Abstract:
A method is disclosed that breaks the “one line, one location” paradigm of teleconferencing in the prior art. The teleconference bridge in the illustrative embodiment is able to utilize more than one audio channel from each location. By having access to more than one audio channel from a teleconference location&#39;s sound field, the bridge is able to create a multi-channel effect during a conference call. When more than one microphone is present in the first sound field, they can be used to create a multi-channel effect in a second or other sound field involved in a conference call that has more than one loudspeaker. With the bridge mimicking the multi-channel imaging in one room with the multi-channel imaging in another, the conference call participants can experience audio depth during a conference call and can experience two-dimensional imaging, depending on the microphone or speaker separation that is present.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of:
         (1) U.S. Patent Application Ser. No. 60/895564, filed on Mar. 19, 2007,
 
which is incorporated herein by reference.
       

    
    
     FIELD OF THE INVENTION 
     The present invention relates to telecommunications in general, and, more particularly, to an improved teleconferencing system. 
     BACKGROUND OF THE INVENTION 
       FIG. 1  depicts a schematic diagram of teleconference system  100  in the prior art. System  100  comprises telecommunications endpoints  101 - 1  through  101 -J, wherein J is an integer greater than one; private branch exchange (PBX)  102 ; telecommunications network  103 ; and teleconference bridge  104 , interconnected as shown. 
     Telecommunications endpoint  101 - j , where j has a value between 1 and J, inclusive, is capable of handling a telephone call for its user. Endpoint  101 - j  is able to call, or to be called by, another endpoint. In order to participate in a conference call, endpoint  101 - j  is able to dial a telephone number that routes to teleconference bridge  104 . Endpoint  101 - j  can be a cellular phone, a conference phone (i.e., “speakerphone”), a deskset, or some other type of telecommunications appliance. 
     Some of endpoints  101 - 1  through  101 -J are PBX terminals, such as those in an office enterprise network, for which telecommunications service is enabled by private branch exchange  102 . 
     Telecommunications network  103  provides the connectivity among endpoints  101 - 1  through  101 -J, exchange  103 , and teleconference bridge  104 . Telecommunications network  103  comprises a transmission network—for example, the Public Switched Telephone Network, which is a complex of telecommunications equipment that is owned and operated by different entities throughout the World. Network  103  can also comprise the Internet or possibly other Internet Protocol-based networks. 
     Teleconference bridge  104  is a server or switch that enables the users of multiple endpoints to communicate with each other during a conference call. Bridge  104  receives audio signals from endpoints that are participating on a conference call, mixes those signals together, and transmits the mixed signals back to the endpoints. 
     As depicted in  FIG. 1 , system  100  is a traditional teleconferencing system for J teleconference locations, where a location is defined by a single endpoint (i.e., endpoint  101 - j ), supported by a teleconference bridge (i.e., bridge  104 ). Some of the endpoints are speakerphones, which are designed specifically to handle conference call communication. Each speakerphone is connected to the bridge via a monophonic, bi-directional channel. If any given speakerphone at a teleconference location has multiple feeds—that is, a main microphone and one or more satellite microphones, or a main loudspeaker and one or more satellite loudspeakers—they are combined at the speakerphone itself into the monophonic channel transmitted by that speakerphone to the bridge. This is depicted in  FIG. 2 , showing an overhead view in which endpoint  101 - 11  is situated on table  202  of conference room  201 . Endpoint  101 - 11 , a speakerphone, comprises satellite microphones  203 - 1  and  203 - 2 , as well as loudspeaker  204 . 
     During operation, the monophonic feed from each endpoint, such as endpoint  101 - 11 , is fed into bridge  104 , which adds the feeds, and the sum is distributed by the bridge back to the speakerphones at the other locations. Each speakerphone at each location receives a signal via a monophonic channel from the bridge, which signal is played out of all loudspeakers connected to that speakerphone. In the operation of any such traditional bridge, the monophonic signal received by any endpoint  101 - j  contains components of one or more other endpoints  101 - k , k≠j, but explicitly excludes components of the signal sent to the bridge by endpoint  101 - j . By doing so, bridge  104  prevents regenerative acoustic feedback that would otherwise occur. 
     Each monophonic, bi-directional channel is associated with a phone line that terminates at the conference bridge. From the bridge&#39;s perspective, each channel and line equates to a different “location,” even though conference call participant who are using a speakerphone and a participant who is using a cell phone might be present in the same conference room. 
     SUMMARY OF THE INVENTION 
     The present invention breaks the “one line, one location” paradigm of teleconferencing in the prior art. The teleconference bridge in the illustrative embodiment is able to utilize more than one audio channel from each location, where there are multiple signal sources present in the room. For example, a participant&#39;s cell phone can be used as a satellite microphone to augment or replace the speakerphone&#39;s microphone at the same location, for the purpose of improving the audio quality experienced on the conference call. As another example, more than one deskset endpoint can be situated in a conference room, each with its own port at the teleconference bridge of the illustrative embodiment; the bridge can mix the audio signals coming from and going to the multiple desksets in a way that optimizes audio quality and avoids acoustic feedback between participating phones located within the same acoustic space. 
     Specifically, by having access to more than one audio channel from a teleconference location&#39;s sound field, the teleconference bridge of the illustrative embodiment is able to create a multi-channel effect during a conference call. When more than one microphone is present in the first sound field (i.e., M 1  is greater than one), they can be used to create a multi-channel effect in a second or other sound field involved in a conference call that has more than one loudspeaker (i.e., N 2  is greater than one). Where a first location has M 1  microphones and a second location has N 2  loudspeakers where M 1 &gt;N 2 , the bridge mixes down to a smaller number of loudspeaker channels. Where a first room has M 1  microphones and a second room has N 2  loudspeakers where M 1 &lt;N 2 , the bridge mixes to a larger number of loudspeaker channels. 
     In accordance with the illustrative embodiment, the bridge is also able to keep track of the relative positions of the endpoints within each sound field, for the purpose of properly imaging the sound fields across teleconference locations. For example, for a three-channel system, the teleconference bridge transmits the signals from microphone channel “A” in the first sound field to loudspeaker channel “A” in the second sound field, the signals from microphone channel B to loudspeaker channel B, and the signals from microphone channel C to loudspeaker channel C. By acquiring and using information about the relative positions of endpoints in a sound field, the bridge ensures that the first sound field represented as microphone channels A-B-C is mimicked in the second sound field as “A-B-C”, and not as “A-C-B”. 
     With the bridge of the illustrative embodiment mimicking the multi-channel imaging in one room with the multi-channel imaging in another, the conference call participants can experience audio depth during a conference call and can experience two-dimensional imaging, depending on the amount of microphone or speaker separation that is present. 
     The illustrative embodiment of the present invention comprises: determining, at a teleconference bridge, the presence of i) a first endpoint and a second endpoint at a first teleconference location, the teleconference bridge being able to receive audio signals from each of the first endpoint and the second endpoint, and ii) a third endpoint and a fourth endpoint at a second teleconference location, the teleconference bridge being able to transmit audio signals to each of the third endpoint and the fourth endpoint, wherein the first teleconference location and the second teleconference location are acoustically isolated from each other; assigning i) the first endpoint and the third endpoint to a first audio channel, and ii) the second endpoint and the fourth endpoint to a second audio channel; receiving i) a first audio signal from the first endpoint, and ii) a second audio signal from the second endpoint; and transmitting i) a third audio signal to all endpoints that have been assigned to the first audio channel at the second teleconference location, wherein the third audio signal is based on the first audio signal, and ii) a fourth audio signal to all endpoints that have been assigned to the second audio channel at the second teleconference location, wherein the fourth audio signal is based on the second audio signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a schematic diagram of teleconference system  100  in the prior art. 
         FIG. 2  depicts an overhead view of conference room  201 , as well as telecommunications endpoint  101 - 11  of system  100 . 
         FIG. 3  depicts schematic diagram of the salient components of teleconference system  300  in accordance with the illustrative embodiment of the present invention. 
         FIG. 4  depicts an overhead view of teleconference location  310 - 2 , as well as telecommunications endpoints  301 - 4  and  301 - 5  of system  300 . 
         FIG. 5  depicts a block diagram of the salient components of teleconference bridge  304 , in accordance with the illustrative embodiment of the present invention. 
         FIG. 6  depicts a flowchart of the overall salient tasks that are related to preparing for, establishing, and managing a teleconference call, as performed by teleconference bridge  304 , in accordance with the illustrative embodiment of the present invention. 
         FIGS. 7A through 7D  depict some illustrative examples of how the number of channels may vary across the teleconference locations, as related to task  601  in  FIG. 6 . 
         FIG. 8  depicts telecommunications endpoints  301 - 6 ,  301 - 7 , and  301 - 8 , some of which might be acoustically collocation with each other, as determined at task  602  of  FIG. 6 . 
         FIGS. 9A through 9D  depict an overview of a chain of events that involve cell phone  301 - 4  being used by a participant to the conference call, as related to task  605  of  FIG. 6 . 
         FIG. 10  depicts a flowchart of the salient subtasks that are related to organizing each teleconference location into receive audio channels and transmit audio channels, as part of task  601 . 
         FIG. 11  depicts a flowchart of the salient subtasks that are related to determining acoustic collocation, as part of task  602 . 
         FIG. 12  depicts a flowchart of the salient subtasks that are related to accounting for cell phone or other portable endpoint, as part of task  605 . 
         FIG. 13  depicts a flowchart of the salient subtasks that are related to acoustically determining the presence of endpoints, as part of task  1001  of  FIG. 10 . 
         FIG. 14  depicts a flowchart of the salient subtasks that are related to determining which endpoints are acoustically collocated with endpoint  301 - j , as part of task  1103  of  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION 
     The following terms are defined for use in this Specification, including the appended claims:
         The term “acoustic collocation,” and its inflected forms, is defined as the state in which a first endpoint is receiving, via its microphone, at least a component of a signal that has been transmitted to a second endpoint and outputted via the second endpoint&#39;s loudspeaker. When this occurs, the first endpoint is said to be “acoustically collocated” with the second endpoint, in addition to being in the same sound field as the second endpoint. Note that in this case, the second endpoint might also be acoustically collocated with the first endpoint, but not necessarily.   The term “sufficient correlation,” and its inflected forms, is defined as the state in which the teleconference bridge of the illustrative embodiment transmits a first audio signal to a second endpoint and receives back a second audio signal from a first endpoint, and the received second signal resembles the transmitted first signal closely enough to conclude that the second signal is related to the first. Determining whether one signal is “sufficiently correlated” with another is one way to determine whether one endpoint is acoustically collocated with another.   The term “acoustic isolation,” and its inflected forms, is defined as the state in which a first teleconference location (e.g., a conference room, the place where someone is conferencing in from their cell phone, etc.) is sufficiently distant from other teleconference locations such that the first location does not acoustically interfere with or is not acoustically interfered on by the other locations during a conference call. The two teleconference locations are said to be in different sound fields from each other.       

       FIG. 3  depicts schematic diagram of the salient components of teleconference system  300  in accordance with the illustrative embodiment of the present invention. System  300  comprises telecommunications endpoints  301 - 1  through  301 -J, wherein J is an integer greater than one; private branch exchange (PBX)  302 ; telecommunications network  303 ; and teleconference bridge  304 , interconnected as shown. 
     Telecommunications endpoint  301 - j , where j has a value between 1 and J, inclusive, is capable of originating, receiving, or otherwise handling a telephone call for its user. Endpoint  301 - j  is able to call, or to be called by, another endpoint. In order to participate in a conference call, endpoint  301 - j  is able to dial a telephone number that routes to teleconference bridge  304 . Endpoint  301 - j  can be an analog telephone, an ISDN terminal, a softphone, an Internet-Protocol phone, a cellular phone, a cordless phone, a PBX deskset, a conference phone (i.e., “speakerphone”), or some other type of telecommunications appliance. It will be clear to those skilled in the art how to make and use endpoint  301 - j.    
     Some of endpoints  301 - 1  through  301 -J are PBX terminals, such as those in an office enterprise network, for which telecommunications service is enabled by private branch exchange  302 , as is well-known in the art. 
     Telecommunications network  303  provides the connectivity among endpoints  301 - 1  through  301 -J, exchange  303 , and teleconference bridge  304 . Network  303  comprises the Public Switched Telephone Network, which is a complex of telecommunications equipment that is owned and operated by different entities throughout the World. In the United States of America, for example, the Public Switched Telephone Network (or “PSTN”) comprises an address space that is defined by ten digits, and, therefore, comprises  10  billion unique addresses or “telephone numbers.” The public switched telephone networks in other countries are similar. In some embodiments, network  303  comprises the Internet or possibly other Internet Protocol-based networks, either in addition to or as opposed to the PSTN. 
     It will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention that comprise various combinations of networks within teleconference system  300 , which networks are public or private, wired or wireless, and circuit-based or packet-based. 
     Teleconference bridge  304  is a server or switch that enables the users of multiple endpoints to communicate with each other during a conference call. Bridge  304  receives audio signals from endpoints that are participating on a conference call, mixes those signals together based on the transfer function associated with each output channel, and transmits the mixed signals back to the endpoints, in accordance with the illustrative embodiment of the present invention. Bridge  304  is described in detail below and with respect to  FIG. 5 . 
     As those who are skilled in the art will appreciate, the techniques of the illustrative embodiment can be implemented at a device other than a teleconference bridge or at a teleconference bridge that is other than a server or switch. 
       FIG. 3  depicts the endpoints that are to participate, or are participating, in a particular conference call, which endpoints are situated at locations  310 - 1  through  310 -L, wherein L is an integer greater than one. Each location  310 - l , where l has a value between 1 and L, inclusive, comprises at least one telecommunications endpoint  301 - j , where j has a value between 1 and J, inclusive. One effect of having J independent endpoints distributed across L teleconference locations is shown in  FIG. 4 , which depicts an overhead view of a conference room. Situated on table  402  of teleconference location  310 - 2  are telecommunications endpoints  301 - 4  and  301 - 5 , each one an independent endpoint that is capable of placing and handling calls. For example, endpoint  301 - 4  might be the cell phone of someone participating on the call, while endpoint  301 - 5  might be a conference room phone that is situated in the conference room at location  310 - 2 . 
       FIG. 5  depicts a block diagram of the salient components of teleconference bridge  304 , in accordance with the illustrative embodiment of the present invention. Bridge  304  comprises receive interface  501 - 1 , transmit interface  501 - 2 , processor  502 , and memory  503 , interconnected as shown. Bridge  304  is capable of performing the tasks described below and with respect to  FIGS. 6 through 14 . 
     Receive interface  501 - 1  and transmit interface  502 - 2  comprise the circuitry that enables bridge  304  to respectively receive signals from and transmit signals to network  303 , in well-known fashion. In accordance with the illustrative embodiment, bridge  304  receives and transmits audio signals that are represented in Internet Protocol packets, in well-known fashion. As those who are skilled in the art will appreciate, in some alternative embodiments bridge  304  receives and transmits audio signals represented in a different format. 
     Processor  502  is a general-purpose processor that is capable of receiving information from receive interface  501 - 1 , of executing instructions stored in memory  503 , of reading data from and writing data into memory  503 , and of transmitting information to transmit interface  501 - 2 . In some alternative embodiments of the present invention, processor  502  might be a special-purpose processor. Processor  502  performs the audio mixing function at bridge  304 , in accordance with the illustrative embodiment of the present invention. As part of the audio mixing function, processor  502  is able to take any input audio signal from any endpoint or other source and mix it into the composite output audio signal to be transmitted to a particular endpoint, for all endpoints to which audio signals are to be transmitted. The specific output signal to a given endpoint is based on the mixer transfer function associated with that output signal, as determined in the illustrative embodiment. 
     Memory  503  stores the instructions and data used by processor  502 , in well-known fashion. Memory  503  might be any combination of dynamic random-access memory (RAM), flash memory, disk drive memory, and so forth. 
     In accordance with the illustrative embodiment, bridge  304  communicates with each endpoint  301 - j  via a different communication port, as is known in the art. As those who are skilled in the art will appreciate, the ports can be implemented in software or in hardware, or both. It will be clear to those skilled in the art how to make and use teleconferencing systems where an endpoint has its own port at bridge  304  or an endpoint shares a port with another endpoint, or both. 
     In accordance with the illustrative embodiment, bridge  304  is able to receive on M L  input channels from endpoints at the combined teleconference locations and to transmit on N L  output channels to endpoints at those locations. The values of M L  and N L  can be equal to or different from each other; in other words, some endpoints might be microphone-only devices, some might be loudspeaker-only devices, and some might comprise both microphones and loudspeakers. Bridge  304  is capable of unidirectional communication with the microphone-only devices or loudspeaker-only devices, and of bi-directional communication with the devices comprising both a microphone and loudspeaker. 
       FIGS. 6 and 10  through  14  depict flowcharts of salient tasks that are related to preparing for, establishing, and managing a teleconference call, as performed by teleconference bridge  304 , in accordance with the illustrative embodiment of the present invention. As those who are skilled in the art will appreciate, some of the tasks that appear in the flowcharts that follow can be performed in parallel or in a different order than that depicted. Moreover, those who are skilled in the art will further appreciate that in some alternative embodiments of the present invention, only a subset of the depicted tasks are performed. 
     Referring now to  FIG. 6 , at task  601  bridge  304  organizes each teleconference location  310 - l  into M l  receive channels and N l  transmit channels, and processes those channels accordingly. In accordance with the illustrative embodiment, when more than one microphone is present in the first sound field (i.e., M 1  is greater than one), the microphones can be used to create a multi-channel effect (e.g., stereo, three-channel, etc.) in the other sound fields that are involved in a conference call. In preparation for the conference call, bridge  304  first determines that multi-channel imaging is possible by detecting the existence of a first sound field with two or more microphones and a second sound field with two or more loudspeakers. 
       FIGS. 7A through 7D  depict illustrative examples of how the number of audio channels may vary across the teleconference locations, as related to task  601 , and as a result, how the mixing of the audio signals will vary between different pairs of teleconference locations. The depicted examples are intended to provide an overview, while details that are related to task  601  are described below and with respect to  FIG. 10 .  FIG. 7A  depicts an example of where M 2  (i.e., the number of microphones at location  310 - 2 ) is equal to N 4  (i.e., the number of loudspeakers at location  310 - 4 ), where M 2  and N 4  are equal to two.  FIG. 7B  depicts an example of where M 2  (i.e., location  310 - 2  microphones) is less than N 3  (i.e., location  310 - 3  loudspeakers); in this situation, bridge  304  mixes to a larger number of loudspeaker channels than microphone channels.  FIG. 7C  depicts an example of where M 4  (i.e., location  310 - 4  microphones) is greater than N 5  (i.e., location  310 - 5  loudspeakers); in this situation, bridge  304  mixes down to a smaller number of loudspeaker channels than microphone channels. And in  FIG. 7D , M 1  (i.e., the number of microphones at location  310 - 1 ) is depicted as being equal to N 3  (i.e., the number of loudspeakers at location  310 - 3 ), where M 1  and N 3  are equal to three. It will be clear to those skilled in the art how to make and use embodiments of the invention that involve other teleconference locations with different numbers of microphones and loudspeakers than those depicted. 
     At task  602 , bridge  304  determines for each location whether two are more endpoints are acoustically collocated, and processes signals for those endpoints accordingly. The purpose of doing so is to determine whether the sound coming from the loudspeaker of any one endpoint will adversely feed back into the microphone of another.  FIG. 8  depicts this possibility of feedback, in which situated on table  802  of teleconference location  310 - 3  are telecommunications endpoints  301 - 6 ,  301 - 7 , and  301 - 8 , each one an independent endpoint that is capable of handling calls. For example, deskset endpoints  301 - 7  and  301 - 8  are close enough that they might cause feedback problems with each other unless their signals are conditioned; the same can be said of cell phone  301 - 6  and deskset  301 - 7 . At the same time, however, cell phone  301 - 6  and deskset  301 - 8  might not cause feedback issues with each other, possibly because they are sufficiently separated. Although  FIG. 8  is intended to provide an example of acoustic collocation, details that are related to task  602  for determining acoustic collocation are described below and with respect to  FIG. 11 . 
     At task  603 , bridge  304  establishes a conference call in well-known fashion. involving endpoints  301 - 1  through  301 -J. 
     At task  604 , bridge  304  during the conference call continually receives audio signals s 1  through s J  from endpoints  301 - 1  through  301 -J, in well-known fashion. 
     At task  605 , bridge  304  determines whether one or more endpoints are appearing at or disappearing from one or more of the teleconference locations, and processes signals for those endpoints accordingly. These endpoints of interest can be cell phones, other types of mobile or portable endpoints, or normally stationary endpoints (e.g., desksets, etc.) that are suddenly plugged into the call at an existing teleconference location. Bridge  304  monitors endpoints because they can conceivably wander in and out of a sound field (or sound fields) involved in the call, as described in the following scenario described with respect to  FIG. 9 , and possibly affect the audio quality experienced by the participants. 
       FIGS. 9A through 9D  depict an overview of a chain of events as related to task  605  that involve endpoint  301 - 4 , a cell phone, being used by a participant of the conference call. Note that details that are related to task  605  are described below and with respect to  FIG. 12 . In this sequence of events, bridge  304  monitors cell phone  301 - 4 . As depicted in  FIG. 9A , the participant approaches teleconference location  310 - 2 . When bridge  304  determines that cell phone  301 - 4  is present in location  310 - 2 &#39;s sound field, the bridge adjusts the signals that it transmits to both cell phone  301 - 4  and to other endpoints in the sound field such as endpoint  301 - 5 , a conference phone. As depicted in  FIG. 9B , the attendee then puts her cell phone on the table in front of her (i.e., on table  902 ). At this point, the cell phone&#39;s microphone is used as a satellite microphone, and there is no need to feed back the audio signals of the conference call to cell phone  301 - 4 . The output of bridge  304  is fed to the loudspeaker of the stand-alone conference phone in the room, namely endpoint  301 - 5 . As depicted in  FIG. 9C , the cell phone user gets up and walks around the room, carrying her cell phone, in which case bridge  304  compensates by possibly resuming the feeding of audio to cell phone  301 - 4 . And as depicted in  FIG. 9D , the cell phone user then leaves the room, possibly to go to another teleconference location that is part of the same conference call in another part of the building; in response, bridge  304  starts feeding the audio signal from the cell phone user to endpoint  301 - 5 . 
     As shown by  FIGS. 9A through 9D , teleconference bridge  304  is able to adapt to the changing relationship between microphones/loudspeakers and locations. Thus, if the attendee decides to leave the room, she can take her cell phone with her and continue participating on the conference call. And if bridge  304  previously was not feeding an output signal to the cell phone, because feeding the signal to the stand-alone loudspeaker was sufficient, the bridge can then start feeding the output signal to the cell phone when the bridge detects that the phone is moving out of the room and away from the loudspeaker. 
     At task  606 , bridge  304  during the conference call continually transmits audio signals x 1  through x J  to endpoints  301 - 1  through  301 -J, in well-known fashion. The transmitted signals are based on various adjustments to the mixer transfer functions that are associated with the output channels, each transfer function being adjusted at one or more of tasks  601 ,  602 , and  605  in accordance with the illustrative embodiment of the present invention. Specifically, each transmitted signal can be based on one or more receive signals (i.e., s 1  through s J ), on a determination that a first endpoint is acoustically collocated with a second endpoint, on two signals being sufficiently correlated with each other, or on something else that affects the output signal&#39;s transfer function. Bridge  304  can exclude at least a component of one or more receive signals from a transmitted signal or can refrain from transmitting a signal entirely. 
     At task  607 , bridge  304  refrains from transmitting an audio signal to one or more endpoints of interest that the bridge at task  605  determined were present, based on the criteria applied at task  605 . 
     At task  608 , bridge  304  determines whether the conference call has finished. If not, task execution proceeds back to task  604 . If the call has finished, task execution ends. 
       FIG. 10  depicts a flowchart of the salient subtasks that are related to organizing each teleconference location into receive audio channels and transmit audio channels, as part of task  601 , in accordance with the illustrative embodiment of the present invention. 
     At task  1001 , bridge  304  determines the presence of endpoints that are to participate in a conference call, as well as the relative positions of the endpoints at each teleconference location. In accordance with the illustrative embodiment, this determination is achieved acoustically, as described below and with respect to  FIG. 13 . 
     As those who are skilled in the art will appreciate, in some alternative embodiments bridge  304  can determine the presence of endpoints at teleconference locations and the endpoints&#39; relative positions via other means. A first alternative means comprises receiving a calling number identifier from each endpoint that is calling into the conference call and looking up the identifier in a database that comprises information on the endpoints, as well as on their teleconference locations and relative positions at each location. A second alternative means comprises receiving information that is spoken or entered (e.g., via endpoint keypad, etc.) from each endpoint at the participating locations, where the information received from an endpoint describes the teleconference location and relative position of that endpoint and of possibly other endpoints. And a third alternative means comprises applying the technique of using geo-location measurements that is described below and with respect to task  1202 . 
     Based on the determined relative positions of the endpoints, bridge  304  then creates, for each teleconference location  310 - l , P i  audio input channels from and Q l  audio output channels to location  310 - l . For example, if a first subset of endpoints at location  310 - l  appear to be situated on the left side of a conference room, a second subset appear to be on the right side, and a third subset appear to be in the middle, then bridge  304  creates a “left” channel, a “right” channel, and a “middle” channel, respectively. 
     At task  1002 , bridge  304  assigns each endpoint at each teleconference location to one of the audio channels created at task  1001 . Each teleconference location  310 - l  comprises M l  microphones and N l  loudspeakers that are assigned to P l  input channels and Q l  output channels associated with location  310 - l , where the values for M l  and P l  can be the same or different and the values for N l  and Q l  can be the same or different. At those who are skilled in the art will appreciate, one or more of the M l  microphones at location  310 - l  can be assigned to a particular audio input channel at bridge  304 , while one or more of the N l  loudspeakers at location  310 - l  can be assigned to a particular audio output channel. Bridge  304  keeps track of the P-to-Q relationship for each pair of teleconference locations and maps the channels accordingly. 
     As those who are skilled in the art will appreciate, after reading this specification, the value for M l  across two or more endpoints can be the same or different and the value for N l  can be the same or different. Furthermore, the value for P l  across two or more endpoints can be the same or different and the value for Q l  can be the same or different. 
     At task  1003 , bridge  304  adjusts one or more transfer functions that govern the output signals to be transmitted during a conference call to the endpoints, based on the determined audio channels and the relative positions of the endpoints across teleconference locations. As part of this task, the mapping is established between each sound field at one location to the corresponding sound field at each other location. For example, for a three-channel system, bridge  304  transmits the signals from microphone channel “A” in the first sound field to loudspeaker channel “A” in the second sound field, the signals from microphone channel B to loudspeaker channel B, and the signals from microphone channel C to loudspeaker channel C. Bridge  304  ensures that the first sound field represented as microphone channels A-B-C is mimicked in the second sound field as “A-B-C”, and not as “A-C-B”. As those who are skilled in the art will appreciate, the same process applies in the other direction, in which the second sound field is represented in multi-channel audio for the listeners in the first sound field. 
     There can be a different number of input channels from one location than there are output channels to another location. When P 1 &gt;Q 2 , bridge  304  mixes the audio from the “extra” input channel into one or more of the output channels. When P 1 &lt;Q 2 , bridge  304  synthesizes an “extra” output channel&#39;s audio from one or more of the input channels. 
     In some embodiments, bridge  304  adjusts the one or more transfer functions to provide cross-channel mixing to coax the other channel&#39;s speakerphone into the receive state, for the purpose of controlling echo. For example, for teleconference location  301 - 4  with each of two independent speakerphones (i.e., endpoints  301 - 9  and  301 - 10 ) receiving a different output audio channel (i.e., “A” and “B”), bridge  304  can provide an attenuated version of the signal being fed to channel A also to channel B, or vice-versa. 
       FIG. 11  depicts a flowchart of the salient subtasks that are related to determining acoustic collocation, as part of task  602 , in accordance with the illustrative embodiment of the present invention. 
     At task  1101 , bridge  304  selects endpoints  301 - 1  through  301 -J to be evaluated. The actual endpoints that are to be evaluated are based on the determination made earlier as to which endpoints are part of the conference call. 
     At task  1102 , bridge  304  initializes endpoint pointer j to 1. 
     At task  1103 , bridge  304  determines which endpoints are acoustically collocated with endpoint  301 - j  and accordingly adjusts the mixer transfer function that corresponds to the output signal to endpoint  301 - j . Task  1103  is described below and with respect to  FIG. 14 . 
     At task  1104 , bridge  304  increments pointer j. 
     At task  1105 , bridge  304  determines whether acoustic collocation has been determined for all J selected endpoints being evaluated. If all endpoints have been checked, task execution proceeds to task  603 . Otherwise, task execution proceeds back to task  1103 . 
       FIG. 12  depicts a flowchart of the salient subtasks that are related to adapting to cell phones or other endpoints that enter or leave a sound field, as part of task  605 , in accordance with the illustrative embodiment of the present invention. 
     At task  1201 , bridge  304  identifies and determines the characteristics of each endpoint of interest—that is, each endpoint that is capable of appearing in, moving in, or disappearing from the sound field in which it is present. For example, the endpoint might be a cell phone or other wireless telephone. The endpoints of interest can be identified to bridge  304  by a conference call participant, by private branch exchange  302 , or through some other means. In accordance with the illustrative embodiment, bridge  304  queries exchange  302  about a particular endpoint that is dialing into the conference call, to which exchange  302  might respond by identifying the endpoint as that of an employee and as being a cell phone in terminal type. In some embodiments, bridge  304  additionally determines one or more other characteristics of the endpoint. 
     At task  1202 , bridge  304  monitors whether the endpoint of interest is acoustically collocated with one or more other endpoints. In accordance with the illustrative embodiment, bridge  304  infers collocation by tracking the geo-location of the endpoint and comparing the endpoint&#39;s geo-location with the geo-locations of one or more conference telephones whose geo-locations have been predetermined and stored in a database accessible by the bridge. It will be clear to those skilled in the art how to determine and store the geo-location of one or more cell phones. When the cell phone comes within a predetermined distance from the conference telephone, as determined from the difference in their geo-locations, bridge  304  infers that the cell phone and conference telephone have become collocated. 
     It will be clear to those skilled in the art, in some alternative embodiments, how to determine acoustic collocation through other means. For example, the tasks described below and with respect to  FIG. 14  can be adapted to acoustically determine collocation of a cell phone with a conference telephone. 
     At task  1203 , bridge  304  determines whether to exclude at least a component of a signal from an output signal (i.e., a signal to be transmitted to one of the endpoints in a conference call). For example, when it is determined that a cell phone is acoustically collocated with a conference telephone, bridge  304  will exclude some or all of the signal received from the cell phone from the signal to be transmitted to the conference telephone, in order to prevent feedback. 
     At task  1204 , bridge  304  determines whether to refrain from transmitting a signal to the endpoint of interest. In accordance with the illustrative embodiment, bridge  304  concludes that an endpoint is being used as a satellite microphone when the endpoint i) is a cell phone, as determined earlier, and ii) either is not moving and/or is within a predetermined distance from (or acoustically collocated with) the conference telephone. In this case, there would be no need to provide an audio signal to the cell phone since no one is using the cell phone to listen. In some embodiments, bridge  304  can determine that a cell phone is not moving by comparing successive geo-location measurements of the phone. In some alternative embodiments, bridge  304  bases the decision to refrain on a predetermined characteristic of a signal that is used to determine acoustic collocation, such as audio level. 
     Conversely, when bridge  304  determines that the endpoint of interest is moving away from the collocated conference phone, the bridge can resume transmitting an audio signal to the endpoint. 
     At task  1205 , bridge  304  adjusts the mixer transfer function for each output channel, based on one or more of the other subtasks that constitute task  605 . Task execution then proceeds to task  606 . 
       FIG. 13  depicts a flowchart of the salient subtasks that are related to acoustically determining the presence of endpoints, as part of task  1001 , in accordance with the illustrative embodiment of the present invention. 
     As part of the tasks that are described here with respect to  FIG. 13 , bridge  304  plays a special audio signal (e.g., a tone, etc.) out of the loudspeaker of each endpoint  301 - 1  through  301 -J. While each tone is played out, bridge  304  listens through the microphones of the other phones. Bridge  304  then correlates the received audio signals with those generated by the bridge. By initializing in this way, bridge  304  is able to determine which of the M L  microphones and N L  loudspeakers are at each of teleconference locations  310 - 1  through  310 -L (i.e., are together in the same sound field), as well as their relative positions at each location. 
     At task  1301 , bridge  304  initializes endpoint pointer j to 1. 
     At task  1302 , bridge  304  transmits a predetermined signal x 0  to endpoint  301 - j . As those who are skilled in the art will appreciate, after reading this specification, signal x 0 &#39;s characteristics (e.g., audio level, frequency, duty cycle, etc.) are selected to allow other endpoints to detect the played signals when those endpoints are proximate to endpoint  301 - j.    
     At task  1303 , bridge  304  receives signals from one or more endpoints, including audio signals being detected by the microphones at the endpoints. The audio signals might comprise predetermined signal x 0  that is being played from endpoint  301 - j &#39;s loudspeaker at the teleconference location currently being assessed. 
     At task  1304 , bridge  304  determines the amount of correlation between predetermined signal x 0  (i.e., the signal being transmitted at endpoint  301 - j ) and each signal received from one or more of the other endpoints. 
     At task  1305 , bridge  304  increments endpoint pointer j. 
     At task  1306 , bridge  304  determines whether all J endpoints have been evaluated. If they have all been evaluated, task execution proceeds to task  1307 . Otherwise, task execution proceeds back to task  1302 . 
     At task  1307 , bridge  304  determines which endpoints are at each teleconference location, based on the signals received at task  1303  for each endpoint  301 - j . In accordance with the illustrative embodiment, bridge  304  examines the signal strength of each received signal and compares the signal strength to a predetermined threshold. If the signal strength exceeds the threshold, the corresponding endpoint is determined to be present at the same teleconference location as endpoint  301 - j  that played the signal via its loudspeaker. As those who are skilled in the art will appreciate, a property of the received signals other than signal strength, such as the amount of correlation, can be used to determine the presence of endpoints at each teleconference location. 
     At task  1308 , bridge  304  determines the relative positions of endpoints  301 - 1  through  301 -J, based on the amounts of correlation between the signals transmitted at task  1302  and the corresponding signals received at task  1303 . In accordance with the illustrative embodiment, bridge  304  examines a characteristic of the correlation between the transmitted signal and each received signal, as determined at task  1304 , and compares the correlation to one or more predetermined thresholds. Depending on which thresholds are exceeded by which received signals, bridge  304  infers relative positions between each endpoint that detects a signal and endpoint  301 - j  that played the transmitted signal. Bridge  304  then infers additional information about the relative positions by performing the threshold test on the other sets of signal correlations that correspond to other received signals compared against the signals played from other endpoints at the same teleconference location. As those who are skilled in the art will appreciate, a property of the received signals other than the amount of correlation can be used to determine the relative positions of the endpoints at each teleconference location. 
     After task  1308 , task execution proceeds to task  1002 . 
       FIG. 14  depicts a flowchart of the salient subtasks that are related to determining which endpoints are acoustically collocated with endpoint  301 - j , as part of task  1103 , in accordance with the illustrative embodiment of the present invention. Determining acoustic collocation is important when two or more endpoints—for example, cell phone  301 - 4  and conference phone  301 - 5 —are at the same conferencing location; the collocated endpoints need not receive each other&#39;s audio, since the participants are in the same sound field and can hear each other directly. 
     As part of the tasks that are described here with respect to  FIG. 14 , bridge  304  plays a special audio signal (e.g., a tone, etc.) out of endpoint  301 - j &#39;s loudspeaker. While the tone is played out, bridge  304  listens through the microphones of the other phones being evaluated with respect to endpoint  301 - j . Bridge  304  then determines whether the received audio signals are sufficiently correlated with the tone generated by the bridge. In accordance with the illustrative embodiment, bridge  304  performs the described tasks before the conference call starts. However, it will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention in which the determination of whether some signals are sufficiently correlated—or the determination of acoustic collocation in general—is performed during the conference call. 
     Although the tasks described here evaluate all endpoints with respect to each endpoint  301 - j , it will be clear to those skilled in the art how to evaluate only a subset of the endpoints on a conference call, for each endpoint  301 - j . For example, once the endpoints at each teleconference location have been determined, it might be advantageous to evaluate only those endpoints at the same teleconference location as endpoint  301 - j.    
     Moreover, in accordance with the illustrative embodiment, the audio signal for determining collocation is transmitted to the endpoints one endpoint at a time. However, it will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention in which the audio signals are transmitted to multiple endpoints concurrently in order to identify multiple sets of collocated endpoints concurrently. For example, those transmitted audio signals can be formed so that they will not interfere with each other (i.e., they are acoustically orthogonal with respect to each other). 
     At task  1401 , bridge  304  transmits a predetermined signal x j  to endpoint  301 - j . As those who are skilled in the art will appreciate, after reading this specification, signal x j &#39;s characteristics (e.g., level, frequency, duty cycle, etc.) are selected to allow other endpoints to pick up the played signals, when those endpoints are proximate to endpoint  301 - j.    
     At task  1402 , bridge  304  initializes endpoint pointer k to 1. 
     At task  1403 , bridge  304  receives a signal s k  from endpoint  301 - k , when k is not equal to j, including audio signals being picked up by endpoint  301 - k &#39;s microphone. The audio signals might comprise predetermined signal x j  that is being transmitted from endpoint  301 - j &#39;s loudspeaker at the teleconference location currently being assessed. 
     At task  1404 , bridge  304  determines the amount of correlation between received signal s k  and predetermined signal x j  (i.e., the signal being transmitted at endpoint  301 - j ). In accordance with the illustrative embodiment, bridge  304  compares the correlation to a predetermined threshold; if the correlation amount exceeds the threshold, received signal s k  is considered to be sufficiently correlated with signal x j . 
     At task  1405 , bridge  304  checks whether signal s k  is sufficiently correlated with signal x j . If there is sufficient correlation, task execution proceeds to task  1406 . Otherwise, task execution proceeds to task  1407 . 
     At task  1406 , bridge  304  adjusts the mixer transfer function that corresponds to the output signals to be transmitted to endpoint  301 - j , to exclude at least a component of a signal received from endpoint  301 - k.    
     At task  1407 , bridge  304  increments endpoint pointer k. 
     At task  1408 , bridge  304  determines whether all endpoints have been evaluated. If they have all been evaluated, task execution proceeds to task  603 . Otherwise, task execution proceeds back to task  1403 . 
     In accordance with the illustrative embodiment, acoustic collocation is determined acoustically, as described with respect to  FIG. 14 . It will be clear to those skilled in the art, after reading this specification, how to determine acoustic collocation through non-acoustic means—for example, by applying the technique of using geo-location measurements that is described above and with respect to task  1202 . 
     It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.