VTT subtitle file information in MPD with SegmentTemplate dynamic configuration is not being called properly

[minna-no-jihye]

origin mpd src : https://d2n9sp52ange51.cloudfront.net/output/non-drm/2025/02/b8950fa2-2623-42c3-b01e-830476ae594d/avc/dash/de1f68352491a0792e3a000861950e466625275d_v1.mpd

videojs demo player path: https://videojs-http-streaming.netlify.app/?debug=false&autoplay=false&muted=false&fluid=false&minified=false&sync-workers=false&liveui=true&llhls=true&url=https%3A%2F%2Fd2n9sp52ange51.cloudfront.net%2Foutput%2Fnon-drm%2F2025%2F02%2Fb8950fa2-2623-42c3-b01e-830476ae594d%2Favc%2Fdash%2Fde1f68352491a0792e3a000861950e466625275d_v1.mpd&type=application%2Fdash%2Bxml&keysystems=&buffer-water=false&exact-manifest-timings=false&pixel-diff-selector=false&network-info=true&dts-offset=false&override-native=true&object-fit=false&use-mms=true&preload=auto&mirror-source=true&forced-subtitles=false&native-text-tracks=false

---

When using the MPD configuration below, subtitles display correctly in the dash.js demo player, but they are not being called in vhs demo player.

 <Representation 
      id="subtitle_ko" 
      bandwidth="256">
    <BaseURL>/output/non-drm/2025/02/b8950fa2-2623-42c3-b01e-830476ae594d/avc/dash/</BaseURL>
    <SegmentTemplate 
        timescale="90000"
        media="93d3aad8-985d-4f07-ac1d-602a41336534_$Number%06d$.vtt" 
        duration="540000" 
        startNumber="0"/>
  </Representation>

After investigation, I found that only the BaseUrl information under Representation is being called, and the segment path is not being dynamically appended to create the full call. Is there a separate option to enable dynamic calling?

Here are the methods I've tried:

1. I removed the BaseURL tag and included both the BaseUrl and dynamic path in the media attribute of SegmentTemplate. This method failed because without BaseUrl, it called an incorrect path.

 <Representation 
      id="subtitle_ko" 
      bandwidth="256">
    <SegmentTemplate 
        timescale="90000"
        media="/output/non-drm/2025/02/b8950fa2-2623-42c3-b01e-830476ae594d/avc/dash/93d3aad8-985d-4f07-ac1d-602a41336534_$Number%06d$.vtt" 
        duration="540000" 
        startNumber="0"/>
  </Representation>

2. I added an initialization attribute value to the SegmentTemplate in the MPD configuration. This resulted in the Subtitle being called, but only after pressing the subtitle button multiple times.
   src :
   https://videojs-http-streaming.netlify.app/?debug=false&autoplay=false&muted=false&fluid=false&minified=false&sync-workers=false&liveui=true&llhls=true&url=https%3A%2F%2Fd2n9sp52ange51.cloudfront.net%2Foutput%2Fnon-drm%2F2025%2F02%2Fb8950fa2-2623-42c3-b01e-830476ae594d%2Favc%2Fdash%2Fde1f68352491a0792e3a000861950e466625275d.mpd&type=application%2Fdash%2Bxml&keysystems=&buffer-water=false&exact-manifest-timings=false&pixel-diff-selector=false&network-info=true&dts-offset=false&override-native=true&object-fit=false&use-mms=true&preload=auto&mirror-source=true&forced-subtitles=false&native-text-tracks=false

<Role schemeIdUri="urn:mpeg:dash:role:2011" value="subtitle"/>

  <Representation 
      id="subtitle_ko" 
      bandwidth="256">
    <BaseURL>/output/non-drm/2025/02/b8950fa2-2623-42c3-b01e-830476ae594d/avc/dash/</BaseURL>  
    <SegmentTemplate 
        timescale="90000" 
        initialization="93d3aad8-985d-4f07-ac1d-602a41336534_init.vtt" 
        media="93d3aad8-985d-4f07-ac1d-602a41336534_$Number%06d$.vtt" 
        duration="540000" 
        startNumber="0"/>
  </Representation>

[gkatsev]

Can confirm that both dash.js and shaka show the captions with the provided source. Though, I believe historically, in DASH content, vtt files were either provided as a single file (for example https://demo.unified-streaming.com/k8s/features/stable/no-handler-origin/tears-of-steel/tears-of-steel-wvtt-sidecar.mpd) or segmented into fmp4 segments (https://demo.unified-streaming.com/k8s/features/stable/video/tears-of-steel/tears-of-steel-wvtt.ism/.mpd).
Both of those, I've confirmed to work on https://videojs-http-streaming.netlify.app/. If you're able, I would recommend changing your subs to conform to one of those methods.
It would be nice to get these segmented vtt files working as well, but can't commit to it being looked at.