CODEWITHSUNDEEP
audiofftdft
Phorce
Phorce

Reputation: 4642

Calculating Hertz of STFT

This question is mainly related to this answer: Here so would be ideal if @Paul R has a chance to look at it.

I have a signal, which I have computed a STFT on. The size of the NFFT is 256 with an overlap of 128. This has produced 21 separate blocks containing the output.

I would like to therefore convert this into Hertz to see what the different Hertz are for each of these points within the signals. I have computed the magnitude using the following formula:

sqrt(output[i][j].re * output[i][j].re + output[i][j].im * output[i][j].im)

This produces the following:

enter image description here

I'm mainly confused with how the next part works. For example I essentially have a 2D vector containing all of the blocks from the STFT (each of size 256). Do I therefore calculate the magnitude for each of the points within the block, calculate the highest number of the magnitude and then use the formula freq = i_max * Fs / N?

If so, would it look something like the following (An example, not actual data):

D0 = {0.23 + 1.58, 
      1.05 + 0.56, 
      0.58 + 1.38, 
      ....., 
      0.58 + 87.6} 
      = mag[0] = sqrt(0.23 * 0.23 + 1.58 * 1.58),
        ....
        ....

This would then produce vector of magnitudes for each of the STFT output. From there, I can calculate which is the highest (Let's say 7) and then I can compute the following:

freq = 7 * 44100 / 4

Where 44100 = Sample rate and 4 = size of the STFT block.

This would then give me what frequency each of the blocks are.

Is this correct, or, am I missing the point completely?

Upvotes: 0

Views: 795

Answers (1)

ederwander
ederwander

Reputation: 3478

You're confusing a few things, your output image seems to be the all 21 blocks (FFT) together 21 * 256 = 5376 points, it's really not help nothing !

You need find the fundamental frequency for every block (NFFT=256) using:

freq = MaxMagnitudeFromThisBlockFFT * Fs / NFFT

For your case:

freq = MaxMagnitudeFromThisBlockFFT * 44100 / 256

At the end you will find 21 frequencies one for each block FFT

PS:

Your NFFT is 256 and your sample rate is 44100, then your frequency resolution is 44100/ 256 = 172,265625, if you want more accurate try NFFT= 2048 or 4096.

Upvotes: 1

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