A causal system is one where the output at any given time depends only on the input values for times . In other words, in a causal system, the output cannot occur before the input is applied.

A system that violates the condition of causality is called a noncausal system.

Example:

is noncausal because is a future input

Why Study Noncausal Systems?

At first glance, noncausal systems may seem illogical. However, nontemporal systems can be both noncausal and realizable. Even in the case of temporal systems, there are situations where the input is “prerecorded”, making “future” inputs available for analysis.