问题
From the question: How do I obtain the frequencies of each value in an FFT?
I have a similar question. I understand the answers to the previous question, but I would like further clarification on frequency. Is frequency the same as the index?
Let's go for an example: let's assume we have an array (1X200) of data in MATLAB. When you apply 'abs(fft)' for that array it gives the same size array as the result (1X200). So, does this mean this array contains magnitude? Does this mean the indices of these magnitudes are the frequencies? Like 1, 2, 3, 4...200? Or, if this assumption is wrong, please tell me how to find the frequency from the magnitude.
回答1:
Instead of using the FFT directly you can use MATLAB's periodogram function, which takes care of a lot of the housekeeping for you, and which will plot the X (frequency axis) correctly if you supply the sample rate. See e.g. this answer.
For clarification though, the index of the FFT corresponds to frequency, and the magnitude of the complex value at each frequency (index) tells you the amplitude of the signal at that frequency.
来源：https://stackoverflow.com/questions/8824021/how-to-find-the-frequency-from-fft-data-in-matlab