Introduction

This unit turns from the study of individual electromagnetic and electronic phenomena to a practical question: how is information actually sent from one place to another? A communication system is any arrangement built to carry a message -- speech, music, a picture, or a stream of digital data -- from a sender to one or more receivers, over some physical distance, with the message reaching the far end recognisably intact. Whether the link is a telephone call over a wire, a radio broadcast reaching thousands of receivers at once, or a satellite link spanning continents, the underlying physical problems are always the same: the message signal must be prepared for transmission, sent through a physical medium that inevitably weakens and disturbs it a little, and then recovered at the far end. This chapter builds up the ideas needed to answer that question in outline: what the system looks like as a block diagram (Section 10.2), why the message signal needs to be modified before it can be sent (Section 10.3), the two commonest ways of modifying it -- amplitude and frequency modulation (Sections 10.4-10.5), how much of the frequency spectrum a signal and a medium each need or can supply (Sections 10.6-10.7), and finally how the transmitted wave physically finds its way through the atmosphere to the receiver (Section 10.8).