function [tgrad,fgrad,s,c]=filterbankphasegrad(f,g,a,L,minlvl)
%FILTERBANKPHASEGRAD Phase gradient of a filterbank representation
% Usage: [tgrad,fgrad,s,c] = filterbankphasegrad(f,g,a,L,minlvl);
% [tgrad,fgrad,s,c] = filterbankphasegrad(f,g,a,L);
% [tgrad,fgrad,s,c] = filterbankphasegrad(f,g,a,minlvl);
% [tgrad,fgrad,s,c] = filterbankphasegrad(f,g,a);
% [tgrad,fgrad,s] = filterbankphasegrad(...)
% [tgrad,fgrad] = filterbankphasegrad(...)
%
% Input parameters:
% f : Signal to be analyzed.
% g : Cell array of filters
% a : Vector of time steps.
% L : Signal length (optional).
% minlvl: Regularization parameter (optional, required < 1).
% Output parameters:
% tgrad : Instantaneous frequency relative to original position.
% fgrad : The negative of the local group delay.
% cs : Filterbank spectrogram.
% c : Filterbank coefficients.
%
% [tgrad,fgrad,s,c] = FILTERBANKPHASEGRAD(f,g,a,L) computes the
% relative instantaneous frequency tgrad and the negative of the group
% delay fgrad of the filterbank spectrogram s obtained from the
% signal f and filterbank parameters g and a.
% Both tgrad and fgrad are specified relative to the original
% coefficient position entirely similar to GABPHASEGRAD.
% fgrad is given in samples, while tgrad is given in normalised
% frequencies such that the absolute frequencies are in the range of ]-1,1].
%
% This routine uses the equivalence of the filterbank coefficients in
% each channel with coefficients obtained from an STFT obtained with a
% certain window (possibly different for every channel). As a consequence
% of this equivalence, the formulas derived in the reference apply.
%
% See also: gabphasegrad
%
% References:
% F. Auger and P. Flandrin. Improving the readability of time-frequency
% and time-scale representations by the reassignment method. IEEE Trans.
% Signal Process., 43(5):1068--1089, 1995.
%
% N. Holighaus, Z. Průša, and P. L. Søndergaard. Reassignment and
% synchrosqueezing for general time-frequency filter banks, subsampling
% and processing. Signal Processing, 125:1--8, 2016. [1]http ]
%
% References
%
% 1. http://www.sciencedirect.com/science/article/pii/S0165168416000141
%
%
% Url: http://ltfat.github.io/doc/filterbank/filterbankphasegrad.html
% Copyright (C) 2005-2023 Peter L. Soendergaard <peter@sonderport.dk> and others.
% This file is part of LTFAT version 2.6.0
%
% 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 3 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, see <http://www.gnu.org/licenses/>.
% AUTHOR : Nicki Holighaus.
complainif_notenoughargs(nargin,3,'FILTERBANKPHASEGRAD');
% Reshape input signal
[f,~,W]=comp_sigreshape_pre(f,'FILTERBANKPHASEGRAD',0);
Ls = size(f,1);
if W>1
error('%s: Only one-channel signals supported.',upper(mfilename));
end
if nargin < 5
if nargin < 4
L = filterbanklength(Ls,a);
minlvl = eps;
else
if ~(isscalar(L) && isnumeric(L) ) && L>0
error('%s: Fourth argument shoud be a positive number.',...
upper(mfilename));
end
if L >= 1
minlvl = eps;
else
minlvl = L;
end;
end
end;
complainif_notposint(L,'L','FILTERBANKPHASEGRAD');
Luser = filterbanklength(L,a);
if Luser~=L
error(['%s: Incorrect transform length L=%i specified. ', ...
'Next valid length is L=%i. See the help of ',...
'FILTERBANKLENGTH for the requirements.'],...
upper(mfilename),L,Luser);
end
% Unify format of coefficients
[g,asan]=filterbankwin(g,a,L,'normal');
% Precompute filters
[gh, gd, g] = comp_phasegradfilters(g, asan, L);
f=postpad(f,L);
c=comp_filterbank(f,g,asan);
% Compute filterbank coefficients with frequency weighted window
ch=comp_filterbank(f,gh,asan);
% Compute filterbank coefficients with time weighted window
cd=comp_filterbank(f,gd,asan);
% Run the computation
[tgrad,fgrad,s] = comp_filterbankphasegrad(c,ch,cd,L,minlvl);