Causal System or not ?

Discussion in 'Homework Help' started by Knigh7, Feb 25, 2014.

  1. Knigh7

    Thread Starter New Member

    Feb 25, 2014
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    I cannot understand why this system isn't causal?
    As a causal system only depends upon its current or previous inputs then this system should also be causal but the solutions say otherwise i don't get why...

    y(t) = x(t/3)

    Also I can understand that the system is stable but I was wondering if someone can show me mathematically as well?

    Thanks...
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    What value of t does y(t) depend on if t = -9 sec?
     
  3. Knigh7

    Thread Starter New Member

    Feb 25, 2014
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    Oh...i see the point you made and it does make sense...but i had the idea since causal signals are zero for t < 0 so we are only supposed to check for t > 0 ?

    And if thats not the case then how can we say that y(t) = x(t) is a causal signal because it can have any value x(t) for example for t = -3 ?
    Do we have to assume that its zero for t < 0?

    Thanks for the reply :)
     
  4. WBahn

    Moderator

    Mar 31, 2012
    17,750
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    Signals are signals. they are neither causal nor non causal. It is systems that are causal or non causal.

    A causal system means that it's response at time t is not dependent on the signal that arrive after time t. It does NOT mean that the signal does not exist after time t.

    Similarly, a causal system's response at time t CAN depend on any part of a signal that occurred at or prior to time t. There is nothing magical about the moment in time that we hang the label t=0 to. What matters is the system response to signals at moments in time relative to the time of interest.
     
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