Final Cut Pro 3: About Sync Adjust and Irregular Audio Rates in DV Clips

Many DV camcorders and decks record audio at sample rates other than the true sample rates of 32 kHz or 48 kHz. Variations in equipment from different manufacturers result in slight, but significant variances from these standard sample rates.

QuickTime expects the sample rate of incoming audio to be exactly 32 kHz or 48 kHz. The discrepancy between the sample rate used by your DV video equipment and the expected sample rate in QuickTime may cause a drift in audio and video sync in your captured clips over many minutes of playback. How noticeable this drift is depends on the DV camcorder or deck you're using.

To avoid this issue in Final Cut Pro and to ensure you have accurate sync in all captured DV clips, make sure you turn on the Sync Adjust Movies Over _ Minutes option in Preferences. When turned on, this option forces Final Cut Pro to count the samples of the audio tracks of all DV clips after they've been captured to determine the sample rate at which they were originally recorded. With the original sample rate determined, Final Cut Pro can then ensure accurate sync no matter how long your captured clip.

Since drifts in sync typically occur in clips with a long duration, you can specify the number of minutes in a clip when this feature should be used. The default value of five minutes is adequate in most cases. When this feature is used by Final Cut Pro, the new sample rate information for your clip is stored in your Final Cut Pro project file, not in the source QuickTime file for this clip on disk. It does not alter the source file in any way. When looking at your clips in the Browser, you can tell if this feature has been used if the Aud Rate column displays an uneven value for your clip. For example, instead of displaying a sample rate of 48.0 kHz for your clip, a sample rate of 48.0258 appears.

Note: There are two types of items with which you shouldn't use the Sync Adjust Movies setting: non-DV clips and clips with timecode breaks.

• Non-DV clips: Clips that you capture from non-DV sources, such as analog tape or from digital sources via SDI, don't have varying sample rates. However, because lost A/V sync due to timecode breaks is still a risk, you should disable the Sync Adjust feature if you're not capturing from a DV source.
• Clips with timecode breaks: Any timecode break, or gap, in the signal recorded to tape results in an invalid sample count if the Sync Adjust feature is enabled. As a result, the A/V sync of this type of clip is lost. The only way to restore sync is to record the clips that come before and after a timecode break separately.