1 <section id="vorbis-spec-comment">
4 $Id: 05-comment.xml,v 1.3 2002/10/25 14:41:44 giles Exp $
5 <emphasis>Last update to this document: July 16, 2002</emphasis>
8 <title>comment field and header specification</title>
11 <section><title>Overview</title>
13 <para>The Vorbis text comment header is the second (of three) header
14 packets that begin a Vorbis bitstream. It is meant for short text
15 comments, not arbitrary metadata; arbitrary metadata belongs in a
16 separate logical bitstream (usually an XML stream type) that provides
17 greater structure and machine parseability.</para>
19 <para>The comment field is meant to be used much like someone jotting a
20 quick note on the bottom of a CDR. It should be a little information to
21 remember the disc by and explain it to others; a short, to-the-point
22 text note that need not only be a couple words, but isn't going to be
23 more than a short paragraph. The essentials, in other words, whatever
24 they turn out to be, eg:
27 <simpara>Honest Bob and the Factory-to-Dealer-Incentives, <citetitle>I'm Still
28 Around</citetitle>, opening for Moxy Früvous, 1997.</simpara>
34 <section><title>Comment encoding</title>
36 <section><title>Structure</title>
39 The comment header is logically a list of eight-bit-clean vectors; the
40 number of vectors is bounded to 2^32-1 and the length of each vector
41 is limited to 2^32-1 bytes. The vector length is encoded; the vector
42 contents themselves are not null terminated. In addition to the vector
43 list, there is a single vector for vendor name (also 8 bit clean,
44 length encoded in 32 bits). The 1.0 release of libvorbis sets the
45 vendor string to "Xiph.Org libVorbis I 20020717".</para>
47 <para>The comment header is decoded as follows:
50 1) [vendor_length] = read an unsigned integer of 32 bits
51 2) [vendor_string] = read a UTF-8 vector as [vendor_length] octets
52 3) [user_comment_list_length] = read an unsigned integer of 32 bits
53 4) iterate [user_comment_list_length] times {
54 5) [length] = read an unsigned integer of 32 bits
55 6) this iteration's user comment = read a UTF-8 vector as [length] octets
57 7) [framing_bit] = read a single bit as boolean
58 8) if ( [framing_bit] unset or end-of-packet ) then ERROR
65 <section><title>Content vector format</title>
68 The comment vectors are structured similarly to a UNIX environment variable.
69 That is, comment fields consist of a field name and a corresponding value and
72 <blockquote><programlisting>
73 comment[0]="ARTIST=me";
74 comment[1]="TITLE=the sound of Vorbis";
75 </programlisting></blockquote>
78 The field name is case-insensitive and may consist of ASCII 0x20
79 through 0x7D, 0x3D ('=') excluded. ASCII 0x41 through 0x5A inclusive
80 (characters A-Z) is to be considered equivalent to ASCII 0x61 through
81 0x7A inclusive (characters a-z).
85 The field name is immediately followed by ASCII 0x3D ('=');
86 this equals sign is used to terminate the field name.
90 0x3D is followed by 8 bit clean UTF-8 encoded value of the
91 field contents to the end of the field.
94 <section><title>Field names</title>
96 <para>Below is a proposed, minimal list of standard filed names with a
97 description of intended use. No single or group of field names is
98 mandatory; a comment header may contain one, all or none of the names
105 <simpara>Track/Work name</simpara>
111 <simpara>The version field may be used to
112 differentiate multiple
113 versions of the same track title in a single collection. (e.g. remix
115 </simpara></listitem>
118 <varlistentry><term>ALBUM</term><listitem><simpara>The collection name to which this track belongs
119 </simpara></listitem>
122 <varlistentry><term>TRACKNUMBER</term>
123 <listitem><simpara>The track number of this piece if part of a specific larger collection or album
124 </simpara></listitem>
127 <varlistentry><term>ARTIST</term>
128 <listitem><simpara>The artist generally considered responsible for the work. In popular music this is usually the performing band or singer. For classical music it would be the composer. For an audio book it would be the author of the original text.
129 </simpara></listitem>
132 <varlistentry><term>PERFORMER</term>
133 <listitem><simpara>The artist(s) who performed the work. In classical music this would be the conductor, orchestra, soloists. In an audio book it would be the actor who did the reading. In popular music this is typically the same as the ARTIST and is omitted.
134 </simpara></listitem>
137 <varlistentry><term>COPYRIGHT</term>
138 <listitem><simpara>Copyright attribution, e.g., '2001 Nobody's Band' or '1999 Jack Moffitt'
139 </simpara></listitem>
142 <varlistentry><term>LICENSE</term>
143 <listitem><simpara>License information, eg, 'All Rights Reserved', 'Any
144 Use Permitted', a URL to a license such as a Creative Commons license
145 ("www.creativecommons.org/blahblah/license.html") or the EFF Open
146 Audio License ('distributed under the terms of the Open Audio
147 License. see http://www.eff.org/IP/Open_licenses/eff_oal.html for
149 </simpara></listitem>
152 <varlistentry><term>ORGANIZATION</term>
153 <listitem><simpara>Name of the organization producing the track (i.e.
155 </simpara></listitem>
158 <varlistentry><term>DESCRIPTION</term>
159 <listitem><simpara>A short text description of the contents
160 </simpara></listitem>
163 <varlistentry><term>GENRE</term>
164 <listitem><simpara>A short text indication of music genre
165 </simpara></listitem>
168 <varlistentry><term>DATE</term>
169 <listitem><simpara>Date the track was recorded
170 </simpara></listitem>
173 <varlistentry><term>LOCATION</term>
174 <listitem><simpara>Location where track was recorded
175 </simpara></listitem>
178 <varlistentry><term>CONTACT</term>
179 <listitem><simpara>Contact information for the creators or distributors of the track. This could be a URL, an email address, the physical address of the producing label.
180 </simpara></listitem>
183 <varlistentry><term>ISRC</term>
184 <listitem><simpara>International Standard Recording Code for the
186 url="http://www.ifpi.org/site-content/online/isrc_intro.html">the ISRC
187 intro page</ulink> for more information on ISRC numbers.
188 </simpara></listitem>
195 <section><title>Implications</title>
197 <para>Field names should not be 'internationalized'; this is a
198 concession to simplicity not an attempt to exclude the majority of
199 the world that doesn't speak English. Field <emphasis>contents</emphasis>
200 however, use the UTF-8 character encoding to allow easy representation of any
203 <para>We have the length of the entirety of the field and restrictions on
204 the field name so that the field name is bounded in a known way. Thus
205 we also have the length of the field contents.</para>
207 <para>Individual 'vendors' may use non-standard field names within
208 reason. The proper use of comment fields should be clear through
209 context at this point. Abuse will be discouraged.</para>
211 <para>There is no vendor-specific prefix to 'nonstandard' field names.
212 Vendors should make some effort to avoid arbitrarily polluting the
213 common namespace. We will generally collect the more useful tags
214 here to help with standardization.</para>
216 <para>Field names are not required to be unique (occur once) within a
217 comment header. As an example, assume a track was recorded by three
218 well know artists; the following is permissible, and encouraged:
220 <blockquote><programlisting>
221 ARTIST=Dizzy Gillespie
224 </programlisting></blockquote>
232 <section><title>Encoding</title>
235 The comment header comprises the entirety of the second bitstream
236 header packet. Unlike the first bitstream header packet, it is not
237 generally the only packet on the second page and may not be restricted
238 to within the second bitstream page. The length of the comment header
239 packet is (practically) unbounded. The comment header packet is not
240 optional; it must be present in the bitstream even if it is
241 effectively empty.</para>
244 The comment header is encoded as follows (as per Ogg's standard
245 bitstream mapping which renders least-significant-bit of the word to be
246 coded into the least significant available bit of the current
247 bitstream octet first):
251 Vendor string length (32 bit unsigned quantity specifying number of octets)
252 </simpara></listitem>
254 Vendor string ([vendor string length] octets coded from beginning of string to end of string, not null terminated)
255 </simpara></listitem>
257 Number of comment fields (32 bit unsigned quantity specifying number of fields)
258 </simpara></listitem>
260 Comment field 0 length (if [Number of comment fields]>0; 32 bit unsigned quantity specifying number of octets)
261 </simpara></listitem>
263 Comment field 0 ([Comment field 0 length] octets coded from beginning of string to end of string, not null terminated)
264 </simpara></listitem>
266 Comment field 1 length (if [Number of comment fields]>1...)...
267 </simpara></listitem>
272 This is actually somewhat easier to describe in code; implementation of the above can be found in <filename>vorbis/lib/info.c</filename>, <function>_vorbis_pack_comment()</function> and <function>_vorbis_unpack_comment()</function>.