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[Bug 17797] <video>: Additional AudioTrack.kind categories are needed to identify tracks where audio descriptions are premixed with main dialogue.

From: <bugzilla@jessica.w3.org>
Date: Wed, 18 Jul 2012 06:47:18 +0000
Message-Id: <E1SrO2Y-0006ay-UB@jessica.w3.org>
To: public-html-bugzilla@w3.org
https://www.w3.org/Bugs/Public/show_bug.cgi?id=17797

Ian 'Hixie' Hickson <ian@hixie.ch> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  Status Whiteboard|XXX                         |

--- Comment #1 from Ian 'Hixie' Hickson <ian@hixie.ch> 2012-07-18 06:47:18 UTC ---
The text from that bug got weirdly truncated. Here's the most relevant comment
though:

=== Bob Lund 2012-02-13 21:55:26 UTC ===
> My intent here is to not add anything until there is at least one format that
> supports this, since there is no point the HTML spec saying to do something
> that can never happen.

Premixed audio descriptions for visually impaired + main dialogue, as required
by the recent FCC ruling
(http://transition.fcc.gov/Daily_Releases/Daily_Business/2011/db0825/FCC-11-126A1.pdf),
is specified in AC-3 audio used in MPEG-2 TS. This spec
(http://www.atsc.org/cms/standards/a_52-2010.pdf) defines how the presence of
this pre-mixed audio track is signaled: in the  AC-3 bit stream information
syntax (section 5.3.2)  field bsmod = 2 signals the visually impaired audio
stream (see Table 5.7 in section 5.4.2.2) and in the AC-3 descriptor field
full_svc = 1 signals a full audio service (section A4.3), meaning the track
includes the main dialogue audio.

This standard is adhered to in North American broadcast channels. These AC-3
signals could also exist when these channels are redistributed over IP.

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Received on Wednesday, 18 July 2012 06:47:24 GMT

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