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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).
 
This works 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.
 
==Proposed Solutions==


RDF + dublin core
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.


XML-encoding (generic rdf or CMML?)
The [[VorbisComment]] page contains improvements to the suggested comment set.


binary encoding (three-component length-encoded binary vectors)
== [[FLAC]] metadata blocks ==
[http://www.shop263.com/i8-1/1.htm ADNT]
[http://www.shop263.com/i8-1/2.htm AGAT贝司]
[http://www.shop263.com/i8-1/3.htm ANT]
[http://www.shop263.com/i8-1/4.htm DDQ]
[http://www.shop263.com/i8-1/5.htm MAA贝司]
[http://www.shop263.com/i8-1/6.htm PU脱膜剂]
[http://www.shop263.com/i8-1/7.htm 油漆]
[http://www.shop263.com/i8-1/8.htm 艾地苯醌]
[http://www.shop263.com/i8-1/9.htm 氨基C酸]
[http://www.shop263.com/i8-1/10.htm 氨基蒽醌]
[http://www.shop263.com/i8-1/11.htm 氨基二醇]
[http://www.shop263.com/i8-1/12.htm 氨基酸]
[http://www.shop263.com/i8-1/13.htm 氨氧化]
[http://www.shop263.com/i8-1/14.htm 拔染印花]
[http://www.shop263.com/i8-1/15.htm 离子染料]
[http://www.shop263.com/i8-1/16.htm 包膜酞白粉]
[http://www.shop263.com/i8-1/17.htm 包膜酞菁蓝]
[http://www.shop263.com/i8-1/18.htm 苯胺黑]
[http://www.shop263.com/i8-1/19.htm 表印]
[http://www.shop263.com/i8-1/20.htm 冰染色酚]
[http://www.shop263.com/i8-1/21.htm 油画颜料]
[http://www.shop263.com/i8-1/22.htm 草酸钴]
[http://www.shop263.com/i8-1/23.htm 超分散剂]
[http://www.shop263.com/i8-1/24.htm 超分散纳米氧化铁]
[http://www.shop263.com/i8-1/25.htm 超能夜光粉]
[http://www.shop263.com/i8-1/26.htm 赤血盐钾]
[http://www.shop263.com/i8-1/27.htm 纯碱]
[http://www.shop263.com/i8-1/28.htm 醋酸锆]
[http://www.shop263.com/i8-1/29.htm 催化剂]
[http://www.shop263.com/i8-1/30.htm 大红粉]
[http://www.shop263.com/i8-1/31.htm 大红色基G]
[http://www.shop263.com/i8-1/32.htm 大红色基RC]
[http://www.shop263.com/i8-1/33.htm 防腐]
[http://www.shop263.com/i8-1/34.htm 镝铁合金]
[http://www.shop263.com/i8-1/35.htm 碘丙炔正丁胺甲酸酯]
[http://www.shop263.com/i8-1/36.htm 电镀铝浆]
[http://www.shop263.com/i8-1/37.htm 靛蓝粉]
[http://www.shop263.com/i8-1/38.htm 靛蓝]
[http://www.shop263.com/i8-1/39.htm 氢溴酸加兰他敏]
[http://www.shop263.com/i8-1/40.htm 对氨基苯甲醚]
[http://www.shop263.com/i8-1/41.htm 对氨基苯乙醚]
[http://www.shop263.com/i8-1/42.htm 对甲苯胺]
[http://www.shop263.com/i8-1/43.htm 对乙基苯甲酰氯]
[http://www.shop263.com/i8-1/44.htm 蒽醌]
[http://www.shop263.com/i8-1/45.htm 蒽醌氧化]
[http://www.shop263.com/i8-1/46.htm 二氨基萘]
[http://www.shop263.com/i8-1/47.htm 二羟甲基丙酸]
[http://www.shop263.com/i8-1/48.htm 二氧化锆]
[http://www.shop263.com/i8-1/49.htm 二氧化硫脲]
[http://www.shop263.com/i8-1/50.htm 二氧化硒]
[http://www.shop263.com/i8-1/51.htm 发光材料]
[http://www.shop263.com/i8-1/52.htm 防腐储罐]
[http://www.shop263.com/i8-1/53.htm 防腐剂]
[http://www.shop263.com/i8-1/54.htm 防焦剂CTP]
[http://www.shop263.com/i8-1/55.htm 防霉剂]
[http://www.shop263.com/i8-1/56.htm 颜料]
[http://www.shop263.com/i8-1/57.htm 纺织印染染料]
[http://www.shop263.com/i8-1/58.htm 纺织印染助剂]
[http://www.shop263.com/i8-1/59.htm 分散剂]
[http://www.shop263.com/i8-1/60.htm 分散红3B]
[http://www.shop263.com/i8-1/61.htm 分散红FB]
[http://www.shop263.com/i8-1/62.htm 分散黄SD]
[http://www.shop263.com/i8-1/63.htm 分散蓝77]
[http://www.shop263.com/i8-1/64.htm 分散染料]
[http://www.shop263.com/i8-1/65.htm 分散染料滤饼]
[http://www.shop263.com/i8-1/66.htm 酚菁兰BGS]
[http://www.shop263.com/i8-1/67.htm 酚菁兰BS]
[http://www.shop263.com/i8-1/68.htm 粉末涂料固化剂]
[http://www.shop263.com/i8-1/69.htm 分散助剂]
[http://www.shop263.com/i8-1/70.htm 氟硅酸钾]
[http://www.shop263.com/i8-1/71.htm 氟硅酸钠]
[http://www.shop263.com/i8-1/72.htm 氟化钠]
[http://www.shop263.com/i8-1/73.htm 氟盐类]
[http://www.shop263.com/i8-1/74.htm 浮石]
[http://www.shop263.com/i8-1/75.htm 付克酰化]
[http://www.shop263.com/i8-1/76.htm 油墨]
[http://www.shop263.com/i8-1/77.htm 钙基膨润土]
[http://www.shop263.com/i8-1/78.htm 感光颜料]
[http://www.shop263.com/i8-1/79.htm 钢衬塑]
[http://www.shop263.com/i8-1/80.htm 钢衬塑管道]
[http://www.shop263.com/i8-1/81.htm 三氯化铁]
[http://www.shop263.com/i8-1/82.htm 拔染染料]
[http://www.shop263.com/i8-1/83.htm 碱性染料]
[http://www.shop263.com/i8-1/84.htm 储运罐]
[http://www.shop263.com/i8-1/85.htm 高钛粉]
[http://www.shop263.com/i8-1/86.htm 陶瓷色釉料]
[http://www.shop263.com/i8-1/87.htm 锆氟酸铵]
[http://www.shop263.com/i8-1/88.htm 锆氟酸钾]
[http://www.shop263.com/i8-1/89.htm 锆英粉]
[http://www.shop263.com/i8-1/90.htm 铬酸酐]
[http://www.shop263.com/i8-1/91.htm 铬酸酐粉]
[http://www.shop263.com/i8-1/92.htm 工程塑料]
[http://www.shop263.com/i8-1/93.htm 工业硫酸]
[http://www.shop263.com/i8-1/94.htm 氯化钴]
[http://www.shop263.com/i8-1/95.htm 固相法]
[http://www.shop263.com/i8-1/96.htm 红色基 ]
[http://www.shop263.com/i8-1/97.htm 三甲氧基甲苯]
[http://www.shop263.com/i8-1/98.htm 氧化铬绿]
[http://www.shop263.com/i8-1/99.htm 氧化钴]
[http://www.shop263.com/i8-1/100.htm 氧化铝]
[http://www.shop263.com/i8-1/101.htm 氧化铁]
[http://www.shop263.com/i8-1/102.htm 氧化铁黑]
[http://www.shop263.com/i8-1/103.htm 氧化铁红]
[http://www.shop263.com/i8-1/104.htm 氧化铁黄]
[http://www.shop263.com/i8-1/105.htm 氧化铁黄]
[http://www.shop263.com/i8-1/106.htm 氧化铁蓝]
[http://www.shop263.com/i8-1/107.htm 氧化铁绿]
[http://www.shop263.com/i8-1/108.htm 氧化铁紫]
[http://www.shop263.com/i8-1/109.htm 氧化铁棕]
[http://www.shop263.com/i8-1/110.htm 氧化锌]
[http://www.shop263.com/i8-1/111.htm 氧氯化锆]
[http://www.shop263.com/i8-1/112.htm 夜光粉]
[http://www.shop263.com/i8-1/113.htm 液体蜡]
[http://www.shop263.com/i8-1/114.htm 一氧化铅]
[http://www.shop263.com/i8-1/115.htm 乙酯]
[http://www.shop263.com/i8-1/116.htm 医药中间体]
[http://www.shop263.com/i8-1/117.htm 乙酸乙酯]
[http://www.shop263.com/i8-1/118.htm 乙氧基喹啉]
[http://www.shop263.com/i8-1/119.htm 乙氧基喹啉粉剂]
[http://www.shop263.com/i8-1/120.htm 易燃易爆气体检测仪
[http://www.shop263.com/i8-1/121.htm 易燃气体检测]
[http://www.shop263.com/i8-1/122.htm 银粉]
[http://www.shop263.com/i8-1/123.htm 印花涂料色浆]
[http://www.shop263.com/i8-1/124.htm 印染助剂]
[http://www.shop263.com/i8-1/125.htm 荧光黄]
[http://www.shop263.com/i8-1/126.htm 荧光桔红]
[http://www.shop263.com/i8-1/127.htm 荧光颜料]
[http://www.shop263.com/i8-1/128.htm 荧光增白剂]
[http://www.shop263.com/i8-1/129.htm 萤光黄8GF]
[http://www.shop263.com/i8-1/130.htm 萤光黄8GF1]
[http://www.shop263.com/i8-1/131.htm 永固红F2R]
[http://www.shop263.com/i8-1/132.htm 永固黄]
[http://www.shop263.com/i8-1/133.htm 永固紫]
[http://www.shop263.com/i8-1/134.htm 永固紫RL]
[http://www.shop263.com/i8-1/135.htm 尼龙辘]
[http://www.shop263.com/i8-1/136.htm 油画颜料]
[http://www.shop263.com/i8-1/137.htm 油墨]
[http://www.shop263.com/i8-1/138.htm 油漆]
[http://www.shop263.com/i8-1/139.htm 油溶]
[http://www.shop263.com/i8-1/140.htm 油溶黄120]
[http://www.shop263.com/i8-1/141.htm 油溶黄163]
[http://www.shop263.com/i8-1/142.htm 油溶黄189]
[http://www.shop263.com/i8-1/143.htm 油溶黄3G]
[http://www.shop263.com/i8-1/144.htm 油溶黄8GF]
[http://www.shop263.com/i8-1/145.htm 油溶黄TP]
[http://www.shop263.com/i8-1/146.htm 油溶染料]
[http://www.shop263.com/i8-1/147.htm 油脂]
[http://www.shop263.com/i8-1/148.htm 催化剂]
[http://www.shop263.com/i8-1/149.htm 有机胺]
[http://www.shop263.com/i8-1/150.htm 有机化工]
[http://www.shop263.com/i8-1/151.htm 有机颜料]
[http://www.shop263.com/i8-1/152.htm 有色金属]
[http://www.shop263.com/i8-1/153.htm 槽罐]
[http://www.shop263.com/i8-1/154.htm 杂环]
[http://www.shop263.com/i8-1/155.htm 蒸煮油墨]
[http://www.shop263.com/i8-1/156.htm 储罐]
[http://www.shop263.com/i8-1/157.htm 直接混纺]
[http://www.shop263.com/i8-1/158.htm 直接耐晒]
[http://www.shop263.com/i8-1/159.htm 直接染料]
[http://www.shop263.com/i8-1/160.htm 直接染料]
[http://www.shop263.com/i8-1/161.htm 直接管]
[http://www.shop263.com/i8-1/162.htm 中铬黄]
[http://www.shop263.com/i8-1/163.htm 中黄FG]
[http://www.shop263.com/i8-1/164.htm 中间体]
[http://www.shop263.com/i8-1/165.htm 中药]
[http://www.shop263.com/i8-1/166.htm 珠光粉]
[http://www.shop263.com/i8-1/167.htm 珠光颜料]
[http://www.shop263.com/i8-1/168.htm 紫酱色基]
[http://www.shop263.com/i8-1/169.htm 紫外吸收剂]
[http://www.shop263.com/i8-1/170.htm 紫外线吸收剂]
[http://www.shop263.com/i8-1/171.htm 阻燃剂]
[http://www.shop263.com/i8-1/172.htm 石墨]
[http://www.shop263.com/i8-1/173.htm 红色基]
[http://www.shop263.com/i8-1/174.htm 化工颜料]
[http://www.shop263.com/i8-1/175.htm 化工中间体]
[http://www.shop263.com/i8-1/176.htm 化学试剂]
[http://www.shop263.com/i8-1/177.htm 还原染料]
[http://www.shop263.com/i8-1/178.htm 油墨原材料]
[http://www.shop263.com/i8-1/179.htm 幻彩颜料]
[http://www.shop263.com/i8-1/180.htm 黄丹]
[http://www.shop263.com/i8-1/181.htm 黄血盐钾]
[http://www.shop263.com/i8-1/182.htm 黄血盐钠]
[http://www.shop263.com/i8-1/183.htm 混凝土减水剂]
[http://www.shop263.com/i8-1/184.htm 活性]
[http://www.shop263.com/i8-1/185.htm 活性白土]
[http://www.shop263.com/i8-1/186.htm 活性蓝]
[http://www.shop263.com/i8-1/187.htm 活性漂土]
[http://www.shop263.com/i8-1/188.htm 光稳定剂]
[http://www.shop263.com/i8-1/189.htm 光稳定剂770]
[http://www.shop263.com/i8-1/190.htm 光引发剂]
[http://www.shop263.com/i8-1/191.htm 颜料]
[http://www.shop263.com/i8-1/192.htm 硅酸锆]
[http://www.shop263.com/i8-1/193.htm 红丹]
[http://www.shop263.com/i8-1/194.htm 红矾钠]
[http://www.shop263.com/i8-1/195.htm 色酚]
[http://www.shop263.com/i8-1/196.htm 金红石钛白粉]
[http://www.shop263.com/i8-1/197.htm 金黄FGR]
[http://www.shop263.com/i8-1/198.htm 金属镝]
[http://www.shop263.com/i8-1/199.htm 金属防锈脂]
[http://www.shop263.com/i8-1/200.htm 金属加工助剂]
[http://www.shop263.com/i8-1/201.htm 金属镧]
[http://www.shop263.com/i8-1/202.htm 金属络合溶剂染料]
[http://www.shop263.com/i8-1/203.htm 金属钕]
[http://www.shop263.com/i8-1/204.htm 金属镨]
[http://www.shop263.com/i8-1/205.htm 金属钐]
[http://www.shop263.com/i8-1/206.htm 金属铽]
[http://www.shop263.com/i8-1/207.htm 金属镱]
[http://www.shop263.com/i8-1/208.htm 腊]
[http://www.shop263.com/i8-1/209.htm 腈纶]
[http://www.shop263.com/i8-1/210.htm 精硒]
[http://www.shop263.com/i8-1/211.htm 精细化工]
[http://www.shop263.com/i8-1/212.htm 漂精片]
[http://www.shop263.com/i8-1/213.htm 聚氨酯]
[http://www.shop263.com/i8-1/214.htm 聚氨酯树脂]
[http://www.shop263.com/i8-1/215.htm 聚乙烯蜡]
[http://www.shop263.com/i8-1/216.htm 聚酯纺前着色剂]
[http://www.shop263.com/i8-1/217.htm 咔唑]
[http://www.shop263.com/i8-1/218.htm 颗粒活性白土]
[http://www.shop263.com/i8-1/219.htm 可膨胀石墨]
[http://www.shop263.com/i8-1/220.htm 空心玻璃微珠]
[http://www.shop263.com/i8-1/221.htm 猛矿]
[http://www.shop263.com/i8-1/222.htm 矿物质]
[http://www.shop263.com/i8-1/223.htm 蓝色盐VB]
[http://www.shop263.com/i8-1/224.htm 镭射粉]
[http://www.shop263.com/i8-1/225.htm 立德粉]
[http://www.shop263.com/i8-1/226.htm 立索尔]
[http://www.shop263.com/i8-1/227.htm 立索尔大红]
[http://www.shop263.com/i8-1/228.htm 抛光腊]
[http://www.shop263.com/i8-1/229.htm 邻氨基苯磺酸]
[http://www.shop263.com/i8-1/230.htm 邻氨基苯甲醚]
[http://www.shop263.com/i8-1/231.htm 邻氨基苯乙醚]
[http://www.shop263.com/i8-1/232.htm 邻氯对硝基苯胺]
[http://www.shop263.com/i8-1/233.htm 硫化]
[http://www.shop263.com/i8-1/234.htm 硫化染料]
[http://www.shop263.com/i8-1/235.htm 硫酸锆]
[http://www.shop263.com/i8-1/236.htm 硫酸钴]
[http://www.shop263.com/i8-1/237.htm 硫酸亚铁]
[http://www.shop263.com/i8-1/238.htm 硫酸氧锆]
[http://www.shop263.com/i8-1/239.htm 卤化]
[http://www.shop263.com/i8-1/240.htm 氯化钴]
[http://www.shop263.com/i8-1/241.htm 氯化聚丙烯]
[http://www.shop263.com/i8-1/242.htm 氯硝]
[http://www.shop263.com/i8-1/243.htm 氯乙酸]
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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.


== [[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 [[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.


== [[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 12:44, 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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