Patent Description:
Patent Document <NUM> discloses an echo suppression device that, in a case of detecting that no signal is being transmitted through a transmitting signal path and that a signal is being transmitted through a receiving signal path, an echo suppressor is used to perform a process of suppressing an echo in a picked-up sound signal.

Patent Document <NUM> discloses a "computer system comprising an audio processing module configured to process an audio signal and output the processed audio signal to audio output means of the system, an echo cancellation module configured to cancel echo from an audio signal received via audio input means of the system; and a control module configured to detect a condition indicative of nonlinear amplitude processing by the audio processing module and control said echo cancellation by the echo cancellation module based on the detected condition" (claim <NUM> thereof).

An "echo suppression device" according to the preamble of claim <NUM> of the present invention is disclosed in Document <NUM>.

However, with the echo suppression device described in Patent Document <NUM>, in a case where performance of a speaker or a speaker amplifier is low, there is a possibility that non-linear echoes increase, uncancelled echoes increase, and a quality of a conversational voice is deteriorated.

The present invention has been made in light of the foregoing, and an object of the present invention is to provide an echo suppression device, an echo suppression method, and an echo suppression program that allow suppressing a non-linear echo and reducing voice deterioration.

In order to solve the problem, an echo suppression device according to the present invention is, for example, an echo suppression device for suppressing an echo generated by inputting a voice signal output from a speaker to a microphone. The echo suppression device includes a level adjustment unit, an echo removal unit, and a double-talk detection unit. The level adjustment unit is provided on a receiving signal path for transferring (also denoted herein as "transmitting" in the description) receiving signals from a far-end terminal to the speaker. The echo removal unit is provided on a transmitting signal path for transferring signals input from the microphone. The echo removal unit is configured to remove a residual echo from a picked-up sound signal output from the microphone. The double-talk detection unit is configured to detect whether signals are in a double-talk state in which the signals are simultaneously transferred to the transmitting signal path and the receiving signal path. The echo suppression device is characterized in that the level adjustment unit includes an attenuator (also denoted a compressor in the description). When the double-talk detection unit has detected the double-talk state, the attenuator is configured to perfom an attenuation (also denoted as a compression in the description) process on a signal greater than a first threshold among the receiving signals the first threshold being greater than zero. When the double-talk detection unit has not detected the double-talk state, the attenuator performs the attenuation process on a signal greater than a second threshold greater than the first threshold among the receiving signals.

The echo suppression device according to the present invention includes the attenuator (compressor) on the receiving signal path for transferring (transmitting) the signal of the receiving signal to the speaker. When the double-talk state has been detected, the attenuator performs the attenuation (compression) process on the signal greater than the first threshold among the receiving signals from the far-end side. Thus, a non-linear echo can be prevented. As a result, voice deterioration can be reduced.

Here, when the double-talk detection unit has not detected that the signal is in the double-talk state, the attenuator may perform the attenuation process on the signal greater than the second threshold greater than the first threshold among the receiving signals. This allows more reliably preventing the non-linear echo.

Here, the level adjustment unit may include a gain adjustment unit that is configured to adjust a gain of the receiving signal. The attenuator may adjust the threshold so that the first threshold becomes small as the gain increases. The attenuator may perform a process on a signal output from the gain adjustment unit. As a result, even when the gain adjustment unit outputs a large voice signal, the attenuator reduces a peak of the voice signal, and thus the non-linear echo can be reduced.

Here, the attenuator may increase attenuatability (also denoted as compressibility in the description) as the gain increases. As a result, even when the gain adjustment unit outputs a large voice signal, the attenuator reduces the peak of the voice signal, and thus the non-linear echo can be reduced.

Here, the attenuator may change attenuatability based on information on a distortion of the speaker. This allows reducing the non-linear echo.

Here, an echo suppressor that performs a process to suppress an echo on a signal from which the residual echo has been removed by the echo removal unit may be further provided. As a result, even when a sound volume of the speaker is set to be large due to a large noise in an external environment or the like and the non-linear echoes are generated many, an echo component can be removed.

Here, the attenuator may compare a value of the receiving signal with a third threshold for each frequency band. When the double-talk detection unit has detected the double-talk state, the attenuator may perform the attenuation process on a signal of the receiving signal having a value greater than the third threshold. Accordingly, a proportion of the signals compressed by attenuator (compressor) is reduced to produce a further natural voice, and thus a telephone call quality can be improved.

In order to solve the problem, an echo suppression method according to the present invention is, for example, an echo suppression method for suppressing an echo in a near-end terminal using the echo suppression device defined above. The echo suppression method includes: detecting whether signals are in a double-talk state in which the signals are simultaneously transferred to the transmitting signal path for transferring a signal input from the microphone and a receiving signal path for transferring a signal to the speaker; performing an attenuation process on a signal greater than a first threshold among receiving signals from a far-end terminal when the double-talk state has been detected; outputting a signal after the attenuation process from the speaker; and removing a residual echo from a picked-up sound signal output from the microphone. As a result, the non-linear echo is allowed to be suppressed and the voice deterioration is allowed to be suppressed.

In order to solve the problem, an echo suppression program according to the present invention is, for example, an echo suppression program for suppressing an echo in a near-end terminal using the echo suppression device defined above. The echo suppression program causes a computer to function asthe double-talk detection unit, the attenuator and the echo removal unit. As a result, the non-linear echo can be suppressed and the voice deterioration can be reduced.

According to the present invention, the non-linear echo can be suppressed and the voice deterioration can be reduced.

Embodiments of an echo suppression device according to the present invention will be described below in detail with reference to the drawings. An echo suppression device is a device suppressing echo generated during a telephone call in a voice communication system.

<FIG> is a diagram schematically illustrating a voice communication system <NUM> provided with an echo suppression device <NUM> according to the first embodiment. The voice communication system <NUM> primarily includes a terminal <NUM> including a microphone <NUM> and a speaker <NUM>, two cell phones <NUM> and <NUM>, a speaker amplifier <NUM>, and the echo suppression device <NUM>.

The voice communication system <NUM> is a system in which a user (a user A on a near-end side) utilizing the terminal <NUM> (near-end terminal) is in voice communication with a user (user B on a far-end side) utilizing the cell phone <NUM> (far-end terminal). A voice signal input via the cell phone <NUM> is amplified and output by the speaker <NUM>, and a voice emitted by the user on the near-end side is collected by the microphone <NUM> and transmitted to the cell phone <NUM>. Thus, the user A can make an amplified voice call (hands-free call) without holding the cell phone <NUM>. The cell phone <NUM> and the cell phone <NUM> are connected together by a common telephone line.

The echo suppression device <NUM> suppresses echoes generated when the voice signals output from the speaker <NUM> are input to the microphone <NUM>. The echo suppression device <NUM> is provided between the terminal <NUM> and the cell phone <NUM>, that is, on a transmitting signal path for transmitting a picked-up sound signal input via the microphone <NUM> from the microphone <NUM> to the cell phone <NUM> and on a receiving signal path for transmitting a receiving signal from the cell phone <NUM> on the far-end side from the cell phone <NUM> to the speaker <NUM>.

The echo suppression device <NUM> may be configured as a dedicated board mounted on a speech terminal or the like (for example, an on-board device, a conference system, or a mobile terminal) in the voice communication system <NUM>. Additionally, the echo suppression device <NUM> may include, for example, computer hardware and software (echo suppression program). The echo suppression program may be stored in advance in, for example, a HDD as a storage medium built into a device, such as a computer, and a ROM in a microcomputer including a CPU, and may be installed from it into a computer. Additionally, the echo suppression program may be temporarily or permanently stored (memorized) in a removable storage medium such as a semiconductor memory, a memory card, an optical disc, a magneto-optical disk, a magnetic disk, or the like.

<FIG> is a block diagram illustrating a general configuration of the echo suppression device <NUM>. The echo suppression device <NUM> primarily includes a level adjustment unit <NUM>, an echo removal unit <NUM>, and a double-talk detection unit <NUM>. In <FIG>, the upper signal path is the transmitting signal path and the lower signal path is the receiving signal path.

The level adjustment unit <NUM> is provided on the receiving signal path. The level adjustment unit <NUM> primarily includes a gain controller <NUM> and a compressor <NUM>.

The gain controller <NUM> is a gain adjustment unit that adjusts a gain of the input receiving signal. Specifically, the gain controller <NUM> adjusts a level (the gain) of amplifying the input signal to adjust a level (a magnitude) of the signal to be output. The gain controller <NUM> may automatically change the gain by noise or the like in an environment where the terminal <NUM> is mounted. Additionally, when an input unit, such as a knob, is operated, the gain controller <NUM> may change the gain based on the position of the input unit.

The signal output from the gain controller <NUM> is input to the compressor <NUM>. The compressor <NUM> amplifies (i.e., compresses) a received signal greater than a threshold among the input receiving signals by a predetermined coefficient (the coefficient is a value smaller than <NUM>) and output the signal. The compressor <NUM> will be described in detail later.

Note that in the present embodiment, the level adjustment unit <NUM> includes the gain controller <NUM> and the compressor <NUM>, but the gain controller <NUM> is not essential. In the absence of the gain controller <NUM>, the receiving signal transmitted from the cell phone <NUM> is directly input to the compressor <NUM>, and the compressor <NUM> only needs to compress the received signal greater than the threshold among the input receiving signals.

The echo removal unit <NUM> is provided on the transmitting signal path to remove a residual echo from the picked-up sound signal output from the microphone <NUM>. The echo removal unit <NUM> is a linear echo canceller that removes the residual echo using an adaptive filter. Specifically, the echo removal unit <NUM> updates a filter coefficient according to a given procedure to generate a pseudo echo signal from a signal transmitted through the receiving signal path, and subtracts the pseudo echo signal from a signal transmitted through the transmitting signal path to remove the residual echo. Note that adaptive filters are well known, and thus description of the adaptive filter is omitted.

Note that in the present embodiment, the adaptive filter is applied to the echo removal unit <NUM>, but the echo removal unit <NUM> can employ another known echo removal technique.

The signal from which the residual echo has been removed by the echo removal unit <NUM> is transmitted to the cell phone <NUM>. The signal from which the residual echo has been removed by the echo removal unit <NUM> is input to the double-talk detection unit <NUM>.

The double-talk detection unit <NUM> detects whether the voice signal input to the echo suppression device <NUM> is in a single-talk state or a double-talk state. Here, the single talk refers to a state (a near-end speech or a far-end speech) in which either the user A or the user B emits a voice and a signal is transmitted to either the transmitting signal path or the receiving signal path. The double talk refers to a state (the near-end speech or the far-end speech) in which both the user A and the user B emit voices and signals are simultaneously transmitted to the transmitting signal path and the receiving signal path.

For example, the double-talk detection unit <NUM> holds a frequency mask generated based on a learning signal. The learning signal is a signal transmitted through the transmitting signal path during one-side speech (the single talk) on the far-end side in which only the sound output from the speaker <NUM> is input to the microphone <NUM>. The frequency mask obtains the maximum value among values of power spectra of a plurality of the input learning signals.

The double-talk detection unit <NUM> compares the value of the power spectrum of the picked-up sound signal with the value of the frequency mask for each frequency band. When the number of frequency bands in which the value of the picked-up sound signal exceeds the value of the frequency mask is equal to or more than a constant value, it is detected that a sound is input from the microphone <NUM> and the signal is transmitted (the near-end speech is present) through the transmitting signal path. The double-talk detection unit <NUM> compares the value of the power spectrum of the receiving signal with the value of the frequency mask for each frequency band. When the number of frequency bands in which the value of the receiving signal exceeds the value of the frequency mask is equal to or more than a constant value, it is detected that the signal is transmitted (the far-end speech is present) through the receiving signal path.

However, the double-talk detection unit <NUM> may detect whether the signal is in the single-talk state or the double-talk state using other various known methods.

The compressor <NUM> will now be described in detail. Results are input from the double-talk detection unit <NUM> to the compressor <NUM>. The compressor <NUM> performs different processes depending on whether the signal is in the double-talk state.

<FIG> is a diagram schematically illustrating a process by the compressor <NUM> when the double-talk state has been detected. When the double-talk detection unit <NUM> has detected the double-talk state, the compressor <NUM> performs a compression process on a signal greater than a threshold I among the receiving signals.

<FIG> is a diagram schematically illustrating a process by the compressor <NUM> when the double-talk state has not been detected. In a case where the double-talk detection unit <NUM> has not detected the double-talk state, the compressor <NUM> performs the compression process on a signal greater than a threshold II among the receiving signals. The threshold II is greater than the threshold I.

In the case of being the double-talk state, an operation by the echo removal unit <NUM> tends to be unstable. Thus, the threshold I is decreased and the peak of the voice signal output from the speaker <NUM> is decreased such that the echo removal unit <NUM> reliably operates. In contrast, when not in the double-talk state, since an uncomfortable feeling is likely to be felt in a voice due to an excessively low peak of the voice signal, the threshold II, which is greater than the threshold I, is used to maintain the quality of the voice.

In <FIG> and <FIG>, the solid line indicates a signal before the compressor <NUM> performs the compression process, and the dotted line indicates a signal after the compressor <NUM> performs the compression process. In the compression process, the compressor <NUM> multiplies the received signal greater than the threshold I or the threshold II among the receiving signals input from the gain controller <NUM> by the coefficient of <NUM> or less, which is predetermined for the receiving signal, thus decreasing an output level of the signal.

As a result, a distorted sound generated by largely vibrating the speaker <NUM>, a housing that holds the speaker <NUM>, a component provided in the housing, or the like, can be reduced. In particular, in a case where performance of the speaker <NUM> or the speaker amplifier <NUM> is low, the terminal <NUM> is small, or the like, a distorted sound is likely to be generated due to vibrations of the speaker <NUM> or the like, but by decreasing the voice level significantly, the distorted sound is reduced.

By reducing the distorted sound at the speaker <NUM>, the signals generated by picking up sounds by the microphone <NUM> and input to the echo removal unit <NUM> hardly contain non-linear echoes, and the echo removal unit <NUM> can sufficiently eliminate the echoes.

Furthermore, the compressor <NUM> does not perform the compression process on the received signal smaller than the threshold I or the threshold II, and outputs the input signal as is. As a result, an uncomfortable feeling due to a change in sound volume of the speaker <NUM> and an interruption of a voice is greatly reduced.

According to the present embodiment, performing the compression process on the signal greater than the threshold I or the threshold II allows preventing the non-linear echoes and stabilizing the behavior of the echo removal unit <NUM>. This allows reducing voice deterioration while reducing uncancelled echoes.

In addition, according to the present embodiment, a non-linear echo is less likely to occur, so it is only necessary to provide the echo removal unit <NUM> that removes a linear echo, and an amount of calculation required to remove echoes can be reduced.

For example, in the absence of the compressor <NUM>, in a case where a non-linear echo is large and the echo is to be cancelled, an echo suppressor is required as in the prior art. Therefore, a large amount of calculation is always required, and the processes are delayed. For example, it is also conceivable to use an echo removal unit using a non-linear adaptive filter, such as a Volterra filter, to cancel the non-linear echoes. However, an enormous amount of calculation (<NUM> times or more than that of a linear echo canceller) is required.

In contrast, according to the present embodiment, by reducing the vibration of the speaker <NUM> or the like, a difference between the receiving signal received by the receiving side and the voice signal output from the speaker <NUM> is decreased, thereby ensuring reducing non-linear echoes and sufficiently removing echoes only by the echo removal unit <NUM>, which is a linear echo canceller.

Note that in the present embodiment, when the double-talk state has been detected, the compressor <NUM> performs the compression process on the receiving signal greater than the threshold I, and when the double-talk state has not been detected, the compression process is performed on the receiving signal greater than the threshold II (greater than the threshold I). However, in the case where the double-talk state has not been detected, performing the compression process on the receiving signal greater than the threshold II is not essential. However, to prevent voice deterioration while reducing non-linear echoes, it is desirable to perform the compression process in both cases where the double-talk state has been detected and the double-talk state has not been detected.

In addition, in the present embodiment, when the double-talk detection unit <NUM> has detected the double-talk state, the compressor <NUM> performs the compression process on the signal greater than the threshold I among the receiving signals, but a threshold that is applied in the double-talk state may be varied depending on the situation.

For example, defining the variable threshold as a threshold Iα, the compressor <NUM> sets the threshold Iα when a gain for amplifying the receiving signal in the gain controller <NUM> is a certain value (defined as a value a) as a threshold Ia. As the gain becomes greater than the value a, the threshold Iα is set to be smaller than the threshold Ia, and as the gain becomes smaller than the value a, the threshold Iα is set to be greater than the threshold Ia. However, the maximum value of the threshold Iα is set to be smaller than the threshold II. As a result, even when a large voice signal is output from the gain controller <NUM>, non-linear echoes can be reduced by reducing the peak of the voice signal by the compressor <NUM>.

In addition, in the present embodiment, while the coefficient (the value smaller than <NUM>) used by the compressor <NUM> in the compression process is constant, the coefficient used in the compression process may be varied depending on the situation.

<FIG> is a diagram schematically illustrating a process by the compressor <NUM> when the gain changes. For example, defining the variable coefficient as a coefficient b, the compressor <NUM> sets the coefficient b when the gain for amplifying the receiving signal in the gain controller <NUM> is the value a as a coefficient c. As the gain becomes greater than the value a, the coefficient b is set to be smaller than the coefficient c, and as the gain becomes smaller than the value a, the coefficient b is set to be greater than the coefficient c. Here, when a percentage of a decrease in output level in the compression process is defined as compressibility, the compressibility increases as the coefficient decreases. That is, as the gain increases, the compressibility increases, and as the gain decreases, the compressibility decreases. As a result, even when a large voice signal is output from the gain controller <NUM>, non-linear echoes can be reduced by reducing the peak of the voice signal by the compressor <NUM>.

For example, the compressor <NUM> may change the compressibility based on information on the distortion of the speaker <NUM>. Here, the information on the distortion of the speaker <NUM> is, for example, a total harmonic distortion factor (or a distortion factor) representing the degree of distortion of the signal. The total harmonic distortion factor indicates that the distortion of the speaker <NUM> is small when the value is small and the distortion of the speaker <NUM> is large when the value is large. Thus, the compressor <NUM> may increase the compressibility when the total harmonic distortion factor is large and decrease the compressibility as the total harmonic distortion factor decreases. As a result, with the use of the speaker <NUM> where distortion is likely to occur, reducing the peak of the voice signal by the compressor <NUM> allows reducing non-linear echoes.

The second embodiment of the present invention has a configuration in which an echo suppressor is provided. Now, an echo suppression device <NUM> according to the second embodiment will be described. The echo suppression device <NUM> is particularly suitable in a case where an external environment of an on-board device or the like possibly varies greatly. Note that the same components as those of the echo suppression device <NUM> according to the first embodiment are denoted by the same reference numerals, and description of the components will be omitted.

<FIG> is a block diagram illustrating a general configuration of the echo suppression device <NUM>. The echo suppression device <NUM> primarily includes the level adjustment unit <NUM>, an equalizer (EQ) <NUM>, the echo removal unit <NUM>, an echo suppressor <NUM>, the double-talk detection unit <NUM>, a noise estimation unit <NUM>, a noise suppression unit <NUM>, and an equalizer (EQ) <NUM>.

The equalizers <NUM>, <NUM> emphasize or reduce a particular frequency band of the voice signal. However, the equalizers <NUM>, <NUM> are not essential.

The echo suppressor <NUM> performs a fast Fourier transform on the signal after the linear echo is cancelled by the echo removal unit <NUM>, performs an echo suppressing process (a process for strongly suppressing echoes) on the signal after the Fast Fourier Transform is performed, and performs an inverse fast Fourier transform on the signal after the echo suppressing process is performed to remove the non-linear echo. The echo suppress processing is well known, and thus, detailed description of the processing is omitted.

In a case where the double-talk detection unit <NUM> detects that a signal is not transmitted through the transmitting signal path but a signal is transmitted through the receiving signal path, the echo suppressor <NUM> may perform a process of suppressing echoes on the signal from which the residual echo has been removed by the echo removal unit <NUM>.

However, in the present embodiment, since the compressor <NUM> reduces non-linear echoes and the echo removal unit <NUM> sufficiently removes echo components, the echo suppressor <NUM> actually operates only when a sound volume of the speaker <NUM> is set to be large, for example, in a case where a voice level of an external noise is large and many non-linear echoes are generated.

Note that, in the present embodiment, the echo suppressor <NUM> performs frequency analysis using the Fast Fourier Transform, but instead of the Fast Fourier Transform, a Discrete Fourier Transform (DFT) can be used for the frequency analysis. In addition, the echo suppressor <NUM> may perform an inverse discrete Fourier transform, instead of the fast Fourier transform.

The noise estimation unit <NUM> estimates a noise component contained in an echo-removed signal converted into a function of a frequency domain by the echo suppressor <NUM>, namely, an estimated noise signal, for each frequency region, and estimates a SN ratio of the echo-removed signal based on a power spectral density of the estimated noise signal, which has been estimated. The noise suppression unit <NUM> suppresses the noise signal in the echo-removed signal based on the power spectral density of the estimated noise signal estimated by the noise estimation unit <NUM> and generates a suppressed signal. Note that the noise estimation unit <NUM> and the noise suppression unit <NUM> are not essential.

According to the present embodiment, an echo component can be removed even in a case where many non-linear echoes are generated. For example, in an on-board device, the external environment possibly changes greatly by driving of a vehicle. When an external noise is large, the sound from the speaker <NUM> is difficult to hear. Therefore, it is necessary to increase the sound volume of the speaker <NUM> and increase a volume of a voice spoken by the user B on the far-end side, and as a result, linear echoes and non-linear echoes increase. The echo removal unit <NUM> can remove the linear echoes, but cannot remove the non-linear echoes. In the present embodiment, since the echo suppressor <NUM> is provided, even when many non-linear echoes are generated, the echo components can be removed.

The third embodiment has a configuration in which an operation of the compressor differs depending on each frequency band. Now, an echo suppression device <NUM> according to the third embodiment will be described. Note that the same components as those of the echo suppression device <NUM> according to the first embodiment are denoted by the same reference numerals, and description of the components will be omitted.

<FIG> is a block diagram illustrating a general configuration of the echo suppression device <NUM>. The echo suppression device <NUM> primarily includes a level adjustment unit 11A, the echo removal unit <NUM>, and the double-talk detection unit <NUM>. The level adjustment unit 11A primarily includes the gain controller <NUM> and a compressor 112A.

The compressor 112A compares the voice level with a threshold for each frequency band, and performs a compression process on a receiving signal in a frequency band in which the voice level is greater than the threshold. The compressor 112A includes a processing unit that performs a Fourier transform and an inverse Fourier transform.

The compressor 112A performs the Fourier transform on the receiving signal to divide power that is an average energy per unit time into power for each frequency band, and calculates a power spectrum that expresses the power for each frequency band as a function of frequency for each unit time. The compressor 112A compares the value of the receiving signal with the threshold for each frequency band, and performs the compression process on a signal in a frequency band greater than or equal to the threshold.

<FIG> is a diagram schematically illustrating a process by the compressor 112A. The solid line in <FIG> indicates the receiving signal. When the double-talk detection unit <NUM> has detected the double-talk state, the compressor 112A performs the compression process on a signal in a frequency band in which the value of the signal is greater than a threshold III. The dotted line in <FIG> indicates a signal after the compressor 112A performs the compression process on the signal greater than the threshold III. Furthermore, in a case where the double-talk detection unit <NUM> has not detected the double-talk state, the compressor 112A performs the compression process on a signal in a frequency band in which the value of the signal is greater than a threshold IV. The dashed line in <FIG> indicates a signal after the compressor 112A performs the compression process on the signal greater than the threshold IV. The threshold IV is greater than the threshold III.

The compressor 112A outputs a signal produced by performing an inverse Fourier transform on the signal after being compressed.

According to the present embodiment, by changing the presence or absence of the compression process for each frequency band, the proportion of the signals compressed by the compressor <NUM> is reduced to produce a further natural voice, and thus a telephone call quality can be improved.

Claim 1:
An echo suppression device (<NUM>) for suppressing an echo generated by inputting a voice signal output from a speaker (<NUM>) to a microphone (<NUM>), the echo suppression device (<NUM>) comprising:
a level adjustment unit (<NUM>) provided on a receiving signal path for transferring receiving signals from a far-end terminal (<NUM>) to the speaker (<NUM>);
an echo removal unit (<NUM>) provided on a transmitting signal path for transferring signals input from the microphone (<NUM>), the echo removal unit (<NUM>) being configured to remove a residual echo from a picked-up sound signal output from the microphone (<NUM>); and
a double-talk detection unit (<NUM>) that is configured to detect whether signals are in a double-talk state in which the signals are simultaneously transferred to the transmitting signal path and the receiving signal path, characterized in that:
the level adjustment unit (<NUM>) includes an attenuator (<NUM>), which, when the double-talk detection unit (<NUM>) has detected the double-talk state, is configured to perform an attenuation process on a signal greater than a first threshold (I) among the receiving signals, the first threshold (I) being greater than zero.