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Replace these with versions built using an older toolset (so as to avoid ABI compatibilities when linking on Ubuntu 14.04 for packaging purposes)
author Chris Cannam
date Fri, 07 Feb 2020 11:51:13 +0000
parents 05aa0afa9217
children
rev   line source
Chris@1 1 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
Chris@1 2 <html>
Chris@1 3 <head>
Chris@1 4
Chris@1 5 <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-15"/>
Chris@1 6 <title>Ogg Vorbis Documentation</title>
Chris@1 7
Chris@1 8 <style type="text/css">
Chris@1 9 body {
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Chris@1 25 #xiphlogo {
Chris@1 26 margin: 30px 0 16px 0;
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Chris@1 28
Chris@1 29 #content p {
Chris@1 30 line-height: 1.4;
Chris@1 31 }
Chris@1 32
Chris@1 33 h1, h1 a, h2, h2 a, h3, h3 a {
Chris@1 34 font-weight: bold;
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Chris@1 36 margin: 1.3em 0 8px 0;
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Chris@1 39 h1 {
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Chris@1 51 li {
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Chris@1 53 }
Chris@1 54
Chris@1 55 #copyright {
Chris@1 56 margin-top: 30px;
Chris@1 57 line-height: 1.5em;
Chris@1 58 text-align: center;
Chris@1 59 font-size: .8em;
Chris@1 60 color: #888888;
Chris@1 61 clear: both;
Chris@1 62 }
Chris@1 63 </style>
Chris@1 64
Chris@1 65 </head>
Chris@1 66
Chris@1 67 <body>
Chris@1 68
Chris@1 69 <div id="xiphlogo">
Chris@1 70 <a href="http://www.xiph.org/"><img src="fish_xiph_org.png" alt="Fish Logo and Xiph.Org"/></a>
Chris@1 71 </div>
Chris@1 72
Chris@1 73 <h1>Ogg Vorbis I format specification: comment field and header specification</h1>
Chris@1 74
Chris@1 75 <h1>Overview</h1>
Chris@1 76
Chris@1 77 <p>The Vorbis text comment header is the second (of three) header
Chris@1 78 packets that begin a Vorbis bitstream. It is meant for short, text
Chris@1 79 comments, not arbitrary metadata; arbitrary metadata belongs in a
Chris@1 80 separate logical bitstream (usually an XML stream type) that provides
Chris@1 81 greater structure and machine parseability.</p>
Chris@1 82
Chris@1 83 <p>The comment field is meant to be used much like someone jotting a
Chris@1 84 quick note on the bottom of a CDR. It should be a little information to
Chris@1 85 remember the disc by and explain it to others; a short, to-the-point
Chris@1 86 text note that need not only be a couple words, but isn't going to be
Chris@1 87 more than a short paragraph. The essentials, in other words, whatever
Chris@1 88 they turn out to be, eg:</p>
Chris@1 89
Chris@1 90 <blockquote><p>
Chris@1 91 "Honest Bob and the Factory-to-Dealer-Incentives, _I'm Still Around_,
Chris@1 92 opening for Moxy Fr&uuml;vous, 1997"
Chris@1 93 </p></blockquote>
Chris@1 94
Chris@1 95 <h1>Comment encoding</h1>
Chris@1 96
Chris@1 97 <h2>Structure</h2>
Chris@1 98
Chris@1 99 <p>The comment header logically is a list of eight-bit-clean vectors; the
Chris@1 100 number of vectors is bounded to 2^32-1 and the length of each vector
Chris@1 101 is limited to 2^32-1 bytes. The vector length is encoded; the vector
Chris@1 102 contents themselves are not null terminated. In addition to the vector
Chris@1 103 list, there is a single vector for vendor name (also 8 bit clean,
Chris@1 104 length encoded in 32 bits). For example, the 1.0 release of libvorbis
Chris@1 105 set the vendor string to "Xiph.Org libVorbis I 20020717".</p>
Chris@1 106
Chris@1 107 <p>The comment header is decoded as follows:</p>
Chris@1 108
Chris@1 109 <pre>
Chris@1 110 1) [vendor_length] = read an unsigned integer of 32 bits
Chris@1 111 2) [vendor_string] = read a UTF-8 vector as [vendor_length] octets
Chris@1 112 3) [user_comment_list_length] = read an unsigned integer of 32 bits
Chris@1 113 4) iterate [user_comment_list_length] times {
Chris@1 114
Chris@1 115 5) [length] = read an unsigned integer of 32 bits
Chris@1 116 6) this iteration's user comment = read a UTF-8 vector as [length] octets
Chris@1 117
Chris@1 118 }
Chris@1 119
Chris@1 120 7) [framing_bit] = read a single bit as boolean
Chris@1 121 8) if ( [framing_bit] unset or end of packet ) then ERROR
Chris@1 122 9) done.
Chris@1 123 </pre>
Chris@1 124
Chris@1 125 <h2>Content vector format</h2>
Chris@1 126
Chris@1 127 <p>The comment vectors are structured similarly to a UNIX environment variable.
Chris@1 128 That is, comment fields consist of a field name and a corresponding value and
Chris@1 129 look like:</p>
Chris@1 130
Chris@1 131 <pre>
Chris@1 132 comment[0]="ARTIST=me";
Chris@1 133 comment[1]="TITLE=the sound of Vorbis";
Chris@1 134 </pre>
Chris@1 135
Chris@1 136 <ul>
Chris@1 137 <li>A case-insensitive field name that may consist of ASCII 0x20 through
Chris@1 138 0x7D, 0x3D ('=') excluded. ASCII 0x41 through 0x5A inclusive (A-Z) is
Chris@1 139 to be considered equivalent to ASCII 0x61 through 0x7A inclusive
Chris@1 140 (a-z).</li>
Chris@1 141 <li>The field name is immediately followed by ASCII 0x3D ('=');
Chris@1 142 this equals sign is used to terminate the field name.</li>
Chris@1 143 <li>0x3D is followed by the 8 bit clean UTF-8 encoded value of the
Chris@1 144 field contents to the end of the field.</li>
Chris@1 145 </ul>
Chris@1 146
Chris@1 147 <h3>Field names</h3>
Chris@1 148
Chris@1 149 <p>Below is a proposed, minimal list of standard field names with a
Chris@1 150 description of intended use. No single or group of field names is
Chris@1 151 mandatory; a comment header may contain one, all or none of the names
Chris@1 152 in this list.</p>
Chris@1 153
Chris@1 154 <dl>
Chris@1 155
Chris@1 156 <dt>TITLE</dt>
Chris@1 157 <dd>Track/Work name</dd>
Chris@1 158
Chris@1 159 <dt>VERSION</dt>
Chris@1 160 <dd>The version field may be used to differentiate multiple
Chris@1 161 versions of the same track title in a single collection.
Chris@1 162 (e.g. remix info)</dd>
Chris@1 163
Chris@1 164 <dt>ALBUM</dt>
Chris@1 165 <dd>The collection name to which this track belongs</dd>
Chris@1 166
Chris@1 167 <dt>TRACKNUMBER</dt>
Chris@1 168 <dd>The track number of this piece if part of a specific larger collection or album</dd>
Chris@1 169
Chris@1 170 <dt>ARTIST</dt>
Chris@1 171 <dd>The artist generally considered responsible for the work. In popular music
Chris@1 172 this is usually the performing band or singer. For classical music it would be
Chris@1 173 the composer. For an audio book it would be the author of the original text.</dd>
Chris@1 174
Chris@1 175 <dt>PERFORMER</dt>
Chris@1 176 <dd>The artist(s) who performed the work. In classical music this would be the
Chris@1 177 conductor, orchestra, soloists. In an audio book it would be the actor who did
Chris@1 178 the reading. In popular music this is typically the same as the ARTIST and
Chris@1 179 is omitted.</dd>
Chris@1 180
Chris@1 181 <dt>COPYRIGHT</dt>
Chris@1 182 <dd>Copyright attribution, e.g., '2001 Nobody's Band' or '1999 Jack Moffitt'</dd>
Chris@1 183
Chris@1 184 <dt>LICENSE</dt>
Chris@1 185 <dd>License information, eg, 'All Rights Reserved', 'Any
Chris@1 186 Use Permitted', a URL to a license such as a Creative Commons license
Chris@1 187 ("www.creativecommons.org/blahblah/license.html") or the EFF Open
Chris@1 188 Audio License ('distributed under the terms of the Open Audio
Chris@1 189 License. see http://www.eff.org/IP/Open_licenses/eff_oal.html for
Chris@1 190 details'), etc.</dd>
Chris@1 191
Chris@1 192 <dt>ORGANIZATION</dt>
Chris@1 193 <dd>Name of the organization producing the track (i.e.
Chris@1 194 the 'record label')</dd>
Chris@1 195
Chris@1 196 <dt>DESCRIPTION</dt>
Chris@1 197 <dd>A short text description of the contents</dd>
Chris@1 198
Chris@1 199 <dt>GENRE</dt>
Chris@1 200 <dd>A short text indication of music genre</dd>
Chris@1 201
Chris@1 202 <dt>DATE</dt>
Chris@1 203 <dd>Date the track was recorded</dd>
Chris@1 204
Chris@1 205 <dt>LOCATION</dt>
Chris@1 206 <dd>Location where track was recorded</dd>
Chris@1 207
Chris@1 208 <dt>CONTACT</dt>
Chris@1 209 <dd>Contact information for the creators or distributors of the track.
Chris@1 210 This could be a URL, an email address, the physical address of
Chris@1 211 the producing label.</dd>
Chris@1 212
Chris@1 213 <dt>ISRC</dt>
Chris@1 214 <dd>ISRC number for the track; see <a href="http://www.ifpi.org/isrc/">the
Chris@1 215 ISRC intro page</a> for more information on ISRC numbers.</dd>
Chris@1 216
Chris@1 217 </dl>
Chris@1 218
Chris@1 219 <h3>Implications</h3>
Chris@1 220
Chris@1 221 <ul>
Chris@1 222 <li>Field names should not be 'internationalized'; this is a
Chris@1 223 concession to simplicity not an attempt to exclude the majority of
Chris@1 224 the world that doesn't speak English. Field <emph>contents</emph>,
Chris@1 225 however, use the UTF-8 character encoding to allow easy representation
Chris@1 226 of any language.</li>
Chris@1 227 <li>We have the length of the entirety of the field and restrictions on
Chris@1 228 the field name so that the field name is bounded in a known way. Thus
Chris@1 229 we also have the length of the field contents.</li>
Chris@1 230 <li>Individual 'vendors' may use non-standard field names within
Chris@1 231 reason. The proper use of comment fields should be clear through
Chris@1 232 context at this point. Abuse will be discouraged.</li>
Chris@1 233 <li>There is no vendor-specific prefix to 'nonstandard' field names.
Chris@1 234 Vendors should make some effort to avoid arbitrarily polluting the
Chris@1 235 common namespace. We will generally collect the more useful tags
Chris@1 236 here to help with standardization.</li>
Chris@1 237 <li>Field names are not required to be unique (occur once) within a
Chris@1 238 comment header. As an example, assume a track was recorded by three
Chris@1 239 well know artists; the following is permissible, and encouraged:
Chris@1 240 <pre>
Chris@1 241 ARTIST=Dizzy Gillespie
Chris@1 242 ARTIST=Sonny Rollins
Chris@1 243 ARTIST=Sonny Stitt
Chris@1 244 </pre></li>
Chris@1 245 </ul>
Chris@1 246
Chris@1 247 <h2>Encoding</h2>
Chris@1 248
Chris@1 249 <p>The comment header comprises the entirety of the second bitstream
Chris@1 250 header packet. Unlike the first bitstream header packet, it is not
Chris@1 251 generally the only packet on the second page and may not be restricted
Chris@1 252 to within the second bitstream page. The length of the comment header
Chris@1 253 packet is (practically) unbounded. The comment header packet is not
Chris@1 254 optional; it must be present in the bitstream even if it is
Chris@1 255 effectively empty.</p>
Chris@1 256
Chris@1 257 <p>The comment header is encoded as follows (as per Ogg's standard
Chris@1 258 bitstream mapping which renders least-significant-bit of the word to be
Chris@1 259 coded into the least significant available bit of the current
Chris@1 260 bitstream octet first):</p>
Chris@1 261
Chris@1 262 <ol>
Chris@1 263 <li>Vendor string length (32 bit unsigned quantity specifying number of octets)</li>
Chris@1 264 <li>Vendor string ([vendor string length] octets coded from beginning of string
Chris@1 265 to end of string, not null terminated)</li>
Chris@1 266 <li>Number of comment fields (32 bit unsigned quantity specifying number of fields)</li>
Chris@1 267 <li>Comment field 0 length (if [Number of comment fields]>0; 32 bit unsigned
Chris@1 268 quantity specifying number of octets)</li>
Chris@1 269 <li>Comment field 0 ([Comment field 0 length] octets coded from beginning of
Chris@1 270 string to end of string, not null terminated)</li>
Chris@1 271 <li>Comment field 1 length (if [Number of comment fields]>1...)...</li>
Chris@1 272 </ol>
Chris@1 273
Chris@1 274 <p>This is actually somewhat easier to describe in code; implementation of the above
Chris@1 275 can be found in vorbis/lib/info.c:_vorbis_pack_comment(),_vorbis_unpack_comment()</p>
Chris@1 276
Chris@1 277 <div id="copyright">
Chris@1 278 The Xiph Fish Logo is a
Chris@1 279 trademark (&trade;) of Xiph.Org.<br/>
Chris@1 280
Chris@1 281 These pages &copy; 1994 - 2005 Xiph.Org. All rights reserved.
Chris@1 282 </div>
Chris@1 283
Chris@1 284 </body>
Chris@1 285 </html>