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Topic: STFT and ISTFT problem. I get noise with the same signal. (Short Time Fourier Transform)
Replies: 0

 Tilemachos Posts: 3 Registered: 6/30/13
STFT and ISTFT problem. I get noise with the same signal. (Short Time Fourier Transform)
Posted: Nov 4, 2013 9:56 AM

Dear friends,

I have a project which goes like this: I have to take a signal (a wav file), then do the STFT (Short Time Fourier Transform) of the signal, and then do the Inverse STFT, and get back the same signal that I had in the first place.

For some reason I get the same signal but with NOISE. I also notticed that the signal that i had at the beggining is smaller than the signal I get at the end by some points.

Here is the code of STFT and ISTFT:

*STFT*

function Xtwz = stft(x, M, R, N, w)

nframes = floor(1+ (length(x)-M)/R);
Xtwz = zeros(N,nframes); % pre-allocate STFT output array
xoff = 0;
zp = zeros(N-M,1); % zero padding (to be inserted)

for m=1:nframes

xt = x(xoff+1:xoff+M); % extract frame of input data
xtw = w .* xt; % apply window to current frame
if (N-M)~=0
xtwz = [zp(1:((N-M)/2)) ; xtw ; zp(((N-M)/2+1):(N-M))];
else
xtwz=xtw;
end
Xtwz(:,m) = fft(xtwz); % STFT for frame m
xoff = xoff + R; % advance in-pointer by hop-size R
end

% Keep half of the spectrum
if rem(N,2)==0
Xtwz = Xtwz(1:(N/2+1),:);
else
Xtwz = Xtwz(1:(N/2+1/2),:);
end

*ISTFT*

function g = istft_til(d,R,N,M)

s = size(d);
nframes = s(2);
xlen = M + (nframes-1)*R;
g = zeros(1,xlen);
i=0;

for b = 1:nframes
ft = d(:,b)';
ft = [ft , conj(ft(N/2:-1:2))];
px = real(ifft(ft));
i=(b-1)*R;
a=px((N-M)/2+1:(N+M)/2);
g((i+1):(i+M)) = g((i+1):(i+M)) + a;
end

end

Does anyone have an idea why is this happening, or how could I fix it???