a causal system?? the biggest contradiction! Hey guys - can someone tell me why i get two contradicting asnwers... i am asked to determine whether the following system is causal or not: y[k+1] + 0.64y[k-1] = x[k] + 2x[k-1] method 1: put k = k-1 (delay the signal by one unit) therefore , y[k] = -0.64y[k-2] +x[k-1] +2x[k-2] it is cleary that this signal is casual because a present ouptut value is dependent on present and past input/output values. method 2: put k = k+1 therefore, 0.64y[k] = y[k+2] +x[k+1] +2x[k] now the system is clearly not causal!!!! whys is this happening!! i can't see any error in my maths. Also , shifting should not alter the signal characteristics!!! please someone help!!! calculating a tranfer function clearly shows the system IS causal-----so what is up with metho 2?????????/ thanks!!!