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I am new to Matlab and signal processing. I am having an issue with defining the frequency range in which the spectrogram is processed. When I am plotting the spectrogram of .wav audio data, the y axis, frequency, spans from zero to around 23 kHz. The useful data I am looking for is in the range of 200-400 Hz. My code snippet is:

[samFa, fs] = audioread('samFa.wav'); %convert audio to numerical data
samFa = samFa(:,1); %take only one channel of numerical output
spectrogram(samFA,2205,1200,12800, fs,'yaxis','MinThreshold',-80);

I don't want to be some noobie that runs into a problem and instantly gives up and posts a duplicate question to stackoverflow, so I have done as much digging as I can, but am at my wit's end.

  • I scoured the documentation for parameters or ways to have Matlab only analyze a subset or range of the data, but found nothing. Additionally, in all of the examples the frequency range seems to automatically adapt to the data set.
  • I know it is possible to just calculate the spectrogram for the entire range of frequencies, and then remove all of the unnecessary data through truncating or manually changing the limits in the plot itself, but changing plotting limits does not help with the numerical data.

  • I went searching through many similar questions, and found an answer all the way from 2012 here: Can I adjust spectogram frequency axes? where the suggested answer was to import a vector of specific frequencies for the spectrogram to analyze. I tried passing a vector of integer values between 200 and 400, and a few other test ranges, but got the error: 

    Error using welchparse>welch_options (line 297)
The sampling frequency must be a scalar.

    I've tried passing the parameter in at different places in the function, with no avail, and don't see anything regarding this parameter in the documentation, leading me to believe that this functionality was possibly removed sometime between 2012 and now.

  • When plotting spectrogram without providing signal frequency, Matlab provides a normalized spectrogram, which only provides a much smaller data window, which I can visually assess to cover the data from 0:5kHz (an artifact of overtones in the audio), so I know that matlab is not finding any data above this range to make the frequency range go to 20kHz
  • I've been trying to learn some signal processing for this project, so I believe the Nyquist frequency should be the maximum frequency that a Fourier transform is able to analyze, to be half the sampling frequency. My recording frequency is sampling at 44,100 Hz, and the spectrogram is ranging to around 22 or 23 kHz, leading me to believe that it's Matlab is noticing my sampling frequency and assuming that it needs to analyze up to such a high range.

For my work I am doing I am needing to produce thousands of spectrograms to then be processed through much further analysis, so it is very time consuming for Matlab to be processing so much unecessary data, and I would expect there to be some functionality in Matlab somehow to get around this.

Sorry for the very long post, but I wanted to fully explain my problem and show that I have done as much work as I could to solve the problem before turning for help. Thank you very much.

yodofizzy
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1 Answers1

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Get the axis handle and set the visual range there:

spectrogram(samFA,2205,1200,12800, fs,'yaxis','MinThreshold',-80);
ax=gca;
ylim(ax, [0.2,0.4]); %kHz

And if you want to calculate specific frequencies range to save time you better use goertzel.

f = 200:10:400;
freq_indices = round(f/fs*N) + 1;   
dft_data = goertzel(data,freq_indices);
Mendi Barel
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  • Note that if you need more than log(N) Goertzel filters for your frequency resolution requirements, using an FFT of size N and throwing away all the unneeded spectrum is usually faster. – hotpaw2 Nov 18 '17 at 18:00