#11314: stuttering audio / strange audio pts when capturing v4l2 stream
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Reporter: chconnor | Type: defect
Status: new | Priority: normal
Component: ffmpeg | Version: 7.0
Keywords: | Blocked By:
Blocking: | Reproduced by developer: 0
Analyzed by developer: 0 |
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(I an non-expert -- apologies in advance if this is my confusion or a bug
in my capture card's driver!)
I am capturing an audio stream and a v4l2 stream from an S/Video capture
card (Hauppauge USB-Live2) like this:
{{{
ffmpeg -report -f alsa -ac 2 -i hw:CARD=Cx231xxAudio,DEV=0 -f v4l2 -i
/dev/video2 -c:a copy -c:v rawvideo broken.avi
}}}
It generates a broken file that has stuttering audio, even though the
individual audio and video streams work fine if demuxed. If I remux the
file like this the result also works:
{{{
ffmpeg -i broken.avi -c:a copy -c:v copy working-remuxed.avi
}}}
If I manually specify the audio sample type and rate at capture, it works:
{{{
ffmpeg -report -f alsa -ac 2 -i hw:CARD=Cx231xxAudio,DEV=0 -f v4l2 -i
/dev/video2 -c:a pcm_s16le -ar 48000 -c:v rawvideo working.avi
}}}
This strikes me as odd since in the broken case ffmpeg correctly
identifies the incoming audio as pcm_s16le and 48kHz.
If I analyze the broken file with ashowinfo, it shows audio pts that are
increasing by 4x the nb_samples. I don't know if this is wrong or not, but
the "working" version has audio pts that increase by 1x the nb_samples.
Broken version (see how first nb_samples is 1024 and pts goes from 0 to
4096):
{{{
$ ffmpeg -i test.avi -vf showinfo -af ashowinfo -fps_mode passthrough
-copyts -f null /dev/null 2> pts.txt
...
[Parsed_ashowinfo_0 @ 0x70654c002f80] n:0 pts:0 pts_time:0 fmt:s16
channels:2 chlayout:stereo rate:48000 nb_samples:1024 checksum:73F22CAC
plane_checksums: [ 73F22CAC ]
[Parsed_ashowinfo_0 @ 0x70654c002f80] n:1 pts:4096 pts_time:0.0853333
fmt:s16 channels:2 chlayout:stereo rate:48000 nb_samples:256
checksum:929680FC plane_checksums: [ 929680FC ]
[Parsed_ashowinfo_0 @ 0x70654c002f80] n:2 pts:5120 pts_time:0.106667
fmt:s16 channels:2 chlayout:stereo rate:48000 nb_samples:1024
checksum:7A55BBD5 plane_checksums: [ 7A55BBD5 ]
}}}
I note that I get non-monotonic DTS warnings when capturing (either broken
or working version).
Attached is a log for working and broken capture, as well as ashowinfo
results for working and broken.
--
Ticket URL: <https://trac.ffmpeg.org/ticket/11314>
FFmpeg <https://ffmpeg.org>
FFmpeg issue tracker
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