A/V desync for possibly broken flv input

[Carl Eugen Hoyos]

A user uploaded a flv sample that plays fine with ffplay but shows progressive A/V desync (10 seconds for the 70 second sample) when transcoding. The same issue can be reproduced with both mplayer -ac ffaac and mplayer -ac faad, vlc regularly inserts silence to keep A/V sync.

$ ffmpeg -i footy.flv -qscale 2 out.avi
ffmpeg version N-77045-ga16243a Copyright (c) 2000-2015 the FFmpeg developers
  built with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl
  libavutil      55.  9.100 / 55.  9.100
  libavcodec     57. 16.101 / 57. 16.101
  libavformat    57. 19.100 / 57. 19.100
  libavdevice    57.  0.100 / 57.  0.100
  libavfilter     6. 20.100 /  6. 20.100
  libswscale      4.  0.100 /  4.  0.100
  libswresample   2.  0.101 /  2.  0.101
  libpostproc    54.  0.100 / 54.  0.100
Input #0, live_flv, from 'footy.flv':
  Metadata:
    Server          : NGINX RTMP (github.com/arut/nginx-rtmp-module)
    displayWidth    : 960
    displayHeight   : 540
    fps             : 29
    profile         : ����������������������������������������������������������������
    level           : ����������������������������������������������������������������
  Duration: 00:00:00.00, start: 0.030000, bitrate: N/A
    Stream #0:0: Video: h264 (Main), yuv420p, 960x540 [SAR 1:1 DAR 16:9], 2349 kb/s, 30.30 fps, 66.67 tbr, 1k tbn, 59.94 tbc
    Stream #0:1: Audio: aac (LC), 48000 Hz, stereo, fltp, 95 kb/s
Please use -q:a or -q:v, -qscale is ambiguous
Output #0, avi, to 'out.avi':
  Metadata:
    Server          : NGINX RTMP (github.com/arut/nginx-rtmp-module)
    displayWidth    : 960
    displayHeight   : 540
    fps             : 29
    profile         : ����������������������������������������������������������������
    level           : ����������������������������������������������������������������
    ISFT            : Lavf57.19.100
    Stream #0:0: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 960x540 [SAR 1:1 DAR 16:9], q=2-31, 200 kb/s, 29.97 fps, 29.97 tbn, 29.97 tbc
    Metadata:
      encoder         : Lavc57.16.101 mpeg4
    Stream #0:1: Audio: ac3 ([0] [0][0] / 0x2000), 48000 Hz, stereo, fltp, 192 kb/s
    Metadata:
      encoder         : Lavc57.16.101 ac3
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> mpeg4 (native))
  Stream #0:1 -> #0:1 (aac (native) -> ac3 (native))
Press [q] to stop, [?] for help
[avi @ 0x2118920] Non-monotonous DTS in output stream 0:1; previous: 7, current: 5; changing to 8. This may result in incorrect timestamps in the output file.
[avi @ 0x2118920] Non-monotonous DTS in output stream 0:1; previous: 8, current: 6; changing to 9. This may result in incorrect timestamps in the output file.

...

...

[avi @ 0x2118920] Non-monotonous DTS in output stream 0:1; previous: 194, current: 192; changing to 195. This may result in incorrect timestamps in the output file.
[avi @ 0x2118920] Non-monotonous DTS in output stream 0:1; previous: 195, current: 193; changing to 196. This may result in incorrect timestamps in the output file.
Past duration 0.823357 too large
Past duration 0.725456 too large

...

...

Past duration 0.620659 too large
Past duration 0.803474 too large
Past duration 0.684608 too large
[live_flv @ 0x21143c0] Packet mismatch 0 21841
Past duration 0.757545 too large
frame= 1969 fps=512 q=2.0 Lsize=   57970kB time=00:01:17.44 bitrate=6132.0kbits/s
video:56253kB audio:1595kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.210521%

[Carl Eugen Hoyos]

Uploaded the sample to ​http://samples.ffmpeg.org/ffmpeg-bugs/trac/ticket5065/
Original description was "flv stream capture that won\'t play the audio well in VLC."