m (it's a draft)
Revision as of 17:03, 14 September 2007
Schemes such as MDMF require an embedding in physical Ogg streams. This page is for development of a specification for embedding metadata streams in Ogg. The final version will probably look like the CMML mapping.
(taken from mailing list discussions)
Should we have a magic number for this? By convention the beginning of stream packet for codecs in Ogg identifies the packet, Strictly speaking we will have a magic number regardless, but should it be:
- '<?xml' the opening of the XML declaration. In this instance the demuxer would pass this upwards to an XML parser which would devide from the rest of the bos packet what to do with it.
- Some other sequence before the XML starets, identifying this as a metadata stream, probably with a version number which will imply the contents to be some form of XML. This may avoid the almost inevitable problem of some implentor assuming '<?xml' is always metadata, it may also reduce the difficulty of writing a demuxer.
c.f. Silvia Pfeiffer on ogg-dev:
I suggest using the solution that CMML has come to use. The XML file is essentially the same as an unencapsulated physical bitstream. Then there is a mapping into a logical bitstream, where some of the default information - in particular the XML header - are split off and put into the bos packet - nothing really needs to go into the eos packet. There's also a magic number and a version number. Also, use the granulepos scheme that we defined for CMML pages- you're going to make your lives easier.
It is a barrier to the widespread introduction of any metadata format that the Vorbis I spec only requires players to support an unaccompanied Vorbis stream; many Ogg Vorbis players will refuse to play augmented streams, especially if the content is not recognised (although many recent players do succeed). As a prelude to development of an Ogg metadata format it will be necessary to encourage developers to introduce more flexible Ogg filters.
To help with testing the following files are available, based on a speculative (and very basic) metadata format. In each case the derivative files are under the same license as the original. Two sets are provided to allow chained stream testing. On some players the seek tests produce an annoying clicking—if you like the music get the originals. Please notice that filenames are mixed case and add a note in discussion if you find a broken link.
- The Ogg-Vorbis-XML version
- The XML/RDF description as a separate document
- With the XML page repeated after every fifth Vorbis page. (This is not a suggested way to add meta data, just a way of testing how players handle seeking in the presence of an unknown stream.)
- With the XML page repeated after every fifth Vorbis page and the stream ending on a meta data page (breaks simpler track-length strategies, again not a suggested format for metadata)