Three-two pull down

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 can't see the addition of this extra video field.

Video

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:

{\frac {23.976}{29.97}}={\frac {4}{5}}

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.

A 3:2 pattern is identical to the one shown above except that it is shifted by one frame. For instance, a cycle that starts with film frame B yields a 3:2 pattern: B1-B2, B2-C1, C2-D1, D2-D2, A1-A2 or 3-2-3-2 or simply 3-2. In other words, there is no difference between the 2-3 and 3-2 patterns. In fact, the "3-2" notation is misleading because according to SMPTE standards for every four-frame film sequence the first frame is scanned twice, not three times.^[1]

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. The 2:3:3:2 pattern is supported by the Panasonic DVX-100B video camera under the name "Advanced Pulldown". Note that just fields are displayed—no frames hence no dirty frames—in interlaced display such as on a CRT. Dirty frames may appear in other methods of displaying the interlaced video.

Audio

The rate of NTSC video (initially color, only, but soon thereafter monochrome and color) is 29.97 frames per second, or one-thousandth slower than 30 frame/s, due to the NTSC color encoding process which mandated that the line rate be a sub-multiple of the 3.579545 MHz color "burst" frequency, or 15734.2637 Hz (29.9700 Hz, frame rate), rather than the (60 Hz) ac "line locked" line rate of 15750.0000… Hz (30.0000… Hz, frame rate). This seemingly odd relationship proved to be essential to eliminating moiré and other image defects.

Although slight, the sync will catch up, and the audio can end up being several seconds out of sync with the image. In order to correct this error, the telecine can either pull up or pull down. A pull up will speed up the sound by 0.1%, used for transferring video to film. A pull down will slow the audio speed down by 0.1%, necessary for transferring film to video.

References

^ Charles Poynton, Digital Video and HDTV: Algorithms and Interfaces., page 430

[1] Charles Poynton, Digital Video and HDTV: Algorithms and Interfaces., page 430

[1]

Video

Audio

See also

References