FSK_Mod

clc
clear all
close all

%vm = input('Enter Amplitude (Message) = ');
%vc = input('Enter Amplitude (Carrier) = ');
fM = input('Enter Message frequency = ');
fC = input('Enter Carrier frequency = ');
deviation = input('Enter Freq. deviation = ');

% The number of bits to send - Frame Length
N = 8;

% Generate the bit stream
str = input ("Give me some text: ", "s");
message = (dec2bin (uint8 (str), N) - "0").'(:).';
disp (message);

%fC = 100e3;
%fM = 10e3;
%deviation = 40e3;
fs = 100 * fC;

t = [0 : 1/fs : length(message) * 1/fM];
% t is too large because the endpoint is included.fs
% ugly fix to address this
t = t(1:end-1);

% Those are prototype-waveforms for a low and a high bit.
t_prototype    = t(1:end/length(message));
low_prototype  = sin(2*pi*(fC-deviation)*t_prototype);
high_prototype = sin(2*pi*(fC+deviation)*t_prototype);

% Piece together protoypes to modulated message
y = [];
for i = message
    if i == 0
        y = [y low_prototype];
    else
        y = [y high_prototype];
    endif;
end

upsampling_factor = fs/fM ;
message_resampled = repmat(message, upsampling_factor, 1)(:)' - 0.5;

subplot(4,1,1);
plot(t, message_resampled, 'linewidth', 1.5, '-r');
refresh;
xlabel('Time [s]');
ylabel('Amplitude [V]');
title('Message Signal waveform');
grid on;
axis tight;

carrier = sin(2*pi*fC*t);
subplot(4,1,2);
plot(t,carrier);
xlabel('Time [s]');
ylabel('Amplitude [V]');
title('Carrier Signal waveform');
grid on;
axis tight;

subplot(4,1,3);
plot(t,y);
xlabel('Time [s]');
ylabel('Amplitude [V]');
title('FM Signal waveform');
grid on;
axis tight;

Nsamps = length (y);
y_fft = abs (fft (y));            %Retain Magnitude
y_fft = y_fft(1:Nsamps/2);        %Discard Half of Points
f_fft = fs*(0:Nsamps/2-1)/Nsamps; %Prepare freq data for plot
subplot (4,1,4);
plot(f_fft, y_fft, 'linewidth', 1.5, '-g');
xlim ([0 2e5]);
ylabel ('Amplitude');
xlabel ('Frequency (Hz)');
title ('Spectrum plot of the FSK modulated signal');
grid on;