Reality Skewed...
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Articles
Whether it is referred to as 3:2 pulldown, 2:3 pulldown, pushup, telecine or any of the many variations on the process an understanding of the procedures involved in converting between standard film and television frame rates can go a long way in being able to anticipate and deal with any difficulties that may occur in using such treated elements in your projects.
Fundamental to understanding and working with 3:2 pulldown material is a clear understanding of the concepts behind the differing frame rates of film and video in their various forms. All visual media show individual images, or frames in rapid succession to achieve the appearance of motion for the viewer. For historical reasons the film and video worlds have settled on different rates for these frames to be shown. These rates are usually described by the number of frames that make up one second of display, referred to as frames-per-second or fps. For film that number is 24. For NTSC television like is shown in the United States the number is usually stated as 30. (Actually the number is really 29.97 for technical reasons.) In other countries that use the PAL format the video frame rate is 25.
When a film is going to be shown on television it needs to be converted from the slower 24 fps to the video standard 30. The ingenious method for this conversion is the mysterious 3:2 pulldown.
Because of the limitations of early video technology video is shown in two time slices, called fields, per 1/30th of a second frame.
To show the 24 frames of real time film on video the film frames must be divided up among the 60 video fields that make up that same time segment of video. both 24 and 60 can be divided by the common denominator of 6 to reveal a 4 to 5 ratio. So a pattern that puts four film frames into five video frames repeatedly would succeed in converting the frame rates.
In the figure above the four film frames and five video frames divided into two fields each are shown. You can see that a pattern of taking three fields of one film frame and two fields of the next repeatedly will achieve the desired conversion. Here is the derivation of the "3:2" name. As with any pattern, it doesn't really matter where you start as long as the same sequence is repeated. (Most film conversion machinery start the process at the frame above labeled 5, then 1,2,3 and 4 creating a "2:3" pattern. But it is the same pattern none the less.)
You can clearly see that the video frames labeled 1, 4 and 5 are the identical matches to the film frames A, C and D. And when playing back frames 2 and 3 together give the viewer the full frame B. Problems only arise when either of these two frames are displayed on their own or are left hanging on an video edit. Many editing systems and editors go to great lengths to avoid these particular frames. Potential problems they could cause can be exemplified like this:
If the segment in green in the head edit point is joined to the segment in green in the tail edit point a problem occurs. The edit in this case would create a film frame impression of a mixture of both B and X, something that didn't exist in the original film footage.
This is only one example of the myriad of problems that could occur with editing film transfers after they are brought into the video domain. Other problems, such as losing sync with audio after several edits or field order confusion, could also occur. Being aware of the issues involved will help to alert one to the dangers involved and help to minimize- or take advantage of- their effects.
