Signal

A signal is a function that represents the variation of a physical quantity with respect to any parameter.

From MATH213

Definition from Lecture 9:

Definition 2: Signals

A signal is a complex-valued function of an independent real variable $t$.

• $t$ usually (but not always) represents time.
• If the domain of the function is $\mathbb{R}$ or some interval of ${\mathbb{R}}^a$ then the signal is said to be a continuous-time (CT) signal.
• If the domain of the function is $\mathbb{Z}$, $\mathbb{Q}$ or other discrete subset of ${\mathbb{R}}^b$ then the signal is said to be a discrete-time (DT) signal.

Definition 3: Systems

Mathematically, a system is a map from a space of input signals to a space of output signals.

Definition 4

A system $S$ is continuous-time (CT) if both its input and output signals are CT.

A system $S$ is discrete-time (FT) if both its input and output signals are DT. $S$ is a hybrid system if ti contains both CT and DT signals.