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time-frequency

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Sravantej
Sravantej il 24 Gen 2012
Commentato: nour yousfi il 6 Set 2016
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

Risposte (1)

Wayne King
Wayne King il 24 Gen 2012
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');
  1 Commento
nour yousfi
nour yousfi il 6 Set 2016
Hi;how can I obtain CWT plots at specific frequencies

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