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Re: Acessibility of <audio> and <video>

From: Henri Sivonen <hsivonen@iki.fi>
Date: Fri, 5 Sep 2008 10:02:00 +0300
Cc: HTML WG <public-html@w3.org>
Message-Id: <5BDFC1DD-18C5-4714-B373-32702406C650@iki.fi>
To: Eric Carlson <eric.carlson@apple.com>

On Sep 4, 2008, at 18:29, Eric Carlson wrote:

> On Sep 4, 2008, at 1:17 AM, Henri Sivonen wrote:
>
>> On Sep 4, 2008, at 01:13, Dave Singer wrote:
>>
>>> 2.1.2 Configuring
>>> Sometimes, similarly, the media format itself can carry optional  
>>> features. An example might be the 3GPP file format (or any file  
>>> format from that family, such as MP4) with a text track in 3GPP  
>>> Timed Text format. Enabling this track (and thereby causing it to  
>>> be presented) may be a way to satisfy a need within a single media  
>>> file.
>>
>> It seems to me that for captioning, an off-by-default track within  
>> the main file is preferable over burned-in open captions, because  
>> tracks within the main file travel better, compress better (and  
>> transferring the captions even when not needed is not burdensome in  
>> terms of relative network bandwidth) and make video more searchable.
>>
>  I am not sure if you are suggesting otherwise, but a a 3GPP Timed  
> Text track is exactly what you describe: a relatively small text  
> track carried within the media file. It may nor may not be enabled  
> by default, that is a decision that is made at authoring time.

Yes, I mean 3GPP Timed Text in the MP4 context. Possibly Kate in an  
Ogg context, but that isn't sorted out yet.

What kind of metadata about captions vs. subtitles, on-by-default vs.  
off-by-default and language can MP4 contain about an 3GPP track? Does  
QuickTime expose in the API whatever the file format can express here?

>>> (In some cases, the media format may also need to disable a track;  
>>> for example, a track providing audio description of video may  
>>> incorporate the standard audio within it, and the normal audio  
>>> track would be disabled if the audio description were enabled.)
>>
>> I would guess that content providers would opt for alternative  
>> files in this case, because additional audio tracks show up on the  
>> bandwidth bill if served even when not needed.
>>
>  This is not necessarily true. Even for progressive download files,  
> some media-subsystems only read the parts of a file necessary for  
> the presentation.

How does this work?

>>> We therefore also need the ability to apply the same preferences  
>>> used for selection, to configuring the file. Note that not all  
>>> media sub-systems will offer the user-agent such an API; that is  
>>> acceptable  for media files associated with those systems, the  
>>> files are not configurable and selection must be used instead.
>>
>> This seems alarming. Does at least one of QuickTime, GStreamer or  
>> DirectShow lack such an API? If one of those lacks such an API, can  
>> such an API be put in place in a timely manner?
>>
>> It seems to me that if automatic selection isn't reliable, content  
>> providers will shy away from an automatic selection system.
>>
>  I believe David is pointing out that not all sub-systems have this  
> capability to emphasize that is it crucial for content authors to be  
> able to structure their markup so one of several files is selected  
> based on the user's stated preferences.

QuickTime, GStreamer and DirectShow seem to be the subsystems that  
make or break the proposal. Is this a problem with those three?

-- 
Henri Sivonen
hsivonen@iki.fi
http://hsivonen.iki.fi/
Received on Friday, 5 September 2008 07:02:43 GMT

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