[FFmpeg-trac] #11314(ffmpeg:new): stuttering audio / strange audio pts when capturing v4l2 stream

[FFmpeg]

#11314: stuttering audio / strange audio pts when capturing v4l2 stream
----------------------------------+----------------------------------
             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>
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