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
2013-11-28
编辑：Wayne King
2013-11-28
0 个投票
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.
4 个评论
Win
2013-11-28
Win
2013-11-28
Wayne King
2013-11-28
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])
Win
2013-11-28
更多回答（2 个）
Wayne King
2013-11-28
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
Wayne King
2013-11-28
编辑：Wayne King
2013-11-28
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.
4 个评论
Win
2013-11-28
I'm not an experienced MATLAB user and I'm not sure what you there: idx1 = 0.2/dt+1; idx2 = idx1+1:(0.4/dt+1); idx3 = (0.4/dt+2):length(t);
Also on using my code: spectrogram(x,128,16,128,FS) I get a figure showing the regions of low and high amplitude clearly and the change in time and frequency as seen in the figure attached. I get a very blurry spectrogram using your code.
Wayne King
2013-11-28
编辑：Wayne King
2013-11-28
I did not worry about optimizing the spectrogram. Your post was that the two function calls produce different results. They do not.
Sorry I made a mistake in the cutoff times, I used 0.2 and 0.4 instead of 2 and 4. I corrected it above and I plotted the results. The figures are identical.
Win
2013-11-28
Ok, maybe I worded my question wrongly
On using the following lines: w = window(@hamming,128); [S,F,T,P] = spectrogram(x,w,16,128,FS); figure surf(T,F,10*log10(P),'edgecolor','none'); axis tight; view(0,90);
I get the figure below. I wanted to know why it's different to the figure above.
Win
2013-11-28
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