bilinear Transforms a s-plane filter (Analog) into a z-plane filter (Digital) using Bilinear transformation [Zb, Za] = bilinear(Sb, Sa, T) [Zb, Zb] = bilinear(Sz, Sp, Sg, T) [Zz, Zp, Zg] = bilinear(...) Sb: Numerator coefficient vector in s-domain. Sa: Denumerator coefficient vector s-domain. Sz: zeros in s-plane. Sp: poles in s-plane. Sg: gain in s-domain. T: Sampling period (double). Zb: Numerator coefficient vector in z-domain. Za: Denumerator coefficient vector z-domain. Zz: zeros in z-plane. Zp: poles in z-plane. Zg: gain in z-domain. a filter design can be transformed from the s-plane to the z-plane while maintaining the band edges by means of the bilinear transform. This maps the left hand side of the s-plane into the interior of the unit circle in z-plane. The mapping is highly non-linear, so you must design your filter with band edges in the s-plane positioned at 2/T tan(w*T/2) so that they will be positioned at w after the bilinear transform is complete. It does following transformation from s-plane to z-plane \begin{eqnarray} s --> \frac{2} {T} \frac {z - 1} {z + 1} \end{eqnarray} Sonu Sharma, RGIT Mumbai (fellow at FOSSEE, IIT Bombay)