STFT

[debor]

 I would like to know  how to use the stft vi to display the spectrum. i have found one vi in the website by name "Moving stft pt.by pt". which calculates point by point  stft and displays the spectrum  but it gives incorrect readings for frequency level above 0.5 .Can somebody help me i am a new user in labview!!

[altenbach]

Well, LabVIEW has two different STFT, one plain and one pt-by-pt. Both have plenty of tuning paramters (sampling, windowing, etc.) that you might need to adjust for your particular situation.

It is difficult to understand what you mean by "incorrect readings for frequency level above 0.5", especially since you don't even give any units for that value.

Can you attach a VI containing some typical data, then tell us why you think the result is incorrect and what you think should be the correct result.

[debor]

Thanks Kudos!

Please find the VI attached .Here if i change the frequency input to the ptbypt sinewave say to 2 or 3 or 10, the spectrum displayed at the output of the STFT is incorrect , even though i scale the spectrum display appropriately .

[altenbach]

Since your time increment is 1 (you tap into [i]), the niquist frequency is 0.5 and anything above will give you an alias frequency. Why don't you place a chart indicator on the output of the "sine ptbypt" and you will see that e.g. with a frequency of "1", you are getting a flat line. You are sampling only once per period.

You are running the loop at 10ms increments, so you should divide [i] by 100 before wiring it to the sine "time" input. You can open the sine VI and look at the code, it is trivially simple. Good luck! 🙂

[debor]

Thanks kudos!!

I think  i would be able to do it now.

Thanks again.