[FFmpeg-user] non monotonous DTS for audio stream

[Albert Netymk]

Hello,

I encountered "non-monotonous DTS" error for audio stream while concating
two mp4 files, which are available at
https://www.dropbox.com/sh/ub3n76mpbcl4aa5/AACEcBlp2MOc7MNMSpQVXXRfa/concat_debug?dl=0&subfolder_nav_tracking=1

I have searched on stackoverflow and the archive of this mailing list, but
still couldn't understand why this occurs. Some explanation on why this
happens or how to further debug it is appreciated.

```
$ ffmpeg -y -f concat -i mylist.txt -c copy output.mp4
ffmpeg version 4.2.3 Copyright (c) 2000-2020 the FFmpeg developers
  built with Apple clang version 11.0.0 (clang-1100.0.33.17)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/4.2.3_1 --enable-shared
--enable-pthreads --enable-version3 --enable-avresample --cc=clang
--host-cflags=-fno-stack-check --host-ldflags= --enable-ffplay
--enable-gnutls --enable-gpl --enable-libaom --enable-libbluray
--enable-libdav1d --enable-libmp3lame --enable-libopus
--enable-librubberband --enable-libsnappy --enable-libsrt
--enable-libtesseract --enable-libtheora --enable-libvidstab
--enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264
--enable-libx265 --enable-libxvid --enable-lzma --enable-libfontconfig
--enable-libfreetype --enable-frei0r --enable-libass
--enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg
--enable-librtmp --enable-libspeex --enable-libsoxr --enable-videotoolbox
--disable-libjack --disable-indev=jack
  libavutil      56. 31.100 / 56. 31.100
  libavcodec     58. 54.100 / 58. 54.100
  libavformat    58. 29.100 / 58. 29.100
  libavdevice    58.  8.100 / 58.  8.100
  libavfilter     7. 57.100 /  7. 57.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  5.100 /  5.  5.100
  libswresample   3.  5.100 /  3.  5.100
  libpostproc    55.  5.100 / 55.  5.100
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fee09803600] Auto-inserting h264_mp4toannexb
bitstream filter
Input #0, concat, from 'mylist.txt':
  Duration: N/A, start: 0.000000, bitrate: 276 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p,
1440x810 [SAR 1:1 DAR 16:9], 147 kb/s, 60 fps, 60 tbr, 15360 tbn, 120 tbc
    Metadata:
      handler_name    : Core Media Video
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz,
stereo, fltp, 129 kb/s
    Metadata:
      handler_name    : Core Media Audio
Output #0, mp4, to 'output.mp4':
  Metadata:
    encoder         : Lavf58.29.100
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p,
1440x810 [SAR 1:1 DAR 16:9], q=2-31, 147 kb/s, 60 fps, 60 tbr, 15360 tbn,
15360 tbc
    Metadata:
      handler_name    : Core Media Video
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz,
stereo, fltp, 129 kb/s
    Metadata:
      handler_name    : Core Media Audio
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #0:1 -> #0:1 (copy)
Press [q] to stop, [?] for help
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fee0a803e00] Auto-inserting h264_mp4toannexb
bitstream filter
[mp4 @ 0x7fee0a803800] Non-monotonous DTS in output stream 0:1; previous:
3414085, current: 3413815; changing to 3414086. This may result in
incorrect timestamps in the output file.
frame= 6123 fps=0.0 q=-1.0 Lsize=    3639kB time=00:01:42.07 bitrate=
292.1kbits/s speed= 513x
video:1862kB audio:1612kB subtitle:0kB other streams:0kB global headers:0kB
muxing overhead: 4.750805%
```

/Albert