Matlab listing: fold.m

The fold() function ``time-aliases'' the noncausal part of a function onto its causal part. When applied to the inverse Fourier transform of a log-spectrum, it converts non-minimum-phase zeros to minimum-phase zeros. (See §J.11 for an example usage.)

Figure J.9: Matlab/Octave fold function.

 

  
function [rw] = fold(r) 
% [rw] = fold(r) 
% Fold left wing of vector in "FFT buffer format" 
% onto right wing 
% J.O. Smith, 1982-2002
  
   [m,n] = size(r);
   if m*n ~= m+n-1
     error('fold.m: input must be a vector'); 
   end
   flipped = 0;
   if (m > n)
     n = m;
     r = r.';
     flipped = 1;
   end
   if n < 3, rw = r; return; 
   elseif mod(n,2)==1, 
       nt = (n+1)/2; 
       rw = [ r(1), r(2:nt) + conj(r(n:-1:nt+1)), ...
             0*ones(1,n-nt) ]; 
   else 
       nt = n/2; 
       rf = [r(2:nt),0]; 
       rf = rf + conj(r(n:-1:nt+1)); 
       rw = [ r(1) , rf , 0*ones(1,n-nt-1) ]; 
   end; 

   if flipped
     rw = rw.';
   end