Re: MPEG-2 TS metadata cue guidelines

On 3/24/14, 2:44 PM, "Silvia Pfeiffer" <silviapfeiffer1@gmail.com> wrote:

>On Tue, Mar 25, 2014 at 7:37 AM, Bob Lund <B.Lund@cablelabs.com> wrote:
>>
>> From: <Clift>, Graham <Graham.Clift@am.sony.com>
>> Date: Monday, March 24, 2014 at 10:11 AM
>> To: Brendan Long <B.Long@cablelabs.com>, Bob Lund
>><b.lund@cablelabs.com>,
>> "public-inbandtracks@w3.org" <public-inbandtracks@w3.org>
>> Subject: RE: MPEG-2 TS metadata cue guidelines
>>
>>
>>
>> I have one suggestion. Instead of playing around with arbitrary endtimes
>> why not set the starttime to the currentTime of playback when the cue is
>> created (making it immediately active when created) and set the endtime
>>to
>> the media resource time
>>
>>
>>
>> What is 'media resource time'?
>>
>> [<Graham>] Well that is terminology you use in the mapping spec. It is
>>the
>> time of the metadata with respect to the MPEG-2 TS timeline (eg PTS)
>>clock.
>> The startTime in contrast would be the HTML5 Media Element currentTime
>>when
>> the cue is created in UA, which is equivalent to
>>
>>                 startTime = endTime - (UA decoder buffering time) + UA
>>cue
>> generation processing time
>>
>>
>>
>> Why not endTime = startTime + 250msec? Seems easier.
>>
>> [<Graham>]
>>
>> I explained the reasoning but I will reiterate. 250msec is just some
>> arbitrary number. Rather than an arbitrary number why not just have the
>> startTime before the mediaresource time to allow the application to
>>prepare
>> for the cue onexit event?
>>
>>
>> Is the following another way of stating your proposal?
>>
>> Let PTS0 be the the MPEG-2 TS PTS corresponding to video.currentTime =
>>0.
>> Let PTS1 be the MPEG-2 TS PTS of the video stream when the UA is ready
>>to
>> create a metadata text track cue from an associated elementary stream.
>>The
>> UA sets cue.startTime = video.currentTime and cue.endTime = max(250msec,
>> PTS1 - PTS0).
>>
>
>
>Using video.currentTime for the startTime is not a good idea: every
>time you run the same file, you potentially get a different cue. This
>can't be tested consistently.

The cues are in-band with a fixed relationship to the media resourcešs
timeline. With respect to a given cue, why would currentTime be different
each time the  stream is received by the UA?

>
>Silvia.

Received on Monday, 24 March 2014 21:23:22 UTC