AdaptationSet for subtitles fed by URL of subtitle file.

[alex-rsk]

We have dynamic MPEG-DASH manifest with SegmentTimeline scheme. It has with two AdaptationSets, each of them refers to video and audio respectively.
I'd like to put a third AdaptationSet for subtitles, WebVTT or TTML , but from some external URL.
In this case, each manifest refresh would contain new pieces for audio and video, but URL remains the same
Something like this:

<?xml version="1.0"?>
<MPD xmlns="urn:mpeg:dash:schema:mpd:2011" minBufferTime="PT1.500S" type="dynamic" availabilityStartTime="2024-12-30T00:00:11.932Z" publishTime="2024-12-30T09:10:42.281Z" minimumUpdatePeriod="PT0H0M3.000S" timeShiftBufferDepth="PT0H1M1.000S" maxSegmentDuration="PT0H0M5.116S" profiles="urn:mpeg:dash:profile:isoff-live:2011,http://dashif.org/guidelines/dash264">>
 <Location>https://video_url.pro/...manifest.mpd</Location>
<Period id="DID1" start="PT0H0M0.000S">
  <AdaptationSet segmentAlignment="true" maxWidth="1920" maxHeight="1080" maxFrameRate="25" par="16:9" mimeType="video/mp4" startWithSAP="1">
   <SegmentTemplate timescale="12800">
    <SegmentTimeline>
     <S .../>
    </SegmentTimeline>
   </SegmentTemplate>
   <Representation id="norm" mimeType="video/mp4" codecs="avc1.640028" width="1920" height="1080" frameRate="25" sar="1:1" bandwidth="808080">
    <SegmentTemplate media="fullhd_seg$Time$.m4s" initialization="fullhd_dashinit.mp4?" timescale="12800" startNumber="1"/>
   </Representation>   
  </AdaptationSet>
  <AdaptationSet segmentAlignment="true" mimeType="audio/mp4" startWithSAP="1">
   <SegmentTemplate media="audio_dash_track2_$Time$.m4s" initialization="audio_dash_track2_init.mp4" timescale="8000" startNumber="1">
    <SegmentTimeline>     
     <S .../>
    </SegmentTimeline>
   </SegmentTemplate>
   <Representation id="audio_track" mimeType="audio/mp4" codecs="mp4a.40.2" audioSamplingRate="8000" bandwidth="28990">
    <AudioChannelConfiguration schemeIdUri="urn:mpeg:dash:23003:3:audio_channel_configuration:2011" value="1"/>
   </Representation>
  </AdaptationSet>
  <AdaptationSet segmentAlignment="true" lang="en" mimeType="application/mp4" startWithSAP="1">
    <URL for subtitle file/> <---------------
  </AdaptationSet>
 </Period>
 <UTCTiming schemeIdUri="urn:mpeg:dash:utc:http-iso:2014" value="https://time.akamai.com/?iso&amp;ms"/>
</MPD>

Is this possible?
Does dash.js able to fetch subtitles and play them in time?

[dsilhavy]

With v5 we added the network interceptor. You can use it to modify network responses before they are processed by dash.js. This should allow you to add an additional AdaptationSet to the MPD. Example of a network interceptor:

• https://reference.dashif.org/dash.js/nightly/samples/network-interceptor/index.html
• https://dashif.org/dash.js/pages/usage/network-interceptor.html

[alex-rsk]

With v5 we added the network interceptor. You can use it to modify network responses before they are processed by dash.js. This >should allow you to add an additional AdaptationSet to the MPD.

Please excuse me for being so dumb, but I still can't get what is the answer to my question in sense of "Would dash.js able to parse and apply contents of subtitle_file.vtt to a live dynamic stream", no matter, how I'd catch some event.

Also, as I see from example above, there is handler of CMCD event, but how I can handle, let's say, manifest refresh event in this case?

[dsilhavy]

• dash.js can handle external vtt files, this is one of our reference streams: https://dash.akamaized.net/akamai/test/caption_test/ElephantsDream/elephants_dream_480p_heaac5_1_https.mpd.
• You can open the example above and set a breakpoint to inspect the response object that is provided to the interceptor function. All requests and responses are proxied through the network interceptor. Changing CMCD data of the outgoing request is just one example on how to use this function.

[alex-rsk]

Thank you for clarification.