On the use of spectrogram function in matlab

from http://stackoverflow.com/questions/19161304/on-the-use-of-spectrogram-function-in-matlab

Consider the following example:

Fs=40;% sampling frequency
x =0:(1/Fs):4;% time domain
y =[sin(2* pi *5* x(x <=2)), sin(2* pi *10* x(x >2))];% signal

    N = length(x);%Length of signal

    NFFT =2^nextpow2(N);%Next power of 2from length of y
    Y = fft(y,NFFT)/N;
    f =Fs/2*linspace(0,1,NFFT/2+1);%Generate the plot, title and labels.
    fh = figure(1);%set(fh,'color','white','visible','off');
    subplot(311);
    plot(x,y,'k');
    xlabel('Time (s)','FontName','Times New Roman','fontsize',10);
    ylabel('Amplitude','FontName','Times New Roman','fontsize',10);set(gca,'FontName','Times New Roman','fontsize',10);%# Frequency domain plots
    subplot(312);
    plot(f,2*abs(Y(1:NFFT/2+1))) 
    xlabel('Frequency (cycles/second)','FontName','Times New Roman','fontsize',10);
    ylabel('Amplitude','FontName','Times New Roman','fontsize',10);set(gca,'FontName','Times New Roman','fontsize',10);

    subplot(313);
    window = x(1:10:end);[S,F,T]= spectrogram(y,window);
    pcolor(T,F,abs(S));shading interp;
    xlabel('Time (s)');
    ylabel('Frequency (cycles/second)');

The STFT (short-time Fourier transform) here does not demonstrate what I expected. I would have expected the y axis to be the same as the xaxis in subplot(312) and the xaxis to be the same as the xaxis in subplot(311).

edited Oct 3 '13 at 14:30

Luis Mendo

asked Oct 3 '13 at 14:01

KatyB

1 Answer

The issue with the x axis is because you are not specifying the sample frequency when calling spectrogram. Try

[S,F,T]= spectrogram(y,window,[],[],Fs);

Also, I think you are misinterpreting the window argument. It refers to window shape (by the way you define it, maybe you are thinking it defines the window positions?). So I would use something like

window = ones(1,10);

(or try window shapes other than rectangular).

As for the y axis, in 313 the y axis is (instantaneous) frequency, whereas in 312 it is spectral amplitude. So they are not comparable. In principle, you could compare the color (z axis) of 313 with the y axis of 312, as both are spectral amplitude; however, their normalization is probably different.

edited Oct 6 '13 at 18:55

answered Oct 3 '13 at 14:24

Luis Mendo

修改

 subplot(313);

%     window = x(1:10:end);

    window = ones(1,80);

   [S,f,t,p] = spectrogram(y,window,[],[],Fs);

   surf(t,f,10*log10(abs(p)),'EdgeColor','none');   

     axis xy; axis tight; colormap(jet); view(0,90);

     xlabel('Time');

     ylabel('Frequency (Hz)');