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This paper reviews the basic physical principles behind: the processing and combination of noisy signals; the quantification of information and of channel capacity; transformations and dualities in the representation of signals in frequency, time and space. The treatment is largely non-mathematical, and is aimed at fostering an insight into the nature of these concepts and their interrelations. In each of its three parts, the paper outlines some of the basic principles and theorems, indicatesÂ¿and in some instances extendsÂ¿the limits of their application, and then relates them to real-life operating conditions and design objectives. It is hoped that this approach will assist readers to see the connection between systems they are analysing, assessing or designing, and the idealized models normally postulated for the development of formal theoretical methods. Special emphasis is given to ways of looking at signal design and processing problems which have proved of particular value in system development in a wide field of applications.