Pulldown is a synchronisation technique that is used to approximate compatibility between two dissimilar formats (usually between film to video), by creating new frames.

Pulldown is added when 24 fps film material is transferred to 60i or 50i tapes using the telecine method. The same images that made up 24 frames of film must now make up 30 (60i, or NTSC) or 25 (50i, or PAL) frames of video. In other words, for every 24 film frames, frames must be added, the number of which depends on the video format:

• Six new video frames transferring to the NTSC format
• One new video frame when transferring to the PAL format

To accomplish this conversion, the film frames are first divided into fields. The fields are then distributed across frames in a regular pattern to create the frames with the smoothest results and least disturbance of the original material.

When transferring to the NTSC format, a pulldown frame is created every four film frames. How the pulldown frame is created varies according to the type of pulldown used:

• In 2:3 pulldown, the first frame is used in two fields of video, while the second is used in three.
• In 3:2 pulldown, the first frame becomes three fields, while the second becomes just two.
• In Advanced pulldown, the second and third frames each become three fields.

Example of a 2:3 and 3:2 pulldown transfer process:

1. The process begins with four frames of film.

   
   

2. The four frames are divided into eight fields.

   
   

3. The fields are resequenced using 2:3 or 3:2 pulldown, duplicatiing fields to create ten fields arranged into five video frames.
   • 2:3 pulldown: third and fourth frames are hybrids.

   
   

   • 3:2 pulldown: second and third frames are hybrids.

   
   

   • Advanced pulldown: third and fourth frames are hybrids.

   
   

In the case of PAL pulldown, the full 24 film frames are used to create a sequence of 25 video frames: the 12^th and 24^th film frames are used in 3 fields of video.

• Original film sequence:

  AA BB CC DD EE FF GG HH II JJ KK LL MM NN OO PP QQ RR SS TT UU VV WW XX

• PAL pulldown: frames 13 through 24 are hybrids.

  AA BB CC DD EE FF GG HH II JJ KK LL LM MN NO OP PQ QR RS ST TU UV VW WX XX

Regardless of the pulldown used, the approach is similar: the converted video consists of sequences of intact film frames separated by hybrid frames.

While the intact frames are identical to their sources, the hybrids consist of fields from two different film frames. If viewed as stills, the hybrid frames appear jittery—because the fields do not originate from the same moment in time. (When played back in the full sequence, the jitter is imperceptible.) When working with 24p sources in Backdraft Conform, it is important to keep in mind the jitter frames. They are the markers for setting up the automatic pulldown removal on capture.