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User defined FFT function parameter not recognized

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farzad
farzad on 29 Nov 2020
Commented: farzad on 29 Nov 2020
Hi All
I am trying to use the FFT function explained in this link
but when I use a nx2 signal and a scalar value for ScanRate I get an error :
Shall someone tell why isnt the function getting my input value
Unrecognized function or variable 'ScanRate'.
% function [frq, amp, phase] = simpleFFT( signal, ScanRate)
% Purpose: perform an FFT of a real-valued input signal, and generate the single-sided
% output, in amplitude and phase, scaled to the same units as the input.
%inputs:
% signal: the signal to transform
% ScanRate: the sampling frequency (in Hertz)
% outputs:
% frq: a vector of frequency points (in Hertz)
% amp: a vector of amplitudes (same units as the input signal)
% phase: a vector of phases (in radians)
function [frq, amp, phase] = simpleFFT( signal, ScanRate)
n = length(signal);
z = fft(signal, n); %do the actual work
%generate the vector of frequencies
halfn = floor(n / 2)+1;
deltaf = 1 / ( n / ScanRate);
frq = (0:(halfn-1)) * deltaf;
% convert from 2 sided spectrum to 1 sided
%(assuming that the input is a real signal)
amp(1) = abs(z(1)) ./ (n);
amp(2:(halfn-1)) = abs(z(2:(halfn-1))) ./ (n / 2);
amp(halfn) = abs(z(halfn)) ./ (n);
phase = angle(z(1:halfn));

  3 Comments

Image Analyst
Image Analyst on 29 Nov 2020
Show us how you instantiated the arguments that you passed in, and the line of code where you actually called the function with those variables.
You didn't just click the green Run triangle did you? Because that's a function where it won't run unless you pass in input arguments, which clicking the green run triangle won't do - it has no idea what you'd want for those variables.
farzad
farzad on 29 Nov 2020
it was a stupid wrong letter typo from me. sorry
when I do
plot( frq,amp)
the maximum value is not 1
applying on other signals with small values, around 1 to 4 mm , I get huge frf values of 180 mm or so
farzad
farzad on 29 Nov 2020
I think it has not mentioned how should be the time and amplitude be arranged in the signal matrix

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Answers (1)

John D'Errico
John D'Errico on 29 Nov 2020
Edited: John D'Errico on 29 Nov 2020
How did you call the function? When you get an error, report the ENTIRE error message, everyting in red. This lets us know how you called the function.
Just because you have a variable named ScanRate in your workspace does not tell MATLAB to automatically pass it into your function.
If you want the function to see the variable, then you NEED to pass it in. Call it like this:
[frq, amp, phase] = simpleFFT( signal, ScanRate);
So you need ot pass in TWO arguments to your function. They can have any names as you pass them in, but inside the function, now ScanRate will be defined.
If you don't, well, then MATLAB will generate an error - the one you got.

  5 Comments

Show 2 older comments
farzad
farzad on 29 Nov 2020
when I do
plot( frq,amp)
the maximum value is not 1
farzad
farzad on 29 Nov 2020
applying on other signals with small values, around 1 to 4 mm , I get huge frf values of 180 mm or so
farzad
farzad on 29 Nov 2020
I think it has not mentioned how should be the time and amplitude be arranged in the signal matrix

Sign in to comment.

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