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I'm trying to complete one task: Download some FM modulated signals from page http://www.w1hkj.com/modes/index.htm. Calculate their spectrograms, i.e. short-time Fourier transform. Observe different FM patterns. Do some literature investigations. Try to recover bits that are coded by different frequency values (states).

At first I displayed a spectrogram:

clc; 
clear; 
close all;

[y,fs] = audioread('Olivia16-500.wav');
spectrogram(y, 512,256,512, fs, 'yaxis'); 

And the other part is: Try to recover bits that are coded by different frequency values (states). I guess I should demodulate signal, so I did the following:

Nx = length(y); 
n= 0:Nx-1;
dt = 1/fs;
t = dt*n;

fc=200;
figure(1)
z=demod(y,fc,fs,'fm');
plot(t,y, 'c', t,z, 'b');
xlabel('Time'); ylabel('Amplitude');
legend('Modulated signal', 'Recovered signal');

And the output:

enter image description here

Did I understand the task correctly or did I miss something? Thank you for any advices.

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    $\begingroup$ you say you should be getting bits, but I don't see bits being output anywhere, so this might be a step in the right direction, but it's not the answer. Other than that, you're asking us to verify you understood an assignment correctly that we've never seen but only heard from you about – and your reproduction of the problem will likely contain the same mistakes in understanding you want to ask about, so this is inanswerable! $\endgroup$ – Marcus Müller Jan 22 at 12:44
  • $\begingroup$ I edited my question by adding exact description of the task $\endgroup$ – cherryBom Jan 22 at 13:13
  • $\begingroup$ I don't believe any of the signals in the page you linked are frequency modulated. $\endgroup$ – MBaz Jan 22 at 14:29
  • $\begingroup$ @MBaz I also have doubts but it's from my professor's book and there is: Examples of different FM modulated HAM signals and their spectra are presented in w1hkj.com/modes/index.htm. $\endgroup$ – cherryBom Jan 22 at 16:29
  • $\begingroup$ Why don't ask your professor then? I'm sure (s)he'll be happy to help you. $\endgroup$ – MBaz Jan 22 at 16:41

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