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

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?

[Star Strider]

If you want to plot them, your time and value arrays have to have the same number of elements. They don’t.

[dpb]

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.

Answer 1

[Vilnis Liepins]

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)));

[Yogesh Babu]

Hello,

Thank you very much for your answer. May i know in that program where should i specify time and value parameter

Answer 2

[Vilnis Liepins]

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.

[Yogesh Babu]

Hello Vilnis Liepins, Thank you very much for your solution.. In your Nedft.m which you have programmed where exactly i should specify the parameters values of 1.Value 2.time

Thanks in advance!!!

[Vilnis Liepins]

Hello again! Input parameters for nedft.m should be specified or calculated outside the function. Example code:

value = [42007935 111212895 184546560 238219725 238219725 184546560 111212895 42007935];

time = [0.5 1.25 2 2.55 4 4.5 6 7];

N=length(value); T=1;

fn=[-ceil((N-1)/2):floor((N-1)/2)]/N/T;

F=nedft(value,time,fn);

value1=round(real(ifft(fftshift(F))));

time1=0:N-1;

plot(time,value,'-g',time1,value1,'-b');

[Yogesh Babu]

Hello When i run your i get the following error Cannot find an exact (case-sensitive) match for 'nedft.m'

The closest match is C:\Users\yogesh\Desktop\NEDFT.M

To change the file extension, cd to the file's folder, type: movefile NEDFT.M NEDFT.m_bad; movefile NEDFT.m_bad NEDFT.m and then cd back.

Let me know how to solve this situation

[Vilnis Liepins]

Nedft.m is just am example script saved as text file and it was created on old Matlab version. I don't have Matlab installed anymore, but I recommend to do something like that: (1) create a new *.m file, (2) copy all the text from nedft.m and paste it into this new matlab file, (3) then save it as nedft.m in your Matlab workspace.

[Yogesh Babu]

Hello, I have followed your intructions but there is still error after doing that. I PASTEDn from old NEDFT file and created new nedft file but there i am getting following error

Error using nedft (line 97) Incorrect size of input argument fn. See Help nedft.

[Vilnis Liepins]

Hello, Try to execute example code posted by me on 19 May 2014 at 11:15. Copy/paste all lines in Matlab window. In result you should get two plots in one window: nonuniform 'value' and uniform 'value1' get from the nedft.m output. Is it works?

[Yogesh Babu]

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]

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.