Apple Final Cut Express 4 - Manual (Page 633)

Chapter 42 Mixing Audio in the Timeline and Viewer 633
VIII
Example: Using Keyframes to Control Pan
Setting keyframes to change pan dynamically works the same way as it does with
levels. You need to set at least two keyframes to effect a change over time.
Changing pan over time is often done to achieve stereo effects such as making a car
sound zoom from left to right, or putting a particular sound effect on one side or the
other of a stereo image. If you want the car sound effect in your edited sequence to
zoom from the left to the right to match the movement of an onscreen car, here are
the steps you would take.
To set up a dynamic stereo pan using keyframes:
1 Open the car sound effect in the Viewer so you can see it in more detail.
2 Move the Viewer playhead to the beginning of the car sound effect’s waveform, right
before the car sound starts playing, and click the Pan Keyframe button to set a keyframe.