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

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???

Thanks in advance.