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Treebeard's Stumper Answer
27 Mar 98

Bringing the Movies Home

The Academy Awards were presented this week. I love film, but I rarely get out to a theater, so I'm dependent on video. Video is different, of course. It's smaller, and you can always press Pause. Film screens are a different shape, so we don't even see the whole picture on TV unless the video is letterboxed into a narrow strip across the screen. It's more puzzling that film is shot at 24 frames per second, while video runs at 30 frames per second. How is film transferred to video without losing frames or changing the timing?

How is film copied to video if they run at different rates of 24 and 30 frames per second? Each frame of video is actually shown on TV as two separate fields. Every other line of a frame is drawn in 1/60th of a second, then the in between lines are drawn. A bit of math solves the stumper: 5/60 = 2/24 = 1/12, so 5 fields of video take the same time as 2 frames of film. The first film frame shows for 3 video fields, the second frame shows for 2 fields, and so on. This may even be noticeable as a slight jitter in video titles and pans.

Note: It's not quite this neat. The original NTSC video standard called for 30 frames per second. When color TV was introduced, this was changed to 29.97 frames per second to allow time for color-burst information to be sent. The result is that video runs a bit slower than film by 1.8 seconds per hour. Close enough!

Film-family kids Chase and Spencer figured this out and explained it to the rest of us at school. We learned that special projectors are available for video production with a "3-2 pulldown feature" that handles the frame timing automatically.

Several kids and adults guessed that video repeats every fourth frame. This would preserve timing, but I think it would be even more jittery, repeating 2 fields instead of one.

Two good Web sources for more information about film to video transfer are:

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Copyright © 1998 by Marc Kummel / mkummel@rain.org