function [y,ys,b]=deltamod(x,t,fs,delta)
%
%   function [y,ys,b]=deltamod(x,t,fs,delta)
%
%   This function will produce: the output bit pattern for the delta
%   modulation of x, at sample rate fs (i.e., fs bits/sec), using a step
%   size of "delta". y is the delta mod approximation, ys is the smoothed
%   signal after a simple LPF at the receiver, and b is the output bit
%   pattern. t is the input time vector associated with input x and outputs y and ys.
 

y=[ ];
delt=t(2)-t(1);       % time between samples in x
delf=1/delt;          % frequency of original samples
num=round(delf/fs);   % this is the number of samples of x that will be 
                        %  associated w/every bit in output
if (num < 2)
    num
    disp(sprintf('Sample frequency looks to be too low, exiting ....'));
    return;
end
b=zeros(1,round(length(x)/num));
y=zeros(size(x));
bindex=1;
currentvalue=0;
y(1:num)=0;

% set up first step...
if (y(1) <= x(1))
    y(1:num)=delta;
    currentvalue=delta;
else
    y(1:num)=-delta;
    currentvalue=-delta;
end
currentindex=num+1;

for i=num+1:num:length(x)
    if (x(i) > currentvalue)
        y(currentindex:currentindex+num)=currentvalue+delta;
        b(bindex)=1;
    else
        y(currentindex:currentindex+num)=currentvalue-delta;
        b(bindex)=0;
    end
    currentvalue=y(currentindex);
    currentindex=currentindex+num;
    bindex=bindex+1;
end

y=y(1:length(x));
N=round(num*2);
ys=filter(1/N*ones(1,N),1,y);