yun-hun-jmai asked . 2021-03-01

How to find corresponding frequencies after viewing amplitude in descending order?

Good day. Below I have a piece of code written.

%% Peak detection Part
% In this part, we have defined a threshold value. So, all peaks above
% threshold value will be detected.
temp_fft = data_filt_ft;
 
% Normalize the FFT
temp_fft = temp_fft/max(temp_fft);
 
% threshold
th = 0.02; % i.e. 1% of maximum amplitude
temp_fft(temp_fft < th) = 0; %you can skip this step by commenting this.
 
% Plot FFT after thresholding
figure; plot(fr, temp_fft);
xlabel('Frequency(Hz)'); ylabel('Amplitude');
grid on;
 
% find peaks of the remaining data
[peak_ft, peak_loc] = findpeaks(temp_fft);
 
% frequency corresponding to peak
[peak_fr] = fr(peak_loc);

%Sort amplitude values into descending order
DescendAmp = sort(peak_ft,'descend');
 
% this fuction returns the fourier transform of the signal
function [ft, f] = fr_t(x, Fs)
L = length(x);
% NFFT = 2^nextpow2(L); % Next power of 2 from length of y
NFFT = L;
 
X = fft(x,NFFT)/NFFT;
f = Fs/2*linspace(0,1,floor(NFFT/2+1));
 
ft = abs(X(1:floor(NFFT/2+1)));
% ft = 20*log10(ft);
% plot(f,ft);
 
end

I am able to sort the amplitudes into descending order but I am unable to view the corresponding frequency values in that order. How can I view the corresponding frequency values in that order?

matlab , signal , signal processing , matlab help

Expert Answer

Prashant Kumar answered . 2025-03-28 01:35:37

To view the corresponding frequency values in descending order of amplitude, you can sort the indices of the amplitude values (peak_ft) and use those indices to reorder the corresponding frequency values (peak_fr). This can be done by sorting the indices based on the amplitude and then using those indices to reorder the frequency values.

Here’s how you can do it:

Modified Code to Sort Frequencies Corresponding to Amplitudes:

% Sort amplitude values into descending order and get the corresponding frequency
[DescendAmp, sortIdx] = sort(peak_ft, 'descend');  % Sort amplitudes and get sorting indices

% Reorder the frequencies using the sorted indices
DescendFreq = peak_fr(sortIdx);

% Now DescendAmp contains the sorted amplitudes, and DescendFreq contains the corresponding frequencies
% You can plot the results or display them

% Plot the corresponding frequency vs. amplitude in descending order
figure; 
plot(DescendFreq, DescendAmp, 'o-');
xlabel('Frequency (Hz)');
ylabel('Amplitude');
title('Sorted Frequency vs. Amplitude');
grid on;

Explanation of the Changes:

Sorting Amplitude and Getting Indices:
- [DescendAmp, sortIdx] = sort(peak_ft, 'descend');
- This line sorts the amplitude values in descending order and returns the sorting indices sortIdx, which tell you how to reorder the original data.
Reordering the Frequencies:
- DescendFreq = peak_fr(sortIdx);
- Using sortIdx, we reorder the frequencies (peak_fr) to match the sorted amplitudes.
Plotting the Results:
- plot(DescendFreq, DescendAmp, 'o-');
- This line creates a plot showing the sorted frequencies against their corresponding amplitudes.

Output:

DescendAmp: Amplitudes sorted in descending order.
DescendFreq: Frequencies corresponding to the sorted amplitudes.

Now, you can directly view the frequencies corresponding to the peaks in descending order of their amplitudes.

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