mel2hz.m

function f = mel2hz(z, htk)
%   f = mel2hz(z, htk)
%   Convert 'mel scale' frequencies into Hz
%   Optional htk = 1 means use the HTK formula
%   else use the formula from Slaney's mfcc.m
% 2005-04-19 dpwe@ee.columbia.edu

if nargin < 2
  htk = 0;
end

if htk == 1
  f = 700*(10.^(z/2595)-1);
else
  
  f_0 = 0; % 133.33333;
  f_sp = 200/3; % 66.66667;
  brkfrq = 1000;
  brkpt  = (brkfrq - f_0)/f_sp;  % starting mel value for log region
  logstep = exp(log(6.4)/27); % the magic 1.0711703 which is the ratio needed to get from 1000 Hz to 6400 Hz in 27 steps, and is *almost* the ratio between 1000 Hz and the preceding linear filter center at 933.33333 Hz (actually 1000/933.33333 = 1.07142857142857 and  exp(log(6.4)/27) = 1.07117028749447)

  linpts = (z < brkpt);

  f = 0*z;

  % fill in parts separately
  f(linpts) = f_0 + f_sp*z(linpts);
  f(~linpts) = brkfrq*exp(log(logstep)*(z(~linpts)-brkpt));

end