The television camera is an optical device not unlike the human eye but, rather than arousing a photochemical response in the visual cells of the retina, light admitted through the camera aperture activates a mosaic of mutually isolated points, usually of a photo-conductive material. Small electrical charges are built up in the mosaic in accordance with the intensity of the incident light.
Red, green, and blue filters cover the three apertures of the colour television camera. Each filter absorbs the light of its complementary colour and enables information about the light-intensity of each separated component to be isolated on three mosaics ready for encoding in the camera. The mosaics are scanned by three beams of electrons (elementary electrical particles) directed to move across them from side to side, and more slowly from top to bottom. The points of the mosaic are encoded at a rate of some five million per second. The filtered components are recorded simultaneously in accurate alignment and the signals combined for transmission to the television receiver(s).
In the receiver the conversion of electronic signals back to light is accomplished traditionally by a cathode-ray tube (CRT) or picture tube. This consists of a large, evacuated glass container with a short cylindrical pipe at one end opening out to a flat viewing screen at the other. At the sealed end of the pipe are three guns, one responding to each colour signal. The current emitted by each gun is regulated by the intensity parts of the transmitted signal. The electron beams are sharply focused by magnet and deflected to scan the screen end of the CRT by two electromagnets. These are controlled in turn by the line and frame markers in the transmitted signal, starting each line and frame of the televised picture at the right moment to give the correct synchronisation with the original camera scan.
In the most common type of CRT, the inner surface of the screen is coated with thousands of tiny, isolated points of phosphorescent materials or crystals each of which releases light in direct proportion to the current received from an electron beam. These phosphors are too fine for the eye to distinguish individually at normal viewing distance and blend optically to give the required colour impression.
The phosphors are arranged in groups of three, known as triads, one of which emits red light when stimulated by its appropriate electron beam. Another emits green and a third emits blue light. Between the electron guns and the phosphor coating is a thin, perforated metal screen, the shadow mask, fixed about 2 cm. from the phosphors. Its grid of perforations is arranged so that each hole lies precisely opposite a phosphor triad. This ensures that each electron beam falls onto the appropriate coloured dots, and ultimately enables a faithful replica of the original scene scanned by the camera to be reproduced as an additive-primary colour synthesis of red, green and blue points of light. An alternative colour tube employs sets of four vertical stripes (red, green, blue and blank); such a system eliminates some of the difficulties in precision manufacture and alignment of the phosphor triads and shadow mask.
In most European systems, the television picture is composed of 625 horizontal lines, and each triad is bombarded by an electron beam 25 times each second. Most American and Japanese systems have 525 lines operated at 30 complete pictures (or frames) per second.
Copyright © 2006 Roy Osborne. All Rights Reserved.
References:
Robert W.G. Hunt (1957), The Reproduction of Colour in Photography, Printing and Television. King's Langley: Fountain Press. Fifth edition 1997.
Kurt Nassau, ed. (1998), Colour for Science, Art and Technology. Amsterdam: Elsevier Press.
William N. Sproson (1983), Colour Science in Television and Display Systems. Bristol: Adam Hilger.
VIDEO
There are several ways in which television signals can be transmitted from the television camera to a receiver. The most common method is by broadcasting on the normal VHF (Very High Frequency) and UHF (Ultra High Frequency) radio wavebands, which are received by television antennae. Secondly, closed-circuit television (CCTV) links the camera directly to one or more receivers or monitors via a wire or fibre-optic cable (similar to a telephone system).
Alternatively, the wire transmitting the signals from the camera can be plugged into a videotape recorder (VTR) and picture and sound recorded on a tape reel, video cassette or videodisc.
The first audiotape recorder (using steel wire) was developed by Vladimir Poulsen as early as 1900. Experiments using coated paper-tape continued in the 1920s by Fritz Pfleumer, and the first magnetised tape recorders appeared in 1932. The videotape recorder was introduced commercially by Ampex in 1956, using magnetic tape similar to that used in audiotape recorders. The videocassette (introduced 1973) has proved immensely popular for recording movie films and network television programmes, though videodiscs, introduced shortly after, have also proved popular and convenient for high-quantity recording.
Standard videotape consists of a polyester ribbon coated with magnetic particles, usually iron oxide; finer, chromium dioxide particles record a finer quality signal and are generally preferred for colour systems. The recording head of the recorder magnetises the tape coating in accordance with the signal it receives from the camera. When the tape is played back, the recorded magnetic pattern induces a current, which reproduces both picture and sound on the television receiver or monitor.
Copyright © 2006 Roy Osborne. All Rights Reserved. |
