# Cut Sinusoidal signal in a integer number of periods

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youjarr on 9 Jul 2019
Answered: Roberto Rubino on 21 Oct 2022
Hey guys,
I have an measurement of three sinusoidal signals.
But it is enough to only work with one in this case.
The measurement doesn´t have an int number of periods and i need to cut for my further work.
To make it more clear here is an picutre:
In this case we have 4 periods.
So i have to find the first negative to positive zero corssing and the last negative to positive zero crossing.
I found this code in the forum but it don´t work with my files but maby it can help u ?!
t = linspace(0,10*pi,200);
x = sin(t);
mcs = x .* circshift(x, [0 -1]);
zxix = find(mcs <= 0);
for k1 = 1:2:size(zxix,2)-1
zx(k1) = interp1(x(zxix(k1):zxix(k1)+1), t(zxix(k1):zxix(k1)+1), 0);
end
figure(1)
plot(t, x)
hold on
plot(zx, zeros(size(zx)), '+r', 'MarkerSize',10)
hold off
grid
I hope u can help me.
##### 2 CommentsShow 1 older commentHide 1 older comment
youjarr on 9 Jul 2019
Edited: youjarr on 9 Jul 2019
With this:
index1 = find(y > 0, 1, 'first');
index2 = find(y > 0, 971, 'last');
yCropped = y(index1:index2);
xCropped = x(index1:index2);
I get what i want but how can i find this 971 automtically?

David Sanchez on 9 Jul 2019
Hi,
bear in mind that the sampling of your curve might play an important role in the game.
t = linspace(0,10*pi,200);
x = sin(t);
zero_crossings_pos = [];
for kind = 1:length(t)-1
if (x(kind) <= 0) && (x(kind+1) > 0)
zero_crossings_pos = [zero_crossings_pos kind];
end
end
figure,
plot(t,x,t,x,'o')
hold on
plot(t(zero_crossings_pos),x(zero_crossings_pos),'r*')
hold off
xlabel('x')
ylabel('sin(x)')
title('sin')
t_crop = t(zero_crossings_pos(2):zero_crossings_pos(5));
x_crop = x(zero_crossings_pos(2):zero_crossings_pos(5));
figure,
plot(t_crop,x_crop)
xlabel('x')
ylabel('sin(x)')
title('Cropped sin')
youjarr on 9 Jul 2019
I tried the code and had to change a small thing but now it works:
t_crop = t(zero_crossings_pos(1,1):zero_crossings_pos(1,end));
x_crop = x(zero_crossings_pos(1,1):zero_crossings_pos(1,end));
Thank you very much.

Roberto Rubino on 21 Oct 2022
Here is a solution equivalent to the one of @David Sanchez, but faster when dealing with large vectors:
t = linspace(0,14*pi,1e6);
x = sin(t+1);
% Find the negative-to-positive crossings ==========================
Signx = sign(x);
zero_crossings_pos = find(diff(Signx)>0);
% =========================================================
t_crop = t(zero_crossings_pos(1)+1:zero_crossings_pos(end));
x_crop = x(zero_crossings_pos(1)+1:zero_crossings_pos(end));
plot(t,x)
hold on
plot(t_crop,x_crop)
legend('non-cropped sine','cropped sine')
xlabel('x')
ylabel('sin(x)')
title('Cropped sin')

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