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%  exa040601b.m,  for  example 4.6.1 and fig 4.6.1(b) 
%  BJ分布在四个域内的形状; 
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clear; 
 
n=128; 
width=4*pi; 
omega=width/n; 
for t=1:n; 
   for f=1:n; 
      tt=(t-n/2)*omega; 
      ff=(f-n/2)*omega; 
      tf=tt*ff; 
      if tf==0 
         g(t,f)=1; 
      else 
         g(t,f)=sin(tf)/tf; 
         end 
   end 
end 
g=fft2(g); 
g=abs(g); 
g=change(g); 
g=rot90(g); 
g=change(g); 
t=-width/2+omega:omega:width/2; 
f=-width/2+omega:omega:width/2; 
 
colormap([0 1 1]) 
subplot(221) 
surf(t,f,g);