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The text below is supposed to be a start point for discussions and is ATM identical to [1]

Contents

Abstract

Embedding xiph codecs like vorbis in containers other than ogg

Current Version

  • 2007-05-23

Status

This is a proposal being discussed. It's currently not ready to be used on real-life situations.


Minimum container requirements

This appendix only explains how to store xiph codecs in containers which support at least one global header per stream, can separate individual codec packets and in principle support the codec, so for example in the case of vorbis that would be variable bitrate and variable number of samples/packet Storage in other containers is outside the scope of this appendix


FIXME non vorbis/theora

Global header:

If the container can store 3 headers per stream in an unambiguous and ordered way then they shall be stored in that way, if OTOH the container is only capable to store a single global header then the 3 codec headers shall be concatenated without any additional header, footer or separator between them to recover the 3 headers from such a global header the following procedure shall be used:

1) search for the 1st occurrence of ID1 the found match and the following ID1_LEN bytes are the 1st header packet
2) search for the 1st occurrence of ID2 after here
3) read an unsigned integer of 32 bits and skip that many bytes
4) [user_comment_list_length] = read an unsigned integer of 32 bits
5) iterate [user_comment_list_length] times {
6) read an unsigned integer of 32 bits and skip that many bytes
}
7) skip 1 byte
8) the match in 2) and what follows until here is the 2nd header packet
9) search for the 1st occurrence of ID3 after here the matching part and what follows is the 3rd header packet

Constants:

Codec ID1 ID2 ID3 ID1_LEN
Vorbis 0x01,'v','o','r','b','i','s' 0x03,'v','o','r','b','i','s' 0x05,'v','o','r','b','i','s' 23
Theora 0x80,'t','h','e','o','r','a' 0x81,'t','h','e','o','r','a' 0x82,'t','h','e','o','r','a' 35

if the container needs an identifier for the global header, for example a 4cc for a global header chunk then glbl shall be used


Storing packets:

Each codec packet shall be stored in exactly one "container packet" and one "container packet" must not contain more then one codec packet "container packet" here means the smallest separatable unit of data in the container


Codec Identifier:

xiph-codec 4-cc id long id
Vorbis vrbs vorbis
Theora ther theora
Tarkin trkn tarkin
Flac flac flac
Speex spex speex

if the container uses 4-character codes 4-cc identifier from the table above shall be used if the container uses arbitrary length strings as identifiers then the long id from the table above shall be used


Examples and Discussions about specific containers

What follows are some notes about specific containers, these notes are just informative as they just repeat what is written above or in the specification of the specific container


Example and Discussion of the avi container

avi supports everything needed to store vorbis, this does not mean that all application will support vorbis in avi as vorbis is rather different from other audio codecs commonly stored in avi ... avi supports a single global header like wav does, the 3 vorbis headers shall be stored in it and only in it as described above dwSampleSize must be set to zero as vorbis is vbr, many applications do this incorrectly for other vbr codecs and consequently vbr audio in avi becomes problematic avi does not have timestamps but each chunk has a constant duration, while vorbis packets can have one of 2 durations, if now the avi header is setup so that each avi chunk has the same duration as the smaller duration of the 2 possibilities in vorbis then simply inserting empty avi chunks will allow every avi chunk to have the correct duration, this is of course not the most beautiful solution but it is the only way to keep things exact, additionally note, that empty chunks have been used since ages in avi to lengthen the duration of video chunks


Example and Discussion of the asf container

asf supports a single global header per stream and has timestamps so storing xiph codecs in it should be possible but asf is patented and microsoft has already threatened individuals so we strongly urge you to avoid this container


Example and Discussion of the matroska container

matroska supports storing 3 headers using a codec specific format, which should be used for storing the 3 headers Note, the above procedure to split one header into 3 works with the vorbis-matroska specific format too


Example and Discussion of the nut container

nut supports a single global header per stream so the 1<->3 merge/split procedure above must be used, except that theres nothing special with storing xiph codecs in nut


Example and Discussion of mpeg-ps / mpeg-ts container

These containers neither support a global header nor provide the neccessary packet separation / framing, so storing xiph codecs in them is outside the scope of this appendix


Example and Discussion of wav container

wav does not provide the necessary packet separation / framing, so storing xiph codecs in it is outside the scope of this appendix


Example and Discussion of the mov container

a single glbl atom shall be placed in the stsd atom in which the the global header shall be stored

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