IFFT of Convolution equivalence
6 ビュー (過去 30 日間)
古いコメントを表示
I was trying out the equation IFFT(x*y) = IFFT(x).IFFT(y), where both 'x' and 'y' are complex numbers. The 'x' and 'y' are 1x8 matrices. The result was different for both sides of the equation. However i was able to prove that x*y = IFFT[ FFT(x).FFT(y)] and FFT(x*y)=FFT(x).FFT(y). What did I miss out when I tried to solve the IFFT relation? Please let me know of any source that has helpful content regarding this relationship
0 件のコメント
採用された回答
Ridwan Alam
2019 年 12 月 18 日
編集済み: Ridwan Alam
2019 年 12 月 18 日
Sorry for the confusion. Here is what I tried:
a1 = randi(50,8,1);
b1 = randi(50,8,1);
x = complex(a1,b1)
a2 = randi(50,8,1);
b2 = randi(50,8,1);
y = complex(a2,b2)
xifft = ifft(x)
yifft = ifft(y)
mifft = xifft.*yifft
z = cconv(x,y,length(x))
zifft = ifft(z)./length(z)
% zifft-mifft is almost zero
Summary: you need circular convolution in frequency domain instead of linear convulation.
More details:
その他の回答 (1 件)
David Goodmanson
2019 年 12 月 18 日
編集済み: David Goodmanson
2019 年 12 月 19 日
Hi JOB/Ridwan
Here is a small example where a and b are padded with zeros, so that regular convolution can be compared with convolution by fft and ifft. (The zeros ensure that for fft and ifft, which do circular convolution, the nonzero parts can't overlap by going 'the other way around the circle.'
a = [(1:3)+i*(2:4) zeros(1,4)]
b = [(3:5)+i*(4:6) zeros(1,4)]
n = length(a)
convab = conv(a,b)
convab1 = ifft(fft(a).*fft(b))
convab2 = n*fft(ifft(a).*ifft(b))
All of these results agree (not counting that the conv result is a longer vector and contains more zeros than the other two). For the convab2 result you have to multiply by an extra factor of n. This is because the Matlab ifft algorithm contains an overall factor of (1/n) and the fft does not.
3 件のコメント
David Goodmanson
2019 年 12 月 19 日
HI Ridwan,
what you say is true, but it's because a &b have different length than convab. One could do a circular convolution of a and b by a method other than ifft and then compare, but this example just pads out a and b to simulate a linear convolution as before, then pads them out again to be the same length as convab:
a = [(1:3)+i*(2:4) zeros(1,4)];
b = [(3:5)+i*(4:6) zeros(1,4)];
n = length(a);
convab = conv(a,b)
nconvab = length(convab);
apad = [a zeros(1,nconvab-n)]
bpad = [b zeros(1,nconvab-n)]
% the following two expressions are the same
ifft(convab)
nconvab*ifft(apad).*ifft(bpad)
参考
カテゴリ
Help Center および File Exchange で Fourier Analysis and Filtering についてさらに検索
製品
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!