Patent Publication Number: US-10764441-B2

Title: Sound signal processing device and sound signal processing method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation Application of PCT Application No. PCT/JP2016/065652, filed May 26, 2016, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a sound signal processing device and a sound signal processing method suitable for a sound emission-reception apparatus used for remote audio conferencing. 
     Description of the Related Art 
     Recently, audio conferencing systems have been put to practical use. Such systems enable exchange of voice signals (sound signals) by use of a sound emission-reception apparatus connected to a network. The sound emission-reception apparatus receives voices of participants in a place (meeting room) by use of a microphone and outputs a sound signal derived from received sound to the network. The sound emission-reception apparatus uses a loudspeaker to convert a sound signal provided by the network into a voice whereby the participants in the meeting room are able to hear the voice. 
     Conceptually, the sound emission-reception apparatus includes a microphone and a loudspeaker. Instead of being provided inside the sound emission-reception apparatus, the microphone and the loudspeaker may be provided outside the sound emission-reception apparatus and connected to the sound emission-reception apparatus. The sound emission-reception apparatus may be understood to include: the microphone; the loudspeaker; and a sound signal processing device that processes sound signals derived from sounds received by the microphone and sound signals output to the loudspeaker. 
     A size of meeting rooms used for audio conferences varies, and a number of participants in such meeting rooms also varies. In a case where a meeting room is spacious and there is a large number of participants, it may be difficult to receive all voices of all the participants and to enable all of the participants to evenly hear sound derived from sound signals provided by the network using the loudspeaker. To overcome this difficulty, a system has been proposed for installation of pods dispersed around a meeting room, with each pod including a microphone and a loudspeaker (refer to  FIG. 15 , and also to the description in the eighth column in U.S. Pat. No. 8,031,853). 
     In the system outlined above, a pod is unable to connect to a network directly, and is required to connect to a base that serves as a host. Thus, when two pods are installed at a distance from each other, it is necessary to provide a base in addition to the two pods. 
     SUMMARY 
     The present invention has been created in view of the above circumstances and has as its object the provision of a sound signal processing method and a sound signal processing device that is able to connect to the network without need for a separate dedicated device, such as a base. 
     In order to achieve the above object, a sound signal processing device according to one aspect of the present invention includes: a microphone terminal to which a sound signal derived from sound received by a microphone is input; a loudspeaker terminal from which a sound signal directed to a loudspeaker is output; a first input terminal to which a sound signal from another device at a proximal-end is input; a first output terminal from which a sound signal directed to the other device at the proximal-end is output; a distal-end input terminal to which a distal-end sound signal is input via a network; a distal-end output terminal from which a sound signal directed to the network is output; a path establisher configured to establish at least one signal path from at least one of the microphone terminal, the first input terminal, or the distal-end input terminal, to at least one of the loudspeaker terminal, the first output terminal, or the distal-end output terminal; and a path indicator configured to indicate to the path establisher a signal path that is to be established. 
     A sound signal processing method according to a second aspect of the present invention is implemented in a device that comprises at least: a microphone terminal to which a sound signal derived from sound received by a microphone is input; a loudspeaker terminal from which a sound signal directed to a loudspeaker is output; a first input terminal to which a sound signal from another device at a proximal-end is input; a first output terminal from which a sound signal directed to the other device is output; a distal-end input terminal to which a distal-end sound signal is input via a network; and a distal-end output terminal from which a sound signal directed to the network is output, the method including: acquiring a connection status of the subject device to the network and a connection status of the other device to the network; and determining, based on the acquired connection statuses, at least one signal path from at least one of the microphone terminal, the first input terminal, or the distal-end input terminal, to at least one of the loudspeaker terminal, the first output terminal, or the distal-end output terminal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a system that includes a sound emission-reception apparatus according to a first embodiment. 
         FIG. 2  is a diagram showing a hardware configuration of the sound emission-reception apparatus. 
         FIG. 3  is a diagram showing functional blocks of the sound emission-reception apparatus. 
         FIG. 4  is a diagram showing an operation sequence of the system. 
         FIG. 5  is a diagram showing signal paths established by a path establisher in the sound emission-reception apparatus. 
         FIG. 6  is a diagram showing signal paths established by the path establisher in the sound emission-reception apparatus. 
         FIG. 7  is a diagram showing a system in which two sound emission-reception apparatuses are connected to a network. 
         FIG. 8  is a diagram showing an operation sequence of the system. 
         FIG. 9  is a diagram showing signal paths established by the path establisher in the sound emission-reception apparatus. 
         FIG. 10  is a diagram showing signal paths established by the path establisher in the sound emission-reception apparatus according to an exemplary application. 
         FIG. 11  is a diagram showing a system that includes a sound emission-reception apparatus according to a second embodiment. 
         FIG. 12  is a diagram showing functional blocks of the sound emission-reception apparatus. 
         FIG. 13  is a diagram showing an operation sequence of the system. 
         FIG. 14  is a diagram showing signal paths established by the path establisher in the sound emission-reception apparatus. 
         FIG. 15  is a diagram showing an operation sequence of a system in which two or more sound emission-reception apparatuses are connected to the network. 
         FIG. 16  is a diagram showing signal paths established by the path establisher in the sound emission-reception apparatus. 
         FIG. 17  is a diagram showing signal paths established by the path establisher in the sound emission-reception apparatus according to an exemplary application. 
         FIG. 18  is a diagram showing functional blocks of a sound emission-reception apparatus according to a third embodiment. 
         FIG. 19  is a diagram showing signal paths established by the path establisher in the sound emission-reception apparatus. 
         FIG. 20  is a diagram showing an exemplary setting of delay devices. 
         FIG. 21  is a diagram showing another example of signal paths established by the path establisher in the sound emission-reception apparatus. 
         FIG. 22  is a diagram showing an exemplary setting of the delay devices in another example. 
         FIG. 23  is a diagram showing signal paths established by the path establisher in the sound emission-reception apparatus according to an exemplary application. 
         FIG. 24  is a diagram showing a connection configuration (example 1) of sound emission-reception apparatuses according to a fourth embodiment. 
         FIG. 25  is a diagram showing another connection configuration (example 2). 
         FIG. 26  is a diagram showing another connection configuration (example 3). 
         FIG. 27  is a diagram showing another connection configuration (example 4). 
         FIG. 28  is a diagram showing functional blocks of the sound emission-reception apparatus according to the fourth embodiment. 
         FIG. 29  is a diagram showing signal paths established by the path establisher in the sound emission-reception apparatus. 
         FIG. 30  is a diagram showing signal paths established by the path establisher in the sound emission-reception apparatus. 
         FIG. 31  is a diagram showing an exemplary setting of delay devices. 
         FIG. 32  is a diagram showing an exemplary setting of delay devices. 
         FIG. 33  is a diagram showing signal paths established by the path establisher in the sound emission-reception apparatus according to an exemplary application. 
         FIG. 34  is a diagram showing signal paths established by the path establisher in the sound emission-reception apparatus according to the exemplary application. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     With reference to accompanying drawings, embodiments of the present invention will now be described below. 
     First Embodiment 
       FIG. 1  is a diagram showing a system that includes a sound emission-reception apparatus according to a first embodiment. 
     In this embodiment, a system  1  includes two sound emission-reception apparatuses  10 . The sound emission-reception apparatuses  10  have the same configuration, with the exception of interior established signal paths, which will be described later. The sound emission-reception apparatuses  10  are installed apart from each other in a place, such as a meeting room. Each sound emission-reception apparatus  10  includes a notification device  130 , such as an LED; and an input device  140 , such as a momentary push-on switch. The two sound emission-reception apparatuses  10  are connected to each other via a cable C. 
     In this embodiment, a single cable C transmits two sound signals. Alternatively, two cables may be used, each of which transmits one sound signal. 
       FIG. 1  shows an example in which one of the two sound emission-reception apparatuses  10  is connected to a network  400  via a PC  300 . Another system (illustration omitted) installed in another place is connected to the network  400 . In this setting, the system  1  and the other system exchange sound signals with each other. 
     As will be described later, each of the two sound emission-reception apparatuses  10  may be connected to the network  400  via the PC  300 . 
     In this description, the term “connection” refers to direct or indirect coupling between two or more elements; and there may be one or more intermediate elements between these two or more elements, with the exception of the sound emission-reception apparatus  10 . A connection between elements may be physical, logical, or a combination of both. For example, the connection between elements may be realized by electric wire, cable, or wiring on a printed circuit board, or may be realized by use of wireless communication, or by a combination of two or more of these forms. 
     In  FIG. 1 , a single sound emission-reception apparatus  10  is connected to the network  400  via the PC  300 . It is to be noted, however, that if the sound emission-reception apparatus  10  is treated as an intermediate element, there is room to assume that the other sound emission-reception apparatus  10  may be connected to the network  400  via the sound emission-reception apparatus  10  and the PC  300 . To exclude the possibility of any such assumption, the sound emission-reception apparatus  10  is not included among the intermediate elements. 
     In the system  1  shown in  FIG. 1 , the sound emission-reception apparatus  10  is connected to the network  400  via the PC  300 , since by using a network connecting capability provided in the PC  300 , the configuration of the sound emission-reception apparatus  10  can be simplified. Alternatively, the sound emission-reception apparatus  10  may be equipped with a network connecting capability and may be directly connected to the network  400 . 
     Relative to the sound emission-reception apparatus  10 , the PC  300  is merely a relay point to the network  400 . Accordingly, in the following description, the presence of the PC  300  will not be discussed, and the point of focus will be whether the sound emission-reception apparatus  10  is connected to the network  400 . 
     A typical example of the network  400  is the Internet. However, examples of the network  400  include an intra-firm LAN (local area network), a wireless telephone network, and a wired telephone network. 
       FIG. 2  shows a hardware configuration of a sound emission-reception apparatus  10 . 
     As shown in  FIG. 2 , the sound emission-reception apparatus  10  includes a microphone  12 , an ADC (analog to digital converter)  14 , a DAC (digital to analog converter)  16 , a loudspeaker  18 , a CPU (central processing unit)  100 , a memory  110 , an I/F (interface)  120 , a notification device  130 , an input device  140 , a communication device  150 , a bus  160 , and a DSP (digital signal processor)  200 . 
     For illustrative purposes, the ADC and the DAC are expressed respectively as AD and DA in the figures. In the following description, the term “device” or “apparatus” may be substituted with a term such as circuitry, unit, or module. 
     The CPU  100  controls each element of the sound emission-reception apparatus  10  by executing a program stored in the memory  110 . In addition to the program, the memory  110  stores temporal data stored by the CPU  100  and the DSP  200 . 
     The microphone  12  receives sounds around the sound emission-reception apparatus  10  to generate an analog sound signal. Specifically, sounds received by the microphone  12  are voices of participants in a meeting room in which the sound emission-reception apparatus  10  is installed. The ADC  14  converts the sound signal derived from sounds received by the microphone  12  into a digital signal and provides the digital signal to the DSP  200 . An ADC  251  converts a sound signal provided from another apparatus via the cable C into a digital signal, and provides the digital signal to the DSP  200 . 
     As will be described later in detail, the DSP  200  performs, with use of signal paths indicated by the CPU  100  (path indicator), computational processing on a sound signal converted by the ADC  14 ; the proximal-end sound signal being converted by the ADC  251 ; and the sound signal being provided by the other distal-end system via the network  400 , the I/F  120 , and the bus  160 . The DSP then outputs the processed sound signals to the DAC  16 , a DAC  261 , the loudspeaker  18 , and the other distal-end system. 
     In the following description, the term “distal-end” refers to a signal and the like that passes through the network  400 ; and the term “proximal-end” refers to a signal and the like that does not pass through the network  400 . 
     The subject apparatus refers to a single sound emission-reception apparatus  10  in focus. The other apparatus refers to a proximal-end sound emission-reception apparatus  10  other than the subject apparatus within the same system. 
     The expression “A toward B” does not exclude a situation where other intermediate elements exist between A and B. 
     The communication device  150  may communicate with the other apparatus by wireless communication, for example. 
     The DAC  16  converts the sound signals processed by the DSP  200  into analog signals and outputs the analog signals. The loudspeaker  18  converts the sound signals converted by the DAC  16  into sounds and outputs the sounds. The DAC  261  converts the sound signals processed by the DSP  200  into analog signals and outputs the analog signals. 
     In this embodiment, the DSP  200  executes signal processing. Before being processed by the DSP  200 , the signals are converted into digital format by the ADC  14  and the ADC  251 , and after being processed by the DSP  200 , the signals are converted back into analog format by the DAC  16  and the DAC  261 . As will be described later, in place of digital signal processing by the DSP  200 , there may be employed analog signal processing. Where analog signal processing is performed, none of the ADC  14 , the ADC  251 , the DAC  16 , and the DAC  261  are required. 
     In  FIG. 2 , a single microphone  12  and a single loudspeaker  18  are illustrated. However, there may be multiple microphones  12  and multiple loudspeakers  18 . 
     In this embodiment, the DSP  200 , the ADC  14 , the ADC  251 , the DAC  16 , and the DAC  261  are described as separate bodies for a purpose of describing signal paths that are established in the DSP  200 . The DSP  200  may house the ADC  14 , the ADC  251 , the DAC  16 , and the DAC  261 . 
       FIG. 3  is a diagram showing functional blocks of the sound emission-reception apparatus  10 , with a focus on the flow of signals. 
     As shown in the figure, a detector  102  and the path indicator  104  are established in the CPU  100  by execution of the program, and a path establisher  202  is established in the DSP  200 . Illustration of the I/F  120  shown in  FIG. 2  is omitted in  FIG. 3  since the I/F  120  is not involved in the flow of signals. 
     The detector  102  detects whether the subject apparatus is connected to the network  400  and is in a condition to be able to exchange sound signals with another system. 
     The detector  102  outputs a detection result to the path indicator  104 . If the detector  102  detects that the subject apparatus is connected to the network  400 , the detector  102  provides sound signals from the other system to a distal-end input terminal  211  of the path establisher  202 , and transfers sound signals output from a distal-end output terminal  213  of the path establisher  202  toward the other system. 
     The path indicator  104  directs the notification device  130  to notify a user, and after the user operates the input device  140 , receives operation information. Here, the user refers to a part or all of the participants in a meeting room in which the sound emission-reception apparatuses  10   a  and  10   b  are installed. 
     The path indicator  104  directs the communication device  150  to exchange information with the sound emission-reception apparatus  10 , which is the other apparatus, and indicates to the path establisher  202  signal paths that are to be established. 
     The path establisher  202  establishes signal paths indicated by the path indicator  104 . As will be described later, the signal paths refer to two or more paths originating from the distal-end input terminal  211 , a microphone terminal  212 , and a first input terminal  221 , and reaching the distal-end output terminal  213 , a loudspeaker terminal  214 , and a first output terminal  231 . Between starting points and end points of the signal paths, signals passing through the signal paths undergo computational processing, such as delay, addition and subtraction, and distribution. 
     The signal paths include paths through which signals are directly provided from starting points to end points and paths through which signals are indirectly provided from starting points to end points, elements such as a delay device or an adder intervening therebetween. 
     The microphone terminal  212  of a sound emission-reception apparatus  10  is a terminal to which a sound signal derived from sound received by the microphone  12  of the sound emission-reception apparatus  10  is input; and the loudspeaker terminal  214  of the sound emission-reception apparatus  10  is a terminal from which a sound signal is output toward the loudspeaker  18  of the sound emission-reception apparatus  10 . 
     The first input terminal  221  is a terminal to which a sound signal from the other apparatus is input, and the first output terminal  231  is a terminal from which a sound signal is output toward the other apparatus. 
     The terminal here refers to a structure to which a signal is input or from which a signal is output. More specifically, the terminal is a signal pin, a part of a wire, or a connector, for example. 
     The microphone  12  and the loudspeaker  18  do not need to be provided inside the sound emission-reception apparatus  10 , and may be provided outside of the sound emission-reception apparatus  10 . Regardless of whether the microphone  12  and the loudspeaker  18  are provided inside or outside the sound emission-reception apparatus  10 , the sound emission-reception apparatus  10  is provided with the microphone terminal  212 , to which sound signals derived from sound received by the microphone  12  are input, and is provided with the loudspeaker terminal  214 , from which sound signals are output toward the loudspeaker  18 . 
     Next, operations of the system  1  will be described. 
     As described above, in this embodiment, there are two cases: a case in which a single sound emission-reception apparatus  10  among two sound emission-reception apparatuses  10  is connected to the network  400 ; and a case in which both of the two sound emission-reception apparatuses  10  are connected to the network  400 . 
     There will be first described operations of the system  1  in a case where a single sound emission-reception apparatus  10  alone is connected to the network  400 . 
       FIG. 4  is a diagram showing an operation sequence of the system  1  in such case.  FIG. 4  shows exchange of information between a sound emission-reception apparatus  10  connected to the network  400  and a sound emission-reception apparatus  10  not connected to the network  400 . For descriptive purposes, “a” is appended to the tails of reference signs of elements in the sound emission-reception apparatus connected to the network  400 , and “b” is appended to the tails of reference signs of elements in the sound emission-reception apparatus not connected to the network  400 . For example, the reference sign of the sound emission-reception apparatus connected to the network  400  is “ 10   a ”, and that of the path indicator therein is “ 104   a ”. The reference sign of the sound emission-reception apparatus not connected to the network  400  is “ 10   b ”, and that of the path indicator therein is “ 104   b”.    
     In the sound emission-reception apparatus  10   a , after the detector  102   a  detects that the sound emission-reception apparatus  10   a  is connected to the network  400 , the detector  102   a  provides the detection result to the path indicator  104   a . After being provided with the detection result, the path indicator  104   a  directs the communication device  150   a  to transmit the detection result to the sound emission-reception apparatus  10   b  (step Sa 11 ). Thus, the detection result that the sound emission-reception apparatus  10   a  is connected to the network is transmitted to the sound emission-reception apparatus  10   b.    
     In the sound emission-reception apparatus  10   b , after the communication device  150   b  receives the detection result of the detector  102   a , the communication device  150   b  transfers the detection result to the path indicator  104   b . The path indicator  104   b , to which the detection result has been transferred, further receives a detection result of the detector  102   b  in the subject sound emission-reception apparatus  10   b . Since the sound emission-reception apparatus  10   b  is not connected to the network  400 , the path indicator  104   b  receives from the detector  102   b  the detection result that the sound emission-reception apparatus  10   b  is not connected to the network  400  (connection not detected). The path indicator  104   b  directs the communication device  150   b  to transmit the detection result of the detector  102   b  to the sound emission-reception apparatus  10   a  (step Sa 16 ). In this way, the detection result that the sound emission-reception apparatus  10   b  is not connected to the network is transmitted to the sound emission-reception apparatus  10   a.    
     In the sound emission-reception apparatus  10   a , after the communication device  150   a  receives the detection result of the detector  102   b , the communication device  150   a  transfers the detection result to the path indicator  104   a . The path indicator  104   a , to which the detection result has been transferred, directs the path establisher  202   a  to establish signal paths used in a master apparatus (prt), and directs the communication device  150   a  to transmit a notification (request) that signal paths of a slave apparatus (chd) used in the sound emission-reception apparatus  10   b  are to be established (step Sa 17 ). 
     The path establisher  202   a  establishes the signal paths of the master apparatus (prt) in accordance with the direction (step Sa 18 ). 
     In the sound emission-reception apparatus  10   b , after the communication device  150   b  receives the notification that the paths of the slave apparatus (chd) are to be established, the communication device  150   b  transfers the notification to the path indicator  104   b . The path indicator  104   b , to which the notification has been transferred, directs the path establisher  202   b  to establish the signal paths of the slave apparatus (chd). In accordance with the direction, the path establisher  202   b  establishes the signal paths of the slave apparatus (chd) (step Sa 19 ). 
     The master apparatus (prt) and the slave apparatus (chd) are different from each other with respect to the signal paths established by the corresponding path establisher  202 . Depending on a situation such as a connection to the network  400 , the master apparatus (prt) may be changed to the slave apparatus (chd), and the slave apparatus (chd) may be changed to the master apparatus (prt). In other words, each of the two sound emission-reception apparatuses  10  is assigned the role of either the master apparatus (prt) or the slave apparatus (chd) depending on the situation. When the situation changes, the role of each sound emission-reception apparatus  10  may be changed to the master apparatus (prt) or to the slave apparatus (chd). 
       FIG. 5  is a diagram showing signal paths established in the path establisher  202   a  and in the path establisher  202   b , and their connection status. 
     As shown in  FIG. 5 , the two sound emission-reception apparatuses  10  are connected via the cable C in the following manner. In the two sound emission-reception apparatuses  10 , the cable C connects the first output terminal  231  of the path establisher  202   a  to the first input terminal  221  of the path establisher  202   b , and connects the first output terminal  231  of the path establisher  202   b  to the first input terminal  221  of the path establisher  202   a.    
     In this way, when connecting the two sound emission-reception apparatuses  10  with the cable C, the user need not consider which of the two sound emission-reception apparatuses  10  is to be the master apparatus (prt) or the slave apparatus (chd). 
     Strictly speaking, the cable C connects the output terminals of the DACs  261  to the input terminals of the ADCs  251 . However, the DACs  261  and the ADCs  251  can be disregarded since the DACs  261  and the ADCs  251  are optional elements and do not affect the signal paths as described above. 
     As shown in  FIG. 5 , in the path establisher  202   a  of the master apparatus (prt), the following two signal paths are established. More specifically, in the path establisher  202   a , there are established:
     (A) a signal path through which a sound signal is provided to the distal-end output terminal  213 , the sound signal being derived by an adder  242  adding together a sound signal that is input to the microphone terminal  212  and is delayed by a delay device  241  (first delay device) and a sound signal input to the first input terminal  221 ; and   (B) a signal path through which a sound signal input to the distal-end input terminal  211  is provided to the first output terminal  231  and through which the sound signal that is input to the distal-end input terminal  211  and is delayed by a delay device  243  (second delay device) is provided to the loudspeaker terminal  214 .   

     In the path establisher  202   b  of the slave apparatus (chd), the following two signal paths are established. More specifically, in the path establisher  202   b , there are established:
     (C) a signal path through which a sound signal input to the microphone terminal  212  is provided to the first output terminal  231 ; and   (D) a signal path through which a sound signal input to the first input terminal  221  is provided to the loudspeaker terminal  214 .   

     In the system  1 , in a case where a single sound emission-reception apparatus  10  alone is connected to the network  400 , the sound emission-reception apparatus  10  is set as the master apparatus (prt), and the other sound emission-reception apparatus  10  is set as the slave apparatus (chd). After the signal paths (A) and (B) are established in the path establisher  202   a  of the master apparatus (prt) and the signal paths (C) and (D) are established in the path establisher  202   b  of the slave apparatus (chd), the following operations are executed. A sound signal derived from sound received by the microphone  12  of the master apparatus (prt) and a sound signal derived from sound received by the microphone  12  of the slave apparatus (chd) are added together by the adder  242 , and the resultant signal is output from the distal-end output terminal  213  of the master apparatus (prt) to another system at a distal-end (network  400 ). A sound signal that is provided by the other system and is input to the distal-end input terminal  211  of the master apparatus (prt) is distributed in the master apparatus (prt), and is output from the loudspeaker  18  of the master apparatus (prt) and from the loudspeaker  18  of the slave apparatus (chd) as sound. In this way, the system  1  is able to exchange sound signals with the other system at a distant location. 
     In  FIG. 4  and  FIG. 5 , an exemplary case is shown in which the sound emission-reception apparatus  10   a  is connected to the network  400  and the sound emission-reception apparatus  10   b  is not connected to the network  400 . In a case where the sound emission-reception apparatus  10   b  is connected to the network  400  and the sound emission-reception apparatus  10   a  is not connected to the network  400 , only the setting as the master apparatus (prt) or the slave apparatus (chd) is switched as shown in  FIG. 6 , and the equivalent circuit of signal paths is the same as the equivalent circuit of signal paths shown in  FIG. 5 . Thus, even if the sound emission-reception apparatus  10   b  alone is connected to the network  400 , there is no change in that sound signals derived from sounds received by the two microphones  12  are added together and a resultant signal is output toward the other system, and that sound signals provided by the other system are distributed and output from the two loudspeakers  18 . 
     Next, operations will be described in a case where both of the two sound emission-reception apparatuses  10  are connected to the network  400  as shown in  FIG. 7 . 
       FIG. 8  is a diagram showing an operation sequence of the system  1  in the present case. 
     Since the two sound emission-reception apparatuses  10  are both connected to the network  400 , the “a” and “b” at the tail of the reference signs are used only to distinguish these two sound emission-reception apparatuses  10 . 
     In the sound emission-reception apparatus  10   a , the detector  102   a  detects connection to the network  400 , and the path indicator  104   a  directs the communication device  150   a  to transmit the detection result to the sound emission-reception apparatus  10   b  (step Sb 11 ). The above step is similar to step Sa 11 . Since the sound emission-reception apparatus  10   b  is also connected to the network  400 , the detector  102   b  detects connection to the network  400  and provides the detection result to the path indicator  104   b , and the path indicator  104   b  directs the communication device  150   b  to transmit the detection result to the sound emission-reception apparatus  10   a  (step Sb 12 ). 
     As a result, information that the sound emission-reception apparatus  10   a  is connected to the network  400  is transmitted to the sound emission-reception apparatus  10   b , and information that the sound emission-reception apparatus  10   b  is connected to the network  400  is transmitted to the sound emission-reception apparatus  10   a.    
     In the sound emission-reception apparatus  10   a , after receiving the information, the communication device  150   a  transfers the information to the path indicator  104   a . Based on the transferred information and the detection result of the detector  102   a , the path indicator  104   a  decides that the subject apparatus and the other apparatus are connected to the network  400 . 
     Similarly, in the sound emission-reception apparatus  10   b , after receiving the information, the communication device  150   b  transfers the information to the path indicator  104   b . Based on the transferred information and the detection result of the detector  102   b , the path indicator  104   b  decides that the subject apparatus and the other apparatus are connected to the network  400 . 
     In this way, each of the sound emission-reception apparatuses  10   a  and  10   b  is able to recognize that both the subject apparatus and the other apparatus are connected to the network  400 . 
     Having decided that both apparatuses are connected to the network  400 , the path indicator  104   a  directs the notification device  130   a  to notify the user (step Sb 13 ). Accordingly, the notification device  130   a  notifies the user that the sound emission-reception apparatus  10   a  is a candidate for selection, by causing an LED, for example, to blink. 
     Similarly, having decided that both apparatuses are connected to the network  400 , the path indicator  104   b  directs the notification device  130   b  to notify the user (step Sb 14 ). Accordingly, the notification device  130   a  notifies the user that the sound emission-reception apparatus  10   b  is a candidate for selection, by causing an LED, for example, to blink. 
     Consequently, the user is prompted to select one of the two sound emission-reception apparatuses  10 . 
     The user operates either the input device  140   a  or the input device  140   b  to select one apparatus among the candidates for selection (step Sb 21 ). Here, description will be given assuming that the user operates the input device  140   a.    
     The reason that one apparatus is selected is to determine a network connection of which sound emission-reception apparatus  10  is to be enabled, among multiple (here two) sound emission-reception apparatuses  10  connected to the network  400 . 
     After the user operates the input device  140   a , the input device  140   a  outputs operation information indicative that the input device  140   a  has been operated. Having received the operation information, the path indicator  104   a  directs the notification device  130   a  to terminate notification to the user and directs the communication device  150   a  to transmit a result of the reception to the sound emission-reception apparatus  10   b  (step Sb 15 ). As a result, the notification device  130   a  causes the LED to go out, and information indicating that the sound emission-reception apparatus  10   a  has been selected by the user is transmitted to the sound emission-reception apparatus  10   b.    
     In the sound emission-reception apparatus  10   b , after the communication device  150   b  receives the information, the information is transferred to the path indicator  104   b . The path indicator  104   b , to which the information has been transferred, directs the notification device  130   b  to terminate notification to the user, and directs the communication device  150   b  to transmit to the sound emission-reception apparatus  10   a  an announcement that the network connection at the subject sound emission-reception apparatus  10   b  will be disabled (step Sb 16 ). 
     In this way, the announcement of disablement is transmitted to the sound emission-reception apparatus  10   a . When disablement is announced, in the sound emission-reception apparatus  10   b , a functional block, illustration of which is omitted in  FIG. 3 , such as the PC  300  or a functional block that controls connection to the network  400 , releases the network connection. 
     In the sound emission-reception apparatus  10   a , after the communication device  150   a  receives the announcement of disablement, the announcement of disablement is transferred to the path indicator  104   a . The path indicator  104   a , to which the announcement of disablement has been transferred, directs the path establisher  202   a  to establish signal paths of the master apparatus (prt) and directs the communication device  150   a  to transmit to the sound emission-reception apparatus  10   b  a notification that signal paths of the slave apparatus (chd) are to be established (step Sb 17 ). 
     Afterward, similarly to  FIG. 4 , the path establisher  202   a  establishes the signal paths of the master apparatus (prt) (step Sb 18 ), and the path establisher  202   b  establishes the signal paths of the slave apparatus (chd) (step Sb 19 ). 
       FIG. 9  is a diagram showing signal paths established in the path establishers  202   a  and  202   b  and connection statuses of the signal paths. Description of  FIG. 5  also applies to  FIG. 9 , except that the network  400  connected to the slave apparatus (chd) is disabled as shown by the dashed line. 
     In this case, when two sound emission-reception apparatuses  10  are connected to the network  400  in the system  1 , the selected sound emission-reception apparatus  10   a  is set as the master apparatus (prt), and the other sound emission-reception apparatus  10   b  is set as the slave apparatus (chd). Accordingly, similarly to a case where a single sound emission-reception apparatus  10  is connected to the network  400 , it is possible to exchange sound signals with the other system at a distant location. 
     In this example, the network connection is enabled for a sound emission-reception apparatus  10  for which the input device  140  has been operated. Alternatively, the network connection may be disabled for a sound emission-reception apparatus  10  for which the input device  140  has been operated. In this configuration, a sound emission-reception apparatus  10  may receive from another sound emission-reception apparatus  10  an announcement of disablement of the network connection, and may enable the network connection after the input device  140  detects the absence of an operation to disable the network connection within a predetermined time. 
     Examples of notification to the user are not limited to blinking of the LED. For example, the notification device  130  may be a matrix display capable of displaying characters and may display a message prompting the user to make a selection, or the notification device  130  may be a voice synthesizing device and may synthesize and output a voice message prompting the user to make a selection, or these forms may be used in combination as appropriate. That is, a manner of notification is not limited to a display (sight), and notification may be achieved by any means that can be sensed by any of the five senses, such as sound (hearing) and vibration (touch). 
     Furthermore, the notification device  130  and the input device  140  may be superimposed on each other by use of a matrix display and a touch panel, for example. 
     In this embodiment, the sound emission-reception apparatus  10  transmits sound signals with the cable C in analog format so that the configuration of the sound input-output apparatus  10  is simplified. In other words, considering that the path establisher  202 , which establishes signal paths of the master apparatus (prt) or the slave apparatus (chd), is realized by computational processing by the DSP  200 , transmission of sound signals in digital format will require synchronization between computational processing and signal transmission in the master apparatus (prt) and computational processing and signal transmission in the slave apparatus (chd), thereby resulting in a complex configuration. 
     As a result of each sound emission-reception apparatus  10  transmitting sound signals in analog format as in the present embodiment, each sound emission-reception apparatus  10  can independently execute its computational processing, thus making it possible to omit an element for synchronization. 
     Sound signals require to be D/A-converted for output, and sound signals require to be A/D-converted for input. Accordingly, sound signals will be delayed for a length of time required for the conversion. 
     For example, a sound signal derived from sound received by the microphone  12  of the slave apparatus (chd) passes through the DAC  261  of the slave apparatus (chd) and the ADC  251  of the master apparatus (prt). Thus, compared to a sound signal derived from sound received by the microphone  12  of the master apparatus (prt), there will be a signal delay corresponding to a length of time required for D/A conversion and A/D conversion. Similarly, a sound signal output toward the loudspeaker  18  of the slave apparatus (chd) passes through the DAC  261  of the master apparatus (prt) and the ADC  251  of the slave apparatus (chd). Thus, compared to a sound signal output toward the loudspeaker  18  of the master apparatus (prt), there will be signal delay corresponding to the length of time required for D/A conversion and A/D conversion. 
     In view of the foregoing, in the present embodiment, a delay time of the delay device  241  is set to be equal to the sum of a delay time that results from analog conversion in the DAC  261  and a delay time that results from digital conversion in the ADC  251 . Additionally, a delay time of the delay device  243  is set to be equal to the sum of a delay time that results from analog conversion in the DAC  261  and a delay time that results from digital conversion in the ADC  251 . 
     In this way, a sound signal derived from sound received by the microphone  12  of the master apparatus (prt) and a sound signal derived from sound received by the microphone  12  of the slave apparatus (chd) are delayed for nearly an equal length of time. The sound signal derived from sound received by the microphone  12  of the master apparatus (prt) and the sound signal derived from sound received by the microphone  12  of the slave apparatus (chd) are added together with little difference in timing and the resultant signal is output toward the network  400 . Accordingly, deterioration of the sound signal can be prevented. Similarly, the sound signal output toward the loudspeaker  18  of the master apparatus (prt) and the sound signal output toward the loudspeaker  18  of the slave apparatus (chd) are delayed for nearly an equal length of time. Thus, it is possible to reduce a difference between timings at which sounds are output from the each of the loudspeakers  18 . 
     Since the microphone  12  and the loudspeaker  18  are close to each other in the sound emission-reception apparatus  10 , an echo is likely to be generated. An exemplary application for suppressing such an echo will now be described. 
       FIG. 10  is a diagram showing signal paths established in the path establishers  202   a  and  202   b  of sound emission-reception apparatuses  10  according to an exemplary application of the first embodiment. As shown in the figure, an echo canceller  244  is provided in each of the master apparatus (prt) and the slave apparatus (chd). 
     The echo canceller  244  first generates a simulated echo component by filtering a sound signal output toward the loudspeaker terminal  214  with filter coefficients that accord with an estimated transfer function of acoustic space from the loudspeaker  18  to the microphone  12 . The echo canceller  244  secondly subtracts the generated simulated echo component from a sound signal input to the microphone terminal  212  to output the resultant signal. 
     By use of the echo canceller  244 , even if sound output from the loudspeaker  18  seeps to and is received by the microphone  12 , the seeping component is subtracted. Consequently, effects of the seeping sound are minimized and deterioration of a sound signal derived from the received sound is suppressed. 
     In the first embodiment, in a case where two sound emission-reception apparatuses  10  are connected to the network  400 , which network connection is to be enabled is determined by an operation performed by the user on the input device  140 . Alternatively, a different method may be employed for determination. For example, one apparatus may be determined randomly from among sound emission-reception apparatuses  10  connected to the network  400 . In a method for random determination, the path indicator  104 , for example, may cause a single random number to be generated in each of two sound emission-reception apparatuses  10  and cause the communication device  150  to transmit the generated random number. The path indicator  104  of a sound emission-reception apparatus  10  compares a random number generated in the subject apparatus with a random number that is generated in the other sound emission-reception apparatus  10  and is received by the communication device  150 . In a case where the random number generated in the subject apparatus is larger than the random number generated in the other apparatus (a single apparatus in this example), for example, the subject apparatus is determined to be the master apparatus (prt) and the other apparatus is determined to be the slave apparatus (chd). If the subject apparatus is determined to be the master apparatus (prt), the network connection of the subject apparatus is enabled, and if the other apparatus is determined to be the slave apparatus (chd), the network connection of the other apparatus is disabled. 
     By the sound signal processing device according to the above embodiment, it is possible to change a signal path established by the path establisher depending on circumstance. For example, it is possible to switch from a signal path used for a master apparatus that connects to the network to a signal path used for a slave apparatus subordinate to the master apparatus. Accordingly, there is no need for either a separate dedicated device that functions as a master apparatus or for a separate dedicated device that serves as a slave apparatus. In other words, according to the above embodiment, without need for a device like a base, a sound signal processing device included in a sound emission-reception apparatus can be connected to a network. 
     Second Embodiment 
     Next, a sound emission-reception apparatus according to a second embodiment will be described. In the second embodiment, the number of sound emission-reception apparatuses  10  forming the system  1  is not limited to two, and the system  1  may be expanded to include two or more sound emission-reception apparatuses  10 . 
       FIG. 11  is a diagram showing a configuration of a system that includes sound emission-reception apparatuses according to the second embodiment. In the illustrated example, the number of sound emission-reception apparatuses constituting the system  1  is four. 
     Similarly to the first embodiment, each sound emission-reception apparatus  10  according to the second embodiment includes a notification device  130  and an input device  140 , and includes a built-in microphone  12  and a built-in loudspeaker  18 . 
     In a case where the four sound emission-reception apparatuses  10  are referred to as A, B, C, and D in order to distinguish them, the four sound emission-reception apparatuses  10  are connected in a manner of A→B→C→D→(A) where “→” represents connection by a cable C. In other words, the four sound emission-reception apparatuses  10  are circularly connected by four cables C. 
     Similarly to the first embodiment, in the second embodiment, a single cable C transmits two sound signals. 
     Among the four sound emission-reception apparatuses  10 , a single sound emission-reception apparatus  10  (the “A” in  FIG. 11 ) is connected to the network  400  via the PC  300 , and exchanges sound signals via the network  400  with other systems (illustration omitted) present in other locations. 
     A hardware configuration of a sound emission-reception apparatus  10  according to the second embodiment differs from that in the first embodiment in the surroundings of the DSP  200 . Accordingly, in the second embodiment, description will be given focusing on functional blocks of the surroundings of the DSP  200 . 
       FIG. 12  is a diagram showing functional blocks of a sound emission-reception apparatus  10  according to the second embodiment. 
     A sound emission-reception apparatus  10  according to the second embodiment differs from that of the first embodiment (see  FIG. 3 ) in that there are provided in the DSP  200  (path establisher  202 ) a second input terminal  222  in addition to the first input terminal  221 , and a second output terminal  232  in addition to the first output terminal  231 . 
     In the description, ordinal numbers, such as “first” and “second”, appearing in names of elements are used to distinguish two or more elements, and are not intended to define their order. For example, with respect to the first input terminal  221  and the second input terminal  222 , one of two input terminals is referred to as the first input terminal  221  and the other one is referred to as the second input terminal  222 . 
     An ADC  252  converts a proximal-end sound signal into a digital signal and provides it to the second input terminal  222 , and a DAC  262  converts a sound signal output from the second output terminal  232  into an analog signal and outputs it toward another apparatus at a proximal-end. 
     Next, operations in the second embodiment will be described. 
     In the second embodiment, there are two possible cases: a case where only a single sound emission-reception apparatus  10  among the multiple (here four) sound emission-reception apparatuses  10  is connected to the network  400 ; and a case where two or more sound emission-reception apparatuses  10  are connected to the network  400 . 
     Here, description will be given first of operations in a case where only a single sound emission-reception apparatus  10  is connected to the network  400 . 
       FIG. 13  is a diagram showing an operation sequence of the system  1  in this case. For descriptive purposes, “a” is appended to the tail of the reference sign of a sound emission-reception apparatus connected to the network  400 , and “b”, “c”, and “d” are appended to the tails of the reference signs of sound emission-reception apparatuses not connected to the network  400 . 
     The operation sequence shown in this figure is essentially the same as that shown in  FIG. 4 , with the exception that the number of the sound emission-reception apparatuses  10  not connected to the network  400  is greater in  FIG. 13  than in  FIG. 4 . 
     In the sound emission-reception apparatus  10   a , after the detector  102   a  detects a connection to the network  400 , the detection result is transmitted to the other sound emission-reception apparatuses  10   b ,  10   c , and  10   d  (step Sc 11 ). Each of the sound emission-reception apparatuses  10   b ,  10   c , and  10   d , which have received the detection result, transmits to the sound emission-reception apparatus  10   a  a detection result (network not detected) indicating that the subject apparatus is not connected to the network  400  (step Sc 16 ). Having received the result “network not detected” from each of the other apparatuses, the sound emission-reception apparatuses  10   b ,  10   c , and  10   d , the sound emission-reception apparatus  10   a  determines signal paths in itself and in other apparatuses (step Sc 17 ). The sound emission-reception apparatus  10   a  sets itself as the master apparatus (prt) (step Sc 18 ) and sets the other apparatuses as slave apparatuses (chd) (step Sc 19 ). 
       FIG. 14  is a diagram showing signal paths established in the path establishers  202   a ,  202   b , . . . , and  202   d , and connection statuses of the signal paths in the second embodiment. In  FIG. 14 , illustration of the path establisher  202   c  is omitted for descriptive purposes. 
     As shown in  FIG. 14 , the four sound emission-reception apparatuses  10  are connected by cables C in the following manner. The first output terminal  231  of an apparatus and the first input terminal  221  of another apparatus are connected by a cable C; and the second output terminal  232  of the apparatus and the second input terminal  222  of the other apparatus are connected by a cable C. All the sound emission-reception apparatuses  10  are circularly connected by cables C, as described above. 
     Accordingly, when connecting the four sound emission-reception apparatuses  10  by the cables C, the user need not consider which of the master apparatus (prt) or the slave apparatus (chd) each of the four sound emission-reception apparatuses  10  is going to form. 
     Similarly to the first embodiment, the DAC  261 , the DAC  262 , the ADC  251 , and the ADC  252  are disregarded in describing connections with the cables C. 
     In the path establisher  202   a  of the master apparatus (prt), four signal paths as described below are established. 
     Specifically, in the path establisher  202   a  of the master apparatus (prt), there are established:
     (A) a signal path through which a sound signal that is input to the microphone terminal  212  and is delayed by the delay device  241  is provided to the first output terminal  231 ;   (B) a signal path through which a sound signal input to the distal-end input terminal  211  is provided to the second output terminal  232 ;   (C) a signal path through which a sound signal input to the first input terminal  221  is provided to the distal-end output terminal  213 ; and   (D) a signal path through which a sound signal that is input to the second input terminal  222  and is delayed by the delay device  243  is provided to the loudspeaker terminal  214 .   

     In the path establisher  202   b  of a slave apparatus (chd), two signal paths described below are established. Specifically, in the path establisher  202   b , there are established:
     (E) a signal path through which a signal is provided to the first output terminal  231 , wherein the signal is derived by the adder  242  adding together a sound signal input to the microphone terminal  212  and delayed by the delay device  241 , and a sound signal input to the first input terminal  221 ; and   (F) a signal path through which a sound signal input to the second input terminal  222  is provided to the second output terminal  232  and through which the sound signal is provided, after being delayed by the delay device  243 , to the loudspeaker terminal  214 .   

     Here, description is given of the path establisher  202   b  as representative of those in the slave apparatus (chd). Similar signal paths are established in the path establishers  202   c  and  202   d.    
     In a case where a single sound emission-reception apparatus  10  alone out of the four apparatuses is connected to the network  400 , this sound emission-reception apparatus  10  is set as the master apparatus (prt) and the other three apparatuses are set as the slave apparatuses (chd). The following operations are executed after signal paths (A), (B), (C), and (D) are established in the path establisher  202   a  of the master apparatus (prt) and signal paths (E) and (F) are established in each of the path establishers  202   b ,  202   c , and  202   d  of the slave apparatuses (chd). 
     A sound signal derived from sound received by the microphone  12  of the single master apparatus (prt) and sound signals derived from sounds received by the microphones  12  of the three slave apparatuses (chd) are added together by the adders  242 , and the resultant signal is output from the distal-end output terminal  213  of the master apparatus (prt) toward the network  400 . A sound signal that is provided from another system and is input to the distal-end input terminal  211  of the master apparatus (prt) is distributed to the three slave apparatus (chd) and to the single master apparatus (prt) sequentially, and sound corresponding to the sound signal is output from each of the loudspeakers  18  of the three slave apparatus (chd) and also from the loudspeaker  18  of the single master apparatus (prt). In this way, the system  1  is able to exchange sound signals with another system at a distant location. 
     Next, description will be given of operations in a case where two or more sound emission-reception apparatuses  10  among the multiple apparatuses are connected to the network  400 . 
       FIG. 15  is a diagram showing an operation sequence of the system in this case. 
     Although all of the sound emission-reception apparatuses  10  are connected to the network  400  in  FIG. 15 , it will suffice if two or more apparatuses are connected to the network  400 . The “a”, “b”, “c”, and “d” at the tails of the reference signs are used to distinguish the four apparatuses. 
     As shown in  FIG. 15 , the operation sequence is essentially the same as that shown in  FIG. 8 , with the exception that the number of the sound emission-reception apparatuses  10  connected to the network  400  is greater in  FIG. 15  than in  FIG. 8 . 
     In the sound emission-reception apparatus  10   a , after the detector  102   a  detects a connection to the network  400 , the detection result is transmitted to the other sound emission-reception apparatuses  10   b ,  10   c , and  10   d  (step Sd 11 ). Each of the other sound emission-reception apparatuses  10   b ,  10   c , and  10   d  also detects a connection to the network  400 , and transmits the detection result to the sound emission-reception apparatuses  10  other than itself (step Sd 12 ). 
     In the sound emission-reception apparatus  10   a , in a case where at least one of the other sound emission-reception apparatus  10   b ,  10   c , or  10   d  is connected to the network  400 , the LED of the notification device  130   a  is caused to blink so as to notify the user that the sound emission-reception apparatus  10   a  is a candidate for selection (step Sd 13 ). 
     In each of the sound emission-reception apparatuses  10   b ,  10   c , and  10   d , in a case where a sound emission-reception apparatus  10  other than the subject apparatus is connected to the network  400 , the LED is caused to blink so as to notify the user that the subject apparatus is a candidate for selection (step Sd 14 ). 
     Among sound emission-reception apparatuses  10  in which the LEDs are blinking, the user operates one of the input device  140   a ,  140   b ,  140   c , or  140   d  to select a single apparatus (step Sb 21 ). Here, description will be given assuming that the user operates the input device  140   a.    
     Having received an operation by the user, the sound emission-reception apparatus  10   a  causes the LED to go out and transmits to the other sound emission-reception apparatuses  10   b ,  10   c , and  10   d  information indicating that the sound emission-reception apparatus  10   a  has been selected by the user (step Sd 15 ). Having received a result indicating that the operation has been received, each of the other sound emission-reception apparatuses  10   b ,  10   c , and  10   d  causes the LED to go out and executes an operation to disable the network connection, and transmits to the sound emission-reception apparatus  10   a  announcement of disablement (step Sd 16 ). 
     After the sound emission-reception apparatus  10   a  receives from all of the other sound emission-reception apparatuses  10   b ,  10   c , and  10   d  the announcement of disablement (or a detection result indicating that the network is not detected), the sound emission-reception apparatus  10   a  determines signal paths in itself and in other apparatuses (step Sd 17 ). The sound emission-reception apparatus  10   a  sets itself as the master apparatus (prt) (step Sd 18 ) and sets other apparatuses as the slave apparatuses (chd) (step Sd 19 ). 
     In this way, also in a case where the four sound emission-reception apparatuses  10  are connected to the network  400 , signal paths as shown in  FIG. 14  are established in the path establishers  202   a ,  202   b , . . . , and  202   d , and the system  1  is able to exchange sound signals with other systems. 
     In this example, a case is described where all the four sound emission-reception apparatuses  10  are connected to the network  400 . However, it is sufficient so long as two or more sound emission-reception apparatuses  10  are connected to the network  400 . Here, a sound emission-reception apparatus  10  that is not connected to the network  400  transmits a result indicating that the network is not detected and the LED in the corresponding notification device  130  is not caused to blink. Accordingly, the sound emission-reception apparatus  10  not connected to the network  400  is excluded from candidates for selection by the user and is set as a slave apparatus (chd). 
     Here, an example is described where the sound emission-reception apparatus  10   a  has been selected. However, even when the sound emission-reception apparatus  10   b , for example, is selected as shown in  FIG. 16 , the equivalent circuit of signal paths is the same as the equivalent circuit of signal paths shown in  FIG. 14 . 
     Furthermore, in the second embodiment, in each path establisher  202 , the delay time of the delay device  241  is set to “n·d”, and the delay time of the delay device  243  is set to “(m−n)d”. Here, the denotation “·” represents multiplication. 
     The denotation “m” represents the number of sound emission-reception apparatuses  10  in the system  1 . The denotation “n” represents a number of the subject apparatus when counted from the master apparatus (prt) along the flow of signals in the ring connection, in a case where the subject apparatus is a slave apparatus (chd). In other words, the “n” represents the number of times of passing through a combination of a DAC  261  (or  262 ) and an ADC  251  (or  252 ), starting from the master apparatus (prt). In a case where the subject apparatus is the master apparatus (prt), the denotation “n” has the same value as the denotation “m”. The denotation “d” represents a sum of the delay time resulting from analog conversion at a single DAC  261  (or  262 ) and the delay time resulting from digital conversion at a single ADC  251  (or  252 ). 
     In the example shown in  FIG. 14 , the denotation “m” is “4”. Since the path establisher  202   a  connected to the network  400  is set as the master apparatus (prt), the denotation “n” with respect to the path establisher  202   a  is “4”, which is the same value as “m”. Accordingly, in the path establisher  202   a , the delay time for the delay device  241  is set to “0” and the delay time of the delay device  243  is set to “0”. 
     The denotation “n” for the path establisher  202   b ,  202   c , and  202   d  are respectively “1”, “2”, and “3”. 
     Accordingly, in the path establisher  202   b , the delay time of the delay device  241  is set to “1·d” and the delay time of the delay device  243  is set to “3·d”. Similarly, in the path establisher  202   c , the delay time of the delay device  241  is set to “2·d” and the delay time of the delay device  243  is set to “2·d”. In the path establisher  202   d , the delay time of the delay device  241  is set to “3·d” and the delay time of the delay device  243  is set to “1·d”. 
     In this way, a sound signal generated by the microphone  12  of the master apparatus (prt) and sound signals generated by the microphones  12  of the slave apparatuses (chd) are added together sequentially, with the sound signals generated in the master apparatus (prt) and in the slave apparatuses (chd) being delayed for nearly an equal length of time. Consequently, effects of delays resulting from DA-conversion and AD-conversion in the sound signal output toward the network  400  can be minimized. 
     Similarly, sound signals output toward the loudspeakers  18  of the slave apparatuses (chd) and a sound signal output toward the loudspeaker  18  of the master apparatus (prt) are delayed for nearly an equal length of time. Consequently, effects of delays resulting from DA-conversion and AD-conversion in the sound output from each loudspeaker  18  can be minimized. 
     The denotation “n” for a sound emission-reception apparatus set as a slave apparatus (chd) may be set by the user, or may be decided by the sound emission-reception apparatus  10  in the following manner. For example, each of the communication devices  150  of the slave apparatuses (chd) communicates with the communication device  150  of the master apparatus (prt), and can decide “n” by detecting a difference between a time of transmission of a test signal from the master apparatus (prt) and a time of arrival of the test signal at the subject apparatus (i.e., time required for DA-conversion and AD-conversion). 
     In the second embodiment described above, the system  1  is able to exchange sound signals with other systems at distant locations. According to the second embodiment in particular, even when a meeting room is large or a number of participants is large, many sound emission-reception apparatuses  10  can be installed for distribution over a wide range, as long as the sound emission-reception apparatuses  10  are circularly connected by cables C. 
     Furthermore, in the second embodiment, in a case where multiple sound emission-reception apparatuses  10  are connected to the network  400 , the user can select the network connection of one of the apparatuses that is to be enabled and the network connections of the other apparatuses that are to be disabled. In this way, the usability to the user can be improved. 
     In the second embodiment, a case is described where the number of connected apparatuses is “4”. However, the number of connected apparatuses may be any number equal to or greater than “2”. When the number of connected apparatuses is “2”, however, the connection configuration by the cables C will be almost the same as that in the first embodiment, and there will be no advantage in having a ring connection. Thus, it is preferable that the number of connected apparatuses be “3” or more. 
       FIG. 17  is a diagram showing signal paths in the sound emission-reception apparatus  10  according to an exemplary application of the second embodiment. As shown in the figure, the echo canceller  244  is provided in each of the master apparatus (prt) and the slave apparatuses (chd). The location, operations, and functions of the echo canceller  244  are similar to those described with reference to  FIG. 10 . 
     Third Embodiment 
     Next, a sound emission-reception apparatus according to a third embodiment will be described. 
     In the first embodiment and the second embodiment, in a case where multiple sound emission-reception apparatuses  10  are connected to the network  400 , a network connection of a single apparatus among them is enabled and network connections of the other apparatuses are disabled. The third embodiment allows network connections by multiple apparatuses. Networks  400  to which the apparatuses are allowed to connect may be of the same type or may be of different types. 
       FIG. 18  is a diagram showing functional blocks of a sound emission-reception apparatus  10  according to the third embodiment. 
     The sound emission-reception apparatus  10  of the third embodiment differs from that of the first embodiment in that the notification device  130  and the input device  140  are not provided (first difference) and in signal paths in the path establisher  202  formed in the DSP  200  (second difference). 
     The first difference results from the fact that elements for selecting one apparatus are unnecessary since multiple apparatuses are allowed to connect to networks in the third embodiment, as described above. 
     The second difference results from the fact that, compared to the first embodiment, there are provided in the DSP  200  (path establisher  202 ) a second input terminal  222 , a third input terminal  223 , and a fourth input terminal  224 , in addition to the first input terminal  221 ; and there are also provided a second output terminal  232 , a third output terminal  233 , and a fourth output terminal  234 , in addition to the first output terminal  231 . 
     Each of the ADCs  251  to  254  converts a proximal-end sound signal into a digital signal and provides the digital signal to the corresponding one of the first input terminal  221  to the fourth input terminal  224 . The DACs  261  to  264  convert sound signals output respectively from the first output terminal  231  to the fourth output terminal  234  into analog signals, and outputs the analog signals to another apparatus at a proximal-end. 
     Next, operations in the third embodiment will be described. In the third embodiment, network connections by multiple apparatuses are allowed, and in the following example, a configuration is described in which six sound emission-reception apparatuses  10  form the system  1  and all the six apparatuses are connected to networks  400 . 
     For descriptive purposes, “a”, “b”, “c”, “d”, “e”, and “f” are appended to the tails of reference signs of the elements in the sound emission-reception apparatuses  10  to distinguish the six sound emission-reception apparatuses  10 . 
     In the third embodiment, one among the six sound emission-reception apparatuses is determined to be the master apparatus (prt), and the remaining five apparatuses are determined to be the slave apparatuses (chd). For descriptive purposes, it is assumed here that the sound emission-reception apparatus  10   a  is determined to be the master apparatus (prt), and the other sound emission-reception apparatuses  10   b ,  10   c ,  10   d ,  10   e , and  10   f  are determined to be the slave apparatuses (chd). 
     An exemplary method to determine the master apparatus (prt) and the slave apparatuses (chd) is described later. 
       FIG. 19  is a diagram showing signal paths established in the path establishers  202   a ,  202   b , . . . , and  202   f  and connection statuses of the signal paths in the third embodiment. For descriptive purposes, illustration of the path establishers  202   c ,  202   d , and  202   e  is omitted in  FIG. 19 . 
     As shown in  FIG. 19 , the six sound emission-reception apparatuses  10  are connected by cables C in the following manner. Cables C connect from the first output terminal  231  of an apparatus to the first input terminal  221  of another apparatus; from the second output terminal  232  of the apparatus to the second input terminal  222  of the other apparatus; from the third output terminal  233  of the apparatus to the third input terminal  223  of the other apparatus; and from the fourth output terminal  234  of the apparatus to the fourth input terminal  224  of the other apparatus. 
     In the third embodiment, a single cable C transmits four sound signals. Alternatively, four cables may be used, each of which transmits a single sound signal. 
     Similarly to the second embodiment, all the sound emission-reception apparatuses  10  are circularly connected by the cables C in the third embodiment. 
     Accordingly, when connecting the six sound emission-reception apparatuses  10  with the cables C, the user need not consider which of the master apparatus (prt) or the slave apparatus (chd) each sound emission-reception apparatus  10  is going to form. 
     Similarly to the first embodiment and the second embodiment, the DACs  261  to  264  and the ADCs  251  to  254  are disregarded in describing connections with the cables C. 
     As shown in  FIG. 19 , four signal paths described below are established in the path establisher  202   a  of the master apparatus (prt). 
     Specifically, in the path establisher  202   a , there are established:
     (A) a signal path through which a sound signal input to the microphone terminal  212  and delayed by the delay device  241  is provided to the first output terminal  231 ;   (B) a signal path through which a sound signal input to the distal-end input terminal  211  is provided to the second output terminal  232 ;   (C) a signal path through which a sound signal is provided to the third output terminal  233  and to the distal-end output terminal  213 , wherein the sound signal is derived by an adder  247  adding together a sound signal input to the first input terminal  221  and a sound signal provided to the second input terminal  222 ; and   (D) a signal path through which the sound signal input to the second input terminal  222  is provided to the fourth output terminal  234  and through which the sound signal is provided, after being delayed by the delay device  243 , to the loudspeaker terminal  214 .   

     In the path establisher  202   b  of a slave apparatus (chd), four signal paths described below are established. Specifically, in the path establisher  202   b , there are established:
     (E) a signal path through which a sound signal is provided to the first output terminal  231 , wherein the sound signal is derived by an adder  245  adding together a sound signal input to the microphone terminal  212  and delayed by the delay device  241  and a sound signal input to the first input terminal  221 ;   (F) a signal path through which a sound signal is provided to the second output terminal  232 , wherein the sound signal is derived by an adder  246  adding together a sound signal input to the distal-end input terminal  211  and a sound signal input to the second input terminal  222 ; and   (G) a signal path through which a sound signal input to the third input terminal  223  is provided to the third output terminal  233  and to the distal-end output terminal  213 ;   (H) a signal path through which a sound signal input to the fourth input terminal  224  is provided to the fourth output terminal  234  and through which the sound signal is provided, after being delayed by the delay device  243 , to the loudspeaker terminal  214 .   

     Here, description is given of the path establisher  202   b  as representative of those in the slave apparatuses (chd). Similar signal paths are established in each of the path establishers  202   c ,  202   d ,  202   e , and  202   f.    
     In a case where a single sound emission-reception apparatus  10  alone, among the six apparatuses, is connected to the network  400 , this sound emission-reception apparatus  10  is set as the master apparatus (prt) and the other five apparatuses are set as the slave apparatuses (chd). The following operations are executed after signal paths (A), (B), (C), and (D) are established in the path establisher  202   a  of the master apparatus (prt) and signal paths (E), (F), (G), and (H) are established in each of the path establishers  202   b ,  202   c ,  202   d ,  202   e , and  202   f  of the slave apparatuses (chd). 
     A sound signal derived from sound received by the microphone  12  of the single master apparatus (prt) and sound signals derived from sounds received by the microphones  12  of the five slave apparatuses (chd) are sequentially added together by the adders  245 , and the resultant signal is provided to the first input terminal  221  of the master apparatus (prt). A sound signal that is provided from the network  400  and is input to the distal-end input terminal  211  of the master apparatus (prt) and sound signals provided from networks  400  and are input to the distal-end input terminals  211  of the five slave apparatuses (chd) are sequentially added together by the adders  246 , and the resultant signal is provided to the second input terminal  222  of the master apparatus (prt). The sound signal provided to the first input terminal  221  of the master apparatus (prt) and the sound signal provided to the second input terminal  222  of the master apparatus (prt) are added together by the adder  247 , and then the resultant signal is output sequentially from the distal-end output terminal  213  of the master apparatus (prt) and from the distal-end output terminal  213  of each slave apparatus (chd) toward the corresponding networks  400 . 
     The sound signal provided to the second input terminal  222  of the master apparatus (prt) is distributed sequentially to the master apparatus (prt) and to the five slave apparatuses (chd), and sound based on the sound signal is output from each of the loudspeakers  18 . In this way, sound based on the signal derived by adding together the sound signals provided from the networks  400  is output from each loudspeaker  18 . 
     In the third embodiment as described above, it is possible to exchange sound signals with other systems at distant locations. According to the third embodiment, similarly to the second embodiment, even when a meeting room is large or a number of participants is large, many sound emission-reception apparatuses  10  can be installed for distribution over a wide range, as long as the sound emission-reception apparatuses  10  are circularly connected by the cables C. 
     In the third embodiment in particular, even when multiple sound emission-reception apparatuses  10  are connected to networks  400 , unlike in the case in the first embodiment and in the second embodiment, there is no need to enable network connection of a single apparatus alone and to disable network connections of other apparatuses. Thus, it is possible to hold a conference via other systems over various networks. 
     In the third embodiment, the number of connected apparatuses is “6”; however, the number may be any number equal to or greater than “2”. When the number of connected apparatuses is “2”, however, the connection configuration by the cables C will be almost the same as that in the first embodiment, and no advantage will be obtained by deploying a ring connection. Thus, it is preferable that the number of connected apparatuses be “3” or more. 
     In the third embodiment, in each path establisher, the delay time of the delay device  241  is set to “n·d” and the delay time of the delay device  243  is set to “(m−n)d”. 
     Here, the denotations “m” and “d” are as described in the second embodiment. The denotation “n” for a slave apparatus (chd) represents a number of the subject apparatus when counted from the master apparatus (prt) along the flow of signals in the ring connection, as in the second embodiment. However, the denotation “n” for the master apparatus (prt) is “0”, which is not the same value as that of the denotation “m”. 
     In the example shown in  FIG. 19 , the denotation “m” is “6”. Since the path establisher  202   a  (sound emission-reception apparatus  10   a ) is set as the master apparatus (prt), the “n” for the path establisher  202   a  is “0”. Accordingly, in the path establisher  202   a , the delay time of the delay device  241  is set to “0” and the delay time of the delay device  243  is set to “6·d”. 
     The denotation “n” for the path establishers  202   b  to  202   f  (sound emission-reception apparatuses  10   b  to  10   f ) are respectively “1” to “6”. Accordingly, in the path establisher  202   b , the delay time of the delay device  241  is set to “1·d” and the delay time of the delay device  243  is set to “5·d”. In the path establisher  202   c , for example, the delay time of the delay device  241  is set to “2·d” and the delay time of the delay device  243  is set to “4·d”.  FIG. 20  is a diagram where coefficients of “d” are expressed as “(p, q)” in each of the sound emission-reception apparatuses  10   a  to  10   f , in a case where the delay time set to the delay device  241  is “p·d” and the delay time set to the delay device  243  is “q·d”. For example, since it is “( 4 ,  2 )” in the sound emission-reception apparatus  10   e , the delay time set to the delay device  241  is “4·d” and the delay time set to the delay device  243  is “2·d”. 
     Since sound signals generated by microphones  12  are added together sequentially after their delays are equalized, it is possible to minimize effects of delay resulting from DA-conversion and AD-conversion in the sound signal output toward the networks  400 . Similarly, since delays of sound signals output toward loudspeakers  18  are equalized, effects of delay resulting from DA-conversion and AD-conversion can be minimized. 
     In the example shown in  FIG. 19 , in the master apparatus (prt), a sound signal input to the first input terminal  221  is provided to the third output terminal  233  without being added together with a sound signal input to the microphone terminal  212 , and a sound signal input to the second input terminal  222  is provided to the fourth output terminal  234  without being added together with a sound signal input to the distal-end input terminal  211 . As shown in  FIG. 21 , however, the sound signal input to the first input terminal  221  may be provided to the third output terminal  233  after being added together with the sound signal input to the microphone terminal  212 ; and the sound signal input to the second input terminal  222  may be provided to the fourth output terminal  234  after being added together with the sound signal input to the distal-end input terminal  211 . 
       FIG. 21  shows an exemplary case where the path establisher  202   f  (sound emission-reception apparatus  10   f ) is set as the master apparatus (prt). 
     In the path establisher  202   f  of the master apparatus (prt), four signal paths described below are established. 
     Specifically, in the path establisher  202   f , there are established:
     (A) a signal path through which a sound signal is provided to the third output terminal  233 , wherein the sound signal is derived by the adders  245 ,  246 , and  247  adding together a sound signal input to the microphone terminal  212  and delayed by the delay device  241 , a sound signal input to the first input terminal  221 , a sound signal input to the second input terminal  222 , and a sound signal input to the distal-end input terminal  211 ;   (B) a signal path through which a sound signal is provided to the fourth output terminal  234 , wherein the sound signal is derived by the adder  246  adding together the sound signal input to the second input terminal  222  and the sound signal input to the distal-end input terminal  211 ;   (C) a signal path through which a sound signal input to the third input terminal  223  is provided to the distal-end output terminal  213 ; and   (D) a signal path through which a sound signal input to the fourth input terminal  224  and delayed by the delay device  243  is provided to the loudspeaker terminal  214 .   

     The signal paths in a slave apparatus (chd) are the same as those shown in  FIG. 19 . 
     In the signal paths shown in  FIG. 21 , sound signals derived from sounds received by the microphones  12  are sequentially added together by the adders  245 . Sound signals provided from networks  400  are sequentially added together by the adders  246 . The sound signals derived from sounds received by the microphones  12  and the sound signals provided from the networks  400  are added together by the adder  247 , and the resultant signal is output from the third output terminal  233  of the master apparatus (prt). A sound signal derived by the adders  246  sequentially adding together the sound signals provided from the networks  400  is output from the fourth output terminal  234  of the master apparatus (prt). 
     In this way, the sound signal output from the third output terminal  233  of the master apparatus (prt) is output from the distal-end output terminal  213  of each of the five slave apparatuses (chd) and from the distal-end output terminal  213  of the master apparatus (prt) toward the corresponding networks  400 . 
     The sound signal output from the fourth output terminal  234  of the master apparatus (prt) is distributed sequentially to the five slave apparatus (chd) and to the master apparatus (prt), and sound based on the sound signal is output from each loudspeaker  18 . In this way, sound based on the signal derived by adding together the sound signals provided from the networks  400  is output from each of the loudspeakers  18 . 
     Accordingly, by establishing the signal paths described above in the master apparatus (prt), it is possible to exchange sound signals with other systems at distant locations. 
     In the signal paths shown in  FIG. 21 , the delay time of the delay device  241  and the delay time of the delay device  243  may be set in accordance in the manner described with reference to  FIG. 19  and  FIG. 20 , assuming that the position of the master apparatus (prt) is shifted by one apparatus in a direction in opposing relation to the direction from the first output terminal  231  to the first input terminal  221 . 
     More specifically, as shown in  FIG. 21 , while the path establisher  202   f  (sound emission-reception apparatus  10   f ) is set as the master apparatus (prt), with respect to the setting of the delay times, the denotation “n” is defined by assuming that the position of the master apparatus (prt) is not at the position of the sound emission-reception apparatus  10   f , but at the position of the sound emission-reception apparatus  10   a , the position shifted by one apparatus from the actual position. 
       FIG. 22  is a diagram in which, with respect to the signal paths shown in  FIG. 21 , the delay time of the delay device  241  and the delay time of the delay device  243  are set similarly to those in  FIG. 20 . Since the master apparatus (prt) is treated as being located at the position of the sound emission-reception apparatus  10   a , which is the position shifted by one apparatus from the sound emission-reception apparatus  10   f , the delay time set for the delay device  241  and the delay time set for the delay device  243  in each of the path establisher  202   a  (sound emission-reception apparatus  10   a ) to the path establisher  202   f  (sound emission-reception apparatus  10   f ) are the same as those shown in  FIG. 20 . 
     In the third embodiment, an exemplary method to determine the master apparatus (prt) and the slave apparatuses (chd) would be to randomly determine one apparatus from among multiple sound emission-reception apparatuses  10  connected to the networks to be the master apparatus (prt), and determine the remaining apparatuses to be the slave apparatuses (chd). 
     In a specific example of such random determination, similarly to the second embodiment, the path indicator  104  in each of the sound emission-reception apparatuses  10  connected to the networks  400  causes a random number to be generated, and the communication device  150  is caused to transmit the generated random number to each of the other sound emission-reception apparatuses  10  connected to the networks  400 . In a case where the random number generated in a sound emission-reception apparatus  10  is, for example, greater than the random numbers that are generated by the other sound emission-reception apparatuses  10  and received by the communication device  150  of the subject sound emission-reception apparatus  10 , the path indicator  104  of the subject sound emission-reception apparatus  10  may determine itself to be the master apparatus (prt) and determine the other apparatuses to be the slave apparatuses (chd). 
     In the third embodiment, in a case where the notification device  130  and the input device  140  are provided, a sound emission-reception apparatus  10  selected by the user may be determined to be the master apparatus (prt). 
       FIG. 23  is a diagram showing signal paths in a sound emission-reception apparatus  10  according to an exemplary application of the third embodiment.  FIG. 23  shows, as an example, signal paths that are set in a slave apparatus (chd). 
     In the third embodiment, sound signals input by multiple networks  400  and sound signals derived from sounds received by multiple microphones  12  are added together, and the resultant sound signal is output toward each of the multiple networks  400 . 
     As a result, components of a sound signal from a network  400  connected to a sound emission-reception apparatus  10  are added to components of sound signals from other networks  400  and are output to the same network  400 , thereby causing signal deterioration such as an echo. 
     In view of the above situation, there are provided a delay device  248  for delaying a sound signal that is from a network  400  and is input to the distal-end input terminal  211 ; and a subtractor  249  for subtracting the sound signal delayed by the delay device  248  from a sound signal to be output toward the network  400  from the distal-end output terminal  213 . 
     Components of a sound signal from a network  400  connected to a single sound emission-reception apparatus  10  circuit the ring connection and are output toward the network  400 . The delay time when the components circuit the ring connection is “m·d”. Thus, by setting the delay times of the delay devices  248  to “m·d”, components of a sound signal input from a network  400  can be removed from a sound signal output toward the network  400 . In this way, deterioration of sound signals output toward the network  400  can be suppressed. 
     In the example shown in  FIG. 23 , there is provided an echo canceller  244 . Similarly to the examples shown in  FIG. 10  and  FIG. 17 , the echo canceller  244  generates a simulated echo component by filtering a sound signal output toward the loudspeaker terminal  214  with filter coefficients that accord with an estimated transfer function of acoustic space from the loudspeaker  18  to the microphone  12 . The echo canceller  244  then subtracts the generated simulated echo component from a sound signal input to the microphone terminal  212  to output the resultant signal. 
     A slave apparatus (chd) is described as an example in this figure. Since the positions of the delay device  248 , the subtractor  249 , and the echo canceller  244  may be the same in the master apparatus (prt) as those in the slave apparatus (chd), description thereof is omitted. 
     Fourth Embodiment 
     Next, a sound emission-reception apparatus according to a fourth embodiment will be described. 
     In the second embodiment and the third embodiment, since the form of connecting multiple sound emission-reception apparatuses  10  with cables C is limited to a ring connection, factors such as a shape of a meeting room, a number of participants, or positioning of the participants are likely to impose constraints on an arrangement of sound emission-reception apparatuses  10 . 
     Description is given below of the fourth embodiment in which such constraints are less likely to be imposed. 
       FIG. 24  is a diagram showing an exemplary connection (example 1) between sound emission-reception apparatuses  10  according to the fourth embodiment. In the example shown in the figure, six sound emission-reception apparatuses  10  are connected in a tree-shaped structure. In order to distinguish the six sound emission-reception apparatuses  10 , “a”, “b”, “c”, “d”, “e”, and “f” are added to the tails of reference signs; and “a”, “b”, “c”, “d”, “e”, and “f” are omitted when the sound emission-reception apparatuses  10  need not be distinguished. 
     As shown in the figure, the sound emission-reception apparatus  10   a  marked with a star is located at the top, and the sound emission-reception apparatuses  10   b  and  10   c  are located one level below. The sound emission-reception apparatuses  10   d  and  10   e  are located one level below the sound emission-reception apparatus  10   b , and the sound emission-reception apparatus  10   f  is located one level below the sound emission-reception apparatus  10   c . In other words, three sound emission-reception apparatuses  10   d ,  10   e , and  10   f  are located at the lowest level; the sound emission-reception apparatus  10   b  is located one level above the sound emission-reception apparatuses  10   d  and  10   e ; the sound emission-reception apparatus  10   c  is located one level above the sound emission-reception apparatus  10   f ; and the sound emission-reception apparatus  10   a  is located one level above the sound emission-reception apparatuses  10   b  and  10   c.    
     A sound emission-reception apparatus  10  located at a higher level and a sound emission-reception apparatus  10  located at a lower level are connected by a cable C. For example, the sound emission-reception apparatus  10   b  is connected to the sound emission-reception apparatus  10   a  at a higher level with a cable C, and is connected to each of the sound emission-reception apparatuses  10   d  and  10   e  at a lower level with cables C. 
     In the fourth embodiment, similarly to the third embodiment, a single cable C transmits four signals. 
     In the fourth embodiment, signal paths established in the path establisher  202  of the single sound emission-reception apparatus  10   a  at the top alone are different from those established in the other sound emission-reception apparatuses  10   b ,  10   c ,  10   d ,  10   e , and  10   f . In order to distinguish them, the sound emission-reception apparatus located at the highest point is referred to as “top node”, and sound emission-reception apparatuses located at other points are referred to as “common nodes”. 
     The terms “high” and “low” are used in the context of viewing the location relative to the top node in a tree-connection structure, and are not indicative of the direction of a sound signal flow. 
     In the fourth embodiment, the number of points at which a cable C is connected to a sound emission-reception apparatus  10  is “3”, for example. In the top node, three points are used for connection to sound emission-reception apparatuses  10  located lower than the top node. In a common node, two out of three points are used for connection to sound emission-reception apparatuses  10  located lower than the subject common node, and the remaining one point is used for connection to a sound emission-reception apparatus  10  located higher than the subject common node. Not all of the three points are always used for connection. In the sound emission-reception apparatus  10   a  shown in  FIG. 24 , only two points are used for connection; and in each of the sound emission-reception apparatuses  10   d ,  10   e , and  10   f , only one point is used for connection. 
     The roles as the top node and the common nodes are not fixed and can be changed flexibly. For example, the sound emission-reception apparatus  10   a  is set as the top node in the connection shown in  FIG. 24 . Alternatively, as in the exemplary connection (example 2) shown in  FIG. 25 , the sound emission-reception apparatus  10   c  may be set as the top node, without changing the connections by the cables C. 
       FIG. 28  is a diagram showing functional blocks of a sound emission-reception apparatus  10  according to the fourth embodiment. 
     The sound emission-reception apparatus  10  of the fourth embodiment differs from that of the first embodiment in that the sound emission-reception apparatus  10  does not include the notification device  130  or the input device  140  (first difference) and in the signal paths in the path establisher  202  established by the DSP  200  (second difference). 
     The first difference results from the fact that the fourth embodiment, similarly to the third embodiment, allows network connections to be made by multiple apparatuses and thus there is no need for elements in selecting one apparatus. 
     The second difference results from the fact that, compared to the first embodiment, there are provided in the path establisher  202  formed by the DSP  200 : a second input terminal  222 , a third input terminal  223 , a fourth input terminal  224 , a fifth input terminal  225 , and a sixth input terminal  226 , in addition to a first input terminal  221 ; and a second output terminal  232 , a third output terminal  233 , a fourth output terminal  234 , a fifth output terminal  235 , and a sixth output terminal  236 , in addition a first output terminal  231 . 
     The ADCs  251  to  256  convert proximal-end sound signals into digital signals and provide the digital signals to the first input terminal  221  to the sixth input terminal  226 , respectively. The DACs  261  to  266  convert sound signals output from the first output terminal  231  to the sixth output terminal  236 , respectively, into analog signals, and output the analog signals toward another apparatus at a proximal-end. 
     For convenience of illustration, the reference sign  222  of the second input terminal to the reference sign  225  of the fifth input terminal, the reference sign  232  of the second output terminal to the reference sign  235  of the fifth output terminal, the reference signs  251  to  256  of the ADCs, and the reference signs  261  to  266  of the DACs, are omitted in  FIG. 28 . 
     Operations in the fourth embodiment will now be described. 
     In the fourth embodiment, one among multiple (here six) sound emission-reception apparatuses is determined to be the top node, and the other five apparatuses are determined to be common nodes. As shown in  FIG. 24 , it is assumed here that the sound emission-reception apparatus  10   a  is determined to be the top node, and the other sound emission-reception apparatuses  10   b ,  10   c ,  10   d ,  10   e , and  10   f  are each determined to be a common node. 
     An exemplary method to determine the top node and the common nodes is described later. 
       FIG. 29  is a diagram showing signal paths established in the path establisher  202  of a sound emission-reception apparatus  10  determined to be the top node.  FIG. 30  is a diagram showing signal paths established in the path establisher  202  of a sound emission-reception apparatus  10  determined to be a common node. 
     In the top node, the number of connection points of cables C to sound emission-reception apparatuses  10  located below is “3”, and the connection points are expressed as “Dn 1 ”, “Dn 2 ”, and “Dn 3 ” in order to distinguish them (refer to  FIG. 29 ). 
     In the top node, the connection point Dn 1  includes the first input terminal  221 , the second input terminal  222 , the first output terminal  231 , and the second output terminal  232 . Similarly, the connection point Dn 2  includes the third input terminal  223 , the fourth input terminal  224 , the third output terminal  233 , and the fourth output terminal  234 ; and the connection point Dn 3  includes the fifth input terminal  225 , the sixth input terminal  226 , the fifth output terminal  235 , and the sixth output terminal  236 . 
     In a common node, since the number of connection points of cables C to sound emission-reception apparatuses  10  located below is “2”, these connection points are expressed as “Dn 1 ” and “Dn 2 ” in order to distinguish them, and a connection point of a cable C to a sound emission-reception apparatus  10  located above is expressed as “Up” (refer to  FIG. 30 ). The connection point Up of a common node replaces the connection point Dn 3  of the top node. Accordingly, the connection point Up of a common node includes the fifth input terminal  225 , the sixth input terminal  226 , the fifth output terminal  235 , and the sixth output terminal  236 . 
     Expressions “Dn 1 ”, “Dn 2 ”, and “Dn 3 ” in the top node and expressions “Dn 1 ”, “Dn 2 ”, and “Up” in a common node are used to distinguish three connection points in a single sound emission-reception apparatus  10  for convenience, and do not intend to indicate particular connection points in a fixed manner. 
     For example, while a certain connection point is serving as “Dn 1 ” in a sound emission-reception apparatus  10  set as a common node, a connection point at the same position as the certain connection point may serve as “Up” in another sound emission-reception apparatus  10  determined to be a common node. 
     In a case where there is a change in the tree-connection structure as described later, a sound emission-reception apparatus  10  may change from a common node to a top node, or change from a top node to a common node. 
     Accordingly, in a case where, for example, a sound emission-reception apparatus  10  is determined to be a common node and a particular connection point therein serves as “Up” for connection to an apparatus located above, the sound emission-reception apparatus  10  may be changed to the top node and the particular connection point may be changed to “Dn 3 ”. Conversely, in a case where a sound emission-reception apparatus  10  is determined to be the top node and a particular connection point therein serves as “Dn 3 ”, the sound emission-reception apparatus  10  may be changed to a common node and the particular connection point may be changed to “Up”. 
     Strictly speaking, connections by the cables C are not from the first output terminal  231  to the sixth output terminal  236  to the first input terminal  221  to the sixth input terminal  226 , but from the DACs  261  to  266  to the ADCs  251  to  256 . As described above, however, since the DACs  261  to  266  and the ADCs  251  to  256  are optional elements and do not affect the signal paths, they are disregarded. 
     As shown in  FIG. 29 , two signal paths described below are established in the path establisher  202  of the top node. 
     Specifically, in the path establisher  202  of the top node, there are established:
     (A) a signal path through which a sound signal is provided to each of the distal-end output terminal  213 , the first output terminal  231 , the third output terminal  233 , and the fifth output terminal  235 , wherein the sound signal is derived by an adder  291  adding together a sound signal output from an adder  283  and a sound signal derived by the adders  271 ,  272 , and  273  adding together a sound signal input to the microphone terminal  212  and delayed by the delay device  241 , a sound signal input to the first input terminal  221 , a sound signal input to the third input terminal  223 , and a sound signal input to the fifth input terminal  225 ; and   (B) a signal path through which a sound signal is provided to each of the input terminal of the adder  291 , the second output terminal  232 , the fourth output terminal  234 , and the sixth output terminal  236 , and to the loudspeaker terminal  214  through the delay device  243 , wherein the sound signal is derived by the adders  281 ,  282 , and  283  adding together a sound signal input to the distal-end input terminal  211 , a sound signal input to the second input terminal  222 , a sound signal input to fourth input terminal  224 , and a sound signal input to the sixth input terminal  226 .   

     The order of addition at the adders  271 ,  272 ,  273 , and  291  is not limited to the above example. Alternatively, a single adder may collectively add together the signals added by these adders. Similarly, the order of addition at the adders  281 ,  282 , and  283  is not limited to the above example, and the signals added by these adders may be added together collectively by a single adder. 
     As shown in  FIG. 30 , four signal paths described below are established in the path establisher  202  of a common node. 
     Specifically, in the path establisher  202  of a common node, there are established:
     (C) a signal path through which a sound signal is provided to the fifth output terminal  235 , wherein the sound signal is derived by the adders  275  and  276  adding together a sound signal input to the microphone terminal  212  and delayed by the delay device  241 , a sound signal input to the first input terminal  221 , and a sound signal input to the third input terminal  223 ;   (D) a signal path through which a sound signal is provided to the sixth output terminal  236 , wherein the sound signal is derived by the adders  285  and  286  adding together a sound signal input to the distal-end input terminal  211 , a sound signal input to the second input terminal  222 , and a sound signal input to the fourth input terminal  224 ;   (E) a signal path through which a sound signal input to the fifth input terminal  225  is provided to each of the first output terminal  231 , the third output terminal  233 , and the distal-end output terminal  213 ; and   (F) a signal path through which a sound signal input to the sixth input terminal  226  is provided to the second output terminal  232  and to the fourth output terminal  234 , and through which the sound signal is provided to the loudspeaker terminal  214  after being delayed by the delay device  243 .   

     The order of addition by the adders  275  and  276  is not limited to the above example, and the signals added by these adders may be collectively added together. Similarly, the order of addition by the adders  285  and  286  is not limited to the above example, and the signals added by these adders may be collectively added together. 
     Although the connection point Up shown in  FIG. 30  may appear to be changed from the connection point Dn 3  shown in  FIG. 29 , any of the connection points Dn 1 , Dn 2 , and Dn 3  may be changed to the connection point Up. Even when a connection point among the connection points Dn 1 , Dn 2 , and Dn 3  that is to be changed to “Up” is not determined in advance, the connection point can be changed to the connection point Up in the following manner. For example, the path indicator  104  may acquire information indicating which sound emission-reception apparatus  10  is connected to which connection point by communicating each other with other apparatuses after being connected with the cables C, may determine the connection point that leads to the top node from the acquired information, and may change the connection point to the “Up”. 
     After signal paths are established in the top node and the common nodes in this way, a sound emission-reception apparatus  10  (common node) at the lowest level provides a sound signal derived from sound received by the microphone  12  to a sound emission-reception apparatus  10  located above, and similarly, provides a sound signal from the network  400  input to the distal-end input terminal  211  to the sound emission-reception apparatus  10  located above. 
     A sound emission-reception apparatus  10  (common node) at a middle level, located at neither the lowest nor the highest level, adds together a sound signal derived from sound received by its own microphone  12  and sound signals that are from microphones  12  and are provided by apparatuses located below; provides the resultant sound signal to a sound emission-reception apparatus  10  located above; adds together a sound signal from the network  400  input to the distal-end input terminal  211  and sound signals from other networks  400  provided by sound emission-reception apparatuses  10  located below; and provides the resultant sound signal to the sound emission-reception apparatus  10  located above. 
     A sound emission-reception apparatus  10  (top node) at the highest level adds together a sound signal derived from sound received by its own microphone  12 , sound signals that are from microphones  12  and are provided by sound emission-reception apparatuses  10  located below, a sound signal from the network  400  input to the distal-end input terminal  211 , and sound signals from networks  400  provided by the sound emission-reception apparatuses  10  located below; provides the resultant sound signal (combined signal of microphone signals and network signals) to the sound emission-reception apparatuses  10  located below and to the distal-end output terminal  213 ; adds together the sound signal from the network  400  input to the distal-end input terminal  211  and the sound signals from the networks  400  provided from the apparatuses located below; and provides the resultant sound signal (combined signal of network signals alone) to the sound emission-reception apparatuses  10  located below and to the loudspeaker terminal  214 . 
     The sound emission-reception apparatus  10  at a middle level provides the combined signal of the microphone and network signals provided by a sound emission-reception apparatus  10  located above to sound emission-reception apparatuses  10  located below and to the distal-end output terminal  213 , and provides the combined signal of the network-alone signals provided by the sound emission-reception apparatus  10  located above to the sound emission-reception apparatuses  10  located below and to its own loudspeaker terminal  214 . 
     A sound emission-reception apparatus  10  at the lowest level provides the distal-end output terminal  213  with the combined signal of the microphone and network signals provided by a sound emission-reception apparatus  10  located above, and provides the loudspeaker terminal  214  with the combined signal of the network-alone signals. 
     As described above, sound signals derived from sounds received by microphones  12  are sequentially added together from the lowest level to a higher level, and sound signals input from the networks  400  are similarly added together sequentially from the lowest level to a higher level. 
     The top node adds together a sound signal derived from sound received by its own microphone  12 , a sound signal that is derived by sequential addition of sound signals from microphones  12  at lower levels, and a sound signal that is derived by sequential addition of sound signals from networks  400  at lower levels; and provides the resultant signal as the combined signal of the microphone and network signals to the lower apparatuses and outputs the resultant signal to the network  400  connected to itself. The top node adds together a sound signal from the network  400  connected to itself and a sound signal that is derived by sequential addition of sound signals from networks  400  at lower levels; and provides the resultant signal as the combined signal of the network-alone signals to the lower apparatuses and causes its own loudspeaker  18  to output sound based on the resultant signal. 
     In the lower levels, the combined signal of the microphone and network signals is sequentially distributed to the lower apparatuses and is output toward each network  400 ; and the combined signal of the network-alone signals is sequentially distributed to the lower apparatuses and is output toward each loudspeaker  18 . 
     In the fourth embodiment, the top node and the common nodes are determined in accordance with a predetermined rule, or determined randomly. 
     An exemplary method for determination in accordance with a predetermined rule may include: detecting a maximum number of nodes in a path among paths, each path originating from a terminal sound emission-reception apparatus  10  at the lowest level in a tree-connection structure, which apparatus has no apparatus connected to it lower than itself, passing through the top node, and reaching another terminal sound emission-reception apparatus  10 ; and determining the sound emission-reception apparatus  10  that is located in the middle of the path to be the top node and determining the other apparatuses to be the common nodes. 
     For example, in the tree-connection structure shown in  FIG. 24 , the path in which the number of nodes is the highest originates from the sound emission-reception apparatus  10   d  (or  10   e ), passing through the sound emission-reception apparatuses  10   b ,  10   a , and  10   c , and reaching the sound emission-reception apparatus  10   f . Thus, the maximum number of nodes is “3”. Accordingly, the sound emission-reception apparatus  10   a , which is located in the middle of the path, is determined to be the top node. 
     Examples of a method to determine the top node and the common nodes randomly include those described in the second embodiment and the third embodiment. 
     The top node and the common nodes are preferably determined at a timing when a change is made to the tree-connection structure. Examples of “when a change is made to the tree-connection structure” include a case where one or more sound emission-reception apparatuses  10  are connected to the tree-connection structure, and also a case where one or more sound emission-reception apparatuses  10  are cut from the tree-connection structure. 
     In the fourth embodiment, a tree-connection structure of an exemplary connection (example 1) shown in  FIG. 24  may be united with a tree-connection structure of an exemplary connection (example 3) shown in  FIG. 26  to create a tree-connection structure of an exemplary connection (example 4) shown in  FIG. 27 . In the exemplary connection (example 3) shown in  FIG. 26 , the sound emission-reception apparatus  10   g  marked with a star is the top node, and the sound emission-reception apparatuses  10   h  and  10   i  are common nodes. 
     When there is created a new tree-connection structure upon unification, one among the sound emission-reception apparatuses  10   a  to  10   i  is determined to be the top node, and the other apparatuses are determined to be common nodes. 
     In a tree-connection structure of the exemplary connection (example 4) shown in  FIG. 27 , the path with the maximum number of nodes is, for example, a path from the sound emission-reception apparatus  10   d  to the sound emission-reception apparatus  10   h , and thus, the maximum number of nodes is “3”. Accordingly, the sound emission-reception apparatus  10   a  that is located in the middle of the path may be determined to be the top node. The sound emission-reception apparatus  10   g , which was the top node before unification, is changed to a common node after unification. 
     In the path establisher  202  in the fourth embodiment, the delay time of the delay device  241  that delays sound signals derived from sound received by the microphone  12  is set to “(i−j)d”; and the delay time of the delay device  243  that delays sound signals output toward the loudspeaker  18  is set to “j·d”. 
     The denotation “d” here is as described in the second embodiment. 
     The denotation “i” is a maximum number of nodes from the top node to a terminal common node. The denotation “j” for a common node is the number of nodes that exist along a path from the subject common node to the top nodes, and the denotation “j” for the top node is zero. 
     In the exemplary connection (example 1) shown in  FIG. 24 , the denotation “i” is “2” because the number of nodes from the sound emission-reception apparatus  10   a  to the sound emission-reception apparatus  10   d  ( 10   e  or  10   f ) would be the highest. The denotation “j” for each of the sound emission-reception apparatuses  10   b  and  10   c  is “1”, and the denotation “j” for each of the sound emission-reception apparatuses  10   d ,  10   e , and  10   f  is “2”. 
     When coefficients of “d” are expressed as “(p, q)” as in  FIG. 20  and  FIG. 22 , the delay time set for the delay device  241  and the delay time set for the delay device  243  are as shown in  FIG. 31 . 
     For example, since “(1, 1)” is shown for each of the sound emission-reception apparatuses  10   b  and  10   c , the delay time set for the delay device  241  is “1·d” and the delay time set for the delay device  243  is “1·d”. 
     In the exemplary connection (example 2) shown in  FIG. 25 , “i” is “3” because the number of nodes from the sound emission-reception apparatus  10   c  to the sound emission-reception apparatus  10   d  (or  10   e ) would be the highest. The denotation “j” for each of the sound emission-reception apparatuses  10   a  and  10   f  is “1”, the denotation “j” for the sound emission-reception apparatus  10   b  is “2”, and the denotation “j” for each of the sound emission-reception apparatuses  10   d  and  10   e  is “3”. 
     Accordingly, coefficients of delay times set for the delay devices  241  and  243  in each of the sound emission-reception apparatuses  10   a  to  10   f  are as shown in  FIG. 32 . 
     By setting delay times of the delay devices  241  and  243  in this way, sound signals generated by multiple microphones  12  are sequentially added together with their delays being equalized, and the resultant signal is returned by the top node to be provided to the lower apparatuses. Accordingly, when the sound signals generated by the multiple microphones  12  are output toward a single network  400 , it is possible to minimize effects of delays resulting from DA-conversion and AD-conversion. Similarly, since the delays of sound signals output from the top node toward each loudspeaker  18  are equalized, it is possible to minimize differences between timings at which sounds are output from multiple loudspeakers  18 . 
     In the fourth embodiment also, it is possible to exchange sound signals with other systems at distant locations. In the fourth embodiment in particular, multiple sound emission-reception apparatuses  10  are connected with cables C in a tree-shaped structure. Thus, factors such as a shape of a meeting room, a number of participants, or a positioning of the participants are not likely to impose constraints on arrangement of sound emission-reception apparatuses  10 . 
     Furthermore, in the fourth embodiment, multiple sound emission-reception apparatus  10  can be connected to networks  400 . Accordingly, there is no need to fixedly connect one sound emission-reception apparatus  10  to a network  400 , and any sound emission-reception apparatus  10  can be connected to a network  400  at need. Accordingly, multiple sound emission-reception apparatuses  10  can be disposed flexibly. 
     In the fourth embodiment, an example is described in which the number of connected apparatuses is “6”. However, the number of connected apparatuses may be any number equal to or greater than “2”. When the number of connected apparatuses is “2”, however, the connection configuration by the cables C will be nearly the same as that in the first embodiment, and there will be no advantage of having a tree-connection structure. Thus, it is preferable that the number of connected apparatuses be “3” or more. 
       FIG. 33  is a diagram showing signal paths in the top node, and  FIG. 34  is a diagram showing signal paths in a common node, among signal paths in sound emission-reception apparatuses  10  according to an exemplary application of the fourth embodiment. 
     Similarly to the third embodiment, in the fourth embodiment, sound signals input from multiple networks  400  and sound signals derived from sounds received by multiple microphones  12  are added together, and the resultant sound signal is output toward each of the multiple networks  400 . Accordingly, signal deterioration such as echo may occur. 
     In view of this situation, there are provided in the top node and in each common node a delay device  248  that delays a sound signal that is from a network  400  and is input to the distal-end input terminal  211 ; and a subtractor  249  that subtracts the sound signal delayed by the delay device  248  from a sound signal to be output from the distal-end output terminal  213  toward the network  400 . 
     In the fourth embodiment, components of a sound signal input to a sound emission-reception apparatus  10  from a network  400  are provided in a round-trip path, commencing from the subject apparatus to the top node and then back to the subject apparatus. The delay time generated in this round-trip is “2j·d”. Thus, by setting the delay time for the delay device  248  to “2j·d” in each of the top node and common nodes, a sound signal input from a network  400  is removed from a sound signal to be output toward the network  400 . In this way, deterioration of sound signals output toward the network  400  can be suppressed. 
     In the examples shown in  FIG. 33  and  FIG. 34 , echo cancellers  244  are provided. Similarly to  FIG. 10 ,  FIG. 17 , and  FIG. 23 , an echo canceller  244  here is configured to generate a simulated echo component by filtering a sound signal output toward the loudspeaker terminal  214  with filter coefficients that accord with an estimated transfer function of acoustic space from the loudspeaker  18  to the microphone  12 ; and to subtract the generated simulated echo component from a sound signal input to the microphone terminal  212  to output the resultant signal. By this configuration, even if sound output from the loudspeaker  18  seeps to and is received by the microphone  12 , effects of the seeping sound can be minimized. 
     In the fourth embodiment, the number of connection points for the cables C at the top node and the common nodes is “3”, but may be “4” or more. For example, when the number of connection points is “5” in a common node, there may be four connection points to sound emission-reception apparatuses  10  at a lower level, and one connection point to a sound emission-reception apparatus  10  at a higher level. 
     Exemplary Applications and Modifications 
     The present invention is not limited to the above embodiments, and can take various applied or modified forms as described below. One or more of the modes of application or modification described below can be combined as appropriate. 
     In the above description, the present invention is described as a sound signal processing device in a sound emission-reception apparatus  10 . However, the present invention may be understood as a sound signal processing method, as well as a sound signal processing device. 
     In the above description, the DSP  200 , the ADC  14 , the ADC  251 , the DAC  16 , and the DAC  261  are described as separate bodies in the first embodiment (refer to  FIG. 2 ), for example. Alternatively, the DSP  200  may house each of these ADCs and each of these DACs. 
     In the above description, the path establisher  202  is formed by the DSP  200 . Alternatively, there may be provided in advance in a sound emission-reception apparatus  10  a circuit that executes the same calculation as the signal paths of the master apparatus (prt) and a circuit that executes the same calculation as the signal paths of the slave apparatus (chd); and depending on the role as the master apparatus (prt) or as the slave apparatus (chd), the two circuits may be switched. Similarly, a circuit for the signal paths of the top node and a circuit for the signal paths of the common node may be provided in a sound emission-reception apparatus  10  in advance and may be switched. 
     In each embodiment, there are provided in the sound emission-reception apparatus  10  multiple ADCs including an ADC that converts sound signals derived from sound received by the microphone  12  and multiple DACs including a DAC that converts sound signals output toward the loudspeaker  18 . Alternatively, physically a single ADC may be operated in a time divisional manner to function as if there are multiple ADCs. Similarly, physically a single DAC may be operated in a time divisional manner to function as if there are multiple DACs. 
     Calculations executed in the path establisher  202  may include, in addition to addition and subtraction of two or more signals, outputting a signal with a greatest amplitude from among two or more signals and discarding other signals. 
     In the above embodiments, the communication device  150  communicates with other apparatuses by radio. Alternatively or additionally, the communication device  150  may utilize wired or infrared communication. Furthermore, the communication device  150  may be used to uniformly set parameters used in sound emission-reception apparatuses  10  in the same system. More specifically, in a case where the volume of sound output from the loudspeaker  18  of a sound emission-reception apparatus  10  is adjusted, the communication device  150  may transmit a parameter indicating the volume to another apparatus. The other apparatus changes its volume to the volume indicated by the received parameter. In this way, the volume may be made uniform among all or a part of the sound emission-reception apparatuses  10  forming the system  1 . 
     In each embodiment, the sound emission-reception apparatus  10  includes the ADCs and the DACs, and analog signals are transmitted through the cables C. Alternatively, the ADCs and the DACs may be omitted and digital signals may be transmitted through the cables C. 
     In the above embodiments, the CPU  100  and the DSP  200  are described as separate bodies. Alternatively, the functions of the CPU  100  and the functions of the DSP may be realized by the same at least one processor. For example, the functions of the DSP  200  may be realized by the CPU  100 . 
     DESCRIPTION OF REFERENCE SIGNS 
       1  . . . system;  10  . . . sound emission-reception apparatus;  12  . . . microphone;  18  . . . loudspeaker;  100  . . . CPU;  102  . . . detector;  104  . . . path indicator;  130  . . . notification device;  140  . . . input device;  200  . . . DSP;  202  . . . path establisher;  211  . . . distal-end input terminal;  212  . . . microphone terminal;  213  . . . distal-end output terminal;  214  . . . loudspeaker terminal;  221  . . . first input terminal;  222  . . . second input terminal;  223  . . . third input terminal;  224  . . . fourth input terminal;  225  . . . fifth input terminal;  226  . . . sixth input terminal;  231  . . . first output terminal;  232  . . . second output terminal;  233  . . . third output terminal;  234  . . . fourth output terminal;  235  . . . fifth output terminal;  236  . . . sixth output terminal.