How do I convert a time domain signal to frequency domain?

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Yogesh Babu
Yogesh Babu 2014-5-12
评论： dpb 2014-5-28
I am having some problems converting a time domain signal into frequency domain--I don't have constant sampling time.
In time domain, is this the way to plot the graph?
value = [42007935 111212895 184546560 238219725 238219725 184546560 111212895 42007935]; time = [0, ,5,1.25,2, 2,5,5, 4, 4.5, 6, 7, 8 ];
plot (time, value);
X and/or Y arguments ignored.
Could anyone help me plot a signal that does not have constant time?
  2 个评论
Star Strider
Star Strider 2014-5-12
If you want to plot them, your time and value arrays have to have the same number of elements. They don’t.
dpb
dpb 2014-5-12
编辑：dpb 2014-5-12
Look for NUFFT (non-uniform FFT) literature -- there's quite a lot.
You surely have the sample times or at least delta-t's, don't you? W/o that you have nothing.
Don't believe TMW has an implementation packaged as yet, anyway.
ADDENDUM:
You might check the File Exchange for submissions and of course there's always a search including Matlab in the search.

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回答（2 个）

Vilnis Liepins
Vilnis Liepins 2014-5-13
Hello,
To convert nonuniform time domain data to frequency domain you can use Extended DFT (program NEDFT.m) available on fileexchange http://www.mathworks.com/matlabcentral/fileexchange/11020-extended-dft
NEDFT call line:
F=nedft(value,time,fn);
where frequencies [fn] could be generated as
fn=[-ceil((N-1)/2):floor((N-1)/2)]/N/T;
As I see from your example [time] data the mean sampling frequency T=1. The length of DFT N usually is set equal to or greater than the length(value) that, as Star Strider commented, should have the same dimensions as [time] array.
After that you also be able to get back constant time version of your data [value1] by applying IFFT to the result of NEDFT, call line:
value1=real(ifft(fftshift(F)));
  1 个评论
Yogesh Babu
Yogesh Babu 2014-5-16
Hello,
Thank you very much for your answer. May i know in that program where should i specify time and value parameter

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Vilnis Liepins
Vilnis Liepins 2014-5-19
编辑：Vilnis Liepins 2014-5-19
Actually they are specified in your post:
>> value = [42007935 111212895 184546560 238219725 238219725 184546560 111212895 42007935];
>> time = [0, ,5,1.25,2, 2,5,5, 4, 4.5, 6, 7, 8 ];
Note that you need to correct the 'time' vector: time(1)<time(2)<time(3)<... and length(value) must be equal to length(time):
- the signal value(1)=42007935 is taken at time(1),
- the signal value(2)=111212895 is taken at time(2), and so on.
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Yogesh Babu
Yogesh Babu 2014-5-28
Hello Vilnis Liepins, I AM UNABLE TO FIND your updates which is on 19 May 2014 at 11:15.Could you please guide where it is exactly
dpb
dpb 2014-5-28
It's about four or five comments past...you only need to use the
"Show N older comments"
button to see the previous that have been collapsed by the viewer by default.

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