A spanned AVCHD clip is a clip that consists of two or more media files. Spanned clips are created whenever you record a single shot that exceeds the file size limit of the recording device.
When this happens, the camcorder continues recording the shot to a new media file. If that file reaches the limit, the camcorder will go on to another new media file, and so on. Note: With spanned clips, you will not see multiple clips in your camcorder's clip browser, because the camera treats the spanned media segments as if they were a single clip.
Because AVCHD media uses an inter-frame compression method, it's necessary for the the camcorder to assess a number of frames (known as a GOP, or Group Of Pictures) in order to record or display any one given frame. When the camcorder stops recording one of a spanned clip's media files and goes on to recording another, it may record the same frames at the end of one segment, as well as at the beginning of the next. This is done so that the GOP structure of the recording can remain intact rather than being broken, ensuring smooth playback on your camcorder.
When Final Cut Pro 6 transfers spanned AVCHD clips to your computer, these extra frames are accounted, but there may be some resulting issues that you should be aware of:
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In the Log and Transfer window, you can preview AVCHD clips to review their contents, and decide whether or not you want to transfer them to your project. If you play a clip that is the second or subsequent segment of a spanned clip, there may be a brief "blink" of black video just as the playback begins. This is normal, and does not indicate a problem with the clip. This may also occur if you scrub the playhead in the preview section of the Log and Transfer window.
- After being transferred to the Browser, some of the clips from spanned clips (specifically, the second and subsequent segments) may have slightly different Media Start and Duration times than indicated in the Log and Transfer window. For example, if the Media Start time of a clip is 00:00:00:00 in the Log and Transfer window, it may appear as 00:00:00:20 or 00:00:00:24 after being transferred to the Browser. In such cases, your clips should always contain slightly more total media than indicated, as opposed to less. All of your footage will be included. The offset in these cases will be less than one second.
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Approximately one half second of audio may be silent at the tail end of all but the last clips in a spanned AVCHD clip. The audio will be present in the redundant section at the beginning of the next clip segment of that spanned clip.
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The second and subsequent clips of a spanned clip may have extra frames added at the front of each clip segment after being transferred. This allows for some overlap, so that the clips can be re-joined in the Timeline. AVCHD doesn't use traditional timecode, so the Media Start times for the spanned clip segments will all be within one second of 00:00:00:00. Because of this, you can't use the clips' timecode to join the spanned clip segments. It may require extra care to re-join the clips in the Timeline with precision.
This document will be updated as more information becomes available.
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