Create 2D Spectrogram in Matlab

Question

I am in need of plotting a 2D spectrogram of a signal in Matlab. I need it for a printed assignment, hence the 3D image makes no sense. However, when the signal is plotted using Spectrogram it automatically produces a 3D plot of the signal.

My Code:

Dataset     = 1;            % Dataset to be analysed
N           = 1024;         % Window size
Beta        = 12;           % Kaiser window beta value (small = narrow main lope)
Overlap     = 800;          % Window overlap
Threshold   = -150;         % Minimum magnitude before threshold

spectrogram(Enclosure{Dataset}(1:end),kaiser(N,Beta),Overlap,2048,fs,'MinThreshold',Threshold,'yaxis');

which produces a graph that looks like this:

But it is seen from the top, and the graph is really showing this:

The reason why i need it to specifically be 2D (and why i don't settle with a screenshot) is because i am using Matlab2tikz to convert Matlab figures into Tikz figures in LaTex. with the 3D images i get figures of +100 Mb and 2D will reduce the size to <1Mb.

Answer 1

You can try the following:

[~,F,T,ps]=spectrogram(Enclosure{Dataset}(1:end),kaiser(N,Beta),Overlap,2048,fs,'MinThreshold',Threshold,'yaxis').
% Output the spectrum in ps

imagesc(T,F,10*log10(ps)) 
% Generate a 2d image

view(270,90)
xlabel('Time [s]')
ylabel('Frequency [Hz]')
c=colorbar;
c.Label.String='Power [dB]';
% Extra setting to make the plot look like the spectrogram

Good luck

Answer 2

I don't know what version of Matlab you are using but in 2015a you should be able to get a handle to the figure with the 3D plot and change the view angle to 2D:

view(0,90);

I've also got an example of how you can make your own 2D plot from the outputs of spectrogram() using a similar method:

x = [0:0.01:100];
y = sin(5*x);
y = awgn(y,0.1);

[S,F,T,P] = spectrogram(y,200,0,length(y)*5,100);

[m,n] = size(P);

figure(2)
surf(F,T,zeros(n,m),P','EdgeColor','none')
view(0,90)
xlabel('Frequency')
ylabel('Time (s)')

The output looks like this:

Hopefully since there is no altitude information, the figure size might be smaller but I can't test that since I don't have Matlab2tikz.

Answer 3

One option is to capture whatever its plotted and then plot it as an image. You can do this using getframe

if you do

F=getframe(gca);
cla;
imshow(F.cdata);

You'll get exactly what you will be seeing before, but as an image.

However I think it defeats a bit the purpose of Matlab2Tikz, as the idea os that you have Tikz code describing your data...