hreshold is required. Each Figure 28.1 A digital communication system amplitude decision will have to be made in the presence of any distortion and interference superimposed upon the data sequence during transmission. The decision making process will be more reliable if it is timed by a clock which distinguishes when each The processing tasks in the transmitting terminal fall into two received symbol has reached an optimum value. The accuracy with categories: which the decision threshold and sample times must be placed will depend upon the severity of distortion suffered during transmission. 1. Source coding is concerned with the structure and statistics of Wideband channels lead to less critical amplitude and time place- the data source. It will involve the conversion of the data into a ment requirements, but they may admit more noise into the receiver. more suitable form for subsequent processing. Word alignment A compromise must therefore be struck. and frame alignment (when multiplexing several data sources The tasks of equalisation, clock extraction and decision making together) can also be regarded as part of the source coder's task. are shown in Figure 28.3. Also shown is the output stage which, 2. Channel coding is concerned with the characteristics of the under the control of the extracted clock, produces a retimed base- transmission channel, it must ensure that the processed data is band waveform. Most digital transmission systems use such a self- compatible with the requirements of the channel. For example, timed fully regenerative arrangement at the distant terminal. A it may be necessary to add redundancy to the transmitted data similar configuration is common at intermediate repeaters (if used), symbols to combat the effects of transmission errors by provid- although untimed amplifiers are being considered for some optical ing some error detection and/or correction capability. Symbol systems. timing information may also need to be added. In addition, coding must ensure that the frequency spectrum of