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Ogg Vorbis comment field specificationLast update to this document: October 2, 2001
The text comment header is the second (of three) header packets that begin a Vorbis bitstream. It is meant for short, text comments, not arbitrary metadata; arbitrary metadata belongs in a separate logical bitream (usually an XML stream type) that provides greater structure and machine parseability.
Comment use rationaleThe comment field is meant to be used much like someone jotting a quick note on the bottom of a CDR. It should be a little information to remember the disc by and explain it to others; a short, to-the-point text note that need not only be a couple words, but isn't going to be more than a short paragraph. The essentials, in other words, whatever they turn out to be, eg:
"Honest Bob and the Factory-to-Dealer-Incentives, _I'm Still Around_, opening for Moxy Fruvous, 1997"
StructureThe comment header logically is a list of eight-bit-clean vectors; the number of vectors is bounded to 2^32-1 and the length of each vector is limited to 2^32-1 bytes. The vector length is encoded; the vector is not null terminated. In addition to the vector list, there is a single vector for vendor name (also 8 bit clean, length encoded in 32 bits). Libvorbis currently sets the vendor string to "Xiphophorus libVorbis I 20010910".
The comment vectors are structured similarly to a UNIX environment variable. That is, comment fields consist of a field name and a field value and look like:
comment="ARTIST=me"; comment="TITLE=the sound of vorbis";
Content vector format
Field namesBelow is a proposed, minimal list of standard filed names with a description of intended use. No single or group of field names is mandatory; a comment header may contain one, all or none of the names in this list.
EncodingThe comment header comprises the entirety of the second bitstream header packet. Unlike the first bitstream header packet, it is not generally the only packet on the second page and may not be restricted to within the second bitstream page. The length of the comment header packet is [practically] unbounded. The comment header packet is not optional; it must be present in the bitstream even if it is effectively empty.
The comment header is encoded as follows (as per Ogg's standard bitstream mapping which renders least-significant-bit of the word to be coded into the least significant available bit of the current bitstream octet first):
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Ogg Vorbis is the first Ogg audio CODEC. Anyone may freely use and distribute the Ogg and Vorbis specification, whether in a private, public or corporate capacity. However, Xiphophorus and the Ogg project (xiph.org) reserve the right to set the Ogg/Vorbis specification and certify specification compliance.
Xiphophorus's Vorbis software CODEC implementation is distributed under the Lesser/Library GNU Public License. This does not restrict third parties from distributing independent implementations of Vorbis software under other licenses.
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