% channel function
clear;
% input active group
load 'activegroup' groupname;
groupname
% input groupname_signal.mat
filename=sprintf('%s_signal.mat',groupname);
load(filename);
% make sure s is real
signal=real(s);
[M,N]=size(signal)
if N~= 1048576
   'Incorrect vector length, should be 1048576'
end;
%
% form filter
fc=N/32;
Norder=8;
fmax=N/2;
n=1:N;
K=1; % filter gain
% low pass filter
[f HLP]=lp_butterworth_oN_dft(fc,K,fmax,N,Norder);
% band pass filter
fo=N/8;
[f2,HBP]=bp_butterworth_oN_dft(fo,fc,K,fmax,N,Norder);
HBP=2*HBP;
% filter signal through channel
S=fft(signal);
H=HLP+HBP; 
R=S.*H;
sn=real(ifft(R));
figure(1);
plot(n,abs(H),n,abs(HLP),n,abs(HBP));
figure(2);
plot(n,log(abs(S)+.1));
figure(3);
plot(n,log(abs(R)+.1));
% find noise deviation
sigma=0.1*(max(sn)-min(sn))
% add noise
w=sigma*randn(1,N);
r=sn+w;
% store result in groupname_r.mat
filename=sprintf('%s_r.mat',groupname);
save(filename,'r');
figure(4);
plot(n,signal);
figure(5);
plot(n,sn);
figure(6);
plot(n,r);