Ogg Vorbis comment field specification

Last update to this document: January 19, 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"

Structure

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

Content vector format

A case-insensitive field name that may consist of ASCII 0x20 through 0x7D, 0x3D ('=') excluded. ASCII 0x41 through 0x5A inclusive (A-Z) is to be considered equivalent to ASCII 0x61 through 0x7A inclusive (a-z).
The field name is immediately followed by ASCII 0x3D ('='); this equals sign is used to terminate the field name.
0x3D is followed by 8 bit clean UTF-8 field contents to the end of the field.

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.

TITLE: Track name
VERSION: The version field may be used to differentiate multiple version of the same track title in a single collection. (e.g. remix info)
ALBUM: The collection name to which this track belongs
TRACKNUMBER: The track number of this piece if part of a specific larger collection or album
ARTIST: Track performer
ORGANIZATION: Name of the organization producing the track (i.e. the 'record label')
DESCRIPTION: A short text description of the contents
GENRE: A short text indication of music genre
DATE: Date the track was recorded
LOCATION: Location where track was recorded
COPYRIGHT: Copyright information

Implications

Field names should not be 'internationalized'; this is a concession to simplicity not an attempt to exclude the majority of the world that doesn't speak English. Field *contents*, however, are represented in UTF-8 to allow easy representation of any language.
We have the length of the entirety of the field and restrictions on the field name so that the field name is bounded in a known way. Thus we also have the length of the field contents.
Individual 'vendors' may use non-standard field names within reason. The proper use of comment fields should be clear through context at this point. Abuse will be discouraged.
There is no vendor-specific prefix to 'nonstandard' field names. Vendors should make some effort to avoid arbitrarily polluting the common namespace.
Field names are not required to be unique (occur once) within a comment header. As an example, assume a track was recorded by three well know artists; the following is permissible, and encouraged:
```
              ARTIST=Dizzy Gillespie 
              ARTIST=Sonny Rollins 
              ARTIST=Sonny Stitt 
```

Encoding

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):

Vendor string length (32 bit unsigned quantity specifying number of octets)
Vendor string ([vendor string length] octets coded from beginning of string to end of string, not null terminated)
Number of comment fields (32 bit unsigned quantity specifying number of fields)
Comment field 0 length (if [Number of comment fields]>0; 32 bit unsigned quantity specifying number of octets)
Comment field 0 ([Comment field 0 length] octets coded from beginning of string to end of string, not null terminated)
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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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.