function [b,a] = f_butterz (F_p,F_s,delta_p,delta_s,ftype,fs,n)

%F_BUTTERZ: Design a Butterworth IIR digital filter
%
% Usage: [b,a] = f_butterz (F_p,F_s,delta_p,delta_s,ftype,...
%                          fs,n)
%
% Inputs: 
%         F_p     = passband cutoff frequency or frequencies
%         F_s     = stopband cutoff frequency or frequencies
%         delta_p = passband ripple
%         delta_s = stopband attenuation
%         ftype    = filter type
%
%                    0 = lowpass
%                    1 = highpass
%                    2 = bandpass (F_p and F_s are vectors)
%                    3 = bandstop (F_p and F_s are vectors)
%
%         fs      = sampling frequency in Hz
%         n       = an optional integer specifying the 
%                   filter order.  If n is not present, an
%                   estimate of order required to meet the 
%                   specifications is used.
% Outputs: 
%          b = 1 by (n+1) coefficient vector of numerator 
%              polynomial 
%          a = 1 by (n+1) coefficient vector of denominator 
%              polynomial
%
% See also: F_CHEBY1Z, F_CHEBY2Z, F_ELLIPTICZ

% Initialize

fs = f_clip (fs,0,fs);
F_p = f_clip (F_p,0,fs/2);
F_s = f_clip (F_s,0,fs/2);
delta_p = f_clip (delta_p,0,delta_p);
delta_s = f_clip (delta_s,0,delta_s);
ftype = f_clip (ftype,0,3);
T = 1/fs;

% Compute normalized lowpass analog filter 

switch (ftype)
    
case 0, r = F_p/F_s;
case 1, r = F_s/F_p;
case 2, r = max([F_s(1)/F_p(1),F_p(2)/F_s(2)]);
case 3, r = max([F_p(1)/F_s(1),F_s(2)/F_p(2)]);
    
end
F_0 = 1/(2*pi);
F_1 = F_0/r;
if nargin < 7
   [bs,as] = f_butters (F_0,F_1,delta_p,delta_s);
else
   [bs,as] = f_butters (F_0,F_1,delta_p,delta_s,n);
end

% Prewarp cutoff frequencies

F_p = tan(pi*F_p*T) / (pi*T);
F_s = tan(pi*F_s*T) / (pi*T);

% Convert to desired frequency-selective filter

switch (ftype)
   
case 0, [B,A] = f_low2lows  (bs,as,F_p);
case 1, [B,A] = f_low2highs (bs,as,F_p);
case 2, [B,A] = f_low2bps   (bs,as,F_p(1),F_p(2));
case 3, [B,A] = f_low2bss   (bs,as,F_p(1),F_p(2));
   
end

% Convert to digital filter

[b,a] = f_bilin  (B,A,fs);