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201 lines
8.9 KiB
Markdown
201 lines
8.9 KiB
Markdown
---
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title: Clocks and synchronization in GStreamer
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...
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# Clocks and synchronization in GStreamer
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When playing complex media, each sound and video sample must be played
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in a specific order at a specific time. For this purpose, GStreamer
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provides a synchronization mechanism.
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GStreamer provides support for the following use cases:
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- Non-live sources with access faster than playback rate. This is the
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case where one is reading media from a file and playing it back in a
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synchronized fashion. In this case, multiple streams need to be
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synchronized, like audio, video and subtitles.
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- Capture and synchronized muxing/mixing of media from multiple live
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sources. This is a typical use case where you record audio and video
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from a microphone/camera and mux it into a file for storage.
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- Streaming from (slow) network streams with buffering. This is the
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typical web streaming case where you access content from a streaming
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server with http.
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- Capture from live source and and playback to live source with
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configurable latency. This is used when, for example, capture from a
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camera, apply an effect and display the result. It is also used when
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streaming low latency content over a network with UDP.
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- Simultaneous live capture and playback from prerecorded content.
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This is used in audio recording cases where you play a previously
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recorded audio and record new samples, the purpose is to have the
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new audio perfectly in sync with the previously recorded data.
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GStreamer uses a `GstClock` object, buffer timestamps and a SEGMENT
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event to synchronize streams in a pipeline as we will see in the next
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sections.
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## Clock running-time
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In a typical computer, there are many sources that can be used as a time
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source, e.g., the system time, soundcards, CPU performance counters, ...
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For this reason, there are many `GstClock` implementations available in
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GStreamer. The clock time doesn't always start from 0 or from some known
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value. Some clocks start counting from some known start date, other
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clocks start counting since last reboot, etc...
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A `GstClock` returns the **absolute-time** according to that clock with
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`gst_clock_get_time ()`. The absolute-time (or clock time) of a clock is
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monotonically increasing. From the absolute-time is a **running-time**
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calculated, which is simply the difference between a previous snapshot
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of the absolute-time called the **base-time**. So:
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running-time = absolute-time - base-time
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A GStreamer `GstPipeline` object maintains a `GstClock` object and a
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base-time when it goes to the PLAYING state. The pipeline gives a handle
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to the selected `GstClock` to each element in the pipeline along with
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selected base-time. The pipeline will select a base-time in such a way
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that the running-time reflects the total time spent in the PLAYING
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state. As a result, when the pipeline is PAUSED, the running-time stands
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still.
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Because all objects in the pipeline have the same clock and base-time,
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they can thus all calculate the running-time according to the pipeline
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clock.
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## Buffer running-time
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To calculate a buffer running-time, we need a buffer timestamp and the
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SEGMENT event that preceeded the buffer. First we can convert the
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SEGMENT event into a `GstSegment` object and then we can use the
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`gst_segment_to_running_time ()` function to perform the calculation of
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the buffer running-time.
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Synchronization is now a matter of making sure that a buffer with a
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certain running-time is played when the clock reaches the same
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running-time. Usually this task is done by sink elements. Sink also have
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to take into account the latency configured in the pipeline and add this
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to the buffer running-time before synchronizing to the pipeline clock.
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Non-live sources timestamp buffers with a running-time starting from 0.
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After a flushing seek, they will produce buffers again from a
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running-time of 0.
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Live sources need to timestamp buffers with a running-time matching the
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pipeline running-time when the first byte of the buffer was captured.
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## Buffer stream-time
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The buffer stream-time, also known as the position in the stream, is
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calculated from the buffer timestamps and the preceding SEGMENT event.
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It represents the time inside the media as a value between 0 and the
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total duration of the media.
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The stream-time is used in:
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- Report the current position in the stream with the POSITION query.
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- The position used in the seek events and queries.
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- The position used to synchronize controlled values.
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The stream-time is never used to synchronize streams, this is only done
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with the running-time.
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## Time overview
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Here is an overview of the various timelines used in GStreamer.
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The image below represents the different times in the pipeline when
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playing a 100ms sample and repeating the part between 50ms and 100ms.
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You can see how the running-time of a buffer always increments
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monotonically along with the clock-time. Buffers are played when their
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running-time is equal to the clock-time - base-time. The stream-time
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represents the position in the stream and jumps backwards when
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repeating.
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## Clock providers
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A clock provider is an element in the pipeline that can provide a
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`GstClock` object. The clock object needs to report an absolute-time
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that is monotonically increasing when the element is in the PLAYING
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state. It is allowed to pause the clock while the element is PAUSED.
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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 *exactly* 1 second *according to the system clock*, the
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soundcard has played back 44.100 samples. This is only true by
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approximation. In fact, the audio device has an internal clock based on
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the number of samples played that we can expose.
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If an element with an internal clock needs to synchronize, it needs to
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estimate when a time according to the pipeline clock will take place
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according to the internal clock. To estimate this, it needs to slave its
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clock to the pipeline clock.
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If the pipeline clock is exactly the internal clock of an element, the
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element can skip the slaving step and directly use the pipeline clock to
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schedule playback. This can be both faster and more accurate. Therefore,
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generally, elements with an internal clock like audio input or output
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devices will be a clock provider for the pipeline.
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When the pipeline goes to the PLAYING state, it will go over all
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elements in the pipeline from sink to source and ask each element if
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they can provide a clock. The last element that can provide a clock will
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be used as the clock provider in the pipeline. This algorithm prefers a
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clock from an audio sink in a typical playback pipeline and a clock from
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source elements in a typical capture pipeline.
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There exist some bus messages to let you know about the clock and clock
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providers in the pipeline. You can see what clock is selected in the
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pipeline by looking at the NEW\_CLOCK message on the bus. When a clock
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provider is removed from the pipeline, a CLOCK\_LOST message is posted
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and the application should go to PAUSED and back to PLAYING to select a
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new clock.
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## Latency
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The latency is the time it takes for a sample captured at timestamp X to
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reach the sink. This time is measured against the clock in the pipeline.
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For pipelines where the only elements that synchronize against the clock
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are the sinks, the latency is always 0 since no other element is
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delaying the buffer.
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For pipelines with live sources, a latency is introduced, mostly because
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of the way a live source works. Consider an audio source, it will start
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capturing the first sample at time 0. If the source pushes buffers with
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44100 samples at a time at 44100Hz it will have collected the buffer at
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second 1. Since the timestamp of the buffer is 0 and the time of the
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clock is now \>= 1 second, the sink will drop this buffer because it is
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too late. Without any latency compensation in the sink, all buffers will
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be dropped.
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### Latency compensation
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Before the pipeline goes to the PLAYING state, it will, in addition to
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selecting a clock and calculating a base-time, calculate the latency in
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the pipeline. It does this by doing a LATENCY query on all the sinks in
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the pipeline. The pipeline then selects the maximum latency in the
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pipeline and configures this with a LATENCY event.
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All sink elements will delay playback by the value in the LATENCY event.
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Since all sinks delay with the same amount of time, they will be
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relative in sync.
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### Dynamic Latency
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Adding/removing elements to/from a pipeline or changing element
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properties can change the latency in a pipeline. An element can request
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a latency change in the pipeline by posting a LATENCY message on the
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bus. The application can then decide to query and redistribute a new
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latency or not. Changing the latency in a pipeline might cause visual or
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audible glitches and should therefore only be done by the application
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when it is allowed.
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