What does "plot(spectrogram(M))" mean?

"plot(spectrogram(M(:,1)))" generates a plot whose y-axis values equal those of the x. The result is a set of lines that spiral outwards from a point in the center.

What is the significance of these axes? What does this result say about the data on a broad level?

Answer 1

[Wayne King]

If the function you are using is the MathWorks' version of spectrogram, then you are plotting a complex-valued function. I don't really see the use case for wanting to do that. I can see wanting to plot the magnitude and phase separately, but that is not what plot(spectrogram(x)) is doing.

First, make sure you are using the MathWorks' spectrogram, enter

>>which spectrogram

and ensure that you get something like:

matlab\toolbox\signal\signal\spectrogram.m

If the above is true, then read the help for spectrogram to see how to plot something meaningful:

      t=0:0.001:2;                    % 2 secs @ 1kHz sample rate
      x=chirp(t,0,1,150);             % Start @ DC, cross 150Hz at t=1sec 
      F = 0:.1:100;
      [y,f,t,p] = spectrogram(x,256,250,F,1E3,'yaxis'); 
      % NOTE: This is the same as calling spectrogram with no outputs.
      surf(t,f,10*log10(abs(p)),'EdgeColor','none');   
      axis xy; axis tight; colormap(jet); view(0,90);
      xlabel('Time');
      ylabel('Frequency (Hz)');