A versatile audio bridge for multimedia conferencing

The paper describes an audio bridge that has been built as part of a multimedia conference bridge. It connects to the public switched telephone network via an ISDN primary rate interface. A generalized mathematical representation of the basic audio bridging function in terms of a matrix multiplication is presented. Not only does it lend itself to efficient implementation on a digital signal processor, but it also allows the provision of a rich set of audio bridging services, including subconferencing, whispering, muting, monitoring, complex gain controls, and pseudo-stereo. A conference control protocol and user signaling interface is implemented which makes the full power and flexibility of the bridge accessible, manageable, and comprehendable to users. A set of privacy principles is formulated and an enforcing mechanism is described in order to prevent abuse of the system. Two graphical user interfaces have been built, one providing centralized control of the bridge, the other integrated into the “Rapport” multimedia conferencing system. The bridge is controlled by a general purpose processor with both LAN and WAN interconnectivity, thus the bridge can be controlled by applications running elsewhere. It can also easily be programmed to implement new bridging services. Apart from the basic audio bridge described, the multimedia bridge which it is part of also performs data bridging, video bridging, and other audio bridging functions