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I was recording screencasts on Ubuntu 16.04 for youtube with my plantronics headset C520-M which worked just nice. I was using ffmpeg successfully.

Now I wanted to add some "live" video explaining things at a whiteboard, to make the video more attractive.

I was planning to record the live view with Logitech HD Pro Webcam C920, which also has builtin microphones that work not that bad. I can not use the headset for sound, since the whiteboard is too far away from the computer... (plus it might look weird).

I was using this command:

ffmpeg -f v4l2 -framerate 25  -video_size 1280x720 -i /dev/video1 \
       -f alsa -ac 2 -thread_queue_size 1024 -i default \
        output.mkv

That always resulted in

[alsa @ 0x69d540] Thread message queue blocking; consider raising the thread_queue_size option (current value: 1024)

The sound then stops at this point for the rest of the recording.

Thus I have a couple of questions:

  • Why does it happen that the audio queue blocks?
  • How far should I raise the thread_queue_size?
  • How can I avoid that the audio queue blocks?
  • Which audio input should I use - actually I tried all of them an the sound stops at some point.

Thanks for any hints.

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