In a simple telecine, white light is shown through exposed and developed motion picture negative (positives can also be used.) As in a projector, the film filters the white light into different colors, according to the image on the film, except in a telecine this image is not projected onto a screen, but is projected onto a charged coupled device (CCD) or vidicon tube. The CCD converts the light into electrical impulses which the telecine electronics modulate into a video signal which can then be recorded onto video tape, or broadcast.
The most complex part of telecine is the synchronization of the mechanical film motion and the electronic video signal. Every time the video part of the telecine samples the light electronically, the film part of the telecine must have a frame in perfect registration and ready to photograph. This is relatively easy when the film is photographed at the same frame rate as the video camera will sample, but when this is not true, a sophisticated procedure is required to change frame rate.
In England and other countries that use the PAL or SECAM video standards, telecine is relatively straightforward: film destined for television is photographed at 25 frames per second, and the PAL video standard broadcasts at 25 frames per second. The transfer from film to video is simple; for every film frame, one video frame is captured. Theatrical features originally photographed at 24 fps are simply sped up by 4% to 25fps. This can cause a noticeable increase in audio pitch, which is sometimes corrected using a pitch shifter.
In the United States and other countries that use the NTSC television standard, the transfer is complicated by the fact that film is generally photographed at 24 frames-per-second (fps), and color NTSC video is broadcasted at 29.97 fps. In order for the film's motion to be accurately rendered on the video signal, the NTSC telecine must use a technique called the 3:2 pulldown to convert from 24 fps to 29.97 fps.
The 3:2 pulldown is accomplished in two steps. The first step is to slow down, or "pulldown" the film motion by 0.1%. This speed change is unnoticeable to the viewer, and makes the film travel at 23.976 fps.
The second step of the 3:2 pulldown is the 3:2 step. At 23.976 fps, there are 4 frames of film for every 5 frames of NTSC video: