power level P2 : frequency of the second largest power level P3 : frequency of the third largest power level N : number of frequency points P AV : mean power level X(j) : power level of j-th frequency Then, based on formula (2) shown below, the ratio of the maximum peak power level to the mean power level is determined (step 204). When the ratio exceeds a predetermined value which is set so as to be regarded as howl (hereinafter called "threshold level"), it is judged that howl is produced at the frequency of the peak power level (steps 205, 206). Formula (2): P SUB = P MAX /P AV P MAX = x (p1) where P SUB : ratio of maximum peak power level to mean power level P MAX : peak power level Howl occurs at a single frequency, but the power levels in the frequency band around the howling frequency are also larger than the power levels in other frequencies. Hence, the mean power level increases with increase of howl. That is, the howl significantly influences the mean power level. Accordingly, by omitting the first through m-th largest power levels in the calculation of the mean power level in formula (1), the ratio of power levels between the howl and non-howling components is increased so that the howl is emphasized. Fig. 3 shows a result of comparing the ratio of the peak power level to the mean power level determined by dividing all power levels by the number of all power levels at all frequencies as in the conventional method, in howling state, and the ratio of the peak power level to the mean power level determined in the method of the invention. In Fig. 3, (a) shows howl waveform, and (b) shows the peak to mean power level ratios. As seen from Fig. 3, according to the invention, the ratio curve has a significant peak when a howl occurs, so that the howl can be accurately detected. Fig. 4 shows the howl waveform and the result of analysis of the waveform by the method of the embodiment. In Fig. 4, (a) shows the howl waveform, (b) shows changes of the ratio of the peak power level to the mean power level by the method of the embodiment, and (c) shows the frequency of the peak power level. Thus, as the howl signal increases, it is known that the ratio of the peak power level to the mean power level increases. For the value of the ratio, accordingly, a proper threshold level is set as shown in Fig. 4 (b). In the howl detecting part 7, when the ratio exceeds the threshold level, it is regarded that howl occurs, and the howling frequency is calculated at the same time (step 207). Since footsteps and other ordinary background noise are wide in the frequency band and the ratio of the peak to mean power levels is smaller than the case of howl, they are not regarded as howl. When the howl is detected and the howling frequency is calculated, the operating part 8 calculates the coefficients for composing such digital filter as to lower the gain of only the howling frequency component in the howl suppressing part 6 (step 208). The calculated coefficients of digital filter are set in the howl suppressing part 6 by the control part 9. In this embodiment, a notch filter is used as the digital filter in the howl suppressing part 6. Alternatively, in the howl suppressing part 6, a graphic equalizer capable of attenuating the howling frequency band component automatically depending on the howling frequency may be used. By this operation, when a howl occurs, its frequency component is removed by the digital filter in the howl suppressing part 6, so that the howl can be suppressed. In this way, by analyzing the frequencies in the frequency analyzing part 7 and the howl detecting part 10, judging the howl from the peak to mean power level ratio, determining the howling frequency, and removing the howling frequency component by the notch filter, the howl can be eliminated even when the background noise is large. In the embodiment, the case of howl caused due to acoustic feedback from one speaker trough one microphone is shown, but the same effects are obtained in the case of using plural microphones and speakers. Hereinafter, a sound amplifying apparatus in another embodiment of the invention is explained below while referring to the drawings. The constitution is the same as the one shown in Fig. 1. In the thus composed sound amplifying apparatus, its operation is explained below. Up to the frequency analyzing part 7, the operation is exactly the same as in the foregoing embodiment. Fig. 5 shows a howl waveform (a) when plural howls occur simultaneously, and the frequency characteristic (b) analyzed by the frequency analyzing part 7. When the howling frequencies are great in number, the mean power level rises, so that the peak to mean power level ratio becomes small, and the howl detection precision is worsened. Accordingly, as the parameters for howl detection, change of peak power level and continuity of peak power frequency are used. Fig. 6 shows howl waveform (a), maximum peak power level change with time (b), and change of maximum peak power frequency with time (c). In the condition that plural howls are produced as in Fig. 6, likewise, the maximum peak power frequency of the maximum peak power level is stable, and the maximum peak power level increases. In this embodiment, as the conditions of howl, the continuity of the frequency of the maximum peak power level, power level increase or decrease of the maximum peak power level, and increase or decrease of the total power level determined in formula (3) are judged. Formula (3): P A = aa*P A + (1-aa) *x (i) 2 where P A : total power level x(i): input signal aa : coefficient satisfying the condition of 0<aa<1 In this embodiment, the value of aa is set around 0.99. It is regarded as howl when the frequency of the maximum peak power level continues over a specific time, the maximum peak power level has increased from the result of previous analysis by the frequency analyzing part 7, and the total power level has also increased from a defined value. The frequency analyzing part 7 analyzes frequencies at specific time intervals. Accordingly, the continuity time of the frequency of the maximum peak power level is determined from the time required for one frequency analysis by the frequency analyzing part 7 and the frequency characteristic of the background noise. In this way, when howl grows, the peak level also increases, and the frequency becomes constant. Therefore, by the above judgement, howl can be detected. According to this method, general noise is hardly mistaken as howl because its peak frequency fluctuations are large and its peak level does not increase monotonously. For pulse-like noise, mistake can be prevented by properly judging the duration time of the peak frequency. The subsequent processing is the same as that in the preceding embodiment. By thus judging the increase or decrease of peak power level and continuity of peak power frequency, in this sound amplifying apparatus, howl can be detected relatively at high precision even in the conditions of noise or plural howls. In the background noise, incidentally, if the level of the low frequency is large, detection errors can be further decreased by detecting and processing the signals being rid of background noise in the low frequency band by using a high pass filter in a later stage of the microphone amplifier 16. A sound amplifying apparatus in a different embodiment of the invention is described below by reference to the accompanying drawings. It is an object of this embodiment to suppress howl by accurately detecting howl even when the background noise is large or echo time is long. Fig. 7 is a block diagram of the sound amplifying apparatus in this embodiment of the invention. In Fig. 7, numeral 11 is a threshold calculating part for calculating the threshold level for detecting howl, and the threshold time to be detected as howl when the frequency of the maximum peak power level continues more than a specific time, and 12 is a threshold control part for setting the threshold level in the howl detecting part 10. The other constituent elements are the same as those in the embodiment of the invention shown in Fig. 1. In the thus composed sound amplifying apparatus, its operation is described below. The sound signal picked up by the microphone 1 is converted into a digital sound signal by the A/D converter 2, and is fed into the howl suppressing part 6 and the frequency analyzing part 7. The frequency analyzing part 7 always analyzes the frequencies of the signal coming out of the A/D converter at specific time intervals. As the method of detection of howl of the howl detecting part 10, the ratio of the peak power level to the mean power level in the frequency region is determined, and when the ratio exceeds a specific threshold level and the duration exceeding the threshold level is over a specific threshold time, it is regarded that howl is produced. As the method of calculation of the threshold level and the threshold time, first the background noise is measured. From the result of frequency analyzing part 7, the threshold level is calculated in each of a plurality of frequency bands by the threshold calculating part 11. Fig. 8 (a) shows a setting example of the threshold level, and Fig. 8 (b) shows a setting example of the threshold time. Howl tends to grow slowly in a low frequency band and grow rapidly in a high frequency band. Herein, in order that the howl detection time may be equal, the threshold time is set shorter in lower frequency bands and longer in higher frequency bands. The determined threshold times and threshold levels are set as the howl judgement conditions in the howl detecting part 10 by the threshold control part 12. In the howl detecting part 10, when the input condition satisfies the howl conditions, it is Judged that howl occurs, and its howling frequency is calculated. Consequently, the operating part 8 calculates such coefficients as to compose a digital filter which lowers the gain of only the howling frequency component in the howl suppressing part 6. The calculated coefficients of digital filter are set in the howl suppressing part 6 by the control part 9. In this way, by analyzing the frequencies in the frequency analyzing part 7, calculating the threshold characteristics depending on the background noise characteristics by the threshold calculating part 11, detecting the howl by the howl detecting part 10, determining its frequency, and removing the howl frequency component by the digital filter, howl can be eliminated even when the background noise is large. The same effects are obtained-also when plural microphones and speakers are used. Instead of the method of setting the threshold level depending on the frequency characteristics of the background noise, the threshold level for howl detection in a band of large level of frequency characteristics may be increased depending on the frequency characteristics of the input signal, and the sensitivity for detecting howl may be lowered, so that detection errors can be decreased. As a further different embodiment of the invention, another sound amplifying apparatus is explained below while referring to the drawings. Fig. 9 is a diagram showing a configuration of a sound amplifying apparatus of this embodiment. Numeral 19 is a voice judging part for judging whether the input sound is voice or non-voice from the signal from the A/D converter 2, and detecting a voice period. The other constituent elements are the same as those in the foregoing embodiments of the invention. In the thus composed sound amplifying apparatus, the operation is described below. Up to the frequency analyzing part 7, the operation is the same as in the embodiment shown in Fig. 1. The voice judging part 19 judges whether the signal picked up by the microphone 1 is voice or non-voice on the basis of the signal from the A/D converter 2. When judged to be voice, the threshold level for detection of howl of the howl detecting part 10 is changed. In this embodiment, when the ratio of the peak power level to the mean power level exceeds a specific threshold level, it is judged that howl occurs. Therefore, the value of the threshold level is lowered during the voice period. When judged that the input sound is voice by the voice judging part 19, the threshold calculating part 11 calculates the threshold level depending on the voice components, and sets the calculated threshold level in the howl detecting part 6 through the threshold level control part 12. The threshold level is set in each of plural frequency bands. As the method of detecting howling, supposing it to be howl when the power level ratio in any frequency band exceeds the threshold level, in the case of voice, generally the voice pitch frequency components (from 200 to 300 Hz in case of women, 130 to around 200 Hz in case of men) may be mistaken as howl. Therefore, the threshold level for detecting howl in the frequency band near the voice pitch is increased by the threshold control part 12, and the detection sensitivity is lowered, so that detection errors can be decreased. Fig. 10 shows examples of frequency characteristics (a) analyzed in the frequency analyzing part 7 in the presence of voice, and the threshold level changing method (b). In the voice part, since the voice pitch frequency is around 250 Hz, the power level near the frequency of 250 Hz is large, so that by the threshold level of the ordinary howl detection, such frequency is misjudged as howl. Accordingly, as an example of voice, by setting the threshold level in the band of the pitch frequency to be larger than the peak level of the voice as shown in Fig. 10 (b), wrong detection of howl can be prevented if the level in the band near 250 Hz becomes larger than the voice pitch. Thus, in the voice portion judged by the voice judging part 19, howl can be detected more precisely by varying the threshold level for detecting howl in the howl detecting part 10. In this embodiment, howl may be detected by using the ratio of the peak power level to the mean power level of the signal picked up by the microphone 1, but various other methods are also possible, such as the method disclosed previously in the invention, and the method of detecting howl simply when the power level exceeds a certain threshold level. Meanwhile, in the embodiment, the threshold level change of howl detection in the case of voice is explained, but wrong detection of howl can be prevented in any acoustic conditions by varying the threshold level for howl detection, depending on the low frequency band large in the background noise level, the band of large noise at specific frequency, or acoustic condition of the room for howl detection. Below is explained a sound amplifying apparatus in still another embodiment of the invention by reference to the drawings. Fig. 11 is a diagram showing a constitution of a sound amplifying apparatus of this embodiment. Numeral 13 is an echo time measuring part, and 14 is a changeover switch for selecting the input signal to the amplifier 4 between the signal from the microphone 1 and a signal for measurement from the echo time measuring part 13. The other construction is the same as in the embodiment of the invention shown in Fig. 9. In the thus composed sound amplifying apparatus, the operation is explained below. First, the background noise and echo time are measured. Measurement of the background noise is the same as the operation in the embodiment shown in Fig. 7. The echo time is measured by the echo time measuring part 13 possessing the function for measuring the generation of a measuring signal and an echo time. The changeover switch 14 is set to the echo time measuring part 13 side by the switch control part 16. In measurement, a measuring signal possessing a band component such as pink noise is generated from the echo time measuring part 13, amplified by the amplifier 4, reproduced through the speaker 5, and picked up by the microphone 1. When the measuring signal is reproduced through the speaker 5 and is sufficiently diffused, the measuring signal is stopped. In the echo time measuring part 13, on the basis of the attenuation waveform of the signal picked up by the microphone 1, the time of attenuation from the original level to -60 dB is determined in each of plural frequency bands. In the threshold calculating part 11, on the basis of the background noise characteristics, the threshold level is determined in the same method as in the embodiment shown in Fig. 7, and the threshold time is calculated according to the measured echo time. To calculate the threshold time, at the frequency longer in echo time, the threshold time is set somewhat shorter because the change of power level is slow, and in a shorter echo time, the threshold time is set slightly longer because power changes are quick. Fig. 12 is an explanatory diagram of an example of setting the threshold time depending on the echo time. In this way, the threshold level and threshold time are determined. When the echo time is measured, and the threshold level and threshold time are determined, the changeover switch 14 is changed to the D/A converter 3 side by the switch control part 18. Hereinafter, the howl detection and suppression actions are the same as those in the embodiment shown in Fig. 7. By detecting howling by using the threshold level and threshold time calculated on the basis of the echo time measured by the echo time measuring part 13, howl can be detected and suppressed more precisely even in a location where the echo time is long. In the embodiment, to detect howl, sound is picked up by the microphone 1, and the ratio of the maximum peak power level to the mean power level of the signal analyzed into frequency components by the frequency analyzing part 7 is used, but simply it may be judged to be howl, for example, when the power level of the signal picked up by the microphone 1 exceeds a certain threshold level, or other various methods may be possible. As the method for measuring the echo time, it may be also possible to measure by using an impulse or chirp signal. Incidentally, if the echo time of the location is known beforehand, instead of the echo time measuring part 13 and changeover switch 14, a memory for storing the echo time may be installed in the constituent block. Fig. 13 shows a constitution of a sound amplifying apparatus in still another embodiment of the invention. Numeral 15 denotes a frequency characteristics measuring part, and 18 is a changeover switch for selecting the input signal into the amplifier 4 between the signal picked up by the microphone 1 and a signal for measuring frequency characteristics coming from the frequency characteristics measuring part 15. The other constitution is the same as that in the preceding embodiment. In the thus composed sound amplifying apparatus, the operation is explained below. First, the frequency characteristics of the room from the speaker 5 to the microphone 1 are measured. The frequency characteristics are measured by the frequency characteristics measuring part 15. By the switch control part 18, the changeover switch 17 is set to the frequency characteristics measuring part 15 side. In measurement, a measuring signal possessing a wide band component such as pink noise is generated from the frequency measuring part 15, amplified by the amplifier 4, and reproduced through the speaker 5. The sound is picked up by the microphone 1, and frequency analyzed by the frequency analyzing part 7. In the threshold calculating part 11, on the basis of the frequency characteristics analyzed by the frequency analyzing part 7, the threshold level is determined. For example, where the distance between the microphone 1 and speaker 5 is long, the power level in a high band is small, so that the threshold level is set low. Fig. 14 shows an example of setting of the threshold level depending on the frequency characteristics. In this way, the threshold level is calculated. Consequently, by the switch control part 18, the changeover switch 17 is set to the D/A converter 3 side. Thereafter, the howling detecting and suppressing actions are the same as those in the embodiment shown in Fig. 7. Thus, using the threshold level calculated on the basis of the frequency characteristics measured by the frequency characteristics measuring part 15, howl is detected, so that howl can be detected more precisely depending on the room conditions, or the frequency characteristics of the room in which the microphone and speaker are placed. In the explanation of the foregoing embodiments, the notch filter is used in the howl suppressing part 6, but the same effects are obtained by using an FIR (finite impulse response) filter.

Claims: A sound amplifying apparatus comprising: a microphone (1) for picking up a sound to obtain a sound signal; an analog-to-digital converter (2) for converting the sound signal from the microphone to a digital sound signal; howl suppressing means (6) including a digital filter for processing the digital sound signal; a digital-to-analog converter (3) for converting a processed digital sound signal from the howl processing means to a processed analog signal; an amplifying means (4) for amplifying the processed analog sound signal to obtain an amplified sound signal; a speaker (5) responsive to the amplified sound signal for generating an amplified sound; frequency analyzing means (7) for frequency analyzing the digital sound signal from the analog-to-digital converter in real time; howl detecting means (10) for detecting a howl contained in the sound signal from a result of frequency analysis by the frequency analyzing means; operation means (8) for calculating coefficients to be set to the digital filter to suppress the howl according to a detection result by the howl detecting means; and control means (9) for setting the calculated coefficients to the digital filter, wherein the howl detecting means (10) detects a maximum peak power level among power levels of the sound signal in a frequency region analyzed by the frequency analyzing means; and characterised in that the howl detecting means (10) calculates an adjusted mean power level of the sound signal by omitting first to m-th largest peak power levels from all power levels in the frequency region, where m is a predetermined integer larger than 0, and calculating a mean value of the remaining power levels, calculates a ratio of the maximum peak power level to the adjusted mean power level, and judges that the maximum power level is a howl component when the ratio is larger than a predetermined threshold level. An apparatus according to claim 1, wherein the howl detecting means (10) judges that the maximum peak power level among the power levels of the sound signal in the frequency region analyzed by the frequency analyzing means (7) is a howl component when a frequency of the maximum peak power level is maintained constant for a predetermined period of time. An apparatus according to claim 1, further comprising threshold control means (11, 12) for controlling the threshold level. An apparatus according to claim 1, wherein the howl detecting means (10) judges that the maximum power level is a howl component when the ratio is larger than a threshold level for a threshold time, and wherein said apparatus further comprises threshold control means (11, 12) for controlling the threshold level and the threshold time. An apparatus according to claim 3 or 4, wherein the threshold control means (11, 12) is responsive to the result of frequency analysis by the frequency analyzing means (7) for controlling the threshold level to be a level dependent on a frequency band in which the frequency of the maximum peak power level is located. An apparatus according to claim 3 or 4, wherein the threshold control means (11, 12) has prepared therein a plurality of threshold levels for a plurality of frequency bands in the frequency region analyzed by the frequency analyzing means (7) depending on a frequency characteristic of a background noise contained in the frequency region, and selects one of the plurality of threshold levels depending on the frequency band in which the frequency of the maximum peak power level is located in response to the result of frequency analysis by the frequency analyzing means. An apparatus according to claim 3 or 4, wherein the threshold control means (11, 12) is responsive to the result of frequency analysis by the frequency analyzing means (7) for changing the threshold level depending on a frequency characteristic of the sound signal. An apparatus according to claim 3 or 4, further comprising a voice judging means (19) responsive to the result of frequency analysis by the frequency analyzing means (7) for judging whether the picked-up sound is a voice or not, wherein the threshold control means (11, 12) is responsive to a judging result by the voice judging means for changing the threshold level in a frequency band containing voice pitch frequency components when the picked-up sound is a voice. An apparatus according to claim 8, wherein the threshold control means (11, 12) increases the threshold level in the frequency band containing voice pitch frequency components when the picked-up sound is a voice. An apparatus according to claim 3 or 4, further comprising a frequency characteristic measuring means (15) for measuring a frequency characteristic of a room in which the microphone (1) and the speaker (5) are located from a position of the speaker to a position of the microphone, wherein the threshold control means (11, 12) is responsive to a measuring result by the frequency characteristic measuring means for changing the threshold level depending on the frequency characteristic of the room. An apparatus according to claim 4, further comprising an echo measuring means (13) for measuring an echo time in a room in which the microphone (1) and the speaker (5) are located, wherein the threshold control means (11, 12) is responsive to a measuring result by the echo measuring means for changing the threshold time depending on the echo time.

Inventors: KAWAMURA AKIHISA, MATSUMOTO MASAHARU, NUMAZU HIROKO, SERIKAWA MITSUHIKO, KAWAMURA, AKIHISA, MATSUMOTO, MASAHARU, NUMAZU, HIROKO, SERIKAWA, MITSUHIKO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1993
Main classification: H04R3
IPC codes: H04R3