Finding zero crossing that are going positive and zero crossing that are going negative

Question

I have a signal that I would like to copy when it:

1) starts at zero crossing going positive

2) copy a set number of points (like 8000)

Answer 1

You can do like this to find "going-up" or "going-down" zero crossings:

%find zero crossings
t1=y(1:n-1);
t2=y(2:n);
tt=t1.*t2;
indx=find(tt<0)

dt        = t2-t1;
indx_up   = find( (tt<0) & (dt>0) ) 
indx_down = find( (tt<0) & (dt<0) )