I'm using Digital Signal Processing Principles, Algorithm and Applications 4th Edition written by Proakis and Manolakis. In Chapter 3, subtopic 3.3.2 it mentions the term "causal signal" which I can't find any definition. I know what a causal system means: a system that output depend entirely on past and present input, not future input but here it's causal signal, not system

So what does causal signal definition?

I tried to look at the Index and it says that the term "Causal Signal" is mentioned in page 85 but in fact, I find nothing in page 85 mentioning it


A system is causal if its output depends only on the current input and past inputs (and not on future inputs). As a consequence, if the system is a linear, time-invariant (LTI) system (which input/output relationship can be completely characterized by an impulse response), that impulse response, $h(t)$ must be zero for all time $t \lt 0$.

Some people define a causal signal, $x(t)$, to be one that can be the impulse response of a causal system: it is zero for all time $t \lt 0$.


same property that the impulse response of a causal, linear and time-invariant (LTI) system has.

  • $\begingroup$ Namely, that the signal $x[n]$ is equal to zero for all $n < 0$. $\endgroup$ – Jason R Nov 18 '15 at 3:43
  • $\begingroup$ and what property is "same property"? $\endgroup$ – Dilip Sarwate Nov 18 '15 at 4:42
  • $\begingroup$ what @JasonR sez. $\endgroup$ – robert bristow-johnson Nov 18 '15 at 4:45

A causal signal is one that is non-zero, only for $t \geq 0$, while it is zero for all negative time(or time instants). While, a non-causal signal is one that is defined over the entire time axis(i.e., non zero for all values of the time.)

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