RFC 3119 (rfc3119) - Page 2 of 19

A More Loss-Tolerant RTP Payload Format for MP3 Audio

RFC 3119     Loss-Tolerant RTP Payload Format for MP3 Audio    June 2001


2. The Structure of MP3 Frames

   In this section we give a brief overview of the structure of a MP3
   frame.  (For more detailed description, see the MPEG 1 audio [3] and
   MPEG 2 audio [4] specifications.)

   Each MPEG audio frame begins with a 4-byte header.  Information
   defined by this header includes:

   -  Whether the audio is MPEG 1 or MPEG 2.
   -  Whether the audio is layer I, II, or III.
      (The remainder of this document assumes layer III, i.e., "MP3"
      frames)
   -  Whether the audio is mono or stereo.
   -  Whether or not there is a 2-byte CRC field following the header.
   -  (indirectly) The size of the frame.

   The following structures appear after the header:

   -  (optionally) A 2-byte CRC field
   -  A "side info" structure.  This has the following length:
      -  32 bytes for MPEG 1 stereo
      -  17 bytes for MPEG 1 mono, or for MPEG 2 stereo
      -  9 bytes for MPEG 2 mono
   -  Encoded audio data, plus optional ancillary data (filling out the
      rest of the frame)

   For the purpose of this document, the "side info" structure is the
   most important, because it defines the location and size of the
   "Application Data Unit" (ADU) that an MP3 decoder will process.  In
   particular, the "side info" structure defines:

   -  "main_data_begin": This is a back-pointer (in bytes) to the start
      of the ADU.  The back-pointer is counted from the beginning of the
      frame, and counts only encoded audio data and any ancillary data
      (i.e., ignoring any header, CRC, or "side info" fields).

   An MP3 decoder processes each ADU independently.  The ADUs will
   generally vary in length, but their average length will, of course,
   be that of the of the MP3 frames (minus the length of the header,
   CRC, and "side info" fields).  (In MPEG literature, this ADU is
   sometimes referred to as a "bit reservoir".)









Finlayson                   Standards Track