When creating a dialogue comp (short for “composite”), the challenge is to find the clips that express the emotion required by the director, and then work with the audio to fit it within the rest of the production. Location audio often offers technical challenges of its own that can threaten the usefulness of the recording. One of the the must difficult to diagnose and solve—at least, for those new to editing audio for picture—is phase cancellation.
A common symptom of phase problems is that your track sounds hollow or quieter when you combine two or more clips. Sometimes this only requires adjusting the volume of each clip to get the best sound. But other times, a track can sound quieter or hollow even when the individual clips are boosted. If this is the case, it can mean the clips are out of phase with each other. Audition CS5.5 can solve this problem easily.
The phase of an audio signal is its particular positive and negative waveform pattern. In A at the top of the illustration at left, two clips of the same waveform are synchronized (as shown by the red X), and their positive and negative pattern is the same, or in phase. This phrase will sound full. In B, the two clips are not synchronized. They’re not off by much, just half a wavelength, which can happen when a source is recorded with two devices or mics—a very common situation with production audio. The positive and negative pattern is the opposite in each clip or out of phase, possibly cancelling each other out, resulting in a quieter or hollow sound. In C, the clips are still not perfectly in sync, but the phase of the clip in Track 2 has been inverted, restoring the in-phase relationship of the waveforms.
Now let’s take a look at how to solve phase problems using Adobe Audition CS5.5.
- Above is a multitrack session with three tracks, each track contains dialog recorded on a different system: on-camera, a second-system digital recorder, and recording from a clip-on lavalier on the talent. The problem: a hollow sound when mixing the three source files.
- Using the Zoom panel tools and the Resizing tool, zoom in on the first clips in tracks. Solo the three tracks by clicking on the Solo button in each track header (illustration).
- Play the clips; the sound of the actor’s voice is hollow and quiet.
- Select the bottom clip, then we bring up the volume on the clip by dragging the Volume Keyframe up until we get a value of +0.0dB or so (illustration).
- Play the section; the sound is hollower than it was before. Changing the volume did not work in this case, so it’s probably a phase problem.
- Double-click on the bottom clip to open it in the Waveform Edit window.
- Choose Effects > Invert.
- Click on the Multitrack icon to return to the Multitrack Editor and play the section again. The sound should now be full, and you can adjust the mix between the clips until you get what the production requires.