Player behavior: SegmentTemplate with $time$ and @duration but no SegmentTimeline

[dsilhavy]

Description

Based on our discussion in the DASH-IF Live Call last week and an issue we addressed recently in dash.js I would like to clarify the right player behavior for content that uses SegmentTemplate with $time$ but does not contain a SegmentTimeline element.

As an example:

 <AdaptationSet
            id="1"
            lang="und"
            contentType="audio"
            segmentAlignment="true"
            mimeType="audio/mp4"
            startWithSAP="1">
            <AudioChannelConfiguration schemeIdUri="urn:mpeg:dash:23003:3:audio_channel_configuration:2011" value="2"/>
            <Role schemeIdUri="urn:mpeg:dash:role:2011" value="main"/>
            <SegmentTemplate
                timescale="48000"
                duration="96096"
                startNumber="1"
                media="$RepresentationID$-$Time$.m4a"
                initialization="$RepresentationID$.init">
            </SegmentTemplate>
            <Representation
                id="stream-audio-1-und-256Kbps"
                bandwidth="256000"
                audioSamplingRate="48000"
                codecs="mp4a.40.2">
            </Representation>
        </AdaptationSet>

Discussion Items

• Does startNumber have an influence on how the player determines the right segment URL. For instance, if startNumber is set to 5 do we need to request a segment with $time$ larger than 0
• What happens if a @presentationTimeOffset is specified? Do we assume that the earliestPresentationTime of the first segment is equal to the @presentationTimeOffset? This would imply that we have an @eptDelta of 0. We then replace $time$ with the value of the @presentationTimeOffset for the first segment. For subsequent requests, the @duration is added.
• Do we need to parse the tfdt.baseMediaDecodeTime and add the sample durations to find the right URL for the next segment?
• Do we want to allow this kind of MPDs in general?
• If we allow this format, do we want to provide test content?

Related Issues

• When using $time$ and @duration in MPD file, the first $time$ value requested by Dash.js is negative. dash.js#4535

[naeoc4509]

According to the contents of '@duration' in '5.3.9.5.2 Initialization Segment information' of ISO IEC 23009-1 : 2022 document below,

There is no rule for MUST pairing '$Number$ : @duration' and '$time$ : sengmentTimeline'

According to the above document, when using @duration, there is a definition of how '$time$' is determined, so I think it is usable.

According to the document, the @startNumber value is affected, but the @presentationTimeOffset value has no effect on both '$time$' and '$number$'.

When using '$time$' and @duration in mpd, 'Shaka player' matches the @presentationTimeOffset value to the 'start time' value of '$time$',
I can't find such content in the above standard document, so I think it is a bug in shaka player related to time calculation.

[haudiobe]

TF 2024/08/16

• Generally we should reduce the number of options. One way would be to prohibit $Time$ and @Number, but use Segment Timeline with r=-1 to have an open ended template.
• It would also be good to get feedback from DVB if they support this mode,
• We will revisit.