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89 lines
4 KiB
XML
89 lines
4 KiB
XML
<chapter id="chapter-clocks">
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<title>Clocks in GStreamer</title>
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<para>
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To maintain sync in pipeline playback (which is the only case where this
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really matters), &GStreamer; uses <emphasis>clocks</emphasis>. Clocks
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are exposed by some elements, whereas other elements are merely clock
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slaves. The primary task of a clock is to represent the time progress
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according to the element exposing the clock, based on its own playback
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rate. If no clock provider is available in a pipeline, the system clock
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is used instead.
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</para>
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<para>
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&GStreamer; derives several times from the clock and the playback state.
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It is important to note, that a <emphasis>clock-time</emphasis> is
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monotonically rising, but the value itself is not meaningful.
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Subtracting the <emphasis>base-time</emphasis> yields the
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<emphasis>running-time</emphasis>. It is the same as the
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<emphasis>stream-time</emphasis> if one plays from start to end at original
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rate. The <emphasis>stream-time</emphasis> indicates the position in the
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media.
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</para>
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<figure float="1" id="chapter-clock-img">
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<title>&GStreamer; clock and various times</title>
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<mediaobject>
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<imageobject>
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<imagedata scale="75" fileref="images/clocks.ℑ" format="&IMAGE;" />
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</imageobject>
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</mediaobject>
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</figure>
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<sect1 id="section-clocks-providers">
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<title>Clock providers</title>
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<para>
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Clock providers exist because they play back media at some rate, and
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this rate is not necessarily the same as the system clock rate. For
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example, a soundcard may playback at 44,1 kHz, but that doesn't mean
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that after <emphasis>exactly</emphasis> 1 second <emphasis>according
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to the system clock</emphasis>, the soundcard has played back 44.100
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samples. This is only true by approximation. Therefore, generally,
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pipelines with an audio output use the audiosink as clock provider.
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This ensures that one second of video will be played back at the same
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rate as that the soundcard plays back 1 second of audio.
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</para>
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<para>
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Whenever some part of the pipeline requires to know the current clock
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time, it will be requested from the clock through
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<function>gst_clock_get_time ()</function>. The clock-time does not
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need to start at 0. The pipeline, which contains the global clock that
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all elements in the pipeline will use, in addition has a <quote>base
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time</quote>, which is the clock time at the the point where the
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pipeline went to the PLAYING state. Each element can subtract the
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<quote>base time</quote> from the clock-time to know the current
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running time.
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</para>
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<para>
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The clock provider is responsible for making sure that the clock time
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always represents the current media time as closely as possible; it
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has to take care of things such as playback latencies, buffering in
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audio-kernel modules, and so on, since all those could affect a/v sync
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and thus decrease the user experience.
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</para>
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</sect1>
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<sect1 id="section-clocks-slaves">
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<title>Clock slaves</title>
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<para>
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Clock slaves get assigned a clock by their containing pipeline. Their
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task is to make sure that media playback follows the time progress as
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represented by this clock as closely as possible. For most elements,
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that will simply mean to wait until the buffer running-time is reached
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before playing back their current sample.
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</para>
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<para>
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The buffer running-time is derived from the buffer timestamp and the
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newsegment event preceeding the buffer. A buffer is played synchronized
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with the clock when the clock's running-time has reached exactly the
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buffer running-time; this can be done with the function
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<function>gst_clock_id_wait ()</function>.
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</para>
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<para>
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For more information on how to write elements that conform to this
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required behaviour, see the Plugin Writer's Guide.
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</para>
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</sect1>
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</chapter>
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