% alias.m


% demonstrate alising effect


% copyright (c) 1997 by Yu Hen Hu


% 


% x(t) = cos(2*pi*F*t) is continuous time signal where F is the 


%   continuous time frequence, t is the time variable


% y(n) = x(nT) = cos(2*pi*F*n*T)= cos(2*pi*fd*n) is sampled signal.


%   where T = 1/fs is sampling period


%         fd = FT is discrete frequency
% Plot range -rg ms to rg ms


% created: 9/29/97




clear all


F=60; fs = 100;  


toend=0;

while toend==0,


   disp(['curret continuous freq. F = ' num2str(F) '; sampling freq fs = ' ...


         num2str(fs) '.']);


   change=input('If want to change, enter 1: ');


   if change == 1,


      tmp=input('enter [F  fs]: ');


      F=tmp(1); fs=tmp(2);

   end

  T = 1000/fs;               % T in ms is sampling period.

  fd = F*T*.001;             % discrete time frequence

  rg=10*T; drg=rg/500;       % rg in ms!  plot 10 samples each side.

  trange=[-rg:drg:rg];       % plot range in continuous time in ms!

                               % symmetric w.r.t. origin

  xrange=[-rg/T:rg/T];       % corresponding range in discrete time samples

  xt=cos(2*pi*F*0.001*trange);

  xn=cos(2*pi*fd*xrange);

  figure(1); hold off; clf; set(1,'Position',[26 304 710 248]); hold on;

  plot(trange,xt,'-');  stem(xrange*T,xn); plot(xrange*T,xn,':');

  xlabel('t    (ms)');  ylabel('x(t) and x(n)')

  title('Illustration of Aliasing Effect')

  toend=input('To plot for another set of frequencies, enter 0: ');

end