figure(3); clf;
plot(y);
xlabel('Index numbers');

% Find a periodic section with some 4-10 periods.
ind_start = 45250; % GET YOUR VALUES HERE!!!
ind_stop  = 45750; % difference about 500-1000
y4 = y(ind_start : ind_stop);


M4 = length(y4);
t4 = [1  : M4]/fs;
plot(t4, y4);
xlabel('time (seconds)');
