Abstract:
A method for identifying among a plurality of devices, through which a voice communication path is connected, a device transmitting an echo signal onto the voice communication path, includes transmitting a predetermined signal to each of the devices from a predetermined portion on the voice communication path, receiving a response signal for the predetermined signal from each of the devices, measuring a period of time for the predetermined signal to travel from the predetermined portion to each of the devices and for the response signal to travel back to the predetermined portion, monitoring a upstream signal and a downstream signal traveling on the voice communication path, extracting an eco component from the downstream signal to determine an echo delay time, and comparing the period of time and the echo delay time to determine a device which causes the echo signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2008-78663, filed on Mar. 25, 2008, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to methods and apparatus for identifying a location of an echo source on a telephone network. 
       BACKGROUND 
       [0003]      FIG. 1  shows two telephone sets  301  and  302  for voice communications which are connected a communication path  303 , such as a telephone network or a voice over Internet protocol (VoIP) network. On the communication path  303 , four devices  310 A,  310 B,  310 C, and  310 D are provided with an example. These four devices  310 A to  310 D might cause an echo when they fall in failure or disorder. 
         [0004]    Since an echo disturbs clear voice conversations on a cell phone network or an IP phone network, carriers usually take countermeasures, such as adjusting or replacing the device, against the echo generation. However, it is difficult to identify which device generates the echo. 
         [0005]    Japanese Patent No. 3310302 discloses one of the echo detecting methods for identifying an echo source, in which an echo is detected at a previous stage to every device on a communication path and an echo source is determined on the detection result. 
         [0006]      FIG. 2A  is a conceptual view of the echo detecting method as disclosed in the publication. 
         [0007]    Echo detecting apparatuses  311   a ,  311   b ,  311   c , and  311   d  are provided at previous stages to all of the devices  310 A,  310 B,  310 C, and  310 D on a communication path  303  similarly to that of  FIG. 1 . The echo detecting apparatuses  311   a ,  311   b ,  311   c , and  311   d  monitor an upstream and a downstream signals transmitted on the communication path  303  to determine whether an echo occurs (“echo present”) or not (“echo absent”). When an echo is detected with the echo detecting apparatuses  311   a  and  311   b  and no echo is detected with the echo detection devices  311   c  and  311   d , as shown in  FIG. 2B , the device  310 B is identified as a source of the echo. 
         [0008]    However, according to the method disclosed in the publication above, echo detecting apparatuses  311   a  to  311   d  should be provided for every device on the communication path  303 , leading to an increase in cost. In addition, the method has a problem that an echo source cannot be identified correctly if plural devices generate an echo at the same time. 
         [0009]      FIGS. 3A and 3B  illustrate a conceptual view and a table similar one shown in  FIGS. 2A and 2B , which explain a problem in the method. 
         [0010]    An assumption of echo occurrence in the devices  310 B and  310 D, such as illustrated in  FIG. 3A , leads the results shown in  FIG. 3B , in which a detection result of all of the four echo detecting apparatuses  311   a ,  311   b ,  311   c , and  311   d  indicates “echo present”. In such a case, the device  310 D is determined as an echo source and an echo generated in the device  310 B would not be detected. 
         [0011]    Further, as disclosed in Japanese Laid-open Patent Publication No. 2003-134005, for example, a technique called “echo canceller” has been proposed. Since the echo canceller absorbs an echo disagreeable to the ear, it is one of promising techniques. However, this technique is not intended to identify an echo source. 
       SUMMARY 
       [0012]    According to one aspect of the invention, a method for identifying among a plurality of devices, through which a voice communication path is connected, a device transmitting an echo signal onto the voice communication path, includes transmitting a predetermined signal to each of the devices from a predetermined portion on the voice communication path, receiving a response signal for the predetermined signal from each of the devices, measuring a period of time for the predetermined signal to travel from the predetermined portion to each of the devices and for the response signal to travel back to the predetermined portion, monitoring a upstream signal and a downstream signal traveling on the voice communication path, extracting an eco component from the downstream signal to determine an echo delay time, and comparing the period of time and the echo delay time to determine a device which causes the echo signal. 
         [0013]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0014]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a diagram illustrating a telephone network including two telephone sets for voice communications and a communication path connecting between the two telephone sets; 
           [0016]      FIG. 2A  is a diagram illustrating a conventional echo detecting method and  FIG. 2B  illustrates results of detection of a presence of an echo; 
           [0017]      FIG. 3A  is a diagram illustrating a problem in the method of  FIGS. 2A and 2B , and  FIG. 3B  illustrates results of detection of a presence of an echo; 
           [0018]      FIG. 4A  is a block diagram illustrating the configuration of an echo source identifying apparatus according to a first embodiment and  FIG. 4B  illustrates a stored data of response delay time; 
           [0019]      FIG. 5  is a diagram illustrating a communication path connected to an echo source identifying apparatus according to a second embodiment; 
           [0020]      FIG. 6  is a block diagram of the configuration of the echo source identifying apparatus of the second embodiment illustrated in one block in  FIG. 5 ; 
           [0021]      FIG. 7  is a diagram illustrating a communication path connected to an echo source identifying apparatus according to a third embodiment; 
           [0022]      FIG. 8  is a block diagram illustrating the configuration of the echo source identifying apparatus of the third embodiment illustrated in one block in  FIG. 7 ; 
           [0023]      FIG. 9  is a diagram illustrating a communication path connected to an echo source identifying apparatus according to a fourth embodiment; 
           [0024]      FIG. 10  is a block diagram illustrating the configuration of the echo source identifying apparatus illustrated in one block in  FIG. 9 ; 
           [0025]      FIG. 11  is an explanatory view of cross-correlation operation according to a fifth embodiment; and 
           [0026]      FIG. 12  is a block diagram illustrating the configuration of an echo source identifying apparatus according to a sixth embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0027]    Preferred embodiments of the present invention will be explained with reference to accompanying drawings. 
         [0028]      FIG. 4A  illustrates a block diagram of the configuration of an echo source identifying apparatus  100 A according to a first embodiment, and a communication path  303  similar to that illustrated in  FIGS. 1 to 3A . 
         [0029]      FIG. 4B  illustrates a table which includes data of response delay time acquired through a response delay data acquiring unit  102 . 
         [0030]    Further, the echo source identifying apparatus  100 A includes an echo delay estimating unit  101 , and an echo source identifying unit  103 . 
         [0031]    The response delay data acquiring unit  102  determines a response delay time which is a round-trip time required for a signal to travel between positions on the path of the echo source identifying apparatus  100 A and of each of the devices  310 A,  310 B,  310 C, and  310 D. The data of response delay times are stored as the resultant information. Since the data of response delay time will be varied with change the configurations of devices on the communication path  303 , the information stored in the response delay data acquiring unit  102  is updated to information of a newly determined response delay time of a device that would be influenced by the change. 
         [0032]    There are several methods for determining a response delay time, such as ones shown bellow, and a desired one may be used. 
         [0033]    (a) The response delay time is acquired by calculating a transmission and a reception times; the transmission time is the time at which any known signal is output from an echo source identifying apparatus  100 A to a communication path; and the reception time is a time at which a device as a measurement target receives the signal. 
         [0034]    (b) A method is for the communication path on which packet communication is established. The response delay time is acquired from a transmission and a reception times; information of the transmission time is included in a packet transmitted to the path from the echo source identifying apparatus  100 A; and the reception time is a time at which the packet is received by a device as a measurement target. 
         [0035]    (c) A method is for the communication path on which packet communication is established. The response delay time is acquired from a transmission and a reception times; the transmission time is a time at which the echo source identifying apparatus  100 A transmits a packet for requesting a response to a device as a measurement target; the reception time is a time at which the echo source identifying apparatus  100 A receives a packet as the response from the device. 
         [0036]    The response delay data acquiring unit  102  of the echo source identifying apparatus  100 A obtains the response delay time corresponding to the time required to transfer a signal between the echo source identifying apparatus  100 A and each of the devices  310 A,  310 B,  310 C, and  310 D, and then stores the information of the response delay time. The response delay time can be determined on the base of any of the above methods in (a) to (c). 
         [0037]    The echo delay estimating unit  101  monitors the up and downstream signals to determine whether an echo is generated based on these signals. When an echo is detected, the echo delay estimating unit  101 , further, estimates an echo delay time from the time of passage of the upstream signal and the time of passage of the downstream signal as an echo. Here is assumed that the echo delay estimating unit  101  estimates the echo delay time as 120 ms and 390 ms, while the echo delay time will be estimated by the use of the algorithm described in the following embodiments. 
         [0038]    Each of the echo delay time, such as 120 ms and 390 ms, determined with the echo delay estimating unit  101  is compared with the response delay time of each of the devices  310 A,  310 B,  310 C, and  310 D by the echo source identifying unit  103 . Then the echo source identifying unit  103  identifies the devices  310 B and  310 D as an echo source, because each of the response delay times of the devices  310 B and  310 D is closer to each of the echo delay times 120 ms and 390 ms than that of the devices  310 A and  310 C. As shown above, each of the response delay times is obtained and stored with the response delay data acquiring unit  102  as illustrated in the table in  FIG. 4B . 
         [0039]    Consequently, even if an echo is generated at plural devices, the plural echo sources can be properly identified. 
         [0040]    The following description is focused on various embodiments where the echo delay estimation algorithm is specified. 
         [0041]      FIG. 5  illustrates a communication path connected to an echo source identifying apparatus according to a second embodiment. 
         [0042]    The communication path  400  illustrated in  FIG. 5  is assumed to be an analog phone network and is connected to an echo source identifying apparatus  100 B. 
         [0043]      FIG. 6  illustrates a block diagram of the configuration of the echo source identifying apparatus  100 B illustrated in one block in  FIG. 5 . 
         [0044]    The echo source identifying apparatus  100 B includes a cross-correlation calculation unit  111 , an echo detection unit  112 , an echo delay amount calculation unit  113 , a response delay amount determination unit  114 , and a delay amount comparison unit  115 . The response delay amount determination unit  114  includes a database (DB)  114   a  that stores the device-specific response delay time measured with the method (a), where the database unit  114   a  includes the table of data of the response delay time corresponding to the device such as ones illustrated in  FIG. 4B . The response delay amount determination unit  114  functions to read the device-specific response delay time from the DB  114   a  to transfer the read data to the delay amount comparison unit  115 . 
         [0045]    The cross-correlation calculation unit  111  receives transmission data (upstream signal) and reception data (downstream signal), and calculates a cross-correlation value Rxy(m) between the transmission data and the reception data based on the following expression: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       R 
                       xy 
                     
                      
                     
                       ( 
                       m 
                       ) 
                     
                   
                   = 
                   
                     
                       ∑ 
                       n 
                     
                      
                     
                       
                         x 
                          
                         
                           ( 
                           n 
                           ) 
                         
                       
                        
                       
                         y 
                          
                         
                           ( 
                           
                             m 
                             - 
                             n 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0046]    In Expression 1, x is reception data, y is transmission data, x(n) is n-th sampled value in the reception data, and y(n) is n-th sampled value in the transmission data. 
         [0047]    The cross-correlation value Rxy(m) is input to the echo detection unit  112  and the echo delay amount calculation unit  113 . 
         [0048]    The echo detection unit  112  determines whether the cross-correlation value Rxy(m) received exceeds a threshold value. If the cross-correlation value Rxy(m) exceeds a threshold value at any value of m and thus, then the echo detection unit  112  decides that an echo occurs. Information of the occurrence of echo and the value of m in the cross-correlation value Rxy(m) that exceeds a threshold value is sent to the echo delay amount calculation unit  113 . 
         [0049]    Upon receiving the information of the occurrence of echo, the echo delay amount calculation unit  113  derives an echo delay amount “delay” from the following expression (2) based on the cross-correlation value Rxy(m) and the value of m. 
         [0000]    
       
         
           
             
               
                 
                   
                     delay 
                     = 
                     
                       arg 
                        
                       
                           
                       
                        
                       
                         
                           max 
                           m 
                         
                          
                         
                           ( 
                           
                             
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                   ( 
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         [0000]    where arg gives m which maximizes R xy (m). 
         [0050]    In the case of calculating the echo delay amount “delay” based on the expression (2), a peak value around the value of m notified from the echo detection unit  112  is determined. Therefore, if the echo detection unit  112  detects that Rxy(m) exceeds a threshold value at plural separate values of m, the echo delay amount “delay” is calculated based on the expression (2) for each of the plural values. Here is assumed that the echo delay amount calculation unit  113  calculates 120 ms as an echo delay amount. 
         [0051]    The echo delay amount calculated with the echo delay amount calculation unit  113  is input to the delay amount comparison unit  115 , which compares the echo delay amount, such as 120 ms from the echo delay amount calculation unit  113  with the device-specific response delay time, such as ones in the table shown in  FIG. 4B  to identify an echo source. The echo delay amount calculated with the echo delay amount calculation unit  113  is, in this example, 120 ms, which is closest to the response delay time 110 ms of the device B in the table shown in  FIG. 4B . Thus, in this case, the device  310 B is identified as an echo source. 
         [0052]    The analog phone network is described here by way of example, but the algorithm described with reference to  FIG. 6  is applicable to an Internet protocol (IP) phone network for packet communications as well. 
         [0053]      FIG. 7  illustrates a communication path such as IP telephone network connected to an echo source identifying apparatus according to a third embodiment of the present invention. 
         [0054]    In the illustrated example, the communication path is assumed an IP phone network for transmitting/receiving packet data, where an echo source identifying apparatus  100 C is connected to the IP phone network. 
         [0055]      FIG. 8  illustrates a block diagram of the configuration of the echo source identifying apparatus  100 C of the third embodiment illustrated in one block in  FIG. 7 . 
         [0056]    The echo source identifying apparatus  100 C in  FIG. 8  includes a frequency analysis unit  121 , a cross-correlation calculation unit  122 , a correlation smoothing unit  123 , an echo detection unit  124 , an echo delay amount calculation unit  125 , a response delay data acquiring unit  126 , and a delay amount comparison unit  127 . 
         [0057]    The frequency analysis unit  121  includes an FFT calculation unit  121   a  for reception data and an FFT calculation unit  121   b  for transmission data. The frequency analysis unit  121  receives reception data and transmission data. The reception data and the transmission data are independently subjected to windowing processing and then Fast Fourier Transform (FFT) processing to thereby calculate a frequency spectrum for each of the reception data and the transmission data and for each of plural frames consecutive along a time axis direction. 
         [0058]    A frequency spectrum X(m, f) of reception data and a frequency spectrum Y(m, f) of transmission data, which are calculated with the frequency analysis unit  121  by using the following expressions (3) to (6). 
         [0000]        x ′( n )= w ( n ) x ( n )  (3) 
         [0000]      X(m,f)         x′(n)  (4) 
         [0000]        y ′( n )= w ( n ) y ( n )  (5) 
         [0000]      Y(m,f)         y′(n)  (6) 
         [0059]    In the expressions (3) to (6), x(n) is reception data, y(n) is transmission data, w(n) is window function (such as Hamming window), n is a sample number, m is a frame number, f is a frequency number, and           is Fourier transformation. The resultant frequency spectrum X(m,f) for the reception data and frequency spectrum Y(m, f) of transmission data are fed to the cross-correlation calculation unit  122  which calculates a cross-correlation value Rxy(m, i) with X(m, f) and Y(m, f) by using the following expression (7). 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       R 
                       xy 
                     
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                       ( 
                       
                         m 
                         , 
                         i 
                       
                       ) 
                     
                   
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                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0060]    As apparent from the expression (7), Rxy(m, i) does not contain a variable “f” because the value is incremented in the direction of f, that is, the direction of frequency, while Rxy(m, i) contains variables “m” and “i” in the direction of frame (that is, the time axis direction). In other words, the expression (7) shows a frame-based cross-correlation value between reception data and transmission data. 
         [0061]    After the cross-correlation calculation unit  122  calculates the cross-correlation value Rxy(m, i), the calculated cross-correlation value Rxy(m, i) is input to the correlation smoothing unit  123  and subjected to the smoothing processing based on the following expression (8). 
         [0000]          R     xy ( m,i )=   R     xy ( m− 1 ,i )×α+ R   xy ( m,i )×(1−α)  (8) 
         [0000]    In the expression (8), α is a smoothing coefficient. 
         [0062]    A cross-correlation value smoothed with the correlation smoothing unit  123  is input to the echo detection unit  124  and the echo delay amount calculation unit  125 . The echo detection unit  124  detects the generation of an echo if the following the expression (9) is satisfied. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       max 
                       m 
                     
                      
                     
                       ( 
                       
                         
                           
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                           ) 
                         
                       
                       ) 
                     
                   
                   &gt; 
                   THR 
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
         [0000]    In the expression 9, THR is a threshold value. 
         [0063]    Then, the value “i” of the peak cross-correlation value which is larger than the threshold value THR, is fed to the echo delay amount calculation unit  125 . 
         [0064]    Similar to the second embodiment, the determination based on the expression (9) is to determine whether the peak local value i exceeds the threshold value THR. For example, if the value i varies, and plural peak local values larger than the threshold value THR are detected, the plural values of “i” are notified to the echo delay amount calculation unit  125 . 
         [0065]    The echo delay amount calculation unit  125  calculates the echo delay amount “delay” based on the following expression (10). 
         [0000]      If    R     xy ( m,i )&gt; THR , then delay= i×N.   (10) 
         [0000]    In the expression (10), “delay” is an echo delay (ms), and N is a frame length (ms). Then the calculated echo delay amount “delay” is sent to the delay amount comparison unit  127 . 
         [0066]    The response delay data acquiring unit  126  obtains the response delay time of each device based on the method (c) described above. More specifically, the response delay data acquiring unit  126  transmits IP packets to each device on the IP phone network  200 , such as devices  310 A,  310 B,  310 C, and  310 D illustrated in  FIG. 7 , and measures each period (response delay time) until receiving each response for the individual IP packets. 
         [0067]    The response delay time of each device thus obtained with the response delay data acquiring unit  126  is sent to the delay amount comparison unit  127 . 
         [0068]    The delay amount comparison unit  127  compares the echo delay amount “delay” with the response delay time of each device to identify a device as an echo source. 
         [0069]      FIG. 9  illustrates a communication path connected to an echo source identifying apparatus according to a fourth embodiment. 
         [0070]    The communication path  420  in  FIG. 9  illustrates a network comprising several different kinds of networks such as an IP phone network  421  and an analog phone network  422 . In the illustrated example, an echo source identifying apparatus  100 D is connected to the communication path  420  for the mixed networks. 
         [0071]      FIG. 10  is a block diagram of the configuration of the echo source identifying apparatus  100 D of the fourth embodiment illustrated in one block in  FIG. 9 . 
         [0072]    The echo source identifying apparatus  100 D includes a frequency analysis unit  131 , a spectrum processing unit  132 , a cross-correlation calculation unit  133 , an echo detection unit  134 , an echo delay amount calculation unit  135 , a response delay data acquiring unit  136 , and a delay amount comparison unit  137 . 
         [0073]    As for the devices such as the devices  310 A,  310 B, and  310 C on the IP communication network  421  in  FIG. 9 , the response delay data acquiring unit  136  measures the response delay time based on the transmission time of a transmission packet and the reception time of a reception packet similar to the above embodiments (see  FIG. 8 ). As for the device such as the device  310 D on the analog communication network  422  in  FIG. 9 , a response delay time is separately measured with the method (a) and stored in a database (DB)  136   a.    
         [0074]    The response delay data acquiring unit  136  notifies the delay amount comparison unit  137  of the response delay time of each device on the IP phone network and on the analog phone network, while the response delay time of the device on the analog phone network is stored in the DB  136   a.    
         [0075]    Reception data and transmission data are input to the frequency analysis unit  131 , in which the frequency analysis unit  131  and two FFT calculation units  131   a  and  131   b  are included. Since the frequency analysis unit  131  and the two FFT calculation units  131   a  and  131   b  are similar to the frequency analysis unit  121  and the two FFT calculation units  121   a  and  121   b  illustrated in  FIG. 8  respectively, no explanation is described. 
         [0076]    The spectrum processing unit  132  in  FIG. 10  normalizes frequency spectra of reception data and transmission data to a numerical value range of −1 to 1 by using the following expressions (11) and (12). 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0077]    Using the frequency spectra normalized with the spectrum, the cross-correlation calculation unit  133  performs cross-correlation operation. The cross-correlation calculation unit  133  has the same function as that of the cross-correlation calculation unit  122  of the third embodiment illustrated in  FIG. 8  except that the cross-correlation operation is performed using the frequency spectra normalized in the spectrum processing unit  132 , and thus the description thereof is not repeated. 
         [0078]    The cross-correlation value calculated with the cross-correlation calculation unit  133  is input to the echo detection unit  134  and the echo delay amount calculation unit  135 . 
         [0079]    The echo detection unit  134  and the echo delay amount calculation unit  135  of the fourth embodiment directly receive the cross-correlation value calculated with the cross-correlation calculation unit  133 . These processes are different from the processes of the third embodiment in which the correlation smoothing unit  123  is provided to feed the smoothed cross-correlation value to the echo detection unit  124  and the echo delay amount calculation unit  125  (see  FIG. 8 ). Since except these differences, the echo detection unit  134  and the echo delay amount calculation unit  135  have the same functions as those of the echo detection unit  124  and the echo delay amount calculation unit  125 , the description thereof is not repeated. 
         [0080]    Further, the delay amount comparison unit  137  is also similar to the delay amount comparison unit  127  in  FIG. 8  and thus, repetitive description thereof is not given. 
         [0081]    Next, an echo source identifying apparatus according to a fifth embodiment of the present invention is described. 
         [0082]    The echo source identifying apparatus of the fifth embodiment is placed under the same environment as that of the echo source identifying apparatus  100 D of the fourth embodiment, and its block configuration is similar to that of  FIG. 10  and thus, the fifth embodiment is described with reference to  FIGS. 9 and 10 . 
         [0083]      FIG. 11  illustrates an explanatory view of cross-correlation operation of the fifth embodiment. 
         [0084]    The cross-correlation calculation unit  133  of the fifth embodiment calculates a cross-correlation value for each of frequency bands obtained by dividing the entire frequency band of the frequency spectrum obtained with the frequency analysis unit  131  by using the following expression (13). 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       R 
                       xy 
                     
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                             f 
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         [0085]    The echo detection unit  134  counts the number of frequency bands where the cross-correlation value of each of the frequency bands is larger than a threshold value among cross-correlation values of the frequency bands calculated with the cross-correlation calculation unit  133 . When the number of frequency bands is larger than a threshold value, the expression (14) is satisfied, it is detected that the echo has been occurred. 
         [0000]    
       
         
           
             
               
                 
                   
                     num 
                      
                     
                       ( 
                       
                         m 
                         , 
                         i 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       num 
                       k 
                     
                      
                     
                       ( 
                       
                         k 
                         | 
                         
                           
                             
                               R 
                               xy 
                             
                              
                             
                               ( 
                               
                                 m 
                                 , 
                                 i 
                                 , 
                                 k 
                               
                               ) 
                             
                           
                           &gt; 
                           THR 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
         [0086]    Further, the echo delay amount calculation unit  135  determines the thus-obtained delay amount as the echo delay time “delay” as shown in the expression (15). 
         [0087]    If num(m,i)&gt;THR2, then an echo is detected by the following expression (15). 
         [0000]      delay= i×N   (15) 
         [0088]    In this example, the number of frequency bands of which the cross-correlation value in the echo detection unit  134  is larger than a threshold value, is counted. However, the number of frequency bands is counted locally in the time axis direction. Accordingly, if plural devices generate an echo, the generation of an echo is detected at each of the plural echo sources, and the echo delay amount calculation unit  135  also calculates the echo delay time at each of the echo sources to identify each of the echo sources. 
         [0089]    In the fifth embodiment, a cross-correlation value is calculated in units of plural frequency bands. Thus, even if noise is mixed into any frequency band, the whole noise rarely influences the whole frequency band group, and the generation of an echo can be detected with higher accuracy. 
         [0090]      FIG. 12  is a block diagram of the configuration of an echo source identifying apparatus according to a sixth embodiment. 
         [0091]    The environment of a communication network connected with an echo source identifying apparatus  100 E of the sixth embodiment is the same as those of the fourth and fifth embodiments (see  FIG. 9 ). 
         [0092]    The following description is made of a difference from the echo source identifying apparatus  100 D of the fourth embodiment. 
         [0093]    The echo source identifying apparatus  100 E of the sixth embodiment shown in  FIG. 12  is provided with a spectrum smoothing unit  142  in place of the spectrum processing unit  132  of the echo source identifying apparatus  100 D of the fourth embodiment shown in  FIG. 10 . 
         [0094]    The spectrum smoothing unit  142  is composed of a smoothing unit  142   a  for smoothing a frequency spectrum of reception data obtained with the FFT calculation unit  141   a  in the frequency analysis unit  141  and a smoothing unit  142   b  for smoothing a frequency spectrum of transmission data obtained with the other FFT calculation unit  141   b . These smoothing processes are performed on the basis of following expressions (16) and (17) for the reception data and the transmission data respectively. 
         [0000]        X ′( m,f )= X ( m− 1 ,f )×α+ X ( m,f )×(1−α)  (16) 
         [0000]        Y ′( m,f )= Y ( m− 1 ,f )×α+ Y ( m,f )×(1−α)  (17) 
         [0000]    In the expressions (16) and (17), α is a smoothing coefficient. 
         [0095]    A cross-correlation calculation unit  143  calculates a cross-correlation value using the frequency spectra smoothing wit the spectrum smoothing unit  142  as follows. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       R 
                       xy 
                     
                      
                     
                       ( 
                       
                         m 
                         , 
                         i 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       ∑ 
                       f 
                     
                      
                     
                       
                         
                           X 
                           ′ 
                         
                          
                         
                           ( 
                           
                             m 
                             , 
                             f 
                           
                           ) 
                         
                       
                        
                       
                         
                           Y 
                           ′ 
                         
                          
                         
                           ( 
                           
                             
                               m 
                               - 
                               i 
                             
                             , 
                             f 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   18 
                   ) 
                 
               
             
           
         
       
     
         [0096]    The other components, such as the frequency analysis unit  141 , the echo detection unit  144 , the echo delay amount calculation unit  145 , the response delay data acquiring unit  146 , the DB  146   a , and the delay amount comparison unit  147  correspond to the components of the echo source identifying apparatus  100 D of the fourth embodiment illustrated in  FIG. 10 . That is, the frequency analysis unit  131 , the echo detection unit  134 , the echo delay amount calculation unit  135 , the response delay data acquiring unit  136 , the DB  136   a , and the response delay data acquiring unit  136 , and thus are not described again. 
         [0097]    As described in the above embodiments, a wide variety of echo source identifying methods are applicable based on cross-correlation operations. 
         [0098]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.