Live MPD unplayable with empty segment timeline for text track

[jakubvojacek]

• The MPD passes the DASH-IF Conformance Tool on http://dashif.org/conformance.html
• The stream has correct Access-Control-Allow-Origin headers (CORS)
• There are no network errors such as 404s in the browser console when trying to play the stream
• The issue observed is not mentioned on https://github.com/Dash-Industry-Forum/dash.js/wiki/FAQ
• The issue occurs in the latest reference client on http://reference.dashif.org/dash.js/ and not just on my page

• Link to playable MPD file: https://edge1.motv.eu///channels/ct-sport-8/live/ct-sport-8.mpd
• Dash.js version: latest
• Browser name/version: chrome latest
• OS name/version: mac

Steps to reproduce

The playback does not start in case of live content that has subtitle track with no subtitles available yet. Sample MPD is:

<?xml version="1.0" encoding="UTF-8"?>
<!--Generated with https://github.com/google/shaka-packager version 5f0868275a-release-->
<MPD xmlns="urn:mpeg:dash:schema:mpd:2011" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xlink="http://www.w3.org/1999/xlink" xsi:schemaLocation="urn:mpeg:dash:schema:mpd:2011 DASH-MPD.xsd" xmlns:cenc="urn:mpeg:cenc:2013" profiles="urn:mpeg:dash:profile:isoff-live:2011" minBufferTime="PT3.2S" type="dynamic" publishTime="2018-07-12T06:43:34Z" availabilityStartTime="2018-07-11T18:18:02Z" minimumUpdatePeriod="PT5S" timeShiftBufferDepth="PT15000S" suggestedPresentationDelay="PT5S">
  <Period id="0" start="PT0S">
    <AdaptationSet id="0" contentType="text" lang="cs" segmentAlignment="true">
      <Role schemeIdUri="urn:mpeg:dash:role:2011" value="subtitle"/>
      <Role schemeIdUri="urn:mpeg:dash:role:2011" value="main"/>
      <Representation id="0" bandwidth="0" codecs="wvtt" mimeType="application/mp4">
        <SegmentTemplate timescale="1000" initialization="ct-sport-8_subtitle_init.mp4" media="subtitle/ttv_$Number$.m4s" startNumber="1"/>
      </Representation>
    </AdaptationSet>
    <AdaptationSet id="1" contentType="video" maxWidth="1920" maxHeight="1080" frameRate="90000/3600" segmentAlignment="true" par="16:9">
      <Representation id="1" bandwidth="400000" codecs="avc1.4d401e" mimeType="video/mp4" sar="1:1" width="640" height="360">
        <SegmentTemplate timescale="90000" initialization="ct-sport-8_video_640_360.mp4" media="video_640_360/ct-sport-8-$Number$.mp4" startNumber="9287">
          <SegmentTimeline>
            <S t="2675297040" d="288000" r="4688"/>
          </SegmentTimeline>
        </SegmentTemplate>
      </Representation>
      <Representation id="2" bandwidth="2500000" codecs="avc1.4d4028" mimeType="video/mp4" sar="1:1" width="1920" height="1080">
        <SegmentTemplate timescale="90000" initialization="ct-sport-8_video_1920_1080.mp4" media="video_1920_1080/ct-sport-8-$Number$.mp4" startNumber="9287">
          <SegmentTimeline>
            <S t="2675297040" d="288000" r="4688"/>
          </SegmentTimeline>
        </SegmentTemplate>
      </Representation>
      <Representation id="3" bandwidth="1000000" codecs="avc1.4d401f" mimeType="video/mp4" sar="1:1" width="960" height="540">
        <SegmentTemplate timescale="90000" initialization="ct-sport-8_video_720_410.mp4" media="video_720_410/ct-sport-8-$Number$.mp4" startNumber="9287">
          <SegmentTimeline>
            <S t="2675297040" d="288000" r="4688"/>
          </SegmentTimeline>
        </SegmentTemplate>
      </Representation>
    </AdaptationSet>
    <AdaptationSet id="2" contentType="audio" lang="cs" segmentAlignment="true">
      <Role schemeIdUri="urn:mpeg:dash:role:2011" value="main"/>
      <Representation id="4" bandwidth="131745" codecs="mp4a.40.2" mimeType="audio/mp4" audioSamplingRate="48000">
        <AudioChannelConfiguration schemeIdUri="urn:mpeg:dash:23003:3:audio_channel_configuration:2011" value="2"/>
        <SegmentTemplate timescale="90000" initialization="ct-sport-8_audio_cs.mp4" media="audio_cs/ct-sport-8-$Number$.mp4" startNumber="9289">
          <SegmentTimeline>
            <S t="2675521778" d="288000" r="3684"/>
            <S t="3736801778" d="281955"/>
            <S t="3737089489" d="288000" r="54"/>
            <S t="3752929489" d="285935"/>
            <S t="3753217404" d="288000" r="804"/>
            <S t="3985057404" d="282036"/>
            <S t="3985345198" d="288000" r="140"/>
          </SegmentTimeline>
        </SegmentTemplate>
      </Representation>
    </AdaptationSet>
  </Period>
  <UTCTiming schemeIdUri="urn:mpeg:dash:utc:http-head:2014" value="https://mw.motv.eu/time.txt"/>
</MPD>

Observed behaviour

The playback is not started, ends up with an error instead. This issue was just recently resolved by the shaka player team in their player shaka-project/shaka-player#1484 and you can sucecssfully test playing the MPD here https://nightly-dot-shaka-player-demo.appspot.com/

Console output

SegmentsUtils.js:238 Uncaught TypeError: Cannot read property 'start' of null
    at decideSegmentListRangeForTemplate (SegmentsUtils.js:238)
    at Object.getSegmentsFromTemplate [as getSegments] (TemplateSegmentsGetter.js:64)
    at Object.getSegments (SegmentsGetter.js:65)
    at updateSegments (DashHandler.js:218)
    at updateSegmentList (DashHandler.js:244)
    at Object.updateRepresentation (DashHandler.js:255)
    at Object.updateData (RepresentationController.js:192)
    at Object.updateData (DashAdapter.js:446)
    at Object.selectMediaInfo (StreamProcessor.js:250)
    at createStreamProcessor (Stream.js:390)

[davemevans]

It passes the validator, but I don't think this is valid - you state there is an empty SegmentTimeline, but that element is not included here, only a SegmentTemplate with no @duration.

The spec is clear that If the Representation contains more than one Media Segment, then either the attribute @duration or the element SegmentTimeline shall be present. Neither of those things is true. Additionally, the DASH-IF IOP are even clearer: exactly one of SegmentTemplate@duration or SegmentTemplate.SegmentTimeline must be present per Representation.

Regardless, it will still fail since media should be available for the entire @timeShiftBufferDepth. Personally, I think dash.js is behaving correctly and that the packager should be producing valid segments with empty samples.

Of course, there is the option to copy the Shaka Player functionality.

[nicosang]

Hi @jakubvojacek and @bbcrddave ,

I rather agree with David. However, the issue seems to be solved by adding a small test of availabilityWindow. @jakubvojacek could you, please, test it on your side?

thanks,

Nico

[jakubvojacek]

@nicosang thank you for the PR. I will try to talk to shaka packager team to fix this issue on their side.