Zero-Crossings example

zerocrossings.m

clear all;
close all;
clc;
tArray=0:.01:4;
%Add some noise for good measure
noise=.1.*randn(1,length(tArray));
vArray=sin(tArray).*sin(3*pi.*tArray)+noise;
%computes the difference of each neighboring
%element in the boolean expression
test=diff(vArray>=0);
%Note: test is one element shorter than vArray
ind=find(test); %Grabs index values where
%test is non-zero

%Compute the linear interpolation
zeroCrossings=[];
for x=1:length(ind)
 rise=vArray(ind(x))-vArray(ind(x)+1);
 run=tArray(ind(x))-tArray(ind(x)+1);
m=rise/run;
b=vArray(ind(x))-m*tArray(ind(x));
zeroCrossings=[zeroCrossings,-b/m];
end
%Plot the results
plot(tArray,vArray,'Marker','.');
hold on
plot(zeroCrossings,zeros(1,length(ind)),'rs')
grid on;