Filter Realizations - Introduction to Digital Signal Processing and Filter Design - page 335

A2D , respectively. So also the coefficients of the numerator and denominator of
G(z) can be obtained by typing b and a , respectively. If we type A1, A2, G ,
or H , the program displays the transfer function as the ratio of two polynomi-
als. The program may not work successfully if the specifications call for a very
narrow transition band or a very high order for the filter, because of numerical
inaccuracy.
%This Matlab Program lp2apx.m obtains the allpass filters
%A 1 (z) and A 2 (z) from the lowpass (Butterworth, Chebyshev I
% and Elliptic) filters of odd order.
clear all
Wp=input('Enter the passband cutoff frequency between 0
and 1');
Ws=input('Enter the stopband cutoff frequency Wp<Ws<1');
Ap=input('Enter the max.attenuation in the passband');
As=input('Enter the min.attenuation in the stopband');
disp('As ftype,type in (1) for Butterworth, (2) for
Chebyshev I, (3) Chebyshev II and (4) for Elliptic
filters');
ftype=input('');
if ftype==1
disp('Butterworth Lowpass Filter');
[N,Wn]=buttord(Wp,Ws,Ap,As);
M=mod(N,2);
if M==0
N=N+1
end
[b,a]=butter(N,Wn);
end
if ftype==2
disp('Chebyshev I Lowpass Filter')
[N,Wn]=cheb1ord(Wp,Ws,Ap,As);
M=mod(N,2);
if M==0
N=N+1
end
[b,a]=cheby1(N,Ap,Wn);
end
if ftype==3
disp('Chebyshev II Lowpass filter')
[N,Wn]=cheb2ord(Wp,Ws,Ap,As);
M=mod(N,2);
if M==0
N=N+1
end