Patent Publication Number: US-8111838-B2

Title: Conferencing apparatus for echo cancellation using a microphone arrangement

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a voice conference apparatus capable of establishing a voice conference between remote places by employing the communication means (the communication network). 
     Very recently, voice conference apparatuses capable of establishing voice conferences between remote places by employing communication means have been gradually popularized. It is desirable that such voice conference apparatuses have been designed in such a way that even when a plurality of persons are present at respective installation places of these voice conference apparatuses, these plural persons can operate the voice conference apparatuses. As a typical apparatus example, the below-mentioned telephone conference apparatuses have been proposed (refer to, for example, patent publication  1 ). That is, one typical telephone conference apparatus includes a plurality of microphone devices for converting audible sounds into electric signals; a loudspeaker for converting the electric signals into audible sounds; and a voice communication network for electrically connecting these microphone devices and the loudspeaker to a telephone line. Each of the microphone devices has such a directional polar sensitivity characteristic which has a high sensitivity with respect to sounds which are radiated from at least one direction, as compared with sounds radiated from other directions. Furthermore, the directional polar sensitivity characteristic has a main lobe, side lobes, and a null present between paired lobes. The loudspeaker is arranged at a position of the null of the directional polar sensitivity characteristic, which is located between the side lobes adjacent to the main lobe. 
     The loudspeaker provided in this telephone conference apparatus has been installed at the null of the polar sensitive patterns as to the main lobe and the side lobes adjacent to the main lobe. As a result, acoustic coupling effects between the loudspeaker and the microphone devices are essentially reduced, so that the telephone conference apparatus can be operated in a full duplex mode, that is to say, while voices of the counter panties are mutually outputted from the loudspeakers so as to hear stories, persons can produce voices by employing the microphone devices. 
     While each of the microphone devices utilized in this telephone conference apparatus has one or plural pieces of microphones, directional microphones are used as the respective microphone devices in order to establish desirable sensitivity characteristics. 
     [Patent Citation 1] U.S. Pat. No. 5,121,426 (JP-A-3-293846) 
     However, the directional polar sensitivity characteristics of the directional microphones have characteristic fluctuations in view of manufacturing aspects, and magnitudes of the main lobes and the side lobes, and also, the null positions are difference from each other as to the respective directional microphones. Also, in such a case that the directional microphones are installed within an apparatus, these directional microphones may be readily influenced by peripheral structural components thereof. Moreover, as to the directional polar sensitivity characteristics of such microphones, the magnitudes of the main lobes and the side lobes, and also, the null positions are changed due to aging effects thereof. As previously described, there is such a problem that the sensitivities of the directional microphones are unstable, and thus, qualities as to the full duplex communication of the telephone conference apparatus are lowered. 
     SUMMARY OF THE INVENTION 
     The present invention has an object to provide a voice conference apparatus having a stable directional polar sensitivity characteristic, by which a quality of a full duplex communication thereof is not lowered. 
     To solve the above-described problem, there is provided a voice conference apparatus according to the present invention, comprising: 
     a microphone unit which has a plurality of omnidirectional microphones for collecting a sound to output a transmission voice signal; 
     a speaker which emits a sound on the basis of a reception voice signal; 
     a communication unit which transmits the transmission voice signal and receives the reception voice signal; and 
     a sensitivity characteristic forming unit which forms a desirable sensitivity characteristic of the microphone unit. 
     With employment of the above-described arrangement, the desirable sensitivity characteristic is formed by the plurality of omnidirectional microphones, so that the sensitivity characteristic can be made stable, and the quality of the full duplex communication is not deteriorated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view for indicating a voice conference apparatus according to an embodiment 1 of the present invention. 
         FIG. 2  is an upper view for showing the voice conference apparatus according to the embodiment 1 of the present invention. 
         FIG. 3  is a diagram for illustratively showing a structural example in which two sets of the voice conference apparatuses according to the embodiment 1 of the present invention are connected to each other. 
         FIG. 4  is a diagram for illustratively representing another structural example in which two sets of the voice conference apparatuses according to the embodiment 1 of the present invention are connected to each other. 
         FIG. 5  is a schematic block diagram for indicating hardware of the voice conference apparatus according to the embodiment 1 of the present invention. 
         FIG. 6  is a schematic block diagram for showing a DSP, a timing control-purpose PLD, a CODEC unit, and a microphone/loudspeaker unit employed in the voice conference apparatus according to the embodiment 1 of the present invention. 
         FIG. 7  is an explanatory diagram for explaining the loudspeaker provided in the embodiment 1 of the present invention. 
         FIG. 8  is a diagram for indicating an arranging relationship between the microphones and the loudspeaker employed in the embodiment 1 of the present invention. 
         FIG. 9  is a schematic diagram for representing process blocks related to the microphone of the DSP provided in the embodiment 1 of the present invention. 
         FIG. 10  is a diagram for indicating another arranging relationship between the microphones and the loudspeaker employed in the embodiment 1 of the present invention. 
         FIG. 11  is a diagram for graphically showing a relationship between an interval of the microphones and a directional pattern as to the voice conference apparatus according to the embodiment 1 of the present invention. 
         FIG. 12  is a diagram for graphically showing a relationship between an interval of the microphones and a sensitivity pattern as to the voice conference apparatus according to the embodiment 1 of the present invention. 
         FIG. 13  is a diagram for illustratively indicating an example as to a correction of the microphone interval as to the voice conference apparatus according to the embodiment 1 of the present invention. 
         FIG. 14  is a diagram for illustratively indicating another example as to a correction of the microphone interval as to the voice conference apparatus according to the embodiment 1 of the present invention. 
         FIG. 15  is a diagram for indicating an arranging relationship between the microphones and the loudspeaker employed in the embodiment 1 of the present invention. 
         FIG. 16  is a diagram for indicating an example as to a correction of a signal delay time as to a voice conference apparatus according to an embodiment 2 of the present invention. 
         FIG. 17  is a diagram for indicating another example as to a correction of a signal delay time as to a voice conference apparatus according to an embodiment 3 of the present invention. 
         FIG. 18  is a perspective view for showing a telephone employed in a voice conference apparatus according to an embodiment 5 of the present invention. 
         FIG. 19  is an upper view for showing a telephone employed in a voice conference apparatus according to the embodiment 5 of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to drawings, a description is made of various embodiments of the present invention. 
     Embodiment 1 
       FIG. 1  is a perspective view for indicating a voice conference apparatus according to an embodiment 1 of the present invention.  FIG. 2  is an upper view for showing the voice conference apparatus according to the embodiment 1 of the present invention, namely, is a diagram when the voice conference apparatus of  FIG. 1  is viewed from an upper plane thereof. 
     In  FIG. 1  and  FIG. 2 , reference numeral  1  shows a voice conference apparatus; reference numerals  2   a  to  2   d  represent sound collecting units for collecting voices of users; reference numeral  3  indicates a speaker unit for reproducing telephone reception voices; reference numeral  4  shows an operation button for performing telephone calling/receiving operations, and various of setting operations; reference numeral  5  represents a display unit for displaying thereon a telephone communication condition, a setting condition, and the like; and reference numeral  6 , or  7  indicates a communication cable which is connected to a communication line of a communication counter party. Namely, the reference numeral  6  is an Ethernet (registered trademark) cable, and the reference numeral  7  is a telephone line. Any one of the communication cables  6  and  7  is utilized in correspondence with communication means. In an actual case, two, or more sets of the voice conference apparatuses which are positionally separated from each other are connected via the communication means to each other so as to be used. 
       FIG. 3  and  FIG. 4  are diagrams for illustratively indicating a structural example in which 2 sets of the voice conference apparatuses according to the embodiment 1 of the present invention are connected to each other, namely, representing such a condition when two sets of the voice conference apparatuses  1   a  and  1   b  are connected to each other through the Ethernet (registered trademark) cable  6 . 
     The voice conference apparatuses  1   a  and  1   b  indicated in  FIG. 3  are identical to the voice conference apparatus  1  shown in  FIG. 1 . The voice conference apparatuses  1   a  and  1   b  are connected via a gateway  10   a  and another gateway  10   b  to the Internet  11 , so that the voice conference apparatuses  1   a  and  1   b  can be telephone-communicated with each other. In the case of  FIG. 3 , voice signals transmitted and received between the voice conference apparatuses  1   a  and  1   b  are such data that digital voice signals are processed as packet data. 
     It should be understood that other terminal apparatuses, a hub, a router, or the like may be alternatively connected to the gateway  10   a , or the gateway  10   b . Also, other terminal apparatuses, a hub, a router, or the like may be alternatively connected between the gateway  10   a  and the voice conference apparatus  1   a , or between the gateway  10   b  and the voice conference apparatus  1   b.    
     Also, as shown in  FIG. 4 , the respective voice conference apparatuses  1   a  and  1   b  may be alternatively connected via telephone lines  7   a  and  7   b  to a public telephone line  12 . In this case, analog voice signals are transmitted and/or received on at least telephone lines  7   a  and  7   b.    
     It should also be noted that the voice conference apparatuses  1   a  and  1   b  according to the embodiment 1 are designed in such a manner that a voice of a user of the relevant voice conference apparatus  1   a , or  1   b , which is inputted to respective sound collecting units (correspond to  2   a  to  2   d  of voice conference apparatus  1  shown in  FIG. 2 ), is not outputted to a speaker unit (corresponding to speaker unit  3  of voice conference apparatus  1  shown in  FIG. 2 ) built in the own voice conference apparatus  1   a , or  1   b . This designing reason is given as follows: That is, in such a case that the voice of the user of the relevant voice conference apparatus  1   a , or  1   b , which is inputted to the built-in microphone, is designed to be outputted from the speaker of the own voice conference apparatus  1   a , or  1   b , the howling phenomenon may readily occur. Nevertheless, if an apparatus by which a howling phenomenon never occurs can be constructed, then the voice of the user of the relevant voice conference apparatus, which is inputted to the built-in microphone, may be alternatively outputted from the speaker of the own voice conference apparatus. 
     With employment of the arrangements shown in  FIG. 1  to  FIG. 4 , while a communication person who uses the voice conference apparatus  1   a  is located apart from another communication person who uses the voice conference apparatus  1   b , these communication persons can establish conversation. It should also be noted that the communication persons who use the respective voice conference apparatuses  1   a  and  1   b  are not limited only to a single person, but also a plurality of persons. 
       FIG. 5  is a schematic block diagram for indicating hardware of the voice conference apparatus  1  according to the embodiment 1 of the present invention. 
     In  FIG. 5 , reference numeral  40  shows a CPU which contains the DSP, reference numeral  41  indicates a program memory which stores thereinto program software for executing various sorts of process operations by the CPU  40 ; and reference numeral  42  represents a work main memory which is required in order to execute various sorts of programs stored in the program memory  41  by the CPU  40 . With employment of these CPU  40  and memories  41  and  42 , packet process operations are performed in levels higher than the MAC layer level, and such output process operations as dial tone and melody are carried out. 
     In this drawing, reference numeral  43  shows a PHY chip for executing a protocol process operation in the physical layer level of Ethernet (registered trademark); and reference numeral  46  indicates a connector for connecting thereto the Ethernet (registered trademark) cable  6 , which is usually called as an “RJ-45.” Packets of voice data which are processed in the CPU  40  are transmitted and/or received via the PHY chip  43 , the connector  46  and the Ethernet (registered trademark) cable  6 . 
     Further, a keyboard  44 , an LCD  45 , and a controller  47  are connected to the CPU  40 . The keyboard  44  is provided inside the operation button  4 , and the LCD  45  is provided inside the display unit  5 . The controller  47  conducts input processing operations of the keyboard  44 . 
     Reference numeral  50  indicates a DSP for executing an echo canceling process operation; reference numeral  51  indicates a program memory which stores thereinto program software for executing various sorts of process operations by the DSP  50 ; and reference numeral  52  represents a work main memory which is required in order to execute various sorts of programs stored in the program memory  51 . 
     A microphone/speaker unit  56  is connected via a timing control-purpose PLD  54  and a CODEC unit  55  to the DSP  50 . After analog input/output signals of the microphone/speaker unit  56  are converted into digital input/output data in the CODEC unit  55 , an echo canceling process operation between microphones and a loudspeaker is carried out in the DSP  50 . A more detailed block diagram as to a partial block  58  for these units will be explained with reference to  FIG. 6 . The microphone/speaker unit  56  includes 8 pieces of microphones and 1 piece of a loudspeaker. 2 pieces of the microphones are installed in each of the sound collecting units  2   a  to  2   d  respectively, and the loudspeaker is installed in the speaker unit  3 , the more detailed arrangements of which will be discussed later with reference to  FIG. 6  and the succeeding drawings. 
     In such a case that the voice conference apparatus  1  according to the embodiment 1 is connected via the telephone line  7  to the public telephone line  12  so as to be operated, a public line I/F unit  57  used to connect the telephone line  7  is furthermore connected with respect to the CODEC unit  55 , as indicated by a dot line of  FIG. 5 . A detailed content of the public line I/F unit  57  is omitted. 
       FIG. 6  is a schematic block diagram for showing more detailed structures than the structures of  FIG. 5  as to the DSP  50 , the timing control-purpose PLD  54 , the CODEC unit  55 , and the microphone/loudspeaker unit  56 . 
     In the embodiment 1, the CODEC unit  55  has two sets of CODEC-IC  55   a  and  55   b . As represented in  FIG. 6 , 8 pieces of omnidirectional microphones  21   a  to  21   d  and  22   a  to  22   d  and a loudspeaker  30  are connected via respective microphone driving circuits  61   a  to  61   d  and  62   a  to  62   d , and also, a speaker amplifying circuit  63  to the CODEC-IC  55   a  and  55   b . In an actual case, both the microphone  21   a  and the microphone driving circuit  61   a , and both the microphone  22   a  and the microphone driving circuit  62   a  are two series of independent circuits. However, in  FIG. 6 , connection lines between the microphone driving circuits  61   a  and  62   a , and between the microphones  21   a  and  22   a , and the CODEC unit  55  are omitted as a single connection line. A similar connecting relationship between the microphones  21   b  to  21   d  and  22   b  to  22   d , the microphone driving circuits  61   b  to  61   d  and  62   b  to  62   d  may be established. 
     Next, a description is made of arranging relationships between the microphones  21 ,  22 , and the loudspeaker  30 , which are contained in the voice conference apparatus  1  according to the embodiment 1. 
       FIG. 7  is an explanatory diagram for explaining a loudspeaker employed in the embodiment 1 of the present invention, namely, for showing an example of the loudspeaker  30 . 
       FIG. 8  is a diagram for representing one arranging relationship between the microphones  21  and  22 , and the loudspeaker  30  in the embodiment 1 of the present invention. Namely,  FIG. 8  represents such a condition that the arranging relationship in such a case that the microphones  21  and  22 , and the loudspeaker  30  shown in  FIG. 7  are built in the voice conference apparatus  1  is viewed from an upper plane of a housing thereof. 
       FIG. 9  is a schematic block diagram for indicating a process block related to the microphones of the DSP  50  in the embodiment 1 of the present invention, namely, shows a process block of a circuit portion related to the microphones  21  and  22  within the DSP  50 . 
       FIG. 10  is a diagram for illustratively showing another arranging relationship between the microphones  21  and  22 , and the loudspeaker  30  employed in the embodiment 1 of the present invention, namely,  FIG. 10  shows the arranging relationship between the microphones  21 ,  22  arranged in one of the sound collecting units  20   a  to  20   d  of the voice conference apparatus  1 , and the loudspeaker  30  arranged in the speaker unit  3 , as viewed along the sectional direction thereof. 
     In the sound collecting units  2   a  to  2   d , and the speaker unit  3  of  FIG. 8 , the microphones  21   a  to  21   d  and  22   a  to  22   d , and also, the loudspeaker  30  are illustrated in such a manner that positions of these structural components  21   a  to  21   d ,  22   a  to  22   d , and  30  can be apparently grasped. However, in the actual case, these microphones  21   a  to  21   d  and  22   a  to  22   d , and loudspeaker  30  are arranged inside the sound collecting units  2   a  to  2   d , and the speaker unit  3 , and therefore, cannot be directly and visibly recognized from the external space. 
     In  FIG. 6 , sound waves (acoustic waves) which are entered to the microphones  21   a  to  21   d  and  22   a  to  22   d  are converted into voltages. The converted voltages are processed by the CODEC unit  55  to be converted into digital signals. These digital signals are processed by the DSP  50  so as to perform thereto the echo canceling process operation. The echo-canceled digital signals are processed by the CPU  40  of  FIG. 5  so as to perform thereto the packet processing operation. And then, the packet-processed digital acoustic signals are transmitted via the PHY chip  43  and the connector  46  to a voice conference apparatus of a counter party which is located on Ethernet (registered trademark), or the Internet (for example, if present voice conference apparatus in  FIG. 3  is apparatus “ 1   a ”, then voice conference apparatus of counter party is apparatus “ 1   b ”). 
       FIG. 7(   a ) is a rear view of the loudspeaker  30 ,  FIG. 7(   b ) is a sectional structural diagram of the loudspeaker  30 , as viewed from the side plane direction, and  FIG. 7(   c ) is a schematic sectional view for showing operations of the loudspeaker  30  in a simple manner. 
     Although the loudspeaker  30  has such a structure as indicated in  FIG. 7(   a ) and  FIG. 7(   b ) in detail, a basic structure of this loudspeaker  30  is illustrated in  FIG. 7(   c ). That is,  FIG. 7(   c ) can explain operations of the loudspeaker  30  based upon cone paper  31  functioning as a diaphragm, a coil  35 , and a magnet  37 . In other words, when an electric voice signal derived from the speaker amplifying circuit  63  in  FIG. 5  is supplied to the coil  35 , the cone paper  31  connected to the coil  35  is vibrated along forward and backward directions, so that the electric voice signal may become sounds in accordance with the Fleming&#39;s rule. The vibration directions of the cone paper  31  are illustratively represented in  FIG. 7(   a ) to  FIG. 7(   c ). 
     Sounds outputted from the loudspeaker  30  are voices which are collected in a voice conference apparatus of a counter party which is located on either Ethernet (registered trademark) or the Internet (for example, if present voice conference apparatus in  FIG. 3  is apparatus “ 1   a ”, then voice conference apparatus of counter party is apparatus “ 1   b ”). The packet data received from the voice conference apparatus  1   a , or  1   b  of the counter party via the connector  46  and the PHY chip  43  shown in  FIG. 6  is processed by the CPU  40  in the packet processing operation. The packet-processed voice data is converted into the analog voice signal via the DSP  50  by the CODEC unit  55 . Thereafter, the analog voice signal amplified by the speaker amplifying circuit  63  is entered to the loudspeaker  30 . 
     In the voice conference apparatus  1  according to the embodiment 1, the microphones  21   a  to  21   d  and  22   a  to  22   d , and loudspeaker  30  are arranged as shown in  FIG. 8 . In other words, the vibration directions of the diaphragms as to the respective first and second microphones  21   a  to  21   d  and  22   a  to  22   d  are intersected substantially perpendicular to propagation directions of compressional waves which are generated from the loudspeaker  30 . Moreover, the second microphones  22   a  to  22   d  are arranged closer to the loudspeaker  30  by a distance “d”, than the first microphones  21   a  to  21   d . In this embodiment 1, it is so assumed that each distance “d” between two sets of the microphones  21   a  and  22   a;    21   b  and  22   b;    21   c  and  22   c;  and  21   d  and  22   d  is equal to ¼ of a wavelength of a maximum process frequency “f”, as represented in (Math. 1).
 
 d=c/ 2 Fs=c/ 4 f= (¼)λ  [Math 1]
 
     c: sound velocity in the air, 
     Fs: sampling frequency used to process input signals from two sets of microphones  21   a  and  22   a  through  21   d  and  22   d  respectively, 
     f: maximum processable frequency, 
     λ: wavelength of maximum processable frequency “f”. 
     The reason why it is desirable to set the distance “d” between each of 2 sets of the microphones  21   a  and  22   a  through  21   d  and  22   d  respectively to approximately d=(¼)λ will now be explained with reference to  FIG. 11 . 
     Circular graphs of  FIGS. 11(   a ) and  11 ( b ) represent polar patterns for indicating directivities in the vicinity of maximum frequencies, which are synthesized by performing signal process operations as to the microphone sets, while it is desirable that sensitivities along dead angle directions become minimum. As shown in the circular graph of  FIG. 11(   b ), if the interval “d” of the microphones  21   a / 22   a  through  21   d / 22   d  respectively becomes larger than ¼λ, then a spatial folding phenomenon may occur along the dead angle direction. On the other hand, as shown in the circular graph of  FIG. 11(   a ), if the interval “d” of the microphones  21   a/   22   a  through  21   d/   22   d  respectively is equal to ¼λ, then the sensitivity along the dead angle direction becomes comparatively low, so that a desirable directivity can be obtained. Conversely, in such a case that the interval “d” of the microphones  21   a/   22   a  through  21   d/   22   d  respectively becomes smaller than ¼λ, as indicated in a graphic representation of  FIG. 12 , since a sensitivity of a main lobe on the side opposite to the dead angle is directly proportional to the interval “d” of the microphones  21   a/   22   a  through  21   d/   22   d  respectively, the sensitivity is lowered, so that noise is relatively increased, and thus, a voice quality is deteriorated. Based upon the above-described explanations, such microphone intervals “d” that both the noise and the voice quality can become optimum may become such an interval value approximated to d=λ/4. 
     While sound velocity “c” within the air is normally 340 m/second, in the voice conference apparatus  1  of the embodiment 1, the maximum processable frequency “f” corresponds to 7 KHz. In this case, the interval “d” between the microphones  21  and  22  becomes approximately 12 mm. 
     A first reason why the maximum processable frequency “f” is selected to be 7 KHz is given as follows: That is, if voice signals having frequencies up to 7 KHz can be processed, then sufficiently satisfactory feelings of sound qualities can be obtained as voice communication operations. It may be sometimes conceived that if the maximum processable frequency “f” is increased higher than 7 KHz, then voice communications with higher sound qualities than the sound qualities may be carried out. However, practically speaking, in such a voice conference apparatus  1  as indicated in the embodiment 1, such a difference of sound quality feelings which can be actually experienced by a user cannot be established. Conversely, as apparent from the (Math. 1), in order to increase the maximum processable frequency “f”, the sampling frequency “Fs” must also be increased, so that a calculation amount by the DSP  50  when the sampling frequency “Fs” is increased is also increased. 
     A second reason why the maximum processable frequency “f” is set to 7 KHz is given as follows: 
     That is, a blocking range of an antialias filter employed in a normal A/D converter is set to a frequency which is lower than ½ of a sampling frequency thereof. As a result, a practically available maximum frequency in an A/D converter having a general sampling frequency of 16 KHz may become approximately 7 KHz. 
     A description is made of such an example that a distance between two microphones is corrected based upon “d=(¼)λ” with reference to  FIG. 13 . In the case that an acoustic center of a speaker is not positioned on extension of a line which connects acoustic centers of two microphones, assuming now that an angle is defined as “θ”, at which a line for connecting the acoustic center of the speaker with an intermediate position between these two microphones is intersected with the extension line of the two microphones, it is desirable that the interval between these two microphones is corrected as “d′=d/cos θ.” As a result, a difference between travel paths in propagation directions of sound waves which reach these two microphones from the acoustic center of the speaker can satisfy the condition of “d=(¼)λ”, so that desirable performance as to a sensitivity and a directivity characteristic can be achieved. For instance, in such a case that d=(¼)λ=12 mm and the angle “θ” is 30 degrees, this distance “d” of the two microphones may be corrected as d′=12 mm/cos 30 degrees=approximately 14 mm. 
     A description is made of another example that a distance “d” between two microphones is corrected with reference to  FIG. 14 . Under such a condition that mixtures of noises having constant magnitudes cannot be avoided (these noises are known as quantizing noises made by fixed point arithmetic, noises of electric boards, etc.), if a distance “d” measured between acoustic centers of two microphones indicated in  FIG. 14(   a ) is set to “d=(¼)λ” represented in a graph of  FIG. 14(   b ), then there are some possibilities that an S/N ratio of an acoustic signal with respect to the noises becomes short. In such a shortage of the S/N ratio, the distance “d” of the two microphones must be corrected as a distance d′=(¼)λ+α&gt;d by which the S/N ratio can be secured. As a consequence, even under such a condition that the adverse influence caused by the noises such as the quantizing noise and the electric noise cannot be neglected, appropriate directivity performance can be achieved. For example, in such a case that d=(¼)λ=12 mm, the distance “d” is corrected as d′=approximately 14 mm. 
     As a concrete example in such a case that the distance “d” is corrected due to the noises, as shown in the graphs of  FIG. 14(   b ), there are some possibilities that the distance “d” is corrected to become such a value slightly longer than “(¼)λ”) in order that the S/N ratio of the acoustic signal to these noises exceeds approximately 35 dB. Since a lower limit value of quantizing noise by general-purpose 16-bit fixed point arithmetic is nearly equal to 35 dB, a maximum directivity characteristic obtained under this condition can be realized. 
     In the microphone units of  FIG. 8 , namely, in the sound collecting units  2   a  to  2   d , each of two sets of the omnidirectional microphone devices  21   a  and  22   a  through  21   d  and  22   d  respectively are arrayed on each of radiation lines  81   a  to  81   d  which connect the respective acoustic centers  82   a  to  82   d  of the sound collecting units  2   a  to  2   d  with the acoustic center  83  of the loudspeaker  30 . Since a total number of omnidirectional microphones employed in each of the sound collecting units  2   a  to  2   d  is selected to be 2, each of the microphone units (namely, respective sound collecting units  2   a  to  2   d ) may be constructed by employing a minimum number of these omnidirectional microphones. As a consequence, the full duplex communication with the high quality can be carried out in a low apparatus cost. 
     In addition, a plurality of microphone units, namely, 4 pieces (in case of embodiment 1) of the sound collecting units  2   a  to  2   d  are arranged on such a concentric plane  86  that the acoustic center  83  of the loudspeaker  30  is located at a center, as viewed from the upper plane of the housing of the voice conference apparatus  1 . While sensitivity characteristics  85   a  to  85   d  of the microphone units, namely, of the sound collecting units  2   a  to  2   d  are substantially identical to each other, at the same time, angles between the adjoining radiation lines  81   a  to  81   d  are equal to each other, and these radiation lines  81   a  to  81   d  connect the acoustic centers  82   a  to  82   d  of these microphone units, namely these sound collecting unit  2   a  to  2   d  with the acoustic center  83  of the loudspeaker  30 . As a consequence, unequally collected sounds as to voices of plural communication persons who are present along any directions can be reduced, so that the full duplex communication having the high quality can be carried out. 
     In addition, the reason why 4 sets of the microphone units, namely the sound collecting units are arranged is caused by that when a table, or a room is overlooked from an upper direction, there are many rectangular shapes, or square shapes. As a consequence, it is possible to assert that this 4-set arrangement of the microphone units can expect at the highest level the uniformity of the sound collections from the respective edges of either the table or the room with employment of a minimum number of these microphone units. As a consequence, the full duplex communication with the high quality can be carried out in a low apparatus cost. 
     Now, the below-mentioned case will be considered: That is, assuming now that a sound collecting direction at an acoustic center of each unit as to the sound collecting units  2   a  to  2   d  is defined as an angle and a magnitude of a sensitivity thereof is defined as a radial direction, sensitivity characteristics  85   a  to  85   d  of these sound collecting units  2   a  to  2   d  are expressed based upon these angles and radial directions. As shown in  FIG. 8 , while orthogonal lines  84   a  to  84   d  are defined as boundaries and these orthogonal lines  84   a  to  84   d  are intersected perpendicular to the respective radiation lines  81   a  to  81   d  and also pass through the acoustic centers  82   a  to  82   d , the sensitivity characteristics  85   a  to  85   d  are formed by employing two sets of these microphones  21   a  to  21   d  and  22   a  to  22   d  respectively of the respective sound collecting units  2   a  to  2   d  by a sensitivity characteristic forming unit (will be explained later) in such a manner that such areas of the sensitivity characteristics  85   a  to  85   d  of the respective sound collecting units  2   a  to  2   d  on the side of the loudspeaker  30  with respect to these boundaries become smaller than other areas thereof. The respective radiation lines  81   a  to  81   d  connect the acoustic centers  82   a  to  82   d  of the sound collecting units  2   a  to  2   d  with the acoustic center of the loudspeaker  30 . In other words, the main lobes of the sensitivity characteristics  85   a  to  85   d  are formed in such directions that the user of the voice conference apparatus  1  considers potentially and unconsciously, namely, formed in directions over the respective radiation lines  81   a  to  81   d , which are located opposite to the loudspeaker  30 . 
     In this example, as to the acoustic centers  82   a  to  82   d  of the sound collecting units  2   a  to  2   d , the below-mentioned points are set in a virtual manner: That is, these points are located over equi-distances from centers of diaphragms of a plurality of omnidirectional microphone devices, which are viewed from vibration directions of these diaphragms, namely, in the embodiment 1, these points are located over the equi-distances from the centers of the diagrams as to each of two sets of the microphones  21   a  to  21   d  and  22   a  to  22   d  respectively in such a case that these microphones  21   a  to  21   d  and  22   a  to  22   d  are viewed from the vibration directions of the diaphragms thereof. Also, as to the acoustic center  83  of the loudspeaker  30 , such a center that the diaphragm of the loudspeaker  30  is viewed from the vibration direction thereof are set in the virtual manner. 
     While the vibration directions of the diaphragms of the respective microphones  21   a  to  21   d  and  22   a  to  22   d  are intersected substantially perpendicular with respect to the propagation direction of the compressional waves generated from the loudspeaker  30 , the second microphones  22   a  to  22   d  are installed closer to the loudspeaker  30  than the first microphones  21   a  to  21   d  provided in the respective sound collecting units  2   a  to  2   d . The below-mentioned sensitivity characteristic forming unit can form such sensitive characteristics of the respective sound collecting units  2   a  to  2   d  as shown in  FIG. 8  due to the arrangement of these microphones  21   a  to  21   d  and  22   a  to  22   d . When three, or more sets of microphones to be arranged in respective sound collecting units  2   a  to  2   d  are provided, if at least one microphone is arranged at a position closer to a loudspeaker than other microphones, while a line intersected perpendicular to radiation lines is defined as a boundary and the radiation lines connect an acoustic center of this loudspeaker with acoustic centers of the respective sound collecting units, an area of a sensitivity characteristic which is formed on the side of the loudspeaker with respect to this boundary becomes smaller than other areas of this sensitivity characteristic. 
     As previously described, since the desirable sensitivity characteristics  85   a  to  85   d  are formed by employing the first omnidirectional microphones  21   a  to  21   d  and the second omnidirectional microphones  22   a  to  22   d  in the respective sound collecting units  2   a  to  2   d , the fluctuations and also the aging changes contained in the sensitivity characteristics  85   a  to  85   d  of the respective sound collecting units  2   a  to  2   d  can be reduced and the sensitivity characteristics  85   a  to  85   d  thereof can be made stable. As a result, the full duplex communication with the high quality can be carried out. Also, since these omnidirectional microphones  21   a  to  21   d  and  22   a  to  22   d  are used, the sound collecting units  2   a  to  2   d  can be made compact, and can eliminate design restrictions thereof, as compared with using of directional microphones. This reason is given as follows: That is, in the case that such directional microphones are employed, these directional microphones can be readily influenced by peripheral structural components thereof, and also, sufficiently large spaces must be secured around these directional microphones, so that the sound collecting units become bulky and the designs thereof are restricted. 
     The sensitivity characteristic forming unit for forming the desirable sensitivity characteristics  85   a  to  85   d  by employing 2 sets of the respective first and second microphones  21   a  to  21   d  and  22   a  to  22   d  employed in the respective sound collecting units  2   a  to  2   d  mainly corresponds to the processing circuit block  59  for processing such input signals from the first microphones  21   a  to  21   d  (will be typically expressed as “first microphone  21 ” in  FIG. 9  and succeeding drawings thereof) provided in the DSP  50  shown in  FIG. 9  in the voice conference apparatus  1  according to the embodiment 1. In  FIG. 9  and the succeeding drawings thereof, the below-mentioned descriptions will be made in such a manner that the respective sound collecting units  2   a  to  2   d  are typically expressed as “ 2 ”; the respective microphones  21   a  to  21   d  and  22   a  to  22   d  are typically expressed as “ 21 ” and “ 22 ”, the acoustic centers  82   a  to  82   d  of the respective sound collecting units  2   a  to  2   d  are typically expressed as  82 ; the respective radiation lines  81   a  to  81   d  which connect the acoustic centers  82   a  to  82   d  of the sound collecting units  2   a  to  2   d  with the acoustic center  83  of the loudspeaker  30  are typically expressed as “ 81 ”; the orthogonal lines  84   a  to  84   d  corresponding thereto are typically expressed as “ 84 ”; and further, the sensitivity characteristics  85   a  to  85   d  of the respective sound collecting units  2   a  to  2   d  are typically expressed as “ 85 .” 
     In  FIG. 9 , an A/D converter  60  and another A/D converter  64  are provided in the CODEC unit  55 . Analog input signals derived from the first microphone  21  and the second microphone  22  inputted into the A/D converters  60  and  64  via the microphone driving circuits  61  and  62  are converted into digital signals. 
     Reference numeral  59  indicates the processing circuit block of the portion related to the present invention, which is constituted by a program stored in the program memory  51  of the DSP  50 . From output data from the A/D converters  60  and  64 , such signals are subtracted which are produced by delaying the opposite output data of these A/D converters  60  and  64  through delay filters  65  and  66 . In the case of the embodiment 1, delay times as to these delay filters  65  and  66  are calculated based upon the following (math. 2).
 
 T   =d/c= ½ Fs= ¼ f   [Math. 2]
 
       T : delay times of delay filters  65  and  66   
     In other words, the delay time is equal to 1 sampling period “1/Fs”, and when a signal waveform having the maximum processable frequency “f” is inputted, the delay filter  65 , or  66  can delay the input signal wave by a delay time equal to ¼ waveform. As a consequence, emphasizing process operations as to voice of a telephone calling person and voice of a telephone receiving person can be optimized, and loads given in reverberation reducing process operations can be furthermore reduced, so that the full duplex communication with the higher quality can be carried out. 
     A calculator  67  subtracts the below-mentioned data from the output data of the A/D converter  64  which A/D-converts the input signal from the first microphone  21  so as to output the subtracted data. The data is obtained by delaying the output data of the A/D converter  64  by the delay time “ T ” by the delay filter  66 , while the A/D converter  64  A/D-converts the input signal from the second microphone  22  which is located closer to the loudspeaker  30  than the first microphone  21 . Since the first microphone  21  is separated from the second microphone  22  by such a distance equal to the ¼ wavelength of the maximum processable frequency “f”, the voices of the telephone receiving persons which are inputted from the loudspeaker  30  to these two microphones  21  and  22  are especially canceled with each other (will be referred to as “main beam” hereinafter). 
     Another calculator  68  subtracts the below-mentioned data from the output data of the A/D converter  64  which A/D converts the input signal derived from the second microphone  22  located closer to the loudspeaker  30  so as to output the subtracted data. The first-mentioned data is obtained by delaying the output data of the A/D converter  63  by the delay time “ T ” by the delay filter  65 , while the A/D converter  63  A/D-converts the input signal from the first microphone  21 . Since the first microphone  21  is separated from the second microphone  22  by such a distance equal to the ¼ wavelength of the maximum processable frequency “f”, the voices of the users (telephone calling persons) which are inputted to these two microphones  21  and  22  from such a direction different from the loudspeaker  30 , more specifically, from a direction opposite from the loudspeaker  30  are canceled with each other (will be referred to as “null beam” hereinafter). Since the delaying/adding process operations are carried out, as to the input to the microphone  22  located closer to the loudspeaker  30 , the voice of the telephone receiving person from the loudspeaker  30  is emphasized, whereas as to the input to the microphone  21  located opposite to the microphone  22 , the voice of the telephone calling person is emphasized. As a result, a subtraction between the voice signal of the telephone receiving person and the reverberation sound of the telephone calling person can be easily carried out in an adaptive filter  69  and a subtracter  70 , which will be explained later, so that the full duplex communication with the high quality can be carried out. 
     The subtracter  70  subtracts such a data from the output data (namely, main beam) of the calculator  67 , while the first-mentioned data is obtained by filtering the output data (namely, null beam) of the calculator  68  by the adaptive filter  69 . As a consequence, the voices which are inputted from the loudspeaker  30  to these two microphones  21  and  22  are furthermore cancelled with each other, and also, the reverberation sounds generated from the circumferential environment of the voice conference apparatus  1  are reduced, so that the voices of the user (telephone calling person) of the voice conference apparatus  1  can be extremely clearly transmitted to the voice conference apparatus  1  owned by a remotely separated talking person. It should also be noted that another adaptive filter (not shown) may be alternatively arranged at a post stage of the subtractor  70  in order to cancel echoes (will be referred to “linear echoes” hereinafter) occurred between the microphones  21  and  22 , and the loudspeaker  30 . 
     While the sounds from the voice conference apparatus  1  owned by the remotely separated talking person corresponding to a communication counter party is outputted from the loudspeaker  30  of the voice conference apparatus  1  with respect to the adaptive filter  69  employed in the voice conference apparatus  1  of the embodiment 1, the user of the voice conference apparatus  1  performs a learning work under such a condition that this user does not talk toward the voice conference apparatus  1 . In this case, a description is made of such a case that an FIR filter is employed as one example of the adaptive filter  69 . 
     First of all, it is possible to assume that a relationship between the output data (main beam) of the calculator  67  and such an echo signal component having a higher correlation with the output data (null beam) of the calculator  68  is theoretically expressed by the below-mentioned (Math. 3):
 
 y   B ( m )= x   A   echo ( m )  [Math. 3]
     y B (m): output data (main beam) of calculator  67  at certain sampling time instant “m”   x A (m) output data (null beam) of calculator  68  at certain sampling time instant “m”   ※ voice of remotely separated talking person under reproduction from speaker   x A   echo (m): echo signal component having higher correlation to null beam “x A (m)”   

     In the (Math. 3), the echo signal component having the higher correlation with the null beam may be completely identical to the null beam if sounds inputted to the microphones  21  and  22  are purely and completely identical to sounds outputted from the loudspeaker  30 . However, in an actual case, various sorts of echo signals are contained in the echo signal component, which are caused by acoustic distortions occurred when electric signals are converted into air vibrations by the loudspeaker  30 , housing vibrations of the voice conference apparatus  1  caused by vibrations of the loudspeaker  30 , and so on. The acoustic distortions are caused by the characteristic frequency and the frequency characteristic as to the loudspeaker  30 , and in particular, high frequency distortions may cause problems in a low-cost loudspeaker. It should also be noted that this echo signal component having the higher correlation with the null beam in the (Math. 3) can be hardly calculated in a direct manner. As a consequence, the adaptive filter  69  synthesizes a quasi-echo signal based upon the null beam in accordance with the following (Math. 4): 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
     The subtracter  70  subtracts the quasi-echo signal from the main beam. As a consequence, there is such a system that the echo signal is attenuated by the main beam. Accordingly, an output signal of the subtracter  70  may be calculated based upon the below-mentioned (Math. 5): 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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                     5 
                   
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     If a predicted error of the adapter filter  69  is zero, then a relationship between a first term of a right hand side and a second term thereof in the (Math. 5) is given as the below-mentioned (Math. 6) and (Math. 7), and the output signal of the subtracter  70  in the (Math. 5) must become zero. 
     
       
         
           
             
               
                 
                   
                     
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     However, in an actual case, since the predicted error is present, the relationship cannot be established based upon the above-explained (Math. 6) and (Math. 7). More specifically, when only a remotely separated talking person makes a voice and a nearly separated talking person does not make a voice, such an output signal of the (Math. 5) is referred to as a “residual echo (error) signal.” The residual echo signal (error signal) is expressed by the below-mentioned (Math. 8). 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                     ⁢ 
                     8 
                   
                   ] 
                 
               
             
           
         
       
     
     In such an adaptive filter, it is important that the filter coefficient is updated (learned) in a direction along which the predicted error is decreased. Several sorts of algorithms of this learning method are known. In a general NLMS method known as better convergence of voice, the filter coefficient is updated as indicated in the below-mentioned (Math. 9). 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             
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     In accordance with this sort of algorithm, the adaptive filter  69  performs the subtracting operation of the echo signal components and the updating operation of the coefficient in a parallel mode so as to continuously attenuate the echo signal components of the null beam mixed in the main beam, so that the adaptive filter  69  can output only such a main beam approximated to the pure main beam, namely only the output signal of the nearly separated talking person. It should also be understood that although the FIR filter is described as one example of the adaptive filter  69  employed in the embodiment 1, the adaptive filter  69  of the voice conference apparatus  1  according to the embodiment 1 is not especially limited only to such an FIR filter. Alternatively, for instance, a frequency domain adaptive filter and a sub-band splitting type adaptive filter may be employed. 
     Conventionally, an adaptive filter is employed in order to perform such an adaptive process operation that a voice of a nearly separated talking person located along a certain direction is more clearly extracted. In contrast, while the adaptive filter  69  employed in the voice conference apparatus  1  of the present embodiment 1 may maintain such a condition that the voices from the loudspeaker  30  can be heard with respect to the nearly separated talking person located around the voice conference apparatus  1 , the adaptive filter  69  may contribute to cancel the echoes of the voices of the remotely separated talking person, which are outputted from the loudspeaker  30  and then are entered to the microphones  21  and  22 . In particular, this adaptive filter  69  can have a merit with respect to so-called “nonlinear echoes” such as echo signals caused by acoustic distortions occurred when electric signals are converted into air vibrations by the loudspeaker  30 ; and echo signals caused by housing vibrations of the voice conference apparatus  1  in connection with vibrations of the loudspeaker  30 . In other words, the sensitivity characteristic forming unit (corresponding to processing circuit block  59  shown in  FIG. 9 ) of the present invention can have such featured points that the sensitivity characteristic of the microphone unit  2  is formed in such a manner that this microphone unit  2  can hardly pick up the voices outputted from the loudspeaker  30 , and furthermore, the non-linear echoes can be reduced. As previously described, in order to increase this echo cancellation effect by the adaptive filter  69 , if the distance “d” between the acoustic centers of these two microphones  21  and  22  is set in accordance with the (Math. 1), then this adaptive filter  69  can achieve the effect with respect especially to distortions having higher frequencies. 
     While one of the sound collecting units  2   a  to  2   d  of the voice conference apparatus  1  shown in  FIG. 8  is typically expressed as the sound collecting unit  2 ,  FIG. 10  is a diagram for indicating an arranging relationship between the microphones  21  and  22  arranged in this sound collecting unit  2  and the loudspeaker  30  arranged in the speaker unit  3 , which is viewed from the sectional direction. Voices collected in a voice conference apparatus of a communication counter party are outputted from the loudspeaker  30 . In other words, vibrations of the cone paper  31  of this loudspeaker  30  may produce compressional waves  38   a  and  38   b  of the air; the produced compressional waves  38   a  and  38   b  are propagated outside of the housing of the voice conference apparatus  1 ; and thus, the voices may be transferred to users who are present around this voice conference apparatus  1 . 
     The vibration directions of the diaphragms  28  of the plurality of microphones  21  and  22  of the sound collecting unit  2  are intersected substantially perpendicular to the propagation direction of the compressional waves  38   a  and  38   b  generated from the loudspeaker  30 . Also, the vibration directions of the diaphragms  28  of the plurality of microphones  21  and  22  of the sound collecting unit  2  are intersected substantially perpendicular to such a microphone unit located immediately above the microphones  21  and  22 , namely, substantially perpendicular to an upper plane of a protection member  20  of the sound collecting unit  2 . The vibration direction of the diaphragm  31  of the loudspeaker  30  is intersected substantially perpendicular to an upper plane of another protection member  3   a  located immediately above the loudspeaker  30 . 
     At this time, although the voices produced from the loudspeaker  30 , namely, the voices collected in the voice conference apparatus of the communication counter party, and the peripheral reverberation are entered to both the microphones  21  and  22 , these voices and reverberation are reduced by the previously explained processing circuit block  59  shown in  FIG. 9 . As a result, the voice of the user who is located around the relevant voice conference apparatus  1  may be extremely clearly transferred to the voice conference apparatus of the communication counter party. 
     If the microphones  21  and  22  are arranged at positions  21   e  and  22   e  as represented in  FIG. 15 , then there are some possibilities that the air compressional waves of the voices which are generated from the loudspeaker  30  and are collected in the voice conference apparatus of the communication counter party can be more or less entered to these microphones  21   e  and  22   e . However, as compared with this merit, a loss of the echo cancellation process result of  FIG. 9  may become extremely large, which is caused by that substantially no distance difference from the loudspeaker  30  can be secured. As a consequence, it is preferable to arrange these two microphones  21  and  22  at the positions represented in  FIG. 10 . 
     It is desirable to arrange that a distance between planes  21   p  and  22   p  having sound collection ports of the plural microphones  21  and  22  of the microphone unit, namely the sound collecting unit  2 , and the upper plane  20   u  of the protection member  20  of the microphone unit, namely the sound collecting unit  2  may become shorter than another distance between the protection member  20  and other plane than the first-mentioned planes  21   p  and  22   p  of the microphones  21  and  22 . As a consequence, the microphone unit, namely the sound collecting unit  2  can mainly collect primary compressional waves from the loudspeaker  30 , and also, can hardly collect reflection sounds within the microphone unit, namely within the sound unit  2 . As a result, the load given to the reverberation reducing process operation can be decreased, so that the full duplex communication with the higher quality can be carried out. 
     It should also be noted that although the voice conference apparatus  1  of the embodiment 1 is equipped with only one set of the loudspeaker  30 , the present invention is not limited only to a single loudspeaker, but plural sets of loudspeakers may be alternatively provided in the voice conference apparatus  1 . In this alternative case, for example, such points which are separated over equi-distances from respective acoustic centers of the plural loudspeakers provided in the speaker unit when being viewed from the upper plane of the voice conference apparatus may be defined as an acoustic center of the speaker unit of the voice conference apparatus. 
     As previously described, in accordance with the embodiment 1, the fluctuations and the aging changes in the sensitivity characteristics  85   a  to  85   d  of the respective sound collecting units  2   a  to  2   d  can be reduced, and the sensitivity characteristics  85   a  to  85   d  can become stable, so that the full duplex communication with the higher quality can be carried out. 
     Embodiment 2 
       FIG. 16  shows an example in which signal delay times are corrected in accordance with an embodiment 2 of the present invention. A different point between  FIG. 16  and  FIG. 10  is given as follows: That is, it is so assumed that a position of a telephone calling person is located parallel to a desk plane and the telephone calling person is present along a direction inclined from an extension line direction of two sets of first and second microphones  21  and  22  by an angle of “θ”, which may reflect a more actual case. From output data from the respective A/D converters  60  and  64 , such digital signals are subtracted which are produced by delaying the output data derived from the opposite A/D converters  60  and  64  through the delay filters  65  and  66 . In the case of the embodiment 2, a delay time “ T   1 ” of the delay filter  65  which is applied to the second microphone  22  may be calculated based upon the below-mentioned (Math. 10), while the second microphone  22  is located closer to the loudspeaker  30  than the first microphone  21 :
 
 T   1   =d/c   [Math. 10]
 
     d: interval between two microphones  21  and  22   
     c: sound velocity in the air 
     On the other hand, a delay time “ T   2 ” of the delay filter  66  which is applied to the first microphone  21  may be calculated based upon the below-mentioned (Math. 11), while the first microphone  21  is located far from the loudspeaker  30  than the second microphone  22  in  FIG. 16 :
 
 T   2   =d· cos θ/ c   [Math. 11]
 
     d: interval between two microphones  21  and  22   
     c: sound velocity in the air 
     θ: angle formed by an extension line of connecting two microphones  21 / 22  to each other and a line extending to a position of telephone calling person from the two microphones  21 / 22   
     That is to say, this delay time “ T   2 ” is shorter than the delay time “ T   1 ”. As a result, the voice signal derived from the direction along which the telephone calling person is actually located is emphasized, so that the sound collecting efficiency can be furthermore improved, and thus, the full duplex communication with the higher equality can be carried out. 
     A description is made of an example as to the effect achieved by setting the delay times in the above-described manner with reference to  FIG. 16 . In this example, assuming now that a microphone unit interval is d=14 mm, and an angle defined between the extension direction of the microphone unit and the desk plane is θ=30 degrees, directivity patterns are exemplified in  FIG. 16 . 
     Firstly, directivity patterns of main beams are indicated. A main beam corresponds to an output signal of such a directivity pattern synthesized in such a manner that a sensitivity thereof is high with respect to a voice direction of a nearly separated talking person, and an opposite side thereof becomes a dead angle. In  FIG. 16(   a ), the main beam has such a purpose that a directivity angle is directed to a direction parallel to the desk plane and indicated by as arrow “a.” In the case of this drawing, such a delay coincident with a difference in reaching times of sound waves between both the microphones  21  and  22  may be given to a microphone signal of such a microphone located closer to the nearly separated talking person than the other microphone in such a manner that the microphone signal of the microphone located closer to the nearly separated talking person is made coincident with the microphone signal of the microphone located far from the nearly separated talking person. In such a case that this delay time “ T   2 ” is given as  T   2 =d/c, as represented in a circular graph of  FIG. 16(   b ), a large amount of sensitivities are also left along the opposite direction of the nearly separated talking person, and a dead angle is insufficiently formed. On the other hand, in the case that the delay time “ T   2 ” is given as  T   2 =d·cos θ/c, as indicated in a circular graph of  FIG. 16(   c ), a sharp dead angle is formed along the opposite direction of the nearly separated talking person. As a result, it can be understood that this case of  FIG. 16(   c ) may provide a better result. 
     Next, directivity patterns of null beams are illustrated. A null beam corresponds to an output signal of such a directivity pattern synthesized in such a manner that a sensitivity thereof is high with respect to a direction of a speaker of the voice conference apparatuses, namely, a maximum reaching direction of an acoustic echo, and an opposite side thereof becomes a dead angle. In  FIG. 16(   a ), the null beam has such a purpose that a directivity angle is directed to a direction of the loudspeaker  30  over the extension line of these two microphones  21  and  22  which is indicated by an angle of “b.” In the case of this drawing, such a delay coincident with a difference between reaching times of sound waves between both the microphones  21  and  22  may be given to a microphone signal of such a microphone located closer to the loudspeaker  30  in such a manner that the microphone signal of the microphone located closer to the loudspeaker  30  is made coincident with the microphone signal of the microphone located far from the loudspeaker  30 . In such a case that this delay time “ T   1 ” is given as  T   1 =d/c, as represented in a circular graph of  FIG. 16(   d ), a sharp dead angle is formed along the opposite direction of the loudspeaker  30 . As a result, it can be understood that this delay time “ T   1 ” becomes appropriate. On the other hand, similar to the main beam side, in such a case that the delay time  T   1 =d·cos θ/c is applied, as indicated in a circular graph of  FIG. 16(   e ), a large amount of sensitivities is also left along the opposite direction of the loudspeaker  30 , and a dead angle is insufficiently formed. 
     As previously described in this embodiment 2, the travel path differences calculated by transforming the delay time “ T ” for synthesizing the directivity along the target sound source directions on the side of the main beam and of the null beam are applied respectively, so that the directivity of the main beam and the directivity of the null beam can be correctly realized. 
     Embodiment 3 
       FIG. 17  shows another example in which signal delay times are corrected in accordance with an embodiment 3 of the present invention. As compared with  FIG. 16 , as shown in  FIG. 17(   b ), such a condition that a position of a telephone calling person is located parallel to a desk plane and the telephone calling person is present along a direction inclined from an extension line direction of two microphones  21  and  22  by an angle of “θ 2 ” is similar to the condition of  FIG. 16(   a ). However, in addition, an acoustic center of a loudspeaker  30  is not located along the extension line direction of these two microphones  21  and  22 , but is located along a direction of an angle “θ 1 .” In the case of the embodiment 3, a delay time “ T   1 ” of the delay filter  65  which is applied to the second microphone  22  may be calculated based upon the below-mentioned (Math. 12), while the second microphone  22  is located closer to the loudspeaker  30  than the first microphone  21 :
 
 T   1   =d ·cos θ 1   /c   [Math. 12]
 
     d: interval between two microphones  21  and  22   
     c: sound velocity in the air 
     θ 1 : angle from extension line of 2 microphones to acoustic center of loudspeaker 
     On the other hand, as represented in  FIG. 17(   b ), a delay time “ T   2 ” of the delay filter  66  which is applied to the first microphone  21  may be calculated based upon the below-mentioned (Math. 13), while the first microphone  21  is located far from the loudspeaker  30  than the second microphone  22 :
 
 T   2   =d ·cos θ 2   /c   [Math. 13]
 
     d: interval between two microphones 
     c: sound velocity in the air 
     θ 2 : angle from extension line of two microphones to position of telephone calling person 
     Since the above-described arrangement is employed, in the main beam, the voice signal derived from such a direction along which the telephone calling person is actually located may be emphasized, and in the null beam, the signal derived from such a direction along which the loudspeaker  30  is actually located may be emphasized. As a result, the sound collecting effect can be furthermore improved, and therefore, the full duplex communication with the higher quality can be carried out. 
     Embodiment 4 
     In the embodiment 1, as indicated in (Math. 1), the distance “d” between two sets of the first microphones  21  ( 21   a  to  21   d ) and the second microphones  22  ( 22   a  to  22   d ) is selected to be approximately ¼ of the wavelength of the maximum processable frequency “f.” Alternatively, this distance “d” may be decreased, while the sampling frequency “Fs” and the maximum processable frequency “f” are not changed. That is to say, instead of the above-explained distance “d”, such a distance “x” calculated based upon the below-mentioned (Math. 14) is defined as such a distance between two sets of the microphones  21  (namely,  21   a  to  21   d ) and the microphones  22  (namely,  22   a  to  22   d ) in an embodiment 4 of the present invention:
 
 x=h·d ( h&lt; 1)  [Math. 14]
 
     In this case, the delay time “ T ” as to the delay filters  65  and  66  provided in the circuit of  FIG. 9  is calculated in accordance with the below-mentioned (Math. 15):
 
 T   =x/c=h/Fs ( h&lt; 1)  [Math. 15]
 
     In other words, the delay time “ T ” may be selected to be such a value obtained by multiplying the sampling period “1/Fs” by “h” (h&lt;1). 
     As previously described, in accordance with the embodiment 4, the distance between two sets of the microphones  21  and  22  arranged in the microphone unit, namely in the sound collecting unit  2  can be furthermore shortened. As a result, it is possible to reduce that reflection sounds different from each other are entered to the respective microphones  21  and  22 , so that the full duplex communication with the higher quality can be carried out. 
     Embodiment 5 
       FIG. 18  is a perspective view for showing a telephone set  113  according to an embodiment 5 of the present invention, namely represents such a telephone set  113  to which any one of the voice conference apparatuses of the embodiments 1, 2, 3, 4 is applied.  FIG. 19  is an upper view of the telephone set  113  according to the embodiment 5 of the present invention, namely shows the telephone set  113  of  FIG. 18  which is viewed from an upper plane thereof. 
     As indicated in  FIG. 18 , similar to the voice conference apparatus  1  of the embodiment 1, the telephone set  113  of this embodiment 5 is equipped with a sound collecting unit  102  and a speaker unit  103 . It should be noted that only one set of the sound collecting unit  102  is employed which is different from the voice conference apparatus  1  of the embodiment 1. 
     In the sound collecting unit  102  and the speaker unit  103  shown in  FIG. 19 , microphone devices  121  and  122  and a loudspeaker  130  are illustrated in such a manner that positions of these structural components can be visually recognized. In an actual case, these structural components are arranged within the sound collecting unit  102  and the speaker unit  103 , and therefore, cannot be directly and visibly recognized. 
     While plural pieces of such telephone sets  113  are employed in a similar manner to the voice conference apparatus  1   a , or  1   b  shown in  FIG. 3 , or  FIG. 4  of the embodiment 1, these plural telephone sets  113  are connected via either the Internet  11  or the public telephone line  12  to each other in order to transmit/receive voice signals of the respective telephone sets  113 . 
     Hardware of the telephone set  113  according to the embodiment 5 has no large different points from those of the voice conference apparatuses  1  shown in  FIG. 5  and  FIG. 6  of the embodiment 1, and therefore, may be realized by merely adding a telephone calling/receiving interface of a handset to these voice conference apparatuses  1 . With respect to 2 pieces of omnidirectional microphone devices  121  and  122  which are installed in the microphone unit, namely the sound collecting unit  102 , and further, the loudspeaker  130  installed in the speaker unit  103 , such microphone devices and a loudspeaker having no large different points from those shown in the embodiment 1 may be employed. 
     In the microphone unit, namely the sound collecting unit  102 , two sets of the omnidirectional microphone devices  121  and  122  are arranged in such a manner that these microphone apparatus  121  and  122  are arranged on a radiation line  181  which connects an acoustic center  182  of the sound collecting unit  102  with an acoustic center  183  of the loudspeaker  130 . 
     Since a total number of omnidirectional microphone devices employed in the sound collecting unit  102  of each of the telephone set  113  is selected to be 2, the microphone unit, namely the sound collecting unit  102  of each of the telephone set  113  may be alternatively constituted by employing a minimum number of such omnidirectional microphones. As a consequence, the full duplex communication with the high quality can be carried out in a low apparatus cost. 
     While the vibration directions of the diaphragms of the respective microphone devices  121  and  122  are intersected substantially perpendicular with respect to the propagation direction of the compressional waves generated from the loudspeaker  130 , the second microphone apparatus  122  is installed closer to the loudspeaker  130  than the first microphone apparatus  121 . It should be noted that a distance between the first microphone apparatus  121  and the second microphone apparatus  122  may be selected to be the distance “d” conducted based upon the (Math. 1) of the embodiment 1, or may be alternatively selected to be the above-explained distance “x” conducted based upon the (Math. 14) of the embodiment 4. 
     While such an orthogonal line  184  is defined as a boundary and this orthogonal line  184  corresponds to a radiation line  181  which connects an acoustic center  182  of the sound collecting unit  102  with another acoustic center  183  of the loudspeaker  130 , such a sensitivity characteristic  185  is formed by such a similar sensitivity characteristic forming unit to the sensitivity characteristic forming unit indicated in the embodiment 1 and the embodiment 2 with employment of two sets of the microphone devices  121  and  122  of the sound collecting unit  102  in such a manner that an area of this sensitivity characteristic  185  of the sound collecting unit  102  on the side of the loudspeaker  130  with respect to this boundary may become smaller than other areas of this sensitivity characteristic  185 . 
     With employment of the above-described arrangement, since the desirable sensitivity characteristic  185  is formed by employing the omnidirectional microphone apparatus  121  and  122  in the respective sound collecting unit  102 , the fluctuations and also the aging changes contained in the sensitivity characteristic  185  of the respective sound collecting units  102  can be reduced and the sensitivity characteristic  185  thereof can be made stable. As a result, the full duplex communication with the high quality can be carried out. Also, the voice signals of the user (telephone calling person) of the telephone set  113  can be extremely clearly transmitted to the telephone set of the telephone counter party. Moreover, since these omnidirectional microphone devices  121  and  122  are used, the sound collecting unit  103  can be made compact, and can eliminate design restrictions thereof, as compared with using of directional microphones. This reason is given as follows: That is, in the case that the directional microphones are employed, these directional microphones can be readily influenced by peripheral structural components thereof, and also, sufficiently large spaces must be secured around these directional microphones, so that the sound collecting unit becomes bulky and the design thereof is restricted. 
     It is desirable to arrange that a distance between a plane containing sound collection ports of the plural microphone devices  121  and  122  of the microphone unit, namely the sound collecting unit  102 , and an upper plane of a protection member of the microphone unit, namely the sound collecting unit  102  may become shorter than another distance between the protection member and other plane than the first-mentioned plane of the microphone devices  121  and  122 . As a consequence, the microphone unit, namely the sound collecting unit  102  can mainly collect primary compressional waves from the loudspeaker  130 , and also, can hardly collect reflection sounds within the microphone unit, namely within the sound unit  102 . As a result, the load given to the reverberation reducing process operation can be decreased, so that the full duplex communication with the higher quality can be carried out. 
     The voice conference apparatuses according to the present invention may be utilized to, for instance, telephone sets, voice conference systems, television conference systems, and the like. 
     Many modifications and variations of the present invention are possible in the light of the above techniques. It is therefore to be understood that within the scope of the invention, the invention may be practiced than as specifically described. The present application is based upon and claims the benefit of priority of Japanese Patent Application Nos. 2007-48762 and 2008-10131 filed on Feb. 28, 2007 and Jan. 21, 2008, respectively, the contents of which are incorporated herein by references in its entirety.