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GStreamer clock

The GStreamer clock is the global time reference that the pipeline uses to decide:

WHEN each buffer should be processed, rendered, or sent

Without a clock: - Buffers would flow as fast as possible - Video would play too fast - Audio/video would drift - Network streams would burst

The clock allows: - Real-time playback - Audio ↔ video sync - Smooth network pacing

Demo: play mp4 file

Change autovideosink sync - True (default) use clock to play the video - False: play the video as fast as possible

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gst-launch-1.0 filesrc location=video.mp4 \
! decodebin \
! videoconvert \
! autovideosink sync=false

Clock + timestamps (critical pair)

Every buffer may carry a timestamp:

  • PTS – when to present
  • DTS – when to decode

The sink compares: buffer PTS vs pipeline clock time

Decision:

  • PTS > now → wait
  • PTS ≈ now → render/send
  • PTS < now → late (drop or rush)

This is why sync=true matters.

Clock source

Only one element provides the clock.

Clock provider priority: - Audio sink (most stable) - Live source (camera, mic, RTP) - System clock (fallback)

Clock in different pipeline types

File playback
  • No audio → system clock
  • With audio → audio sink clock
Demo
the output from the above filesrc pipe
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Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...
Redistribute latency...
Got context from element 'nvh264dec0': gst.cuda.context=context, gst.cuda.context=(GstCudaContext)"\(GstCudaContext\)\ cudacontext1", cuda-device-id=(uint)0;
Redistribute latency...
Redistribute latency...
Pipeline is PREROLLED ...
Setting pipeline to PLAYING ...
Redistribute latency...
New clock: GstSystemClock
Redistribute latency...
Got EOS from element "pipeline0".
Execution ended after 0:00:00.711912146
Setting pipeline to NULL ...
Freeing pipeline ..

Camera

  • Camera provides clock
  • No preroll
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gst-launch-1.0 v4l2src ! videoconvert ! autovideosink

RTP / UDP streaming

Sender
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appsrc is-live=true do-timestamp=true !
...
udpsink sync=true
  • Sender timestamps buffers
  • Sink uses clock to pace packets
Receiver
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udpsrc !
rtpjitterbuffer !
...
autovideosink
  • rtpjitterbuffer re-aligns timestamps
  • Sink syncs to clock

Live vs non-live sources (very important)

Source is-live Clock behavior
filesrc false preroll, seekable
v4l2src true no preroll
udpsrc true real-time
appsrc (live) true you control time

preroll

Preroll is the process where a pipeline buffers just enough data so that playback can start smoothly at time 0.