Separate ast and start URL parameters

[tobbee]

Make it possible to set start time separate from availabilityStartTime.

start time is to be interpreted as the timeline offset for media timeline.

In addition, it should be possible to specify that the first period starts at a later time.

For the most general case, we need three URL parameters, all defined in NTP seconds
(relative to epoch start 1970-01-01T00:00:00Z).

1. start - this is the anchor point for the media timeline relative to epoch start
2. ast (availabilityStartTime) >= start is the start and reference point for the mediaTimeline in the MPD. It is defined relative to epoch start.
3. fpstart (firstPeriodStart >= ast is the time that the first period starts. It should be AST when the timeShiftBufferDepth makes the current interval overlap with ast. PresentationTimeOffset values must be set so that the media timeline is correct.

presentationTimeOffset must be set relative to timelineStart (start), so it may be be non-zero even for a period starting at the availabilityStartTime. This mimics the situation where one wants to have the availabilityStartTime point to the start of a program in a live channel, but the media timeline is relative to some other time like the Epoch start.

Constraints:

   start <= ast <= fpstart

If only start is specified (could be via startrel), ast and fpstart will automatically change to the same value.

With more values specified, unspecified values are automatically shifted to fulfill the constraints.
If specified values are not compatible, an error is returned.

It would also be useful to have relative values for ast astrel and fpstart fpstartrel,
to be automatically replaced by absolute values in a Location header similarly to startrel.

Implications for live 2 VoD

Currently, one can specify a start and stop time and have the MPD switch from dynamic to static as the stop time is passed. The start time corresponds to AST. This means that this emulates a stand-alone live event.

To emulate a live program in a linear channel, one should set AST to the time of the program start, but set a presentationTimeOffest to the time of the start in order to be able to have identical segments for the live channel and for the program. The shifted AST makes it impossible for the viewer to search past the start of the program. Furthermore, the timeShiftBufferDepth should ideally be set to the length of the program so that it is possible to search the full available program duration up to it is complete.

When updating this, it would be useful to also remove UTCTiming as the MPD type transitions from dynamic to static.

[haudiobe]

Test Call:

• good to have done
• possibly also add large time codes (starting from 1970)