fft second argument function matlab

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Nuchto
Nuchto 2017-12-16
评论： Star Strider 2017-12-17
Hi,
To do the fft I noticed in my homework the second argument is the sampling frequency but I don't understand why. Anyone with a simple explanation? Thank you.

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回答（2 个）

Jan
Jan 2017-12-16
编辑：Jan 2017-12-16
Are you talking about the built-in function fft? Then the 2nd argument is not the sampling frequency. See https://www.mathworks.com/help/matlab/ref/fft.html:
Y = fft(X,n) returns the n-point DFT.
The meaning of n is explained by the formula: https://www.mathworks.com/help/matlab/ref/fft.html#buuutyt-6: It is the sum over n elements.
If you do not mean the built-in fft function, please explain this, because then the readers cannot know, what you are talking about.
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Nuchto
Nuchto 2017-12-16
Thanks, Jan. In my notes from class, I have:
R=fft(audioclip.audio(winbeginning:winbeginning+winLength),audioclip.fs);
As you can see, the audioclip.audio and audioclip.fs are the output s from audioread. Does this make sense? Winbeginning and winlength are just the windows for which the fft is computed.
Jan
Jan 2017-12-17
Well, this might make sense, or in other words, this need not be nonsense. Of course you can use the sampling frequency to define the number of elements for the Fourier transformation. The question is, if you really want this. So please explain, what this code line should achieve. Without knowing this detail, it is impossible to decide, if the code is correct or not. If e.g. winLength is 1, calling fft is nonsense.

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Star Strider
Star Strider 2017-12-16
The audioread (link) function returns only the (Nx2) data, and sampling frequency.
Your function returns a segment of length ‘winLength’ of the data, and (apparently) calculates the Fourier transform of length ‘Fs’. Here, 'Fs' is most likely a 1-second window of the signal.
See fft (link) for documentation on the Fourier transform.
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Nuchto
Nuchto 2017-12-17
The fft function is the Matlab inbuilt function. N argument is the n-point value.
Star Strider
Star Strider 2017-12-17
I am aware of that.
If you use the sampling frequency ‘fs’ as the length of the Fourier transform, you may not get all the frequency resolution you otherwise would if your signal is more than one second long. I always use either the length of the signal, or nextpow2 of the length to specify the Fourier transform length.

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