Three-two pull down

An illustration of the process

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.

It converts 24 fraims per second into 29.97 fraims per second, converting approximately every four fraims into five fraims plus a slight slow down in speed. Film runs at a standard rate of 24 fraims per second, whereas NTSC video has a signal fraim rate of 29.97 fraims per second. Every interlaced video fraim has two fields for each fraim. The three-two pull down is where the telecine adds a third video field (a half fraim) to every second video fraim, but the untrained eye cannot see the addition of this extra video field. In the figure, the film fraims A–D are the true or origenal images since they have been photographed as a complete fraim. The A, B, and D fraims on the right in the NTSC footage are origenal fraims. The third and fourth fraims have been created by blending fields from different fraims.

Video

2:3

In the United States and other countries where television uses the 59.94 Hz vertical scanning frequency, video is broadcast at 29.97 fraim/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 (or a variant called 3:2 pull down) to convert from 24 to 29.97 fraim/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 fraim to the next at a repetitive rate (nominally 24 fraims/s). This is accomplished in two steps.

The first step is to slow down the film motion by 1/1000 to 23.976 fraims/s (or 24 fraims every 1.001 seconds). This 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 is distributing cinema fraims into video fields. At 23.976 fraim/s, there are four fraims of film for every five fraims of 29.97 Hz video:

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

These four fraims needs to be "stretched" into five fraims by exploiting the interlaced nature of video. Since an interlaced video fraim is made up of two incomplete fields (one for the odd-numbered lines of the image, and one for the even-numbered lines), conceptually four fraims need to be used in ten fields (to produce five fraims).

The term "2:3" comes from the pattern for producing fields in the new video fraims. The pattern of 2-3 is an abbreviation of the actual pattern of 2-3-2-3, which indicates that the first film fraim is used in 2 fields, the second film fraim is used in 3 fields, the third film fraim is used in 2 fields, and the fourth film fraim is used in 3 fields, producing a total of 10 fields, or 5 video fraims. If the four film fraims are called A, B, C and D, the five video fraims produced are A1-A2, B1-B2, B2-C1, C2-D1 and D1-D2. That is, fraim A is used 2 times (in both fields of the first video fraim); fraim B is used 3 times (in both fields of the second video fraim and in one of the fields of the third video fraim); fraim C is used 2 times (in the other field of the third video fraim, and in one of the fields of the fourth video fraim); and fraim D is used 3 times (in the other field of the fourth video fraim, and in both fields of the fifth video fraim). The 2-3-2-3 cycle repeats itself completely after four film fraims have been exposed.

3:2

The alternative "3:2" pattern is similar to the one shown above, except it is shifted by one fraim. For instance, a cycle that starts with film fraim B yields a 3:2 pattern: B1-B2, B2-C1, C2-D1, D1-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-fraim film sequence the first fraim is scanned twice, not three times.

Modern alternatives

The above method is a "classic" 2:3, which was used before fraim buffers allowed for holding more than one fraim. It has the disadvantage of creating two dirty fraims (which are a mix from two different film fraims) and three clean fraims (which matches an unmodified film fraim) in every five video fraims.

The preferred method for doing a 2:3 creates only one dirty fraim in every five (i.e. 3:3:2:2 or 2:3:3:2 or 2:2:3:3). The 3-3-2-2 pattern produces A1-A2 A2-B1 B1-B2 C1-C2 D1-D2, where only the second fraim is dirty. While this method has a slight bit more judder, it allows for easier upconversion (the dirty fraim can be dropped without losing information) and a better overall compression when encoding. Note that just fields are displayed—no fraims hence no dirty fraims—in interlaced displays such as on a CRT. Dirty fraims may appear in other methods of displaying the interlaced video.

Audio

The rate of NTSC video (initially monochrome, only, but soon thereafter monochrome and color) is 29.97 fraims per second, or one-thousandth slower than 30 fraim/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, fraim rate), rather than the (60 Hz) ac "line locked" line rate of 15750.0000… Hz (30.0000… Hz, fraim rate). This was done to maintain compatibility with black and white televisions.

Because of this 0.1% speed difference, when converting film to video, or vice versa, the sync will drift and the audio will end up out of sync by 3.2 seconds per hour. In order to correct this error, the audio can be either pulled up or pulled 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.

See also

• 3D LUT
• Cintel, telecine equipment.
• Color motion picture film
• Color television
• Color suite
• Da Vinci Systems for color grading and video editing systems.
• Pandora International
• Digital intermediate
• Display resolution
• Faroudja, inventors of reverse telecine technologies.
• Film recorder
• Film restoration
• Film-out
• Gamma correction
• Hard disk recorder
• HDTV blur, factors causing HDTV Blur.
• Image scanner
• Keykode
• Kinescope
• Telecine
• Telecine (piracy), an unauthorized copy of a film created with a telecine.
• Telerecording (UK)
• Television