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Determining a system's causality using its impulse response

Signal Processing Asked on October 24, 2021

I have the following input-output relation for a system:

$$y(t) = Odd Part Of [x(t)]$$

My question is: Is the system causal?

What my approach has been:

I expressed $y(t)$ alternatively as:
$$y(t) = frac{x(t) – x(-t)}{2}tag{1}$$

Here, when I substitute $x(t)$ with the impulse function, I get the impulse response as $0$ because the impulse function is an even function. Its odd part is $0$. This leads me to believe that the system is causal as the impulse response is zero for negative time.

However, when I substitute $t$ with $-t$ in Eq. $(1)$, I find that for negative time, the output depends on the input at a future time. This would lead me to believe that the system is non-causal.

So my question is actually two-fold here:

  1. How do I reconcile the two seemingly contradictory results?

  2. Why is the impulse response of the system 0? What’s the difference between this system and having no system at all?

Any help/pointers would be sincerely appreciated.

One Answer

Clearly, for negative values of $t$, the system needs to know the future in order to determine its output. Hence, the system can't be causal.

Since the system is also time-varying (show it!), its response to an impulse doesn't say much about its general behavior, unlike it would be the case for a linear time-invariant (LTI) system. So the given system's response to an impulse being zero for $t<0$ doesn't say anything at all about causality or any other general properties of the system.

Takeaway: don't use a system's impulse response for drawing conclusions about its properties before having verified that the system is LTI and can in fact be characterized by its impulse response.

Answered by Matt L. on October 24, 2021

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