Telecine

Telecine is the process of transferring motion picture film to a video format, such as television, or a machine used to complete this process. Telecine enables a motion picture, captured originally on film, to be viewed with standard video equipment, such as televisions and video cassette decks. This has allowed producers and distributors working in film to release their products on video and allowed producers to use video production equipment to complete their film projects.

Basic principle

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 projected not onto a screen, but onto a charge-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.

Frame rate differences

In countries that use the PAL or SECAM video standards, film destined for television is photographed at 25 frames per second. The PAL video standard broadcasts at 25 frames per second, so the transfer from film to video is simple; for every film frame, one video frame is captured. Theatrical features originally photographed at 24 frame/s are simply sped up by 4% to 25 frame/s. 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, film is generally photographed at 24 frame/s. Color NTSC video is broadcast at 29.97 frame/s. For the film's motion to be accurately rendered on the video signal, an NTSC telecine must use a technique called the 3:2 pulldown to convert from 24 to 29.97 frame/s.

Similar techniques must be used for films shot at "silent speeds" of less than 24 frame/s, which include most silent movies themselves as well as many home movies.

Some High Definition video recorders can record directly to 24fps so no interlacing is required. You can find out more at High-Definition Telecine

3:2 Pulldown

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 frame/s.

The second step of the 3:2 pulldown is the 3:2 step. At 23.976 frame/s, there are 4 frames of film for every 5 frames of NTSC video:

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

These four frames are "stretched" into five by exploiting the interlaced nature of NTSC video. For every NTSC frame, there are actually two complete images or "fields," one for the odd-numbered lines of the image, and one for the even-numbered lines. There are, therefore, ten fields for every 4 film frames, and the telecine alternately places one film frame across two fields, the next across three, the next across two, and so on. The cycle repeats itself completely after four film frames have been exposed, and in the telecine cycle these are called the "A," "B," "C," and "D" frames, thus:

File:Three-two pulldown diagram.png

Note that the pattern in this example is actually 2-3, 2-3. The name "3:2 pulldown" is an archaic reference to the pattern that was used by older telecine equipment. The modern telecine uses a 2-3 technique.

"3:2 pulldown" creates error in the video signal that can be clearly seen in the above image. This is one reason why NTSC films viewed at home rarely appear as smooth as when viewed in a cinema. Some line doublers are capable of reversing 3:2 pulldown, thereby recreating an error-free video stream.