III. HOW TV WORKS
The TV camera converts the light of an image, and the sound, into an electronic signal, which for pre-recorded programmes can then be transferred to videotape. The image is converted into electricity in the camera tube. This is achieved by focusing the light of the image on a light-sensitive surface. An electron beam scans over the image, and converts each light value into a corresponding electrical charge.
Each signal is made up of image, sound, and a synchronization pulse, which ensures that the image and sound of the camera or videotape at the transmission end are coordinated with those of the TV set. These elements are amplified by a device in the TV set, and are separated out. The sound is converted back from a carried wave to an audio signal current and is amplified into the loudspeaker. The synchronizing signal is picked up by a separate circuit. The picture signal goes to the picture tube. At the back of this funnel-shaped vacuum tube are three electron guns that fire electron beams at a coated fluorescent screen at the front of the tube (the TV screen) and illuminate it. The electron gun is a metal cylinder that fires streams of electrons at the back of the screen. A positive element (anode) within the gun pulls the negative electrons away from a cathode. The TV picture signal determines how many of these electrons pass through an aperture, and on to the back of the screen. Thus the brightness of each element on the screen is controlled. Each beam corresponds to one of three primary colours: red, blue, and green. By combining these colours in various ways, all the colours that are seen on a TV screen can be produced.
The synchronizing signal controls the movement of a scanning spot, which moves in a horizontal line, activating electronic dots formed by chemicals called phosphors on the back of the screen, before returning to the beginning of the next line in a deactivated form, and then moving across again. The scanning process takes place at great speed.
The image signal contains information about how bright to make each picture element (the brightness signal), and in colour TV, information too about which combination of red, blue, and green to create (hue) and how strong or pastel each colour should be (saturation). The hue and saturation information are known as the colour signal. There are over a million phosphor dots on any colour TV screen, arranged in tiny triangles of red, blue, and green. The electron beams are made only to strike their own colour. The red beam strikes the red dot of each triangle, and so on.
The control centre of any TV station determines which images are to be transmitted, and the signal is attached to a pattern of waves (called carrier waves) of a particular frequency and wavelength, and sent out from the broadcast station’s transmission antenna. In the United States, for example, the government allocates each terrestrial channel 6 MHz of frequency between 54 and 88 MHz and 174 and 216 MHz on the electromagnetic spectrum.
A TV channel needs a much greater bandwidth (amount of spectrum space) than radio because it needs to be able to carry visual information as well as audio. The TV signal and the carrier wave are relayed via stations, usually based on hills and mountains. A receiving antenna (roof aerial, satellite dish, or cable receiving box) picks up the signals. The viewer selects which channel’s frequency to tune the TV to, but unlike radio, where stations advertise their frequency (for FM stations, it is usually between 88 and 108 MHz), TV viewers rarely know the allocation of their favoured channels, as the television controls are pre-set.
D. Scanning Systems
The main US TV system (referred to as NTSC, after the US government’s standards committee) produces 30 images of 525 lines per second, but as with all systems across the world, each image consists of two interlaced scanning dots, working on alternate lines. This interlaced system produces less flicker than the single scanning of an entire image. Thus 60 half-pictures per second are actually produced, complying with the 60 Hz powerline frequency that is standard in the United States.
In Europe, 50 Hz is the electrical standard, and the two main European systems (PAL and SECAM) produce 50 half-images per second, making 25 whole images. The human eye perceives this rate of single images as forming continuous motion. PAL, a German development, is widely used in Western Europe; SECAM is used in France, and in much of Eastern Europe. Both have 625 lines, though France experimented with an 819-line SECAM system for a while.
Recent experiments in Japan, Europe, and the United States have aimed at developing high definition TV (HDTV) based on picture resolutions of 1,125 and even 1,250 lines. HDTV is capable of being shown on a much larger screen, without loss of quality, and in a greater ratio of width to height. For the foreseeable future, HDTV seems likely to remain an expensive option, but may eventually become more widely adopted.