Programme 1

Background

Digital communication

Digital communication uses a universal language, consisting only of 1s and 0s, which can be generated and decoded by computers, mobile phones, televisions, faxes, CD players and many other digital systems. The stream of 1s and 0s (bit stream) encodes the information being conveyed, whether it be text, sound or pictures. It is created by a digital encoder and can be conveyed by a variety of carriers: by electrical signals through wires, by radio waves through transmitters and satellites, and by light waves through space and through optical fibres. At its destination, digital decoders convert the information in the bit stream back to its original form.

Multiplexing

Multiplexing is used to send several streams of information at the same time, by combining the separate bit streams into a single, complex signal. The individual bit streams are recovered by a decoder at the receiving end. Digital signals are usually multiplexed using a system known as time-division multiplexing or TDM. Many signals can be carried at the same time by breaking each one into short segments. The multiplexer builds these segments into a composite signal in a fixed order, repeating the order over and over again until all of the segments are encoded into the complex sequence containing all the information from all the messages. At the other end a demultiplexer separates out the individual signals and sends them on to their destinations.

TDM is crucial to the operation of the Internet, where demand varies from minute to minute. By adjusting the time intervals between sampling of different messages, TDM allows the maximum number of people to use the web simultaneously. So at busy times, when demand is high, individual bit streams take longer to multiplex and web pages take longer to download. At the receiving end, even the fastest modem can only download information at the rate at which it arrives.

Electronic noise

All electronic systems suffer from a low-level random voltage known as electronic noise. The noise mixes with and contaminates the signal being carried. The degree of contamination is expressed as the signal-to-noise ratio. Once noise is added to an analogue signal, it is hard to remove. A fuzzy radio signal or a TV picture looking like a snowstorm are sure signs of analogue signals with a low signal-to-noise ratio. Analogue noise reduction systems do exist: for example, the Dolby system found on most audio systems.

The great advantage of digital television and audio is that noise is much easier to remove. Provided the contamination is not too great, the decoder can decide whether each bit is likely to be a 1 or a 0 – so the noise can be removed altogether. One of the advantages of digital television is the quality of sound and picture.

See the ‘whatis?’ website (http://www.whatis.com/index.htm) for more information.