Understanding Time Invariance in Signals

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Discussion Overview

The discussion revolves around the concept of time invariance in signals, specifically examining the conditions under which a signal is considered time invariant or time variant. Participants explore various examples and techniques for determining time invariance, with a focus on the signal x[-n].

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses difficulty in determining time invariance and requests techniques applicable to various signals.
  • Another participant suggests that if the parameters beside a function contain a t term, the signal is time variant; if they are constants, the signal is time invariant.
  • Some participants assert that x[-n] is time invariant, questioning the reasoning behind its classification as time variant.
  • A participant references a solution manual that states x[-n] is not time invariant, indicating reliance on external sources for validation.
  • There is a discussion about the meaning of 'n', with one participant clarifying that it represents discrete time values.
  • A more detailed explanation of the test for time invariance is provided, illustrating the process and showing that x[-n] does not satisfy the conditions for time invariance.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether x[-n] is time invariant or not. Some argue in favor of its invariance, while others provide reasoning that supports its classification as time variant.

Contextual Notes

Participants reference external materials and solution manuals, indicating that their understanding may depend on these sources. The discussion includes various interpretations of the parameters involved in the signals.

kolycholy
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i usually have such a hard time determining whether a signal is time invariant or not ...

for example, why would x[-n] not be time-invariant?

please don't just tell me why x[-n] would not be time invariant ...
tell me techniques that I can apply to other signals too
 
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look at the parameters beside your function if they contain a t term then your signal is time varient while if the parameters are constants then the signal is time invarient.
 
angel23 said:
look at the parameters beside your function if they contain a t term then your signal is time varient while if the parameters are constants then the signal is time invarient.
that makes sense ... but then tell me why x[-n] is not time invariant?
 
do you see any t terms beside the function??
it is time invarient. why r u sure it isn't time invarient?
you can use this site to see the graph for check. http://www.jhu.edu/~signals/sys/resulta939.html

(i used unit step as an example)
 
Last edited by a moderator:
angel23 said:
do you see any t terms beside the function??
it is time invarient. why r u sure it isn't time invarient?
you can use this site to see the graph for check. http://www.jhu.edu/~signals/sys/resulta939.html

(i used unit step as an example)
i am so sure it isn't time invariant, because the solution manual said so ...
 
Last edited by a moderator:
What is 'n'?
 
desA said:
What is 'n'?
n is just time, but it assumes discrete value only
 
So, you've answered your own question.
 
desA said:
So, you've answered your own question.
no i did not ... please enlighten me ...
 
  • #10
i am sure it is time invarient my mind says so.
 
  • #11
The key is in understanding the test for time invariance.

To test: x[n] > DELAY > x[n-n0] > SYSTEM > w[n]
|
>>> SYSTEM > y[n] > DELAY > y[n-n0]

w[n] and y[n-n0] are equal if the system is time invariant

in the case of y[n]=x[-n], for the top approach, delaying the system results in n-n0 then we apply the system's effect of reversing JUST n, so w[n]=x[-n-n0]. With the second path, we apply the system and get y[n]=x[-n] and then apply the delay to get y[n-n0]=x[-(n-n0)]=x[-n+n0].

Since x[-n-n0] is not the same as x[-n+n0] the system is time VARIANT.
 

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