2.1 INTRODUCTION

As we saw in Chapter 1, a communications system passes information from an information source to an information sink. The source converts the information into an electrical signal that is transmitted across the communications channel and is then converted by the sink back into information that can be understood by the recipient at the source.

In any communications system, the information produced by the source is represented in some physical signal that can be processed, transmitted, and recovered. Broadly, these signals fall into one of two categories: analog and digital. An analog signal varies continuously in time and amplitude, taking on an infinite range of possible values that are directly related to the underlying physical quantity being represented, such as sound pressure or light intensity. In contrast, a digital signal represents information using a finite set of discrete values—most commonly two levels corresponding to binary symbols. While analog signals closely mirror the physical phenomena they describe, digital signals offer greater robustness to noise, easier storage and processing, and compatibility with modern compression, encryption, and error-control techniques. Consequently, although many real-world sources are inherently analog, most contemporary communications systems ultimately operate on digital representations of source information.

Before considering how information is represented digitally, it is necessary to understand the behavior of analog signals, since they form the fundamental physical basis of many communications systems. Moreover, even when information is ultimately processed or transmitted in digital form, it is carried by analog electrical waveforms within transmitters, channels, and receivers. For these reasons, we begin by examining the properties of analog signals and the basic tools used to describe and analyze them.