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Dash-Industry-Forum / dash.js

A reference client implementation for the playback of MPEG DASH via Javascript and compliant browsers.
http://reference.dashif.org/dash.js/nightly/samples/dash-if-reference-player/index.html
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Should #t MPD Anchors on dynamic manifests not be modified by the availabilityStartTime? #3034

Closed robertbryer closed 3 years ago

robertbryer commented 5 years ago
Steps to reproduce

Get a unix epoch from within the last 5 minutes and use it as the #t= anchor in this link: http://reference.dashif.org/dash.js/v2.9.3/samples/dash-if-reference-player/index.html?url=http://smp-scratch.tools.bbc.co.uk/bob/samples/livesim-1-minute-availability.mpd#t=1563886800s

And then load the stream.

This is a modified version of the livesim MPD to change the availabilityStartTime to +1 minute.

Observed behaviour

The playback starts 1 minute prior to the value you chose for the #t anchor. This is the value of the availabilityStartTime. The original livesim MPD has a value of 0 for the availabilityStartTime, so you won't notice this using the normal livesim manifest.

You can see exactly where the start time is modified here: https://github.com/Dash-Industry-Forum/dash.js/blob/3549b41141c345172b51e1513530c23705151f6f/src/streaming/controllers/PlaybackController.js#L371

It doesn't appear to make that much sense to have time anchors like this reference against the availability timeline - but it's been about 5 years since this code was written. Is it just that this hasn't been noticed because all our samples don't have an availabilityStartTime or is there some use case here I don't understand? Also is there any apparent danger to changing this behaviour after so long?

sandersaares commented 5 years ago

MPD anchors of the form t=1234 are an offset from the start of the first period. Optionally a period parameter can be used to indicate which period is the base.

The syntax t=posix:123456 indicates an offset from the Unix epoch.

Note also, there should be no "s" at the end of the value.

Ref MPEGDASH C.4 and IOP v4.3 chapter 4.3.3.5.

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