Ogg Vorbis comment field specification

Last update to this document: February 9, 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 metadata stream (usually an XML stream type).

Comment use rationale

The 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"


The 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 20010119".

The comment vectors are structured similarly to a UNIX environment. That is, comment fields consist of a field name and a field value and look like:

comment[1]="TITLE=the sound of vorbis"; 

Content vector format

Field names

Below 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.
Track name
The version field may be used to differentiate multiple version of the same track title in a single collection. (e.g. remix info)
The collection name to which this track belongs
The track number of this piece if part of a specific larger collection or album
Track performer
Name of the organization producing the track (i.e. the 'record label')
A short text description of the contents
A short text indication of music genre
Date the track was recorded
Location where track was recorded
Copyright information
ISRC number for the track; see the ISRC intro page for more information on ISRC numbers.



The 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):

  1. Vendor string length (32 bit unsigned quantity specifying number of octets)
  2. Vendor string ([vendor string length] octets coded from beginning of string to end of string, not null terminated)
  3. Number of comment fields (32 bit unsigned quantity specifying number of fields)
  4. Comment field 0 length (if [Number of comment fields]>0; 32 bit unsigned quantity specifying number of octets)
  5. Comment field 0 ([Comment field 0 length] octets coded from beginning of string to end of string, not null terminated)
  6. Comment field 1 length (if [Number of comment fields]>1...)...
This is actually somewhat easier to describe in code; implementation of the above can be found in vorbis/lib/info.c:_vorbis_pack_comment(),_vorbis_unpack_comment()
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