MATLAB and Simulink resources for Arduino, LEGO, and Raspberry Pi

# How to find frequency from image of spectrogram?

Asked by Sunil Shahi on 30 Jun 2013

Here is what I have done so far.

[~,~,~,P]=spectrogram(W(:,end),tres,tres/2,fres,fs); %tres is time resolution %and fres is frequency resolution.

I=flipud(-log(P)); %here 'I' gives the spectrogram image in a matrix.

imshow(I,[]); %this line will display the image.

now my question is if I choose a pixel co-ordinate from the image matrix I how can I get its frequency?

## Products

Answer by Wayne King on 30 Jun 2013
Edited by Wayne King on 30 Jun 2013

If you output the vector of frequencies from spectrogram(), you will have the frequency information.

```      t=0:0.001:2;
x=chirp(t,0,1,150);
[y,f,t,p] = spectrogram(x,256,250,256,1000);```

Each row of p corresponds to the frequency at the corresponding element of the vector, f. But keep in mind, that it's only as good as the frequency resolution you have. If you're looking for a technique that gives you instantaneous frequency and time behavior for a signal, it does not exist. You can only obtain information about a time-frequency rectangle of a minimum area.