Struggling with fft of a signal.

I am ultimently wanting to get the velocity profile from the signal (data file is too big to attach), but currently I am trying to find the frequency of the selected data. If ther is a better way let me know.

Answer 1

[Dave B]

As signal was defined with signal=readtable(... I bet signal is a table, I think you want to pass in one of the listed types to fft. What's in signal?

class(signal) % will tell you what class the variable is, I think it's a table

Is it a table with one variable? Is the variable called Var1

head(signal) % wlil show you the top few rows of the table, I think it's a single Variable called Var1

Assuming table is a variable called Var1, and that the variable contains numeric data (i.e. one of the listed types in the error message you received), consider

x = fft(signal.Var1, n);

But of course if it's a different variable you'll need to use that name, and if it's not numeric you'll have to convert it one way or another.

Also I'm a little wary of your use of pwelch on the fft result, are you sure that's what you wanted?

Answer 2

[shawn finley]

Yes it is a table, below is a image of some of the data. I am looking at CH4 & time data in the code. I was wanting to use the fft to pull the frequency but i seem to have confused myself with the fft example.

[Dave B]

@shawn finley - Looking at your screenshot I suspect it's Var2 where I had Var1 above, but to be clear the question 'is it a table?' is about the MATLAB representation, not the excel one...

Does this command work? What does it produce?

head(signal)

[shawn finley]

also when i impliment the head () it returns the below response.

ans=

8x3 table

x_0_0002 x_0_0003 x0_03

________ ________ _______

-0.0002 -0.0026016 0.0099219

-0.0002 -0.0026 0.01

-0.0002 -0.0026 0.03

-0.0002 -0.0030016 0.0099219

-0.0002 -0.0034016 0.0099219

-0.0002 -0.0034 0.03

-0.0002 -0.0026016 -0.010078

-0.0002 -0.0022 -0.01

[shawn finley]

yes when i use class() it returns 'tabel'

[Dave B]

I'll just write this again in case it's unclear, because this is the answer to your original post:

• The reason you got an error: fft needs numeric information, you were passing in a table.
• The solution is: extract the numeric information that you want the transform of, and pass that in to fft.
• Probably you don't want to call pwelch on the result of fft...unless that's what you want, the PSD of the DFT.

The values in your table don't seem to line up, and it looks to me like the variable names (the part above the line) are coming from values. To do an fft on the data you've imported here, e.g. for the second vairable, you'd do:

f = fft(signal.x_0_0003, n);

But I'd suggest you work a bit on cleaning up the import: maybe include the row with the variable names? or use the 'ReadVariableNames' option to explicitly specify that you don't want them? Scrolling back and forth between your screenshot and imported data I couldn't see any obvious way to line them up.

[shawn finley]

i hvae removed the pwelch because i believe that you are corret that is not what i am looking for. I below i have my new code and what is is currently outputting. Now just to make sure I am not doing something completely wron the fft need both the time column and the CH4 column to work correct (i.e. can't be just the CH4 column)? I will work on the cleanning up the import. THANKS!!

[Dave B]

I think if you point to the correct part of your spreadsheet it should show CH4 for the variable name and that will simplify your life significantly.

The one you pasted in the image has a different name (one less zero) for the second variable from the text pasted above (?) hence the error.

[shawn finley]

Thanks for your time and quick response. I have working code!!