durbin.m

FS1016源代码

% MATLAB SIMULATION OF NSA FS-1016 CELP v3.2
% COPYRIGHT (C) 1995-99 ANDREAS SPANIAS AND TED PAINTER
%
% This Copyright applies only to this particular MATLAB implementation
% of the FS-1016 CELP coder.  The MATLAB software is intended only for educational
% purposes.  No other use is intended or authorized.  This is not a public
% domain program and distribution to individuals or networks is strictly
% prohibited.  Be aware that use of the standard in any form is goverened
% by rules of the US DoD.  Therefore patents and royalties may apply to
% authors, companies, or committees associated with this standard, FS-1016.  For
% questions regarding the MATLAB implementation please contact Andreas
% Spanias at  [phone].  For questions on rules,
% royalties, or patents associated with the standard, please contact the DoD.
%
% ALL DERIVATIVE WORKS MUST INCLUDE THIS COPYRIGHT NOTICE.
%
% ******************************************************************
% DURBIN
%
% PORTED TO MATLAB FROM CELP 3.2a C RELEASE
% 6-8-94
%
% ******************************************************************
%
% DESCRIPTION
%
% Durbin recursion: Uses autocorrelation lags to solve for predictor
%                   coefficients
%
% DESIGN NOTES
%
% This performs the classical Durbin recursion on the correlation
% sequence r(0), r(1), r(2) . . . to obtain n reflection coefficients (rc).
%
% The sign convention used defines the first reflection coefficient
% as the normalized first autocorrelation coefficient, which results
% in positive values of rc(0) for voiced speech.
%
% REFERENCES
%
% Parsons, Voice and Speech Processing, McGraw-Hill, 1987, p. 160 & 378.
%
% VARIABLES
%
% INPUTS
%   c0       -      Autocorrelation low order lag, r(0)
%   c        -      Higher order autocorrelation lags, r(1) - r(n)
%   n        -      Predictor order
%
% OUTPUTS
%   a        -      LPC coefficients (1 to n+1) (direct form predictor)
%
% INTERNALS
%   i        -      Index of recursion
%   rc       -      Reflection coefficients
%   alpha    -      Intermediate result of recursion
%   beta     -      Intermediate result of recursion
%   tmp      -      Intermediate result of recursion
%
% GLOBALS
%   MAXNO    -      LPC predictor order
%
% ******************************************************************

function a = durbin( c0, c, a, n )

% DECLARE GLOBAL CONSTANTS
global MAXNO

% INIT LOCAL VARIABLES
rc = zeros( MAXNO, 1 );
tmp = zeros( MAXNO, 1 );

% TRAP ZERO ENERGY CONDITION
if c0 <= 0.00
    rc( 1:n ) = 0.00;
    return
end

% INITIALIZATION
alpha = c0;
a(2) = -c(1) / c0;
rc(1) = a(2);
beta = c(1);

% RECURSION
for i = 1:n-1
    alpha = alpha + ( beta * rc(i) );
    beta = c( i+1 );
    beta = beta + sum( c( 1:i ) .* a( i+1:-1:2 ) );
    rc( i+1 ) = -beta / alpha;
    tmp( 1:i ) = rc( i+1 ) .* a( i+1:-1:2 );
    a( 2:i+1 ) = a( 2:i+1 ) + tmp( 1:i );
    a( i+2 ) = rc( i+1 );
end