Spectrogram windowing- why do the 2 methods generate different graphs?

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Win 2013-11-28

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评论： Win 2013-11-28

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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Answer 1

Wayne King 2013-11-28

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编辑：Wayne King 2013-11-28

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 2013-11-28

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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

Ok, that helps, thanks! :)

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Answer 2

Wayne King 2013-11-28

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在 MATLAB Online 中打开

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

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Win 2013-11-28

在 MATLAB Online 中打开

I am using the spectrogram function but in the documentation it is said that hamming is the default window. Hence I tried to reproduce it using the second function. Here are my codes:

clear all; FS = 100; w1=2; w2=5; w3=40; i=1; for t=0:1/FS:10

    if t<=2
        x(i)=5*sin(2*pi*w1*t);
    end
    if (t>2)&& (t<=4)
        x(i)=2*sin(2*pi*w2*t);
    end
    if (t>4)&& (t<=10)
        x(i)=sin(2*pi*w3*t);
    end
    i=i+1;
end
%Method 1       
%spectrogram(x,128,16,128,FS)          
%Method 2
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);

Win 2013-11-28

On typing which spectrogram I get: C:\Program Files\MATLAB\R2012a\toolbox\signal\signal\spectrogram.m and on typing which hamming I get C:\Program Files\MATLAB\R2012a\toolbox\signal\signal\hamming.m

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Answer 3

Wayne King 2013-11-28

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https://ww2.mathworks.cn/matlabcentral/answers/107954-spectrogram-windowing-why-do-the-2-methods-generate-different-graphs#answer_116671

编辑：Wayne King 2013-11-28

在 MATLAB Online 中打开

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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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