Cut Sinusoidal signal in a integer number of periods

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.

Thanks in advance

[youjarr]

Now I got this:

with this:

index1 = find(y > 0, 1, 'first');
index2 = find(y > 0, 1, 'last');
yCropped = y(index1:index2);
xCropped = x(index1:index2);

But I still need to remove one half period... .

[youjarr]

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?

Answer 1

[David Sanchez]

Hi,

bear in mind that the sampling of your curve might play an important role in the game.

The code below might help you out:

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]

Thanks for your answer.

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.

Answer 2

[Roberto Rubino]

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')