Image Processing Reference
In-Depth Information
Editing of the video must be sensitive to these transfers. For example, editing any-
where other than at 5-frame intervals changes the cadence of the starting frame of a 5-frame
sequence. The viewer may not notice the change in cadence, but it does make subsequent
processing of that footage very difficult. If you are editing interlaced video, then after an
inverse telecine process the field dominance could be compromised and fields after the edit
may exhibit a twittering effect. As long as the edit point is an integer multiple of 5 frames,
then this problem will not occur. That constraint is a very large burden to place on a film
editor. It is much better to compress from the unedited original material.
Pulldown—50 Hz Frame Rate
The techniques that work for transferring film to 30 fps video will not work for transfer
to 25 fps video in Europe. The European TV frame rate is much closer to the correct frame
rate for film. To use a pulldown effect requires that we introduce just one additional
frame per second. This produces a noticeable flicker in the movement, so it is not used as
a solution.
Transfers to 30 fps video run the film a little slower, and that does not materially alter
the sound quality. Running 24 fps film at 25 fps looks okay visually because the motion is
only slightly faster. The pitch of the sound is increased noticeably and must be corrected.
Pitch correction is tricky because you must preserve synchronization or the lip sync gets
off track.
You are also freed from the cadence and field dominance issues because there are no
pulldown frame cadences to consider.
Line-Based Telecine Transfer
A more sophisticated approach is to index the film at 24 fps and scan the appropriate lines
of the raster. This results in a frame of video containing portions of two frames of film.
There is no obvious pulldown happening. Determining which part of a composite frame
belongs to one or the other of the original 24 FPS source is very hard indeed unless there
are some obvious clues. This would require some sophisticated image processing software
to compare one frame of video with another and see which lines have changed between
successive frames or corresponding odd or even fields.
Building Your Own Telecine System
It might be feasible (and fun, perhaps) to construct your own film scanner using some
components that are cannibalized from other sources.
Choosing a Film Scanner
The most expensive part of building your own system will be the film scanner. You will
require one that has a straight path through the unit, and you will also have to manufac-
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