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low-overhead mode

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Jean-Marc Valin 2009-03-06 10:42:07 -05:00
parent 595f809862
commit ee1c9c52e2
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doc/ietf/draft-valin-celt-rtp-profile.xml
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@ -618,6 +618,36 @@ mapping.
</section>
<section anchor="Low-Overhead Mode" title="Low-Overhead Mode">
<t>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.
</t>
<t>
For example a low-overhead surround configuration could be signaled as:
<list style="empty">
<t>m=audio 8008 RTP/AVP 97</t>
<t>a=ptime: 5</t>
<t>a=rtpmap:97 CELT/48000/6</t>
<t>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</t>
</list>
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.
</t>
</section>
<section anchor="RTP Payload Types" title="RTP Payload Types">