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==Motivation==
This page aims to give an overview of the current state of metadata in Ogg and the ongoing projects towards improving it. The different components work in concert; for example [[Ogg Skeleton]] provides important infrastructure for [[CMML]], [[VorbisComment]] is simple to use and program, while the draft [[M3F|Multimedia Metadata Format (M3F)]] provides more sophisticated information.
 
== [[VorbisComment]]s ==


All the Xiph.org codecs have some internal mechanism for including metadata about the current stream.
All the Xiph.org codecs have some internal mechanism for including metadata about the current stream.
Generally, this is one of the codec headers, and in the words of the [http://www.xiph.org/ogg/vorbis/doc/v-comment.html vorbis spec],  
Generally, this is one of the codec headers, and in the words of the [http://www.xiph.org/vorbis/doc/v-comment.html vorbis spec],  
"It is meant for short, text comments...much like someone jotting a quick note on the bottom of a CDR."
"It is meant for short, text comments ... much like someone jotting a quick note on the bottom of a CDR." A single VorbisComment can store upto 2^64 bytes (16 exabytes).
 
VorbisComments store metadata describing the stream in key=value pairs, such as "ARTIST=Elvis", "TITLE=Blue Suede Shoes". Multiple copies of any given key are allowed (for example you can specify ARTIST several times for multiple performers). The specification has several suggested keys: TITLE, VERSION, ALBUM, TRACKNUMBER, ARTIST, PERFORMER, COPYRIGHT, LICENSE, ORGANIZATION, DESCRIPTION, GENRE, DATE, LOCATION, CONTACT, ISRC. See the [http://www.xiph.org/vorbis/doc/v-comment.html specification] for the intent of each one.
 
The [[VorbisComment]] page contains improvements to the suggested comment set.
 
== [[FLAC]] metadata blocks ==
 
Metadata is included in the FLAC codec as METADATA_BLOCK_DATA. Seven types of metadata block are defined: 
#''METADATA_BLOCK_STREAMINFO'': Sample rate, number of channels, etc.
#''METADATA_BLOCK_PADDING'': Nul padding.
#''METADATA_BLOCK_APPLICATION'': Third-party applications can register an ID. Metadata is typically 32-bit integers, but any datatypes can be specified.
#''METADATA_BLOCK_SEEKTABLE'': For one or more seek points.
#''METADATA_BLOCK_VORBIS_COMMENT'': Also known as FLAC tags, the contents of a VorbisComment packet. Note that the 32-bit field lengths are little-endian coded according to the Vorbis spec, as opposed to the usual big-endian coding of fixed-length integers in the rest of FLAC. FLAC metadata blocks are limited to 2^24 bytes (16 megabytes) and a VorbisComment packet in FLAC must fit within that limit.
#''METADATA_BLOCK_CUESHEET'': Typically, but not necessarily, for CD-DA (Red Book) cuesheets.
#''METADATA_BLOCK_PICTURE'': For binary picture data.


This works well enough for most things, and can be overloaded/abused (depending on your point of view)
== [[Ogg Skeleton]] ==
for most other things. But there are three major requirements that point to the design of an external
metadata format; one that can be interleaved with the other streams in a container.


* ''Machinability'' There are a number of items of metadata that a player will want to parse and take action on. While there are usually 'convention' schemes for doing this with the embedded comment headers, this is much easier if there is a separate metadata stream designed for such use, instead of having to do best-effort parsing of natural language comments. For example, a video file with multiple audio tracks can specify the language of each one; a player than can parse these reliably can match them against a language preference list configured by the user to automatically select and begin playback of the best option.
[[Ogg Skeleton]] provides metadata useful for handling Ogg streams. This includes information like mime-types and mapping for granulepos which allows seeking streams without the need for the demuxer to understand them. The latest version, [[Ogg Skeleton 4]], also provides a keyframe index to enable faster seeking over high latency networks.


* ''Kitchen Sink'' There are a minority of people who care passionately about having every detail about a track available. In the sense of conserving such information, and providing an equivalent to liner notes for online distribution, this is a goal worth supporting. However, the simple unstructured key-value pairs offered by the inline metadata are unwieldy for this level of detail. How do you tell the 2nd unit Assistant Director from the USA unit Assistant Director? How do you indicate which artist played tenor sax in the solo?
Ogg Skeleton allows for attachment of message header fields, given as name-value pairs, that contain some sort of protocol messages about the logical bitstream. This is intended for decode related stuff, such as the screen size for a video bitstream or the number of channels for an audio bitstream.


* ''Addressability'' The internal comment metadata headers are by necessity attached to a single content stream. This is useful for some appication, but a limitation in others. In a multiplexed stream, which set of comments refers to the collection as a whole? (By convention, in Ogg, it's the first logical bitstream occuring, but we can do better.) A separate metadata stream type must address this issue of collective metadata while still allowing description of individual streams. It should also allow temporal addressability, so that changes can be described. Because the in-stream comment metadata are part of the codec headers, it cannot change over the course of the stream, and allowing additional comment packets elsewhere in the stream presents seeking challenges. In the Ogg container this can be resolved by inserting a chain boundary, but this is a poor option for very-low-bitrate streams and unreliable transports such as RTP.
== [[OggKate]] ==


==Proposed Solutions==
[[OggKate]] was originally designed for karaoke and text. The stream can carry text and images, and these can be animated.


RDF + dublin core
== [[CMML]] (deprecated) ==


XML-encoding (generic rdf or CMML?)
CMML is not used anymore; use [[OggKate]] instead. The [[CMML|Continuous Media Markup Language]] allowed time-based marking up of media streams, at its simplest this allowed you to divide media files into clips and provide information about each clip.
binary encoding (three-component length-encoded binary vectors)
[http://www.shop263.com/i-1/1511.htm 二茂铁]
[http://www.shop263.com/i-1/1512.htm 二萘酚]
[http://www.shop263.com/i-1/1513.htm 二羟基苯甲酸]
[http://www.shop263.com/i-1/1514.htm 二羟基苯腈]
[http://www.shop263.com/i-1/1515.htm 二羟甲基丙酸]
[http://www.shop263.com/i-1/1516.htm 二羟甲基海因]
[http://www.shop263.com/i-1/1517.htm 二氢芳樟醇]
[http://www.shop263.com/i-1/1518.htm 二氢黄樟素]
[http://www.shop263.com/i-1/1519.htm 二氢月桂烯]
[http://www.shop263.com/i-1/1520.htm 二氢月桂烯醇]
[http://www.shop263.com/i-1/1521.htm 二巯基丙磺酸钠]
[http://www.shop263.com/i-1/1522.htm 二壬基萘磺酸钡]
[http://www.shop263.com/i-1/1523.htm 二壬基萘磺酸钡盐]
[http://www.shop263.com/i-1/1524.htm 二叔丁基过氧化]
[http://www.shop263.com/i-1/1525.htm 二叔丁基过氧化氢]
[http://www.shop263.com/i-1/1526.htm 二特丁基对苯二酚]
[http://www.shop263.com/i-1/1527.htm 二烯丙基胺]
[http://www.shop263.com/i-1/1528.htm 二硝基甲苯]
[http://www.shop263.com/i-1/1529.htm 二辛酯]
[http://www.shop263.com/i-1/1530.htm 二溴丁烷]
[http://www.shop263.com/i-1/1531.htm 二溴海因]
[http://www.shop263.com/i-1/1532.htm 二溴基氰乙酰胺]
[http://www.shop263.com/i-1/1533.htm 二盐基亚磷酸]
[http://www.shop263.com/i-1/1534.htm 二盐基亚磷酸铅]
[http://www.shop263.com/i-1/1535.htm 二氧丙烷]
[http://www.shop263.com/i-1/1536.htm 二氧化锆]
[http://www.shop263.com/i-1/1537.htm 二氧化硫脲]
[http://www.shop263.com/i-1/1538.htm 二氧化碳]
[http://www.shop263.com/i-1/1539.htm 二氧六环]
[http://www.shop263.com/i-1/1540.htm 二氧七环]
[http://www.shop263.com/i-1/1541.htm 二氧五环]
[http://www.shop263.com/i-1/1542.htm 二乙二醇二醋酸酯]
[http://www.shop263.com/i-1/1543.htm 二乙基氨基甲基丙烯酸乙酯]
[http://www.shop263.com/i-1/1544.htm 二乙基苯胺]
[http://www.shop263.com/i-1/1545.htm 二乙基二硫]
[http://www.shop263.com/i-1/1546.htm 二乙基羟胺]
[http://www.shop263.com/i-1/1547.htm 二乙醚]
[http://www.shop263.com/i-1/1548.htm 二乙烯三胺]
[http://www.shop263.com/i-1/1549.htm 二异丙醇胺]
[http://www.shop263.com/i-1/1550.htm 二异丙基苯胺]
[http://www.shop263.com/i-1/1551.htm 二异丙基酚]
[http://www.shop263.com/i-1/1552.htm 二异丁基酮]
[http://www.shop263.com/i-1/1553.htm 二异丁酯]
[http://www.shop263.com/i-1/1554.htm 二元醇醚类]
[http://www.shop263.com/i-1/1555.htm 二月桂酸]
[http://www.shop263.com/i-1/1556.htm 发光粉]
[http://www.shop263.com/i-1/1557.htm 发泡剂]
[http://www.shop263.com/i-1/1558.htm 发泡阻燃材料]
[http://www.shop263.com/i-1/1559.htm 发烟硫酸]
[http://www.shop263.com/i-1/1560.htm 钒锆黄]
[http://www.shop263.com/i-1/1561.htm 反丁烯二酸]
[http://www.shop263.com/i-1/1562.htm 反式对甲基环己甲]
[http://www.shop263.com/i-1/1563.htm 泛醇]
[http://www.shop263.com/i-1/1564.htm 泛解酸内酯]
[http://www.shop263.com/i-1/1565.htm 泛酸钙]
[http://www.shop263.com/i-1/1566.htm 芳基硅烷]
[http://www.shop263.com/i-1/1567.htm 芳烃溶剂油]
[http://www.shop263.com/i-1/1568.htm 芳香酸甲酯]
[http://www.shop263.com/i-1/1569.htm 芳樟醇]
[http://www.shop263.com/i-1/1570.htm 防白水]
[http://www.shop263.com/i-1/1571.htm 防腐油漆]
[http://www.shop263.com/i-1/1572.htm 防火堵料]
[http://www.shop263.com/i-1/1573.htm 防火涂料]
[http://www.shop263.com/i-1/1574.htm 防老剂]
[http://www.shop263.com/i-1/1575.htm 防霉剂]
[http://www.shop263.com/i-1/1576.htm 防水粘合材料]
[http://www.shop263.com/i-1/1577.htm 防锈蜡]
[http://www.shop263.com/i-1/1578.htm 仿陶瓷的花盆]
[http://www.shop263.com/i-1/1579.htm 纺丝油剂]
[http://www.shop263.com/i-1/1580.htm 纺织蜡]
[http://www.shop263.com/i-1/1581.htm 纺织印染助剂]
[http://www.shop263.com/i-1/1582.htm 纺织用油]
[http://www.shop263.com/i-1/1583.htm 非离子表面活性剂]
[http://www.shop263.com/i-1/1584.htm 非洛地平]
[http://www.shop263.com/i-1/1585.htm 非诺贝特]
[http://www.shop263.com/i-1/1586.htm 非诺贝特酸]
[http://www.shop263.com/i-1/1587.htm 菲醌]
[http://www.shop263.com/i-1/1588.htm 废硫酸]
[http://www.shop263.com/i-1/1589.htm 酚醛树脂]
[http://www.shop263.com/i-1/1590.htm 阳极电泳漆]
[http://www.shop263.com/i-1/1591.htm 封口蜡]
[http://www.shop263.com/i-1/1592.htm 蜂胶]
[http://www.shop263.com/i-1/1593.htm 蜂蜡]
[http://www.shop263.com/i-1/1594.htm 蜂王浆]
[http://www.shop263.com/i-1/1595.htm 呋喃]
[http://www.shop263.com/i-1/1596.htm 呋喃甲醇]
[http://www.shop263.com/i-1/1597.htm 呋喃甲醛]
[http://www.shop263.com/i-1/1598.htm 呋喃树脂]
[http://www.shop263.com/i-1/1599.htm 水合肼]
[http://www.shop263.com/i-1/1600.htm 水肌酸]
[http://www.shop263.com/i-1/1601.htm 水基清防蜡剂]
[http://www.shop263.com/i-1/1602.htm 水解聚丙烯腈]
[http://www.shop263.com/i-1/1603.htm 水晶]
[http://www.shop263.com/i-1/1604.htm 水晶蜡]
[http://www.shop263.com/i-1/1605.htm 水泥]
[http://www.shop263.com/i-1/1606.htm 水泥分散剂]
[http://www.shop263.com/i-1/1607.htm 水泥缓凝剂]
[http://www.shop263.com/i-1/1608.htm 水溶性氮酮]
[http://www.shop263.com/i-1/1609.htm 氧基硅烷]
[http://www.shop263.com/i-1/1610.htm 水溶性硅油]
[http://www.shop263.com/i-1/1611.htm 水乳剂]
[http://www.shop263.com/i-1/1612.htm 水性木]
[http://www.shop263.com/i-1/1613.htm 水性上光油]
[http://www.shop263.com/i-1/1614.htm 水性亚光型光油]
[http://www.shop263.com/i-1/1615.htm 水杨酸]
[http://www.shop263.com/i-1/1616.htm 水杨酸苯酯]
[http://www.shop263.com/i-1/1617.htm 水杨酸甲酯]
[http://www.shop263.com/i-1/1618.htm 水杨酸辛酯]
[http://www.shop263.com/i-1/1619.htm 水杨酰胺]
[http://www.shop263.com/i-1/1620.htm 顺丁胶]
[http://www.shop263.com/i-1/1621.htm 顺丁烯二酸酐]
[http://www.shop263.com/i-1/1622.htm 氟苯]
[http://www.shop263.com/i-1/1623.htm 氟苯基]
[http://www.shop263.com/i-1/1624.htm 氟代苯]
[http://www.shop263.com/i-1/1625.htm 氟代二苯甲酮]
[http://www.shop263.com/i-1/1626.htm 氟代烷基硅烷]
[http://www.shop263.com/i-1/1627.htm 氟锆酸钾]
[http://www.shop263.com/i-1/1628.htm 氟硅酸钠]
[http://www.shop263.com/i-1/1629.htm 氟化铵]
[http://www.shop263.com/i-1/1630.htm 氟化钙]
[http://www.shop263.com/i-1/1631.htm 氟化钾]
[http://www.shop263.com/i-1/1632.htm 氟化铝]
[http://www.shop263.com/i-1/1633.htm 氟化钠]
[http://www.shop263.com/i-1/1634.htm 氟化氢铵]
[http://www.shop263.com/i-1/1635.htm 氟化氢钾]
[http://www.shop263.com/i-1/1636.htm 氟化氢纳]
[http://www.shop263.com/i-1/1637.htm 氟化锶]
[http://www.shop263.com/i-1/1638.htm 氟磺酸]
[http://www.shop263.com/i-1/1639.htm 氟甲苯]
[http://www.shop263.com/i-1/1640.htm 氟尿嘧啶]
[http://www.shop263.com/i-1/1641.htm 氟硼酸钾]
[http://www.shop263.com/i-1/1642.htm 氟硼酸钠]
[http://www.shop263.com/i-1/1643.htm 氟西汀]
[http://www.shop263.com/i-1/1644.htm 氟硝基苯]
[http://www.shop263.com/i-1/1645.htm 氟乙酸甲酯]
[http://www.shop263.com/i-1/1646.htm 氟致冷剂]
[http://www.shop263.com/i-1/1647.htm 复合高效抗磨减摩剂]
[http://www.shop263.com/i-1/1648.htm 复合胶]
[http://www.shop263.com/i-1/1649.htm 复合聚酯膜]
[http://www.shop263.com/i-1/1650.htm 复合铅盐稳定剂]
[http://www.shop263.com/i-1/1651.htm 复混肥]
[http://www.shop263.com/i-1/1652.htm 复配型果蔬保鲜剂]
[http://www.shop263.com/i-1/1653.htm 富镧氯化稀土]
[http://www.shop263.com/i-1/1654.htm 富马酸]
[http://www.shop263.com/i-1/1655.htm 改性防沉剂]
[http://www.shop263.com/i-1/1656.htm 改性三聚]
[http://www.shop263.com/i-1/1657.htm 改质沥青]
[http://www.shop263.com/i-1/1658.htm 钙基膨润土]
[http://www.shop263.com/i-1/1659.htm 干酪素]
[http://www.shop263.com/i-1/1660.htm 干湿强剂]
[http://www.shop263.com/i-1/1661.htm 甘氨酸]
[http://www.shop263.com/i-1/1662.htm 甘氨酸钠]
[http://www.shop263.com/i-1/1663.htm 甘宝素]
[http://www.shop263.com/i-1/1664.htm 甘露醇]
[http://www.shop263.com/i-1/1665.htm 甘油]
[http://www.shop263.com/i-1/1666.htm 感光化学品]
[http://www.shop263.com/i-1/1667.htm 钢蜡]
[http://www.shop263.com/i-1/1668.htm 高补强白碳黑]
[http://www.shop263.com/i-1/1669.htm 高纯氨水]
[http://www.shop263.com/i-1/1670.htm 高纯硅酸钠溶液]
[http://www.shop263.com/i-1/1671.htm 高纯氢]
[http://www.shop263.com/i-1/1672.htm 高纯碳酸锶]
[http://www.shop263.com/i-1/1673.htm 高纯稀土]
[http://www.shop263.com/i-1/1674.htm 高沸点芳烃溶剂]
[http://www.shop263.com/i-1/1675.htm 高分子复合粘合剂]
[http://www.shop263.com/i-1/1676.htm 高光滑爽型水性光油]
[http://www.shop263.com/i-1/1677.htm 高光亮水性光油]
[http://www.shop263.com/i-1/1678.htm 高回弹泡沫]
[http://www.shop263.com/i-1/1679.htm 高氯化聚乙烯]
[http://www.shop263.com/i-1/1680.htm 高氯酸锂]
[http://www.shop263.com/i-1/1681.htm 高锰酸钾]
[http://www.shop263.com/i-1/1682.htm 高水溶性硫酸钾]
[http://www.shop263.com/i-1/1683.htm 高温导热油]
[http://www.shop263.com/i-1/1684.htm 高效活性聚合物驱油剂]
[http://www.shop263.com/i-1/1685.htm 高效节油剂]
[http://www.shop263.com/i-1/1686.htm 高效渗透促进剂]
[http://www.shop263.com/i-1/1687.htm 高效脱模剂]
[http://www.shop263.com/i-1/1688.htm 高沼气煤矿水胶炸药]
[http://www.shop263.com/i-1/1689.htm 锆氟酸铵]
[http://www.shop263.com/i-1/1690.htm 锆氟酸钾]
[http://www.shop263.com/i-1/1691.htm 锆刚玉]
[http://www.shop263.com/i-1/1692.htm 锆英粉]
[http://www.shop263.com/i-1/1693.htm 隔膜烧碱]
[http://www.shop263.com/i-1/1694.htm 铬矿]
[http://www.shop263.com/i-1/1695.htm 铬酸酐]
[http://www.shop263.com/i-1/1696.htm 铬酸锶]
[http://www.shop263.com/i-1/1697.htm 庚醇]
[http://www.shop263.com/i-1/1698.htm 庚醛]
[http://www.shop263.com/i-1/1699.htm 庚酸]
[http://www.shop263.com/i-1/1700.htm 庚酰氯]
[http://www.shop263.com/i-1/1701.htm 工程塑料]
[http://www.shop263.com/i-1/1702.htm 工程塑料母料]
[http://www.shop263.com/i-1/1703.htm 工业氨水]
[http://www.shop263.com/i-1/1704.htm 工业白油]
[http://www.shop263.com/i-1/1705.htm 工业冰醋酸]
[http://www.shop263.com/i-1/1706.htm 工业冰乙酸]
[http://www.shop263.com/i-1/1707.htm 工业硅酸铅]
[http://www.shop263.com/i-1/1708.htm 工业级戊二酸]
[http://www.shop263.com/i-1/1709.htm 工业甲基萘]
[http://www.shop263.com/i-1/1700.htm 工业磷酸]
[http://www.shop263.com/i-1/1711.htm 工业氯化铵]
[http://www.shop263.com/i-1/1712.htm 工业氯化钾]
[http://www.shop263.com/i-1/1713.htm 工业氯乙酸]
[http://www.shop263.com/i-1/1714.htm 工业清洗剂]
[http://www.shop263.com/i-1/1715.htm 工业水杨酸]
[http://www.shop263.com/i-1/1716.htm 工业涂料]
[http://www.shop263.com/i-1/1717.htm 工业硝酸钾]
[http://www.shop263.com/i-1/1718.htm 工业盐酸]
[http://www.shop263.com/i-1/1719.htm 工业乙酸酐]
[http://www.shop263.com/i-1/1720.htm 工业用甲醇]
[http://www.shop263.com/i-1/1721.htm 工业用油]
[http://www.shop263.com/i-1/1722.htm 工业用油酸]
[http://www.shop263.com/i-1/1723.htm 工艺品树脂]
[http://www.shop263.com/i-1/1724.htm 工艺品专用漆]
[http://www.shop263.com/i-1/1725.htm 功夫酸]
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== [[M3F]] (unused draft) ==


[[M3F|Multimedia Metadata Format]] is not being developed anymore; use [[VorbisComment]] instead.


The format was intended to replace VorbisComments for the use of ''structured'' metadata, allowing VorbisComments to revert to its orginally intended use of "short, text comments ... much like someone jotting a quick note on the bottom of a CDR."


[[M3F]] for the Ogg container was a draft specification which aimed to provide metadata for media streams. The exact aims of this project were still under development, but they included being able to describe artist relationships to a piece more accurately as well as providing the structure to encourage more reliable metadata.


== [[XMLEmbedding]] (unused draft) ==


To implement XML metadata in Ogg (as for [[M3F]]), a mapping to Ogg streams is needed. The use of XML metadata will also open the way for the inclusion of technologies such as:
* RDF + dublin core
* [http://www.adobe.com/products/xmp/ XMP]
* [http://wiki.musicbrainz.org/MusicBrainzXMLMetaData MusicBrainz]
* [http://www.w3.org/Graphics/SVG/ SVG]
This specification is still not past the discussion stage.


== Aims of advanced metadata ==


VorbisComments work well enough for most things, and can be overloaded/abused (depending on your point of view) for most other things. But there are three major requirements that point to the design of an external metadata format; one that can be interleaved with the other streams in a container.


* '''Machinability:''' There are a number of items of metadata that a player will want to parse and take action on. While there are usually 'convention' schemes for doing this with the embedded comment headers, this is much easier if there is a separate metadata stream designed for such use, instead of having to do best-effort parsing of natural language comments. For example, a video file with multiple audio tracks can specify the language of each one; a player than can parse these reliably can match them against a language preference list configured by the user to automatically select and begin playback of the best option.


* '''Kitchen Sink:''' There are a minority of people who care passionately about having every detail about a track available. In the sense of conserving such information, and providing an equivalent to liner notes for online distribution, this is a goal worth supporting. However, the simple unstructured key-value pairs offered by the inline metadata are unwieldy for this level of detail. How do you tell the 2nd unit Assistant Director from the USA unit Assistant Director? How do you indicate which artist played tenor sax in the solo?


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* '''Addressability:''' The internal comment metadata headers are by necessity attached to a single content stream. This is useful for some appication, but a limitation in others. In a multiplexed stream, which set of comments refers to the collection as a whole? (By convention, in Ogg, it's the first logical bitstream occuring, but we can do better.) A separate metadata stream type must address this issue of collective metadata while still allowing description of individual streams. It should also allow temporal addressability, so that changes can be described. Because the in-stream comment metadata are part of the codec headers, it cannot change over the course of the stream, and allowing additional comment packets elsewhere in the stream presents seeking challenges. In the Ogg container this can be resolved by inserting a chain boundary, but this is a poor option for very-low-bitrate streams and unreliable transports such as RTP.
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Latest revision as of 11:44, 24 July 2013

This page aims to give an overview of the current state of metadata in Ogg and the ongoing projects towards improving it. The different components work in concert; for example Ogg Skeleton provides important infrastructure for CMML, VorbisComment is simple to use and program, while the draft Multimedia Metadata Format (M3F) provides more sophisticated information.

VorbisComments

All the Xiph.org codecs have some internal mechanism for including metadata about the current stream. Generally, this is one of the codec headers, and in the words of the vorbis spec, "It is meant for short, text comments ... much like someone jotting a quick note on the bottom of a CDR." A single VorbisComment can store upto 2^64 bytes (16 exabytes).

VorbisComments store metadata describing the stream in key=value pairs, such as "ARTIST=Elvis", "TITLE=Blue Suede Shoes". Multiple copies of any given key are allowed (for example you can specify ARTIST several times for multiple performers). The specification has several suggested keys: TITLE, VERSION, ALBUM, TRACKNUMBER, ARTIST, PERFORMER, COPYRIGHT, LICENSE, ORGANIZATION, DESCRIPTION, GENRE, DATE, LOCATION, CONTACT, ISRC. See the specification for the intent of each one.

The VorbisComment page contains improvements to the suggested comment set.

FLAC metadata blocks

Metadata is included in the FLAC codec as METADATA_BLOCK_DATA. Seven types of metadata block are defined:

  1. METADATA_BLOCK_STREAMINFO: Sample rate, number of channels, etc.
  2. METADATA_BLOCK_PADDING: Nul padding.
  3. METADATA_BLOCK_APPLICATION: Third-party applications can register an ID. Metadata is typically 32-bit integers, but any datatypes can be specified.
  4. METADATA_BLOCK_SEEKTABLE: For one or more seek points.
  5. METADATA_BLOCK_VORBIS_COMMENT: Also known as FLAC tags, the contents of a VorbisComment packet. Note that the 32-bit field lengths are little-endian coded according to the Vorbis spec, as opposed to the usual big-endian coding of fixed-length integers in the rest of FLAC. FLAC metadata blocks are limited to 2^24 bytes (16 megabytes) and a VorbisComment packet in FLAC must fit within that limit.
  6. METADATA_BLOCK_CUESHEET: Typically, but not necessarily, for CD-DA (Red Book) cuesheets.
  7. METADATA_BLOCK_PICTURE: For binary picture data.

Ogg Skeleton

Ogg Skeleton provides metadata useful for handling Ogg streams. This includes information like mime-types and mapping for granulepos which allows seeking streams without the need for the demuxer to understand them. The latest version, Ogg Skeleton 4, also provides a keyframe index to enable faster seeking over high latency networks.

Ogg Skeleton allows for attachment of message header fields, given as name-value pairs, that contain some sort of protocol messages about the logical bitstream. This is intended for decode related stuff, such as the screen size for a video bitstream or the number of channels for an audio bitstream.

OggKate

OggKate was originally designed for karaoke and text. The stream can carry text and images, and these can be animated.

CMML (deprecated)

CMML is not used anymore; use OggKate instead. The Continuous Media Markup Language allowed time-based marking up of media streams, at its simplest this allowed you to divide media files into clips and provide information about each clip.

M3F (unused draft)

Multimedia Metadata Format is not being developed anymore; use VorbisComment instead.

The format was intended to replace VorbisComments for the use of structured metadata, allowing VorbisComments to revert to its orginally intended use of "short, text comments ... much like someone jotting a quick note on the bottom of a CDR."

M3F for the Ogg container was a draft specification which aimed to provide metadata for media streams. The exact aims of this project were still under development, but they included being able to describe artist relationships to a piece more accurately as well as providing the structure to encourage more reliable metadata.

XMLEmbedding (unused draft)

To implement XML metadata in Ogg (as for M3F), a mapping to Ogg streams is needed. The use of XML metadata will also open the way for the inclusion of technologies such as:

This specification is still not past the discussion stage.

Aims of advanced metadata

VorbisComments work well enough for most things, and can be overloaded/abused (depending on your point of view) for most other things. But there are three major requirements that point to the design of an external metadata format; one that can be interleaved with the other streams in a container.

  • Machinability: There are a number of items of metadata that a player will want to parse and take action on. While there are usually 'convention' schemes for doing this with the embedded comment headers, this is much easier if there is a separate metadata stream designed for such use, instead of having to do best-effort parsing of natural language comments. For example, a video file with multiple audio tracks can specify the language of each one; a player than can parse these reliably can match them against a language preference list configured by the user to automatically select and begin playback of the best option.
  • Kitchen Sink: There are a minority of people who care passionately about having every detail about a track available. In the sense of conserving such information, and providing an equivalent to liner notes for online distribution, this is a goal worth supporting. However, the simple unstructured key-value pairs offered by the inline metadata are unwieldy for this level of detail. How do you tell the 2nd unit Assistant Director from the USA unit Assistant Director? How do you indicate which artist played tenor sax in the solo?
  • Addressability: The internal comment metadata headers are by necessity attached to a single content stream. This is useful for some appication, but a limitation in others. In a multiplexed stream, which set of comments refers to the collection as a whole? (By convention, in Ogg, it's the first logical bitstream occuring, but we can do better.) A separate metadata stream type must address this issue of collective metadata while still allowing description of individual streams. It should also allow temporal addressability, so that changes can be described. Because the in-stream comment metadata are part of the codec headers, it cannot change over the course of the stream, and allowing additional comment packets elsewhere in the stream presents seeking challenges. In the Ogg container this can be resolved by inserting a chain boundary, but this is a poor option for very-low-bitrate streams and unreliable transports such as RTP.
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