FFT gives different answers according to what axis its applied to
2 views (last 30 days)
Show older comments
I am seeing some behaviour with fft which I did not expect. It appears the output of the FFT depends on the axis to which it is applied.
In the following example - doing the FFT of the same data vector differs depending on whether you do fft, axis 1, data (100,1) compared to doing fft on axis 2 with the data shaped (1,100). I would expect these to be the same?
r = randn(100,1);
rf = fft(r);
rf1 = fft(r,[],1);
rt = r';
rtf = fft(rt,[],2);
sum(abs(rf-rf1))
sum(abs(rf-rtf'))
Even more worrying - ifft behaves the same way, so when reconstructing the original random array:
irf = ifft(rf);
irtf = ifft(rtf'); % this doesn't match irf
irtf2 = ifft(rtf,[],2); % this does match irf
Shouldn't irtf and irtf2 be the same? This seems quite dangerous to me - I thought it is standard to transpose data as required in Matlab and I would not expect to see differences in the output here.
0 Comments
Accepted Answer
Guillaume
on 20 Feb 2015
Edited: Guillaume
on 20 Feb 2015
The results are the same. Your problem is with the usage of ' aka ctranspose which computes the conjugate transpose of the input, i.e. not only transpose the vector but returns the conjugate of complex values.
r = randn(100, 1);
isequal(fft(r), fft(r, [], 1)) %return true
isequal(fft(r), fft(r.', [], 2).') %return true
edit: d'oh. Got my demo wrong!
0 Comments
More Answers (1)
Thomas Koelen
on 20 Feb 2015
Edited: Thomas Koelen
on 20 Feb 2015
fft behaves the same in:
clc
close all
clear all
A=rand(1,100);
B=A';
X1=abs(fft(A));
X2=abs(fft(B));
figure(1)
plot(X1)
figure(2)
plot(X2)
Try this.
0 Comments
See Also
Categories
Find more on Logical in Help Center and File Exchange
Products
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!