Re: Sourcing language information for media tracks

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> 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.
>