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Updated 18/11/2012Home » Video Glossary » Pulldown

Pulldown

Film is generally shot and projected at 24 frames per second (fps), so when film frames are converted to NTSC video, the rate must be modified to play at 29.97 fps. During the telecine process, 12 fields are added to each 24 frames of film (12 fields = 6 frames) so the same images that made up 24 frames of film then comprise 30 frames of video.Video plays at a speed of 29.97 fps so the film actually runs at 23.976 fps when transferred to video.

2:2 pulldown

In countries that use the PAL or SECAM video standards, film destined for television are photographed at 25 frames per second. The PAL video standard broadcasts at 25 frames per second, so the transfer from film to video is simple; for every film frame, one video frame is captured.

2:2 pulldown

2:3 pulldown

In the United States and other countries, video is broadcast at 29.97 fps. For the film's motion to be accurately rendered on the video signal, a telecine must use a technique called the 2:3 pulldown, also known as 3:2 pulldown, to convert from 24 to 29.97 fps.

The first step is to slow down the film motion by 1/1000 to 23.976 frames/s. The difference in speed is imperceptible to the viewer. For a two-hour film, play time is extended by 7.2 seconds.

The second step of the 2:3 pulldown is distributing cinema frames into video fields. At 23.976 frame/s, there are four frames of film for every five frames of 60 Hz video:

These four frames are stretched into five by exploiting the interlaced nature of 60 Hz video. For every frame, there are actually two incomplete images or fields, one for the odd-numbered lines of the image, and one for the even-numbered lines. There are, therefore, ten fields for every four film frames, which are called A, B, C, and D. The telecine alternately places A frame across two fields, B frame across three fields, C frame across two fields and D frame across three fields. This can be written as A-A-B-B-B-C-C-D-D-D or 2-3-2-3 or simply 2-3. The cycle repeats itself completely after four film frames have been exposed:

2:3 pulldown

The above method is a classic 2:3, which was used before frame buffers allowed for holding more than one frame. The preferred method for doing a 2:3 creates only one dirty frame in every five (i.e. 3:3:2:2 or 2:3:3:2 or 2:2:3:3); while this method has a slight bit more judder, it allows for easier upconversion (the dirty frame can be dropped without losing information) and a better overall compression when encoding.