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Customizing Multimedia with a SMIL
By John Maxwell Hobbs
 While
many people are familiar with XML as a means of abstracting data from
its HTML presentation, few seem aware of such capabilities in the realm
of multimedia. If you find yourself tailoring presentations to
different display or throughput considerations, or you're creating
multiple language versions of presentations, you'll want to explore the
Synchronized Multimedia Integration Language (SMIL). In particular,
SMIL's <switch> tag can simplify your life and make customizing
presentations easier. SMIL is a recommendation from the World Wide Web Consortium (W3C) for the easy implementation of sophisticated time-based multimedia content on the Web. SMIL is an XML
extension and currently in version 1.0. While is still relatively
young, it has SMIL is currently supported for Windows, Unix and
Macintosh by Oratex and for Windows and Macintosh by Real Networks.
For customizing media delivery to smart phones and communicators, SMIL really shines when compared to other systems.
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| The basics of SMIL creation have been covered in an earlier article:
The Synchronized Multimedia Interface Language.
Unlike monolithic multimedia presentation formats like Shockwave, SMIL
is a distributed format, like HTML. A .smil file is a text file in a
declarative format that instructs the SMIL browser to fetch source
multimedia files from various locations and render them at the
specified times for the specified durations. This allows the
presentation to be easily modified after creation, simply by replacing
a source file, rather than having to re-author the entire presentation. Switching your presentations The
<switch> element in SMIL takes advantage of this to allow
customized presentations to be created based on bandwidth availability,
hardware constraints, and user preferences. The <switch> element has three criteria: - A valid SMIL element
- A valid media type
- A valid test attribute
and takes the following form:
<switch>
<option1 test-attribute="media1">
<option2 test-attribute="media2">
</option2>
</option1>
</switch>
| Valid test attributes are: | | system-bitrate
the bandwidth available to the system, determined either through bandwidth detection, or by user preference settings system-captions
a "closed-captioning" option determined by user preference settings system-language
a language selection option determined by user preference settings and
selected from comma-separated list of language names as defined in
[RFC1766] system-overdub-or-caption
a user selected
option that determines a preference for language translation via
subtitles or overdubs when a translation option is available system-required
this is in place for future extensions to SMIL system-screen-size
allows the browser to determine if the device is able to display a
presentation of a specific size. The test attribute is stated as
"screen-size-val ::= screen-height"X"screen-width" with the size being
specified in pixels system-screen-depth
allows the presentation to specific the bit depth of the screen color palette required to display specific media. |
Customizing a Presentation for Bandwidth One
of the biggest headaches facing multimedia content developers is the
need for multiple presentations to deal with a wide variety of
connection speeds. This headache is doubled when creating content that
is updated often, as with newscasts. The system-bitrate test attribute
makes it easy to deal with this. The following code fragment
demonstrates how to easily deliver audio to a variety of bandwidths:
<switch>
<audio src="lan-audio.au" system-bitrate="75000">
<audio src="56k-audio.au" system-bitrate="47000">
<audio src="/28k-audio.au" system-bitrate="20000">
</audio>
</audio>
</audio>
</switch>
The
same audio material has been encoded at three target bitrates - one for
LAN users, one for 56k modem users, and the third for 28k modem users.
When creating the presentation, the choices should be ordered from the
most desirable to the least desirable, with the last item being a
fail-safe choice whenever possible. In certain situations, such as with
video, when the bandwidth is very low the last option may need to be an
alternative media type as in the example below:
<switch>
<video src="lan-video.rm" system-bitrate="75000">
<video src="56k-video.rm" system-bitrate="47000">
<video src="28k-video.rm" system-bitrate="20000">
<img src="617511_files/slide.html">
</video>
</video>
</video>
</switch> Since
the .jpg file in the example above is the fallback choice, a test
attribute is not necessary. If a choice within a <switch> tag
does not have a test attribute specified, it will always function as
the default choice if all of the other test attributes return a value
of "FALSE." The
system-bitrate test attribute can also be used in a more sophisticated
fashion to select between complete presentations optimized for specific
bandwidths, as in this example
. Customizing for Devices As
non-PC based access to the Internet spreads through the use of smart
phones and communicators, it will become necessary to customize media
delivery for their varied capabilities. This is where SMIL shines
compared to other systems. Take a look at this example
. The
SMIL browser determines the display capabilities of the receiving
device, and sends the appropriate version of the source file. If none
of the test attributes are met, it defaults to an audio description of
the video file. This option is particularly useful for someone
accessing the content through a telephone-to-web gateway. Adjusting for User Preferences
The Internet
is an international communications network, and your users will
likely speak a variety of languages. SMIL allows this issue
to be addressed in a variety of ways. Perhaps the simplest is
to provide audio tracks in a variety of languages.
It is possible
to list multiple languages in a test attribute.
Although this
is perhaps the easiest way to address the language issue, it
may not be the most aesthetically pleasing to all viewers. In
the case of entertainment or artistic presentations, many users
prefer to read subtitles while listening to the original audio
track.
It is also possible to accommodates
hearing-impaired viewers who want the presentation to be close-captioned
in the original language.
As
you can see from the above examples, the very simple parameters of the
<switch> element can be combined to create very complex
behaviors, ranging from approaches taken out of expediency, to those of
presentations that allow for user customization. Through the use of
these techniques, highly interactive presentations, similar to those
becoming available on DVD systems, can be created. Through the use of
the system-caption-or-overdub attribute, director's comments can be
added to the presentation of a film. The system-captions attribute can
be used not just to provide close-captioning for the hearing impaired,
but used in a different fashion can provide audio description for the
visually impaired as well. With the ability to provide alternative
media types based on bandwidth and hardware constraints, aesthetically
pleasing content can be delivered to a wider spectrum of users than was
previously possible.
About the author: John Maxwell Hobbs
is a musician and has been working with computer multimedia for over
fifteen years. He is currently Head of Creative Development at Ericsson CyberLab New York. His interactive composition Web Phases
was one of the winners of ASCI's Digital '98 competition. He recently
completed a Java-based interactive musical instrument in collaboration
with artist/programmer Mark Napier called Ripple.
He is also on the board of directors of Vanguard Visions, an
organization dedicated to fostering the work of artists experimenting
with technology and serves on the Digital Arts sub-committee of the
Mayor's Council on New Media in New York City. He is the former
Producing Director for The Kitchen.
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