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328 lines
13 KiB
Text
328 lines
13 KiB
Text
Latency
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-------
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The latency is the time it takes for a sample captured at timestamp 0 to reach the
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sink. This time is measured against the clock in the pipeline. For pipelines
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where the only elements that synchronize against the clock are the sinks, the
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latency is always 0 since no other element is delaying the buffer.
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For pipelines with live sources, a latency is introduced, mostly because of the
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way a live source works. Consider an audio source, it will start capturing the
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first sample at time 0. If the source pushes buffers with 44100 samples at a
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time at 44100Hz it will have collected the buffer at second 1.
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Since the timestamp of the buffer is 0 and the time of the clock is now >= 1
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second, the sink will drop this buffer because it is too late.
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Without any latency compensation in the sink, all buffers will be dropped.
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The situation becomes more complex in the presence of:
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- 2 live sources connected to 2 live sinks with different latencies
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* audio/video capture with synchronized live preview.
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* added latencies due to effects (delays, resamplers...)
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- 1 live source connected to 2 live sinks
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* firewire DV
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* RTP, with added latencies because of jitter buffers.
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- mixed live source and non-live source scenarios.
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* synchronized audio capture with non-live playback. (overdubs,..)
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- clock slaving in the sinks due to the live sources providing their own
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clocks.
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To perform the needed latency corrections in the above scenarios, we must
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develop an algorithm to calculate a global latency for the pipeline. The
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algorithm must be extensible so that it can optimize the latency at runtime.
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It must also be possible to disable or tune the algorithm based on specific
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application needs (required minimal latency).
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Pipelines without latency compensation
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--------------------------------------
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We show some examples to demonstrate the problem of latency in typical
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capture pipelines.
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- Example 1
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An audio capture/playback pipeline.
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asrc: audio source, provides a clock
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asink audio sink, provides a clock
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.--------------------------.
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| pipeline |
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| .------. .-------. |
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| | asrc | | asink | |
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| | src -> sink | |
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| '------' '-------' |
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'--------------------------'
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NULL->READY:
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asink: NULL->READY: probes device, returns SUCCESS
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asrc: NULL->READY: probes device, returns SUCCESS
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READY->PAUSED:
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asink: READY:->PAUSED open device, returns ASYNC
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asrc: READY->PAUSED: open device, returns NO_PREROLL
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* Since the source is a live source, it will only produce data in the
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PLAYING state. To note this fact, it returns NO_PREROLL from the state change
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function.
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* This sink returns ASYNC because it can only complete the state change to
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PAUSED when it receives the first buffer.
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At this point the pipeline is not processing data and the clock is not
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running. Unless a new action is performed on the pipeline, this situation will
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never change.
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PAUSED->PLAYING:
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asrc clock selected because it is the most upstream clock provider. asink can
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only provide a clock when it received the first buffer and configured the
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device with the samplerate in the caps.
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asink: PAUSED:->PLAYING, sets pending state to PLAYING, returns ASYNC becaus
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it is not prerolled. The sink will commit state to
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PLAYING when it prerolls.
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asrc: PAUSED->PLAYING: starts pushing buffers.
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* since the sink is still performing a state change from READY -> PAUSED, it
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remains ASYNC. The pending state will be set to PLAYING.
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* The clock starts running as soon as all the elements have been set to
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PLAYING.
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* the source is a live source with a latency. Since it is synchronized with
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the clock, it will produce a buffer with timestamp 0 and duration D after
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time D, ie. it will only be able to produce the last sample of the buffer
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(with timestamp D) at time D. This latency depends on the size of the
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buffer.
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* the sink will receive the buffer with timestamp 0 at time >= D. At this
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point the buffer is too late already and might be dropped. This state of
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constantly dropping data will not change unless a constant latency
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correction is added to the incomming buffer timestamps.
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The problem is due to the fact that the sink is set to (pending) PLAYING
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without being prerolled, which only happens in live pipelines.
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- Example 2
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An audio/video capture/playback pipeline. We capture both audio and video and
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have them played back synchronized again.
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asrc: audio source, provides a clock
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asink audio sink, provides a clock
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vsrc: video source
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vsink video sink
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.--------------------------.
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| pipeline |
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| .------. .-------. |
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| | asrc | | asink | |
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| | src -> sink | |
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| '------' '-------' |
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| .------. .-------. |
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| | vsrc | | vsink | |
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| | src -> sink | |
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| '------' '-------' |
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'--------------------------'
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The state changes happen in the same way as example 1. Both sinks end up with
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pending state of PLAYING and a return value of ASYNC until they receive the
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first buffer.
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For audio and video to be played in sync, both sinks must compensate for the
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latency of its source but must also use exactly the same latency correction.
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Suppose asrc has a latency of 20ms and vsrc a latency of 33ms, the total
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latency in the pipeline has to be at least 33ms. This also means that the
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pipeline must have at least a 33 - 20 = 13ms buffering on the audio stream or
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else the audio src will underrun while the audiosink waits for the previous
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sample to play.
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- Example 3
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An example of the combination of a non-live (file) and a live source (vsrc)
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connected to live sinks (vsink, sink).
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.--------------------------.
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| pipeline |
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| .------. .-------. |
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| | file | | sink | |
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| | src -> sink | |
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| '------' '-------' |
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| .------. .-------. |
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| | vsrc | | vsink | |
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| | src -> sink | |
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| '------' '-------' |
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'--------------------------'
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The state changes happen in the same way as example 1. Except sink will be
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able to preroll (commit its state to PAUSED).
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In this case sink will have no latency but vsink will. The total latency
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should be that of vsink.
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Note that because of the presence of a live source (vsrc), the pipeline can be
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set to playing before sink is able to preroll. Without compensation for the
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live source, this might lead to synchronisation problems because the latency
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should be configured in the element before it can go to PLAYING.
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- Example 4
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An example of the combination of a non-live and a live source. The non-live
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source is connected to a live sink and the live source to a non-live sink.
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.--------------------------.
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| pipeline |
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| .------. .-------. |
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| | file | | sink | |
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| | src -> sink | |
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| '------' '-------' |
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| .------. .-------. |
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| | vsrc | | files | |
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| | src -> sink | |
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| '------' '-------' |
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'--------------------------'
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The state changes happen in the same way as example 3. Sink will be
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able to preroll (commit its state to PAUSED). files will not be able to
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preroll.
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sink will have no latency since it is not connected to a live source. files
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does not do synchronisation so it does not care about latency.
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The total latency in the pipeline is 0. The vsrc captures in sync with the
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playback in sink.
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As in example 3, sink can only be set to PLAYING after it successfully
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prerolled.
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State Changes revised
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---------------------
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As a first step in a generic solution we propose to modify the state changes so
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that no sink is set to PLAYING before it is prerolled.
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In order to do this, the pipeline (at the GstBin level) keeps track of all
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elements that require preroll (the ones that return ASYNC from the state
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change). These elements posted a ASYNC_START message without a matching
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ASYNC_DONE message.
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The pipeline will not change the state of the elements that are still doing an
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ASYNC state change.
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When an ASYNC element prerolls, it commits its state to PAUSED and posts an
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ASYNC_DONE message. The pipeline notices this ASYNC_DONE message and matches it
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with the ASYNC_START message it cached for the corresponding element.
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When all ASYNC_START messages are matched with an ASYNC_DONE message, the
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pipeline proceeds with setting the elements to the final state again.
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The base time of the element was already set by the pipeline when it changed the
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NO_PREROLL element to PLAYING. This operation has to be performed in the
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separate async state change thread (like the one currently used for going from
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PAUSED->PLAYING in a non-live pipeline).
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implications:
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- the current async_play vmethod in basesink can be deprecated since we now
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always call the state change function when going from PAUSED->PLAYING. We
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keep this method however to remain backward compatible.
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Latency compensation
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--------------------
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As an extension to the revised state changes we can perform latency calculation
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and compensation before we proceed to the PLAYING state.
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When the pipeline collected all ASYNC_DONE messages it can calculate the global
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latency as follows:
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- perform a latency query on all sinks.
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- latency = MAX (all min latencies)
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- if MIN (all max latencies) < latency we have an impossible situation and we
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must generate an error indicating that this pipeline cannot be played. This
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usually means that there is not enough buffering in some chain of the
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pipeline. A queue can be added to those chains.
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The sinks gather this information with a LATENCY query upstream. Intermediate
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elements pass the query upstream and add the amount of latency they add to the
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result.
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ex1:
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sink1: [20 - 20]
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sink2: [33 - 40]
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MAX (20, 33) = 33
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MIN (20, 40) = 20 < 33 -> impossible
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ex2:
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sink1: [20 - 50]
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sink2: [33 - 40]
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MAX (20, 33) = 33
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MIN (50, 40) = 40 >= 33 -> latency = 33
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The latency is set on the pipeline by sending a LATENCY event to the sinks
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in the pipeline. This event configures the total latency on the sinks. The
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sink forwards this LATENCY event upstream so that intermediate elements can
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configure themselves as well.
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After this step, the pipeline continues setting the pending state on its
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elements.
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A sink adds the latency value, received in the LATENCY event, to
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the times used for synchronizing against the clock. This will effectively
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delay the rendering of the buffer with the required latency. Since this delay is
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the same for all sinks, all sinks will render data relatively synchronised.
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Flushing a playing pipeline
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---------------------------
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Using the new state change mechanism we can implement resynchronisation after an
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uncontrolled FLUSH in (part of) a pipeline. Indeed, when a flush is performed on
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a PLAYING live element, a new base time must be distributed to this element.
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A flush in a pipeline can happen in the following cases:
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- flushing seek in the pipeline
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- performed by the application on the pipeline
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- performed by the application on an element
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- flush preformed by an element
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- after receiving a navigation event (DVD, ...)
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When a playing sink is flushed by a FLUSH_START event, an ASYNC_START message is
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posted by the element. As part of the message, the fact that the element got
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flushed is included. The element also goes to a pending PAUSED state and has to
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be set to the PLAYING state again later.
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The ASYNC_START message is kept by the parent bin. When the element prerolls,
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it posts an ASYNC_DONE message.
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When all ASYNC_START messages are matched with an ASYNC_DONE message, the bin
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will capture a new base_time from the clock and will bring all the sinks back to
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PLAYING after setting the new base time on them. It's also possible
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to perform additional latency calculations and adjustments before doing this.
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Dynamically adjusting latency
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-----------------------------
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An element that want to change the latency in the pipeline can do this by
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posting a LATENCY message on the bus. This message instructs the pipeline to:
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- query the latency in the pipeline (which might now have changed) with a
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LATENCY query.
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- redistribute a new global latency to all elements with a LATENCY event.
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A use case where the latency in a pipeline can change could be a network element
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that observes an increased inter packet arrival jitter or excessive packet loss
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and decides to increase its internal buffering (and thus the latency). The
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element must post a LATENCY message and perform the additional latency
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adjustments when it receives the LATENCY event from the downstream peer element.
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In a similar way can the latency be decreased when network conditions are
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improving again.
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Latency adjustments will introduce glitches in playback in the sinks and must
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only be performed in special conditions.
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