FFT doesn't always have frequency in all frames

1. How do you read frequencies in FFT?
2. How does sample rate affect FFT?
3. How many samples do I need for FFT?

How do you read frequencies in FFT?

The first bin in the FFT is DC (0 Hz), the second bin is Fs / N , where Fs is the sample rate and N is the size of the FFT. The next bin is 2 * Fs / N . To express this in general terms, the nth bin is n * Fs / N .

How does sample rate affect FFT?

The amplitude of the DFT (FFT) is proportional to the number of samples. Therefore, if you sample for twice as long at the same sampling frequency, or if you sample for the same duraiton but twice as fast, you will have twice as many data points, and the DFT amplitude will be twice as large. See examples below.

How many samples do I need for FFT?

The number of samples (N) in the FFT must be an integer power of 2. Therefore, N = 2^p, where p is a positive integer. This rule minimizes the number of multiplications—and therefore the computation time—needed to compute the coefficients of the Fourier series.