Oggless

2007-06-04T21:54:40Z

Seelie: /* Example and Discussion of the avi container */

The text below is supposed to be a start point for discussions and is ATM identical to [http://svn.mplayerhq.hu/nut/docs/oggless-xiph-codecs.txt?view=log]

==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

Some Links:
* [http://svn.xiph.org/tags/vorbisacm_20020708/ vorbisacm]
* [http://www.alexander-noe.com/video/documentation/avi.pdf AVI File Format] (PDF), 5.7 (p.21) VFR Audio - Storing Vorbis in AVI

====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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