3:2 pulldown

Three-two pull down (3:2 pull down) is a term used in filmmaking and television production for the post-production process of transferring film to video. Film runs at a standard rate of 24 frames per second, whereas NTSC video has a signal frame rate of 29.97 frames per second. Every interlaced video frame has two fields for each frame. The three-two pull down is where the telecine adds a third video field to every second video frame, but the untrained eye cannot see the addition of this extra video field.

In the United States and other countries where television uses the 59.94 Hz vertical scanning frequency, video is broadcast at 29.97 frame/s. For the film's motion to be accurately rendered on the video signal, a telecine must use a technique called the 2:3 pull down, also known as 3:2 pull down, to convert from 24 to 29.97 frame/s.

The term “pulldown” comes from the mechanical process of “pulling” (physically moving) the film downward within the film portion of the transport mechanism, to advance it from one frame to the next at a repetitive rate (nominally 24 frames/s). This is accomplished in two steps. The first step is to slow down the film motion by 1/1000 to 23.976 frames/s (or 24 frames every 1.001 seconds). 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 29.97 Hz video:

These four frames are “stretched” into five by exploiting the interlaced nature of 60 Hz video.

Every original film frame can be considered to consist of two incomplete images or fields, one for the odd-numbered lines of the image, and one for the even-numbered lines. There are, therefore, eight fields for every four film frames, which are called A, B, C, and D. But these eight fields have to be "stretched" to ten fields by repeating two of the fields. The telecine alternately places A frame across two fields (A1, A2), B frame across three fields (B1, B2, B2), C frame across two fields (C1, C2), and D frame across three fields (D1, D2, D2). This can be written as A1-A2, B1-B2, B2-C1, C2-D1, D2-D2, or 2-3-2-3 or simply 2-3. The cycle repeats itself completely after four film frames have been exposed.

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