Telecine and Inverse Telecine
Telecine is the process of transferring motion picture film into electronic form. The term is also used to describe the machine used in this process. Traditional motion picture film runs at exactly 24 fps progressive scan, which doesn't convert to the 59.94 fps of NTSC or the 50 fps of PAL.
Converting from film to PAL is easy. The video is sped up 4 percent to 25 fps and converted into progressive PAL.
The telecine process for NTSC is more complex: the film is first slowed down 0.1 percent to 23.976 fps and is then converted to 59.94 fps by using 3:2 pulldown (Figure 4.1). In this process, the first frame of film becomes three fields of video, the next frame becomes two fields of video, the next becomes three fields, and so on, resulting in two out of five frames having fields that come from different source frames.
Figure 4.1 In the 3:2 pulldown process inherent to NTSC telecine, each 4 frames of film become 10 fields/5 frames of video by using a 3-then-2 field pattern to maintain the smooth motion of the film in the newly created video.
To correctly compress video that has been telecined, it is crucial to be able to identify these duplicate frames and then remove them. Inverse telecine basically reconstructs the four frames from every five to turn the source back into progressive video.