flt - general second order digital filter (poles and zeros)

flt output[b] input[bvpn] gain[bvpn] 
 a0[bvpn] a1[bvpn] a2[bvpn] b1[bvpn] b2[bvpn] 
  t1[dpv] t2[dpv] t3[dpv] t4[dpv] ;

	General second-order filter:

y[i] = gain * (a0*x[i] + a1*x[i-1] + a2*x[i-2] + b1*y[i-1] + b2*y[i-2])

(y[i] is the current output, y[i-1] is the previous output, x[i-1] is
the previous input, etc.) The filter requires 5 coefficients, as shown,
and 4 temporary locations (t1 through t4), which are normally d's.