Causality
For a system to be causal: when a signal is inputted into the system, then the output signal (at any given time) only depends on the input signal in present or past time. The output signal does not depend on the input signal in future time.
If when the input signal is input into the system and the output signal depends on the input signal in future time, then the system is non-causal.
Example of a Causal System
Y(t) = x(t - 5) <-- y(t) depends on x(t) 5 units in the past, so therefore it does not depend on the future ==> causal
Y(t) = 5*x(t) + (t + 3) <-- y(t) still only depends on the input in present time
Example of non-causal system
Y(t) = x(t + 5) <-- y(t) depends on x(t) 5 units in the future, so therefore it is causal.
