Presentation Time Offset

The presentationTimeOffset is an attribute which can be encountered in an MPD (the "manifest" of the DASH streaming technology).

Overview

Simply put, this attribute allows to correct an offset present in the media segments once those are decoded.

One of the possible usecase would be creating an on demand MPD from a subsection of an already-existing content, without modifying directly the concerned segments nor their (possibly time-based) URLs.

Another main usecase is when handling multi-Periods MPDs. Segments in newer Periods already need to consider an offset, corresponding to the start of the given Period. In those cases, the presentationTimeOffset might allows to "cancel" out this offset. This can be useful if the corresponding segments already define the right time.

Simple example

For example, let's imagine some on-demand content with a duration of 2 hours. To stay simple, this content begins at 00:00:00.000 and ends at 01:08:00.000 (1 hour and 8 minutes).

CONTENT:

00:00:00.000                                                        01:08:00.000
    |====================================================================|

Now let's say that we want to create a new on-demand content, which is only a sub-part from this content. For example, we will take the subpart going from 00:05:24.000 to 00:12:54.000 (for a duration of 00:07:30.000).


00:00:00.000                                                        02:00:00.000
    |====|------|========================================================|
            ^
      Subpart going from 00:05:24 to 00:12:54.000

Because we might not want to use money uselessly, we want to create this new content simply by creating a new MPD, and without touching the already created segments, nor their URLs.

In that condition, we will still need the client to know that this content actually have an offset of 00:05:24.000. If it does not know that, we will just think that the content begins at a default 00:00:00.000 time.

Letting the client think that the content begins at the default 00:00:00.000 time could lead to several issues:

  • it might not be able to request the right first segments (as the URLs could be time-based)

  • even if it does, it might not be able to actually play the content, as we're pushing segments corresponding to a 00:05:24.000 while the browser is still waiting for the 00:00:00.000 ones (in that case, we would just have an infinite buffering state).

  • even if it does, the client timeline will announce a wrong time, offseted 5 minutes and 24 seconds too late.

This is where the presentationTimeOffset comes into play. In our simple example, this value will just announce an offset of 00:05:24.000 (under the form of an integer with a timescale to convert it into seconds), and the client will know what to do.

What the client has to do here is:

  • begin to play at 0 secods
  • ask the right segments, by adding this offset to the one it thinks it needs
  • remove the offset from the segment before decoding it

Time conversions

The presentationTimeOffset is linked to multiple other time attributes of an MPD, especially the start of the Period concerned, and of course the time of the segment.

We will enounce below a simple equation which put their relation into perspective.

To understand this equation, we will need to define some variables:

Variable Definition
PTO The "presentationTimeOffset" attribute of the MPD
mediaTime The start time announced in the segment
TS Timescale used by PTO and segmentTime, to transform them into seconds
periodStart Start time of the given period, in seconds
presentationTime The time at which the segment will be shown, in seconds 
    mediaTime        PTO
  -------------  -  -----  +  periodStart  =  presentationTime
       TS            TS

Easier conversion: the timestampOffset

As seen in the previous chapter, to convert the media time (time announced in the segments) into the presentation time (time that will be shown to the user), you will need to use both also include three other variables:

  • the start of the period

  • the presentationTimeOffset

  • the timescale used by the presentationTimeOffset and the media time

As a convenient plus, those three variables rarely change for a given period.

To simplify the conversion, we can thus define a new variable using those three. This is what the timestampOffset is all about.

Let's go back to the equations in the previous chapters, to isolate those three into the really simple equation: mediaTime/TS + timestampOffset = presentationTime (you can refer to the previous chapter to understand what those variables means)


  mediaTime       PTO
 -----------  -  -----  +  periodStart  =  presentationTime
     TS           TS

  mediaTime           PTO
 -----------  + ( -  -----  +  periodStart ) =  presentationTime
     TS               TS

                          PTO                                       PTO
  timestampOffset  =  -  -----  +  periodStart  =  periodStart  -  -----
                          TS                                        TS

With timestampOffset defined, it becomes easy to go back and forth between the mediaTime and the presentationTime:

                       mediaTime
presentationTime  =   -----------  +  timestampOffset
                          TS

mediaTime  =  (  presentationTime  -  timestampOffset  )  *  TS

As an added bonus, SourceBuffers defined in the HTML5 MediaSource Extentions also have a timestampOffset property , which means exactly the same thing as defined here!

In the RxPlayer

Now that we have all of those concepts out of the way, how are we going to use it, in the RxPlayer?

The RxPlayer has A LOT of time-related values defined for a given segment:

  • the time defined in the segment itself (mediaTime)

  • the time displayed when playing it in the HTMLMediaElement (presentationTime)

  • the time possibly set in the request (requestSegmentTime)

  • the time as announced in the corresponding attribute of the manifest (manifestTime)

  • the time used in the corresponding Segment Object in the RxPlayer (playerTime)

  • the time used in the buffered APIs of a HTMLMediaElement or SourceBuffer (bufferedTime)

  • ...

As it turns out it's a lot simpler once you make two isolated groups:

  • the manifest group, which uses the non-offseted mediaTime.

    In this group you have:

    • the mediaTime (duh)
    • the manifestTime
    • the requestSegmentTime

  • the real time group, which uses the offseted presentationTime.

    In this group you have:

    • the presentationTime
    • the playerTime
    • the bufferedTime

The manifest group is then only used in the transports code of the RxPlayer. Meanwhile, the real time group is used everywhere else.

It's actually the transports code that does most of the conversion for the rest of the code (removing the offset when requesting new segments, re-adding it once the segment is downloaded.

To be able to offset those segments in the SourceBuffer, those are still informed of course of the timestampOffset by the transports code. Then, this timestampOffset will be exploited only by the final decoding code.

Presentation Time Offset

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