time-frequency

hi, i have a signal whose frequency is varying with time, i need to know the frequencies at all the times. I need a plot of the signal frequency vs signal time. Can anyone tell me how to get that plot

Answer 1

[Wayne King]

Hi Sravantej, You have to understand that there are constraints on how precise you can obtain a time-frequency analysis of a signal.

Having said that, you can use spectrogram in the Signal Processing Toolbox and/or cwtft in the Wavelet Toolbox.

Here is an example with spectrogram:

   t=0:0.001:2;                    
   x=chirp(t,0,1,150);              
   [y,f,t,p] = spectrogram(x,256,250,1000); 
   surf(t,f,10*log10(abs(p)),'EdgeColor','none');   
   axis xy; axis tight; colormap(jet); view(0,90);
   xlabel('Time');
   ylabel('Frequency (Hz)');

Here is an example with cwtft from the Wavelet Toolbox:

   N = 1024;
   t = linspace(0,1,N);
   dt =1/(N-1);
   Y = sin(8*pi*t).*(t<=0.5) + sin(16*pi*t).*(t>0.5);
   s0  = 6*dt;  ds = 0.15;  NbSc = 50;
   wname = 'morl';
   SCA = {s0,ds,NbSc};
   cwtsig = cwtft({Y,dt},'scales',SCA,'wavelet',wname);
   MorletFourierFactor = 4*pi/(6+sqrt(2+6^2));
   Scales = cwtsig.scales.*MorletFourierFactor;
   Freq = 1./Scales;
   imagesc(t,[],abs(cwtsig.cfs));
   indices = get(gca,'ytick');
   set(gca,'yticklabel',Freq(indices));
   xlabel('Time'); ylabel('Hz');
   title('Time-Frequency Analysis with CWT');

[nour yousfi]

Hi;how can I obtain CWT plots at specific frequencies