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# are there any noncausal,nonstable systems practically

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#### wajahat

Hi all, i want to know are there any noncausal ,nonstable systems practically.
if there are no such systems practically then why do me study them.
Any response in this regard would be helpful
regards
wajahat

of course unstable systems are there but due to their instability, their activity lasts for a short time and then die off e.g a detonated bomb just before exploding.

Concerning noncausal systems I suppose, it's unlikely that they exist and are used anywhere in real life. At least, I've never collided with such exotic cases. Imagine, the output signal of such system, according to its definition, should arise earlier, than the input one, provided the system had zero initial conditions. It's impossible, because it contradicts the law of energy preservation.

The second situation unstability is very actual. If the system possesse the reverse link (for example, transistor cascades) THEN IT MAY BE CAPABLE OF SELF-SIMULATION, which is caused by unstability. There are special criterions for continious-time and discrete-time signals, allowing to distinguish such systems.

With respect,

Dmitrij

non causal systems do exist in practice but the can NOT BE REAL TIME SYSTEMS(very logical!). you can have a block of data stored in your memory and u can perform any non-causal algorithm on that data using Matlab.

a lot of or almost every system has some initial value storage..non causal..
e.g. integrator,differentiator...

An example of Non-Causal system is Missile guidance system or Missile tracking system. In this, we have to determine future samples of the output (future position) depending on certain samples of Past and present inputs.

Maybe there is some confusion about the concept of non-causal system.
A system is said causal if its output at time T is independent from the input at time t>T, in other words if the output is the product of and only of the present or past input values and not of the future ones.
Hence non causal system is the one whose output depends from future input values: for this reason in the real world I don't think that non causal systems exist.

For what concerns the answer given by signal.processor I do not think this could be an example of non causal system, because use of memory storage to improve the control is not equal to saying that the output will depend from the future inputs, in facts he falls into a contradiction concluding his example of non causality writing "we have to determine future samples of the output depending on certain samples of Past and present inputs" which is the definition of a causal system!

Also the answer by rag_perfect is misleading: if you are talking of initial condition it is implicit in the word initial that these values are specified and fixed for t<=T0 where T0 is the starting time for the input and output but there is no specified dependence between them, so I don't see the relationship with the concept of non causality of a system.

Non causal system is a theoretical abstraction which has (as far I know) limited application in real systems.

Regards
Mowgli

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