LabVIEW
STFT Understanding
Hello
I am analyzing the accelaration of a free vibration of a concrete column. This is the signal.
This gives the Power Spectrum.
and this the STFT
What's my problem?
Well, in the Power spectrum you can see a large component at about 3 Hz (Fundamental Frequency) and this sameone, in the STFT, you can barely see it
My question is, why does this happen?
Another problem i have, related to the provious one, are the type of spectrum (Amplitude or Power) and values (RMS or Peak) that the STFT gives.
I read the LABview Help thousand of times but it doesn't say anyhing about that. I suposse that it gives Power Spectrum RMS. Am i right?
We have absolutely no idea what parameters you are using. Maybe attach your VI and your data so we can analyze what's happening.
(Btw, it's LabVIEW, not LABview.)
Sorry, I don't have a computer with the advanced signal processing toolkit in LabVIEW 2021. I only have 2020.
Can you do he following:
- Set the controls to reasonable values and make them the default. No need to tell me the settings.
- Use save for previous to save in LabVIEW 2020.
- Attach the down-converted code once more. It will be in a new folder.
Since the STFT uses a small sliding window, very low frequencies are masked, of course. You cannot tell the frequency of a banana.
Sorry, I don't have the vibration toolkit so I cannot run the upstream code. Please add an indicator (right-click wire...create indicators), run the VI, and turn the indicator into a constant. Save for previous and attach once more.
(I cannot tell what determines the loop rate. It is not reasonable to burn the CPU forever repeating the same code over and over with the same inputs.)
You have 1000 points per second, so a 3Hz signal has a period of ~333 points. Your window is a 512 point Blackman which is much (much!) less than 333 points at half height, so your 3Hz signal ends up as banana with no full cycles remaining.
Tell me if im correct, what you are telling me is this
What would be the solution? Decrece the Sample rate?
@FCance wrote:
There, thanks for tips.
Sorry, this is again in LabVIEW 2021 so I cannot open it on my current computer (despite the file name).
One possibility would be to increase the window size, but then you lose time resolution. I am sure there are more adaptive solutions, but I haven't studied it. How about something based on wavelets? Do you have the advanced signal processing toolkit?
