Detect main part of a signal

Hi, I would like to receive some help about one question:

Need to know how to detect the real signal in an EMG file (as image 1).

Till now, I know how to find the absolute center with this code:

[F, C]=size(D.data);

center=(F-mod(F,2))/2;

x=x(center-1500:center+1500,:);

and then I search results on both sides of that middle point, but as you can see, it doesn't return symetric values (image 2).

How can I add something to that code to detect the interesting area?

Thanks, regards

Answer 1

[Star Strider]

I usually use the envelope function, usually using the 'peak' option, and then test for the first instance of those (ususlly the upper envelope) exceds a ceertain threshold.

To illustrate —

Fs = 1E+3;
L = 60;
t = linspace(0, Fs*L, Fs*L+1).'/Fs;
EMG = randn(size(t))/10;
N  = numel(t((t>=15 & t<=45)));
EMG(t>=15 & t<=45) = randn(N,1);

figure
plot(t, EMG)
grid
xlabel('Time')
ylabel('Amplitude')
title('Original')

[eu,ed] = envelope(EMG, 50, 'peak');

thrshld = max(eu(t>0 & t<5));

sigstart = find(eu >= thrshld*1.5, 1, 'first');
sigend = find(eu >= thrshld*1.5, 1, 'last');

figure
plot(t, EMG, DisplayName="Signal")
hold on
plot(t, eu, '-r', DisplayName="Upper Envelope")
hold off
grid
xline(t([sigstart sigend]), '--k', LineWidth=2, DisplayName="Signal Borders")
xlabel('Time')
ylabel('Amplitude')
title('Original With upper Envelope & Signal Borders')
legend('Location','best')

.

[Julio]

Thank you!

[Star Strider]

As always, my pleasure!