why the number of point of sfft and istft is different、

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neal paze 2021-9-5

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https://ww2.mathworks.cn/matlabcentral/answers/1447044-why-the-number-of-point-of-sfft-and-istft-is-different

编辑： David Goodmanson 2021-9-30

I do stft to a sound,then I use the result of stft to do istft,but i find the sample point is different.

The original sound is 338160*1 double,after istft,the rebuild sound is 338112*1 double.How can I rebuild sound in the same as the point of original point?

I have try others' sound,all the number loss(original point -rebuild point) is 48.such as 338160-338112=48.

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Answer 1

David Goodmanson 2021-9-5

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https://ww2.mathworks.cn/matlabcentral/answers/1447044-why-the-number-of-point-of-sfft-and-istft-is-different#answer_781359

编辑：David Goodmanson 2021-9-30

在 MATLAB Online 中打开

Hi neal,

The documentation for stft says that if you want the number of sample points to be unaffected, then if

length of the signal = n,

width of the window = w

number of overlap points = 'over'

then (n-over)/(w-over) must an integer. But things are a lot easier to deal with by using the equivalent condition,

(n-w)/(w-over) = integer

For that to be true, whatever the prime factors of (n-w) are, then (w-over) must be composed of a product of some ot those prime factors.

n = 338160;
w = 256;
factor(n-w)
  ans =  2     2     2     2     7     7   431

If (w-over), the unoverlapped portion of the window, is denoted by z, then fortunately for a window of length 256, there are a lof of available factors for z. In your case, though, z = 64 does not divide (n-w) so things don't work, as you found out. Here are possibilities for z, with z = 64 also included:

format short g
z = [49 8*7 64 2*49 16*7];
over = w-z
Ratio = (n-w)./(w-over)         % should be an integer
fracn = over/w                  % overlap fraction
   over = 207   200   192   158   144
   Ratio = 6896         6034       5279.8         3448         3017
   fracn = 0.80859      0.78125         0.75      0.61719       0.5625

so there are some choices available. If you really like 75% overlap exactly and don't care about 256 for window length, then

n = 338160;
w = 240;
over = 180;   % z = 60
factor(n-w)
Ratio = (n-w)./(w-over)
ans =    2     2     2     2     2     2     2     2     2     2     2     3     5    11
Ratio =   5632