Patent Publication Number: US-10313824-B2

Title: Audio processing device for processing audio, audio processing method, and program

Description:
BACKGROUND 
     Field of the Disclosure 
     The present disclosure relates to an audio processing device for processing audio, an audio processing method, and a program. 
     Description of the Related Art 
     A two-channel stereo audio collection device with directional characteristics in a right-to-left direction of the device has been known. The method for providing these directional characteristics is roughly classified into the method for collecting sound by means of a directional microphone and the method for generating directional characteristics from audio signals collected by multiple non-directional microphones by stereo feeling emphasis processing. The method using the directional microphone includes, for example, the method for collecting sound by two unidirectional microphones arranged to face in directions targeted for the directional characteristics, and the method for collecting sound for two opposing directions by a single bidirectional microphone. The directional microphone has an advantage that the directional characteristics are acoustically provided, and on the other hand, has characteristics that noise is easily generated due to vibration. Thus, a method has been employed, in which a portable device such as a video camera is equipped with a non-directional microphone and a stereo feeling of a collected audio signal is emphasized by signal processing (see Japanese Patent Laid-Open No. 2001-189999). 
     However, in Japanese Patent Laid-Open No. 2001-189999, when the processing of emphasizing the stereo feeling is performed, there is a problem that a low-frequency component is greatly damped as compared to a high-frequency component. For this reason, when the level of the low-frequency component is adjusted, there is, on the other hand, a problem that noise as the low-frequency component, such as floor noise, increases. 
     SUMMARY 
     An audio processing device includes a first low-pass filter unit configured to output a low-frequency component of a first right channel audio signal; a second low-pass filter unit configured to output a low-frequency component of a first left channel audio signal; a first subtraction unit configured to subtract an output signal of the second low-pass filter unit from the first right channel audio signal, thereby outputting a second right channel audio signal; a second subtraction unit configured to subtract an output signal of the first low-pass filter unit from the first left channel audio signal, thereby outputting a second left channel audio signal; a first addition unit configured to add up the first right channel audio signal and the first left channel audio signal; a third low-pass filter unit configured to output a low-frequency component of an output signal of the first addition unit; a first amplification unit configured to amplify an output signal of the third low-pass filter unit; a control unit configured to control the amplification factor of the first amplification unit based on the second right channel audio signal and the second left channel audio signal; a second addition unit configured to add up the second right channel audio signal and an output signal of the first amplification unit; and a third addition unit configured to add up the second left channel audio signal and the output signal of the first amplification unit. 
     Further features of the present disclosure will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a configuration example of a sound collection device according to one or more aspects of the present disclosure. 
         FIG. 2  is a diagram of a configuration example of an audio processing unit according to one or more aspects of the present disclosure. 
         FIG. 3  shows graphs of frequency characteristics with respect to sensitivity and a noise floor level. 
         FIG. 4  shows a graph of the frequency characteristics with respect to an equalizer amplification factor. 
         FIG. 5  shows graphs of the frequency characteristics with respect to sensitivity and a noise floor level. 
         FIG. 6  shows graphs of the frequency characteristics with respect to sensitivity and a noise floor level. 
         FIG. 7  is a flowchart of processing by a low-frequency component determination unit  404 . 
         FIG. 8  shows graphs of the frequency characteristics with respect to sensitivity and a noise floor level. 
         FIG. 9  is a diagram of a configuration example of an audio processing unit according to one or more aspects of the present disclosure. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
       FIG. 1  is a diagram of a configuration example of a sound collection device  10  according to a first embodiment of the present disclosure. The sound collection device  10  has a central processing unit (CPU)  2 , a program ROM  3 , a memory  4 , a display  5 , an operation unit  6 , a sound collection unit  7 , an audio processing unit  8 , and a recording unit  9 , these components being connected together via an internal bus  1 . Note that the sound collection device  10  further has a battery, a recording medium drive, etc. 
     The internal bus  1  is a versatile bus for inputting/outputting various types of data, a control signal, an instruction signal, etc. to/from each block of the sound collection device  10 . The CPU  2  is a calculation processing device configured to control operation of the sound collection device  10 . The CPU  2  is configured to input an instruction from a user via the operation unit  6  to execute later-described various programs and to perform display control of the display  5 . The program ROM  3  is configured to store data and the programs for an operation processing procedure of the CPU  2  (e.g., the programs for the processing of starting up the sound collection device  10 , basic input/output processing, and each type of processing described later). The memory  4  is used as a work area of the CPU  2 . 
     The display  5  is a display unit configured to provide a graphic user interface (GUI). The operation unit  6  is a touch panel using multiple manipulators arranged at a housing of the sound collection device  10  or employing a technique by means of a resistance film or an electrostatic capacitor on a surface of the display  5 . The touch panel can instruct various types of operation input to the CPU  2  by a combination of a display image of the display  5  and a detection region of the touch panel. 
     The sound collection unit  7  is configured to collect audio around the sound collection device  10  by built-in microphones, thereby converting an analog audio signal of the collected audio into a digital signal and outputting the digital signal to the audio processing unit  8 . The audio processing unit  8  is an audio processing device as a microcomputer configured to execute the following processing programs, and is configured to execute processing necessary for audio recording/reproduction. Alternatively, the audio processing unit  8  may be configured to execute the following processing in response to program execution by the CPU  2 . The audio processing unit  8  temporarily stores, in the memory, the digital audio signal output from the sound collection unit  7 , and performs stereo feeling emphasis processing. In addition, the audio processing unit  8  also performs audio processing such as the effect processing of providing a special effect to audio, level optimization processing, and noise reduction processing. 
     The CPU  2  outputs the audio signal processed by the audio processing unit  8  to the recording unit  9  for recording, and outputs the audio signal to a not-shown output unit for reproduced output or monitor audio output. The recording unit  9  is configured to convert the audio signal into a data format suitable for recording and to write the data in a recording medium such as a data tape, an optical disc, or a flash memory. Moreover, the recording unit  9  is configured to read the data stored in the recording medium. 
       FIG. 2  is a diagram of a functional configuration example of the audio processing unit  8 . The audio processing unit  8  has a stereo feeling emphasis unit  100 , a low-frequency monophonic generation unit  200 , a low-frequency sound volume adjustment unit  300 , a low-frequency component selection unit  400 , and a frequency band synthesizing unit  500 . The audio processing unit  8  performs the stereo feeling emphasis processing. The audio processing unit  8  inputs, as input  1  and input  2 , a right channel audio signal and a left channel audio signal obtained by the sound collection unit  7 . The sound collection unit  7  includes multiple non-directional microphones. The audio processing unit  8  performs, by not-shown processing blocks, signal level amplification processing, wind noise reduction processing, etc. for the audio signals obtained by the sound collection unit  7 , thereby inputting the right channel audio signal (the input  1 ) and the left channel audio signal (the input  2 ). 
     The stereo feeling emphasis unit  100  has low-pass filters (hereinafter referred to as “LPFs”)  101 ,  102 , dampers (damping units)  103 ,  104 , and subtracters  105 ,  106 . The LPF  101  is configured to perform low-pass filter processing for the right channel audio signal (the input  1 ), thereby outputting a low-frequency component of the right channel audio signal. The LPF  102  is configured to perform the low-pass filter processing for the left channel audio signal (the input  2 ), thereby outputting a low-frequency component of the left channel audio signal. The damper  103  is configured to damp the output signal of the LPF  101  to a predetermined level. The damper  104  is configured to damp the output signal of the LPF  102  to a predetermined level. The subtracter  105  is configured to subtract the output signal of the damper  104  from the right channel audio signal (the input  1 ), thereby outputting the right channel audio signal with an emphasized stereo feeling. The subtracter  106  is configured to subtract the output signal of the damper  103  from the left channel audio signal (the input  2 ), thereby outputting the left channel audio signal with an emphasized stereo feeling. 
     Note that delay elements may be provided instead of the LPFs  101 ,  102 . Moreover, the degree of emphasis of the stereo feeling changes according to a distance between the two non-directional microphones and settings for the cutoff frequencies of the LPFs  101 ,  102  and the damping rates of the dampers  103 ,  104 . 
     By the processing of the stereo feeling emphasis unit  100 , a signal with a smaller phase difference between the two channel audio signals is greatly damped. For audio signals at two certain points, a lower frequency results in a smaller phase difference. As described above, the stereo feeling emphasis unit  100  outputs signals whose low-frequency components have been greatly damped as compared to high-frequency components. 
       FIG. 3  shows graphs of an example of frequency characteristics of the output signal of the subtracter  105  with respect to sensitivity  701  and a noise floor level  702 . The output signal of the subtracter  105  will be described by way of example, but the same applies to the output signal of the subtracter  106 . For the sensitivity  701 , the horizontal axis represents a frequency, and the vertical axis represents the level of sensitivity of the output signal with respect to the input signal. For the noise floor level  702 , the horizontal axis represents the frequency, and the vertical axis represents a noise floor level. The sensitivity  701  shows that the sensitivity is lower in a low frequency range and is higher in a high frequency range and shows a changing slope within a range of about 500 Hz to about 2 kHz between the low and high frequency ranges. On the other hand, the noise floor level  702  shows that the noise level in an intermediate frequency range is relatively high and the noise level in the low frequency range is substantially equal to that in the high frequency range. 
     In  FIG. 2 , the low-frequency sound volume adjustment unit  300  has equalizers (equalizer units, hereinafter referred to as “EQs”)  301 ,  302  and amplifiers (amplification units)  303  to  305 . The EQ  301  is configured to receive the output signal of the subtracter  105 , thereby correcting a sensitivity difference in the sensitivity  701  among frequency bands. The EQ  302  is configured to receive the output signal of the subtracter  106 , thereby correcting a sensitivity difference in the sensitivity  701  between frequency bands. The amplifier  303  is configured to amplify the output signal of the EQ  301 . The amplifier  304  is configured to amplify the output signal of the EQ  302 . 
       FIG. 4  shows a graph of the frequency characteristics with respect to the amplification factor of the EQ  301 . The EQ  301  will be described by way of example, but the same applies to the EQ  302 . The frequency characteristics with respect to the amplification factor of the EQ  301  show, for example, such characteristics that the amplification factor changes within a range of about 500 Hz to about 2 kHz such that the changing slope of the frequency characteristics with respect to the sensitivity  701  of  FIG. 3  is inverted, and also show such characteristics that the amplification factor is higher in the low frequency range and is lower in the high frequency range. The EQ  301  is configured such that a low-frequency component of the output signal of the subtracter  105  is amplified at a higher amplification factor than that of a high-frequency component. The EQ  302  is configured such that a low-frequency component of the output signal of the subtracter  106  is amplified at a higher amplification factor than that of a high-frequency component. 
     Note that upon digital signal processing, the EQ  301  damps, taking the amplification factor of the component with a higher amplification factor as 1, the component with a lower amplification factor for preventing the amplified data from exceeding a digital full scale. However, as a result, the high-frequency component is damped with reference to the signal level of the low-frequency component damped by the stereo feeling emphasis processing, and for this reason, the signal level is lowered across the entire frequency band. For this reason, the amplifier  303  amplifies the output signal of the EQ  301  to a proper level with the frequency characteristics being held across the entire frequency band, thereby adjusting frequency band balance. The amplifier  304  amplifies the output signal of the EQ  302  to a proper level with the frequency characteristics being held across the entire frequency band, thereby adjusting the frequency band balance. 
       FIG. 5  shows graphs of the frequency characteristics of the output signal of the amplifier  303  with respect to sensitivity  901  and a noise floor level  902 . The sensitivity  901  shows a smaller difference between the low and high frequency ranges as compared to the sensitivity  701 . Note that an LPF with a high cutoff frequency may be provided at a subsequent stage of each of the amplifiers  303 ,  304  to perform adjustment for further flattening the entire frequency band. On the other hand, the noise floor level  902  is higher on a low frequency side. This is because the damped low-frequency signal is amplified by the EQ  301 , and accordingly, the noise floor level  902  is similarly elevated. 
     When the audio collected by the microphones is loud, the floor noise indicated by the noise floor level  902  is embedded in such collected audio, and therefore, is less captured. Thus, such noise is small enough to be ignored. On the other hand, when the audio collected by the microphones is quiet, the floor noise is easily captured. Moreover, low-frequency floor noise more easily catches one&#39;s ears as compared to high-frequency floor noise, and therefore, provides the impression of feeling louder noise. 
     For this reason, the frequency band synthesizing unit  500  is configured to process the audio signal such that the stereo feeling is emphasized while the floor noise becomes less noticeable. In the present embodiment, a low-frequency component with a low noise floor level is generated and utilized. The low-frequency monophonic generation unit  200  has an adder  201  and an LPF  202 . The adder  201  is configured to add up the right channel audio signal (the input  1 ) and the left channel audio signal (the input  2 ), thereby outputting a monophonic signal. Since the output signal of the adder  201  is the monophonic signal obtained by simple addition, a low-frequency component is not specifically damped, and a white noise component as floor noise is damped by several dB. With this configuration, the adder  201  can provide a signal with a low noise floor level and a favorable signal-to-noise ratio. The LPF  202  is configured to perform the low-pass filter processing with a predetermined cutoff frequency for the monophonic signal output from the adder  201 , thereby outputting a low-frequency component of the monophonic signal. The amplifier  305  is configured to amplify the output signal of the LPF  202  at an amplification factor set by the low-frequency component selection unit  400 . 
       FIG. 6  shows graphs of the frequency characteristics of the output signal of the LPF  202  with respect to sensitivity  601  and a noise floor level  602 . The sensitivity  601  and the noise floor level  602  represent the sensitivity and noise floor level of the low-frequency component of the monophonic signal output from the LPF  202 . Note that in a case where overflow of a calculation result of the adder  201  is concerned, a level converter such as a bit shift may be provided at a former stage of the adder  201 . The same applies to other adders. 
     As described above, when the audio collected by the microphones is loud, even if the noise floor levels of the low-frequency components is elevated by correction of the EQs  301 ,  302 , such noise is embedded in the collected audio, and therefore, is less captured. Thus, the noise is less noticeable. This state causes no problem because the stereo feeling is also provided to the low-frequency audio. 
     On the other hand, when the audio collected by the microphones is quiet, the floor noise is easily captured, and therefore, such an audio signal needs to be replaced with the above-described monophonic signal with a low noise floor level and a favorable signal-to-noise ratio. For this reason, in the present embodiment, the low-frequency component selection unit  400  is provided to detect the state of the input audio, thereby switching, based on a detection result, the processing between the processing of providing the audio with the stereo feeling and the processing of providing the audio with a reduced noise floor level. 
     The low-frequency component selection unit  400  has LPFs  401 ,  402 , a level detection unit  403 , a low-frequency component determination unit  404 , a subtracter  405 , and an absolute value acquisition unit (ABS)  406 . The LPF  401  is configured to perform the low-pass filter processing for the output signal of the subtracter  105 , thereby outputting a low-frequency component of the output signal of the subtracter  105 . The LPF  402  is configured to perform the low-pass filter processing for the output signal of the subtracter  106 , thereby outputting a low-frequency component of the output signal of the subtracter  106 . The level detection unit  403  is configured to output, to the low-frequency component determination unit  404 , a greater one of the output signals of the LPFs  401 ,  402 . The subtracter  405  is configured to subtract the output signal of the LPF  402  from the output signal of the LPF  401 . The absolute value acquisition unit  406  is configured to acquire the absolute value of the output signal of the subtracter  405 , thereby outputting such an absolute value to the low-frequency component determination unit  404 . That is, the absolute value acquisition unit  406  outputs a difference between the output signals of the LPFs  401 ,  402  to the low-frequency component determination unit  404 . 
     The low-frequency component determination unit  404  is configured to determine the amount of low-frequency monophonic component to be multiplexed with the right and left channel audio signals based on the signal level output from the level detection unit  403  and the absolute value output from the absolute value acquisition unit  406 . Moreover, the low-frequency component determination unit  404  is configured to set the amplification factors of the amplifiers  303  to  305  and the amplification factors of the EQs  301 ,  302  according to the determined low-frequency monophonic component amount. An example where the low-frequency component determination unit  404  performs determination based on the levels of low-frequency components of the two signals subjected to the stereo feeling emphasis processing and a level difference between these two signals will be described herein. 
     The frequency band synthesizing unit  500  has an adder  501  and an adder  502 . The adder  501  is configured to add up the output signal of the amplifier  303  and the output signal of the amplifier  305 , thereby outputting the right channel audio signal (output  1 ) with the emphasized stereo feeling. The adder  502  is configured to add up the output signal of the amplifier  304  and the output signal of the amplifier  305 , thereby outputting the left channel audio signal (output  2 ) with the emphasized stereo feeling. 
       FIG. 7  is a flowchart of the processing of the low-frequency component determination unit  404 . The low-frequency component determination unit  404  determines the amount of addition of the low-frequency monophonic signal to the stereo feeling emphasis processing signal. First, at a step S 11 , the low-frequency component determination unit  404  determines whether or not the level of signal output from the level detection unit  403  is smaller than a first predetermined value. In a case where the low-frequency component determination unit  404  determines that the signal level is smaller than the first predetermined value, the low-frequency component determination unit  404  proceeds the processing to a step S 12 . In a case where the low-frequency component determination unit  404  determines that the signal level is not smaller than the first predetermined value, the low-frequency component determination unit  404  proceeds the processing to a step S 16 . 
     At the step S 12 , the low-frequency component determination unit  404  determines whether or not the level of signal output from the level detection unit  403  is smaller than a second predetermined value. The second predetermined value is smaller than the first predetermined value. In a case where the low-frequency component determination unit  404  determines that the signal level is smaller than the second predetermined value, the low-frequency component determination unit  404  proceeds the processing to a step S 13 . In a case where the low-frequency component determination unit  404  determines that the signal level is not smaller than the second predetermined value, the low-frequency component determination unit  404  proceeds the processing to a step S 14 . 
     The first predetermined value described herein is such a level that the audio collected by the microphones is equivalent to 80 dBspl. The second predetermined value is such a level that the audio collected by the microphones is equivalent to 40 dBspl. These values have been described by way of example, and suitable values are set considering the noise floor level of the low-frequency component subjected to the stereo feeling emphasis processing, for example. 
     At the step S 16 , the low-frequency component determination unit  404  determines whether or not the absolute value output from the absolute value acquisition unit  406  is smaller than a first predetermined value. In a case where the low-frequency component determination unit  404  determines that the absolute value is smaller than the first predetermined value, the low-frequency component determination unit  404  proceeds the processing to a step S 15 . In a case where the low-frequency component determination unit  404  determines that the absolute value is not smaller than the first predetermined value, the low-frequency component determination unit  404  proceeds the processing to a step S 17 . 
     At the step S 14 , the low-frequency component determination unit  404  determines whether or not the absolute value output from the absolute value acquisition unit  406  is smaller than a second predetermined value. In a case where the low-frequency component determination unit  404  determines that the absolute value is smaller than the second predetermined value, the low-frequency component determination unit  404  proceeds the processing to the step S 13 . In a case where the low-frequency component determination unit  404  determines that the absolute value is not smaller than the second predetermined value, the low-frequency component determination unit  404  proceeds the processing to the step S 15 . 
     At the step S 13 , the low-frequency component determination unit  404  determines that the signal level of the low-frequency component is lowest, and sets the amplification factors of the EQs  301 ,  302  and the amplifiers  303  to  305  such that a great amount of low-frequency monophonic signal to be multiplexed with the right and left channel audio signals is set. The amplification factor of the amplifier  305  is increased. For example, the EQs  301 ,  302  output, without correction, the signals with the damped low frequency components. The amplifiers  303  to  305  adjust the frequency band balance such that the damped low frequency components output from the EQs  301 ,  302  are complemented with the monophonic component output from the LPF  202 . At this point, the adder  501  adds up the signal output with the characteristics of  FIG. 3  from the amplifier  303  and the signal output with the characteristics of  FIG. 6  from the amplifier  305 , thereby outputting the signal with the characteristics of  FIG. 8 . 
       FIG. 8  shows graphs of the frequency characteristics of the output signal of the adder  501  with respect to sensitivity  801  and a noise floor level  802 . The same applies to the output signal of the adder  502  and the output signal of the adder  501 . The adder  501  adds up the output signal of the amplifier  303  and the output signal of the amplifier  305 . The sensitivity  801  shows more improved sensitivity balance between the high and low frequency ranges as compared to the sensitivity  901  of  FIG. 5 . The noise floor level  802  shows that the noise floor level in the low frequency range is held at a lower state as compared to the noise floor level  902  of  FIG. 5 . 
     At the step S 17 , the low-frequency component determination unit  404  sets the amplification factors of the EQs  301 ,  302  and the amplifiers  303  to  305  such that the amount of low-frequency monophonic signal to be multiplexed with the right and left channel audio signals is zero. The amplification factor of the amplifier  305  is zero. The adders  501 ,  502  each output only the components subjected to the stereo feeling emphasis processing without multiplexing the low-frequency monophonic signal output from the amplifier  305  with the output signals of the amplifiers  303 ,  304 . 
     At the step S 15 , the low-frequency component determination unit  404  determines as lack of the stereo feeling, and sets the amplification factors of the EQs  301 ,  302  and the amplifiers  303  to  305  such that the amount of low-frequency monophonic signal to be multiplexed with the right and left channel audio signals is at an intermediate level. The amplification factor of the amplifier  305  is set at an intermediate level. The adders  501 ,  502  each multiplex the low-frequency monophonic signal output from the amplifier  305  with the right and left channel audio signals output from the amplifiers  303 ,  304 , thereby reducing a noise feeling. 
     Note that as described above, the determination conditions of the steps S 14  and S 16  can be omitted. For example, at the step S 11 , in a case where the low-frequency component determination unit  404  determines that the level of signal output from the level detection unit  403  is not smaller than the first predetermined value, the low-frequency component determination unit  404  proceeds the processing to the step S 17 . At the step S 12 , in a case where the low-frequency component determination unit  404  determines that the level of signal output from the level detection unit  403  is not smaller than the second predetermined value, the low-frequency component determination unit  404  proceeds the processing to the step S 15 . In this case, the low-frequency component determination unit  404  sets the amplification factors of the EQs  301 ,  302  and the amplifiers  303  to  305  according to the level of signal output from the level detection unit  403 . 
     The low-frequency component determination unit  404  controls the amplification factors of the amplifiers  303  to  305  such that the amplification factor of the amplifier  305  increases and the amplification factors of the amplifiers  303 ,  304  decrease with a decrease in the level of signal output from the level detection unit  403 . 
     With the output signal of the absolute value acquisition unit  406  as the added determination condition, the low-frequency component determination unit  404  can more finely determine the effect of use of the stereo feeling emphasized signal for the low-frequency component. Note that the determination condition which is the output signal of the absolute value acquisition unit  406  does not provide, in terms of which one of the collected audio or the floor noise is more easily captured, much influence as compared to the determination condition which is the level of signal output from the level detection unit  403 . Thus, even when the determination condition which is the output signal of the absolute value acquisition unit  406  is omitted, a proper effect can be expected. 
     The low-frequency component determination unit  404  may use the output signal of the LPF  202  instead of the output signal of the level detection unit  403 . In this case, the low-frequency component determination unit  404  sets the amplification factors of the amplifiers  303  to  305  and the amplification factors of the EQs  301 ,  302  based on the output signal of the LPF  202  and the output signal of the absolute value acquisition unit  406 . That is, the low-frequency component determination unit  404  sets the amplification factors of the amplifiers  303  to  305  and the amplification factors of the EQs  301 ,  302  based on the low-frequency component of the monophonic signal output from the LPF  202  and a difference between the output signals of the LPFs  401 ,  402 . 
     The steps S 14  and S 16  may be omitted, and the low-frequency component determination unit  404  may set the amplification factors of the amplifiers  303  to  305  and the amplification factors of the EQs  301 ,  302  based on the output signal of the LPF  202 . That is, the low-frequency component determination unit  404  may sets the amplification factors of the amplifiers  303  to  305  and the amplification factors of the EQs  301 ,  302  based on the low-frequency component of the monophonic signal output from the LPF  202 . 
     The low-frequency component determination unit  404  controls the amplification factors of the amplifiers  303  to  305  such that the amplification factor of the amplifier  305  increases and the amplification factors of the amplifiers  303 ,  304  decrease with a decrease in the level of signal output from the LPF  202 . 
     Note that at the step S 17 , the low-frequency component determination unit  404  sets, to zero, the amount of low-frequency monophonic component to be multiplexed, but may set such that the amount of low-frequency monophonic component to be multiplexed is smaller than that of the step S 15 . 
     Note that a greater number of predetermined values for comparison with the signal level and predetermined values for comparison with the absolute value can result in more detailed control steps for the amount of low-frequency monophonic signal to be multiplexed. Conversely, a smaller number of predetermined values for comparison with the signal level and predetermined values for comparison with the absolute value can change the amount of low-frequency monophonic signal to be multiplexed only in a limited state. For example, at the step S 16 , the first predetermined value is set to zero. In this case, when the signal level is greater than the first predetermined value, the processing inevitably proceeds to the step S 17 , and the adders  501 ,  502  can output only the components subjected to the stereo feeling emphasis processing without multiplexing of the low-frequency monophonic signal. 
     Note that in a case where each of the adders  501 ,  502  mixes both of the low-frequency component subjected to the stereo feeling emphasis processing and the low-frequency monophonic signal, the low-frequency component determination unit  404  also adjusts the amplification factors of the EQs  301 ,  302  according to the amount of low-frequency monophonic signal to be multiplexed. That is, when the amount of low-frequency monophonic signal to be multiplexed is great, the low-frequency component determination unit  404  decreases the amplification factors of the EQs  301 ,  302 , thereby reducing elevation of the noise floor level. When the amplification factors of the EQs  301 ,  302  are changed, the output levels of the EQs  301 ,  302  are also changed. Thus, according to such a change, the amplification factors of the amplifiers  303 ,  304  are also adjusted. 
     The amplifiers  303  to  305  can be used to resemble the volume control for adjusting the output amount of each signal. Thus, when the amount of low-frequency monophonic signal to be multiplexed is changed, a time constant is applied to a change in the output levels of the amplifiers  303  to  305 , and a change is made such that a stereo component and a monophonic component are cross-faded. This can ease a rapid change in noise floor and directional characteristics. 
     The adder  501  adds up the output signal of the amplifier  303  and the output signal of the amplifier  305 , thereby outputting the right channel audio signal (the output  1 ) with the emphasized stereo feeling. The adder  502  adds up the output signal of the amplifier  304  and the output signal of the amplifier  305 , thereby outputting the left channel audio signal (the output  2 ) with the emphasized stereo feeling. 
     According to the present embodiment, the audio processing unit  8  can output the audio with reduced low-frequency floor noise when the audio collected by the microphones is quiet, and can output the audio with the emphasized stereo feeling even in the low frequency range when the audio collected by the microphones is loud. 
     Second Embodiment 
       FIG. 9  is a diagram of a function configuration example of an audio processing unit  8  according to a second embodiment of the present disclosure. The audio processing unit  8  of  FIG. 2  of the first embodiment has the EQs  301 ,  302 , but the audio processing unit  8  of  FIG. 9  of the second embodiment has no EQs  301 ,  302 . The audio processing unit  8  of  FIG. 9  is similar to the audio processing unit  8  of  FIG. 2  in a stereo feeling emphasis unit  100 , a low-frequency monophonic generation unit  200 , and a low-frequency component selection unit  400 , and is different from the audio processing unit  8  of  FIG. 2  in a low-frequency sound volume adjustment unit  300  and a frequency band synthesizing unit  500 . Hereinafter, differences of the present embodiment from the first embodiment will be described. 
     The low-frequency sound volume adjustment unit  300  has high-pass filters (high-pass filter units, hereinafter referred to as “HPFs”)  311 ,  313 , LPFs (Low-pass filter units)  312 ,  314 , amplifiers  315  to  319 , and adders  320 ,  321 . The HPF  311  is configured to perform high-pass filter processing with a predetermined cutoff frequency for an output signal of a subtracter  105 , thereby outputting a high-frequency component of the output signal of the subtracter  105 . The HPF  313  is configured to perform the high-pass filter processing with a predetermined cutoff frequency for an output signal of a subtracter  106 , thereby outputting a high-frequency component of the output signal of the subtracter  106 . The LPF  312  is configured to perform low-pass filter processing with a predetermined cutoff frequency for the output signal of the subtracter  105 , thereby outputting a low-frequency component of the output signal of the subtracter  105 . The LPF  314  is configured to perform the low-pass filter processing with a predetermined cutoff frequency for the output signal of the subtracter  106 , thereby outputting a low-frequency component of the output signal of the subtracter  106 . 
     The cutoff frequencies of the HPFs  311 ,  313  and the LPFs  312 ,  314  are set considering easiness of adjustment in a case where a frequency band for which a stereo feeling needs to be left upon multiplexing of a low-frequency monophonic signal is adjusted or frequency characteristics are adjusted at a subsequent stage of the frequency band synthesizing unit  500 . In a case where the output signals of the subtracters  105 ,  106  show the frequency characteristics shown in  FIG. 4 , the cutoff frequencies are set within a range of 500 to 2 kHz. Note that considering the above-described point, when other frequencies are proper, the cutoff frequencies may be set accordingly. Moreover, when the same cutoff frequency of the LPFs  312 ,  314  is applied to the cutoff frequency of the LPF  202 , low-frequency components with equivalent frequency bands are provided via each LPF. 
     The amplifier  315  is configured to amplify the output signal of the HPF  311 . The amplifier  316  is configured to amplify the output signal of the LPF  312 . The amplifier  317  is configured to amplify the output signal of the HPF  313 . The amplifier  318  is configured to amplify the output signal of the LPF  314 . The amplifier  319  is configured to amplify the output signal of the LPF  202 . The amplifiers  315 ,  317  amplify the high-frequency components of the signals, and therefore, the amplification factors of the amplifiers  315 ,  317  are fixed regardless of the amount of multiplexing of the low-frequency monophonic signal. On the other hand, the amplifiers  316 ,  318 ,  319  change the amplification factors thereof according to the amount of multiplexing of the low-frequency monophonic signal. 
     As in the processing of the flowchart of  FIG. 7 , a low-frequency component determination unit  404  determines the amount of multiplexing of the low-frequency monophonic signal, and sets the amplification factors of the amplifiers  316 ,  318 ,  319 . The low-frequency component determination unit  404  decreases the amplification factors of the amplifiers  316 ,  318  for increasing the amplification factor of the amplifier  319 , and increases the amplification factors of the amplifiers  316 ,  318  for decreasing the amplification factor of the amplifier  319 . A time constant is applied to a change in the amplification factors of the amplifiers  316 ,  318 ,  319 , and a change is made such that a stereo component and a monophonic component are cross-faded. This can ease a rapid change in noise floor and directional characteristics. 
     The adder  320  is configured to add up the output signal of the amplifier  316  and the output signal of the amplifier  319 , thereby outputting a low-frequency component of a right channel audio signal. The adder  321  is configured to add up the output signal of the amplifier  318  and the output signal of the amplifier  319 , thereby outputting a low-frequency component of a left channel audio signal. 
     The frequency band synthesizing unit  500  has an adder  511  and an adder  512 . The adder  511  is configured to add up the output signal of the amplifier  315  and the output signal of the adder  320 , thereby outputting the right channel audio signal (output  1 ) with the entire frequency band. The adder  512  is configured to add up the output signal of the amplifier  317  and the output signal of the adder  321 , thereby outputting the left channel audio signal (output  2 ) with the entire frequency band. The output signals of the adders  511 ,  512  show the frequency characteristics with respect to the sensitivity  801  and the noise floor level  802  as shown in  FIG. 8 . 
     Note that in the low-frequency component selection unit  400 , the signal low-frequency components targeted for level detection of a level detection unit  403  and subtraction of a subtracter  405  are the same as those for which gain adjustment needs to be performed by the amplifiers  316 ,  318 . Thus, in the low-frequency component selection unit  400 , the level detection unit  403  and the subtracter  405  can each perform level detection and subtraction based on the output signals of the LPFs  312 ,  314 . In this case, the LPFs  401 ,  402  can be omitted. 
     Embodiment(s) of the present disclosure can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     While the present disclosure has been described with reference to exemplary embodiments, the scope of the following claims are to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2017-099887, filed May 19, 2017, which is hereby incorporated by reference herein in its entirety.