APPENDIX A
336
Measuring Time and Frame Size
The timebase affects the way clips are represented in the Project and Timeline windows. For
example, the tick marks in the Timeline window’s time ruler represent the timebase. A clip
in Source view uses the timebase at which it was saved; Premiere duplicates or skips frames
to adjust a source clip timebase to the project timebase. For this reason, source clips work
best if their timebases match the project timebase at the time you import them. For situa-
tions where the frame rate may differ from the timebase, see ,“Understanding frame rates in
relation to the timebase” on page 336.
Because all time values in your project are calculated using the timebase, set the project
timebase correctly before you begin editing. Changing the timebase in the middle of a project
is not recommended as it changes the precision of time calculations, which can cause existing
edit points or markers to shift, or may change clip durations slightly.
Understanding frame rates in relation to the timebase
The
frame rate
can have different meanings depending on whether you are talking about
source clips or the final video:
•
For source clips, the frame rate is the number of visual samples created per second when
the clip was originally shot (using a camera) or rendered (using animation software).
•
For final video, the frame rate is the number of frames an exported clip or the Timeline will
display each second when played back.
For traditional media, set the frame rate to match the timebase of the target media; specify
24 fps for editing motion-picture film, 25 fps for PAL and SECAM video, and 29.97 fps for
NTSC video. The depiction of motion on screen can only be as precise as the original frame
rate of the source clips. Increasing the frame rate of final video will not by itself make motion
appear smoother.
When the frame rate of original clips, the timebase, and the frame rate you specify for playback
or export all match, you usually don’t have to think about frame rate. When they don’t match,
the relationship between those three factors becomes important in the following ways:
•
When the frame rate of a source clip doesn’t match the timebase, Premiere compares the clip
frame rate to the timebase to determine how the original frames can be represented in the
Timeline. For example, if a clip shot at 24 fps is used in a project with the timebase set to 30,
c00.book for PS Page 336 Tuesday, March 31, 1998 1:28 PM
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