How do I iterate through a line while skipping a set number of values from the beginning of the line?

% Hello, I'm given a data set of EMG signals. I created a running average mean line for the signal defined by variable "y". I want to in a way be able to iterate through each value along the average mean line y, but skipping say the first 50 values at the beginning of the line. This is because what I am trying to do is detect a drop in the average mean line y when it crosses the threshold "j" in this case. But if the starting average mean line y always start at a value of 0, it will always present itself in the elseif case of "concussion".

% MATLAB Code

% EMG = Trial1

function [dMean, dMax, dMin] = EMGFilter6(EMG)

EMG_2 = EMG;

t = EMG_2(:,1); % time

x = EMG_2(:,2); % signal

d = abs(x);

windowSize = 1000;

b = (1/windowSize)*ones(1,windowSize);

a = 1;

dMean = mean(d);

dMax = max(d);

dMin = min(d);

j = 0.01*dMean;

p = 'concussion';

l = 'no concussion';

y = filter(b,a,d);

for k = i:y

plot(t,d,'color','r')

hold on

plot(t,y,'color','b')

legend('Input Data','Filtered Data')

end

for g = i:y

if g>j

disp(l)

elseif g<j

disp(p)

elseif g == j

disp(l)

end

end

end

[Hendrik-Jan Kuijt]

I think you can do this with the movmean() function. I think you can use the enpoints option and set it to 'discard' so that you only get results if there is 50 samples of 'y' in the average.

More documentation here: https://www.mathworks.com/help/matlab/ref/movmean.html

Answer 1

[Mathieu NOE]

hello

maybe this code could be of some help (to extract snipsets of data based on threshold comparison with running rms value of the signal)

each snipset of length above min_contiguous_samples and of rms amplitude above threshold is stored in cell array data_store (the colored individual segement in bottom picture) :

code :

%% load data
load data.mat
time = Trial1(:,1);
data = Trial1(:,2);
samples = length(data);
dt = mean(diff(time));


%% parameters
min_contiguous_samples = 1000; % store "red" segments only if they are at least this length
threshold = 0.05; % 1 = max (100%)  of rms value
samples_befor = 100; % add some samples before the "start" point detected by code below
samples_after = 100; % add some samples after the "end" point detected by code below
%% main loop %%%%
% running rms (buffered) parameters : 
rms_buffer = 500;  % nb of samples in one buffer (buffer size) 
shift = 1; % = rms_buffer-overlap;    % nb of samples between 2 contiguous buffers  
    for ci=1:fix((samples-rms_buffer)/shift +1)
        start_index = 1+(ci-1)*shift;
        stop_index = min(start_index+ rms_buffer-1,samples);
        time_index(ci) = round((start_index+stop_index)/2);  % time index expressed as sample unit (dt = 1 in this simulation)
        rms_data(ci) = my_rms(data(start_index:stop_index));  % 
    end
    time_rms = time(time_index);
    ind = (rms_data>threshold*max(rms_data)); 
    
    % now define start en end point of "red" segments
    [begin,ends] = find_start_end_group(ind);
    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 
    
    % define for plot the red rms data 
    time2 = time_rms(ind);
    rms_data2 = rms_data(ind);
    % define the begin / ending x, y values of raw data 
    time2_begin = time_rms(begin);
    data_begin = interp1(time,data,time2_begin);
    time2_ends = time_rms(ends);
    data_ends = interp1(time,data,time2_ends);
    
figure(1),
plot(time,data,'k',time2,rms_data2,'r.',time2_begin,data_begin,'*c',time2_ends,data_ends,'*m','MarkerSize',12);
legend('signal',['rms above ' num2str(threshold*100) ' %'] ,'begin points','end points');
% store each "red" segment separately (in cell array)
figure(2), hold on
for ci = 1:length(begin)
    ind = (time>=time2_begin(ci)-samples_befor*dt & time<=time2_ends(ci)+samples_after*dt);
    xx = time(ind);
    yy = data(ind);
    data_store{ci} = [xx(:) yy(:)]; % 2 columns : time / data
    plot(xx,yy);
end
hold off
%%%% end of main file %%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
function [begin,ends] = find_start_end_group(ind)
% This locates the beginning /ending points of data groups
    D = diff([0,ind,0]);
    begin = find(D == 1);
    ends = find(D == -1) - 1;
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
function x_rms = my_rms(x)
    x_rms = sqrt(mean(x.^2));
    
end