What is the process of broadcasting?


There are thousands of different television channels around the world, broadcasting entertainment, news, information and sport. Television programmes are created at television stations. Each station normally broadcasts several separate channels. Some programmes, such as news and sports, are broadcast live, which means the viewers see action as it happens. Most programmes are recorded on videotape and broadcast at a later date. Some programmes are a combination of live and recorded action.

A television camera electronically divides an image of the scene it is pointing at into hundreds of narrow horizontal lines, each made up of hundreds of small dots of colour. It creates an electrical signal that represents the colours of all the dots. It repeats this process 25 or 30 times a second to create a continuous picture signal.

Pictures from the cameras in a television studio, and from cameras at outside broadcasts, such as sporting events, are fed to a control room, where they appear on screens. Here, live pictures from cameras, pictures from videotape (such as short news reports) and computer graphics are mixed to create the signal for the pictures that will be broadcast. Sound from studio microphones or audio tape is also added.

There are several ways of broadcasting signals. In each case, the signal is modulated before it is sent, with different channels using different carrier signals. The receiver tunes in to the signal from the channel the viewer wants to watch. Many signals now travel in digital rather than analogue form. This allows many more channels to be broadcast, and eliminates the interference that often makes pictures sent using analogue signals fuzzy.

In terrestrial television, the signal goes to a transmitter where it is turned into a radio signal that is spread out in all directions. The signal can be detected by an aerial of any receiver within range of the transmitter. In cable television, the signal travels from a cable television station through a network of underground cables that link directly to receivers connected to the network. In satellite television, the signal is beamed by microwaves to a satellite high above the Earth. The satellite detects the signal with its own aerial and re-transmits it so that it can be picked up by receivers on the Earth’s surface. In webcasting, television and video pictures are transmitted over the Internet. The pictures are first converted into a digital video format and then made available on a website.

In closed-circuit television (CCTV), signals are not broadcast at all. Instead, they go directly from the camera to a receiver. CCTV is used for security systems, with the pictures being recorded as well as viewed.

Terrestrial television signals are broadcast from transmitters at the top of tall masts, often on hill tops, and detected by aerials placed high up on roofs. This gives the signals a clear route from transmitter to aerial. But in mountainous areas the signals are often blocked by hills. This is not a problem with satellite television, where the signals come down from a satellite high in the sky to small aerial dishes aimed accurately at the satellite.

Interactive television is television in which the viewer can send information back to the television station, normally via a telephone line. The combination of digital television and a telephone line also allows viewers to access the Internet.

Television broadcasting satellites sit directly above the Earth’s equator, about 36,000 kilometres up, in an orbit called a geostationary orbit. They orbit at the same rate as the Earth turn, which means they always stay above the same point on the Earth’s surface. Television signals are beamed to them by microwaves from dish aerials at a ground station. The satellite transmits a broad signal beam which covers a wide area when it hits the surface. Any receiver in the area can detect the signal.

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