
 if (istep==0)                  # Only filter on a valid timestep
  y0= sin2;                     # Curve to be filtered

  for iCoeff= 1:nval_butter1 delay_butter1(iCoeff,2)= delay_butter1(iCoeff,1); endfor        # Time shift

  y1= b_butter1(1)*y0 + delay_butter1(2,2);
  for iCoeff= 2:nval_butter1-1
   delay_butter1(iCoeff,1)= b_butter1(iCoeff)*y0 - a_butter1(iCoeff)*y1 + delay_butter1(iCoeff+1,2);
  endfor
  delay_butter1(nval_butter1,1)= b_butter1(nval_butter1)*y0 - a_butter1(nval_butter1)*y1;
 endif

 sin2_butter1= y1;    # Output value

