Spectrogram: HOW TO CONTROL ZERO-PADDING for individual fft's of an ULTRA LONG sequence

Question

francois heslot 2021-12-15

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https://ww2.mathworks.cn/matlabcentral/answers/1611265-spectrogram-how-to-control-zero-padding-for-individual-fft-s-of-an-ultra-long-sequence

回答： Bjorn Gustavsson 2021-12-15

Hello,

I have a long audio sequence x at sampling rate fs, and would like to perform a spectrogram on it; (fs = 44000, and this is a several second recording).

I want this spectrogram

to be built from half-overlapping individual audio snips of say 3000 audio data points
I want to have the individual FFT of say length nFFT= 2^16 , i.e. with a very substantial amount of zero padding (for very fine interpolation purpose).

So in this case I want a specific control on the zero-padding, i.e. a zero-padding that is NOT governed by the criterium nFFT > size(x).

Can this be done using matlab spectrogram ?

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Answer 1

Chunru 2021-12-15

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https://ww2.mathworks.cn/matlabcentral/answers/1611265-spectrogram-how-to-control-zero-padding-for-individual-fft-s-of-an-ultra-long-sequence#answer_855515

在 MATLAB Online 中打开

"doc spectrogram" for details.

[x, fs] = audioread('RockGuitar-16-44p1-stereo-72secs.wav');

whos

Name Size Bytes Class Attributes fs 1x1 8 double x 3195904x2 51134464 double

% zeros will be padded automatically

% win overlap nfft

spectrogram(x(:,1), 3000, 1500, 2^16, fs);

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Answer 2

Fabia Bayer 2021-12-15

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https://ww2.mathworks.cn/matlabcentral/answers/1611265-spectrogram-how-to-control-zero-padding-for-individual-fft-s-of-an-ultra-long-sequence#answer_855500

在 MATLAB Online 中打开

Hello,

if I understand your question correctly, you want segments of length n_w = 3000, each overlapping by 1500 samples, and then zero-padded with 2^16-3000 zeros for your FFT.

This should be technically possible using

n_w = 3000;
n_fft = 2^16
window = ones(1, n_w) %no window - add here whichever window of length n_w, i.e. hamming, hanning etc you prefer
spectrogram(x, window, [], n_fft)

However, you may want to think about whether this solution actually computes what you are looking for. A high amount of zero-padding increases the frequency resolution of your result, but it also dismorphs yor result. A signal that is zero almost everywhere, as the one you specify, will reflect this in its FFT. So it may be worth thinking again whether adding this enormous zero padding truly is superior to increasing the frequency resolution by taking longer (potentially more overlapping) segments from your signals.