bilinear Transform a s-plane filter specification into a z-plane specification [ZB, ZA] = bilinear (SB, SA, T) [ZB, ZA] = bilinear (SZ, SP, SG, T) [ZZ, ZP, ZG] = bilinear (...) Transform a s-plane filter specification into a z-plane specification. Filters can be specified in either zero-pole-gain or transfer function form. The input form does not have to match the output form. 1/T is the sampling frequency represented in the z plane. Note: this differs from the bilinear function in the signal processing toolbox, which uses 1/T rather than T. Theory: Given a piecewise flat filter design, you can transform it 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. 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.