Zero-crossing calculation

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Jayanta Deb
Jayanta Deb on 12 Aug 2017
Commented: GIULIA CISOTTO on 3 Nov 2020
Hi all, I need a script which can calculate zero crossing of the signal. I have the value of x and y. The script should be able to calculate the exact position of zero crossing points. Help!

Accepted Answer

Star Strider
Star Strider on 12 Aug 2017
This is reasonably robust:
function ZC = ZeroX(x,y)
zci = @(v) find(v(:).*circshift(v(:), [-1 0]) <= 0); % Returns Approximate Zero-Crossing Indices Of Argument Vector
zxidx = zci(y);
for k1 = 1:numel(zxidx)
idxrng = max([1 zxidx(k1)-1]):min([zxidx(k1)+1 numel(y)]);
xrng = x(idxrng);
yrng = y(idxrng);
ZC(k1) = interp1( yrng(:), xrng(:), 0, 'linear', 'extrap' );
x = linspace(0, 11*pi, 42); % Create Data
y = sin(x); % Create Data
ZC = ZeroX(x,y);
plot(x, y, '-r')
hold on
plot(ZC, zeros(size(ZC)), 'pg')
hold off
GIULIA CISOTTO on 3 Nov 2020
Dear Ike Brown,
thank you for sharing your code. However, I would like to let you know that it seems to incorrectly detect ZC points.
I wrote an alternative Matlab script (myZC.m) to find the ZC. Here, you can find it.
To test the difference between ZeroX and myZC, you can run a modified version of your toy example as follows:
x = linspace(0, 11*pi, 42); % Create Data
y = sin(x); % Create Data
n = 1 + 2.*randn(1,length(y)); % add Gaussian noise
y = y+n;
% myZC
[myZC, myZCx, myZCy, ys] = myZC(x,y);
% Comparison with ZeroX
ZC = ZeroX(x,y);
% Print ZC values
[length(myZC), length(ZC)]
% Plot figure
figure, hold all
plot(x,y,'k'), xlabel('No. samples'), ylabel('Amplitude [A.U.]')
plot(x(myZC), zeros(size(myZC)), 'bs', 'markerface', 'b')
plot(myZCx, myZCy, 'cd', 'markerface', 'c')
plot(ZC, zeros(size(ZC)), 'pr'), grigrid on, legend('y', 'ys', 'myZC', 'myZC-refined', 'ZC')
In attach, you can also find a representative plot: you can notice that ZeroX can mostly correctly detect the zero-crossigs, but sometimes it misses one (between sample no.17 and sample no.18) or it detects a false one (at sample 27).
ZeroX returns a correct number of zero-crossings, but when plotted, they are mismatched to the raw signal, while myZC can correctly compute the number of zero-crossing and precisely get their locations.

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More Answers (1)

MSP on 12 Aug 2017
GIULIA CISOTTO on 3 Nov 2020
Dear Shubhashree, this is not actually true: zero-crossing is defined as the number of times a signal changes its sign. If you apply your computation, then you might have an incorrect number of ZC. Let's consider a signal with a number of consecutive samples set to zero: then, you will add all of them to the ZC counter. However, you would be wrong in that. Please, check my answer above for the correct computation of ZC.

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