ABSTRACT
In Chapter 2, we discussed the basic continuous-time and discrete-time signals. In this chapter, we introduce the concept of systems. The main categories in which systems are classified are given and the basic properties of systems are described. A system is a concept with a very wide meaning used in various expressions of everyday life. From an electrical engineering perspective, a system is an entity that manipulates one or more signals to perform an operation and returns the operation result as one or more signals. A more formal definition is that a continuous-time or discrete-time system is an entity that transforms an input signal x(t) or x[n] into an output signal y(t) or y[n] according to a specified operation. The transformation of x(t) or x[n] is denoted by y(t) ¼ S{x(t)} or y[n] ¼ S{x[n]}, where S denotes the system. In simple words, one can say that a system responds to an input signal x(t) or x[n] by an output signal y(t) or y[n] (see Figures 3.1 and 3.2). From a signals and systems perspective, a system is considered a black box, which means
that it is examined in terms of its input, output, and properties without the need to know its internal working. An example of a system is an electrical guitar. Consider the music note played by the guitarist as the input signal. The electrical guitar, the amplifier, and the sound boxes constitute the system. The output signal is the actual sound that the audience hears. The output signal is different (louder, i.e., has a higher amplitude and usually a longer duration) from the input signal. Considering a black-box approach, we are interested in the input and the output signals and not the internal implementation of the system (e.g., the circuit of the amplifier).
