I am trying to figure out the frequencies in a Pulse Code Modulated (PCM) signal. The PCM samples are in .wav files. I am trying to produce a spectrograph of the frequencies in the files. I have coded the Fast Fourier Transform, but that only gives me the coefficients. What should I multiply times the output of the FFT to get the frequencies?

Thanks for your help. I have been fumbling through textbooks, but although I have found some great information on how the FFT works, I have not been able to find the last step.

 

To do this you'll need to know the sampling frequency and note that you'll need to consider only the first half of your FFT output vector
(This part correspoonds to the positive frequencies).

The frquencies are:

f=(Fs/N) * k

Fs - your sampling frequency
N - the length of your FFT
k- the vector index

For example: Fs = 1Khz, N=1024, so:
First fft coefficient corresponds to 0 Hz.
Second fft coefficient corresponds to 0.97Hz
.
.
.
512'th fft coefficient corresponds to 500Hz.

Hope this helps