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34 Dialogic
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Multimedia Software for ATCA Release 1.0 Release Update, Rev 06 — January 23, 2008
Dialogic Corporation
• “Octet-aligned,” to make the packet parsing easier for the AMR application.
The AMR RTP payload format is specified in the new IPM_AUDIO_CODER_OPTIONS_INFO
AudioCoderOptionsInfo structure (in the IPM_MEDIA structure). The
MEDIATYPE_AUDIO_LOCAL_CODER_OPTIONS_INFO media type controls the receive side,
and the MEDIATYPE_AUDIO_REMOTE_CODER_OPTIONS_INFO media type controls the
transmit side.
The IPM_MEDIA structure contains a union with an IPM_AUDIO_CODER_OPTIONS_INFO
structure. The AMR RTP payload format is controlled by ORing in either
CODER_OPT_AMR_EFFICIENT or CODER_OPT_AMR_OCTET in the
AudioCoderOptionsInfo.unCoderOptions field.
Note: Specifying an AMR RTP payload is mandatory and the formats are mutually exclusive.
Example 1
In this example, the host application wants to transmit at the bit rate requested by the incoming
CMR value. The following diagram depicts this use case, where the local side is the host
application.
Note: While the diagram above implies an immediate reaction to a CMR from the other side, in reality,
the other side’s response to a CMR may take a few packets.
The sequence of activities is as follows:
1. The host is transmitting at the preferred bit rate set by the host application via the remote audio
coder settings, in this case 7.4 kbit/s.
2. The host sets its CMR value to 7.95 kbit/s via the local audio coder setting.
3. The host sets its RTP payload to efficient bandwidth and CMR tracking mode.
4. The host receives its first packet from the remote side with the CMR value higher than the
preferred rate.
5. The host changes the transmitted bit rate to match the CMR value.
Local Side Remote Side
FT=4 (7.4 kbit/s), CMR=5 (7.95 kbit/s)
FT=5 (7.95 kbit/s), CMR=6 (10.2 kbit/s)
FT=6 (10.2 kbt/s), CMR=5 (7.95 kbit/s)