Adobe 65021048 User Manual page 391

ADOBE PREMIERE PRO CS3 385
User Guide
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
intermediate 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.
Choosing an optimal compression setting is a balancing act that varies depending on the type of video material, the
target delivery format, and the intended audience. 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. Often, the optimal compression setting
is arrived at through trial and error.
About data rate
With some video and audio codecs, you can specify the data rate, also called the bit rate, which controls the amount
of video information that must be processed each second during playback. Specifying a data rate actually sets the
maximum data rate, because the actual data rate varies depending on the visual content of each frame.
To maximize the quality of encoded video, set the data rate as high as the target delivery medium can support. If
you plan to stream video to an audience using dial-up Internet access, this may be as low as 20 kilobits per second;
however, if you plan to distribute video on DVD, it may be as high as 7 megabits per second. The data rate you
specify depends on the purpose of the video. The following list describes data rate guidelines for some uses:
The data rate should maximize quality while fitting the entire program within the space available
DVD production
on the DVD.
The data rate should fall within the capabilities of the computer and hard disk that
Non-DV videotape production
perform the final playback to tape.
If the final video will be played back from hard disks, determine the typical data transfer rate of
Hard-disk playback
your audience's hard disks and set the data rate accordingly. If you're exporting video to be used in another editing
system or to be imported into a compositing application, you'll want to export at the maximum quality. Use a
lossless codec or the codec supported by your video capture card, and specify the data rate that the editing system
supports for video capture and editing.
The data rate for video played from a CD-ROM depends on the speed of the drive. For example,
CD-ROM playback
if you're preparing a final video file for a quad-speed CD-ROM drive (600 kilobytes per second), you might specify
between 300 and 500 kilobytes per second to account for both the data rate of the drive and for the system overhead
required to move the data.
The data rate can be 1 megabit per second or faster, depending on the speed of the intranet.
Intranet playback
The data rate should account for real-world performance at the target data rate. For
Streaming video over the web
example, the data rate for streaming video designed for a 56-kilobit-per-second connection is often set to 40 kilobits
per second. The difference accounts for factors such as data volume and line quality that often prevent telephone-
based Internet connections from consistently achieving their stated data rate. For broadband connections, set the
data rate for streaming video to 128 kilobits per second.
The data rate is less important than the size of the video file on disk, because
Downloading a video file over the web
the main concern is how long it takes to download the file. However, it still may be desirable to reduce the data rate
for downloaded video because doing so reduces the size of the video file, making it download faster.
April 1, 2008