2.1 INTRODUCTION

In this chapter analog-to-digital (A/D) and digital-to-analog (D/A) conversion techniques of audio, television, and frequency division multiplexed (FDM) signals are reviewed. Due to the relatively long equipment and propagation delay of long-distance systems and particularly of satellite systems, the effect of voice echo is annoying to the user. For this reason, recently developed echo suppression and cancellation subsystems are also described.

Signal processing and multiplexing form a large and complex subject. Books have been written that present the theoretical fundamentals and applications of signal processing [Oppenheim, 2.4; Bellamy, 2.5; Feher, 2.6]. In this section we limit our presentation to a brief review of the principles of operation, followed by applications of signal-processing and multiplexing subsystems used in telecommunications systems. This review should facilitate the comprehension of the material presented in Chapters 3 and 4.

The most frequently employed A/D conversion methods used in communication systems are:

The A/D converter, located in the transmitter, is also known as the encoder, or simply coder. The D/A converter, located in the receiver, is known as the decoder. The word codec is derived from coder/decoder.

In addition to the basic conversion methods just listed, more-involved codes have been developed. Frequently used acronyms include the following: