Re: WebVTT, Regions and live streams.

Hi Bill,

replies inline.

On Fri, Sep 18, 2015 at 1:34 PM, May, Bill <bill.may@mlb.com> wrote:
> Hello,
>
> My name is Bill May; I’m an engineer at MLB Advanced Media looking into how to expand Closed-Captions into a world-wide solution.
>
> We’re looking for a user experience similar to closed captions; the ability to have 1 word/letter at a time displayed, support for roll-up type caption for both VOD and live (event and linear) presentations.  We use HTTP Live Streaming (HLS) as our video protocol.
>
> CEA-708 captions are are one possible solution, but we believe that it will get harder as we add more languages.  We’d like to unbundle the captions from the other media.
>
> So, it will come as little surprise that I’m thinking of webVTT.
>
> With the later versions of the specification, WebVTT has done an excellent job of translating 608/708 and the properties required into webVTT using the region attributes, but only for a completed VOD type presentation.
>
> Solutions like HLS and DASH use a duration based segmentation to provide (near) live streams.  When we need to provide webVTT cues for a live stream, the direction isn’t very clear.  Specifically:
>
> 1). How to handle a EDM (clear screen).
> 2). what to do at the end of a segment/beginning of the next when the closed caption line spans the segment.
>
> case 1: (EDM)
> When we use the region syntax, I have been assuming is that each cue gets a start time, and an end time that encompasses the 16 seconds maximum time that the closed caption specifications state.
>
> That way, as the captions are added, the oldest ones will roll out of the region, even if they have time left.  If captions aren’t added, they have (Please correct me if my assumption of the region is wrong).
>
> However, that doesn’t let me enter a clear screen command.  There’s no way to change the end time of those earlier cues.
>
> One possible solution for this is to add a bunch of short lived cues with non-breaking space, but I do not believe that this is acceptable due to the background artifacts.


EDM (clear screen) isn't really something that maps easily to a
file-based format. It comes from a time where captions were
command-based. The way to achieve a "clear screen" is to carefully
author the cue durations such that at the time that the EDM is
supposed to happen, that is the end time of the cues. It has to be
pre-authored and can't be changed later. If you don't add any more
captions to a region, the old ones will disappear and the newer ones
will simply not scroll up, since they don't get pushed by new ones.
When the last one ends, the region is cleared.



> case 2 (cues that overlap segmentation)
> As for the segmentation, assume we have the following: Assuming a region with 2 lines, and let’s say we want to push out each word every 3/4 second (by using the timing mechanisms).  (I’ve left out any cue settings to make it a bit clearer)
>
> 00:00:00.000 —-> 00:00:17.500
> Caption <00:00:00.750>Line <00:00:01.500>1
>
> 00:00:03.000 —> 00:00:22.000
> Caption <00:00:03.750>Line <00:00:04.5>2<00:00:5.25>Longer<00:00:6.000>extended
>
> If we need to provide segmentation at 5 second intervals, the segmentation process does not have the words “Longer” and “Extended”; they haven’t been entered by the captioner.


Why do you need to provide segmentation at 5 second intervals?


>  It also doesn’t know what the end time should be, as it should be 16 seconds from the end of the last word.

You can choose the end time to be whatever suits best. Is your cue now
supposed to be a max of 5 seconds long or 16 seconds long?


> Creating a new cue that starts for “Longer” and “extended” will cause the “Caption Line 2” part to scroll; we want it to continue on the same line.


That's not the problem of the cue, but the problem that you want to
segment at 5 second intervals.


> I’d like to know if there is a clear solution to these problems, and if not, if additions to the specification can be added to handle these cases.

It should all work with the adjustment of the end time as needed,
unless I am missing something.

Cheers,
Silvia.

>
> Thanks,
> Bill May
> MLB Advanced Media

Received on Wednesday, 30 September 2015 04:54:36 UTC