EXISTING TV TECHNOLOGY
television transmissions are broadcast as analogue signals. While most
people in the UK get a great terrestrial television signal reception
in their home with analogue, a large number of homes receive a poorer
signal. This is because the strength of an analogue wave decreases over
distance, can be effected by interference from other sources (such as
overhead electric cables) and can be blocked by large objects such as
buildings and hills. To receive the best possible signal, you need your
roof-top antenna to be in clear view of your nearest transmitter mast.
Satellite television uses an external dish to receive the signals. The dish is pointed directly at the orbiting satellite so a clear signal can always get through. However, satellite signals are effected by weather conditions and by pollution in the earth's atmosphere - conditions that prevent the analogue signal from being received in full and causing a snow-storm pattern on your TV screen. Cable television uses a fixed fibre-optic cable to send signals directly to your decoder box and suffers little of the signal loss experienced by terrestrial of satellite services. However, existing cable services are restricted in the number of channels they can offer - only a finite amount of information can be passed down the cable at any time.
DIGITAL TELEVISION TECHNOLOGY
Digital television goes a long way to address the problems experienced with current analogue services. Each analogue channel is compressed and converted into a digital data stream using the MPEG-2 compression system, making the amount of information to be transmitted much smaller. The MPEG system only transmits the parts of the picture that changes from one frame to the next, rather than sending a complete new frame. Because the space needed for a digital channel is less than an analogue channel, several digital signals can be transmitted side-by-side in the space previously occupied by a single channel in analogue. Atmospheric interference has little or no effect on a digital signal, so practically all the information is received in tact. When received, the digital signal is expanded into its original form and converted to analogue for viewing.
The whole point of this digital system is to get a clear, high quality signal to the receiver without any loss of information, in addition to creating much more bandwidth for transmitting extra channels and services.