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Special techniques
Many variations of the basic techniques of recording television program material were developed in sports telecasting. The first to be introduced was the “instant replay” method, in which a magnetic recording is made simultaneously with the live-action pickup. When a noteworthy episode occurs, the live coverage is interrupted and the recording is broadcast, followed by a switch back to live action. Often the recording is made from a camera viewing the action from a different angle. Other variations include the slow-motion and stop-action techniques, in which magnetic recording plays the basic role. The magnetic recordings for these kinds of temporary storage are usually made on rotating discs.
Use has been made, particularly in sports broadcasting, of split-screen techniques and the related methods of inserting a portion of the image from another camera into an area cut out from the main image. These techniques employ an electronic switching circuit that turns off the signal circuit of one camera for a portion of several line scans while simultaneously turning on the signal circuit of another camera, the outputs of the two cameras being combined before the signal is broadcast. The timing of the electronic switch is adjusted to blank out, on successive line scans of the first camera, an area of the desired size and shape. The timing may be shifted during the performance and the area changed accordingly. One example of this technique is the wipe, which removes the image from one camera while inserting the image from another, with a sharp, moving boundary between them.
The technology and techniques of interactive computer graphics are used to create the graphics and text broadcast over television, particularly in news and weather programs. The material created using the computer is stored in a temporary buffer memory, from which it is then converted into the scanned version needed to be inserted into the television picture. Many of the animated main titles for television programs are created on computers and involve sophisticated shading, colouring, and other effects.
Flying spot scanner
A form of television pickup device, used to record images from film transparencies, either still or motion-picture, is the flying spot scanner. The light source is a cathode-ray tube (CRT) in which a beam of electrons, deflected in the standard scanning pattern, produces a spot on the fluorescent phosphor surface. The light from this spot is focused optically on the surface of the photographic film transparency to be recorded. As the image of the spot moves, it traces out a scanning line across the film, and the amount of light emerging from the other side of the film at each point is determined by the degree of transparency of the film at that point. The emerging light is focused onto a photoelectric cell, which produces a current proportional to the light entering it.
This current thus takes on a succession of values proportional to the successive values of film density along each line in the scanning pattern; in other words, it is the picture signal current. No storage action occurs, so the light from the CRT must be very intense and the optical design very efficient to secure noise-free reproduction. If an optical immobilizer is used, the flying spot system may be used with motion-picture film, as described below.
Motion-picture recording
Telecine, the recording on videotape of films originally produced for the cinema, is an important activity in television broadcasting, in the videotape rental market, and even in the home-movie market. In this technique the film is projected onto an image sensor for conversion into a video signal. Telecine film projectors fall into two classes, continuous and intermittent, according to the type of film motion.