SpHEAR/sphear_fir_design.m

%%
%% *SpHEAR Project
%%
%% sphear_fir_design: design a matrix of FIR filters that equalizes the microphone
%%
%% usage:
%%   [FIR] = sphear_fir_design(F, A2BF, n, Fs, Fl, W, XYZ)
%%
%% F: array of filter section center frequencies
%% A2BF: vector of A2B matrixes, one for each frequency
%% n: order of the FIR filter being created
%% maxboost: max high and low frequency boost in dB of the final filter
%% Fs: sampling rate
%% Fl: frequency limit, below it use A2B matrixes, above it use WXYZ shapes
%%
%% for the first run do not supply W/XYZ, render the filters and then run
%% sphear_fir_extract to extract the W and XYZ filter shapes, finally run
%% this again with the extra parameters below
%%
%% W: extracted shape of W filter (used above Fl)
%% XYZ: extracted shape of X, Y and Z filters (used above Fl)
%%
%% Copyright 2016, Fernando Lopez-Lezcano, All Rights Reserved
%% nando@ccrma.stanford.edu
%%
%%  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/>.

function [FIR] = sphear_fir_design(F, A2BF, n, maxboost, Fs, Fl, W, XYZ)

  fig = figure;

  %% all b format components
  for b = 1:4
    %% all capsules
    for c = 1:4

      for d = 1:size(A2BF(:,1,1))(1)
	if (F(d) <= Fl)
	  %% below the limit frequency we use the A2B coefficients
	  vlast = A2BF(d, c, b);
	  dlast = d;
	  if (exist("W") == 1)
	    wlast = W(d);
	  end
	  if (exist("XYZ") == 1)
	    xyzlast = XYZ(d);
	  end
	else
	  %% above the limit frequency we interpolate the W and XYZ
	  %% filters if they exist, otherwise keep the last valid value
	  if (b == 1)
	    if (exist("W") == 1)
	      A2BF(d, c, b) = vlast * (W(d)/wlast);
	    else
	      A2BF(d, c, b) = vlast;
	    end
	  else
	    if (exist("XYZ") == 1)
	      A2BF(d, c, b) = vlast * (XYZ(d)/xyzlast);
	    else
	      A2BF(d, c, b) = vlast;
	    end
	  end
	end
      end

      %% fit a spline to the W and XYZ segments and extrapolate
      %% highest frequency if it does not exist in the measurements
      %%
      %% the 200Hz spacing of the points is arbitrary, we should
      %% select something that makes more sense
      fh = (F(dlast-1)/(Fs/2):200/(Fs/2):1)';
      %% make the end really 1.0
      fh(end) = 1;
      rh = interp1(F(dlast-1:end)/(Fs/2), A2BF(dlast-1:end,c,b)', fh, "cubic", "extrap");

      %% we design the spline taking into account one additional point
      %% on the bottom edge so that it will fit better with the existing
      %% low frequency points, now we need to find the exact transition
      %% frequency in the interpolated array
      for idx = 1:size(fh(:))
	if (fh(idx) <= F(dlast)/(Fs/2))
	  istart = idx;
	end
      end
      
      %% extract the low frequency A2B coefficients
      fl = [0 F(1:dlast-1)']' ./ (Fs/2);
      rl = [A2BF(1,c,b) A2BF(1:dlast-1,c,b)']';

      %% concatenate low and high frequency arrays
      fi = cat(1, fl, fh(istart:end));
      ri = cat(1, rl, rh(istart:end));

      printf("low frequency boost %f, high frequency boost %f\n", 20*log10(ri(1)), 20*log10(ri(end)));
      %% force a max boost at low and high frequencies
      if (maxboost != 0)
	if (abs(ri(1)) > 10^(maxboost/20))
	  printf("limit low freq boost from %f to %f dB (%f)\n", 20*log10(ri(1)), maxboost, ri(1));
	  ri(1) = sign(ri(1)) * 10^(maxboost/20);
	  printf("new limit %f\n", ri(1));
	end
	if (abs(ri(end)) > 10^(maxboost/20))
	  printf("limit high freq boost from %f dB to %f dB (%f)\n", 20*log10(ri(end)), maxboost, ri(end));
	  ri(end) = sign(ri(end)) * 10^(maxboost/20);
	  printf("new limit %f\n", ri(end));
	end
      end
      
      %% plot real measured points
      axis([300, 24000]);
      grid on;
      if (b == 1)
	semilogx(F, 20*log10(abs(A2BF(:,c,b)))','*k');
      else
	semilogx(F, 20*log10(abs(A2BF(:,c,b)))','*k');
      end
      hold on;
      %% plot extrapolated spline
      if (b == 1)
	semilogx(fi*(Fs/2), 20*log10(abs(ri)), 'b');
      else
	semilogx(fi*(Fs/2), 20*log10(abs(ri)), 'r');
      end
      
      %% design the filter
      FIR(b, c, :) = fir2(n, fi, ri);
    end
  end
  
  if (exist("W") == 1)
    print(fig, '-dpng','-color',["figures/w_xyz_filter_full.png"]);
  else
    print(fig, '-dpng','-color',["figures/w_xyz_filter_low.png"]);
  end
end