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[Bug 13357] <video>: Additional AudioTrack.kind categories are needed to identify tracks where audio descriptions are premixed with main dialogue.

From: <bugzilla@jessica.w3.org>
Date: Mon, 13 Feb 2012 21:55:29 +0000
To: public-html-bugzilla@w3.org
Message-Id: <E1Rx3rt-0002VP-AG@jessica.w3.org>

--- Comment #13 from Bob Lund <b.lund@cablelabs.com> 2012-02-13 21:55:26 UTC ---
(In reply to comment #12)
> 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
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 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 Monday, 13 February 2012 21:55:32 UTC

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