1 <chapter id="chapter-clocks">
2 <title>Clocks in GStreamer</title>
5 To maintain sync in pipeline playback (which is the only case where this
6 really matters), &GStreamer; uses <emphasis>clocks</emphasis>. Clocks
7 are exposed by some elements, whereas other elements are merely clock
8 slaves. The primary task of a clock is to represent the time progress
9 according to the element exposing the clock, based on its own playback
10 rate. If no clock provider is available in a pipeline, the system clock
15 &GStreamer; derives several times from the clock and the playback state.
16 It is important to note, that a <emphasis>clock-time</emphasis> is
17 monotonically rising, but the value itself is not meaningful.
18 Subtracting the <emphasis>base-time</emphasis> yields the
19 <emphasis>running-time</emphasis>. It is the same as the
20 <emphasis>stream-time</emphasis> if one plays from start to end at original
21 rate. The <emphasis>stream-time</emphasis> indicates the position in the
25 <figure float="1" id="chapter-clock-img">
26 <title>&GStreamer; clock and various times</title>
29 <imagedata scale="75" fileref="images/clocks.ℑ" format="&IMAGE;" />
34 <sect1 id="section-clocks-providers">
35 <title>Clock providers</title>
38 Clock providers exist because they play back media at some rate, and
39 this rate is not necessarily the same as the system clock rate. For
40 example, a soundcard may playback at 44,1 kHz, but that doesn't mean
41 that after <emphasis>exactly</emphasis> 1 second <emphasis>according
42 to the system clock</emphasis>, the soundcard has played back 44.100
43 samples. This is only true by approximation. Therefore, generally,
44 pipelines with an audio output use the audiosink as clock provider.
45 This ensures that one second of video will be played back at the same
46 rate as that the soundcard plays back 1 second of audio.
49 Whenever some part of the pipeline requires to know the current clock
50 time, it will be requested from the clock through
51 <function>gst_clock_get_time ()</function>. The clock-time does not
52 need to start at 0. The pipeline, which contains the global clock that
53 all elements in the pipeline will use, in addition has a <quote>base
54 time</quote>, which is the clock time at the the point where media time
55 is starting from zero. This timestamp is subtracted from the clock
56 time, and that value is returned by <function>_get_time ()</function>.
59 The clock provider is responsible for making sure that the clock time
60 always represents the current media time as closely as possible; it
61 has to take care of things such as playback latencies, buffering in
62 audio-kernel modules, and so on, since all those could affect a/v sync
63 and thus decrease the user experience.
67 <sect1 id="section-clocks-slaves">
68 <title>Clock slaves</title>
70 Clock slaves get assigned a clock by their containing pipeline. Their
71 task is to make sure that media playback follows the time progress as
72 represented by this clock as closely as possible. For most elements,
73 that will simply mean to wait until a certain time is reached before
74 playing back their current sample; this can be done with the function
75 <function>gst_clock_id_wait ()</function>. Some elements may need to
76 support dropping samples too, however.
79 For more information on how to write elements that conform to this
80 required behaviour, see the Plugin Writer's Guide.