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Re: Sourcing language information for media tracks

From: Yosuke Funahashi <yosuke@w3.org>
Date: Fri, 11 Mar 2016 20:28:51 +0900
To: Philip J├Ągenstedt <philipj@opera.com>, Sergey Volk <servolk@chromium.org>, Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Cc: "public-inbandtracks@w3.org" <public-inbandtracks@w3.org>, Matt Wolenetz <wolenetz@chromium.org>
Message-ID: <56E2ABF3.6080501@w3.org>
I can set up a Github repo for this spec under the W3C account If you prefer.


On 3/11/16 5:53 PM, Philip J├Ągenstedt wrote:
> Silvia, any chance of maintaining this spec on GitHub so that you can accept
> PRs? Not that I'm volunteering, of course, but this seems like the kind of thing
> where it'd be appropriate :)
> On Fri, Mar 11, 2016 at 8:26 AM, Sergey Volk <servolk@chromium.org
> <mailto:servolk@chromium.org>> wrote:
>     Hi,
>     I've been working on adding media track support in Chromium and was
>     trying to use  the spec at
>     https://dev.w3.org/html5/html-sourcing-inband-tracks/ for reading
>     various media track properties.
>     I've ran into a few issues (see the discussion at
>     https://codereview.chromium.org/1735003004/ for details):
>     1. The current draft says that for MPEG-4 ISOBMFF files the language
>     for audio/video tracks should be: "Content of the language field in
>     the MediaHeaderBox."
>     But according to various sources (e.g.
>     https://developer.apple.com/library/mac/documentation/QuickTime/QTFF/QTFFChap2/qtff2.html,
>     https://www.scribd.com/doc/10911307/26/mdhd-box, etc) the language
>     field in MDHD box is a zero-padded 16-bit value that encodes a
>     3-letter ISO 639-2/T language code:
>     3-character code specifying language(see ISO 639-2/T); each character
>     isinterpreted as 0x60 + (5 bit) code to yield an ASCII character.
>     Yet according to the HTML5 spec (see
>     https://html.spec.whatwg.org/multipage/embedded-content.html#dom-audiotrack-language)
>     A/V track language attributes must return BCP-47 language tag strings,
>     not the 3-character ISO 639-2/T codes.
>     So the inband-tracks-sourcing spec should probably mention that the
>     language read from MDHD.language field needs to be translated into
>     BCP-47.
>     2. Again, according to multiple sources on the internet (for example
>     see
>     https://developer.apple.com/library/mac/documentation/QuickTime/QTFF/QTFFChap2/qtff2.html).
>     There is a new way to specify language info for media tracks in MP4
>     files, see the 'Extended Language Tag Atom' section in the link above.
>     The ELNG box is optional, but when present it will have a BCP-47
>     language info. So when the ELNG box is present in the .mp4 file, it
>     should probably be preferred over the language field of MDHD box.

Yosuke Funahashi
Web and TV IG co-chair, Entertainment vertical champion,
HTML Media Extensions WG team contact at W3C
Project Associate Professor at Keio University
Received on Friday, 11 March 2016 11:29:03 UTC

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