title: ffmpeg: Concatenating webm files - output file shows first video only
body: <p>I have 6 webm files (video/audio) which I captured through WebRTC (web browser getUserMedia API). Individually they all play fine. They're all 15 seconds in length and 2MB in size each.</p>
  
  <p>When I concatenate them with ffmpeg using concat demuxer (<a href="https://trac.ffmpeg.org/wiki/Concatenate" rel="nofollow">documentation</a>), the resulting output file is 12MB (wchich I expect), but when I play it , it only plays the first video and then it stops after 15 seconds. Tried playing it with Google Chrome as well as VLC.</p>
  
  <p>This is the ffmpeg command I am using:</p>
  
  <pre><code>ffmpeg -f concat -i mylist.txt -c copy output3.webm
  </code></pre>
  
  <p>And here is mylist.txt:</p>
  
  <pre><code>file 'tmpD08D.webm'
  file 'tmpD08E.webm'
  file 'tmpD08F.webm'
  file 'tmpD090.webm'
  file 'tmpD091.webm'
  file 'tmpD0A1.webm'
  </code></pre>
  
  <p>Here is the ffmpeg output:</p>
  
  <pre><code>c:\Temp\files&gt;ffmpeg -f concat -i mylist.txt -c copy output4.webm
  ffmpeg version N-72383-g7206b94 Copyright (c) 2000-2015 the FFmpeg developers
    built with gcc 4.9.2 (GCC)
    configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-av
  isynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enab
  le-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --
  enable-libdcadec --enable-libfreetype --enable-libgme --enable-libgsm --enable-l
  ibilbc --enable-libmodplug --enable-libmfx --enable-libmp3lame --enable-libopenc
  ore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --ena
  ble-librtmp --enable-libschroedinger --enable-libsoxr --enable-libspeex --enable
  -libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-aacenc --enabl
  e-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --enable
  -libwebp --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --e
  nable-lzma --enable-decklink --enable-zlib
    libavutil      54. 26.100 / 54. 26.100
    libavcodec     56. 41.100 / 56. 41.100
    libavformat    56. 33.101 / 56. 33.101
    libavdevice    56.  4.100 / 56.  4.100
    libavfilter     5. 16.101 /  5. 16.101
    libswscale      3.  1.101 /  3.  1.101
    libswresample   1.  1.100 /  1.  1.100
    libpostproc    53.  3.100 / 53.  3.100
  Input #0, concat, from 'mylist.txt':
    Duration: N/A, start: 0.000000, bitrate: N/A
      Stream #0:0: Audio: opus, 48000 Hz, mono, fltp
      Stream #0:1: Video: vp8, yuv420p, 640x480, SAR 1:1 DAR 4:3, 30 fps, 30 tbr,
  1k tbn, 1k tbc
  [webm @ 00000000003a5fe0] Codec for stream 0 does not use global headers but con
  tainer format requires global headers
  [webm @ 00000000003a5fe0] Codec for stream 1 does not use global headers but con
  tainer format requires global headers
  Output #0, webm, to 'output4.webm':
    Metadata:
      encoder         : Lavf56.33.101
      Stream #0:0: Video: vp8, yuv420p, 640x480 [SAR 1:1 DAR 4:3], q=2-31, 30 fps,
   30 tbr, 1k tbn, 1k tbc
      Stream #0:1: Audio: opus, 48000 Hz, mono
  Stream mapping:
    Stream #0:1 -&gt; #0:0 (copy)
    Stream #0:0 -&gt; #0:1 (copy)
  Press [q] to stop, [?] for help
  [concat @ 0000000000361e20] DTS 0 &lt; 14911 out of order
  [webm @ 00000000003a5fe0] Non-monotonous DTS in output stream 0:0; previous: 149
  11, current: 0; changing to 14911. This may result in incorrect timestamps in th
  e output file.
  [webm @ 00000000003a5fe0] Non-monotonous DTS in output stream 0:0; previous: 149
  11, current: 48; changing to 14911. This may result in incorrect timestamps in t
  he output file.
  [webm @ 00000000003a5fe0] Non-monotonous DTS in output stream 0:1; previous: 148
  69, current: 59; changing to 14869. This may result in incorrect timestamps in t
  he output file.
  </code></pre>
  
  <p>Note that I see a lot of "Non-monotonous DTS in output stream " errors in the ffmpeg output.</p>
  
  <p>What am I doing wrong here?</p>