How to plot the diagonal of a spectrum as a 2d plot?

I have a power spectral density spectrogram with x=time, y=frequency, c=power spectral density. My plot has very high PSD values in a linear trend on it and I want to plot the frequency and time that correspond to that high PSD linear trend. How can I plot this? I used this to plot the spectrogram:

tfSpectra = zeros(nf2,totalStack);
tft = nptsGap.*dt.*(0:totalStack-1) + twin/2;
tff = 1./dt./nf.*(0:nf2-1);
for i = 1:totalStack
    index = 1+(i-1)*nptsGap : (i-1)*nptsGap+nptsSegment;
    fdata = mean(abs(fft(uxt(index,:),nf,1)), 2);
    tfSpectra(:,i) = fdata(1:nf2);
end
imagesc(tft,tff,10*log10(tfSpectra))
h = colorbar;
set(get(h,'label'),'string','PSD (dB/Hz)','FontSize',14);
caxis([-30,15])

Answer 1

[Balavignesh]

Hi Brianna,

I understand that you have a power spectral density (PSD) spectrogram and you would like to plot the frequency and time that correspond to a high PSD linear trend.

I would suggest you calculate the maximum PSD value and its corresponding indices in the 'tfSpectra' Matrix using the inbuilt functions 'max' and 'find'. After retrieving the frequency and time values corresponding to the maximum PSD, you could plot the spectrogram using the 'imagesc' function. In addition, you could also add markers or annotations to indicate the frequency and time corresponding to the maximum PSD.

The following example code may help you achieve this:

% Sample value of tfSpectra. Use your own data
tfSpectra = [1 2 3; 4 5 6; 7 8 9];

% Calculate maximum PSD value and its corresponding indices
[maxPSD, maxIndex] = max(tfSpectra(:));
[maxFreqIndex, maxTimeIndex] = ind2sub(size(tfSpectra), maxIndex);

% Retrieve frequency and time values corresponding to the maximum PSD
tft = [0 1 2];  % Example time values
tff = [10 20 30];  % Example frequency values
maxFreq = tff(maxFreqIndex);
maxTime = tft(maxTimeIndex);

% Plot the spectrogram
imagesc(tft, tff, 10*log10(tfSpectra));
h = colorbar;
set(get(h, 'label'), 'string', 'PSD (dB/Hz)', 'FontSize', 14);
caxis([-30, 15]);

% Add markers and annotations for the maximum PSD
hold on;
plot(maxTime, maxFreq, 'ro', 'MarkerSize', 10, 'LineWidth', 2);
text(maxTime, maxFreq, sprintf('(%0.2f, %0.2f)', maxTime, maxFreq), 'Color', 'r', 'FontSize', 12, 'VerticalAlignment', 'bottom');
hold off;

Kindly have a look at the following documentation links to have more information on:

• 'max' function: https://www.mathworks.com/help/matlab/ref/max.html
• 'imagesc' function: https://www.mathworks.com/help/matlab/ref/imagesc.html

Hope that helps!

Balavignesh