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269 lines
8.9 KiB
Markdown
269 lines
8.9 KiB
Markdown
# Synchronisation
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This document outlines the techniques used for doing synchronised
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playback of multiple streams.
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Synchronisation in a `GstPipeline` is achieved using the following 3
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components:
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- a `GstClock`, which is global for all elements in a `GstPipeline`.
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- Timestamps on a `GstBuffer`.
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- the SEGMENT event preceding the buffers.
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## A GstClock
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This object provides a counter that represents the current time in
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nanoseconds. This value is called the `absolute_time`. A `GstClock`
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always counts time upwards and does not necessarily start at 0.
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Different sources exist for this counter:
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- the system time (with `g_get_current_time()` and with microsecond
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accuracy)
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- monotonic time (with `g_get_monotonic_time()` with microsecond
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accuracy)
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- an audio device (based on number of samples played)
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- a network source based on packets received + timestamps in those
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packets (a typical example is an RTP source)
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- …
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In GStreamer any element can provide a `GstClock` object that can be used
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in the pipeline. The `GstPipeline` object will select a clock from all the
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providers and will distribute it to all other elements (see
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[gstpipeline](additional/design/gstpipeline.md)).
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While it is possible, it is not recommended to create a clock derived
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from the contents of a stream (for example, create a clock from the PCR
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in an mpeg-ts stream).
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## Running time
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After a pipeline selected a clock it will maintain the `running_time`
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based on the selected clock. This `running_time` represents the total
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time spent in the PLAYING state and is calculated as follows:
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- If the pipeline is NULL/READY, the `running_time` is undefined.
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- In PAUSED, the `running_time` remains at the time when it was last
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PAUSED. When the stream is `PAUSED` for the first time, the
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`running_time` is 0.
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- In PLAYING, the `running_time` is the delta between the
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`absolute_time` and the base time. The base time is defined as the
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`absolute_time` minus the `running_time` at the time when the pipeline
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is set to `PLAYING`.
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- after a flushing seek, the `running_time` is set to 0 (see
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[seeking](additional/design/seeking.md)). This is accomplished by redistributing a new
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base\_time to the elements that got flushed.
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This algorithm captures the `running_time` when the pipeline is set from
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`PLAYING` to `PAUSED` and restores this time based on the current
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`absolute_time` when going back to `PLAYING`. This allows for both clocks
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that progress when in the `PAUSED` state (systemclock) and clocks that
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don’t (audioclock).
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The clock and pipeline now provide a `running_time` to all elements that
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want to perform synchronisation. Indeed, the running time can be
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observed in each element (during the PLAYING state) as:
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```
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C.running_time = absolute_time - base_time
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```
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We note `C.running_time` as the `running_time` obtained by looking at the
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clock. This value is monotonically increasing at the rate of the clock.
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## Timestamps
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The `GstBuffer` timestamps and the preceding SEGMENT event (See
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[streams](additional/design/streams.md)) define a transformation of the buffer timestamps
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to `running_time` as follows:
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The following notation is used:
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**B**: `GstBuffer`
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- B.timestamp = buffer timestamp (`GST_BUFFER_PTS` or `GST_BUFFER_DTS`)
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**S**: SEGMENT event preceding the buffers.
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- S.start: start field in the SEGMENT event. This is the lowest allowed
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timestamp.
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- S.stop: stop field in the SEGMENT event. This is the highers allowed
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timestamp.
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- S.rate: rate field of SEGMENT event. This is the playback rate.
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- S.base: a base time for the time. This is the total elapsed `running_time`
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of any previous segments.
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- S.offset: an offset to apply to S.start or S.stop. This is the amount that
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has already been elapsed in the segment.
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Valid buffers for synchronisation are those with B.timestamp between
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`S.start` and `S.stop` (after applying the `S.offset`). All other buffers
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outside this range should be dropped or clipped to these boundaries (see
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also [segments](additional/design/segments.md)).
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The following transformation to `running_time` exist:
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```
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if (S.rate > 0.0)
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B.running_time = (B.timestamp - (S.start + S.offset)) / ABS (S.rate) + S.base
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=>
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B.timestamp = (B.running_time - S.base) * ABS (S.rate) + S.start + S.offset
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else
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B.running_time = ((S.stop - S.offset) - B.timestamp) / ABS (S.rate) + S.base
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=>
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B.timestamp = S.stop - S.offset - ((B.running_time - S.base) * ABS (S.rate))
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```
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We write `B.running_time` as the `running_time` obtained from the `SEGMENT`
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event and the buffers of that segment.
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The first displayable buffer will yield a value of 0 (since `B.timestamp
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== S.start and S.offset and S.base == 0`).
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For `S.rate > 1.0`, the timestamps will be scaled down to increase the
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playback rate. Likewise, a rate between 0.0 and 1.0 will slow down
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playback.
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For negative rates, timestamps are received stop S.stop to `S.start` so
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that the first buffer received will be transformed into `B.running_time`
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of 0 (`B.timestamp == S.stop and S.base == 0`).
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This makes it so that `B.running_time` is always monotonically increasing
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starting from 0 with both positive and negative rates.
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## Synchronisation
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As we have seen, we can get a `running_time`:
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- using the clock and the element’s `base_time` with:
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```
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C.running_time = absolute_time - base_time
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```
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- using the buffer timestamp and the preceding `SEGMENT` event as (assuming
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positive playback rate):
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```
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B.running_time = (B.timestamp - (S.start + S.offset)) / ABS (S.rate) + S.base
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```
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We prefix C. and B. before the two running times to note how they were
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calculated.
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The task of synchronized playback is to make sure that we play a buffer
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with `B.running_time` at the moment when the clock reaches the same
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`C.running_time`.
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Thus the following must hold:
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```
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B.running_time = C.running_time
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```
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expaning:
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```
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B.running_time = absolute_time - base_time
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```
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or:
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```
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absolute_time = B.running_time + base_time
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```
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The `absolute_time` when a buffer with `B.running_time` should be played
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is noted with `B.sync_time`. Thus:
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```
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B.sync_time = B.running_time + base_time
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```
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One then waits for the clock to reach `B.sync_time` before rendering the
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buffer in the sink (See also [clocks](additional/design/clocks.md)).
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For multiple streams this means that buffers with the same `running_time`
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are to be displayed at the same time.
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A demuxer must make sure that the `SEGMENT` it emits on its output pads
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yield the same `running_time` for buffers that should be played
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synchronized. This usually means sending the same `SEGMENT` on all pads
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and making sure that the synchronized buffers have the same timestamps.
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## Stream time
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The stream time is also known as the position in the stream and is a
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value between 0 and the total duration of the media file.
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It is the stream time that is used for:
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- report the `POSITION` query in the pipeline
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- the position used in seek events/queries
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- the position used to synchronize controller values
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Additional fields in the `SEGMENT` are used:
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- `S.time`: time field in the `SEGMENT` event. This the stream-time of
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`S.start`
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- `S.applied_rate`: The rate already applied to the segment.
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Stream time is calculated using the buffer times and the preceding
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`SEGMENT` event as follows:
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```
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stream_time = (B.timestamp - S.start) * ABS (S.applied_rate) + S.time
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=> B.timestamp = (stream_time - S.time) / ABS(S.applied_rate) + S.start
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```
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For negative rates, `B.timestamp` will go backwards from `S.stop` to
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`S.start`, making the stream time go backwards:
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```
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stream_time = (S.stop - B.timestamp) * ABS(S.applied_rate) + S.time
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=> B.timestamp = S.stop - (stream_time - S.time) / ABS(S.applied_rate)
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```
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In the `PLAYING` state, it is also possible to use the pipeline clock to
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derive the current `stream_time`.
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Give the two formulas above to match the clock times with buffer
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timestamps allows us to rewrite the above formula for `stream_time` (and
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for positive rates).
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```
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C.running_time = absolute_time - base_time
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B.running_time = (B.timestamp - (S.start + S.offset)) / ABS (S.rate) + S.base
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=>
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(B.timestamp - (S.start + S.offset)) / ABS (S.rate) + S.base = absolute_time - base_time;
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=>
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(B.timestamp - (S.start + S.offset)) / ABS (S.rate) = absolute_time - base_time - S.base;
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=>
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(B.timestamp - (S.start + S.offset)) = (absolute_time - base_time - S.base) * ABS (S.rate)
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=>
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(B.timestamp - S.start) = S.offset + (absolute_time - base_time - S.base) * ABS (S.rate)
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filling (B.timestamp - S.start) in the above formule for stream time
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=>
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stream_time = (S.offset + (absolute_time - base_time - S.base) * ABS (S.rate)) * ABS (S.applied_rate) + S.time
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```
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This last formula is typically used in sinks to report the current
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position in an accurate and efficient way.
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Note that the stream time is never used for synchronisation against the
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clock.
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