Audio Mixer

Within a conference, the audio mixer is responsible for selecting the input streams and summing these streams into a mixed output stream. This section provides a detailed view into the various modules that comprise it.

The audio mixer is the core component in the media plane. It is responsible for selecting incoming audio streams, summing them, and distributing the summed output back to the participants.

When mixing audio streams in a large conference, the audio mixer selects only a subset of the input streams; typically, the mixer selects three or four of the loudest streams for summation. The reason is because the human ear is capable of differentiating between only three or four distinct talkers. The mixer discards the remaining audio streams.

The mixer may also include an event reporting mechanism, allowing it to communicate internal events to other system components. For example, a conferencing system component might need to display on a web interface which participants are currently speaking. Another event of interest is media streaming failure detection. Media failure events can notify the signaling stack to initiate a call disconnection.

In addition to creating an output stream containing the three or four loudest participants, the audio mixer must also satisfy another requirement: participants who are included in the mix should not hear their own audio in the mix. Some delay occurs in the summation and return of the composite audio, and therefore, participants in the mix could potentially hear a delayed echo of their own streams played back to them, after mixing.

To avoid self-echo, each endpoint that contributes a stream for the audio mix receives a unique output stream, which does not contain audio from the same participant.

This approach is referred to as N-1 summation, where N is the number of mixed streams, and the stream returned to a conferee is the summation of the mixed streams, minus the stream contributed by that individual.

Figure 2-5, which illustrates the basic components of an audio conferencing system, shows an example in which the mixer has four input streams: stream 1, stream 2, stream 3, and stream 4. Streams 1, 2, and 3 have been selected for summation, but stream 4 has not. Figure 2-5 illustrates how endpoints that have streams selected for summation (mixing) receive a special mix in which the audio from their own stream is omitted. For the device assigned to steam 1, the mixed stream is the summation of stream 2 and steam 3. For stream 2, the mixed stream is the summation of stream 1 and stream 3, and so on. For stream 4, which is not contributing to the mix, the return stream is made up of all three contributing streams (that is, steam 1, stream 2, and stream 3).

Figure 2-5 Audio Mixer Block Diagram

Stream 1 Stream 2 Stream 3 Stream 4

Figure 2-5 Audio Mixer Block Diagram

Stream 1 Stream 2 Stream 3 Stream 4

The following sections describe the components of the audio mixer block. Network (IP/UDP) Module

The User Datagram Protocol (UDP) stack is responsible for sending and receiving RTP packets to and from the remote devices. Each incoming stream requires one socket. An endpoint signals the desired address and port for an incoming stream to the remote endpoint during the initial call setup. For H.323, the endpoint includes this information in the H.245 signaling. For SIP, the endpoint lists this information in the SDP part (offer/answer) of the SIP signaling.

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