W3C home > Mailing lists > Public > public-html-a11y@w3.org > February 2011

Re: using TTML for caption delivery, discussion

From: David Singer <singer@apple.com>
Date: Sun, 13 Feb 2011 17:37:57 +0800
Cc: HTML Accessibility Task Force <public-html-a11y@w3.org>
Message-Id: <788A65E3-F802-4BB3-B1C6-F8FC9A1FA10C@apple.com>
To: Sean Hayes <Sean.Hayes@microsoft.com>

On Feb 13, 2011, at 8:23 , Sean Hayes wrote:

> Point 3. Single or chunked delivery
> Since the typical size of a caption file is only on the order of 10s of Kb, maybe 100Kb or so  for a long form movie, actually receiving it all up front and parsing it in one go isn't that much of a problem, and generally an advantage. The only time it would be an issue is in delivery of live content where you don't know the captions in advance, and as far as I can tell that's not a use case that is supported by the <video> tag  today. 

The video tag can point at anything, including RTSP controlled streams, and particularly it can point at chunked-over-HTTP manifest files, and even when pointing at a http: URL for a media file, byte-range access for time ranges can work.  Nothing in HTML says it has to be an http: URL, and nothing says it has to be a simple from-the-beginning simple download.

> Integrating TTML into MPEG4 again is fairly easy due to the small size, it can simply all fit in one XML box. Or be delivered as multiple segments in a trak. This has been defined for DECE and could be adopted into MPEG.

Whole documents do sound 'heavy' though.

> Point 4. Profiles.
> There is a fairly comprehensive profiling mechanism built into TTML,

but it only seems to allow covering language features, not characteristics of the stream (like, that it's in time order) or other functional aspects (like, CSS styling support), right?

> So I don't believe we actually achieve very much in the real world by trying to make a decision now. In a few years we may be able to see which format is gaining most ground in practice and make a decision then. The thing to do today is to ship HTML5 so that captioning is not precluded, allow pioneering content authors to write caption content in any format they choose, and wait and see how the browser vendors do on implementing <track> natively over time.

Total agreement here.  I have heard rumblings of a suggested mandate for TTML, whereas I would prefer to agree with you and get some experience doing captioning in specific and accessibility support in general before we see mandates. 

> Cheers,

cheers indeed, thanks for the thoughtful reply.

David Singer
Multimedia and Software Standards, Apple Inc.
Received on Sunday, 13 February 2011 09:39:25 UTC

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