rfc3640.txt

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3.3.  Usage of this Specification

3.3.1.  General

   Usage of this specification requires definition of a mode.  A mode
   defines how to use this specification, as deemed appropriate.
   Senders MUST signal the applied mode via the MIME format parameter
   "mode", as specified in section 4.1.  This specification defines a
   generic mode that can be used for any MPEG-4 stream, as well as
   specific modes for the transportation of MPEG-4 CELP and MPEG-4 AAC
   streams, defined in ISO/IEC 14496-3 [1].

   When use of this payload format is signaled using SDP [5], an
   "rtpmap" attribute is part of that signaling.  The same requirements
   apply for the rtpmap attribute in any mode compliant to this
   specification.  The general form of an rtpmap attribute is:

   a=rtpmap:<payload type> <encoding name>/<clock rate>[/<encoding
             parameters>]

   For audio streams, <encoding parameters> specifies the number of
   audio channels: 2 for stereo material (see RFC 2327 [5]) and 1 for
   mono.  Provided no additional parameters are needed, this parameter
   may be omitted for mono material, hence its default value is 1.




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3.3.2.  The Generic Mode

   The generic mode can be used for any MPEG-4 stream.  In this mode, no
   mode-specific constraints are applied; hence, in the generic mode,
   the full flexibility of this specification can be exploited.  The
   generic mode is signaled by mode=generic.

   An example is given below for the transportation of a BIFS-Anim
   stream.  In this example carriage of multiple BIFS-Anim Access Units
   is allowed in one RTP packet.  The AU-header contains the AU-size
   field, the CTS-flag and, if the CTS flag is set to 1, the CTS-delta
   field.  The number of bits of the AU-size and the CTS-delta fields
   are 10 and 16, respectively.  The AU-header also contains the RAP-
   flag and the Stream-state of 4 bits.  This results in an AU-header
   with a total size of two or four octets per BIFS-Anim AU.  The RTP
   time stamp uses a 1 kHz clock.  Note that the media type name is
   video, because the BIFS-Anim stream is part of an audio-visual
   presentation.  For conventions on media type names, see section 4.1.

   In detail:

   m=video 49230 RTP/AVP 96
   a=rtpmap:96 mpeg4-generic/1000
   a=fmtp:96 streamtype=3; profile-level-id=1807; mode=generic;
   objectType=2; config=0842237F24001FB400094002C0; sizeLength=10;
   CTSDeltaLength=16; randomAccessIndication=1;
   streamStateIndication=4

   Note: The a=fmtp line has been wrapped to fit the page, it comprises
   a single line in the SDP file.

   The hexadecimal value of the "config" parameter is the
   BIFSConfiguration() as defined in ISO/IEC 14496-1.  The
   BIFSConfiguration() specifies that the BIFS stream is a BIFS-Anim
   stream.  For the description of MIME parameters, see section 4.1.

3.3.3.  Constant Bit-rate CELP

   This mode is signaled by mode=CELP-cbr.  In this mode, one or more
   complete CELP frames of fixed size can be transported in one RTP
   packet; interleaving MUST NOT be used with this mode.  The RTP
   payload consists of one or more concatenated CELP frames, each of
   equal size.  CELP frames MUST NOT be fragmented when using this mode.
   Both the AU Header Section and the Auxiliary Section MUST be empty.

   The MIME format parameter constantSize MUST be provided to specify
   the length of each CELP frame.




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   For example:

   m=audio 49230 RTP/AVP 96
   a=rtpmap:96 mpeg4-generic/16000/1
   a=fmtp:96 streamtype=5; profile-level-id=14; mode=CELP-cbr; config=
   440E00; constantSize=27; constantDuration=240

   Note: The a=fmtp line has been wrapped to fit the page, it comprises
   a single line in the SDP file.

   The hexadecimal value of the "config" parameter is the
   AudioSpecificConfig()as defined in ISO/IEC 14496-3.
   AudioSpecificConfig() specifies a mono CELP stream with a sampling
   rate of 16 kHz at a fixed bitrate of 14.4 kb/s and 6 sub-frames per
   CELP frame.  For the description of MIME parameters, see section 4.1.

3.3.4.  Variable Bit-rate CELP

   This mode is signaled by mode=CELP-vbr.  With this mode, one or more
   complete CELP frames of variable size can be transported in one RTP
   packet with OPTIONAL interleaving.  In this mode, the largest
   possible value for AU-size is greater than the maximum CELP frame
   size. Because CELP frames are very small, there is no support for
   fragmentation of CELP frames.  Hence, CELP frames MUST NOT be
   fragmented when using this mode.

   In this mode, the RTP payload consists of the AU Header Section,
   followed by one or more concatenated CELP frames.  The Auxiliary
   Section MUST be empty.  For each CELP frame contained in the payload,
   there MUST be a one octet AU-header in the AU Header Section to
   provide:

   a) the size of each CELP frame in the payload and

   b) index information for computing the sequence (and hence timing) of
      each CELP frame.

   Transport of CELP frames requires that the AU-size field be coded
   with 6 bits.  Therefore, in this mode 6 bits are allocated to the
   AU-size field, and 2 bits to the AU-Index(-delta) field.  Each AU-
   Index field MUST be coded with the value 0.  In the AU Header
   Section, the concatenated AU-headers are preceded by the 16-bit AU-
   headers-length field, as specified in section 3.2.1.

   In addition to the required MIME format parameters, the following
   parameters MUST be present: sizeLength, indexLength, and
   indexDeltaLength.  CELP frames always have a fixed duration per
   Access Unit; when interleaving in this mode, this specific duration



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   MUST be signaled by the MIME format parameter constantDuration.  In
   addition, the parameter maxDisplacement MUST be present when
   interleaving.

   For example:

   m=audio 49230 RTP/AVP 96
   a=rtpmap:96 mpeg4-generic/16000/1
   a=fmtp:96 streamtype=5; profile-level-id=14; mode=CELP-vbr; config=
   440F20; sizeLength=6; indexLength=2; indexDeltaLength=2;
   constantDuration=160; maxDisplacement=5

   Note: The a=fmtp line has been wrapped to fit the page; it comprises
   a single line in the SDP file.

   The hexadecimal value of the "config" parameter is the
   AudioSpecificConfig() as defined in ISO/IEC 14496-3.
   AudioSpecificConfig() specifies a mono CELP stream with a sampling
   rate of 16 kHz, at a bitrate that varies between 13.9 and 16.2 kb/s
   and with 4 sub-frames per CELP frame.  For the description of MIME
   parameters, see section 4.1.

3.3.5.  Low Bit-rate AAC

   This mode is signaled by mode=AAC-lbr.  This mode supports the
   transportation of one or more complete AAC frames of variable size.
   In this mode, the AAC frames are allowed to be interleaved and hence
   receivers MUST support de-interleaving.  The maximum size of an AAC
   frame in this mode is 63 octets.  AAC frames MUST NOT be fragmented
   when using this mode.  Hence, when using this mode, encoders MUST
   ensure that the size of each AAC frame is at most 63 octets.

   The payload configuration in this mode is the same as in the variable
   bit-rate CELP mode as defined in 3.3.4.  The RTP payload consists of
   the AU Header Section, followed by concatenated AAC frames.  The
   Auxiliary Section MUST be empty.  For each AAC frame contained in the
   payload, the one octet AU-header MUST provide:

   a) the size of each AAC frame in the payload and

   b) index information for computing the sequence (and hence timing) of
      each AAC frame.

   In the AU-header Section, the concatenated AU-headers MUST be
   preceded by the 16-bit AU-headers-length field, as specified in
   section 3.2.1.





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   In addition to the required MIME format parameters, the following
   parameters MUST be present: sizeLength, indexLength, and
   indexDeltaLength.  AAC frames always have a fixed duration per Access
   Unit; when interleaving in this mode, this specific duration MUST be
   signaled by the MIME format parameter constantDuration.  In addition,
   the parameter maxDisplacement MUST be present when interleaving.

   For example:

   m=audio 49230 RTP/AVP 96
   a=rtpmap:96 mpeg4-generic/22050/1
   a=fmtp:96 streamtype=5; profile-level-id=14; mode=AAC-lbr; config=
   1388; sizeLength=6; indexLength=2; indexDeltaLength=2;
   constantDuration=1024; maxDisplacement=5

   Note: The a=fmtp line has been wrapped to fit the page; it comprises
   a single line in the SDP file.

   The hexadecimal value of the "config" parameter is the
   AudioSpecificConfig(), as defined in ISO/IEC 14496-3.
   AudioSpecificConfig() specifies a mono AAC stream with a sampling
   rate of 22.05 kHz.  For the description of MIME parameters, see
   section 4.1.

3.3.6.  High Bit-rate AAC

   This mode is signaled by mode=AAC-hbr.  This mode supports the
   transportation of variable size AAC frames.  In one RTP packet,
   either one or more complete AAC frames are carried, or a single
   fragment of an AAC frame is carried.  In this mode, the AAC frames
   are allowed to be interleaved and hence receivers MUST support de-
   interleaving.  The maximum size of an AAC frame in this mode is 8191
   octets.

   In this mode, the RTP payload consists of the AU Header Section,
   followed by either one AAC frame, several concatenated AAC frames or
   one fragmented AAC frame.  The Auxiliary Section MUST be empty.  For
   each AAC frame contained in the payload, there MUST be an AU-header
   in the AU Header Section to provide:

   a) the size of each AAC frame in the payload and

   b) index information for computing the sequence (and hence timing) of
      each AAC frame.

   To code the maximum size of an AAC frame requires 13 bits.
   Therefore, in this configuration 13 bits are allocated to the AU-
   size, and 3 bits to the AU-Index(-delta) field.  Thus, each AU-header



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   has a size of 2 octets.  Each AU-Index field MUST be coded with the
   value 0.  In the AU Header Section, the concatenated AU-headers MUST
   be preceded by the 16-bit AU-headers-length field, as specified in
   section 3.2.1.

   In addition to the required MIME format parameters, the following
   parameters MUST be present: sizeLength, indexLength, and
   indexDeltaLength.  AAC frames always have a fixed duration per Access
   Unit; when interleaving in this mode, this specific duration MUST be
   signaled by the MIME format parameter constantDuration.  In addition,
   the parameter maxDisplacement MUST be present when interleaving.

   For example:

   m=audio 49230 RTP/AVP 96
   a=rtpmap:96 mpeg4-generic/48000/6
   a=fmtp:96 streamtype=5; profile-level-id=16; mode=AAC-hbr;
   config=11B0; sizeLength=13; indexLength=3;
   indexDeltaLength=3; constantDuration=1024

   Note: The a=fmtp line has been wrapped to fit the page; it comprises
   a single line in the SDP file.

   The hexadecimal value of the "config" parameter is the
   AudioSpecificConfig(), as defined in ISO/IEC 14496-3.
   AudioSpecificConfig() specifies a 5.1 channel AAC stream with a
   sampling rate of 48 kHz.  For t