function falias = alias(f,fs)
% ALIAS generates an aliased frequency that corresponds to the 
% frequency and sampling rate given
%
% FALIAS = ALIAS(F,FS) 
%
% returns the baseband frequency (between 0 and fs/2) FALIAS for a 
% given frequency F and sampling frequency FS

if(f < 0 | fs < 0)
   error('Negative frequencies not allowed.');
end
if (f > 0 & f < fs/2)
   error('No aliasing.');     % signal frequency f within Nyquist limits
end 

k = round(f/fs);              % find nearest multiple of sampling frequency
falias = abs(k*fs-f);         % calculate baseband alias frequency