How does the FFT zero pad

Question

Avelino Amado 2019-5-23

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/463702-how-does-the-fft-zero-pad

评论： Matt J 2019-5-23

hw_ex.mat

I am using the Hilbert function for an analysis, and I would like to use the FFT method to get the imaginary part. What I am unsure of is how the FFT function is zero padding? I would like there to be an even number of zeros on each end of my data so that when I apply a window, my data are centered. Any help on this would be greatly appreciated.

example of code I'm using (i've attached an example mat file with the variable "prox")

a= hilbert(hann(length(prox)).*prox,2^8);

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Matt J 2019-5-23

3
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/463702-how-does-the-fft-zero-pad#answer_376377

编辑：Matt J 2019-5-23

在 MATLAB Online 中打开

Y = fft(X,n);

is the same as

Xp=X;
Xp(length(X)+1:n)=0;
Y=fft(Xp);

8 个评论
显示 6更早的评论隐藏 6更早的评论

Matt J 2019-5-23

编辑：Matt J 2019-5-23

在 MATLAB Online 中打开

They are simply inverses of one another. ifftshift is what you use if your signal origin is sampled in the center of the input array, X. In this example, the original continuous signal, before sampling, had value 4 at t=0.

X =
     0     0     0     0     1     2     3     4     3     2     1     0     0     0     0

By applying ifftshift, the samples are circulantly shifted so that the t-origin is sampled at the beginning of the array, which is where fft() expects it to be.

>> ifftshift(X)
ans =
     4     3     2     1     0     0     0     0     0     0     0     0     1     2     3

If we now feed this to fft(), the output spectrum will also have its origin (the frequency origin) at the beginning of the array

>> fft(ifftshift(X))
ans =
  Columns 1 through 10
   16.0000   12.7758    5.9786    1.0000    0.0783    1.0000    1.0000    0.1673    0.1673    1.0000
  Columns 11 through 15
    1.0000    0.0783    1.0000    5.9786   12.7758

But this is often not how we like to look at things, so fftshift can be used to shift the origin to the center of the array again,

>> fftshift(ans)
ans =
  Columns 1 through 10
    0.1673    1.0000    1.0000    0.0783    1.0000    5.9786   12.7758   16.0000   12.7758    5.9786
  Columns 11 through 15
    1.0000    0.0783    1.0000    1.0000    0.1673

Avelino Amado 2019-5-23

I have done the fft, ifftshift, but what I'm still unsure of is when I apply a window. My understanding would be I would window after I pad with zeros, is that correct?

Matt J 2019-5-23

That is something that only you can know (because it is your algorithm). But ifftshift would normally be done right before fft.

请先登录，再进行评论。

How does the FFT zero pad

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

采纳的回答

8 个评论
显示 6更早的评论隐藏 6更早的评论

更多回答（0 个）

另请参阅

类别

标签

Community Treasure Hunt

How does the FFT zero pad

0 个评论 显示 -2更早的评论隐藏 -2更早的评论

采纳的回答

8 个评论 显示 6更早的评论隐藏 6更早的评论

更多回答（0 个）

另请参阅

类别

标签

Community Treasure Hunt

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

8 个评论
显示 6更早的评论隐藏 6更早的评论