MATLAB: Spectrogram time axis samples to actual time

convertsamplessampling ratespectrogramtimetime axis

Hi. I'm trying to plot the result of a spectrogram like: y-axis frequency and x-axis for time, but instead of time in seconds, it plots samples and I don't know how to convert samples to time. Normally, it would be Sample_number / Sampling_rate, but the original signal has 7813 samples and the spectrogram has only 961. Any ideas? Thank you.

Best Answer

Include the sampling frequency, and the result is a time-frequency plot (rather than an index-frequency plot). See for example: Spectrogram and Instantaneous Frequency (link). Note that fs must be the fifth input to spectrogram. To input a sample rate and still use the default values of the preceding optional arguments, specify these arguments as empty, [].

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MATLAB: How to plot spectrogram on log frequency scale

You have to get the data spectrogram produces and plot it separately:

T = 0:0.001:2;
X = chirp(T,100,1,200,'q');
[S,F,T] = spectrogram(X,128,120,128,1E3,'yaxis');
figure(1)
surf(T,F,abs(S))
view([0 90])
axis tight
xlabel('Time')
ylabel('Frequency')
set(gca,'YScale','log')
title 'Quadratic chirp'

This is the chirp signal from the spectrogram documentation. See the ‘Examples’ section for the linear scale of the same signal.

MATLAB: Get the frequency sequence of an audio file

Use the spectrogram function with outputs.

From the documentation:

[s,f,t] = spectrogram(_,fs) returns a vector of cyclical frequencies, f, expressed in terms of the sample rate, fs.

Best Answer

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MATLAB: How to plot spectrogram on log frequency scale

MATLAB: Get the frequency sequence of an audio file

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