RealMedia: Difference between revisions
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This is the old audio-only RealAudio file format. | This is the old audio-only RealAudio file format. | ||
A very similar structure is also used to describe audio streams in RM files. | A very similar structure is also used to describe audio streams in RM files. | ||
The audio data part is just a stream of bytes with no structure. | |||
There is no index in .ra files. | |||
=== RealAudio 1.0 file (.ra version 3) === | === RealAudio 1.0 file (.ra version 3) === |
Revision as of 14:20, 26 February 2006
- Extensions: rm, ra, rmvb
- 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
This is the old audio-only RealAudio file format. A very similar structure is also used to describe audio streams in RM files.
The audio data part is just a stream of bytes with no structure. There is no index in .ra files.
RealAudio 1.0 file (.ra version 3)
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.
byte[4] Header signature ('.', 'r', 'a', 0xfd) word Version (always 3) word Header size - 7 byte[10] Unknown dword Data size byte Title string length byte[] Title string byte Author string length byte[] Author string byte Copyright string length byte[] Copyright string byte Comment string length byte[] Comment string byte Unknown * byte Fourcc string length (always 4) * byte[] Fourcc string (always "lpcJ") * Audio data
Notes:
- Fields marked with * may be missing. Based on the only known sample with no FourCC, it's assumed that all these fields are either present or missing.
- The informative fields (title, author, copyright and comment) can have zero length.
RealAudio 2.0 file (.ra version 4)
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.
Possible fourcc values are 28_8, dnet and sipr.
byte[4] Header signature ('.', 'r', 'a', 0xfd) word Version (always 4) word Unused (always 0) byte[4] ra4 signature (always ".ra4") dword Data size - 0x27 word Version2 (always equal to version) dword Header size - 16 word Codec flavor dword Coded Frame Size byte[12] Unknown word Sub packet h word Frame Size word Sub Packet Size word Unknown word Sample rate word Unknown word Sample size word Channels byte genr string length (always 4) byte[] genr string byte Fourcc string length (always 4) byte[] Fourcc string byte[3] Unknown byte Title string length byte[] Title string byte Author string length byte[] Author string byte Copyright string length byte[] Copyright string byte Comment string length byte[] Comment string Audio Data
Notes:
- The 0x27 in data size is the size of the fixed-length part of the header (up to channels).
- The informative fields (title, author, copyright and comment) can have zero length.
.ra version 5
While .ra header can have version 5, there are no known RealAudio files with this format, and it's not known if they really exist.
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
- lpcJ - RealAudio 1.0 (VSELP)
- 28_8 - RealAudio 2.0 (LD-CELP)
- dnet - AC3
- sipr - Sipro
- cook - Cook
- atrc - ATRAC
- ralf - RealAudio Lossless Format
- raac - LC-AAC
- racp - HE-AAC