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Multi-bitrate Quicktime SMIL files

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From: Terry Thompson
Date: Jun 25, 2007 6:50PM


Hi All,

Does anyone have a sample SMIL file that serves captioned videos to users at
different bitrates? Here's what I'm trying to accomplish:

1. I have three different Quicktime videos, each encoded at different
bitrates.

2. The physical dimensions vary for each of the videos, which affects how
many words will fit on a line. Therefore, I also have three different
caption files (qt.txt).

3. I'd like to deliver the correct version of both the video and caption
text, based on user's bandwidth as detected by Quicktime (I don't want to
burden users with having to select the correct version).

I know that in the SMIL file, <switch> in combination with the
system-bitrate attribute allows me to present different objects for
different bitrates, but I'm not clear how best to use <switch>. Here's
pseudo-code for what I have currently:

<head>
<layout>
<switch>
<!-- define three <root-layout> elements, one for each system-bitrate -->
</switch>
<!-- define all possible regions with id's like "video_modem",
"video_high", etc.-->
</layout>
</head>
<body>
<par>
<switch>
<!--choice of <video> elements, one for each system-bitrate.
Each has a unique region attribute corresponding with
one of the regions defined above -->
</switch>
<switch>
<!--choice of <textstream> elements, same approach as with video.-->
</switch>
</par>
</body>

This actually works ok for the modem version, but for the higher bandwidth
versions the Quicktime Player window is the same size as the modem version -
too small to accommodate the larger video. It's as if the <root_layout>
element doesn't respond to being included within <switch>. I've tried 101
variations on the above, which usually fixes one problem but creates
another. I think I just need to study a proven template if others have
already done this.

Thanks for any help!
Terry

Terry Thompson
Technology Specialist, DO-IT
University of Washington
<EMAIL REMOVED>