DASH - Understanding MPD at availabilityStartTime

[Romain Picard]

Le 11.02.2016 17:18, Arjen Veenhuizen a écrit :
> Timing is still a heavily discussed topic in the DASH standardisation
> organisation but I'll try to answer your question. It depends on the 
> profile
> you use but assuming MP4 or MPEG-2 TS profile in MPEG DASH :
> 
> “When the MPD is updated, the value of MPD at availabilityStartTime shall 
> be
> the same in the original and the updated MPD.”
> 
> So when it updates the MPD by removing or adding stuff you shouldn’t 
> update
> this value.
> 
> Also, If I understand you correctly, you assume that Segment 
> availability
> Start time = MPD at availabilityStartTime. But it is a bit more complex. 
> It is
> actually :
> 
> Segment availability start time = MPD at availabilityStartTime + 
> PeriodStart +
> MediaSegment[i].startTime + MediaSegment[i].duration

There are only few available dash live sources for now, but see this one 
with a 3 hours timeshift buffer:
http://bitlivedemo-a.akamaihd.net/mpds/stream.php?streamkey=bitcodin

the availabilityStartTime field is updated each time the MPD is 
refreshed, and the period Start value is fixed to "PT0S". Maybe this is 
not correct but considering the description of the dash specification 
there are probably other sources with this behavior.


> 
> Basically, this means that you can access a segment before the content 
> it
> describes in UTC time is over, aka you can look into the future:).
> 
> I hope this answers your questions at least a little bit.
> 
> 
> 
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