TELEVISION

an electronic system for transmitting still or moving images and sound to receivers that project a view of the images on a picture tube or screen and re-create the sound. The effect of a television system is to extend the senses of vision and hearing beyond their natural limits. Television systems are designed, therefore, to embrace the essential capabilities of these senses, with appropriate compromises between the quality of the reproduction and the costs involved. The aspects of natural vision that must be considered in a television system include the ability of the human eye to distinguish the brightness, colours, details, sizes, shapes, and positions of the objects in the scene before it. The aspects of hearing include the ability of the ear to distinguish the pitch, loudness, and distribution of sounds. The television system must also be designed to override, within reasonable limits, the effects of interference and to minimize visual and aural distortions in the transmission and reproduction processes. The particular compromises adopted for public television service are embodied in the television standards adopted and enforced in each country by the government agency responsible for broadcasting. Television technology deals with the fact that human vision employs many hundreds of thousands of separate electrical circuits, in the optic nerve from the retina to the brain, to convey simultaneously in two dimensions the whole content of the scene on which the eye is focussed, whereas in electrical communications it is feasible to employ only one such circuit ( i.e., the broadcast channel) to connect the transmitter and the receiver. This fundamental disparity is overcome in television by a process of image analysis and synthesis, whereby the scene to be televised is first translated into an electrical image, and the latter is then broken up into an orderly sequence of electrical impulses that are sent over the channel one after the other. At the receiver the impulses are translated back into a corresponding sequence of lights and shadows, and these are reassembled in their correct positions on the viewing screen. This sequential reproduction of visual images is feasible only because the visual sense displays persistence; that is, the brain retains the impression of illumination for about 0.1 second after the source of light is removed from the eye. If, therefore, the process of image synthesis occurs within less than 0.1 second, the eye is unaware that the picture is being reassembled piecemeal, and it appears as if the whole surface of the viewing screen were continuously illuminated. By the same token, it is then possible to re-create more than ten complete pictures per second and to simulate thereby the motion of the scene so that it appears to be continuous. In practice, to depict rapid motion smoothly, it is customary to transmit from 25 to 30 complete pictures per second. To provide detail sufficient to accommodate a wide range of subject matter, each picture is analyzed into 300,000 or more elementary details. This analysis implies that the rate at which these details are transmitted over the television system exceeds 4,000,000 per second. To provide a system suitable for public use and also capable of such speed has required the full resources of modern electronic technology.