1.1 Introduction

Historically, a signal meant any set of signs, symbols, or physical gesticulations that transmitted information or messages. The first electronic transmission of information was in the form of Morse code. In the most general sense a signal can be embodied in two forms: (1) some measured or observed behavior or physical property of a phenomenon that contains information about that phenomenon or (2) a signal that can be generated by a manufactured system and have the information encoded. Signals can vary over time or space. Our daily existence is replete with the presence of signals, and they occur not only in man-made systems but also in human and natural systems. A simple natural signal is the measurement of air temperature over time, as shown in Figure 1.1. Study of the fluctuations in temperature informs us about some characteristics of our environment. A much more complex phenomenon is speech. Speech is intelligence transmitted through a variation over time in the intensity of sound waves. Figure 1.2 shows an example of the intensity of a waveform associated with a typical sentence. Each sound has a different characteristic waveshape that conveys different information to the listener. In television systems the signal is the variation in electromagnetic wave intensity that encodes the picture information. In human systems, measurements of heart and skeletal muscle activity in the form of electrocardiographic and electromyographic voltages are signals. With respect to these last three examples, the objective of signal analysis is to process these signals...