OTHER TECH

Hardly a month goes by when we don't receive a reader letter or e-mail taking us to task for discussing the "2:3 pulldown" functions of DVD players instead of the more commonly found "3:2 pulldown." Let me immediately put the confused at ease by saying that both terms refer to the same thing. Why we use 2:3 instead of 3:2 requires a short tutorial on "pulldown."

The term itself comes from film. In a motion-picture camera, a "pulldown claw" engages the film's sprocket holes and literally pulls the film into position for exposure. Since most film cameras have the reels positioned vertically, with the feeding reel on top, the action is downward, hence pulldown. (Imax equipment has horizontally mounted reels. "Clawsideways," anyone?) The pulldown action occurs 24 times a second during normal film shooting (that is, other than time-lapse or slow-motion sequences). This produces a film frame rate of 24 frames per second (fps), each frame lasting 1/24 second. By comparison, the video frame rate for standard analog TV is 30 fps (rounded off from 29.97 fps), with the scan lines of an entire 1/30-second frame transmitted in two alternating ("interlaced") sets, first the odd-numbered lines in a 1/60-second video field and then the even-numbered lines. Now, clearly 24 fps doesn't equal 30 fps, nor does 1/24 second equal 1/30 second. So to show a film on television you have a problem. You can't simply point a video camera at a running film and have it come out right.

The solution is to use an uneven pulldown sequence, as shown in the diagram. A telecine machine used to convert film into video signals takes two sequential film frames, A and B, and converts frame A into two video fields and frame B into three video fields. The whole sequence is then repeated for the next two film frames, and so on. The resulting signal gets recorded on DVD, usually with the third B field (marked X in the diagram) omitted, as it duplicates the first B field. Two fields, then three fields - 2:3! It all comes out even at the end since the duration of two film frames is the same as that of five video fields: 2/24 = 5/60.

PDF: Diagram