% EXPERIMENT 4
% PULSE WIDTH MODULATION AND DEMODULATION

clc;
clear all;
close all;

fm = 1000;          % Message frequency
fs = 10000;         % Sampling frequency
Fs = 200000;        % Sampling rate

t = 0:1/Fs:0.01;
Ts = 1/fs;

% Message signal (2 Vpp)
m = sin(2*pi*fm*t);

N = round(Ts*Fs);

pwm = zeros(size(t));
decoded = zeros(size(t));

% PWM Generation and Demodulation
for k = 0:floor(length(t)/N)-1

    index = k*N + 1;

    % Duty cycle variation
    duty = 0.5 + 0.4*m(index);

    % Pulse width
    width = round(duty*N);

    finish = min(index+width-1,length(t));

    pwm(index:finish) = 1;

    % Demodulation
    recovered = (width/N - 0.5)/0.4;

    decoded(index:min(index+N-1,length(t))) = recovered;

end

% FFT based Low Pass Filter
L = length(t);

Y = fftshift(fft(decoded));

f = ((0:L-1)-floor(L/2))*(Fs/L);

cutoff = 2000;
Y(abs(f)>cutoff) = 0;

demodulated = real(ifft(ifftshift(Y)));

% Plotting
figure(1);

subplot(3,1,1);
plot(t,m,'b','LineWidth',1.5);
title('Original Message Signal');
xlabel('Time (seconds)');
ylabel('Amplitude');
grid on;

subplot(3,1,2);
plot(t,pwm,'r');
title('PWM Modulated Signal');
xlabel('Time (seconds)');
ylabel('Amplitude');
grid on;

subplot(3,1,3);
plot(t,demodulated,'g','LineWidth',1.5);
title('PWM Demodulated Signal');
xlabel('Time (seconds)');
ylabel('Amplitude');
grid on;

disp('Experiment 4 Completed Successfully');