function [ Lambda ] = convModel( W, H )
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Name: convModel
% Date: March 2018
% Programmer: Christian Dittmar
%
% Convolutive NMF model implementing the eq. (4) from [1]. Note that it can
% also be used to compute the standard NMF model in case the number of time
% frames of the templates equals one.
%
% References:
% [1] Christian Dittmar and Meinard Müller "Reverse Engineering the Amen
% Break " Score-informed Separation and Restoration applied to Drum
% Recordings" IEEE/ACM Transactions on Audio, Speech, and Language Processing,
% 24(9): 1531-1543, 2016.
%
% Input:  W          tensor holding the spectral templates which can be
%                    interpreted as a set of spectrogram snippets with
%                    dimensions: numBins x numComp x numTemplateFrames
%         H          corresponding activations with dimensions:
%                    numComponents x numTargetFrames
%
% Output: approxV    approximated spectrogram matrix
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% If you use the 'NMF toolbox' please refer to:
% [1] Patricio López-Serrano, Christian Dittmar, Yiğitcan Özer, and Meinard
%     Müller
%     NMF Toolbox: Music Processing Applications of Nonnegative Matrix
%     Factorization
%     In Proceedings of the International Conference on Digital Audio Effects
%     (DAFx), 2019.
%
% License:
% This file is part of 'NMF toolbox'.
% https://www.audiolabs-erlangen.de/resources/MIR/NMFtoolbox/
% 'NMF toolbox' is free software: you can redistribute it and/or modify it
% under the terms of the GNU General Public License as published by the
% the Free Software Foundation, either version 3 of the License, or (at
% your option) any later version.
%
% 'NMF toolbox' 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 'NMF toolbox'. If not, see http://www.gnu.org/licenses/.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% check parameters
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% the more explicit matrix multiplication will be used
[numBins, numComp, numTemplateFrames] = size(W);
[numComp, numFrames] = size(H);

% initialize with zeros
Lambda = zeros(numBins,numFrames);

% this is doing the math as described in [2], eq (4)
% the alernative conv2() method does not show speed advantages
for k = 1:numTemplateFrames
  multResult = squeeze(W(:,:,k))*shiftOperator(H,k-1);
  Lambda = Lambda+multResult;
end

% add small positive constant
Lambda = Lambda + eps;