% demo of aliasing

Fs = 1 ;
Ts = 1/Fs ;
T = 5 ;   % display 5 seconds

t = 0:Ts:T ;         % sample times
tover = 0:0.005:T ;  % time scale for oversampling (analog signal)

f = 0.417 * Fs ;     % start somewhere

% set up graphics
close all
scrsz = get(0,'ScreenSize') ; 

% top figure is for selecting frequency 
figure('OuterPosition',[1 3*scrsz(4)/4 scrsz(3) scrsz(4)/4]) ; % figure 1
rectangle('Position', [0 0 0.5 1], 'FaceColor', [0.9 0.9 0.9]) % nonaliased area
rectangle('Position', [-0.5 0 0.5 1], 'FaceColor', [0.95 0.95 0.95])  % negative nonaliased area
axis([-T T 0 1]) ;
set(gca, 'GridAlpha', 1)
grid on
set(gca,'YTickLabel',[])
hold on

% bottom figure is for displaying signals
figure('OuterPosition',[1 1 scrsz(3) 3*scrsz(4)/4]) ; % figure 2
axis([0 T -1 1]);
hold on

% loop on sine frequencies
while f > 0
    figure(1)
    [f,~] = ginput(1) ; % input desired frequency
    if f<0
        return % stop
    end
    w = 2*pi*f ;
    cla  % erase previous
    rectangle('Position', [0 0 0.5 1], 'FaceColor', [0.9 0.9 0.9])  % nonaliased area
    rectangle('Position', [-0.5 0 0.5 1], 'FaceColor', [0.95 0.95 0.95])  % negative nonaliased area
    grid on
    scatter(f, 0.5, 200, 'b', 'filled')

    s = sin(w*t) ;
    sover = sin(w*tover);
    
    % display analog and sampled signal
    figure(2)
    cla  % erase previous
    plot(t,s,'or', 'MarkerFaceColor','r', 'MarkerSize',15);  % plot sample values
    plot(tover,sover,'b') % plot oversampled sine (analog signal)
    
    % now show aliased sinusoid (if there is one) 
    if f > 0.5   % over Nyquist frequency
        hold on
        falias = f - Fs ;
        while falias > 0.5*Fs
            falias = falias - Fs ;
        end
        walias = 2*pi*falias ;
        salias = sin(walias*tover);
        figure(1)
        scatter(falias, 0.5, 200, 'g', 'filled') % show aliased frequency
        figure(2)
        hold on
        plot(tover,salias, 'g')  % plot alised sine
    end
    
   
end