title: FFMPEG Webvtt m3u8 download into single vtt file, ignore the "X-TIMESTAMP-MAP=MPEGTS=" (timestamp sync problem)
body: <p>I want to download the subtitle of a video, it has separated m3u8 file for subtitle:</p>
  <p><em><strong>webvtt source in m3u8 format</strong></em>: <a href="https://sdn-global-streaming-cache.3qsdn.com/9378/files/19/05/1199332/Dqt6jZBkvX2nLyY4CGhxmpK89PbQRFV7-drm-aes.ism/Dqt6jZBkvX2nLyY4CGhxmpK89PbQRFV7-drm-aes-textstream_deu=1000.m3u8" rel="nofollow noreferrer">https://sdn-global-streaming-cache.3qsdn.com/9378/files/19/05/1199332/Dqt6jZBkvX2nLyY4CGhxmpK89PbQRFV7-drm-aes.ism/Dqt6jZBkvX2nLyY4CGhxmpK89PbQRFV7-drm-aes-textstream_deu=1000.m3u8</a></p>
  <p>looks like:</p>
  <pre><code>#EXTM3U
  #EXT-X-VERSION:4
  ## Created with Unified Streaming Platform(version=1.9.5)
  #EXT-X-PLAYLIST-TYPE:VOD
  #EXT-X-MEDIA-SEQUENCE:1
  #EXT-X-INDEPENDENT-SEGMENTS
  #EXT-X-TARGETDURATION:989
  #USP-X-TIMESTAMP-MAP:MPEGTS=900000,LOCAL=1970-01-01T00:00:00Z
  #EXTINF:24.72, no desc
  Dqt6jZBkvX2nLyY4CGhxmpK89PbQRFV7-drm-aes-textstream_deu=1000-1.webvtt
  #EXTINF:2.56, no desc
  Dqt6jZBkvX2nLyY4CGhxmpK89PbQRFV7-drm-aes-textstream_deu=1000-13.webvtt
  #EXTINF:50.88, no desc
  Dqt6jZBkvX2nLyY4CGhxmpK89PbQRFV7-drm-aes-textstream_deu=1000-14.webvtt
  #EXTINF:3.079, no desc
  Dqt6jZBkvX2nLyY4CGhxmpK89PbQRFV7-drm-aes-textstream_deu=1000-40.webvtt
  #EXTINF:60.401, no desc
  ....
  </code></pre>
  <p><strong>Then use the command to download the multiple webvtts into one vtt file &quot;EN.vtt&quot;:</strong></p>
  <pre><code>ffmpeg -i  &quot;https://sdn-global-streaming-cache.3qsdn.com/9378/files/19/05/1199332/Dqt6jZBkvX2nLyY4CGhxmpK89PbQRFV7-drm-aes.ism/Dqt6jZBkvX2nLyY4CGhxmpK89PbQRFV7-drm-aes-textstream_deu=1000.m3u8&quot;  &quot;DE.vtt&quot;
  </code></pre>
  <p><strong>In output &quot;DE.vtt&quot;  the X-TIMESTAMP-MAP=MPEGTS:xxxxxx,LOCAL:00:00:00.000 from each webvtt file (webvtt in m3u8) are ignored.</strong></p>
  <p><strong>so that the time Synchronisation is broken, the start time of each Segment always begins from 00:00:00, looks like:</strong></p>
  <pre><code>WEBVTT
  
  00:00.000 --&gt; 00:02.560
  So viel Macht.
  
  00:00.000 --&gt; 00:03.079
  Was er sagte.
  
  00:00.000 --&gt; 00:04.680
  oder
  die Schönheit
  
  00:00.000 --&gt; 00:01.440
  Er sagte immer:
  
  00:00.000 --&gt; 00:03.840
  Auf seinen Klang 
  
  00:00.000 --&gt; 00:07.519
  Seine Paranoia.
  </code></pre>
  <p><strong>Could ffmpeg support the relative time stamp to convert into absolute time stamp?
  Or use other method to download perfect webvvt file?</strong></p>