Oggless

Abstract

This page documents how to embed the different Xiph codecs like Vorbis or Theora in containers other than Ogg. It's still in the drafting stage, and most of it was taken from a version that was previously a part of the NUT Subversion repository. Said document was originally authored by Michael Niedermayer.

Current Version

• 2007-07-21

Status

Information on how to convert losslessly from one format to another is missing, as is Ogg timestamping. Regardless, it is mostly finished and likely won't be updated further.

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.

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 of storing 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 fourcc 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 separable data unit in the container.

Codec Identifier:

Xiph codec	fourcc 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, the fourcc 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 applications 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 set up in a way 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:

• Vorbisacm
• 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. This should be used for storing the 3 headers.

Note that 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 there is 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 do they provide the necessary packet separation / framing. Storing Xiph codecs in them is thus 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.