Working with Audio at the Subframe Level

While the smallest unit of video is a single frame, the smallest adjustable unit of audio in Final Cut Pro is 1/100 of a frame. Audio level and pan keyframes, as well as the sync between the video and audio tracks of a clip, can be set with an accuracy of 1/100 of a frame.

Viewing an Audio Clip at Single-Frame Resolution

When you’re editing an audio clip in the Viewer, you can zoom in so far that the playhead is the width of a single video frame on the waveform. You can use this single-frame view, along with the subframe scrubbing function in Final Cut Pro, to match the beats of musical clips you edit together; or you can use it to set subframe keyframes to eliminate pops or clicks that occur at the audio edit points you’ve chosen. If you do not hear audio from the monitoring speakers while scrubbing, you may need to turn up the volume, or you may have audio turned off (see Turning Off the Audio Scrubbing Sounds).

Subframe Synchronization of Audio and Video

When you’re synchronizing audio and video, audio that’s as little as 300 samples off perfect sync can be noticeable. For this reason, Final Cut Pro allows you to resynchronize your audio in increments of 1/100 of a frame. There are several cases in which this will come in handy:

  • When editing music clips together, it’s essential that you edit them together precisely on the beat. Even a subtle offset can upset the rhythm.

  • Video and audio that were recorded from separate sources and resynchronized on tape may be subtly out of sync.

  • Audio that was recorded with a microphone far away from the audio source might have an offset between the video and audio, because sound moves through the air more slowly than light.

To navigate through a clip by subframe units
  1. Open a clip in the Viewer.

  2. Hold down the Shift key while dragging the playhead within the duration of a single video frame.

    This allows you to move the playhead in increments of 1/100 of a frame. When you set a new edit point in the clip, the clip’s audio item slips a fraction of a frame, establishing a new sync relationship between the video and audio items.

To slip an audio clip item by subframe units
  1. Open a clip with both video and audio items in the Viewer, then click the audio tab.

  2. Move the playhead to the In or Out point of the clip, then press Command-Equal Sign (=) to zoom in on the audio waveform as far as possible.

    Figure. Audio tab of the Viewer showing a wide playhead bar representing one video frame at full magnification.
  3. Press the Shift key and position the playhead within the frame to a new point with better sync.

    Figure. Audio tab of the Viewer showing the playhead repositioned within the frame at a new point with better sync.
  4. Click the Mark In or Mark Out button.

Final Cut Pro slips the audio item in the clip by the offset from the subframe position of the playhead to the boundary of the previous whole frame.