Omit detrending in certain portion of signal

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Konvictus177
Konvictus177 on 22 Mar 2022
Commented: Mathieu NOE on 24 Mar 2022
Hello,
I have a signal with a certain portion that is much higher than the rest of the signal.
How can I avoid applying detrending to this portion but apply detrending to everything to the left and right of this signal? Blue portion should be detrended. Red portion should not be detrended and stay at this or a similar level.
What if I have two of these red portions within my signal? For example the lower peak portion in the middle and the higher portion to the right. Do not detrend both portions?
Thanks.
  2 Comments
Konvictus177
Konvictus177 on 22 Mar 2022
Here is the signal.
Currently I would do simple detrending but this would detrend the entire signal. I want the portions that are high (red) and the lower peak portion in the middle not to be detrended. Detrending the entire signal elevates my lower peak and lowers the higher portion of the signal. Espcially the poitns surrounding the higher portion.
% detrend signal
signal_detrended = detrend(signal,9);
% plot test signal
figure()
plot(signal)
hold on
plot(signal_detrended)
legend("before detrending","after detrending")

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Accepted Answer

Mathieu NOE
Mathieu NOE on 23 Mar 2022
hello
may I suggest a small improvement here - this avoid finding manually the samples that shoudn't be detrended
so whatever the amount and x position of the peaks, this code should work :
load('signal.mat');
n = 1:length(signal);
n = n';
signalS = smoothdata(signal,'movmedian',200);
I = abs(signal-signalS) < 5e-3;
signal_detrended = detrend(signal(I), 'SamplePoints', n(I));
signal_combined = zeros(length(signal), 1);
signal_combined(I) = signal_detrended;
signal_combined(~I) = signal(~I);
figure(1), plot(n, signal,n, signalS,n, signal_combined)
legend('raw','smoothed','partial detrend');

More Answers (1)

Benjamin Thompson
Benjamin Thompson on 22 Mar 2022
You can use an index vector and the 'SamplePoints' option for the detrend function:
>> n = 1:length(signal);
>>n = n';
>> I = (n < 4000) | ((n > 4500)) & (n < 8500);
>> signal_detrended = detrend(signal(I), 'SamplePoints', n(I));
>> hold off;
>> figure, plot(n, signal)
>> grid on, zoom on
>> hold on
>> plot(n(I), signal_detrended, 'r')
  2 Comments
Benjamin Thompson
Benjamin Thompson on 22 Mar 2022
This is just some basic MATLAB commands, but here you are. The index vector defines the set of what was detrended and what was not.
>> signal_combined = zeros(length(signal), 1);
>> signal_combined(I) = signal_detrended;
>> signal_combined(~I) = signal(~I);
>> figure, plot(n, signal_combined)

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