bilinear: Bilinear transformation

View source: R/bilinear.R

bilinearR Documentation

Bilinear transformation


Transform a s-plane filter specification into a z-plane specification.


## Default S3 method:
bilinear(Sz, Sp, Sg, T, ...)

## S3 method for class 'Zpg'
bilinear(Sz, T, ...)

## S3 method for class 'Arma'
bilinear(Sz, T, ...)



In the generic case, a model to be transformed. In the default case, a vector containing the zeros in a pole-zero-gain model.


a vector containing the poles in a pole-zero-gain model.


a vector containing the gain in a pole-zero-gain model.


the sampling frequency represented in the z plane.


Arguments passed to the generic function.


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.

The bilinear transform is:

z = \frac{1 + sT/2}{1 - sT/2}

s = \frac{T}{2}\frac{z - 1}{z + 1}

Please note that a pole and a zero at the same place exactly cancel. This is significant since the bilinear transform creates numerous extra poles and zeros, most of which cancel. Those which do not cancel have a “fill-in” effect, extending the shorter of the sets to have the same number of as the longer of the sets of poles and zeros (or at least split the difference in the case of the band pass filter). There may be other opportunistic cancellations, but it will not check for them.

Also note that any pole on the unit circle or beyond will result in an unstable filter. Because of cancellation, this will only happen if the number of poles is smaller than the number of zeros. The analytic design methods all yield more poles than zeros, so this will not be a problem.


For the default case or for bilinear.Zpg, an object of class “Zpg”, containing the list elements:


complex vector of the zeros of the transformed model


complex vector of the poles of the transformed model


gain of the transformed model

For bilinear.Arma, an object of class “Arma”, containing the list elements:


moving average (MA) polynomial coefficients


autoregressive (AR) polynomial coefficients


Original Octave version by Paul Kienzle Conversion to R by Tom Short.


Proakis & Manolakis (1992). Digital Signal Processing. New York: Macmillan Publishing Company.

Octave Forge

See Also

Zpg, sftrans, Arma

signal documentation built on June 26, 2024, 9:06 a.m.

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