TABLE OF CONTENTS Most discrete-time signals come from sampling a continuous-time signal, such as speech and audio signals, radar and sonar data, and seismic and biological signals. The process of converting these signals into digital form is called analog-to-digital (A/D) conversion. The reverse process of reconstructing an analog signal from its samples is known as digital-to-analog (D/A) conversion. This chapter examines the issues related to A/D and D/A conversion. Fundamental to this discussion is the sampling theorem, which gives precise conditions under which an analog signal may be uniquely represented in terms of its samples.
An A/D converter transforms an analog signal into a digital sequence. The input to the A/D converter, x a( t), is a real-valued function of a continuous variable, t. Thus, for each value of t, the function x a( t) may be any real number. The output of the A/D is a bit stream that corresponds to a discrete-time sequence, x( n), with an amplitude that is quantized, for each value of n, to one of a finite number of possible values. The components of an A/D converter are shown in Fig. 3-1. The first is the sampler, which is sometimes referred to as a continuous-to-discrete (C/D) converter, or ideal A/D converter. The sampler converts the continuous-time signal x a( t) into a discrete-time sequence x( n) by extracting the values of x
