MATLAB: Find peaks in a specific portion of data

Question

Hello,

I have sets of data with a group of artifacts at the begging and end (see below – ignore the middle artifact):

I need to isolate the portions of the data with the start/end artifiacts as I'll be removing them. Right now I'm identifying the group of artifcats manually by literally zooming in on them and seeing exactly which point they start and end:

xlim_br_start = [1000 9000];
xlim_br_end = [320000 350000];

But because I have multiple sets of data like this that have these start/end artifacts but, with slightly varying start/end times, I would like to automate the process of isolating them so I can remove them.

Does someone know how to go about doing this? Any advice is appreciated.

Thanks in avance.

Answer 1

Option 1: set threshold

If the outliers always have such an enormous amplitude relative to the signal, an easy approach would be to set a threshold value that is several times larger than the expected signal. Any points that exceed the threshold are identified as outliers.

% Create data
y = rand(1,10000) ./ 50; 
y(randi(200,1,30)+300) = rand(1,30)/2 + 0.5; 
x = linspace(0, 3.5, 10000); 
threshold = 0.2; 
% Plot data
figure
subplot(2,1,1)
plot(x,y, 'b-')
% Mark threshold
hold on
plot(xlim,threshold*[1,1], 'r-')
%identify outliers

isOut = y > threshold; 
% Plot data without outliers
subplot(2,1,2)
plot(x(~isOut), y(~isOut), 'b-')

Option 2: use isoutlier()

isoutlier(): https://www.mathworks.com/help/matlab/ref/isoutlier.html

This function identifies outliers. Pros: It's more flexible if the thresholding method doesn't work. Cons: It may require tweeking (see input options) and if the SNR is low, it may not be sufficient.

%Same as my example above except...
%identify outliers
isOut = isoutlier(y);