Demod_FM

clear all;
clc;
clf;

fm = 1/(0.5e-3);                      % freq of message signal
Am = 10;                         % amplitude

fc = 100000;                     % freq of carrier signal
Ac = 10;

Fs = 4*fc;                          % sampling freq
n = 1:(Fs/fm)*0.5;

signal_a = Am*sin( 2*pi*fm/Fs*n ) - 5;       % signal(1)

signal_single = [ signal_a  ];

signal = [ signal_single signal_single signal_single signal_single signal_single ];

n = 0:length(signal)-1;

subplot(411); plot(n, signal);      % message signal

%% Modulation
t = n/Fs;
T = 1/Fs;

delta_f = 50000;                     % dewiacja cz?stotliwo?ci [Hz]
kf = delta_f / max(signal);
beta = delta_f/fm;

calka = cumsum(signal)*T;

mod_fm = Ac*cos(2*pi*fc*t + 2*pi*kf*calka);

subplot(412); plot(n, mod_fm);

%% Demodulation
dem(1)=0;

for i = 1:length(mod_fm)-1
    dem(i+1) = mod_fm(i+1) - mod_fm(i);
end

f_3dB = 10000 ;
rzad = 3;
[B,A] = butter( rzad , f_3dB/Fs*2);

demod_fm = ( ( sign( dem )+1 ) / 2).*dem;
demod = filter(B,A,demod_fm);

%% Spectrum

spectrum = abs(fft(mod_fm));
subplot(413); plot(n, spectrum,'-x');
subplot(414); plot(n, demod);

%audiowrite('audio1.wav', demod, 48000);
%x, Fs = audioread