%% Copyright (C) 2005 Julius O. Smith III
%%
%% This program is free software; you can redistribute it and/or modify
%% it under the terms of the GNU General Public License as published by
%% the Free Software Foundation; either version 2 of the License, or
%% (at your option) any later version.
%%
%% This program is distributed in the hope that it will be useful,
%% but WITHOUT ANY WARRANTY; without even the implied warranty of
%% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
%% GNU General Public License for more details.
%%
%% You should have received a copy of the GNU General Public License
%% along with this program; if not, write to the Free Software
%% Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA

%% -*- texinfo -*-
%% @deftypefn {Function File} {[@var{sos}, @var{g}] =} tf2sos (@var{B}, @var{A})
%% Convert direct-form filter coefficients to series second-order sections.
%%
%% INPUTS:
%% @var{B} and @var{A} are vectors specifying the digital filter @math{H(z) = B(z)/A(z)}.
%% See @code{filter()} for documentation of the @var{B} and @var{A} 
%% filter coefficients.
%%
%% RETURNED:
%% @var{sos} = matrix of series second-order sections, one per row:@*
%% @var{sos} = [@var{B1}.' @var{A1}.'; ...; @var{BN}.' @var{AN}.'], where@*
%% @code{@var{B1}.'==[b0 b1 b2] and @var{A1}.'==[1 a1 a2]} for 
%% section 1, etc.@*
%% b0 must be nonzero for each section (zeros at infinity not supported).
%% @var{Bscale} is an overall gain factor that effectively scales
%% any one of the @var{B}i vectors.
%%
%% EXAMPLE:
%% @example
%% B=[1 0 0 0 0 1]; 
%% A=[1 0 0 0 0 .9];
%% [sos,g] = tf2sos(B,A)
%%
%% sos =
%%
%%    1.00000   0.61803   1.00000   1.00000   0.60515   0.95873
%%    1.00000  -1.61803   1.00000   1.00000  -1.58430   0.95873
%%    1.00000   1.00000  -0.00000   1.00000   0.97915  -0.00000
%%
%% g = 1
%%
%% @end example
%%
%% @seealso{sos2tf zp2sos sos2pz zp2tf tf2zp}
%% @end deftypefn

function [sos,g] = tf2sos(B,A)

[z,p,g] = tf2zp(B(:)',A(:)');
sos = zp2sos(z,p,g);

%!test
%! B=[1 0 0 0 0 1]; A=[1 0 0 0 0 .9];
%! [sos,g] = tf2sos(B,A);
%! [Bh,Ah] = sos2tf(sos,g);
%! assert({Bh,Ah},{B,A},100*eps);
