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Adding a Source

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This Example creates a Pipeline like this:

  audiotestsrc is-live=true ! audiomixer ! autoaudiosink

It installs Pad-Probes after the audiotestsrc and the audiomixer, which log the PTS-Timestamps of the Buffers flowing through the Element's src-Pads.

After 2 seconds, another audiotestsrc is created with such a Pad-Probe and linked to the audiomixer.

On the Speaker/Headphone you can hear the 220 Hz-Zone from the first audiotestsrc, which after 2 Seconds gets mixed with the 440 Hz Tone from the second audiotestsrc.

On the Console you can see that the Timestamps from the second audiotestsrc start around the 0:00:02 seconds mark, because we configured it to be a true live-source.

The most important Lines have been marked as such:

  1. We schedule the execution on the GLib Event-Loop by using Glib.idle_add. On the Console you can see, that the Sequence runs in its own Sequence-Thread but the second Test-Source is actually added from the MainThread.

    It seems that this exact example also works without this, because some of the Methods are Thread-Safe by Documentation (ie. Bin.add) and some are by luck (ie GObject.Object.set_property), but not following this pattern can lead to unexpected and hard to find issues.

    From the console you can also see, that the Pad-Probes are called synchronously from their respective Elements' Streaming-Threads (Shown as Dummy-[n]).

  2. Forcing the audiotestsrc to be a Live-Source makes it produce Buffers with timestamps starting at the current Running-Time of the Pipeline. A Non-Live-Source would start ad 0:00:00 and run faster then the other sources, until it would have caught up to the current Running-Time.

    Try to comment the Line marked with (2) out and look at the Console-Output around the 0:00:02 seconds mark

  3. Furthermore it is important that all other Sources in the Pipeline are also in Live-Mode, here this is ensured by setting the is-live property of testsrc1. If one of the Sources connected to the same downstream Elements (here for example the audsiomixer) are not live, they will produce as many buffers as the sink allows them to. In a pipeline like the following, where neither source nor sink enforces live-behaviour, the timestamp the audiomixer is working with might be way ahead of those produced by your newly added live-source: audiotestsrc ! audiomixer ! wavenc ! filesink …

    But even if your sink is live, like an alsasink for example, the timestamp at the audiomixer might be multiple seconds ahead of your new Live-Source. GStreamer will then halt the downstream elements until you new Live-Source has caught up, possibly dropping data from other Live-Sources and not sending any Buffers out to the sink.

    If you are dealing with sources that do not support live behaviour, for example a filesrc, you should place an identity-Element with the sync-Property set to True right after it, so that it behaves like a live-source to downstream elements like audiomixers.

  4. An Element does not automatically take over its parent state. Also, not all Elements in a Pipeline have to have the same state. In this case the new audiotestsrc-Element starts in NULL state and is added as such to the Pipeline. Once it is added, its state is synced to the pipeline (PLAYING) which makes it switch from NULL through READY and PAUSED to PLAYING, where it then starts generating buffers and sending them downstream to the audiomixer.