Time position in STFT output

Question

How can I understand in which time position in STFT function output am I?

For example, if I have 3987 time frame in the output of STFT, my window length is 625 (hamming), my hop size is 125 and the length of signal is 2 second.

• How can I estimate for example the first 5 milisecond or a window of 10 milisecond in the ouput of STFT? I know that the the number of time frames are calculated by

  coln = 1+fix((xlen-wlen)/h);

• But how is the equation in my situation?

• Can I say that each time frame has a length of hop size?

Answer 1

Assuming 0-based vectors, hop size $H$, and window size $M$, the frame number $N_f$ will contain the DFT of a window that starts on sample $n_1 = N_f \times H$ and ends on sample $n_2 = n_1 + M -1$.

That means that for each STFT frame, you shift the window by $H$ samples. Note that $H$ and $M$ not necessarily coincide.