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HTMLTimingObject - reference point

From: Ingar Mæhlum Arntzen <ingar.arntzen@gmail.com>
Date: Thu, 11 Jun 2015 18:23:36 +0200
Message-ID: <CAOFBLLp7WBoZbxRANbrDNUDYJMuJCwg4EyLkJqc6X-RW56gd0A@mail.gmail.com>
To: public-webtiming@w3.org, oskar.vandeventer@tno.nl
Hi all.

Oskar van Deventer of TNO has indicated an unclarity in the
 HTMLTimingObject draft spec.

Oskar has been involved in sync activities of DVB CSS and HbbTV 2.0. He is
now working with Media Orchestration in the context of MPEG, where the
HTMLTimingObject is one of the considered technologies.

The unclarity refers to a "reference point", a concept discussed in DVB CSS
but not in the timing object spec.

To indicate the issue, consider the following use case: A user agent
(STB/TV) receives a media stream and seeks to maintain a HTMLTimingObject
as a representation of the current playback position. To do this timestamps
need to be extracted from the stream data. The question is, what exactly
defines current playback position? The last block currently decoded? Or,
perhaps do we rather mean when pixels hit the screen?

This is the reference point, and dis-agreement on reference point may be a
source of (small) sync errors. In DVB CSS the reference point is defined as
when pixels hit screen, or when audio is output from speakers. Since
timestamps can not be extracted at these locations, they must instead be
extracted earlier in the processing chain and then adjusted according to
expected downstream latency.

The HTMLTimingObject draft currently does not define a reference point.

My suggestion is that we simply adopt the definition used by DVB CSS and
update the draft spec to reflect this.

This implies that the HTMLTimingObject (director of playback) ideally
represents the effects of timed operations (rather than their initiation).
It follows that components that take direction from the timing object and
has a non-negligible latency for operations to take effect, must issue
operations slightly in advance (according to expected latency).

If there are no objections, I'll go ahead and update the spec shortly.

Thank you Oskar for bringing this issue to our attention.


Best,

Ingar
Received on Thursday, 11 June 2015 16:24:06 UTC

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