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From: Jerry Avins on 5 Feb 2010 11:23 rsk wrote: > All > > This might have been discussed multiple times. However, I am new comer to > the site. > > My question is given below. I wanted to put plots but could not find the > way to do it. > > > > I took a sine curve from 0-1 s( 1Hz)and 'fft'ed it in MATLAB. I get unit > magnitude at 1 Hz as expected. Sampling rate 1024. > > Then I zero padded it till 2 sec. So, total lines = 2048. The fft then > showed major peak(0.5 magnitude) at 2 Hz with other higher frequencies > present. > > All articles on zero padding say that, zero padding does not add more > information but just makes existing information more fine. Going by that, > the fft in second case should also have shown peak at 1 Hz. May I know > where did I go wrong? > > In Summary, should there be a difference in the usual sine wave and zero > padded sine wave with respect to frequency spectrum? You tell us what you intended to do, but I can't analyze your result without knowing what you actually did. I can guess, though. Have you misinterpreted the frequencies associated with the bins in the second case? zero padding will make twice as many bins, so what was 1 Hz before becomes 0.5 Hz after padding. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
From: Jerry Avins on 6 Feb 2010 11:19
rsk wrote: ... > I am still confused with the outcome. Did you mean, even if I got 0.5 Hz as > the frequency, it is actually 1 HZ?. Also, why did the amplitude change? An FFT provides a series of numbers in sequence. Doubling the count of the input data doubles the count of the output data. How is a frequency assigned to a particular output datum? Hint: the answer depends on the sampling frequency and the time interval. Zero padding effectively doubles the sample rate. Jerry -- Engineering is the art of making what you want from things you can get. ����������������������������������������������������������������������� |