# how to extend time after fft ?

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omar thamer 2013년 11월 8일
댓글: omar thamer 2013년 11월 13일
I have a signal runs for 500 seconds ( 1:1:500 sec) after i did the fft, i want to go back to time domain and see how the signal looks like for 1000 Seconds (1:1:1000) ? Any advice how to extend the time ? Is it better to do it in frequency domain ? Is it simply to add the sines and cosines with loops ?
Thank you

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### 채택된 답변

Vilnis Liepins 2013년 11월 11일
ifft(edft(YourData,1500),1500)
Signal for 1000 sec will be ifft output 1:1000.
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omar thamer 2013년 11월 12일
Thank you Vilnis, really helped a lot.Though, results not as expected for real signal but absolutely true for periodic computer generated one. I am wondering if you can advice on my signal which i attached here. As you can see a lot o discontinuities and no clear center frequency.
Any advice how to get periodicity to my signal ? I tried with STFT it also yields no results.
omar thamer 2013년 11월 13일
Its not working properly .. Have a look to my code:
t=1:1:400;
x=sin(2*pi*t*1/20);
X=edft(x,500);
Y=ifft(X,500);
the outcome for extended signal has different magnitude and frequency.

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### 추가 답변 (1개)

Walter Roberson 2013년 11월 8일
Try
ifft(fft(YourData), 1000)
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omar thamer 2013년 11월 8일
it will be padded with zeros then and return a complex vector. Its not working
Walter Roberson 2013년 11월 11일
fft() assumes that its data is periodic, so to get 1000 seconds of output, ifft() to get the 500 second output and then replicate it.

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