butter

% Generate a butterworth filter.
% Default is a discrete space (Z) filter.
% 
% [b,a] = butter(n, Wc)
%    low pass filter with cutoff pi*Wc radians
%
% [b,a] = butter(n, Wc, 'high')
%    high pass filter with cutoff pi*Wc radians
%
% [b,a] = butter(n, [Wl, Wh])
%    band pass filter with edges pi*Wl and pi*Wh radians
%
% [b,a] = butter(n, [Wl, Wh], 'stop')
%    band reject filter with edges pi*Wl and pi*Wh radians
%
% [z,p,g] = butter(...)
%    return filter as zero-pole-gain rather than coefficients of the
%    numerator and denominator polynomials.
% 
% [...] = butter(...,'s')
%     return a Laplace space filter, W can be larger than 1.
% 
% [a,b,c,d] = butter(...)
%  return  state-space matrices 
%
% References: 
%
% Proakis & Manolakis (1992). Digital Signal Processing. New York:
% Macmillan Publishing Company.