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-   -   causal and time variant (http://www.physicsforums.com/showthread.php?t=49650)

skan Oct25-04 09:30 PM

causal and time variant
 
1.g(t) = f(t) + f(-t)
2. g(t)= f(t/2)
can someone pls explain why these functions are time variant and non causal

ynot Nov1-04 07:07 PM

Here are my guesses:

They're time variant because they vary with the time variable "t".

The first one is non-causal because because it uses -t (negative time), so the function could have a value before time 0, usually the definition of when an initial event happens. The output could change before the stimulating event.

The second one is non causal because the division of time by 2. This means that the output could change at t=1 second, caused by an event at 2 seconds. This would be non-causal because the output would occur before the stimulating event.

The basic definition of causal is that the output cannot change before an initial event, and all event functions, (closing of a switch, pushing a button..) are defined for t => 0.


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