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54 lines
2.6 KiB
XML
54 lines
2.6 KiB
XML
<chapter id="chapter-advanced-tagging">
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<title>Tagging (Metadata and Streaminfo)</title>
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<para>
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Tags are pieces of information stored in a stream that are not the content
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itself, butthey rather <emphasis>describe</emphasis> the content. Most
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media container formats support tagging in one way or another. Ogg uses
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VorbisComment for this, MP3 uses ID3, AVI and WAV use RIFF's INFO list
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chunk, etc. GStreamer provides a general way for elements to read tags from
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the stream and expose this to the user. The tags (at least the metadata)
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will be part of the stream inside the pipeline. The consequence of this is
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that transcoding of files from one format to another will automatically
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preserve tags, as long as the input and output format elements both support
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tagging.
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</para>
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<para>
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Tags are separated in two categories in GStreamer, even though applications
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won't notice anything of this. The first are called <emphasis>metadata</emphasis>,
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the second are called <emphasis>streaminfo</emphasis>. Metadata are tags
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that describe the non-technical parts of stream content. They can be
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changed without needing to re-encode the stream completely. Examples are
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<quote>author</quote>, <quote>title</quote> or <quote>album</quote>. The
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container format might still need to be re-written for the tags to fit in,
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though. Streaminfo, on the other hand, are tags that describe the stream
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contents technically. To change them, the stream needs to be re-encoded.
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Examples are <quote>codec</quote> or <quote>bitrate</quote>. Note that some
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container formats (like ID3) store various streaminfo tags as metadata in
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the file container, which means that they can be changed so that they don't
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match the content in the file anymore. Still, they are called metadata
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because <emphasis>technically</emphasis>, they can be changed without
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re-encoding the whole stream, even though that makes them invalid. Files
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with such metadata tags will have the same tag twice: once as metadata,
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once as streaminfo.
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</para>
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<para>
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A tag reading element is called <classname>TagGetter</classname> in
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&GStreamer;. A tag writer is called <classname>TagSetter</classname>. An
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element supporting both can be used in a tag editor for quick tag changing.
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</para>
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<sect1 id="section-tagging-read" xreflabel="Reading Tags from Streams">
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<title>Reading Tags from Streams</title>
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<para>
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WRITEME
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</para>
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</sect1>
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<sect1 id="section-tagging-write" xreflabel="Writing Tags to Streams">
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<title>Writing Tags to Streams</title>
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<para>
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WRITEME
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</para>
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</sect1>
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</chapter>
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