Abstract:
According to an aspect of an embodiment, a multipoint communication apparatus comprises: a receiving unit for receiving a plurality of signals containing voice signal elements and noise signal elements from the plurality of other communication apparatuses, respectively; a noise estimating unit for estimating the plurality of noise signal elements in the plurality of received signals, respectively; a gain control unit for adjusting gains of the plurality of signals passing therethrough so as to equalize the levels of the plurality of noise signal elements outputted from the gain element on the basis of the estimated levels of the noise signal elements; a combining unit for combining the plurality of signals passed through the gain control unit; and an output unit for outputting a sound signal on the basis of the a combined signal by the combining unit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sound volume control unit and a method for automatically adjusting sound volume of sound signals. 
     2. Description of the Related Art 
     A multi-point speech system for speaking among two or more points by using cellular phones, IP (Internet Protocol) telephones, television conference systems and the like has come to be used lately. In such multi-point speech system, sound volume of receiving signals may differ per point due to sensitivity of microphones of transmitting-side units of the respective points even if reproducing volume of a receiving-side unit is set at certain level. 
       FIG. 19  shows an exemplary configuration of such multi-point speech system. Transmitting-side telephones  101  and  102  as well as a receiving-side telephone  104  are connected to a communication network  103 . The telephone  104  receives a voice signal S 1  from the telephone  101  and a voice signal S 2  from the telephone  102  and a speaker  105  converts output signals and outputs as voice. 
     When volume of the voice signal S 1  is large and volume of the voice signal S 2  is small at this time, volume of the voices outputted out of the speaker  105  differs between those from the telephone  101  and the telephone  102 . Then, it has been desired to automatically adjust the volumes of the receiving signals in order to make the volumes of sounds of the all points even. 
       FIG. 20  shows a prior art sound volume control method using automatic gain controls (AGC). The AGC is a function for automatically adjusting an amplification factor (gain) of an amplifying circuit so that volume of an output is adjusted to a desirable level even when amplitude of an input signal fluctuates. 
     A mixer  203  performs mixing (addition) in this volume control method after leveling the volumes of the receiving signals S 1  and S 2  of the respective points by the AGCs  201  and  202 , respectively. Thereby, it becomes possible to correct the difference between the volumes of the points. Various configurations have been proposed as the configuration of the AGCs  201  and  202 . 
       FIG. 21  is a structural view of an AGC described in a non-patent literature, Peter L. Chu, “VOICE-ACTIVATED AGC FOR TELECONFERENCING” proceedings ICASSP96 vol. 2, pp. 929-932, 1996. According to this configuration, a frame electric power calculating section  301  divides an input signal into frames of 20 ms and calculates energy (frame power) within each frame. Next, a maximum value calculating section  302  calculates a maximum value of the frame power from the past to the present time and a gain calculating section  303  calculates a gain from a difference of powers between the maximum value and a target level. Then, a multiplier  304  multiplies the gain with the input signal to generate an output signal. 
     However, although the volumes of voices of speakers in the output signals of the AGC are almost leveled in all of the points, volume of noise that depends on an ambient environment differs per each point. Still more, a SNR (Signal-to-Noise Ratio) of each point does not change. Accordingly, a SNR of an output signal after mixing is adjusted to a value of a point where the SNR of the receiving signal is least among all of the points. Therefore, when there is such point where the SNR is small, the SNR of all of the points becomes small and it becomes hard to catch the voices. 
     Japanese Patent Application Laid-open No. 2004-133403 relates to a voice signal processing apparatus that samples voices that form a conversation in a conversation state in which a plurality of voices and noises are mixed and raises an output volume of its voice or lowers a volume of other sounds. 
     Japanese Patent Application Laid-open No. 2004-507141 relates to a method for processing a voice signal to overcome background noise not related to the voice signal, Japanese Patent Application Laid-open No. 2002-223268 relates to a voice control unit for obtaining receiving voice from which a discomfort feeling is eliminated without being buried in an ambient background noise and Japanese Patent Application Laid-open No. 2002-1575100 relates to an adaptive noise suppressing voice coding apparatus that detects and eliminates noises within a present speech. 
     SUMMARY 
     According to an aspect of an embodiment, a multipoint communication apparatus comprises: a receiving unit for receiving a plurality of signals containing voice signal elements and noise signal elements from the plurality of other communication apparatuses, respectively; a noise estimating unit for estimating the plurality of noise signal elements in the plurality of received signals, respectively; a gain control unit for adjusting gains of the plurality of signals passing therethrough so as to equalize the levels of the plurality of noise signal elements outputted from the gain element on the basis of the estimated levels of the noise signal elements; a combining unit for combining the plurality of signals passed through the gain control unit; and an output unit for outputting a sound signal on the basis of the a combined signal by the combining unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a structural view of a first multi-point teleconferencing apparatus; 
         FIG. 2  is a structural view of an inter-point sound volume control section; 
         FIG. 3  is a structural view of a first AGC; 
         FIG. 4  illustrates charts showing power spectrums of a voice section; 
         FIG. 5  illustrates charts showing powers after mixing; 
         FIG. 6  is a structural view of a receiving section of a cellular phone; 
         FIG. 7  is a structural view of a second AGC; 
         FIG. 8  is a graph showing a relationship between a multiplication factor and SNR; 
         FIG. 9  is a structural view of a second multi-point teleconferencing apparatus; 
         FIG. 10  illustrates charts showing first volume control of the invention; 
         FIG. 11  illustrates charts showing prior art first volume control; 
         FIG. 12  is a structural view of a, third multi-point teleconferencing apparatus; 
         FIG. 13  illustrates charts showing second volume control of the invention; 
         FIG. 14  illustrates charts showing prior art second volume control; 
         FIG. 15  is a structural view of a fourth multi-point teleconferencing apparatus; 
         FIG. 16  illustrates charts showing third volume control of the invention; 
         FIG. 17  illustrates charts showing prior art third volume control; 
         FIG. 18  is a structural view of a third AGC; 
         FIG. 19  is a structural view of a prior art multi-point teleconference system; 
         FIG. 20  is a structural view of a prior art sound volume control method; and 
         FIG. 21  is a structural view of a prior art AGC. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments for carrying out the invention will be explained below in detail with reference to the drawings. 
       FIG. 1  shows an exemplary configuration of a sound volume control unit (multi-point teleconferencing apparatus) applied to a multi-point teleconferencing system. A teleconference is carried out among three points in this case. This multi-point teleconferencing apparatus has noise volume estimating sections  401  and  402 , an inter-point volume control section  403 , AGCs  404  and  405  and a mixer  406  and outputs an output signal by adjusting sound volumes (levels) of receiving signals from two points. 
     The noise volume estimating sections  401  and  402  calculate levels of noises contained respectively in the receiving signals S 1  and S 2  and output calculation results to the inter-point volume control section  403 . Based on the calculation results of the noise level, the inter-point volume control section  403  controls levels of the receiving signals S 1  and S 2  so that the noise levels of the receiving signals S 1  and S 2  are made even and output them to the AGCs  404  and  405 , respectively. 
     The AGCs  404  and  405  adjust gains of the signals outputted out of the inter-point volume control section  403  to a target level and the mixer  406  performs mixing on the output signals from the AGCs  404  and  405  and then outputs an output signal. 
       FIG. 2  shows an exemplary configuration of the inter-point volume control section  403  in  FIG. 1 . The inter-point volume control section  403  has amplifiers  501  and  502  and a gain calculating section  503 . The amplifiers  501  and  502  amplify the receiving signals S 1  and S 2 , respectively, and output them to the AGCs  404  and  405 . The gain calculating section  503  adjusts gains of the amplifiers  501  and  502  based on the calculation results of the noise volume estimating sections  401  and  402 . 
     The gain calculating section  503  calculates the gains of the amplifiers  501  and  502  so as to adjust the noise level of each receiving signal to a reference level of noise level of a point having a highest SNR and sets the obtained gain to each of the amplifiers. It facilitates to catch a voice of the point where the SNR is large and the voice levels are made even among the points. It is noted that a minimum value of the noise levels or a predetermined noise level of the plurality of points may be used as the reference level instead of the noise level of the point having the highest SNR. 
       FIG. 3  shows an exemplary configuration of the AGCs  404  and  405  in  FIG. 1 . This AGC includes a fast Fourier transforming section  601  (FFT), a noise spectrum estimating section  602 , a SNR estimating section  603 , a target gain calculating section  604 , a gain calculating section  605 , a multiplier  606  and an inverse fast Fourier transforming section  607  (IFFT) and controls an amplification amount of each band corresponding to the SNR and a target level in a frequency domain. Each domain corresponds to each discrete value of the frequency for example. 
     The FFT  601  transforms the input signal from a temporal domain to the frequency domain by orthogonal transformation to find a power spectrum (input spectrum) of input sounds. Specifically, the FFT  601  divides the input signal into a plurality of bands to calculate a band signal of each band and finds the power per band from each band signal to calculate the input spectrum. 
     The noise spectrum estimating section  602  outputs a spectrum of non-voice section containing only noise signals in the input spectrum as a noise spectrum and the SNR estimating section  603  finds a SNR spectrum that is a ratio between the input spectrum and the noise spectrum. The SNR spectrum represents the SNR of each band. 
     The target gain calculating section  604  calculates a target gain so that volume of the output signal is adjusted to the target level from the input spectrum and the target level and the gain calculating section  605  calculates a gain of each band from the target gain and the SNR spectrum. At this time, the gain calculating section  605  calculates the gain of each band so that the gain is adjusted to the target gain in the band whose SNR is high and so that the gain is adjusted to 1 in the band whose SNR is low, i.e., so as not to be amplified. 
     The multiplier  606  multiplies the gain of each band with each band of the input spectrum to adjust the level of each band. The IFFT  607  generates an output signal by transforming the input signal from the frequency domain to the temporal domain by orthogonal inverse transformation. 
     Such AGC allows only voice components whose SNR is high to be adjusted to the target level without changing the noise components whose SNR is low. 
       FIG. 4  shows power spectrums of a voice section containing voices, other than noise, such as a voice of a speaker. A power spectrum  701  of the input sound of the AGC contains a power spectrum  702  of the voice component. 
     The power spectrum  701  changes like a power spectrum  703  in the prior art AGC shown in  FIG. 21  because the whole band is amplified homogeneously. In this case, the noise component is also amplified together with the voice component and sound offensive to ears is outputted. In addition to that, fluctuation of the level of the noise component occurs, causing sound more offensive to the ears, because the gain changes between the voice section and the non-voice section. 
     In contrary to that, the power spectrum  701  changes like a power spectrum  704  in the AGC in  FIG. 3  because the bands whose SNR is low are not amplified and only the bands whose SNR is high are selectively amplified. Accordingly, it becomes possible to amplify only the voice components without amplifying the noise components. 
       FIG. 5  shows powers of the output signals after mixing the receiving signals of the two points. Although the sound level becomes constant by adjusting the receiving signals S 1  and S 2  by the AGCs in the prior art volume control method shown in  FIG. 20 , the SNR after mixing becomes low. In contrary to that, the SNR after mixing becomes large as compared to the prior art in the volume control method in  FIG. 1  by applying the AGC after adjusting the noise levels between the points. 
     Although  FIG. 1  shows the case when the receiving signals from the two points are processed, the same applies to a case when receiving signals of three or more points are processed. 
       FIG. 6  shows an exemplary configuration of a case when the AGC in  FIG. 3  is applied to a receiving section of a cellular phone. A decoding section  901  decodes a receiving signal and outputs it to an AGC  902  and the AGC  902  adjusts a gain of the signal outputted out of the decoding section  901  and outputs it to a speaker  903 . 
       FIG. 7  shows an exemplary configuration of the AGC  902  in  FIG. 6 . This AGC has the FFT  601 , the noise spectrum estimating section  602 , the SNR estimating section  603 , the gain calculating section  605 , the multiplier  606 , the IFFT  607 , a voice action detecting section  1001  (VAD), an average voice level calculating section  1002  and a target gain calculating section  1003 . 
     The VAD  1001  detects voice sections and non-voice sections from an input signal and the average voice level calculating section  1002  calculates an average voice level from an average level of the voice sections. The target gain calculating section  1003  calculates a target gain G 0  from a ratio between the average voice level and a target level. 
     The FFT  601  calculates an input spectrum by performing the FFT on the input signal. The noise spectrum estimating section  602  detects the voice sections and the non-voice sections by detecting a speech action from the input spectrum and calculates a noise spectrum from an average level of the non-voice sections. The SNR estimating section  603  calculates a SNR spectrum from the input spectrum and the noise spectrum. 
     The gain calculating section  605  calculates a gain G(i) from an i-th band from the target gain G 0  and the SNR spectrum by the following expression:
 
 G ( i )= G 0×β( i ) ( i= 1, 2, . . . ,  n )
 
     Where, the i-th factor β(i) is determined corresponding to the SNR of the i-th band contained in the SNR spectrum. 
     β(i) takes a value as shown in  FIG. 8  for example when G 0  and G(i) are represented by logarithmic gain. In this case, β(i) of the band whose SNR is 0 to 2 dB is zero and β(i) of the band whose SNR exceeds 6 dB is 1. β(i) of the band whose SNR is 2 to 6 dB assumes a value from zero to 1. Accordingly, G(i) of the band whose SNR is 0 to 2 dB turns out to be 0, G(i) of the band whose SNR exceeds 6 dB coincides with G 0  and G(i) of the band whose SNR is 2 to 6 dB assumes a value from 0 to G 0 . 
     The multiplier  606  multiplies G(i) with the power of the i-th band in the input spectrum to calculate an output spectrum and the IFFT  607  perform the IFFT on the output spectrum to generate an output signal. 
       FIG. 9  shows an exemplary configuration in which the AGC in  FIG. 7  is applied to the multi-point teleconferencing apparatus. A teleconference is carried out among three points in this case. The multi-point teleconferencing apparatus has a noise volume estimating section  1201 , an inter-point volume control section  1202 , AGCs  1203  and  1204  and a mixer  1205 . 
     An operation of the noise volume estimating section  1201  is the same with the noise volume estimating sections  401  and  402  in  FIG. 1  and operations of the inter-point volume control section  1202  and the mixer  1205  are the same with those of the inter-point volume control section  403  and the mixer  406  in  FIG. 1 . The AGCs  1203  and  1204  have the same configuration with that shown in  FIG. 7 . 
       FIG. 10  illustrates exemplary sound control in the multi-point teleconferencing apparatus in  FIG. 9 . The noise volume estimating section  1201  calculates a noise level of each point at first. The noise volume estimating section  1201  calculates a noise level  1303  (−60 dBov) from power  1301  of the receiving signal S 1  and calculates a noise level  1304  (−80 dBov) from power  1302  of the receiving signal S 2 . In this case, the noise level  1304  corresponds to a minimum value of the noise level. 
     Then, the inter-point volume control section  1202  attenuates the receiving signal S 1  so that the noise level  1303  coincides with the noise level  1304 . Thereby, the power  1301  of the receiving signal S 1  changes like power  1305 . 
     Next, the AGC  1203  amplifies only the voice components of the receiving signal S 1  to a target level  1306  (−26 dBov) Thereby, the power  1305  of the receiving signal S 1  changes like power  1307 . Meanwhile, the voice components of the receiving signal S 2  have been already reaching to the target level  1306 . 
     Next, the mixer  1205  generates the output signal by mixing adjusted signals from the AGCs  1203  and  1204 . A voice level of the generated output signal is −26 dBov and a noise level thereof is −80 dBov. 
     In contrary to that, according to the prior art volume control method shown in  FIG. 20 , the AGC  201  amplifies the receiving signal S 1  to the target level  1306  as shown in  FIG. 11 . Thereby, the power  1301  of the receiving signal S 1  changes like power  1401  and a noise level  1303  rises to a noise level  1402  (−40 dBov). 
     Next, the mixer  203  outputs the output signal by mixing the adjusted signals from the AGCs  201  and  202 . A voice level of the generated output signal is −26 dBov and a noise level is −40 dBov. 
     Thus, although the noise level after mixing is −40 dBov in the prior art, it is −80 dBov in the invention and the apparatus of the invention can lower the noise level. 
       FIG. 12  shows another exemplary configuration of the multi-point teleconferencing apparatus. This multi-point teleconferencing apparatus has a configuration in which the inter-point volume control section  1202  in the configuration in  FIG. 9  is replaced with an inter-point volume control section  1502  and a SNR estimating section  1501  is added. 
     The SNR estimating section  1501  calculates SNRs of the receiving signals S 1  and S 2  and outputs them to the inter-point volume control section  1502 . The inter-point volume control section  1502  finds a maximum value of the SNRs from the SNR estimating section  1501  and calculates a target noise level from a difference between the maximum value of the SNR and a target level. Then, the inter-point volume control section  1502  adjusts the level of the receiving signal of each point so that the noise levels of all of the points are adjusted to the target level. 
       FIG. 13  shows an exemplary volume control of the multi-point teleconferencing apparatus in  FIG. 12 . The noise volume estimating section  1201  calculates a noise level of each point at first and the SNR estimating section  1501  calculates SNR of each point. They calculate a voice level  1603  (−50 dBov) and a noise level  1605  (−60 dBov) from power  1601  of the receiving signal S 1 , and the SNR of the receiving signal S 1  turns out to be 10 dB. They also calculate a voice level  1604  (−60 dBov) and a noise level  1606  (−80 dBov) from power  1602  of the receiving signal S 2  and, the SNR of the receiving signal S 2  turns out to be 20 dB. 
     Next, the inter-point volume control section  1502  calculates a target noise level  1608  (−46 dBov) by subtracting the maximum value 20 dB of the SNR from a target level  1607  (−26 dBov). Then, the receiving signals S 1  and S 2  are amplified respectively so that the noise levels  1606  and  1606  coincide with the target noise level  1608 . Thereby, the power  1601  of the receiving signal S 1  changes like power  1609  and a voice level  1611  thereof turns out to be −36 dBov. The power  1602  of the receiving signal S 2  changes like power  1610  and a voice level thereof coincides with the target level  1607  (−26 dBov). 
     Next, the AGC  1203  amplifies only the voice component of the receiving signal S 1  to the target level  1607  (−26 dBov). Thereby, the power  1609  of the receiving signal S 1  changes like power  1612 . Meanwhile, the voice component of the receiving signal S 2  has been already reaching to the target level  1607 . 
     Next, the mixer  1205  generates the output signal by mixing adjusted signals from the AGCs  1203  and  1204 . A sound level of the generated output signal is −26 dBov and a noise level thereof is −46 dBov. 
     In contrary to that, according to the prior art volume control method shown in  FIG. 20 , the AGC  201  amplifies the receiving signal S 1  to the target level  1607  as shown in  FIG. 14  and the AGC  202  amplifies the receiving signal S 2  to the target level  1607 . Thereby, power  1601  of the receiving signal S 1  changes like power  1701  and a noise level  1605  rises to a noise level  1703  (−36 dBov). Also, power  1602  of the receiving signal S 2  changes like power  1702  and a noise level  1606  rises to a noise level  1704  (−46 dBov). 
     Next, the mixer  203  outputs the output signal by mixing the adjusted signals from the AGCs  201  and  202 . A voice level of the generated output signal is −26 dBov and a noise level is −36 dBov. 
     Thus, although the noise level after mixing is −36 dBov in the prior art, it is −46 dBov in the invention and the apparatus of the invention can lower the noise level. 
       FIG. 15  shows a still other exemplary configuration of the multi-point teleconferencing apparatus. A teleconference is carried out among four points in this case. This multi-point teleconferencing apparatus has a noise volume estimating section  1801 , a SNR estimating section  1802 , an inter-point volume control section  1803 , AGCs  1804  through  1806  and a mixer  1807  and generates an output signal from three receiving signals S 1  through S 3  in the same manner with the multi-point teleconferencing apparatus in  FIG. 12 . 
       FIG. 16  shows exemplary volume control in the multi-point teleconferencing apparatus in  FIG. 15 . The noise volume estimating section  1801  calculates noise levels of the respective points at first and the SNR estimating section  1802  calculates SNRs of the respective point. Voice levels of the receiving signals S 1 , S 2  and S 3  are −10 dBov, −30 dBov and −40 dBov ( 1901 ), respectively, and noise levels thereof are −30 dBov, −60 dBov and −80 dBov ( 1902 ), respectively. Accordingly, SNRs thereof are 20 dB, 30 dB and 40 dB ( 1903 ), respectively. 
     Next, the inter-point volume control section  1803  subtracts a SNR maximum value of 40 dB from the target level (−25 dBov) to calculate a target noise level (−65 dBov) Then, it adjusts the receiving signals S 1 , S 2  and S 3  so that the respective noise levels coincide with the target noise level. Thereby, the voice levels of the receiving signals S 1 , S 2  and S 3  turn out to be −45 dBov, −35 dBov and −25 dBov ( 1904 ) and the noise level turns out to be −65 dBov ( 1905 ). 
     Next, the AGCs  1804  and  1805  amplify only the voice components of the receiving signals S 1  and S 2  to the target level (−25 dBov). Thereby, the voice level of the receiving signals S 1 , S 2  and S 3  turn out to be −25 dBov ( 1906 ). Meanwhile, the noise level is kept to be −65 dBov ( 1907 ). 
     Next, the mixer  1807  outputs the output signal by mixing the adjusted signals from the AGCs  1804  through  1806 . A voice level of the generated output signal turns out to be −25 dBov ( 1908 ) and a noise level thereof turns out to be −65 dBov ( 1909 ). 
       FIG. 17  shows the prior art exemplary volume control shown in  FIG. 20 . Voice levels ( 2001 ) and noise levels ( 2002 ) of receiving signals S 1 , S 2  and S 3  are the same with those shown in  FIG. 16  in this case. The AGCs adjust these receiving signals to the target level (−25 dBov). Thereby, voice levels of the receiving signals S 1 , S 2  and S 3  turn out to be −25 dBov ( 2003 ) and noise levels thereof turn out to be −45 dBov, −55 dBov and −65 dBov ( 2004 ), respectively. 
     Next, mixing is performed on the adjusted signals to generate an output signal. A voice level of the generated output signal turns out to be −25 dBov ( 2005 ) and a noise level thereof turns out to be −45 dBov ( 2006 ). 
     Thus, while the noise level after mixing is −45 dBov according to the prior art, it is 65 dBov according to the invention and the apparatus of the invention can lower the noise level. 
     Although  FIGS. 12 and 15  show the configurations for processing the receiving signals from two and three points, respectively, the same applies to a configuration for processing receiving signals from four or more points. 
       FIG. 18  shows another exemplary configuration of the AGCs shown in  FIGS. 6 ,  9 ,  12  and  15 . A voice action is detected based on an input spectrum instead of receiving signals in this configuration. 
     The AGC has the FFT  601 , the SNR estimating section  603 , the gain calculating section  605 , the multiplier  606 , the IFFT  607 , a noise spectrum estimating section  2101 , a VAD  2102  and a target gain calculating section  2103 . Among them, operations of the FFT  601 , the SNR estimating section  603 , the gain calculating section  605 , the multiplier  606  and the IFFT  607  are the same with those shown in  FIG. 7 . 
     The VAD  2102  detects voice sections and non-voice sections from the input spectrum and the noise spectrum estimating section calculates a noise spectrum from an average level of the non-voice sections and outputs it to the SNR estimating section  603 . The target gain calculating section  2103  calculates an average voice level from an average level of the voice sections, calculates a target gain G 0  from a ratio between the average voice level and the target level and outputs it to the gain calculating section  605 . 
     By the way, voice may distort if the voice is amplified to much by the AGC and the SNR drops if the voice is attenuated. Then, it is desirable to set upper and lower limits for the calculated target gain. When the calculated target gain exceeds the upper limit, the target gain is replaced with a value of the upper limit and when the calculated target gain is lower than the lower limit, the target gain is replaced with a value of the lower limit. Zero dB is used as the lower limit of the target gain for example. 
     The voice may also distort when the noise level of a certain point is too small because the SNR becomes large and a large gain is set and the voice is clipped when the noise level is too large in the multi-point teleconferencing apparatus. Then, it is desirable to provide a noise superimposing section for superimposing low level noise to a receiving signal inputted to the inter-point volume control section. Thereby, it becomes possible to assure noise of certain level or more for each point. 
     It is also possible to set upper and lower limits for the noise level of each point, instead of superimposing the low level noise. In this case, when the noise level exceeds the upper limit, the noise level is replaced with a value of the upper limit and when the noise level is lower than the lower limit, the noise level is replaced with a value of the lower limit. 
     Although the target gain calculating section within the AGC has calculated the target gain from the target level common to the all bands in the embodiments described above, it is also possible, instead of that, to calculate the target gain different per each band from the target level different per each band. 
     Still more, although the AGC has performed the automatic gain control based on the power spectrum of the input sound, it may perform the automatic gain control based on an amplitude spectrum of the input sound instead. In this case, the FFT outputs the amplitude spectrum of the input sound as an input spectrum and the multiplier generates an output spectrum by multiplying G(i) with the i-th band in the input spectrum. 
     A multipoint communication apparatus also achieves the performance of adjusting gains of signals receiving from a plurality of other communication apparatuses by processing at a CPU in the multipoint communication apparatus.