Apple M8728Z/A User Manual - Page 30
Working with PAL Video, Synchronizing the Audio with the Video, Determining How to Prepare Source
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Note that there is not a one-to-one correspondence between film frames and video frames after this pull-down occurs. The duration of a video frame is four-fifths the duration of a film frame. Because of this discrepancy, if you tried to match a specific number of whole video frames to some number of whole film frames, the durations would seldom match perfectly. In order to maintain overall synchronization, there is usually some fraction of a film frame that must be either added to or subtracted from the duration of the next edit. This means that in the cut list, Cinema Tools occasionally has to add or subtract a film frame from the end of a cut in order to maintain synchronization. For this reason, if you edit 3:2 pull-down video, the Cinema Tools cut list is only accurate to within +/- 1 frame on each edit. This accuracy issue is easily resolved by using the Reverse Telecine feature (or third-party hardware or software) to remove the extra fields and restore the film's original 24 fps rate before you begin editing digitally, providing a one-to-one relationship between the video and film frames. Setting the Final Cut Pro editing timebase in the Sequence Preset Editor to 24 fps (or 23.98 fps-see Synchronizing the Audio with the Video) allows you to edit the video and generate a very accurate cut list. See Determining How to Prepare Source Clips for Editing for more information about issues related to these options. What's an A Frame? You will see and hear references to "A" frames whenever you are involved with 3:2 pull-down video. As the previous illustration shows, the A frame is the only one that has all its fields contained within one video frame. The others (B, C, and D frames) all appear in two video frames. Because the A frame is the start of the video five-frame pattern, it is highly desirable to have one as the first frame in all video clips. It's common practice to have A frames at non-drop frame timecode numbers ending in "5" and "0." See About A Frames for more information. Working with PAL Video The PAL video frame rate is exactly 25 fps. There are two methods used when transferring film to PAL: running the film at 25 fps (referred to as the 24 @ 25 method), and adding two extra fields per second (similar to NTSC's 3:2 pull-down, referred to as the 24 & 1 method, or the 24 @ 25 pull-down method). 30 Chapter 2 Before You Begin Your Film Project