function tau=firstzero(y);  %function tau=firstzero(y);%%find the first zero of the autocorrelation function of y.%%Michael Small%3/3/2005%ensmall@polyu.edu.hk  len=50;  [r,t]=acorr(y,len);lY=length(y);%lY=1000;while (all(r(2:(end-1))>0)),  len=len*2;  if (len==2*lY),    disp('WARNING : No minium found in firstzero(y)');    tau=1;    return;  end;  if (len>lY)    len=lY;  end;  [r,t]=acorr(y,len);end;r=r((len+1):end);t0=find(r>0);t0=t0(find(diff(t0)>1));if length(t0)>0,  t0=t0(1);  t1=t0+1;else  t0=min(find(r<0));  t1=t0+1;end;if t0==(len+1),  tau=t0;  return;end;if (-r(t1)>=r(t0))  tau=t0;else,  tau=t1;end;tau=tau-1;%if tau>(len/4),%  tau=1;%end;