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128 lines
5.6 KiB
Plaintext
128 lines
5.6 KiB
Plaintext
Title Embedding xiph codecs like vorbis in containers other then ogg
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Version 2006-07-30 (draft)
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Status this is not a standard or otherwise accepted by xiph or any other
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group, one day when we have a fully working implementation, did
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enough testing and so on we might submit it to IETF? to become an
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RFC ...
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furthermore this document has been submitted to vorbis-dev and
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so far has been ignored, maybe they where too busy maybe xiph
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wants to prevent their open codecs from being used in containers
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other then their own?
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Author Michael Niedermayer (michaelni at gmx dot at)
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License GPL + GFDL + anything neeeded to turn this into a open standard
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like a RFC
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Minimum container requirments:
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This appendix only explains how to store xiph codecs in containers which
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support at least one global header per stream, can seperate individual codec
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packets and in principle support the codec, so for example in the case of
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vorbis that would be variable bitrate and variable number of samples/packet
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Storage in other containers is outside the scope of this appendix
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FIXME non vorbis
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Global header:
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If the container can store 3 headers per stream in an unambiguos and ordered
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way then they shall be stored in that way, if OTOH the container is only
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capable to store a single global header then the 3 codec headers shall be
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concatenated without any additional header, footer or seperator between them
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to recover the 3 headers from such a global header the following procedure
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shall be used:
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1) search for the 1st occurance of 01,'v','o','r','b','i','s'
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the found match and the following 23 bytes are the 1st header packet
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2) search for the 1st occurance of 03,'v','o','r','b','i','s' after here
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3) read an unsigned integer of 32 bits and skip that many bytes
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4) [user_comment_list_length] = read an unsigned integer of 32 bits
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5) iterate [user_comment_list_length] times {
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6) read an unsigned integer of 32 bits and skip that many bytes
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}
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7) skip 1 byte
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8) the match in 2) and what follows until here is the 2nd header packet
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9) search for the 1st occurance of 05,'v','o','r','b','i','s' after here
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the matching part and what follows is the 3rd header packet
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if the container needs an identifer for the global header, for example a 4cc
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for a global header chunk then glbl shall be used
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Storing packets:
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Each codec packet shall be stored in exactly one "container packet"
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and one "container packet" must not contain more then one codec packet
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"container packet" here means the smallest seperateable unit of data in the
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container
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Codec Identifer:
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xiph-codec 4-cc id long id
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Vorbis vrbs vorbis
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Theora ther theora
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Tarkin trkn tarkin
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Flac flac flac
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Speex spex speex
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if the container uses 4-character codes 4-cc identifer from the table above
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shall be used
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if the container uses arbitrary length strings as identifers then the long
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id from the table above shall be used
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Examples and Disscussions about specific containers
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What follows are some notes about specific containers, these notes are just
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informative as they just repeat what is written above or in the
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specification of the specific container
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Example and Disscussion of the avi container
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avi supports everything needed to store vorbis, this does not mean that all
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application will support vorbis in avi as vorbis is rather different from
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other audio codecs commonly stored in avi ...
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avi supports a single global header like wav does, the 3 vorbis headers
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shall be stored in it and only in it as described above
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dwSampleSize must be set to zero as vorbis is vbr, many applications do
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this incorrectly for other vbr codecs and consequently vbr audio in avi
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becomes problematic
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avi does not have timestamps but each chunk has a constant duration, while
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vorbis packets can have one of 2 durations, if now the avi header is setup
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so that each avi chunk has the same duration as the smaller duration of
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the 2 possibilities in vorbis then simply inserting empty avi chunks will
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allow every avi chunk to have the correct duration, this is of course
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not the most beautifull solution but it is the only way to keep things
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exact, additionally note, that empty chunks have been used since ages
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in avi to lengthen the duration of video chunks
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Example and Disscussion of the asf container
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asf supports a single global header per stream and has timestamps so
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storing xiph codecs in it should be possible but asf is patented and
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microsoft has already threatened individuals so we strongly urge you to
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avoid this container
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Example and Disscussion of the matroska container
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matroska supports storing 3 headers using a codec specific
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format, which should be used for storing the 3 headers
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Note, the above procedure to split one header into 3 works with the
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vorbis-matroska specific format too
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Example and Disscussion of the nut container
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nut supports a single global header per stream so the 1<->3 merge/split
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procedure above must be used, except that theres nothing special with
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storing xiph codecs in nut
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Example and Disscussion of mpeg-ps / mpeg-ts container
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These containers neither support a global header nor provide the neccessary
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packet seperation / framing, so storing xiph codecs in them is outside the
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scope of this appendix
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Example and Disscussion of wav container
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wav does not provide the neccessary packet seperation / framing, so storing
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xiph codecs in it is outside the scope of this appendix
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Example and Disscussion of the mov container
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a single glbl atom shall be placed in the stsd atom in which the the global
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header shall be stored
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