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Re: Action item. definition and use of Clean audio in European television

From: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Date: Sat, 14 May 2011 13:49:36 +1000
Message-ID: <BANLkTinBpj1g9aY9zHYF0vXC93fK8DT8tg@mail.gmail.com>
To: Sean Hayes <Sean.Hayes@microsoft.com>
Cc: "HTML Accessibility Task Force (public-html-a11y@w3.org)" <public-html-a11y@w3.org>
On Sat, May 14, 2011 at 1:33 PM, Silvia Pfeiffer
<silviapfeiffer1@gmail.com> wrote:
> Ha, that's funny: have we been using the wrong term all the time?
> In out requirements it's called "clear audio" (and that's also what we
> have requested in the bug, see
> http://www.w3.org/Bugs/Public/show_bug.cgi?id=12544 ). In these
> documents it's called "clean audio".
> I think we will need to change our documents/request for @kind label...
> I've now (using the proper name) also found this document:
> http://www.etsi.org/deliver/etsi_ts/101100_101199/101154/01.09.01_60/ts_101154v010901p.pdf
> which is "Digital Video Broadcasting (DVB); Specification for the use
> of Video and Audio Coding in Broadcasting Applications based on the
> MPEG-2 Transport Stream" . Annex E talks about Clean Audio as a
> supplementary audio service in the form of receiver-mixed data. This
> is in contrast to broadcast-mixed delivery, which would be as a
> separate alternative audio stream, which DVB does not support.
> So, it seems that while we are asking for a @kind="clean audio" label
> on audio tracks that are delivered separately to the main audio track
> and provide a better understandable audio mix, this isn't actually
> what DVB supports (and I assume DVB is the only way in which clean
> audio is currently realized?).
> So, the question is: what do we do in HTML5 to support the kind of
> clean audio tracks that DVB expects.
> IIUC, the receiver-mixed means of delivery of clean audio (and audio
> description for that matter) delivers it through multi-channel audio
> but within the main audio track. This means, on a 3, 5.1 or more audio
> channel file, some of these channels are reserved for non-main audio.

Actually, that's incorrect for audio description. A "supplementary
audio" channel is an additional audio channel, so it would take up the
number of audio file with stereo, they would deliver 3 channels, for
one with 5.1 it would have 7 channels etc. I think though for clean
audio it is still true that they use center channel for speech only in
a 5.1 file.

Now the big question is: how common is all this. And would we
encourage people to rather provide multitrack than multichannel for
accessibility needs?


> There is a marker of "visual impaired commentary" for AD and "hearing
> impaired" for clean audio used on the individual channels that have a
> separate use.
> The images shown in the spec show the use of clean audio only in the
> case of 5.1 channels with the center channel used for clean audio,
> while when using audio description it shows the use of 3 channels:
> left, right, and a mono audio description.The user has the possibility
> to separately increase the volume of the audio description and of the
> clean audio.
> If such a file with multiple audio channels was to be used in a HTML5
> page, we wouldn't get the same level of control that DVB has defined,
> because there is no control that allows to change the volume mix of
> multi-channel audio. It is important to understand that there is a
> difference between multi-channel audio and multi-track audio:
> multi-channel audio is encoded within one audio track just by
> interleaving the channels within that track. In HTML5 we don't have a
> means to change the volume of the individual channels, but only to
> change the volume of a track.
> Cheers,
> Silvia.
> On Fri, May 13, 2011 at 7:59 PM, Sean Hayes <Sean.Hayes@microsoft.com> wrote:
>> Here is a summary of the European digital television definition for clean
>> audio [adapted from a forthcoming edition of TS 101 154 from the DVB]. The
>> system relies on client side mixing, under control of the programme-maker.
>> Clean Audio refers to audio providing improved intelligibility. It is
>> targeted for viewers with hearing impairments, but can as well serve as
>> improvement for listening in noisy environments. an important requirement
>> is to adjust, on a passage by passage basis, the relative level of programme
>> sound in the mix which the secondary audio user hears. The programme maker
>> is best able to determine the level under controlled conditions when
>> authoring the secondary audio information to modulate the level of programme
>> sound in the secondary audio capable receiver so suitable secondary audio
>> information is thus transmitted within the secondary audio stream.
>> Individual secondary audio users will have different aural acuity, and there
>> are, in practice, differences in audio signal level for different home
>> receivers. An essential requirement is for the user to be able to adjust the
>> volume to suit his/her condition.
>> A descriptor is sent in a service to indicate a clean audio service. The
>> level by which the main audio service should be attenuated is sent in the
>> metadata (on a per channel basis). The client should accentuate the levels
>> of the channel containing the dialogue and attenuate the other channels,
>> accordingly. The level of the centre channel added should additionally be
>> under user control to allow individual tailoring of the sound for
>> audibility.
>> Background:
>> http://www.rnid.org.uk/VirtualContent/84923/background_noise_PDF.pdf
>> European HD TV requirements: http://tech.ebu.ch/docs/tech/tech3333.pdf [p15]
Received on Saturday, 14 May 2011 03:50:24 UTC

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