Spectrogram windowing- why do the 2 methods generate different graphs?
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Method 1 spectrogram(x,128,16,128,FS)
Method 2 w = window(@hamming,128); [S,F,T,P] = spectrogram(x,w,16,128,FS);
Since I cannot change the window type in method 1, I am using method 2. I would like to know why there is a BIG difference in the figures generated.
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Wayne King
le 28 Nov 2013
Modifié(e) : Wayne King
le 28 Nov 2013
That is a different question. The answer to that question is that calling spectrogram() with no output arguments is the same as providing the 'xaxis' option for FREQLOCATION (this is explained in the help).
That means that instead of having frequency on the y-axis as your second figure does and my figures too, you have frequency on the x-axis and time on the y-axis.
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Wayne King
le 28 Nov 2013
yes, you can manipulate the color axis by setting limits and or manipulating the color map. Also, you should overlap your windows more.
For example:
[S,F,T,P] = spectrogram(x,128,64,128,FS);
surf(T,F,10*log10(P),'edgecolor','none'); axis tight;
view(0,90); caxis([-90 5])
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Wayne King
le 28 Nov 2013
Are you sure you are using MathWorks' version of spectrogram() and/or hamming()? Because I see no difference at all (and there should not be)
x = randn(1000,1);
FS = 1;
w = window(@hamming,128); [S,F,T,P] = spectrogram(x,w,16,128,FS);
[S1,F1,T1,P1] = spectrogram(x,128,16,128,FS);
isequal(S,S1) % returns a 1
isequal(P,P1) % also returns a 1
If you enter
>>which spectrogram
and
>>which hamming
Do you get paths that end in:
matlab\toolbox\signal\signal
2 commentaires
Wayne King
le 28 Nov 2013
Modifié(e) : Wayne King
le 28 Nov 2013
Here I use your signal and they are identical:
FS = 100;
w1=2; w2=5; w3=40;
t=0:1/FS:10;
dt = 1/FS;
x = zeros(size(t));
idx1 = 2/dt+1;
idx2 = idx1+1:(4/dt+1);
idx3 = (4/dt+2):length(t);
x(1:idx1) = 5*sin(2*pi*w1*t(1:idx1));
x(idx2) = 2*sin(2*pi*w2*t(idx2));
x(idx3) = sin(2*pi*w3*t(idx3));
w = window(@hamming,128); [S,F,T,P] = spectrogram(x,w,16,128,FS);
[S1,F1,T1,P1] = spectrogram(x,128,16,128,FS);
isequal(S,S1) % returns a 1
isequal(P,P1) % also returns a 1
figure
surf(T,F,10*log10(P),'edgecolor','none'); axis tight;
view(0,90);
figure
surf(T,F,10*log10(P1),'edgecolor','none'); axis tight;
view(0,90);
And yes Hamming is the default window.
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