在g上的GStreamer管道中添加和删除音频源

Question

我编写了一个小Python脚本，它使用一个Adder插件将两个源流混合在一起。

启动程序后，您将听到由audiotestsrc插件生成的1kHz音调。当您按Enter键时，另一个500Hz的测试音连接到Adder上，所以您可以同时听到它们。（顺便说一句，我真的不明白为什么我要把管道重新设置到播放状态来听混音。有没有什么方法可以在不重启管道的情况下插入新的源代码？）

当你再次按回车键时，1kHz的音调应该从混音中删除，500Hz的音调应该继续播放，但是我什么也听不到了。最后一行是调试输出中的pulse pulsesink.c:528:gst_pulsering_stream_underflow_cb:<pulseaudio_output> Got underflow。我真不知道下一步该怎么做。

以下是完整的源代码：

#!/usr/bin/python
# On-the-go source removal doesn't work this way with GStreamer. Why?

import gobject;
gobject.threads_init()
import gst;

if __name__ == "__main__":
    pipe = gst.Pipeline("mypipe")

    adder = gst.element_factory_make("adder","audiomixer")
    pipe.add(adder)

    buzzer = gst.element_factory_make("audiotestsrc","buzzer")
    buzzer.set_property("freq",1000)
    pipe.add(buzzer)

    pulse = gst.element_factory_make("pulsesink", "pulseaudio_output")
    pipe.add(pulse)

    buzzer.link(adder)
    adder.link(pulse)
    pipe.set_state(gst.STATE_PLAYING)

    raw_input("1kHz test sound. Press <ENTER> to continue.")

    buzzer2=gst.element_factory_make("audiotestsrc","buzzer2")
    buzzer2.set_property("freq",500)

    pipe.add(buzzer2)
    buzzer2.link(adder)
    pipe.set_state(gst.STATE_PLAYING)

    raw_input("1kHz + 500Hz test sound playing simoultenously. Press <ENTER> to continue.")

    buzzer.unlink(adder)
    pipe.set_state(gst.STATE_PLAYING)

    raw_input("Only 500Hz test sound. Press <ENTER> to stop.")

Answer 1

我自己找到了解决办法。我必须使用带有Adder的请求板，并使用GStreamer的pad blocking功能。

下面是工作源代码，其中包含一些说明：

#!/usr/bin/python

import gobject;
gobject.threads_init()
import gst;

if __name__ == "__main__":
    # First create our pipeline
    pipe = gst.Pipeline("mypipe")

    # Create a software mixer with "Adder"
    adder = gst.element_factory_make("adder","audiomixer")
    pipe.add(adder)

    # Gather a request sink pad on the mixer
    sinkpad1=adder.get_request_pad("sink%d")

    # Create the first buzzer..
    buzzer1 = gst.element_factory_make("audiotestsrc","buzzer1")
    buzzer1.set_property("freq",1000)
    pipe.add(buzzer1)
    # .. and connect it's source pad to the previously gathered request pad
    buzzersrc1=buzzer1.get_pad("src")
    buzzersrc1.link(sinkpad1)

    # Add some output
    output = gst.element_factory_make("autoaudiosink", "audio_out")
    pipe.add(output)
    adder.link(output)

    # Start the playback
    pipe.set_state(gst.STATE_PLAYING)

    raw_input("1kHz test sound. Press <ENTER> to continue.")

    # Get an another request sink pad on the mixer
    sinkpad2=adder.get_request_pad("sink%d")

    # Create an another buzzer and connect it the same way
    buzzer2 = gst.element_factory_make("audiotestsrc","buzzer2")
    buzzer2.set_property("freq",500)
    pipe.add(buzzer2)

    buzzersrc2=buzzer2.get_pad("src")
    buzzersrc2.link(sinkpad2)

    # Start the second buzzer (other ways streaming stops because of starvation)
    buzzer2.set_state(gst.STATE_PLAYING)

    raw_input("1kHz + 500Hz test sound playing simoultenously. Press <ENTER> to continue.")

    # Before removing a source, we must use pad blocking to prevent state changes
    buzzersrc1.set_blocked(True)
    # Stop the first buzzer
    buzzer1.set_state(gst.STATE_NULL)
    # Unlink from the mixer
    buzzersrc1.unlink(sinkpad1)
    # Release the mixers first sink pad
    adder.release_request_pad(sinkpad1)
    # Because here none of the Adder's sink pads block, streaming continues

    raw_input("Only 500Hz test sound. Press <ENTER> to stop.")