RFC 2658 (rfc2658) - Page 2 of 10

RTP Payload Format for PureVoice(tm) Audio

RFC 2658       RTP Payload Format for PureVoice(tm) Audio    August 1999


   The Qcelp CODEC [1] compresses each 20 milliseconds of 8000 Hz, 16-
   bit sampled input speech into one of four different size output
   frames:  Rate 1 (266 bits), Rate 1/2 (124 bits), Rate 1/4 (54 bits)
   or Rate 1/8 (20 bits).  The CODEC chooses the output frame rate based
   on analysis of the input speech and the current operating mode
   (either normal or reduced rate).  For typical speech patterns, this
   results in an average output of 6.8 k bits/sec for normal mode and
   4.7 k bits/sec for reduced rate mode.

3 RTP/Qcelp Packet Format

   The RTP timestamp is in 1/8000 of a second units.  The RTP payload
   data for the Qcelp CODEC has the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      RTP Header [2]                           |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |RR | LLL | NNN |                                               |
   +-+-+-+-+-+-+-+-+       one or more codec data frames           |
   |                             ....                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The RTP header has the expected values as described in [2].  The
   extension bit is not set and this payload type never sets the marker
   bit.  The codec data frames are aligned on octet boundaries.  When
   interleaving is in use and/or multiple codec data frames are present
   in a single RTP packet, the timestamp is, as always, that of the
   oldest data represented in the RTP packet.  The other fields have the
   following meaning:

   Reserved (RR): 2 bits
      MUST be set to zero by sender, SHOULD be ignored by receiver.

   Interleave (LLL): 3 bits
      MUST have a value between 0 and 5 inclusive.  The remaining two
      values (6 and 7) MUST not be used by senders.  If this field is
      non-zero, interleaving is enabled.  All receivers MUST support
      interleaving.  Senders MAY support interleaving.  Senders that do
      not support interleaving MUST set field LLL and NNN to zero.

   Interleave Index (NNN): 3 bits
      MUST have a value less than or equal to the value of LLL.  Values
      of NNN greater than the value of LLL are invalid.






K. McKay                    Standards Track