I have a Logitech C920 webcam that provides properly formatted h264 video, and a mic hooked up to an ASUS Xonar external USB sound card. I can read both and mux their data into a single file like this:
gst-launch-1.0 -e \ mp4mux name=muxy ! filesink location=/tmp/out.mp4 \ alsasrc device='hw:Device,0' do-timestamp=true ! audio/x-raw,rate=48000 ! audioconvert ! queue ! lamemp3enc ! muxy.audio_0 \ v4l2src do-timestamp=true ! video/x-h264,framerate=30/1,height=720 ! h264parse ! queue ! muxy.video_0
...but then I get poorly synchronized audio/video. The audio flow consistently starts with 250ms of garbage noise, and the resulting mp4 video is 250ms (7 or 8 frames at 30fps) out of sync.
Seems like the sources start simultaneously, but the sound card inserts 250ms of initialization junk every time. Or perhaps, the camera takes 250ms longer to start up but reports an incorrect start of stream flag. Or, maybe the clocks in my devices are out of sync for some reason. I don't know how to figure out the difference between these (and other) potential root causes.
Whatever the cause, I'd like to patch over the symptoms at least. I've been trying to do any of the following in the gstreamer pipeline, any of which would satisfy my requirements:
- Cut out the first 250ms of audio
- Delay the video by 250ms or 7 frames
- Synchronize the audio and video timestamps properly with attributes like alsasrc slave-method or v4l2src io-mode
And I'm apparently doing it wrong. Nothing works. No matter what, I always end up with the video running 250ms/7 frames ahead of the audio. Adding the queue elements reportedly fixed the sync issue as mediainfo now reports Duration values for audio and wideo within 20ms of each other, which would be acceptable. But that's not how the resulting videos actually work. Clap my hands, the noise arrives late.
This can be fixed in post processing but why not avoid the hassle and get it right, straight from the gst pipeline? I'm all out of tricks and just about ready to fall back to fixing every single video's sync by hand instead. Any ideas out there?
Thanks for any help, tips, ideas.