about non-causal systems

Does non-causal systems are real? If not, then can anyone give me example of the same?:razz:8-O

Re: about non-causal sysems

HI..

Try this ...

https://www.edaboard.com/threads/129148/

You should be cautious with the definition of a non-causal system.
For example, there are real circuits that exhibit a negative group delay. That means that they respond to a certain kind of input with a negative delay, which means: in advance to the time the input appears.

Hi,
A causal system is one in which the output depends only on current or past inputs, but not future inputs. Similarly, an anti-causal system is one in which the output depends only on current or future inputs, but not past inputs. Finally, a non-causal system is one in which the output depends on both past and future inputs. All "real time" systems must be causal, since they can not have future inputs available to them.


One may think the idea of future inputs does not seem to
make much physical sense; however, we have only been
dealing with time as our dependent variable so far, which is
not always the case. Imagine rather that we wanted to do
image processing. Then the dependent variable might represent
pixel positions to the left and right (the "future") of the current
position on the image, and we would not necessarily have a causal system.

Thanks for your reply,my friend. But i heard about some systems mainly in case of communication that for reducing noise we have to make anti-noise signals which will cancel the noise with very less error. So,in case of such systems we have to predict future outputs and we have to make anti-noise signals. So can we call such systems as ''non-causal" systems?

chakradeo.abhishek said:

Thanks for your reply,my friend. But i heard about some systems mainly in case of communication that for reducing noise we have to make anti-noise signals which will cancel the noise with very less error. So,in case of such systems we have to predict future outputs and we have to make anti-noise signals. So can we call such systems as ''non-causal" systems?

Click to expand...

This is a totally different thing. Noise is random, it can be added in your signal at any time and from any possible source. In communication When we talk, the DSP (Digital signal processor) is doing all the calculation in real time. Now if you are transmitting a signal of say 20HZ to 20000Hz, every frequency falling outside of this limit would be a noise and you can filter them using low pass and high pass filters. Some noises attack your signal's magnitude by adding in it. If its a predetermined noise then you can remove it but if its random then sorry you can't. For example, in a strong stormy weather cell phone signals almost vanishes. another example is when you talk then it happens sometime that you hear breaking voice of the listener,it is the noise which is interrupting the signal. In short how your system is going to know that what are you going to say next? So this system is not possible.

Hi,
Do not mix the Input with Noise. Input can be a noise also (as the noise is amplified after inputting, like in signal jammers). Noise can also be with the input. The noise can not be predicted where as signal input can be.