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ADOBE PREMIERE PRO 2.0
374
User Guide
Streaming media delivers video over the web or other network without downloading a file to a hard
Streaming video
disk, comparable to the way a traditional broadcast works. The bitrate—and therefore the quality—of streaming
video is constrained by the bandwidth of the network or modem. When streaming video over the web, you can
specify a higher bitrate if you know your audience has broadband Internet access, such as DSL or cable modem
service. To provide versions tailored to the bitrate limits of different viewing scenarios, you can use the encoder's
Audiences or Alternates feature. Streaming video is most effectively deployed over an office's intranet, where
high-speed bandwidths are more common and consistent. Macromedia Flash Video, QuickTime, Windows Media,
and RealMedia file types include streaming media formats.
Understanding video compression and data rate
Generally speaking, encoding video and audio to a digital format involves balancing quality with effective storage
and playback. For this reason, most formats use compression to reduce file size and data rate. Without compression,
a single frame of standard-definition video consumes nearly 1 MB (megabyte) of storage. At NTSC's frame rate of
approximately 30 frames per second, uncompressed video requires a data rate of nearly 30 MB per second, and 45
seconds of footage consumes about 1 GB of storage. By comparison, a standard NTSC DV file's data rate is about 3.6
MB per second, and it takes about 5 minutes of footage to consume 1 GB of storage. Understanding the effects of
compression on quality and data rate will help you select the best format and settings for the task at hand.
About compression
Compression is essential to reduce the size of movies that could otherwise be so that they can be stored, transmitted,
and played back effectively. When exporting or rendering a movie file for playback on a specific type of device at a
certain bandwidth, you choose a compressor/decompressor (also known as an encoder/decoder), or codec, to
compress the information and generate a file readable by that type of device at that bandwidth.
A wide range of codecs is available; no single codec is the best for all situations. For example, the best codec for
compressing cartoon animation is generally not efficient for compressing live-action video. When compressing a
movie file, you can fine-tune it for the best-quality playback on a computer, video playback device, the web, or from
a DVD player. Depending on which encoder you use, you may be able to reduce the size of compressed files by
removing artifacts that interfere with compression, such as random camera motion and excessive film grain.
The codec you use must be available to your entire audience. For instance, if you use a hardware codec on a capture
card, your audience must have the same capture card installed, or a software codec that emulates it.
For more information about compression, search for the Adobe DV Compression Primer (English only) on the
Adobe website.
About compression keyframes
Compression keyframes are different from the keyframes that you use to control track or clip properties such as
audio volume or clip rotation. Compression keyframes are automatically placed during export at regular intervals in
the movie. During compression, they are stored as complete frames. The frames between the keyframes, called inter-
mediate frames, are compared to the previous frame and only the changed data is stored. This process can greatly
reduce file size, depending on the spacing of the keyframes. Fewer keyframes and more intermediate frames result
in smaller file sizes but produce lower-quality images and motion. More keyframes and fewer intermediate frames
result in significantly larger file sizes but produce higher-quality images and motion.
Choosing compression settings is a balancing act that varies depending on the type of video material, the target
delivery format, and the intended audience. Often, the optimal compression setting is arrived at through trial and
error.
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