Final Cut Pro 4: Irregular Audio Rates and A/V Sync in DV Clips
This document discusses why a DV clip may have an audio rate that differs slightly from the expected rate.
Products Affected
Final Cut Pro
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 can result in slight but significant variances from these standard sample rates.
The QuickTime DV capture tools expect the sample rate of an incoming DV stream to contain locked audio at a sample rate of exactly 32k or 48k. When this rate differs, the audio and video will drift out of sync after several minutes of playback. While not perceptible on short clips, longer clips will generally display a visible drift in A/V sync after 15 minutes or so, depending on the difference between the expected and actual sample rates.
In previous versions of Final Cut Pro, the "Sync Adjust Movies Over [n] Minutes" option could be enabled in the preferences, triggering an adjustment in the audio sample rate based on the amount of audio recorded to disk during capture.
In Final Cut Pro 4, this preference has been replaced by automatic audio rate analysis, specific to DV media. This new rate analysis mechanism accurately determines the real sample rate, as recorded by the camera/deck, by examining the DV stream directly. For media captured in Final Cut Pro 4, this analysis is performed during capture, with the resulting media file updated to permanently reflect the actual sample rate. DV media files captured in previous versions of Final Cut Pro will be analyzed when imported or opened, with the actual sample rate used within Final Cut Pro; but no changes are written to the media file itself.
Note: This rate adjustment will not have any effect on loss of A/V sync caused by capturing over a segment of blank or bad tape.
Related Documents
The QuickTime DV capture tools expect the sample rate of an incoming DV stream to contain locked audio at a sample rate of exactly 32k or 48k. When this rate differs, the audio and video will drift out of sync after several minutes of playback. While not perceptible on short clips, longer clips will generally display a visible drift in A/V sync after 15 minutes or so, depending on the difference between the expected and actual sample rates.
In previous versions of Final Cut Pro, the "Sync Adjust Movies Over [n] Minutes" option could be enabled in the preferences, triggering an adjustment in the audio sample rate based on the amount of audio recorded to disk during capture.
In Final Cut Pro 4, this preference has been replaced by automatic audio rate analysis, specific to DV media. This new rate analysis mechanism accurately determines the real sample rate, as recorded by the camera/deck, by examining the DV stream directly. For media captured in Final Cut Pro 4, this analysis is performed during capture, with the resulting media file updated to permanently reflect the actual sample rate. DV media files captured in previous versions of Final Cut Pro will be analyzed when imported or opened, with the actual sample rate used within Final Cut Pro; but no changes are written to the media file itself.
Note: This rate adjustment will not have any effect on loss of A/V sync caused by capturing over a segment of blank or bad tape.
Related Documents
93327 "Final Cut Pro 4: A/V Sync Issues With Capturing DV Clips Over Blank or Bad Tape"