Matlab listing: invmyspectrogram.m

function a = invmyspectrogram(b,hop)
%INVMYSPECTROGRAM Resynthesize a signal from its spectrogram.
%   A = INVMYSPECTROGRAM(B,NHOP)
%   B = complex array of STFT values as generated by MYSPECTROGRAM.
%   The number of rows of B is taken to be the FFT size, NFFT.
%   INVMYSPECTROGRAM resynthesizes A by inverting each frame of the
%   FFT in B, and overlap-adding them to the output array A.
%   NHOP is the overlap-add offset between successive IFFT frames.
%
%   See also: MYSPECTROGRAM

[nfft,nframes] = size(b);

No2 = nfft/2; % nfft assumed even
a = zeros(1, nfft+(nframes-1)*hop);
xoff = 0 - No2; % output time offset = half of FFT size
for col = 1:nframes
  fftframe = b(:,col);
  xzp = ifft(fftframe);
  % xzp = real(xzp); % if signal known to be real
  x = [xzp(nfft-No2+1:nfft); xzp(1:No2)];
  if xoff<0 % FFT's "negative-time indices" are out of range
    ix = 1:xoff+nfft;
    a(ix) = a(ix) + x(1-xoff:nfft)'; % partial frames out
  else
    ix = xoff+1:xoff+nfft;
    a(ix) = a(ix) + x';  % overlap-add reconstruction
  end
  xoff = xoff + hop;
end

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About the Author: Julius Orion Smith III
Julius Smith's background is in electrical engineering (BS Rice 1975, PhD Stanford 1983). He is presently Professor of Music and Associate Professor (by courtesy) of Electrical Engineering at Stanford's Center for Computer Research in Music and Acoustics (CCRMA), teaching courses and pursuing research related to signal processing applied to music and audio systems. See http://ccrma.stanford.edu/~jos/ for details.