Classification of Signals

Signals are categorized as Continuous-time or discrete-time, Periodic or aperiodic, Analog or digital, and Causal or noncausal.

A continuous-time signal holds value at every moment, while a discrete-time signal holds value only at specific moments.

Discrete-time signals, often denoted by x of n, where n is an integer, usually represent phenomena with inherently discrete variables.

A continuous-time periodic signal, such as a sinusoid, repeats itself every T seconds, adhering to the periodicity condition. Any signal that does not satisfy the periodicity condition is called an aperiodic signal.

A discrete-time periodic signal, with a period of positive integer N, remains unaltered after a time shift of N. A discrete-time periodic signal can be represented as a sum of complex exponentials, particularly when analyzed using Fourier series.

An analog signal is a continuous-time signal with amplitude values that can vary continuously within a given range. A digital signal is a discrete-time signal with amplitude values restricted to a finite set of possible levels.

A signal is considered causal if it is zero for all negative time values. A signal that exists for negative time values is called noncausal.