Find flat regions in a signal above origin (time series data)

Hi all, I would like to detect flat regions in a time series data above the origin. For your reference I have attached a figure and highlighted the region of interest in green color. I have also included the sample data for the same.

I have tried the following code and got the below figure, where it identifes too many flat regions and I dont know how to adjust it to get the above results. Any help in this regard is highly appreciated.

load data.mat;
x=1:numel(data);
y=data;

z = y;
thresh = 0.05; % height threshold
% find peaks
[pks,locs] = findpeaks(z,'MinPeakProminence',thresh);
% remove signal noise between peaks
for ii = 1:length(locs)-1
    zz = z(locs(ii)+1:locs(ii+1)-1);
    zz(abs(zz) < thresh) = 0;
    z(locs(ii)+1:locs(ii+1)-1) = zz;
end
% plot
plot(x,y);
hold on
plot(x,z);
plot(x(locs),pks,'og');
legend('original signal','modified signal','peaks')

Answer 1

[Mathieu NOE]

hello

IMHO, you don't need the peaks code (and the for loop) to make the low amplitude signals to zero

you can do it directly on the raw signal

then I added a new portion of code to detect the flat sections that are longer than min_contiguous_samples contiguous samples

so you can also discard if needed the short ones

see the demo below :

load data.mat;
x=1:numel(data);
y=data;
z = y;
thresh = 0.05; % height threshold

%% your for loop simplified : 
ind = abs(z) < thresh; % NB we need ind in the new code below
z(ind) = 0;

%% new code
min_contiguous_samples = 300; % select segments only if they are at least this length => detect signal  = 1
detect_signal = zeros(size(x))+0.5; % initialisation

    % now define start en end point of  segments above threshold
        %%%%%%%%%%
        % This locates the beginning /ending points of data groups
        D = diff([0;ind;0]);
        begin = find(D == 1);
        ends = find(D == -1) - 1;
        %%%%%%%%%%
        
    length_ind = ends - begin;
    ind2= length_ind>min_contiguous_samples; % check if their length is valid (above min_contiguous_samples value)
    begin = begin(ind2); % selected  points 
    ends = ends(ind2); % selected  points 
    
    x2 = x(ind);
    y2 = y(ind);
    % define the begin / ending x, y values of raw data 
    x2_begin = x(begin);
    y_begin = interp1(x,y,x2_begin);
    x2_ends = x(ends);
    y_ends = interp1(x,y,x2_ends);
    
for ci = 1:length(begin)
    ind = (x>=x2_begin(ci) & x<=x2_ends(ci));
    detect_signal(ind) = 1;
end

% plot
plot(x,y,x,z,x,detect_signal);xlim([2 2.3]*1e5);ylim([-1.5 1.5]);
legend('original signal','modified signal','flat sections detected')

[Ganesh Naik]

Dear Mathieu, thanks for your help, much appreciated. I will go through this code in detail. I need a small favor from you. I would like to store start and end of the detected flat regions from the original data in a vector, could you please help me with it. One more thing, so by adjusting the contigiuos sample length i can adjust the flat regions right?

I am going to accept this answer soon, thanks again.

Kind regards

[Mathieu NOE]

hello Ganesh

look at the arrays x2_begin and x2_ends (already in my code)

those values are the start and stop x values of the valid flat regions

valid here means according to the min specified length by min_contiguous_samples

all the best

[Ganesh Naik]

Dear Mahtieu, thanks for the answer, yes I can create a vector using x2_begin and x2_ends data. I will also adjust the min_contiguous_samples as per my requirements.

Thanks and regards

[Mathieu NOE]

ok

as always, my pleasure !