Signals and Systems

signal

I don't understand this problem, can anyone explain this in detail?
a system is memory-less if the output depends only on the present values of the input. how can you tell by just looking at the function? how do you determine that? the same thing for causality , output depends on the past and present values on the input.
Can you give an example where a system is not memory-less and non causal . ?
Thanks
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Best Answer

An example for a system with memory could be this counter with \$a_0=0\$

$$a_{n} = a_{n-1} + 1$$

Calculating \$a_{n}\$ requires knowledge about the previous state \$a_{n-1}\$ (except for the case of \$a_{0}\$)

A counter without a memory could look like this

$$a_n(n) = n$$

\$a_{n}\$ can be calculated directly from the input n. Knowledge about a previous state is not required. Both examples are doing the same thing and you might be tempted to say they are the same thing, but under the consideration of memory-less-ness1 they are not the same.

A filter may or may not be easy to implement in one way or the other. At first glance, requiring memory appears to be entirely disadvantageous. However, being able to access previous state directly can make an algorithm very quick for example.

A nice example are video codecs. To save space, some video codes only look at the differences between one frame and the next one. Of course this makes it hard to go to an arbitrary frame and jump around in the video, because in theory, every previous frame needs to be calculated. But if you are just watching the video continuously in one go, you will not experience that problem. Encoding a video this way can make its file size smaller.

Knowing that something is stateless2 gives you an advantage, because you know that you should always get the same result for the same input. If you don't, there's either something wrong with that part of your system or if you can guarantee that its working properly then there must be something else that has a memory and causing the differences.

A counter that breaks causality could look like this

$$a_{n} = a_{n+1} - 1$$

That's almost ridiculous. I cannot tell you the number without knowing the next number, but that in turn requires knowledge about its next number...where does this end...or rather start? \$\infty -1\$? What value does that have? Such filters cannot be built.


1 is that a word?

2 I guess that's a better word