INTRODUCTION

Modulation may be defined generally as the representation of information in a form suitable for transmission. One might define analog modulation as the modulation of analog information, and predominantly voice and other audio signals. However, this is a bit vague since digital modulation – the topic of the next chapter – may be used for precisely the same purpose. A possible distinction between analog and digital modulation might be made in the following way: Digital modulation requires the information to be transmitted to be represented in digital form at some point in the modulation process, whereas this is not required in analog modulation. Therefore, analog modulation might be best defined simply as modulation that is not better described as digital modulation.

This chapter provides a brief but reasonably complete introduction to analog modulation as it is used in modern communications systems. First, some preliminaries: Section 5.2 provides a brief overview of the overarching concept of sinusoidal carrier modulation. Then Sections 5.3 and 5.4 introduce the concept of complex baseband representation, which provides useful insight into the analysis of sinusoidal carrier modulation, including the effects of propagation and noise. Most commonly-used analog modulations may be classified as either amplitude modulation, single sideband modulation, or frequency modulation; these are described in Sections 5.5, 5.6, and 5.7, respectively. This chapter concludes with a brief discussion of techniques which are employed to improve the quality of audio signals in analog modulation systems.

SINUSOIDAL CARRIER MODULATION

Generally speaking, modulation schemes (now referring to both analog and digital modulations) fall predominantly into just two classes: baseband modulation and sinusoidal carrier modulation. Examples of baseband modulation include pulse modulation – used primarily in wired communication systems such as Ethernet – and impulse modulation – the basis for most ultrawideband (UWB) wireless communications systems. In impulse modulation, information is represented by the spacing between impulses, and the impulses are extremely narrow so as to make the bandwidth of the signal extraordinarily large. Present-day UWB impulse radio systems are limited to short-range, low-power applications beyond the scope of this book; reader is referred to [64] for more information.