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Multimedia techniques

From: Madeleine Rothberg <Madeleine_Rothberg@wgbh.org>
Date: 9 Nov 1999 16:38:38 -0500
Message-ID: <n1269956723.55830@wgbh.org>
To: "W3C-WAI-UA" <w3c-wai-ua@w3.org>

Here are some techniques for checkpoint 3 on multimedia.
I'm sorry these are late. I hope they can still be useful.
They are below with line breaks and attached as text without
line breaks in case that is more useful to Ian.

-Madeleine


Techniques for checkpoint 3.1 
For SMIL, see specific techniques for Checkpoints 3.7, 3.8, 
and 3.9.

Techniques for checkpoint 3.4 
Provide access to a static list of time dependent links, 
including information about the context of the link. For 
example, provide the time at which the link appeared along 
with a way to easily jump to that portion of the 
presentation.

Provide easy-to-use controls (including both mouse and 
keyboard commands) to allow viewers to pause the presentation 
and advance and rewind by small and large time increments.  

Provide a mode in which all active elements are highlighted 
in some way and can be navigated sequentially. For example, 
use a status bar to indicate the presence of active elements 
and allow the user to navigate among them with the keyboard 
or mouse to identify each element when the presentation is 
moving and when it is paused.
 
Techniques for checkpoint 3.7 
It is important that any continuous equivalent tracks be 
rendered synchronously with the primary content. This ensures 
that users with disabilities can use the primary and 
equivalent content in combination. For example, if a hard-of-
hearing user is watching a video and reading captions, it is 
important for the captions to be in sync with the audio so 
that the viewer can use any residual hearing. For audio 
description, it is crucial that the primary audio track and 
the audio description track be kept in sync to avoid having 
them both play at once, which would reduce the clarity of the 
presentation.

SMIL
User agents which play SMIL presentations should take 
advantage of a variety of access features defined in SMIL. A 
W3C note on access features of SMIL 1.0 documents those 
features currently recommended [reference]. A future version 
of SMIL (known currently as SMIL Boston) is in development 
and additional access features may be available when this 
specification becomes a W3C Recommendation. 

As defined in SMIL 1.0, SMIL players should allow users to 
turn closed captions on and off by implementing the test 
attribute system-captions which takes the values "on" and 
"off." For example, include in the player preferences a way 
for users to indicate that they wish to view captions, when 
available. SMIL files with captions available should use the 
following syntax:
<textstream alt="English captions for My Favorite Movie"
                            system-captions="on"
                            src="closed-caps.txt"/> 
In this case, when the user has requested captions, this 
textstream should be rendered, and when they have not it 
should not be rendered. 

SMIL 1.0 does not provide a test attribute to control audio 
description in the same way as captions. 

Another test attribute, system-overdub-or-captions, allows 
the user to choose between alternate language text or sound. 
This attribute specifies whether subtitles or overdub should 
be rendered for people who are watching a presentation where 
the audio may be in a language in which they are not fluent. 
This attribute can have two values: "overdub", which selects 
for substitution of one voice track for another, and 
"subtitle", which means that the user prefers the display of 
subtitles. However, this attribute should not be used to 
determine if users need closed captions. When both are 
available, deaf users will prefer to view captions, which 
contain additional information on music, sound effects, and 
who is speaking, which are not included in subtitles since 
those are intended for hearing people.

QuickTime
User agents which play QuickTime movies should provide the 
user with a way to turn on and off the different tracks 
embedded in the movie. Authors may use these alternate tracks 
to provide alternative equivalents for use by viewers with 
disabilities. The Apple QuickTime player currently provides 
this feature through the menu item "Enable Tracks."

Microsoft Windows Media Object
User agents which play Microsoft Windows Media Object 
presentations should provide support for Synchronized 
Accessible Media Interchange (SAMI), a protocol for creating 
and displaying caption text synchronized with a multimedia 
presentation. Users should be given a way to indicate their 
preference for viewing captions. In addition, user agents 
which play Microsoft Windows Media Object presentations 
should enable viewers to turn on and off other alternative 
equivalents, including audio description and alternate video 
tracks.

Other Formats
Other video or animation formats should incorporate similar 
features. At a minimum, users who are blind and users who are 
deaf need to be able to turn on and off audio description and 
and captions. The interface to set these preferences must be 
accessible. Information on how to author accessible tracks 
should be included in documentation about the media player.


Techniques for checkpoint 3.8 
Provide an interface which displays all available tracks, 
with as much identifying information as the author has 
provided, and allow users to choose which tracks are 
rendered. For example, if the author has provided "alt" or 
"title" for various tracks, use that information to construct 
the list of tracks.

Provide an interface which allows users to indicate their 
preferred language separately for each kind of continuous 
equivalent. Users with disabilities may need to choose the 
language they are most familiar with in order to understand a 
presentation which may not include all equivalent tracks in 
all desired languages. In addition, international users may 
prefer to hear the program audio in its original language 
while reading captions in their first language, fulfulling 
the function of subtitles or to improve foreign language 
comprehension. In classrooms, teachers may wish to control 
the language of various multimedia elements to achieve 
specific eduational goals.
 
Techniques for checkpoint 3.9
See specific techniques for checkpoint 3.8.



Received on Tuesday, 9 November 1999 16:36:56 GMT

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