Re: derivative of an input signal

[Przemek Klosowski]

On 05/07/2013 08:41 AM, primi wrote:
> i hav a low pass filter output..i need to find the derivative of it, becoz
> the derivative converts
> the slowly changing output of the LPF into a more defined and faster
> changing waveform.
>
> some one please help me.iam new to octave.
>
> i tried diff(), gradient() and polyder()..bt i didint get the required
> output..

Please provide more information about your problem: what data do you 
have initially, and why are the results you get not satisfactory?

Basically, if you have a vector X with evenly spaced samples of some 
signal x(t) for t=t0+dt*i where i=1...N, then diff(X)/dt will be an 
approximation to the time derivative of that signal. How good is this 
approximation is determined by how fast the signal changes with respect 
to the time interval between the samples. You say it's slow, so it 
should be OK.

If your samples are not evenly spaced in time and the time interval 
values are in vector dt (which then would be only 1..N-1 long), you can 
do diff(X)./dt. Again, it depends on what your input data is.