% hilopassdemo.m
% show smoothing an differecing as a moving mask 

pausetime = .1;

% Define filters
L  = [1,1]/2;
H  = [1,-1]/2;
n  = size(L,2); 

%define signal
N = 128;
Dt = 0:(N-1); 
t = (0:(N-1))/N;
X = sin(5*t) + 0.75*cos(50*t)+0.5*unitnois(size(t));
mx = 2;
mn = -2;

% inital plot

figure(1)
subplot(2,1,1)
plot(Dt,X);
title('original signal')
axis([0,N-1,mn,mx])
subplot(2,1,2)
plot(Dt,0);
axis([0,N-1,mn,mx])
title('window for filtered signals')
disp('Press a key for moving filtering demo')
pause

% now the filtering demo

Xl = [];
Xh = [];

for j = 1:N 
   inds = j:(-1):(j-n+1);
   inds = rem(inds + N-1,N)+1;
   Xl =[Xl,X(inds)*L'];
   Xh =[Xh,X(inds)*H'];
   Dtf = 0:(j-1);
   
   subplot(2,1,1)
   plot(Dt,X,'b-',inds-1,X(inds),'r.');
   axis([0,N-1,mn,mx])
   title('current selected values')
   subplot(2,1,2)
   plot(Dtf,Xl,'r-',Dtf,Xh,'g-');
   axis([0,N-1,mn,mx])
   title('filtered signals')
   pause(pausetime)
   end