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1 function map=specbasis(edges,M)
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2 % specbasis - Create frequency mapping matrix
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3 %
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4 % as_fmap ::
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5 % [[L-1]] ~ 'array of L-1 bin edge frequencies, in FT bins',
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6 % M:natural ~ 'number of bins in linear-freq spectrum'
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7 % ->
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8 % [[L,M]] ~ 'L-by-M sparse array'.
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9 %
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10 % Converts a filterbank specification into a sparse matrix for
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11 % mutliplying with a linear-frequency spectrogram.
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12 %
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13 % Note, frequencies must measured in bins of the target spectrum,
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14 % Also, the first and last of the L bands are catch-alls which
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15 % get all energy below the bottom edge and above the top edge
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16 % respectively. If, eg, the bottom edge is 0, then the bottom
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17 % band will be empty.
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18
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19 E=repmat([0; edges(:); M-1],1,M);
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20 I=repmat(0:M-1,length(edges)+1,1);
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21 map=sparse(phi(E(2:end,:)-I)-phi(E(1:end-1,:)-I));
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22 map=map.*(map>8e-15); % squeeze out more small values
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23 map(:,2:end-1)=map(:,2:end-1)/2; % make up for top and bottom bins
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24
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25 % ____
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26 % this is piecewise linear ramp : ___/
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27 % It models the response of an FFT bin to a band edge as the edge
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28 % moves across the frequency range. The response of a bin to a
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29 % band is computed as the sum of responses due to the left and
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30 % right edges, ie, for the ith FT bin,
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31 % response=phi(e1-i)-phi(e2-i)
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32 % where e1 and e2 are the lower and upper edges of the band
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33 function y=phi(x), y=abs(x+0.5)-abs(x-0.5);
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34
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