# Tag Info

Accepted

### Why are anticausal systems even defined?

Of course they don't exist. But we can stop time and use systems that would be non-causal if we hadn't stopped time. Stop time? Yes, just store your data and work offline / non-realtime. Or work on ...
• 90.5k
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### How can an impulse generate an output in the past time frame?

As mentioned in SakSath's answer a system with $h[n]\neq 0$ for $n<0$ is non-causal. Such a system cannot be implemented in real-time. However, you could use such a system for offline processing. ...
• 90.5k
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### Is there a relation between an analytic signal (signal processing) and an analytic function (complex analysis)?

There is a relationship between these two concepts. Let the complex function $f(z)$ be analytic on and inside a simple closed curve $C$ in the complex plane. Then Cauchy's integral formula states that ...
• 90.5k
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### Non-causality of fractional delays

It's not the delay itself that causes the total discrete-time system to be non-causal. In continuous time we simply have an impulse response $\delta(t-t_0)$, which is clearly causal for $t_0>0$. ...
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### Non-causality of fractional delays

Just an expansion on Matt's great answer: you have stumbled into the "dirty little secret of sampling" :-) In order to sample without aliasing the signal has to be bandlimited. However, any ...
• 45.6k
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### Why does causality imply that the system function is analytic?

New answer: I provide a new answer because I believe that this is a clearer and more direct way of explaining the relation between causality of the impulse response and analyticity of the ...
• 90.5k
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• 4,295
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### For a system to be causal, number of finite zeros <= number of finite poles. Why?

If the number of finite zeros is not greater than the number of finite poles then the transfer function is proper, i.e., the degree of the numerator polynomial is not greater than the degree of the ...
• 90.5k

### Why can't a causal digital filter have an infinitely sharp transition between the passband and the stopband?

This is three years later, but since I don't see the real answer posted here, I will post it. The correct answer is that if we are literally interpreting the original statement as a purely ...

### Why can't a causal digital filter have an infinitely sharp transition between the passband and the stopband?

My answer implicitly refers to ideal brickwall lowpass filters which do have infinetely sharp transition bands. For other possible interpretations, refer to other answers. In practice, neither analog, ...
• 28.3k
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### Does "improper" imply that a system cannot be stable and causal?

An improper system cannot be causal and stable. If the order of the numerator is greater than the order of the denominator, you'll always have at least one pole at infinity. Consequently, not all ...
• 90.5k
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### How to conclude LTI, causality and BIBO stability of a system represented by a differential equation?

I take your equation: $\frac{d^2y}{dt^2}-3\frac{dy}{dt}+2y(t)=x(t)$ Laplace transform will be: $s^2Y(S) - 3sY(S) +2Y(s) = X(s)$ Now I can find transfer function: $H(s) \triangleq \frac{Y(s)}{X(s)}$...
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### What is a memoryless system?

Confusion may arise, for causal systems, from mistaking "having negative signal values (amplitudes)" and "depending on negative time indices". A strict memory-less system depends ...
Accepted

### Realization of a filter based on its transfer function

A transfer function is called realizable if it can be implemented by a causal and stable system. The given frequency response is continuous and doesn't have any impulses, so the corresponding system ...
• 90.5k