RealMedia

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Revision as of 06:23, 22 February 2006 by 205.201.57.114 (talk)
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  • Extensions: rm, ra
  • Company: Real

Multimedia container format developed by Real and used almost exclusively for codecs developed by Real.

The old .ra files are just for audio. The newer RealMedia (.rm) files are for audio and video.

RA Format

RealAudio 1.0 file:

This is from the very first version of RealAudio (1994). These files can only contain 8kbps VSELP audio data. A FourCC (lpcJ) may be present, but it is ignored.

bytes 0-3  '.', 'r', 'a', 0xfd
bytes 4-5  0x00, 0x03
bytes 6-7  Pointer to start of compressed audio data
bytes 8-21 Unknown
bytes 22+:
 Title string, preceded by a length byte
 Author string, preceded by a length byte
 Copyright string, preceded by a length byte
 Comment string, preceded by a length byte
 ...
 Audio data

RealAudio 2.0 file:

This is second version of the RealAudio file format. It is distinguished from the above by the value in byte 5 (=0x04). This type of file must contain a valid FourCC to identify the audio codec.

bytes 0-3   '.', 'r', 'a', 0xfd
bytes 4-7   0x00, 0x04, 0x00, 0x00
bytes 8-11  ".ra4"
bytes 12-15 Data size (??)
bytes 16-17 Version (??)
bytes 18-21 Header size (??)
bytes 22-23 Codec flavor
bytes 24-27 Coded Frame Size
bytes 28-41 ???
bytes 42-43 Frame Size
bytes 44-45 Sub Packet Size
bytes 46-47 ??
bytes 48-49 Sample rate
bytes 50-53 ??
bytes 54-55 Channels
bytes 56+   Type and FourCC

Version '5':

Same as above, except byte 5 = 0x05 and there are 6 more bytes after 'sub packet size', and the FourCC is not preceeded by a length byte.

RealMedia Format

This is the newer format which stores both audio and video. All multi-byte numbers are stored in big endian format.

A RealMedia file consists of a series of chunks. Each chunk has the following format:

bytes 0-3    chunk type (FOURCC)
bytes 4-7    chunk size, including 8-byte preamble
bytes n..    chunk payload

Real chunk types:

  • .RMF: RealMedia file header
  • PROP: File properties
  • MDPR: Stream properties
  • CONT: Content description/metadata
  • DATA: File data
  • INDX: File index
  • '.','r','a',0xFD: Legacy RealAudio encapsulation
  • VIDO

Codecs

Codecs in RealMedia are identified by the following four character codes:

Audio
Video
  • rv10 - H.263
  • rv13 - H.263
  • rv20 - H.263+
  • rv30 - Unknown format
  • rv40 - Unknown format