[FFmpeg-trac] #3615(ffmpeg:new): -y option affects audio/video skew during screen capture (alsa+x11grab)

FFmpeg trac at avcodec.org
Sat Nov 16 23:24:15 EET 2019

#3615: -y option affects audio/video skew during screen capture (alsa+x11grab)
             Reporter:  grepfor     |                    Owner:
                 Type:  defect      |                   Status:  new
             Priority:  normal      |                Component:  ffmpeg
              Version:  git-master  |               Resolution:
             Keywords:              |               Blocked By:
             Blocking:              |  Reproduced by developer:  0
Analyzed by developer:  0           |
Changes (by madscientist159):

 * version:  2.2.1 => git-master


 I am able to confirm this bug exists in the latest FFmpeg GIT, *and* that
 it affects more than just screen capture.

 Easiest way to see it in action is to use one alsa and one video4linux
 input into some kind of stream muxer (which one doesn't seem to matter),
 then wait for a bit on the "Overwrite?" screen.  If you wait on that
 screen, and the capture hardware doesn't have massive buffers available,
 you'll see an ALSA XRUN indicating the audio device was open and running
 the entire time the prompt was shown.  Passing -y produces no XRUN.

 My primary concern right now is that I've been trying to track down a
 subtle, constant A/V desync with properly timestamped ALSA/V4L buffers
 (hardware synced clocks), and I'm wondering if this issue is related.  If
 pausing on the Overwrite prompt causes buffer overrun, I wonder if the
 small amount of code executed between the ALSA device opening and the V4L
 device opening with -y passed could still be introducing the slight
 (~20ms) constant desync.

 I'm not giving a full command line because capture devices will vary
 between users, and a live source does seem to be required to trigger the
 issue (x11 screen grab, V4L capture, etc.).

Ticket URL: <https://trac.ffmpeg.org/ticket/3615#comment:5>
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