Transmitting Television

Television signals can be transmitted using terrestrial ground transmission, cable or satellites.

Terrestrial television uses radio and microwave techniques to transmit a composite signal of amplitude modulated (AM) pictures and frequency modulated (FM) audio via an antenna which covers a geographical limited area by line-of-sight. A receiving antenna enables the decoded picture signal to be displayed on a television screen and the audio signal through a loudspeaker. The International Telecommunications Union allocates frequencies for terrestrial broadcasting to individual governments, who then reallocate bandwidths to broadcasting corporations. The bandwidth available or terrestrial transmission is, therefore, limited. This has implications for using terrestrial transmission for educational purposes, as educational programmes have to compete for airtime with programmes designed for different purposes.

Cable is another means of transmitting television. It is in use in the United States of America, Canada, and Europe, but is not used in South Africa. Cable is a local distribution mechanism that transmits the video signal via a fibre optic or copper wire cable directly to television sets. Cable television can link into national television networks by receiving terrestrial or satellite transmission, and relaying these via cable.

Satellite television makes use of a satellite in a stationary orbit around the earth that acts like a giant mirror. The video signal is beamed up to the satellite by a ground station and then beamed down again over a large geographical area (known as the 'footprint' of the satellite). The satellite signal is received by the parabolic receiving dish of the user. Satellite transmission is limited by the number of satellites available and by available frequencies for transmission. Satellites have to broadcast at frequencies that do not interfere with either terrestrial broadcast frequencies or other satellite broadcasts. As with terrestrial broadcasts, there is therefore limited bandwidth available for satellite television transmission, although this is much larger than the bandwidth available for terrestrial signals.