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Time Invariance

If the cascade

$ x(t) \to timedelay \to sys \to z(t) $

yields the same output as the cascade

$ x(t) \to sys \to timedelay \to z(t) $

for any $ t_{0} $, then the system is called "time invariant".

Example of Time Invariant system

$ X(t)\to sys \to Y(t)=15*X(t) $

$ X(t) \to timedelay \to Y(t)=X(t-t_{0}) \to sys \to Z(t)=15*Y(t)=15*X(t-t_{0}) $

$ X(t) \to sys \to Y(t)=15*X(t) \to timedelay \to Z(t)=15*Y(t-t_{0})=15*X(t-t_{0}) $

Since both the outputs are same we can say that the given system is time invariant.


Example of a Non-Time Invariant system

$ X(t)\to sys \to Y(t)=15*X(2t) $

$ X(t) \to timedelay \to Y(t)=X(t-t_{0}) \to sys \to Z(t)=15*Y(2t)=15*X(2t-t_{0}) $

$ X(t) \to sys \to Y(t)=15*X(2t) \to timedelay \to Z(t)=15*Y(t-t_{0})=15*X(2(t-t_{0}))=15*X(2t-2t_{0}) $

Since both the outputs are different or are not equal we can say that the given system is not time invariant.

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Ph.D. 2007, working on developing cool imaging technologies for digital cameras, camera phones, and video surveillance cameras.

Buyue Zhang