dir2cas.m

It a filter. Buterwarth filter. very important tool in gsm

function[b0,B,A]=dir2cas(b,a);
b0=b(1);
b=b/b0;
a0=a(1);
a=a/a0;
b0=b0/a0;
M=length(b); 
N=length(a);
if N>M
    b=[b zeros(1,N-M)];
elseif M>N
    a=[a zeros(1,M-N)];N=M;
else
    NM=0;
end
K=floor(N/2);
B=zeros(K,3);
A=zeros(K,3);
if K*2==N;
    b=[b 0];
    a=[a 0];
end
broots=cplxpair(roots(b));
aroots=cplxpair(roots(a));
for i=1:2:2*K
    Brow=broots(i:1:i+1,:);
    Brow=real(poly(Brow));
    B(fix((i+1)/2),:)=Brow;
    Arow=aroots(i:1:i+1,:);
    Arow=real(poly(Arow));
    A(fix((i+1)/2),:)=Arow;
end