Bridging Film and Television
Pull down (or pulldown) is a crucial technique in the post-production process of telecine, which is the transfer of motion picture film to video. Film is traditionally shot at a standard of 24 frames per second (fps), which gives it its characteristic ‘cinematic’ motion. However, broadcast television standards operate at different frame rates. The NTSC standard, used in North America, Japan, and other regions, runs at approximately 29.97 fps (or 60 interlaced fields per second). The PAL standard, used in Europe and much of the rest of the world, runs at 25 fps. Pull down is the mechanism that reconciles this mathematical discrepancy.
The most common form is the 3:2 pull down, used to convert 24 fps film to 29.97 fps NTSC video. The process works by distributing the 24 film frames across approximately 60 video fields. This is achieved by alternating the duration for which each film frame is scanned. The first film frame is scanned for three video fields, the second for two, the third for three, the fourth for two, and so on. This ‘3-2-3-2’ pattern effectively stretches 4 frames of film to fit into 5 frames of video (or 10 fields), thus converting 24 frames into approximately 30 (29.97).
| Film Frame | Video Fields | Description |
|---|---|---|
| 1 | 3 (A, B, C) | The first frame is held for 3 fields. |
| 2 | 2 (D, E) | The second frame is held for 2 fields. |
| 3 | 3 (F, G, H) | The third frame is held for 3 fields. |
| 4 | 2 (I, J) | The fourth frame is held for 2 fields. |
This process, while effective, introduces a slight, almost imperceptible judder into the motion because the frames are not displayed for a uniform duration. This is one of the subtle reasons why a film viewed on an old NTSC television can feel slightly different from its theatrical presentation.
Other Forms and Modern Contexts
For converting 24 fps film to the 25 fps PAL standard, the process is much simpler. The film is simply sped up by 4%, a change so minor that it is generally unnoticeable to the viewer, although it does cause a slight increase in audio pitch, which is often corrected using pitch shifting algorithms. This is sometimes referred to as a 2:2:2:2:2:2:2:2:2:2:2:3 pull down to account for the slight mismatch over a long period, but in practice, it’s a straightforward speed increase.
In the modern era of digital filmmaking and progressive-scan displays (like LCDs and OLEDs), complex pull down cadences are becoming less common. Many modern televisions and players can display content at its native 24 fps frame rate, eliminating the need for 3:2 pull down and its associated motion judder. However, the principles are still fundamental to broadcast television and the process of ‘inverse telecine’ (removing the added pull down frames) is a critical step when mastering film-originated content for Blu-ray and streaming to restore the original 24 progressive frames.