function [r,xtilde]=dpcm_ek(x,a)
%差分編碼
[m,nx]=size(x);  

[p,na]=size(a);
if p==1 & na > 1, a = a'; p=na;  end  

r=round(x(1:p)); xtilde=r;
for t=p+1:m,
   xhat(t)=a'*xtilde(t-1:-1:t-p);
   r(t)=round(x(t)-xhat(t));
   xtilde(t)=xhat(t)+r(t);
end