# How can I detect constant signal patch from whole signal by using matlab script ?

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Kaustubh hike on 17 Sep 2021
Edited: Image Analyst on 17 Sep 2021
Hi,
How can I detect constant signal patch from whole signal by using matlab script ? just like shown in red color in in below plot
I would need to detect that constant signal range as well as start and end point ? KSSV on 17 Sep 2021
x = 0:0.1:10;
y = gaussmf(x,[2 5]);
% Introduce constant value
[val,idx] = max(y) ;
idx0 = idx:idx+10 ;
y(idx0) = val ;
% Get the constant data indices
idx1 = find(diff(y)==0) ;
[idx0' [idx1(1)-1 idx1]']
##### 2 CommentsShowHide 1 older comment
KSSV on 17 Sep 2021
I have shown the logic....Instead of (x,y) you use your data. It should work fine..If not share your data.

Image Analyst on 17 Sep 2021
Edited: Image Analyst on 17 Sep 2021
If the constant is fairly flat, and is always in the upper portion (because you are not interested in flat parts near the minimum values at the bottom) you can find the difference and threshold it and use find(). Untested code (because you forgot to attach your data):
subplot(3, 1, 1);
plot(x, y);
grid on;
% Find difference from previous element.
diffValues = diff(y);
subplot(3, 1, 2);
plot(diffValues, 'b-');
% Find max, min, and mid values.
minValue = min(y);
maxValue = max(y);
midValue = (maxValue + minValue) / 2;
% Threshold
threshold = 15; % What ever value defines "flat" for you.
% Find indexes in the upper half that are "flat"
indexes = (y > midValue) & (diffValues < threshold);
subplot(3, 1, 3);
plot(indexes, 'b-');
% Find starting and stopping index
firstIndex = find(indexes, 1, 'first')
lastIndex = find(indexes, 1, 'last')
% Draw vertical lines on the plot
xline(firstIndex, 'Color', 'r', 'LineWidth', 2);
xline(lastIndex, 'Color', 'r', 'LineWidth', 2)

R2017b

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