# Causal System or not ?

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

1. ### Knigh7 Thread Starter New Member

Feb 25, 2014
6
0
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
18,096
4,920
What value of t does y(t) depend on if t = -9 sec?

3. ### Knigh7 Thread Starter New Member

Feb 25, 2014
6
0
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?