Reproduction of visual images at a distance using radio waves. For transmission, a television camera converts the pattern of light it takes in into a pattern of electrical charges. This is scanned line by line by a beam of electrons from an electron gun, resulting in variable electrical signals that represent the picture. These signals are combined with a radio carrier wave and broadcast as electromagnetic waves. The TV aerial picks up the wave and feeds it to the receiver (TV set). This sepa
rates out the vision signals, which pass to a cathode-ray tube where a beam of electrons is made to scan across the screen line by line, mirroring the action of the electron gun in the TV camera. The result is a recreation of the pattern of light that entered the camera.
rates out the vision signals, which pass to a cathode-ray tube where a beam of electrons is made to scan across the screen line by line, mirroring the action of the electron gun in the TV camera. The result is a recreation of the pattern of light that entered the camera.Receiving aerials
Because the wavelength of any television signal is short a resonant aerial becomes possible, and this usually consists of a half-wave aerial made of light alloy or steel tube, fed at the centre with low-impedance coaxial or balanced cable. Greater gain is obtained if a reflector element is added, and quite complicated arrays are used in areas of weak signal strength.
These aerials are mounted either vertically or horizontally to conform with the polarization of the transmitting aerials. Tubing is used for the elements, since an aerial made of wire would be too sharply resonant, with resulting loss of bandwidth, and therefore poor picture definition.
Television channels
In addition to transmissions received by all viewers, the 1970s and 1980s saw the growth of pay-television cable networks, which are received only by subscribers, and of devices, such as those used in the Qube system (USA), which allow the viewers' opinions to be transmitted instantaneously to the studio via a response button, so that, for example, a home viewing audience can vote in a talent competition. The number of programme channels continues to increase, following the introduction of satellite-beamed TV signals and digital television.
Further use of TV sets has been brought about by videotext and the use of video recorders to tape progra
mmes for playback later or to play pre-recorded video cassettes, and by their use as computer screens and for security systems. Extended-definition television gives a clear enlargement from a microscopic camera and was first used in 1989 in neurosurgery to enable medical students to watch brain operations.
mmes for playback later or to play pre-recorded video cassettes, and by their use as computer screens and for security systems. Extended-definition television gives a clear enlargement from a microscopic camera and was first used in 1989 in neurosurgery to enable medical students to watch brain operations.
