Re: [help] FFT & Spectrogram

[febty febriani]

2009/7/2 Judd Storrs <address@hidden>

On Thu, Jul 2, 2009 at 2:01 AM, febty febriani <address@hidden> wrote:

octave <<EOF
Fs=1;                          
fh=fopen("result.dat","r");
x=fscanf(fh,"%lf");
x0=hanning(4096);                  
a=(x(1:4096)).*(x0);                  
b=fft(a,8192);                      
c=abs(b(1:4096));
c=c/max(max(c))    ;                  
d=((1:4096)/4096)*(Fs/2);                                      
subplot(2,1,1);plot(d,c);
subplot(2,2,1);imagesc(flipud(20*log10(c)));
EOF

What I suspect is happening here is when octave reaches the EOF it exits and doesn't draw the plots. Since there also aren't any commands to save the plots to a file that is why you don't see any output. You can perhaps add a "print" statement to get the plots to file http://www.network-theory.co.uk/docs/octave3/octave_163.html

Or if you would rather watch the plots you can add

drawnow ;
sleep(10) ;

before the EOF to pause 10 seconds before exiting. For the second subplot you may want "subplot(2,1,2)" instead of "subplot(2,2,1)"

Hope this helps,

--judd

thanks very much, it is very helpful for me. I fixed the script. I can see the FFT result. For the spectrogram, I just saw blank image. From Octave prompt, I got error like below :

GNUPLOT (plot_image):  Image grid must be at least 2 x 2.


GNUPLOT (plot_image):  Image grid must be at least 2 x 2.


my script is

###
octave <<EOF
Fs=1;    
n=4096;                        
fh=fopen("result.dat","r");
y=fscanf(fh,"%lf");
window=hanning(n);                    
y1=(y(1:n)).*(window);                    
y2=fft(y1,n*2);    
[S, f, t] = specgram(y, n, Fs, window, length(window)/2);                    
S=abs(y2(1:n));
S=S/max(max(S));                    
x=((1:n)/n)*(Fs/2);                                            
subplot(2,1,1);plot(x,S);axis([0 0.01]);title("200901");xlabel("Frequency");ylabel("Amplitude");
subplot(2,1,2);imagesc(flipud(20*log10(S)));
print -dpng output.png;
drawnow;
sleep(10);
EOF
###

any idea why I got such error?
any help is appreciated. Thanks very much.
-- 

******
febty febriani
Indonesian Institute of Sciences
Research Center for Physics
Kompleks PUSPIPTEK
Serpong, Indonesia, 15314