create FFT size

[celynn]

Hi, Can anyone advise me how to create my FFT size from my 1D array say 40926?

[celynn]

Can someone please advise me. i am desperately in need for these application to use in a FFT power spectrum.vi

My sample size is 40962 and i am aware that the FFT length takes in 2^n length i have no idea how to i break up my 1D array into a desired(Fast FFT size) block  and create a wave form.vi to input into the FFT power spectrum?

[LV_Pro]

There are several articles on the National Instruments website that refer to this subject. I would advise looking into them before you attempt to use the FFT. The FFT is a very powerful tool, but as with most powerful tools if it is not understood the results you get, while appearing useful, may be invalid. I haven't used LV's FFT tools recently, but in the past you could feed any size data (within reason) to the function. If it was a 2^N length an FFT was performed, other sizes would cause it to be evaluated with a DFT, with a sizable increase in execution time. As this was on pretty slow machines when I was doing it (100MHz Pentium) it was pretty critical that the data conform to the "power of two" criteria. One technique, to be used cautiously, is padding with zeros. Doing a search on this website (NI) using "FFT padding" returns a number of articles, i.e.:http://zone.ni.com/devzone/conceptd.nsf/webmain/81227DF4C2952C1A86256CA80053F322, which way the pros and cons of padding.

P.M.

[Joe_H]

could not type fast enough go with what LV says.

Message Edited by jhoskins on 09-01-2005 09:19 AM

[celynn]

yes i have read. i was thinking how to break my array into blocks?

i)Divide with a constant ie 10 to get approxi 4096?

 ii)follow form a 2D array ?

iii)feed into the FFT power spectrum.vi ?

[celynn]

Hi,

The sampling frequency is 1000Hz and FFT length will be 4096. How do i create the vi for that range from my samples?

[altenbach]

Many LabVIEW versions ago, you would get a huge performance hit unless the array size was an integer power of two.

This is no longer true! Many other sizes will give you very similar speeds using a mixed radix Cooley-Tukey algorithm, the speed vs. size dependecy is a bit jagged, but often it is not worth worrying about it. 🙂

The NI algoritms seem competitive with e.g. the FFTW . Make sure you're not wasting your time 😉

Have you benchmarked your transform time?

[celynn]

There is a reason why i want to spilt my sample size because apparently when i bulid a coherence function using FFT Power spectrum and FFT cross.vi the results was all 1. And i read that somehow i got to spilt the data into the FFT size.

[altenbach]

The result should not depend on the data size. If it does, something else is wrong. :(

Could you attach a small example (containing your data in a diagram constant)?

[celynn]

In the attached vi. i only use 2 1D array.