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function [hs,pols,zers,gain]=analpf(n,fdesign,rp,omega)
//[hs,pols,zers,gain]=analpf(n,fdesign,rp,omega)
//Creates analog low-pass filter with cut-off frequency at omega
// n :filter order (pos. integer)
// fdesign :filter design method
// : fdesign=('butt','cheb1','cheb2','ellip')
// rp :2-vector of error values for cheb1, cheb2, and
// :ellip filters where only rp(1) is used for
// :cheb1 case, only rp(2) is used for cheb2 case, and
// :rp(1) and rp(2) are both used for ellip case.
// : 0<rp(1),rp(2)<1
// :for cheb1 filters: 1-rp(1)<ripple<1 in passband
// :for cheb2 filters: 0<ripple<rp(2) in stopband
// :for ellip filters: 1-rp(1)<ripple<1 in passband
// : 0<ripple<rp(2) in stopband
// omega :cut-off frequency of low-pass filter in Hertz
// hs :rational polynomial transfer function
// pols :poles of transfer function
// zers :zeros of transfer function
// gain :gain of transfer function
//
// hs=gain*poly(zers,'s')/poly(pols,'s')
//
//!
// author: C. Bunks date: 9 Sept 1988
// Copyright INRIA
select fdesign
case 'butt' then
[pols,gain]=zpbutt(n,omega);
zers=[];
hs=gain/real(poly(pols,'s'));
case 'cheb1' then
epsilon=sqrt(-1+1/(1-rp(1))**2);
[pols,gain]=zpch1(n,epsilon,omega);
zers=[];
hs=gain/real(poly(pols,'s'));
case 'cheb2' then
att=1/rp(2);
[zers,pols,gain]=zpch2(n,att,omega);
hs=gain*real(poly(zers,'s'))./real(poly(pols,'s'));
case 'ellip' then
epsilon=sqrt(-1+1/(1-rp(1))**2);
att=1/rp(2);
m=find_freq(epsilon,att,n);
omegar=omega/sqrt(m);
[zers,pols,gain]=zpell(epsilon,att,omega,omegar);
hs=gain*real(poly(zers,'s'))./real(poly(pols,'s'));
else
error('Unknown design type --- program termination'),
end
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