mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-11-18 15:51:11 +00:00
228 lines
8.1 KiB
Text
228 lines
8.1 KiB
Text
Synchronisation
|
|
---------------
|
|
|
|
This document outlines the techniques used for doing synchronised playback of
|
|
multiple streams.
|
|
|
|
Synchronisation in a GstPipeline is achieved using the following 3 components:
|
|
|
|
- a GstClock, which is global for all elements in a GstPipeline.
|
|
- Timestamps on a GstBuffer.
|
|
- the SEGMENT event preceding the buffers.
|
|
|
|
|
|
A GstClock
|
|
~~~~~~~~~~
|
|
|
|
This object provides a counter that represents the current time in nanoseconds.
|
|
This value is called the absolute_time.
|
|
|
|
Different sources exist for this counter:
|
|
|
|
- the system time (with g_get_current_time() and with microsecond accuracy)
|
|
- monotonic time (with g_get_monotonic_time () with microsecond accuracy)
|
|
- an audio device (based on number of samples played)
|
|
- a network source based on packets received + timestamps in those packets (a
|
|
typical example is an RTP source)
|
|
- ...
|
|
|
|
In GStreamer any element can provide a GstClock object that can be used in the
|
|
pipeline. The GstPipeline object will select a clock from all the providers and
|
|
will distribute it to all other elements (see part-gstpipeline.txt).
|
|
|
|
A GstClock always counts time upwards and does not necessarily start at 0.
|
|
|
|
While it is possible, it is not recommended to create a clock derived from the
|
|
contents of a stream (for example, create a clock from the PCR in an mpeg-ts
|
|
stream).
|
|
|
|
|
|
Running time
|
|
~~~~~~~~~~~~
|
|
|
|
After a pipeline selected a clock it will maintain the running_time based on the
|
|
selected clock. This running_time represents the total time spent in the PLAYING
|
|
state and is calculated as follows:
|
|
|
|
- If the pipeline is NULL/READY, the running_time is undefined.
|
|
- In PAUSED, the running_time remains at the time when it was last
|
|
PAUSED. When the stream is PAUSED for the first time, the running_time
|
|
is 0.
|
|
- In PLAYING, the running_time is the delta between the absolute_time
|
|
and the base time. The base time is defined as the absolute_time minus
|
|
the running_time at the time when the pipeline is set to PLAYING.
|
|
- after a flushing seek, the running_time is set to 0 (see part-seeking.txt).
|
|
This is accomplished by redistributing a new base_time to the elements that
|
|
got flushed.
|
|
|
|
This algorithm captures the running_time when the pipeline is set from PLAYING
|
|
to PAUSED and restores this time based on the current absolute_time when going
|
|
back to PLAYING. This allows for both clocks that progress when in the PAUSED
|
|
state (systemclock) and clocks that don't (audioclock).
|
|
|
|
The clock and pipeline now provide a running_time to all elements that want to
|
|
perform synchronisation. Indeed, the running time can be observed in each
|
|
element (during the PLAYING state) as:
|
|
|
|
C.running_time = absolute_time - base_time
|
|
|
|
We note C.running_time as the running_time obtained by looking at the clock.
|
|
This value is monotonically increasing at the rate of the clock.
|
|
|
|
|
|
Timestamps
|
|
~~~~~~~~~~
|
|
|
|
The GstBuffer timestamps and the preceeding SEGMENT event (See
|
|
part-streams.txt) define a transformation of the buffer timestamps to
|
|
running_time as follows:
|
|
|
|
The following notation is used:
|
|
|
|
B: GstBuffer
|
|
- B.timestamp = buffer timestamp (GST_BUFFER_PTS or GST_BUFFER_DTS)
|
|
|
|
S: SEGMENT event preceeding the buffers.
|
|
- S.start: start field in the SEGMENT event. This is the lowest allowed
|
|
timestamp.
|
|
- S.stop: stop field in the SEGMENT event. This is the highers allowed
|
|
timestamp.
|
|
- S.rate: rate field of SEGMENT event. This is the desired playback rate.
|
|
- S.base: a base time for the time. This is the total elapsed running_time of any
|
|
previous segments.
|
|
- S.offset: an offset to apply to S.start or S.stop. This is the amount that
|
|
has already been elapsed in the segment.
|
|
|
|
Valid buffers for synchronisation are those with B.timestamp between S.start
|
|
and S.stop (after applying the S.offset). All other buffers outside this range
|
|
should be dropped or clipped to these boundaries (see also part-segments.txt).
|
|
|
|
The following transformation to running_time exist:
|
|
|
|
if (S.rate > 0.0)
|
|
B.running_time = (B.timestamp - (S.start + S.offset)) / ABS (S.rate) + S.base
|
|
else
|
|
B.running_time = ((S.stop - S.offset) - B.timestamp) / ABS (S.rate) + S.base
|
|
|
|
We write B.running_time as the running_time obtained from the SEGMENT event
|
|
and the buffers of that segment.
|
|
|
|
The first displayable buffer will yield a value of 0 (since B.timestamp ==
|
|
S.start and S.offset and S.base == 0).
|
|
|
|
For S.rate > 1.0, the timestamps will be scaled down to increase the playback
|
|
rate. Likewise, a rate between 0.0 and 1.0 will slow down playback.
|
|
|
|
For negative rates, timestamps are received stop S.stop to S.start so that the
|
|
first buffer received will be transformed into B.running_time of 0 (B.timestamp ==
|
|
S.stop and S.accum == 0).
|
|
|
|
This makes it so that B.running_time is always monotonically increasing
|
|
starting from 0 with both positive and negative rates.
|
|
|
|
|
|
Synchronisation
|
|
~~~~~~~~~~~~~~~
|
|
|
|
As we have seen, we can get a running_time:
|
|
|
|
- using the clock and the element's base_time with:
|
|
|
|
C.running_time = absolute_time - base_time
|
|
|
|
- using the buffer timestamp and the preceeding SEGMENT event as (assuming
|
|
positive playback rate):
|
|
|
|
B.running_time = (B.timestamp - (S.start + S.offset)) / ABS (S.rate) + S.base
|
|
|
|
We prefix C. and B. before the two running times to note how they were
|
|
calculated.
|
|
|
|
The task of synchronized playback is to make sure that we play a buffer with
|
|
B.running_time at the moment when the clock reaches the same C.running_time.
|
|
|
|
Thus the following must hold:
|
|
|
|
B.running_time = C.running_time
|
|
|
|
expaning:
|
|
|
|
B.running_time = absolute_time - base_time
|
|
|
|
or:
|
|
|
|
absolute_time = B.running_time + base_time
|
|
|
|
The absolute_time when a buffer with B.running_time should be played is noted
|
|
with B.sync_time. Thus:
|
|
|
|
B.sync_time = B.running_time + base_time
|
|
|
|
One then waits for the clock to reach B.sync_time before rendering the buffer in
|
|
the sink (See also part-clocks.txt).
|
|
|
|
For multiple streams this means that buffers with the same running_time are to
|
|
be displayed at the same time.
|
|
|
|
A demuxer must make sure that the SEGMENT it emits on its output pads yield
|
|
the same running_time for buffers that should be played synchronized. This
|
|
usually means sending the same SEGMENT on all pads and making sure that the
|
|
synchronized buffers have the same timestamps.
|
|
|
|
|
|
Stream time
|
|
~~~~~~~~~~~
|
|
|
|
The stream time is also known as the position in the stream and is a value
|
|
between 0 and the total duration of the media file.
|
|
|
|
It is the stream time that is used for:
|
|
|
|
- report the POSITION query in the pipeline
|
|
- the position used in seek events/queries
|
|
- the position used to synchronize controller values
|
|
|
|
Additional fields in the SEGMENT are used:
|
|
|
|
- S.time: time field in the SEGMENT event. This the stream-time of S.start
|
|
- S.applied_rate: The rate already applied to the stream.
|
|
|
|
Stream time is calculated using the buffer times and the preceeding SEGMENT
|
|
event as follows:
|
|
|
|
stream_time = (B.timestamp - S.start) * ABS (S.applied_rate) + S.time
|
|
|
|
For negative rates, B.timestamp will go backwards from S.stop to S.start,
|
|
making the stream time go backwards.
|
|
|
|
In the PLAYING state, it is also possible to use the pipeline clock to derive
|
|
the current stream_time.
|
|
|
|
Give the two formulas above to match the clock times with buffer timestamps
|
|
allows us to rewrite the above formula for stream_time (and for positive rates).
|
|
|
|
C.running_time = absolute_time - base_time
|
|
B.running_time = (B.timestamp - (S.start + S.offset)) / ABS (S.rate) + S.base
|
|
|
|
=>
|
|
(B.timestamp - (S.start + S.offset)) / ABS (S.rate) + S.base = absolute_time - base_time;
|
|
|
|
=>
|
|
(B.timestamp - (S.start + S.offset)) / ABS (S.rate) = absolute_time - base_time - S.base;
|
|
|
|
=>
|
|
(B.timestamp - (S.start + S.offset)) = (absolute_time - base_time - S.base) * ABS (S.rate)
|
|
|
|
=>
|
|
(B.timestamp - S.start) = S.offset + (absolute_time - base_time - S.base) * ABS (S.rate)
|
|
|
|
filling (B.timestamp - S.start) in the above formule for stream time
|
|
|
|
=>
|
|
stream_time = (S.offset + (absolute_time - base_time - S.base) * ABS (S.rate)) * S.abs_applied_rate + S.time
|
|
|
|
This last formula is typically used in sinks to report the current position in
|
|
an accurate and efficient way.
|
|
|
|
Note that the stream time is never used for synchronisation against the clock.
|
|
|