function u=am_mod(a,m,ts,fc)
%		u=am_mod(a,m,ts,fc)
%AM_MOD  	takes signal m sampled at ts and carrier
%		freq. fc as input and returns the AM modulated
%		signal. "a" is the modulation index.
%		and ts << 1/2fc. 

t=[0:length(m)-1]*ts;
c=cos(2*pi*fc.*t);                 
m_n=m/max(abs(m)); 
u=(1+a*m_n).*c;          