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Re: [media] handling multitrack audio / video

From: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Date: Mon, 1 Nov 2010 12:14:25 +1100
Message-ID: <AANLkTin3erJgwoYgAm4vtPnvL43iGXrvgJX-PX-KKtm_@mail.gmail.com>
To: Philip Jägenstedt <philipj@opera.com>
Cc: public-html-a11y@w3.org
On Fri, Oct 29, 2010 at 2:08 AM, Philip Jägenstedt <philipj@opera.com> wrote:
> On Thu, 28 Oct 2010 14:46:32 +0200, Geoff Freed <geoff_freed@wgbh.org>
> wrote:
>> On Thu, 28 Oct 2010 13:05:57 +0200, Philip Jägenstedt <philipj@opera.com>
>> wrote:
>>> It's
>>> beyond this most basic case I'd like to understand the actual use cases.
>>> To clarify, option 2 would allow things like this, borrowing SMIL syntax
>>> as seen in SVG:
>>> <video id="v" src="video.webm"></video>
>>> <video begin="v.begin+10s" src="video2.webm"></video>
>>> <!-- video and video2 should be synchronized with a 10s offset -->
>>> or
>>> <video id="v" src="video.webm"></video>
>>> <video begin="v.end" src="video2.webm"></video>
>>> <!-- video and video2 should play gapless back-to-back -->
>>> Are there compelling reasons to complicate things to this extent? The
>>> last example could be abused to achieve gapless playback between chunks in a
>>> HTTP live streaming setup, but I'm not a fan of the solution myself.
>> I think there are compelling cases which are likely to occur in production
>> environment because they are more efficient than the example I outlined
>> above.  For example, an author could store the same three descriptions
>> discretely, rather than in a single audio file, and then fire each one at
>> the appropriate point in the timeline, in a manner similar to the one you've
>> noted above:
>> <video id="v" src="video.webm"></video>
>> <audio sync="v.begin+15s" src="description1.webm"></audio>
>> <audio sync="v.begin+30s" src="description2.webm"></audio>
>> <audio sync="v.begin+45s" src="description3.webm"></audio>
> Rights, it's easy to see how it could be used. If the implementation cost is
> worth what you get, I expect that similar implementations already exist in
> desktop applications. Are there any implementations of such a system in
> widespread use and does it actually get the sync right down to the sample?

Jeroen from JWPlayer/Longtail Video has implemented something for
audio descriptions, where audio descriptions come in separate files
and are synchronized through markup - I believe the synchronization is
done in the JWplayer in Flash, see
. AFAIK this is the most used platform for providing audio
descriptions on the Web at this point in time - I've seen it use in
government Websites around the globe.

If it can be done in Flash in an acceptable quality, I would think
browsers should be able to do it. I can ask Jeroen for more
implementation details if necessary - AFAIK he said there was frequent
re-synchronization of the secondary resource to the main resource,
which continues playback at its existing speed.

Received on Monday, 1 November 2010 01:15:19 UTC

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