Adobe PREMIERE 5 User Manual page 357

ADOBE PREMIERE 5.0 349
User Guide
a version of this technique that is used by many codecs. As you increase spatial compression,
the data rate and file size decrease, and the picture loses sharpness and definition. For many
codecs, the degree of spatial compression is controlled by the Quality and Data Rate options—
lowering the values for these options increases spatial compression. In some codecs, Quality
and Data Rate are interrelated so that changing one affects the other.
Temporal compression
Temporal (time) compression looks for ways to compact the description of the changes during
a sequence of frames. It does this by looking for patterns and repetition over time.
For example, in a video clip of a person speaking in front of a static background, temporal
compression will notice that the only pixels that change from frame to frame are those forming
the face of the speaker. All the other pixels don't change (when the camera is motionless).
Instead of describing every pixel in every frame, temporal compression describes all the pixels
in the first frame, and then for each frame that follows, describes only the pixels that are
different from the previous frame. This technique is called frame differencing.
When most of the pixels in a frame are different from the previous frame, it's preferable to
describe the entire frame again. Each whole frame is called a keyframe, which sets a new starting
point for frame differencing. Many codecs can create keyframes at an interval you specify, and
some codecs can also insert keyframes at markers you set in the Timeline window in Premiere.
Some codecs automatically create a keyframe for a frame that is visually very different from the
previous frame. As you specify fewer keyframes, the data rate and file size decreases, and so
does the picture quality. The degree of temporal compression is usually controlled by a codec's
Quality option and by keyframe settings—lowering the value for these options increases
temporal compression. For information on setting keyframe options, see "Setting keyframes"
on page 302.