Real time Numeric Derivative

[alipekcan]

Hi, There has been a discussion on
http://www.cplusplus.com/forum/general/103788/

about taking the first derivative but this is applied to a already measure data as I understand.

Is this method for real-time?
Alg#3 = 4/3 (f(xn+1)-f(xn-1))/2t - 1/3 (f(xn+2) - f(xn-2)) / 4t

In real-time case isn't it that if I take xn+2 as xn then I will get t*2 delay for the derivative calculations.

Do I have to do some kind of prediction for the next step?
I need to calculate a good approximation of the derivative in real time.
Backwards Euler :f(xn)-f(xn-1)/ t will start to fail if Sampling (t) increases.
Does anyone know a good method for derivative in real-time applications?

Thanks in advance.