The time response of a system is the output signal(s) as a function of time, following the application of a set of prescribed input signals, under specified operating conditions In a continuous-time system (Fig. 2.1), the input and output signals are continuous time. A discrete-time system (Fig. 2.2) has discrete-time input and output signals. A hybrid-time system (Fig. 2.3) is one in which both continuous-time and discrete-time signals appear at the inputs and outputs. An A/D converter is an example of a hybrid system: The input is continuous-time, but the output is discrete-time. Therefore, a continuous-time system is one in which continuous-time input signals are transformed into continuous-time output signals, and a discrete-time system is one that transforms discrete-time input signals into discrete-time output signals. A continuous-time system is symbolically represented as x(t) —+ y(t), and a discrete-time system is represented as x (k) —+ y(k) or Xk —Y Yk, where k stands for the integer sample number.