Abstract: The feedback caused between the output and the input of an amplification path is reduced by providing a delay in the amplification path, passing through the amplification path a signal having an auto-correlation function which is substantially a delta function, correlating the said signal before being delayed in the delay with the signal after being delayed in the delay to produce a plurality of correlation coefficients, modifying the signal in the amplification path to provide a modified signal, the modification being provided by a transversal filter controlled by the said plurality of correlation coefficients, and combining the modified signal with the signal in the amplification path so as to reduce the effect of the feedback.

Abstract: A howling detection and prevention circuit which receives an output of a microphone as its input signal and detects howling therein includes a computing section which divides frequency of the input signal into a plurality of frequency bands on the basis of a predetermined sampling period and computes power of each of the frequency bands, an identifying section which sequentially shifts the frequency band and identifies whether howling exists or not in accordance with a predetermined condition by employing a value of the computed power of each frequency band, and a gain adjusting section which, when howling has been detected as a result of the identifying, adjusts gain of the frequency band in which the howling has been detected to prevent the howling. A loudspeaker system employing this howling detection and prevention circuit is also provided.

Abstract: Sound is converted into an electrical signal by a microphone and is converted into an inaudible, digitally modulated signal that is combined with the electrical signal from the microphone, amplified, and converted into sound waves by a speaker. Any sound traveling from the speaker back to the microphone includes the inaudible component representing the original sound. The inaudible component is separated from the audible components, and the original sound is reconstructed in a digital demodulator. The reconstructed original sound is subtracted from the signal from the microphone, thereby reducing any echo and canceling feedback. Digital modulation includes any form of shift keying, including coherent and noncoherent techniques for modulating frequency, phase, or amplitude.

Abstract: Disclosed howling eliminating apparatus comprises frequency analyzing means 32 for detecting a frequency which causes howling, band eliminating means 20 for eliminating a band including the howling frequency detected by the frequency analyzing means, and controlling means 30 for controlling both the frequency analyzing means and the band eliminating means in their characteristics. The frequency analyzing means is controlled to divide input frequencies into a plurality of wide bands, detect howling status in each of the divided wide bands to isolate one wide band suspected to include the howling frequency. When the one wide band causing the howling is isolated, the frequency analyzing means divides the suspected wide band into a plurality of narrow bands, and detects the howling frequency in one of the narrow bands. The band eliminating means is controlled to eliminate the narrow band including the howling frequency. The apparatus thus shortens the time it takes to detect howling with a minimum of hardware.

Abstract: The present invention is embodied in a method and apparatus for eliminating audio feedback which includes an active microphone coupled to an amplifier which transmits signals received at the active microphone to a parametric speaker for broadcasting. The apparatus comprises at least one transducer for detecting at least one sonic frequency and generating an electrical signal representative of the at least one sonic frequency, a processor for receiving the electrical signal and generating a first ultrasonic frequency which has modulated thereon the at least one sonic frequency, a parametric demodulator for recovering the at least one sonic frequency from the first ultrasonic frequency, and a speaker for directly emitting the at least one sonic frequency.

Abstract: Techniques for reducing unwanted acoustic feedback in a space are carried out by an adaptive notch filter algorithm that adjusts a notch to a plurality of different notch values in order to locate feedback. The results obtained by performing the algorithm at various notch values are compared. Based on the comparison, the parameters for the algorithm are adjusted for processing of the input signals to reduce the feedback.

Abstract: An acoustic transmission line speaker enclosure with concentric cylindrical structures establishes acoustic coupling between a rear-traveling sound wave and a surrounding air mass. Inherent rigidity or high bending resistance of the cylindrical structure allows use of very thin walled cylinders without a massive and large overall enclosure. An audio amplifier tunable to a listening room removes very low narrow frequency band components of an audio signal. Listening room cavity resonance is measured by injecting a frequency-varying sound wave into the listening room while detecting peak sound energy within the room. The filter the eliminates from audio signal frequencies associated with listening room cavity resonance.

Abstract: A method (FIGS. 6-8) for detecting and attenuating N feedback frequencies in a digitized signal uses a tree structure containing a plurality of staged filters. In a step (602), an array of digital filters (FIG. 8) having N branches (40) is constructed. The array is arranged in a tree structure with each branch (40) having several stages (42, 44, and 46). Many of the N filters are used simultaneously in multiple different branches of the tree structure thus reducing the total number of filters required to detect all N feedback frequencies. Within each branch, N-1 of the N filters are notch filters, and each of the N- 1 notch filters attenuates the digitized signal at one of the N feedback frequencies. The remaining one filter in each of the N branches is a bandpass filter that passes the remaining of the N feedback frequency. Therefore, each branch of the tree passes a unique feedback frequency absent of all other N-1 feedback frequencies.

Abstract: Noise reducing circuits for electronic receiving instruments, such as telephone receivers in headsets or handsets that are used in noisy locations, provide compensation of the set's receiver unit-to-error microphone transfer function to enhance the noise reduction. Further circuits provide pre-conditioning of the incoming voice signal to make the noise cancellation more effective. The tendency of these noise cancelling circuits to oscillate is substantially lessened by added circuitry which rapidly detects onset of oscillation and momentarily reduces the noise cancellation without interrupting the incoming speech path altogether.

Abstract: Echo suppression is accomplished by comparing the ratio of the microphone (101) and speaker (121) energies to a dynamic threshold that varies with the background noise (310) at a second communication device (130), and transmitting locally generated noise when the potential for echo exists. The use of the dynamic threshold provides for echo suppression with significantly better doubletalk performance.

Abstract: A signal amplification system wherein a signal coming from a microphone is applied to a loudspeaker via a signal processing system. The signal processing system includes an estimator for estimating the transfer function from the loudspeaker to the microphone. The equalizer is set to a transfer function that is reciprocal to the transfer function from the loudspeaker to the microphone to equalize the transfer function from the microphone to the listeners. The estimator may include an adaptive filter which has a transfer function that approaches the transfer function from the loudspeaker to the microphone.

Abstract: In a signal amplifier system, a microphone (2) is connected to an echo canceller (16) via a decorrelator (6). The output signal of the echo canceller (16) is amplified by an amplifier (14) and fed to a loudspeaker (18). The echo canceller (16) is included to avoid instability caused by undesired feedback of the signal coming from the loudspeaker (18) through a feedback path (11). To improve the stabilizing effect of the echo canceller (16), the decorrelator (6) is included for decorrelating the signal coming from the microphone (2) and the signal transmitted by the loudspeaker (18).

Abstract: In an apparatus for amplifying a sound while suppressing a howling noise, a microphone collects a sound and converts the collected sound into a corresponding input signal. An amplifier amplifies the input signal to generate the sound through a loudspeaker. A plurality of equalizers are interposed between the microphone and the amplifier, each equalizer having a variable attenuation frequency for attenuatively filtering the input signal around the attenuation frequency. Detection is conducted for detecting a plurality of noise spectrum peaks of the howling noise contained in the collected sound. Then, an adjustment is conducted adjusting respective variable attenuation frequencies of the plurality of the equalizers correspondingly to the plurality of the detected noise spectrum peaks to thereby remove the howling noise from the generated sound.

Abstract: Acoustic feedback is removed from an audio signal (50) by digitizing the audio signal (50) to produce a digitized audio signal (54). The digitized audio signal (54) is then filtered with an adaptive bandpass filter (56) to detect the frequency of the acoustic feedback, where the adaptive bandpass falter (56) is aligned with the feedback based on a phase relationship between the input and the output of the adaptive bandpass filter (56). A notch filter (58) is then configured based on the frequency of the acoustic feedback, and the digitized audio signal (54) is filtered with the notch filter (58) to attenuate the feedback. The feedback-attenuated digitized signal (62) is converted to a feedback-attenuated analog signal (70).

Abstract: A howling prevention apparatus prevents a howling noise which is generated in a sound amplification system using a microphone and a loudspeaker. The howling prevention apparatus utilizes a plural number of first to n-th equalizers, which have a variable frequency response and which modulate an output of the microphone. A detector detects a frequency point at which a loop gain of the system reaches a peak while supplying a standard noise signal to the loudspeaker to produce a test sound and measuring the sound collected by the microphone through the series of the equalizers. A frequency response of the first equalizer is adjusted to suppress a noise peak at and around a frequency point detected by the detector. Then, a frequency response of the second equalizer is also adjusted to suppress a noise peak at and around another frequency point detected by the detector. Lastly, a frequency response of the n-th equalizer is adjusted by subsequently repeating the above setting procedure.

Abstract: An audio system for multimedia computer systems and method for use are provided including an audio module having speakers, a microphone and associated circuitry. The audio module is adapted to be interposed between a monitor and a monitor pedestal such that the audio module swivels and tilts in concert with the monitor. The circuitry is adapted to drive the audio module speakers and to drive only one speaker and the microphone in a speaker phone mode.

Abstract: A hearing aid with digital, electronic compensation for acoustic feedback includes a microphone, a preamplifier, a digital compensation circuit and output amplifier and a transducer. The digital compensation circuit includes a noise generator for the insertion of noise, and an adjustable, digital filter for the adaptation of the feedback signal. The adaptation takes place using a correlation circuit which includes a digital circuit to carry out a statistical evaluation of the filter coefficients in the correlation circuit, and changes the feedback function in accordance with this evaluation.

Abstract: An A/D converter converts an analog signal to a digital signal. A plurality of cascade-connected notch filters include a first notch filter which is connected to the output of the A/D converter. A D/A converter is connected to the output of the last stage notch filter for converting a digital signal to an analog signal. The output of the last stage notch filter is connected to the input of a fast Fourier transform unit for analyzing the frequency. Analysis results of the fast Fourier transform unit are supplied to a detector. A coefficient having the same center frequency as that of a peak frequency outputted from the detector is selected from a coefficient memory and it is transferred to a second coefficient memory. Thus, the frequencies of the notch filters are set to eliminate howling.

Abstract: An audio interface garment includes systems for attenuating the influence of sound waves generated by audio output devices on output signals from a plurality of input devices an. In one embodiment, input signals which are applied to the audio output devices are combined by a mixer to form a mixed audio signal. A plurality of Widrow-Hoff least mean square adaptive filters each form a corresponding filtered signal based upon the mixed audio signal and the output signal from a corresponding one of the input devices. A plurality of processed signals are formed by differencing each filtered signal from the corresponding output signal. The weight values of the adaptive filters are modified according to the least mean square method. The processed signals provide signals in which the first sound waves are attenuated.

Abstract: The acoustical-mechanical disturbance feedback between the electrical-acoustical converter and the acoustical-electrical converter of a hearing aid apparatus is compensated by means of an adaptive compensator filter which feeds back a signal derived from the output of an amplification filter to its input. At the input side thereby the signal from the acoustical-to-electrical converter and the output signal of the adaptive compensator filter are substracted at a difference forming unit, the output of which being led to the input of the amplification filter. The difference is thereby formed in time domain, and time domain to frequency domain transform is performed at the output side of the difference forming unit, accordingly inverse frequency domain to time domain transform at the electric input side of the electrical-to-acoustical converter.

Abstract: Sound is converted into an electrical signal by a microphone and is converted into an inaudible, pulse width modulated signal that is combined with the electrical signal from the microphone, amplified, and converted into sound waves by a speaker. The pulse width modulator includes an A/D converter coupled to a shift register in a digital encoder. Any sound travelling from the speaker back to the microphone includes the inaudible component representing the original sound. The inaudible component is separated from the audible components, and the original sound is reconstructed in a pulse width demodulator including a shift register in a digital decoder coupled to a D/A converter. The reconstructed original sound is subtracted from the signal from the microphone, thereby reducing any echo and cancelling feedback. The apparatus includes amplitude correction circuitry for flattening the frequency response of the apparatus and includes phase correction circuitry for eliminating phase shifts in the apparatus.

Abstract: A sound field control device includes an acoustic feedback signal path including a chamber and one or more loudspeaker or loudspeakers and one or more microphone or microphones provided in the chamber, the loudspeakers being disposed at a predetermined distance from the microphones, and at least either the loudspeakers or the microphones being provided in the plural, an electrical feedback signal path for feeding back a collected sound signal collected by the microphone or microphones to the loudspeaker or loudspeakers, and a collected sound signal supply control circuit provided in the electrical feedback signal path for changing, with lapse of time, a signal level of each collected sound signal fed back from the microphones to the loudspeakers. The transmission route of the collected sound signal is completely switched or the ratio of distribution of the collected sound signal is changed. Coloration in hearing is thereby reduced and a margin of howling is expanded without causing unnaturalness in hearing.

Abstract: A method for active noise reduction based on destructive interference of sound waves in order to reduce the energy in a sound field employs two omnidirectional microphones (M1, M2) provided in connection with a loudspeaker. The acoustic feedback of the microphones is eliminated by a closed loop consisting of the microphones and the loudspeaker. The loudspeaker used is an open loudspeaker with a dipole characteristic, thus causing one of the microphones to be more sensitive to the far field and thereby to the noise which has to be suppressed. The method is implemented by a device which comprises a digital signal processor (DSP) for processing the microphone signals and which transmits an output signal to the loudspeaker where the feedback component from the loudspeaker is substantially eliminated, while the output signal's phase and amplitude are adjusted in such a manner that an effective cancellation of the noise is obtained in an area around the loudspeaker's near field.

Abstract: Disclosed is a mixer console which is capable of providing a mix-minus audio monitor signal to several locations from which an input audio signal is provided. The mix-minus audio signal consists of the sum of all active input channels except for the input channel to which the mix-minus audio signal is provided. Thus, the mix-minus signal permits monitoring without feedback or echo effects.

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. A coefficient calculator calculates coefficients to be input to the digital filter to suppress the howl according to a detection result by the howl detector, and a controller inputs 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.

Abstract: An audio processing system for use in teleconferencing systems having high and low transmission delays. The audio processing system utilizes complementary comb filters in combination with auxiliary echo suppressors, a frequency scaler, expander/gates and automatic level controlling devices to substantially improve acoustic stability margin and reduce far-end talker echo.

Abstract: A two channel audio switcher includes two analog switches, which may be implemented as a CMOS integrated circuit, receiving at their control input ports a complementary pair of drive signals, typically square wave, so as to toggle their outputs at a supersonic rate, which may be made variable. The switcher may receive as input two channels of a stereophonic source, two independent audio sources or a common monophonic source. The outputs typically drive an audio output unit such as the transmitter of a radio broadcasting system, an audio recording system, or amplifiers, controls and loudspeakers of a sound reinforcement system in an auditorium or studio. The system may be monophonic or stereophonic. In a stereophonic a.m. broadcasting system such as the Kahn independent sideband system, alternate switching allows each sideband to be modulated to full available power, thus allowing an increase in the apparent radiated power.

Abstract: A sound system with a howling-prevention function comprises an all-pass filter having group delay characteristics, which vary with the elapse of time, provided on a line used for the transmission of an audio signal from a microphone. Owing to the provision of such a sound system, any deterioration in the sound quality with respect to high frequencies and a chorus phenomenon is not produced, thereby making it possible to prevent howling without causing inferior sound quality.

Abstract: An improved echo cancelling device for reducing the effects of acoustic feedback between a loudspeaker and microphone in a communication system. The device includes an adjustable filter for receiving a loudspeaker signal and generating in response thereto an echo estimation signal. The device subtracts the echo estimation signal from the microphone signal to produce an echo corrected microphone signal. During periods of time when the microphone signal is substantially derived from acoustic feedback between the loudspeaker and the microphone, the device adjusts transfer characteristics of the filter to reduce the echo corrected microphone signal. The improvement includes estimating from the adjusted transfer characteristics an energy transfer ratio representative of the ratio of the energy of the microphone signal to the energy of the loudspeaker signal.

Abstract: A system for reproducing a sound field simulating a real acousting space includes a sound collecting device, a sound recording device and a sound reproducing device. The sound collecting device includes first and second collection systems, the first collection system having non-directional microphones at respective corners of a cube defining a hypothetical three-dimensional space in the real acoustic space and the second collection system having directional microphones of the same number as the non-directional microphones concentrically within the three-dimensional space with directivity of each of the directional microphones being set in the direction of one of the non-directional microphones with which the directional microphone constitutes a pair. Collected sounds by the first and second collection systems of the sound collection device are mixed with each other and recorded on a recording medium by the sound recording device, and further provided on the sound reproducing device.

Abstract: A hearing aid includes a filter in an electrical feedback path, the characteristics of which filter are calculated to model acoustic coupling between the receiver and microphone of the aid. A limiter is inserted in the main electrical pathway between the microphone and the receiver to provide stability in the presence of sudden sound bursts. A noise signal is injected continuously into the electrical circuit and is used to adapt the characteristics of the filter to accommodate changes in the acoustic coupling. The level of the noise signal can be varied to match changes in residual signal level to maintain signal to noise ratio and to optimize rate of adaption commensurate with satisfactory hearing function while the noise itself is unobtrusive to the user.

Abstract: A sound field is generated responsive to direct sound picked up by a direct-sound collecting microphone and indirect-sound picked up by an indirect-sound collecting microphone. Respective processing circuits are provided for signal processing the sound signals of the direct-sound collecting microphone and the indirect-sound collecting microphone. An adaptive filter is provided having a transfer function equal to a transfer function from a reproducing speaker to the indirect-sound collecting microphone. The output of the signal processing circuit associated with the indirect-sound collecting microphone is filtered by the adaptive filter, and the output of the adaptive filter is subtracted from the output of the indirect-sound collecting microphone. The output of the subtractor is then applied to the signal processing circuit associated with the indirect-sound collecting microphone for signal processing.

Abstract: A feed-back noise-eliminating microphone circuit is provided. The circuit includes a positive-phase pick-up circuit to pick up sound source signal for amplification through a positive-phase amplifier for further output through a balance adjusting circuit and a power amplifier. The circuit further includes a reverse-phase pick-up circuit to pick up feedback signals from the loudspeaker for amplification through a reverse-phase amplifier, wherein feedback noise from the reverse-phase amplifier is offset at said balance adjusting circuit by the output signal from the positive-phase amplifier.

Abstract: A preferred duplex communications device includes a speaker for transmitting a signal received from a remote source, first and second microphones for transducing received acoustical signals wherein the first microphone is positioned significantly closer to the speaker than the second microphone so that the first microphone receives significantly more acoustical signal strength from the speaker than the second microphone. A first amplifier is connected to the first microphone and a second amplifier is connected to the second microphone wherein the second amplifier has a gain relative to a gain of the first amplifier such that the output signals of the first and second amplifiers are generally equal with respect to signals received by way of the speaker. A filter is then connected to the outputs of the first and second amplifiers to filter the signals transduced from the speaker thereby preventing transmission to the remote source of essentially all of the signals from the speaker.

Abstract: A system for controlling the second field in auditorium having the feature that stage and audience seating areas are different acoustically, which includes a first assisted acoustics system whereby acoustical energy from the stage area is input, and then controlled acoustic energy is supplied to the audience seating area, and a second assisted acoustics which is provided independently of the first electronic acoustical augmentation system, whereby acoustical energy from the audience seating area is input, and then controlled acoustic energy is supplied to the stage area. Each assisted acoustics system includes acoustic energy input devices and acoustic energy output devices whereby a uniform rate of power decay coefficient can be effected throughout the hall, including spaces under balconies and the like. Significantly improved the degree of acoustic similarity between the stage area and audience seating area is achieved by controlling reverberation characteristics.

Abstract: A loudspeaking telephone station (speakerphone) includes a loudspeaker and one or more microphones within the same housing. The microphones are directional, each having a polar response characteristic that includes a major lobe, one or more side lobes, and nulls between pairs of lobes. The loudspeaker is positioned in the null of the polar response characteristic that resides between the major lobe and an adjacent side lobe. The microphone apparatus is positioned so that its major lobe is aimed in a direction that is generally perpendicular to the direction that the loudspeaker is aimed. Means are provided for increasing the distance between input sound ports of a first-order-gradient (FOG) microphone and thereby improving its sensitivity. A pair of such improved FOG microphones are used in assembling a second-order-gradient microphone. Full duplex operation is achieved when a pair of echo cancelers are added to further reduce the coupling between the transmit and receive directions of the speakerphone.

Abstract: Electro-acoustic system for improving the acoustic of a predetermined room, said system comprising a microphone array having a plurality of microphones and a loudspeaker array having a plurality of loudspeakers, as well as a signal processing unit, interposed between said arrays, said signal processing unit having means for generating reflections, whereby at least one of the microphones is directed in such a manner that it receives at least reflected sound from a sound source in the predetermined room and/or that at least one of the loudspeakers is directed at a reflecting surface in the predetermined room.

Abstract: For improved sound reinforcement, especially of the human voice, in situations where clarity and full audience coverage are important, an integrated structure supports a microphone and a speaker unit in an anti-feedback spatial relationship. The single speaker unit, mounted in an elevated position facing the audience, acts as a well-localized "point source", the microphone being located nearby in a working location to the rear of speaker. Despite this proximity, acoustic feedback and resultant "howl" tendencies are suppressed by providing both the microphone and the speaker unit with hypercardioid directivity and locating them in an offset back-to-back relationship such that alignment of their "null" regions combines their attenuation, minimizing feedback. In one embodiment, a lectern table supports the microphone in a working position and mounts an offset vertical pole carrying the speaker unit. The system amplifier may be housed within the lectern table.

Abstract: A reverberation imparting device for electro-acoustically enhancing reverberation in acoustic space comprises a microphone disposed in the acoustic space, a loudspeaker disposed in the acoustic space for diffusing the sound picked up by the microphone, and feedback means comprising a signal processing circuit for electrically processing an electric signal corresponding to the sound picked up by the microphone, an output of the signal processing circuit being supplied to the loudspeaker. The microphone, feedback means and loudspeaker form a feedback loop. The signal processing circuit comprises a circuit for subjecting impulse responses of finite length to a convolution operation. Time axis of reflected sounds is extended and extension of reverberation time thereby is realized without depending upon loop gain. Density of reflected sounds is increased by subjecting impulse responses of finite length to a convolution operation whereby separation of reflected sounds is prevented.

Abstract: An electronic filter for an electroacoustic system. The system has a microphone for generating an electrical output from external sounds and an electrically driven transducer for emitting sound. Some of the sound emitted by the transducer returns to the microphone means to add a feedback contribution to its electical output. The electronic filter includes a first circuit for electronic processing of the electrical output of the microphone to produce a filtered signal. An adaptive filter, interconnected with the first circuit, performs electronic processing of the filtered signal to produce an adaptive output to the first circuit to substantially offset the feedback contribution in the electrical output of the microphone, and the adaptive filter includes means for adapting only in response to polarities of signals supplied to and from the first circuit.

Abstract: A muting circuit for a transconductance amplifier has a switch connected across the signal load of the amplifier to short circuit the output.

Abstract: A voice enhancer system for a car includes a pair of microphones positioned adjacent to the front seats of the car, and a pair of loudspeakers positioned adjacent to the rear seats. The microphones are connected to the loudspeakers, via an amplifier/electrical conditioning unit, by electrical wiring positioned behind the car's upholstery. The conditioning unit, which includes filters and a frequency shifter is adjusted to prevent acoustical `howl-around` within the enclosure of the car. For voice enhancement from the rear to the front of the car, microphones may be positioned adjacent to the rear seats, and loudspeakers may be positioned adjacent to the front seats, these components being connected together, via a second amplifier/electrical conditioning unit, by electrical wiring.

Abstract: A howling protective apparatus comprising a phase fluctuation control means, a wave peak comparison means and a timer circuit, wherein said phase fluctuation circuit is controlled by an output from said timer circuit; a howling protective apparatus comprising a gain control means, a wave peak comparison means and a changing means for varying a coefficient of integration; a howling protective apparatus comprising a phase fluctuation circuit, a variable gain circuit and a wave peak comparison circuit, wherein said phase fluctuation circuit and the variable gain circuit are controlled by an output from said wave peak comparison circuit.

Abstract: A modular acoustic sound system for a room for two-way conversations without the need for switching. The system includes pairs of microphones each connected to a phase shifter so that in-phase signals inputted to the microphone pair are shifted 180.degree. apart. The out-of-phase signals are input to a summing circuit having an amplified output which drives a plurality of loudspeakers. A portion of the audible loudspeaker outputs enters the microphone pair, is phase shifted, and cancelled out in the summing amplifier. Any number of loudspeaker/microphone pairs operate in the system and any microphone may be spoken into to drive every loudspeaker.

Abstract: An amplifier in which the gain (A.sub.i) is automatically controlled to a maximal value comprises means (D.sub.i) for detecting ringing tones and means (V.sub.i) for reducing the gain upon the detection of ringing tones. Also proposed is a circuit (D.sub.i) for detecting ringing tones. A dynamic equilibrium can be obtained by alternately reducing the gain in the event of ringing tones and raising the gain in the absence of ringing tones.

Abstract: A circuit arrangement for suppressing oscillations, such as acoustic feedback in a hearing aid, has a circuit which recognizes the presence of oscillations in a useful signal, an oscillatory frequency search circuit, and an oscillation modifying circuit controlled by the search circuit. The oscillation modifying circuit suppresses oscillations by filtering. Drift effects are avoided by a frequency clamp-on sub-circuit in the search circuit, which retains the frequency of the recognized oscillation at the modifying circuit, even when the oscillatory signal at the input of the search circuit disappears.

Abstract: A sound system, for a band or the like, incorporates an anechoic enclosure which allows a performer to use the same desired speaker color under all performing conditions and which insulates the audience and other pick-up microphones from each sound-source's speaker and insulates the pickup microphone for each sound source from the ambient room sounds. According to the system, a sound source, such as a musical instrument, outputs a first sound signal which is amplified and fed to a speaker in an anechoic enclosure. In response to the speaker output, a microphone in the anechoic chamber outputs a second sound signal which is amplified and distributed to room speakers. In its exemplary embodiment, the anechoic enclosure comprises four laminated layers: an inner layer of rigid closed-cell foam surrounding a chamber; a layer of denser flexible, closed-cell foam; a second layer of rigid closed-cell foam; and an outer hard, rigid layer. The enclosure may be opened so that the speaker therein may be heard.

Abstract: A communication system, such as a hearing aid or a public address system, may include a microphone for inputting audio information to the system, an amplifier for amplifying audio frequency signals inputted to the microphone, and a speaker for outputting amplified audio frequency signals into the environment which provides an acoustic feedback path between the speaker and the microphone. The invention provides an identification circuit for dynamically identifying those parameters associated only with acoustic feedback, and a correction circuit whose transfer function is established in accordance with the parameters identified by the identification circuit. The transfer function of the correction circuit is such that the effect of acoustic feedback is cancelled from the transfer function of communication system. The identification circuit is constructed and arranged to identify said parameters in response to a turn-on of the system, or to an automatically-sensed threshold change in gain of the amplifier.

Abstract: An automatically controlled amplifier arrangement comprises a controlled amplifier unit (35), a detection unit (2) for detecting oscillations of the arrangement in the case of acoustic feedback during operation, and a control unit (3) which, in response to the detection unit (2), controls the gain of the amplifier unit (35). The controlled amplifier unit comprises separate control means for separately controlling the gain for different frequency bands (1', 1", 1'") below the howling limit. Prior to this a pre-adjustment is effected. Further, a circuit is proposed which increases the gain during preadjustment until oscillation occurs and which subsequently reduces the gain (if desired for each frequency band) until an adjustable margin below the howling limit is obtained (if desired for each frequency band).

Abstract: An amplifier for acoustic signals provided with means for suppressing undesired acoustic feedback signals in a rapid manner. The amplifier includes a high-pass filter which separates a part of the signal to be amplified to obtain a control signal for reducing the gain of the amplifier when an acoustic feedback signal is detected. The output of the high-pass filter is connected to a periodicity detector, e.g. a single peak detector, which discriminates between an acoustic feedback signal having a regular periodicity and a signal having an irregular character, such as noise or speech. The control signal is derived from the output of the periodicity detector.