Film and Video 3/2 pulldown explained


Describe 3/2 pulldown without waving my hands. OK. I'm up for it.

3/2 pulldown refers to the odd dance that movie projectors do to project a 24-frame film into a 30-frame video system. The modern, cutting-edge, digital application for 3/2 is to produce 30-frame sample videos from special-effects computer picture frames, designed to go to 24 frame film.

In a normal 24-frame movie theatre, each film frame is projected twice to make the flicker so fast that you don't see it. There's a rotating shutter in the lamphouse allowing light to go through 48 times per second; two flashes per frame. This is 2/2 pulldown, although nobody calls it that. Two flashes, two more, two, two, two...

For video use, speed up the light shutter so it's flashing 60 times per second. Change the film transport system (the pull-down) to allow one film frame to stay visible for three flashes, and the next film frame for only two. Three, two, three, two....

If you plot this out on graph paper (as I know you're doing right now) you see that each *video* frame gets two flashes of light, so video is happy. The *film* is still traveling exactly 24 frames per second if you average it over any two frames, so film is happy. Both systems are stable and all the nasty artifacts disappear.

Almost all.

Examine the video frames slowly one at a time and you find that sometimes a video frame is produced from two *different* film frames. Action will seem to flicker and vibrate on these frames. These are what I call the "fudge frames" and they, too, follow the 3/2 rule. Perfect, perfect, perfect, fudge, fudge, [repeat]. If you play this video at normal speed, you don't see the weird frames and action looks smooth.

The graphics people at work do their special effects on a client's scanned negative. They work on each frame individually, one at a time. The show is designed to eventually run 24-frames per second, but nobody ever sees that inside the computers.

The problem comes not when they carefully convert all their frames back to film for the client, but before that, when they have to present all their frames to a video system so the client can tell how things are going before the final transfer to film.

If the artists do a direct frame-for-frame transfer to videoland, the resulting videotape plays 25% too fast. This is Not a Good Thing. There are methods of slowing the video down, but they all have visible artifacts.

The systems people have devised a method of digitally creating 3/2 pulldown. This process creates all the necessary fudge frames so the video transfer plays at the right speed. It even looks correct if the video fields are inspected one at a time.

The picture illustrates the 3/2 distribution of film images in the video parade. It also illustrates the non-linear editor "field grab" technique which insures that the editor gets the correct film frame sequence regardless of the scrambled video frames.

The letters in the illustration are not accidental. Please note that the "A" film frame is the only one that exactly fills a video frame. B, C, and D all straddle video frame boundries.

6/26/97
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