diff --git a/doc/ietf/draft-valin-celt-rtp-profile.xml b/doc/ietf/draft-valin-celt-rtp-profile.xml index 718d434f..b037050d 100644 --- a/doc/ietf/draft-valin-celt-rtp-profile.xml +++ b/doc/ietf/draft-valin-celt-rtp-profile.xml @@ -618,6 +618,36 @@ mapping. +
+ +A low-overhead mode is defined to make more efficient use of bandwidth +when transmitting CELT frames. In that mode none of the length values need to be transmitted. +One the a=fmtp: parameter low-overhead: +is defined and contains a single frame size, followed by a '/', followed by the number +of frames (per channel) per packet, followed by a '/', followed by a comma-separated +list of the number of bytes per frame for each stream defined in the channel mapping. +The frame-size: parameter MUST not be specified and SHOULD be ignored if encountered +in an SDP offer or answer. The ptime:, maxptime: and b=AS: parameters SHOULD also be +ignored since the low-overhead: parameter makes them redundant. When the low-overhead: +parameter is specified, the length of each frame MUST NOT be encoded in the payload and +the bit-rate MUST NOT be changed during the session. + + + +For example a low-overhead surround configuration could be signaled as: + +m=audio 8008 RTP/AVP 97 +a=ptime: 5 +a=rtpmap:97 CELT/48000/6 +a=fmtp:97 low-overhead=256/1/86,86,43,30;mapping=2,2,1,1/L,R,LR,RR,C,MLFE/ITU-RBS.775-1 + + +In this example, 4 bytes per packet would be saved. This corresponds to a 6 kbit/s +reduction in the overhead, although the 60 kbit/s overhead of the IP, UDP and RTP +headers is still present. + + +