Performing a 3:2 pull-down, Ab c d – Apple Cinema Tools 3 User Manual
Page 23
Chapter 1
Before You Begin Your Project
23
Performing a 3:2 Pull-Down
The most common approach to distributing film’s 24 fps among NTSC video’s 30 fps is
to perform a 3:2 pull-down (also known as a 2:3:2:3 pull-down). If you alternate
recording two fields of one film frame and then three fields of the next, the 24 frames
in one second of film end up filling the 30 frames in one second of video.
As shown above, the 3:2 pattern (actually a 2:3:2:3 pattern since frame A is recorded to
two fields followed by frame B recorded to three fields) repeats after four film frames.
Virtually all high-end commercials, movies, and non-live television shows use this
process prior to being broadcast.
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 t 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.
A
B
A
B B
C
C
D D D
Film frames
One second
A
B
C
D A
D A
B
C
D A
B
C
D A
B
C
D
A B
C D
B
C
NTSC video frames (divided into fields)
A A B B B C C D D D A A B B B C C D D D A A B B
C C D D D A A B
B
B
C
C
D D D A
B
A
B B
C
C
D D
B
D
A
B
C
D
A A
B
B
B
C
C
D D
D
Field
1
Field
2
Field
1
Field
2
Field
1
Field
2
Field
1
Field
2
Field
1
Field
2
UP01101.Book Page 23 Thursday, March 10, 2005 3:16 PM