What Is 3:2 Pulldown?

by Dave Pincek, Vice President of Product Development

Film runs at a rate of 24 frames per second, and video that adheres to the NTSC television standard runs at 30 frames a second. When film is converted to NTSC video, the frame rates need to be matched in order for the film-to-video conversion to take place. This process of matching the film frame rate to the video frame rate is called "3:2 pulldown."

As you recall, NTSC video uses interlaced scanning, with half of the lines scanned in the first field and the other half scanned in the second field. Therefore, two video fields make a video frame.

Figure 1

Figure 1 illustrates the method of transferring each frame of film to video fields/frames. The left side of the illustration shows the film frames, and the right side shows the video fields that make up each video frame. Two video fields make one video frame.

As the dark set of arrows shows, the first frame of film is transferred to three fields of video (or one-and-one-half video frames). Then, film frame #2 is transferred to two video fields. Thus, it is called "3:2 pulldown."

In the illustration, film frame #3 is transferred to three fields, and film frame #4 goes to the next two video fields, and so on, alternating between three and two video fields per film frame. Consequently, when 24 film frames are processed, 30 video frames have been used, so the rates have been matched.

It is important to understand 3:2 pulldown when dealing with scan doublers and quadruplers. Scan doublers and quadruplers work on the premise that two fields equal one frame. With 3:2 pulldown, this premise isn't accurate, and using standard scan doubling or quadrupling techniques will not result in the best possible image. For this reason, various manufacturers of scan doublers and quadruplers state that they have a 3:2 pulldown feature. This means that their scan doubler or quadrupler is able to detect any video that was made from film, and it will apply a specific scan doubling or quadrupling technique that optimizes the conversion of video made with the 3:2 pulldown process. This will provide a better image than simply applying normal scan doubling or quadrupling techniques.

PAL and SECAM television systems run at 25 frames per second. They are close enough to the film frame rate that 3:2 pulldown isn't required. One film frame is transferred to one video frame. The slight speed discrepancy makes a movie slightly shorter when it is transferred to video, but the speed variation is so slight it will not be noticed in viewing.

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