Causal and Noncausal Systems

A system is called causal if its output y ( t ) at an arbitrary time t = t,, depends on only
the input x ( t ) for t It o. That is, the output of a causal system at the present time depends
on only the present and/or past values of the input, not on its future values. Thus, in a
causal system, it is not possible to obtain an output before an input is applied to the
system. A system is called noncausal if it is not causal. Examples of noncausal systems are


Note that all memoryless systems are causal, but not vice versa.