Telecine
Telecine, or TK, is the process of transferring film into video and is performed in a color suite. The term is also used to refer to the equipment used in this post-production process.
Telecine enables a motion picture, captured originally on film stock, to be viewed with standard video equipment, such as television sets, video cassette recorders, DVD, Blu-ray or computers. Initially, this allowed television broadcasters to produce programs using film, usually 16-mm stock, but transmit them in the same format and quality as other forms of television production. Furthermore, telecine allows film producers, television producers and film distributors working in the film industry to release their productions on video and allows producers to use video production equipment to complete their filmmaking projects.
Within the film industry, it is also referred to as a TK, TC having already been used to designate timecode. Motion picture film scanners are similar to telecines.
History
With the advent of popular broadcast television, producers realized they needed more than live television programming. By turning to film-originated material, they would have access to the wealth of films made for the cinema in addition to recorded television programming on film that could be aired at different times. However, the difference in frame rates between film and television meant that simply playing a film into a television camera would result in flickering.The kinescope was used to record the image from a television display to film, synchronized to the TV scan rate. The film could then be shown directly into a video camera for retransmission. Non-live programming could also be filmed using the kinescope, edited mechanically as normal, and then played back for TV. As the film was run at the same speed as the television, the flickering was eliminated. Various displays, including projectors for these video-rate films, slide projectors and film cameras were often combined into a film chain, allowing the broadcaster to cue up various forms of media and switch between them by moving a mirror or prism. Color was supported by using a multi-tube video camera, prisms, and filters to separate the original color signal and feed the red, green and blue to individual tubes.
However, this still left film shot at cinema frame rates as a problem. The obvious solution is to simply speed up the film to match the television frame rates, but this, at least in the case of NTSC, requires a change that is rather obvious to the eye and ear. The simple solution is to periodically play a selected frame twice. For NTSC, the difference in frame rates can be corrected by showing every fourth frame of film twice. This solution does require the sound to be handled separately. A more advanced technique is to use 2:3 pulldown, discussed below, which turns every second frame of the film into three fields of video, which results in a slightly smoother display. PAL uses a similar system, 2:2 pulldown. However, during the analog broadcasting period, the 24 frames per second film was shown at a slightly faster 25 frames per second rate to match the PAL video signal. This resulted in a fractionally higher-pitched audio soundtrack, and resulted in feature films having a slightly shorter duration, by being shown 1 frame per second faster.
In recent decades, telecine has primarily been a film-to-storage process, as opposed to film-to-air. Changes since the 1950s have primarily been in terms of equipment and physical formats; the basic concept remains the same. Home movies originally on film may be transferred to video tape using this technique.
Frame rate differences
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 video and film frame rates differ, a sophisticated procedure is required.2:2 pulldown
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 frames per second are shown at 25 frames per second. While this is usually not noticed in the picture, the 4% increase in playback speed causes a slightly noticeable increase in audio pitch by about 0.707 semitones. This can be corrected using time stretching algorithms, which speed up audio while preserving pitch.
2:2 pulldown is also used to transfer shows and films photographed at 30 frames per second, like Friends and Oklahoma!, to NTSC video, which has ≈59.94 Hz scanning rate. This requires playback speed to be slowed by a tenth of a percent.
2:3 pulldown
In the United States and other countries where television uses the 59.94 Hz vertical scanning frequency, video is broadcast at ≈29.97. For the film's motion to be accurately rendered on the video signal, a telecine must use a technique called the 2:3 pulldown, also known as 3:2 pulldown, to convert from 24 to ≈29.97.The term pulldown comes from the mechanical process of pulling the film downward within the film portion of the transport mechanism, to advance it from one frame to the next at a given rate. This is accomplished in two steps. The first step is to slow down the film motion by NTSC's ratio to . The difference in speed is imperceptible to the viewer. For a two-hour film, play time is extended by 7.2 seconds. If the total playback time must be kept exact, a single frame can be dropped every 1000 frames.
The second step of the 2:3 pulldown is distributing cinema frames into video fields. At 23.976, there are four frames of film for every five frames of 29.97 video:
These four film frames are stretched into five video frames by exploiting the interlaced nature of 60 Hz video. For every video frame, there are actually two incomplete 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 four film frames, which are called A, B, C, and D. The telecine alternately places frame A across two fields, frame B across three fields, frame C across two fields and frame D across three fields. This can be written as A-A-B-B-B-C-C-D-D-D or 2-3-2-3 or simply 2–3. The cycle repeats itself completely after four film frames.
A 3:2 pulldown pattern is identical to the one described above except that it is shifted by one frame. For instance, a cycle that starts with film frame B yields a 3:2 pattern: B-B-B-C-C-D-D-D-A-A 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-frame film sequence, the first frame is scanned twice, not three times.
The above method is a classic 2:3, which was used before frame buffers allowed for holding more than one frame. The preferred method for doing a 2:3 creates only one dirty frame in every five ; while this method has slightly more judder, it allows for easier upconversion and a better overall compression when encoding. The 2:3:3:2 pattern is supported by the Panasonic DVX-100B video camera under the name "Advanced Pulldown". Note that just fields are displayed—no frames, hence no dirty frames—in an interlaced display, such as on a CRT. Dirty frames may appear in other methods of displaying the interlaced video.
Euro pulldown
A new method called 2:2:2:2:2:2:2:2:2:2:2:3, Euro, 12:1 or 24:1 pulldown, can be used in order to convert 24 material to 25. Usually, this involves a film to PAL transfer without the aforementioned 4% speedup. For film at 24, there are 24 frames of film for every 25 frames of PAL video. In order to accommodate this mismatch in frame rate, 24 frames of film have to be distributed over 50 PAL fields. This can be accomplished by inserting a pulldown field every 12 frames, thus effectively spreading 12 frames of film over 25 fields of PAL video.This method was born out of a frustration with the faster, higher-pitched soundtracks that traditionally accompanied films transferred for PAL and SECAM audiences. A few motion pictures are beginning to be telecined this way. It is particularly suited for films where the soundtrack is of special importance.
Other pulldown patterns
Similar techniques must be used for films shot at silent speeds of less than 24, which includes home movie formats as well as silent film.- 16 to NTSC 30 : pulldown should be 3:4:4:4 or the film may be run at 15 then pulldown should be 4:4. As motion pictures shot at this framerate are silent, there is no audio that is affected.
- 16 to PAL 25: pulldown should be 3:3:3:3:3:3:3:4
- 18 to NTSC 30: pulldown should be 3:3:4
- 20 to NTSC 30: pulldown should be 3:3
- 20 to PAL 25: pulldown should be 3:2
- 27.5 to NTSC 30: pulldown should be 3:2:2:2:2
- 27.5 to PAL 25: pulldown should be 1:2:2:2:2
- 24 to 96 : pulldown is 4:4
- 24 to 120 : pulldown is 5:5
- 24 to 120 : pulldown is 6:4