# Signal

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

In Digital Circuits, usually a signal is a variation of electrical quantity with respect to time.

### From MATH213

Signal

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 $R$ or some interval of $R$ (or any uncountable subset) then the signal is said to be a
continuous-time (CT)signal- If the domain of the function is $Z$, $Q$ or other discrete subset of $R$ (any countable subset of $R$) then the signal is said to be a
discrete-time (DT)signal

What are some examples?

Physical variable such as voltages, currents, displacements, forces, etc. are continuous time.

Quantities such as those seen in economics (houses, money etc.), digital hardware, anything stored digitally are are discrete time.