# Identify Zerocrossing Indices near to the peak of the other signal matlab

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Ganesh Naik on 27 May 2022
Commented: Star Strider on 28 May 2022
Hi all, I have two signals (data attached) , for first signal I plot the peaks of the signal and for the second one I find zero crossings of the signal. My aim is to find the zero crossing indices (start and stop) closer to the peak of the first signal. In this example (please refer to the figure), peak for the first signal is around 4.567 and the zerocrossings (closest ones) are closer to 4.564 and 4.571 respectively. I would like to get these indices (start and stop) for the signal with all the peak points. For your reference, I have added data, code for peak and zerocrossing and also figure.
https://www.dropbox.com/s/i8s52no5ivdo6gm/data.mat?dl=0
Any help in this regard is highly appreciated. t=1:length(Data(:,1));
[pk,lk] = findpeaks(Data(:,1),t,'MinPeakProminence',200);
ax1=subplot(211);
plot(t,Data(:,1),lk,pk,'o')
y = Data(:,2);
zci = @(v) find(v(:).*circshift(v(:), [-1 0]) <= 0);
zx = zci(y); % Approximate Zero-Crossing Indices
figure(1);
ax2=subplot(212)
plot(t, y, '-r')
hold on
plot(t(zx), y(zx), 'bp'); hold on;
% plot(t,Pepi);
hold off
grid
legend('Signal', 'Approximate Zero-Crossings')

Star Strider on 27 May 2022
The file is too large to download here, and the online Run feature still has problems with .mat files.
That aside, finding the zero-crossings (using simulated data) is straightforward
t = linspace(0, 10, 500);
s = sum((1:10)'.*(sin((1:10)'*pi*t)));
zxi = find(diff(sign(s))); % Approximate Zero-Crossing Indices
for k = 1:numel(zxi)
idxrng = max(1,zxi(k)-2) : min(numel(s),zxi(k)+2);
t_exact(k) = interp1(s(idxrng), t(idxrng), 0);
end
t_exact
t_exact = 1×100
0 0.1367 0.2346 0.3311 0.4270 0.5226 0.6180 0.7136 0.8092 0.9047 0.9999 1.0953 1.1909 1.2865 1.3819 1.4773 1.5730 1.6691 1.7656 1.8637 1.9999 2.1370 2.2348 2.3312 2.4269 2.5224 2.6181 2.7137 2.8092 2.9045
figure
plot(t, s)
hold on
plot(t(zxi), s(zxi), 'xg')
plot(t_exact, zeros(size(t_exact)), '+r')
hold off
grid
legend('Signal','Approximate Zero-Crossing Indices','Interpolated Exact Zero-Crossings', 'Location','best') figure
plot(t, s)
hold on
plot(t(zxi), s(zxi), 'sg', 'MarkerSize',10)
plot(t_exact, zeros(size(t_exact)), 'xr', 'MarkerSize',10)
hold off
grid
legend('Signal','Approximate Zero-Crossing Indices','Interpolated Exact Zero-Crossings', 'Location','best')
xlim([3 5]) I changed the zero-crossing index code from my previous posts. The one I use here is more robust and does not encounter the ‘wrap-around’ problems of my original version.
.
Star Strider on 28 May 2022
As always, my pleasure!

KSSV on 27 May 2022
Multiple options:
1. Use knnsearch
2. Use interp1
3. Use InterX: https://in.mathworks.com/matlabcentral/fileexchange/22441-curve-intersections
I would suggest to use third option.
Ganesh Naik on 27 May 2022
Hi KSSV thanks for your answer. I have two separate signals. Signal 1 generates peak points and signal 2 is a noisy signal and from it I get several zero crossing points. My aim is to pick up two closest zero crossing points from signal 2 for every peak of signal 1. Unfortunately signal 1 and signal 2 dont intersects and hence I cant use the third option that you suggested.

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