FFT on dim > 1 returning negative values?

2012 4 18
2 个回答
回答已采纳
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1 个投票

I have a matrix that's [NxT] and I want the fft over the second dimension. But if I do this:
X = rand(100,200);
F1 = fft(X,[],2);
F2 = fft(X')';
figure;
subplot(2,1,1);
plot(real(F2)-real(F1));
subplot(2,1,2);
plot(imag(F2)-imag(F1));
Then, you can see, imag(F1) = -imag(F2). That is, F1 = conj(F2)
Is this a bug or am I misunderstanding something?

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 采纳的回答

Sean de Wolski
Sean de Wolski 2012-4-18

0 个投票

This is caused because you are taking the complex conjugate transpose ('). If you take the regular transpose (.') you will see the behavior you are expecting:
X = rand(100,200);
F1 = fft(X,[],2);
F2 = fft(X.').'; %note the .'
figure;
subplot(2,1,1);
plot(real(F2)-real(F1));
subplot(2,1,2);
plot(imag(F2)-imag(F1));
And for more info:
doc ctranspose %complex conjugate transpose
doc transpose %regular one

更多回答（1 个）

Keith
Keith 2012-4-18

0 个投票

Whooooooooooooooooooooooooa. I did NOT know that! I've been using it for years. You seriously just blew my mind.
Now I get to go spend the next year going through all of my code.
Thanks so much for setting me straight!

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