STFT of one particular frequency

How can I compute and plot an STFT of a signal and extract only data of one frequency not the whole spectrum?

Answer 1

[Wayne King]

Each row in the output of spectrogram() is the information at one DFT bin as a function of the time segments.

   Fs = 1000;
   t = 0:1/Fs:5-1/Fs;
   x = cos(2*pi*100*t).*(t<1)+randn(size(t));
   [xhat,f,T,psd_est] = spectrogram(x,200,150,200,Fs);

Now, you just select the row of the xhat (complex-valued) or psd_est matrix that contains your frequency of interest. Here I'll select 100 Hz.

plot(T,psd_est(21,:))

You see that 100 Hz essentially drops out after T = 1 second, which is exactly what we expect.