Overlap data

[wojtas601]

Hello

I have record data in text files (1 channel (1 vector), 16384 S/s (16384Hz), band 5000Hz). This dates represent vibration.

I would like to overlap this data as I shown on picture (attachment). It will be much longer vector then I shown of course (16384 samples per vector)-it’s example. It will be possible to change overlap value (25%, 50%, 75%).

My question is: what I ought to know before I start my project? I wont to do FFT analysis after. It will be mathematically correct analysis?

[Craig_D]

I see no problem with shifting your data like that.  After you read the file into arrays in LabVIEW, you can use the split 1D array VI to return the portion of the array you want.  You can determine the index of the split using the percentage input and the length of the array and rounding to the nearest integer.  You can then concatenate this array with the new data you want to add.

In terms of the FFT, you can take your resulting array and build it into a waveform using the reciprocal of the rate as the dt parameter.  The result will be a mathematically correct FFT if you are sure of the acquisition rate.  In terms of this being the correct analysis, it depends on what you are trying to measure.  Think about it this way, if you take 1000 points of a 60 Hz sine wave and concatenate it with 1000 points of a 120 Hz sine wave, the resulting FFT power spectrum graph will show two spikes of equal height: one at 60 Hz and one at 120 Hz.

Please let me know if you have any more questions!

Regards,

Craig Davis

Applications Engineer

National Instruments

http://www.ni.com/support