I was going through my signal and system notes.they say $y(t)=x(t/2)$ is a non causal system? As non causal system depend on future inputs. how $t=t/2$ is future value of time? i could not understand how? could somebody please explain it to me?


2 Answers 2


Because for negative values of $t$ you have, for example, $y(-2) = x(-1)$ which depends on a future value of $x(t)$ at $t=-1$ for the current value of $y(t)$ at $t=-2$.

Note that $t=-1$ represents a future time for $t=-2$.

  • 1
    $\begingroup$ You beat me to it ... $\endgroup$
    – Matt L.
    Commented Jan 17, 2018 at 10:56


Use a negative time $t$ to see that $y(t)$ depends on future input values.


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