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Variable Bitrate Encoding
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What is Variable Bitrate (VBR)?
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When movies are compressed, each frame of the video must be encoded to a certain number of bytes. There are
several techniques for allocating the bytes for each frame.
- Fixed bitrate is used by certain codecs (like Cinepak), which attempt to allocate approximately
the same number of bytes per frame.
- Variable bitrate (VBR) is supported by other codecs (such as MPEG-2 and Sorenson), and attempts to give
each frame the optimum number of bytes, while still meeting set constraints (such as the overall data rate
of the movie, and the maximum peak data rate).
Almost every DVD-Video feature film available today uses variable bitrate encoding.
The need to acheive the best possible quality in a given space is important even on a 4.7GB disc;
it is even more important on a 650MB CD-ROM or a 1MB web file.
These two samples are a clear example of why you should consider using the Developer Edition
of the Sorenson Codec with Media Cleaner Pro. Both clips are compressed with the Sorenson Video codec and the
QDesign Music Codec at 12KBps (a good rate for realtime playback over an ISDN line).
The "Basic Edition" clip is compressed to a fixed bitrate using the Basic Version of the Sorenson
Codec; the "Developer Edition with Media Cleaner Pro" was produced with the Developer Edition of the
Sorenson Codec and Media Cleaner Pro 3.0's advanced data rate features. Media Cleaner Pro is the only tool
which has access to these advanced features.
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Try playing each movie and look at the difference between them. (On a faster machine, you may be able to
play both movies at the same time.) This clip is especially well-suited to Variable Bitrate encoding (VBR),
because it contains a variety of content, but the technique is useful on a wide range of movies.
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How to Get Sorenson and Media Cleaner
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The Sorenson Video Developer Edition is now shipping, and can be ordered through
Terran's online store. Media Cleaner Pro 3.0
is under development; if you order the upgrade today,
you will get copies of the beta until 3.0 ships.
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