Abstract: A sound management enclosure, production, and recapture device may include a generally symmetrical design and may be constructed and arranged to optionally internally mount components therein. The sound management device may be constructed and arranged to alter, focus, or dissipate sound waves within a predictable environment in order to enhance the associated effects. Sound capturing devices, such as microphones, may be placed within the sound management enclosure for transmission to an external system for recording or projection.

Abstract: An audio encoding and decoding method includes obtaining a channel combination scheme for a current frame, obtaining an encoding mode of the current frame based on a downmix mode of a previous frame and the channel combination scheme for the current frame, performing time-domain downmix processing on left and right channel signals of the current frame based on the encoding mode of the current frame to obtain primary and secondary channel signals of the current frame, and encoding the primary and secondary channel signals of the current frame.

Abstract: A multi-channel audio decoder for providing at least two output audio signals on the basis of an encoded representation is configured to perform a weighted combination of a downmix signal, a decorrelated signal and a residual signal, to obtain one of the output audio signals. The multi-channel audio decoder is configured to determine a weight describing a contribution of the decorrelated signal in the weighted combination in dependence on the residual signal. A multi-channel audio encoder for providing an encoded representation of a multi-channel audio signal is configured to obtain a downmix signal on the basis of the multi-channel audio signal, to provide parameters describing dependencies between the channels of the multi-channel audio signal, and to provide a residual signal. The multi-channel audio encoder is configured to vary an amount of residual signal included into the encoded representation in dependence on the multi-channel audio signal.

Abstract: The present document relates to audio decoding and/or audio transcoding. In particular, the present document relates to a scheme for efficiently decoding a number M of audio channels from a bitstream comprising a higher number N of audio channels. In this context a method and system for merging a first and a second source set of spectral band replication (SBR) parameters to a target set of SBR parameters is described. The first and second source set comprise a first and second frequency band partitioning, respectively, which are different from one another. The first source set comprises a first set of energy related values associated with frequency bands of the first frequency band partitioning. The second source set comprises a second set of energy related values associated with frequency bands of the second frequency band partitioning. The target set comprises a target energy related value associated with an elementary frequency band.

Abstract: An audio decoding device includes a processor; and a memory which stores a plurality of instructions, which when executed by the processor, cause the processor to execute, decoding, using a first channel signal and a second channel signal included in a plurality of channels of an audio signal having a first frequency range and a second frequency range, a first prediction coefficient of the first frequency range and a second prediction coefficient of the second frequency range, both selected from a code book when prediction-encoding a third channel signal that is not subjected to prediction encoding and that is included in the plurality of channels; decoding a residual signal included in the first frequency range, the residual signal representing an error occurring in prediction encoding; and prediction-decoding the third channel signal subjected to prediction-encoding in the second frequency range from the first channel signal, the second channel signal.

Abstract: A system and method for switching a mode of a receiver between a monophonic and stereophonic mode is disclosed. The method can include generating a residual signal indicative of a noise level associated with a pilot energy signal, calculating a residual block energy level of the residual signal over an observation interval, generating a monitor signal based on a number of times the pilot energy signal is less than a pilot energy threshold during the observation interval and switching the mode of the receiver based on the residual block energy level and the monitor signal.

Abstract: A receiver may estimate a channel condition based on a difference between a pilot energy signal and a filtered pilot energy signal. The receiver may further switch to a stereophonic mode when both the difference and an energy level of the pilot energy signal satisfy a predetermined requirement of the receiver, and switch to a monophonic mode when either the difference or the energy level of the pilot energy signal fail to satisfy the predetermined requirement of the receiver.

Abstract: An audio receiver may include a mono/stereo detector that causes the audio receiver to output either a monophonic or a stereophonic signal based on a difference between a pilot energy signal and a filtered pilot energy signal. The audio receiver includes a filter that filters the pilot energy signal to generate a filtered pilot energy signal, wherein a variable leakage factor associated with the filter is used to minimize a noise level of the filtered pilot energy signal and to reduce a response time of the audio receiver.

Abstract: A method of encoding an audio signal, where signals including two or more channel signals are downmixed to a mono signal, the mono signal is divided into a low-frequency signal and a high-frequency signal, the low-frequency signal is encoded through algebraic code excited linear prediction (ACELP) or transform coded excitation (TCX), and the high-frequency signal is encoded using the low-frequency signal. A method of decoding of an audio signal, a low-frequency signal encoded through ACELP or TCX is decoded, a high-frequency signal is decoded using the low-frequency signal, the low-frequency signal and the high-frequency signal are combined to generate a mono signal, and the mono signal is upmixed by decoding spatial parameters regarding signals including two or more channel signals.

Abstract: A multi-channel synthesizer includes a post processor for determining post processed reconstruction parameters or quantities derived from the reconstruction parameter for an actual time portion of the input signal so that the post processed reconstruction parameter or the post processed quantity is different from the corresponding quantized and inversely quantized reconstruction parameter in that the value of the post processed reconstruction parameter or the derived quantity is not bound by the quantization step size. A multi-channel reconstructor uses the post-processed reconstruction parameter for reconstructing the multi-channel output signal.

Abstract: An audio subsystem in a mobile communication device has a speaker. In one implementation, the audio subsystem has a higher-gain power amplifier, a lower-gain power amplifier, and a switching system configured to route signals produced by the lower-gain power amplifier to the speaker while the device is operative in a first mode and configured to route signals produced by the higher-gain power amplifier to the speaker while the device is operative in a second mode. In another implementation, the audio subsystem has a power amplifier coupled to the speaker and a power supply control controlled to provide voltage at a first voltage level to the power amplifier while the device is operative in a first mode and controlled to provide voltage at a second, higher, voltage level to the power amplifier while the device is operative in a second mode.

Abstract: A method of encoding an audio signal, where signals including two or more channel signals are downmixed to a mono signal, the mono signal is divided into a low-frequency signal and a high-frequency signal, the low-frequency signal is encoded through algebraic code excited linear prediction (ACELP) or transform coded excitation (TCX), and the high-frequency signal is encoded using the low-frequency signal. A method of decoding of an audio signal, a low-frequency signal encoded through ACELP or TCX is decoded, a high-frequency signal is decoded using the low-frequency signal, the low-frequency signal and the high-frequency signal are combined to generate a mono signal, and the mono signal is upmixed by decoding spatial parameters regarding signals including two or more channel signals.

Abstract: The present invention includes apparatuses and methods comprising a means for detecting the presence of speakers and microphones coupled to a portable multi-function device (such as Apple's iPhone™). In response, a portable multi-function device can adapt its output depending on the nature of the coupled headset device. In particular, a portable multi-function device containing the present invention can, upon detecting only one speaker in a coupled headset accessory device, combine the multiple channels of a stereo audio signal into a single mono audio signal. Likewise, a portable multi-function device containing the present invention can alert users to the absence of a coupled microphone.

Abstract: Signal of a channel selected in accordance with a cue instruction is supplied to a cue bus. Cue signal processing section provided in the cue bus can perform one or more signal processing operations, selected from among a predetermined plurality of different signal processing operations, such as “Delay”, “Insert” and “Equalizer”. For each of a plurality of channel types, a set of setting information is stored which contains information for setting ON or OFF of individual ones of the plurality of different signal processing operations. The set of setting information corresponding to the type of the channel selected via the cue instruction is referenced, and one or more signal processing operations is determined in such a manner that each signal processing operation set in an ON state in the setting file in correspondence with the type of the selected channel is performed on the signal supplied to the cue bus.

Abstract: A method of encoding an audio signal, where signals including two or more channel signals are downmixed to a mono signal, the mono signal is divided into a low-frequency signal and a high-frequency signal, the low-frequency signal is encoded through algebraic code excited linear prediction (ACELP) or transform coded excitation (TCX), and the high-frequency signal is encoded using the low-frequency signal. A method of decoding of an audio signal, a low-frequency signal encoded through ACELP or TCX is decoded, a high-frequency signal is decoded using the low-frequency signal, the low-frequency signal and the high-frequency signal are combined to generate a mono signal, and the mono signal is upmixed by decoding spatial parameters regarding signals including two or more channel signals.

Abstract: An encoding method and apparatus and a decoding method and apparatus are provided. The decoding method includes extracting a compatible down-mix signal optimized for a first multi-channel decoder from the input bitstream, converting the compatible down-mix signal to be optimized for a second multi-channel signal by performing a compatibility processing operation on the compatible down-mix signal, and generating a three-dimensional (3D) down-mix signal by performing a 3D rendering operation on the converted down-mix signal. Accordingly, it is possible to efficiently encode multi-channel signals with 3D effects and to adaptively restore and reproduce audio signals with optimum sound quality according to the characteristics of an audio reproduction environment.

Abstract: According to an aspect of the invention, a portable terminal includes: a first audio input-output unit and a second audio input-output unit configured to receive audio when a mode is set as an input mode and output audio when the mode is set as an output mode; and a setting unit configured to set the respective modes of the first audio input-output unit and the second audio input-output unit.

Abstract: An FM receiver and a pilot detector thereof, and a method for determining a type of a processed signal are provided. The FM receiver comprises a demodulator, a band pass filter (BPF), and a pilot detector. The demodulator generates a multiplexed (MPX) signal. The BPF filters the MPX signal to generate a pilot signal. The pilot detector comprises a signal addition device and a comparator. The signal addition device adds the processed signal for a period of time and to generate a result signal in response to the addition. The comparator compares the result signal with a reference to determine whether the type is stereo. The method comprises the steps of: adding the processed signal for a period of time; generating a result signal in response to the addition; and comparing the result signal with a reference to determine whether the type is stereo.

Abstract: A receiver may estimate a channel condition based on a difference between a pilot energy signal and a filtered pilot energy signal. The receiver may further switch to a stereophonic mode when both the difference and an energy level of the pilot energy signal satisfy a predetermined requirement of the receiver, and switch to a monophonic mode when either the difference or the energy level of the pilot energy signal fail to satisfy the predetermined requirement of the receiver.

Abstract: A system and method for switching a mode of a receiver between a monophonic and stereophonic mode is disclosed. The method can include generating a residual signal indicative of a noise level associated with a pilot energy signal, calculating a residual block energy level of the residual signal over an observation interval, generating a monitor signal based on a number of times the pilot energy signal is less than a pilot energy threshold during the observation interval and switching the mode of the receiver based on the residual block energy level and the monitor signal.

Abstract: An audio receiver may include a mono/stereo detector that causes the audio receiver to output either a monophonic or a stereophonic signal based on a difference between a pilot energy signal and a filtered pilot energy signal. The audio receiver includes a filter that filters the pilot energy signal to generate a filtered pilot energy signal, wherein a variable leakage factor associated with the filter is used to minimize a noise level of the filtered pilot energy signal and to reduce a response time of the audio receiver.

Abstract: A method for providing a 3D audio work includes providing a one-ear HRTF filter and a related function synthesizer storing a related function therein, and inputting sound signals into the one-ear HRTF filter. The sound signals are converted into one-ear output sound signals which are received by one ear and synthesized to output sound signals for the other ear. A method for providing the related function includes inputting sound signals into HRTF filters of opposite ears and obtaining output sound signals which respectively act as raw signals and target signals. The raw signals are synthesized by a synthesizer to output sound signals which compare with the target signals. A related function registered in the synthesizer is accordingly regulated so as to obtain the related function which satisfies a minimum difference between the output sound signals from the synthesizer and the target signals.

Abstract: The sound reproduction of a multichannel sound reproduction system with a plurality of speakers which is connected to the output of an FM stereo receiver is controlled by a control signal derived from the reception quality. Preferably, the control signal from the FM stereo receiver for controlling the stereo and mono components is also employed to control the multichannel sound reproduction system. For example, stereo, pseudo-stereo or mono reproduction are provided in the multichannel sound reproduction system in response to the stereo component within the output signal.

Abstract: A FM output portable audio player is provided for digitally coupling audio signals generated therein to be broadcast through a radio receiver system, such as an automobile radio. The audio player includes an audio signal source for generating digital audio output signals. Digital audio interface circuitry is coupled directly to the digital audio output signals and includes a digital FM stereo/RDS multiplexer for modulating the digital audio output signals on a RF carrier to produce a modulated audio signal. An internal antenna is coupled to the digital FM stereo/RDS multiplexer. The portable audio player is capable of wirelessly transmitting the modulated signal to the radio receiver system. The digital audio interface circuitry is formed of integrated circuit chips which provides for lower cost, a compact size, and a higher audio quality.

Abstract: A sound signal processor and a method for processing a sound signal using the same enables the elimination of a plop noise generated in the course of a sound mode checking operation. The sound signal processor receives a sound intermediate frequency (SIF) signal and includes a channel mode discriminator for determining a current sound mode of the received SIF signal; a mode transition checker for performing a checking operation to determine whether the current sound mode has resulted from a transition from a first sound mode to a second sound mode; an amplitude measurement circuit for measuring an amplitude of the received SIF signal; and a controller for controlling a drive of the mode transition checker according to the measured amplitude. If the sound channel mode of a currently input sound signal is a mono channel mode, the amplitude of the input sound signal is measured, and the measured amplitude is compared to a reference level.

Abstract: A method of denoise a stereo signal comprising a stereo sum signal and a stereo difference signal, performs a frequency selective stereo to mono blending based on the masking effect of the human auditory system. Therefore, a stereo signal noise reducer, comprising a first filter bank (1) to split the stereo difference signal (l?r) into a plurality of subbands, respective first multipliers (CO, . . . , CN) to weight each of the subbands of the stereo difference signal with a respective corresponding control signal (CO, . . . , CN), and a first adder (3) to sum all weighted subbands of the stereo difference signal (l?r) to build a frequency selective weighted stereo difference signal (diff), within which a number and width of the subbands obtained via the first filter bank (1) are choosen according to the properties of the human auditory system, further comprises a weighting factor determination unit which determines a respective control signal (CO, . . .

Abstract: A cochlear implant system in which information about the periodicity of a received audio signal in each filter channel is used as a factor in determining the rate of stimulation applied to a tonotopically placed electrode corresponding to the relevant filter channel. As such, the stimulation rate for each electrode is related to the periodicity of the signal in the filter channel corresponding to that electrode. This may allow for improved perception of pitch by the implant recipient.

Abstract: A method for selecting an audio source when capturing audio data with at least one capture device includes connecting an audio source to an audio input port of the capture device, turning off the output of the audio source, calculating first signal strengths of the audio signals detected through the audio input ports of the capture device, turning on the output of the audio source, calculating second signal strengths of the audio signals detected through the audio input ports of the capture device, calculating a signal change between the first signal strength and the second signal strength of each audio input port, and selecting the audio input port with the largest signal change.

Abstract: A Japanese BTSC audio broadcast signal has three modes of transmission: mono, stereo, or dual mono. The control channel centering at 3.5 fH is one of the three channels in the J-BTSC signal, and contains information to indicate to the decoder which one of the three modes the audio transmission is in. The present invention uses a bandpass filter directly in the AM band, followed by envelope filtering and a decision circuit. Therefore, the need for AM demodulation and AM carrier detection is eliminated.

Abstract: When a plug of a monaural microphone is connected to an external microphone input section, a right channel terminal is connected to a GND terminal, and a voltage Vdet will be a GND potential. A DC voltage detection section detects this GND potential, and outputs, for example, an H level signal as a detection signal. In response to this detection signal, a system control section controls a DSP so that, for example, an audio signal of the left channel is recorded by use of both right and left recording channels.

Abstract: A method is provided for increasing the quality of audio reproduction of a binaural rendering in a passenger environment having a number of seats, each seat having an audio system including a headphone jack for interconnection with a headphone device. The method comprising the steps of: playing predetermined audio tones over the audio system, and interconnecting a testing system to the headphone jack of the seats to monitor the signal received by the headphone jack to determine the audio quality of reproduction of the seat. The invention extends to an audio monitoring device, an audio monitoring system, and a storage medium carrying said audio tones.

Abstract: A test signal generator adapted to generate a test signal for an audio program having at least a left and right program track, the test signal generator including a memory means holding digital data descriptive of a unique image; and converter adapted to convert the digital data into a pseudo audio test signal having a left test track and right test track. The pseudo audio test signal is attached or mixed with the respective audio program and the resulting composite signal is recorded on a respective media and/or transmitted by other means to a remote location. At the remote location, the test signal is separated and displayed using the left track and right tracks as two orthogonal components to generate a reproduction of the image. The relative orientation, spatial and geometric characteristics of the reproduced image is indicative of various parameters of the received audio program.

Abstract: The invention relates to a method, signal processing device and computer program for stereo widening (SW) of stereo format signals to become suitable for headphone listening. The invention also relates to a mobile appliance performing signal processing according to the invention. According to the invention a separate monophonic signal path (ME) is formed in order to equalize the frequency spectrum of the monophonic component of the left and right output signals (Lout,Rout) by at least extracting from the left and right input signals (Lin,Rin) an at least substantially monophonic signal component contained in said signals (Lin,Rin), processing the extracted monophonic signal component to obtain a processed monophonic signal component, and combining said processed monophonic signal component with at least one of the left (Lout) or the right (Rout) output signals.

Abstract: A portable computer system includes an audio circuit and a common audio input socket coupled to the audio circuit. This audio circuit has a controller and two amplifiers. Two types of external audio devices is to be coupled to the common audio input socket. The controller controls the two amplifiers alternately depending upon the audio type of the external audio device coupled to the common audio input socket. In a preferred embodiment, a first amplifier is provided for amplifying stereo audio signals and a second amplifier for amplifying a mono audio signal. When the controller detects a mono audio device, the microcontroller disables the first amplifier. Detecting a stereo audio device, the controller disables the second amplifier.

Abstract: The filtering coefficients of a frequency domain stereophonic echo canceller are adapted by a method which takes account of the cross-correlation between the input signals relating to the two channels. In particular, the adaptation process takes account of the coherence function, reducing the problems commonly encountered with stereophonic cancellation schemes when relatively correlated input signals occur.

Abstract: A multi-channel stereo converter is described comprising stereo magnitude determining means for generating a stereo information signal (a/b; &rgr;), which represents a degree of stereo between audio input signals (L, R), and transforming means for transforming said audio signals (L, R) based on said stereo information signal (a/b; &rgr;) into at least a surround signal (S). A space mapping interpretation is presented and an audio center signal may be derived from the stereo input signals as well. The result is more flexibility in application and design, without substantial cross talk in the audio signals.

Abstract: Multiple attributes of a metered signal are indicated by a level meter including a matrix-connected array of luminescence devices, suitable circuitry or software for obtaining at least three attributes of a metered signal, a control signal generator for generating a respective control signal corresponding to each of the obtained attributes of the metered signal, and a driver for driving the matrix-connected array of luminescence devices in accordance with the first, second, and third control signals.

Abstract: The present invention discloses a differential audio device applicable to notebook computers. The device uses differential signal characters to improve the audio signal quality and its transmission distance. The device contains at least an audio controller to output audio signals, a single-ended signal to differential signal converter to convert audio signals into differential signals, a differential signal to single-ended signal converter to convert differential signals into audio signals for output.

Abstract: Audio-processing devices are known generally. When such an audio device wants to use as input signals from another device de audio device has to determine whether the received input signals are mono signals or stereo signals.

Abstract: Apparatus for analyzing audio signals from a stereo pair of audio channels, comprises: means for detecting the magnitudes of the audio signals of the two audio channels; first detector for detecting a degree of phase correlation between the audio signals of the two audio channels; and second detector for generating an indicator color for display in respect of the audio channels at a time of test, the indicator color having a hue, intensity and/or saturation dependent on at least the relative magnitudes of and the degree of phase correlation between the audio signals of the two audio channels at the time of test.

Abstract: Monaural inputs are accepted for presentation to a pair of speakers. The output sound image from that pair of speakers is such that a listener will perceive that the sound is stereo and that certain sounds come from a non-existent center speaker and that other sounds come from locations beyond the physical boundaries of the two speakers. The system operates to preserve full tone range at both speakers and does not split the sound from a particular instrument between both speakers. The system can be used to provide sound signals to the rear speakers of a multiple speaker system when the sound input to the rear speakers is either monaural or stereo.

Abstract: An apparatus for detecting a pilot signal. A phase detector detects a phase difference between a filtered amplitude-modulated pilot signal and a frequency divided signal, and a low pass filter low-pass-filters the phase difference. A voltage-controlled oscillator outputs a clock signal having a frequency determined by an output signal of the low-pass filter and outputs the clock signal having the frequency determined by an oscillation control signal. A (1/N) frequency divider divides the switching signal and outputs the (1/N) divided signal as a frequency divided signal, and an envelope detector detects an envelope of the filtered amplitude-modulated pilot signal and outputs the detected envelope as the pilot signal. The frequency of the pilot signal has information used to discriminate a broadcasting state. Therefore, the manufacturing cost and the number of pins in an integrated circuit can both be reduced.

Abstract: For professional audio operations such as recording and broadcasting, an LED bar-graph display instrument monitors average and peak loudness levels of stereophonic audio signals that have been encoded in serial digital format such as AES/EBU digital audio format. Internal audio processing circuitry, that can be implemented digitally with a custom chip gate array set, receives as input a serial stream of digital stereo audio data and converts this to ballistically conditioned logarithmic average and peak levels which are simultaneous displayed, generally simulating the ballistics of contemporary standard electronically displayed loudness meters such as the Dorrough analog model. The peak hold can be switched manually or internally between three hold durations: indefinite, 3 seconds or zero.

Abstract: An improved monaural signal judgement circuit which is capable of judging whether or not an input signal is monaural when signals from a television, a video cassette tape recorder, an audio cassette tape recorder, a compact disc player, or the like are inputted to the monaural signal judgement circuit and of converting the process into a pseudo stereo function when the input signal is monaural, which includes a wave form shaping circuit for separately wave-form-shaping a left-side signal and a right-side signal inputted thereto; an operation circuit for computing a signal level difference of the wave-form-shaped right-side and left-side signals; a control signal setting circuit for setting a control signal in accordance with an output of the operation circuit; an input control circuit connected between an input side of the operation circuit and an output side of the control signal setting circuit for adjusting a signal level at an input side of the operation circuit in accordance with a control signal applied

Abstract: In a mono/stereo input circuit for use in an audio equipment, and more particularly in a circuit for automatically recognizing and receiving mono/stereo audio signals, although a mono connector is inserted into a stereo jack, such an insertion of the mono connector is automatically recognized and then a stereo circuit can operate as a form of a mono circuit. The circuit for automatically recognizing and receiving the mono/stereo audio signals utilizes a fact that one connection terminal of the jack is grounded or not according to a type of the connector such as a mono connector and a stereo connector which is inserted into the jack. Accordingly, a resistor used as a voltage generator generates a generation voltage according to such a connector recognition. Also, using a difference between the generated voltage, the analog switches are turned on or off, to obtain a mono or a stereo audio signal processing circuit according to a type of the inserted connector.

Abstract: A stereo and dual audio signal identifying circuit including an FM detector receiving a second intermediate frequency signal, a bandpass filter, an AM detector. The circuit further including an amplifier having a first input receiving an output signal from the AM detector, and a second input receiving a signal derived from a series connected resistor and DC-filtering capacitor connected between the second input and ground, and having a feedback resistor connected between the second amplifier input and the amplifier. The amplifier drives a second stage of the circuit including a voltage controlled oscillator, a phase detector and a low-pass filter. The particular disposition of the DC-filtering capacitor within the circuit allows the capacitor to be externally connected to a single pin in an IC package incorporating the circuit.

Abstract: A stereo and dual audio signal identifying system comprising, an FM detector, bandpass filter, and AM detector which generate a filter/detected input to a first PLL having a 210 Hz reference signal. The first PLL generates a first input to a comparator and a second PLL generates a second input to the comparator. The comparator generates an output signal indicative of the presence of stereo and dual audio signals.

Abstract: A music/voice discriminating apparatus is composed of a signal processing portion for effecting the signal processing upon input acoustic signals, a music/voice deciding portion for discriminating whether the input acoustic signals are music or voice, a first signal processing portion for optimally setting acoustic parameters for the signal processing respectively for music or voice, and a second signal processing portion for controlling the acoustic parameters of the first signal processing portion in accordance with the decision results of the music/voice deciding portion so that it may become a desirable value set in the second parameter setting portion.

Abstract: A method and apparatus used for determining phasing correlation between left-channel and right-channel signals of a stereophonic audio signal is provided. The determination of the phase correlation is facilitated by deriving centrally localized components contained in the audio signal to provide an indication representative of phase correlation existing between the left-channel and right-channel signals.

Abstract: A stereo/monaural detection apparatus for detecting whether two-channel input audio signals are stereo or monaural, wherein the level difference between the input audio signals is calculated, and after the signal representing the level difference is discriminated with a predetermined hysteresis maintained, a stereo/monaural detection is performed in accordance with the result of such discrimination, thereby preventing an erroneous detection that may otherwise be caused by any level difference variation during a short time as in a case where the sound field is positioned at the center in the stereo signals. And the discrimination with the hysteresis is partially inhibited in response to the signal obtained by discriminating the level sum of the input audio signals at a predetermined reference level, hence preventing instability of the operation when the audio input level is extremely feeble.