function H = mmse(gamma,xihat)
% MMSE - MMSE spectral amplitude estimation (Ephraim & Malah, 1983) 
%    H = MMSE(GAMMA,XIHAT) returns the MMSE spectral amplitude estimation
%    filter H given a posteriori signal-to-noise ratio (SNR) GAMMA and an
%    estimate XIHAT of a priori SNR.  GAMMA and XIHAT may be arrays but must
%    be of equal dimension.
%
% References:
% 
% Y. Ephraim and D. Malah.  Speech enhancement using optimal non-linear
% spectral amplitude estimation.  In Proceedings of the IEEE International
% Conference on Acoustics, Speech and Signal Processing, volume 3, pages
% 1118-1121, 1983.
%
% Y. Ephraim and D. Malah.  Speech enhancement using a minimum mean-square
% error short-time spectral amplitude estimator.  IEEE Transactions on
% Acoustics, Speech and Signal Processing, ASSP-32(6):1109-1121, December
% 1984.

% Author: Patrick J. Wolfe
%         Signal Processing Group
%         Cambridge University Engineering Department
%         p.wolfe@ieee.org
%
% Copyright (c) 2000 by Patrick J. Wolfe
% $Revision: 1.2 $  $Date: 2001/01/24 13:09:23 $

% MMSE spectral amplitude estimation filter calculation
%
upsilon = gamma.*xihat./(1+xihat);
M = (1+upsilon).*besseli(0,upsilon./2,1) + upsilon.*besseli(1,upsilon./2,1);
H = sqrt(pi.*upsilon)./(2.*gamma).*M;