#9776: pulse and x11grab: audio is around 2 seconds behind video
-------------------------------------+-------------------------------------
Reporter: merulasnyde | Type: defect
Status: new | Priority: normal
Component: avdevice | Version: git-
Keywords: pulse | master
pulseaudio avdevice delay desync | Blocked By:
audio audio/video |
Blocking: | Reproduced by developer: 0
Analyzed by developer: 0 |
-------------------------------------+-------------------------------------
Summary of the bug: When recording audio+video of my screen, the captured
audio is around 2 seconds behind the video.
How to reproduce:
{{{
% ffmpeg -f pulse -ac 2 -channel_layout stereo -i default -r 60
-video_size 640x480 -f x11grab -i :0.0+0,0 -c:a libvorbis -b:a 192k -c:v
libx264 -b:v 2M delayed_audio.mkv
* Start the capture (possibly with some audio already playing so
pavucontrol can be checked to see whether audio is being recorded)
* Move something that can produce audio at demand into the captured
region, e.g. a media player with a play/pause button.
* Periodically push the play/pause button to start and stop audio.
* Check the recorded video and see whether the audio start+stop matches
the pushes of the button
}}}
{{{
ffmpeg version f77ac5131cd1c623aa54ec432cba0d922e9aa470
ffmpeg version N-106835-gf77ac5131c Copyright (c) 2000-2022 the FFmpeg
developers
built with gcc 11.2.0 (Gentoo Hardened 11.2.0 p1)
configuration: --enable-libx264 --enable-libvorbis --enable-libpulse
--enable-gpl --enable-ffmpeg --enable-version3 --enable-nonfree --disable-
ffplay --disable-ffprobe --disable-libx265 --disable-libv4l2 --disable-
libvpx --disable-nvenc --disable-nvdec --disable-cuda --disable-vaapi
--disable-librtmp --disable-ffnvcodec --disable-hwaccels --disable-filters
--enable-filter=scale --enable-filter=aresample --disable-decoders
--disable-encoders --enable-decoder=rawvideo --enable-decoder=pcm_s16le
--enable-encoder=libx264 --enable-encoder=libvorbis --disable-doc
libavutil 57. 24.101 / 57. 24.101
libavcodec 59. 27.100 / 59. 27.100
libavformat 59. 23.100 / 59. 23.100
libavdevice 59. 6.100 / 59. 6.100
libavfilter 8. 38.100 / 8. 38.100
libswscale 6. 6.100 / 6. 6.100
libswresample 4. 6.100 / 4. 6.100
libpostproc 56. 5.100 / 56. 5.100
}}}
It was fine in 4.3 and stopped working in 4.4-something.
I ran a bisect between 4.3 and some bad commit in 4.4, using the strategy
outlined above to determine whether the commit is good or bad.
The bisect revealed this to be a (first) bad commit:
b2d0826513c5e76f9bad2f1f0c809bc5c8e58b0c
{{{
Author: Marton Balint <[email protected]>
Date: Sat Feb 6 19:48:51 2021 +0100
avdevice/pulse_audio_dec: do not read undersized frames
Keep on reading fragments until we got fragment_size amount of data,
otherwise
we might get frames with 1-2 samples only if pa_stream_peek is called
slightly
less frequently than sample rate.
Note that fragments might contain a lot less data than fragment_size,
so
reading multiple fragments to get fragment_size amount of data is
intentional.
Signed-off-by: Marton Balint <[email protected]>
}}}
I was also able to reproduce it on a precompiled binary of (the same)
master N-106835-gf77ac5131c-20220509 prior to the bisect.
Because it seems to be a pulse issue I imagine there is some extra info
that could be useful (e.g. pertaining to my pulseaudio installation).
--
Ticket URL: <https://trac.ffmpeg.org/ticket/9776>
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