About compression keyframes – Adobe Premiere Elements 12 User Manual
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Streaming video over the web
Playing back from a handheld device
Downloading a video file over the web
or private network that uses Internet network protocols. Because they are limited in scope, intranets generally use higher-quality communications
lines than standard telephone lines, so they are usually much faster than the Internet.
Though there are fewer users with dial-up connections than in previous years, you should still consider tailoring
your data rate to users still employing this method of accessing the Internet if you want your file to be viewable to the largest number of users.
Streaming video on the web is constrained by the limited bandwidth (56 KB or less) of most consumer modems. Use a higher bitrate if you know
your audience has broadband Internet access, such as DSL or cable modem service.
The data rate is very important due to the relatively small size, capacity, and lesser speed of handheld
devices. The data rate can range from 8 to 90 kilobits per second, depending upon the device. You can choose preset QuickTime export options in
Premiere Elements to export a file optimized for playback on a handheld device.
The data rate is less important than the size of the video file because 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.
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.
When you export a movie, Premiere Elements automatically places compression keyframes at regular intervals in the movie. During compression,
these keyframes are stored as complete frames. The frames between the keyframes are called intermediate frames. Premiere Elements compares
each intermediate frame to the frame before it and stores only the data that is different. 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 with lower-quality images and playback. More
keyframes and fewer intermediate frames result in significantly larger file sizes with higher-quality images and playback.
For example, a video of a talking person has a smaller file size than a video with lots of action, because only the mouth and tiny facial expressions
change frame to frame. In contrast, a video of a sporting event requires numerous keyframes and intermediate frames, because the action
changes considerably frame to frame. This results in either a larger file size or lower quality playback, depending on how much you compress the
video.
Choosing compression settings is a balancing act. You need to adjust the setting 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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