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Dash-Industry-Forum / Guidelines-TimingModel

DASH-IF implementation guidelines: the DASH timing model
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Question about Example 10 #52

Closed panpawel88 closed 4 years ago

panpawel88 commented 4 years ago

Hello,

I have a question about Example 10, which starts with following description:

The example defines a sample timeline with a timescale of 1000 units per second, with the period starting at position 900

... and contains following MPD:

\<MPD xmlns="urn:mpeg\:dash\:schema:mpd:2011"> \<Period duration="PT900S">

I cannot understand why the period starts at position 900. Period@start is not defined and there is no previous periods.

Could you clarify why in the Example 10, the period starts at position 900?

sandersaares commented 4 years ago

The example is correct but perhaps confusingly crafted. I will attempt to improve it.

As for the reason, the example tries to say that the period starts at position 900 on the sample timeline (because presentationTimeOffset is 900). On the MPD timeline, it indeed starts at 0 and lasts until 900s.

The two timelines are independent and I suspect the vague wording and the use of the same numeric value 900 are causing confusion here.

sandersaares commented 4 years ago

I attempted to improve the wording to make it clear that this talks about the sample timeline. Please see the referenced PR above and comment on whether it looks better.

I did not change the misleading 900 and 900 as I did not have the time to double-check the math right now, but ideally this should also get done.

panpawel88 commented 4 years ago

Thanks for explanation. You're right - I didn't spot that there are 2 different 900's in this example. I created a diagram to make sure that I understand your example correctly:

Example10

Is my understanding correct? If yes, then maybe there is a mistake in:

and the period end point is approximately 69 milliseconds after the segment start point of the last media segment.

Because, the last media segment start point is 900625 (225 (segments) x 4001 (duration) + 400 (first segment start point)) and the period end point is 900900, which gives us 275 milliseconds difference.

sandersaares commented 4 years ago

Yes, what you say looks correct.

I am not sure where that 69 came from but your logic about 275 looks sensible to me. I will correct it to say 275.

panpawel88 commented 4 years ago

Thanks once again for clarification! PR looks good to me.