crossing

Detect zero (or any other) level crossing of signals.

Description

Detect the zero (or any other level) crossing of a given signal, either in discrete data points or using linear interpolation.

• MATLAB release: MATLAB 7.4 (R2007a)

[Mike Price]

 

[Andre Frotta Muller]

Very good!!!!

Thanks.

[Rajeeva Kumar]

 

[Zach Lewis]

I found that with discrete data, it can be off 1 data point to the actual closest value (closest to 0). I've added this as a check
[CC,II] = min(abs([S(ind-1) S(ind) S(ind+1)]),[],2); %find closest value to zero around crossing
ind = ind+(II-2); %update indices

[Jonathan Kellogg]

I found that if you wanted it to linearly interpolate between the t points that you give it, you need to go through the code and everywhere you see par.interpolation you need to change to just par or it breaks on the 'linear' string that put in. Otherwise works well for me.

[Alan Cumming]

I found the same as Zach Lewis, it doesn't really find the correct point (it's one point off), and the interpolation doesn't work either (not only because the function specification line is wrong, either).

[Christian Rothleitner]

In general a good algorithm. Just an improvement: First search for exact zeros. Otherwise you will count twice. Here the extension:
ind0 = find(S==0); %find amplitudes which are exactly zero
S1 = S(1:end-1) .* S(2:end);
ind1 = find(S1< 0);
ind_all_unsort = [ind0 ind1];
ind = sort(ind_all_unsort);%sorts values in ascending order
t0 = t(ind);

[Howard Fishman]

For data far from 1, the eps test me not either not pass when you want to interpolate or pass when you shouldn't,

Change line 45 to
if abs(S(ind(ii))) > eps(S(ind(ii)))

Also change line 28 to
if nargin < 3 || isempty(level)

The condition of a peak (not a crossing) at exactly zero as described by Christian Rothleitner can be handled with the following

[t0, indx] = unique(t0);
ind = ind(indx);

[Sergio GM]

It worked for me! Thanks.

[Islam Mohammed]

if you need the number of crossing, NC.

NC = length(ind1) + length(ind0);

this is because if the signal cross zero exactly at the sample time it will not be calculated in ind1.

[Abhinav Jha]

Is zero crossing, by definition, supposed to return all the indices at which the value of signal stays zero continually, or just the point at which zero is reached ? For eg if
a = [1 2 -1 0 0 0 1]
then should the return indices be
ind = [2 4 5 6],
or only
ind = [2 4].
Doesn't zero crossings mean the points at which zero is crossed ?

[Urs J]

Hi

Search for zeros in a=[-1 1], and it will "crash" at line 99 because index is set to 0.

[René Truelsen]

Excellent. Just what I was looking for. A function to find the zero-crossings of my LoG filtered signal

[Hartmut Wziontek]

Nice source, saved some time for me.
However, I'd suggest to following patch to avoid indexes out of range:

*** crossing.m.orig Tue Aug 26 07:41:32 2008
--- crossing.m Tue Aug 26 13:38:27 2008
*************** end
*** 96,103 ****
  % Addition:
  % Some people like to get the data points closest to the zero crossing,
  % so we return these as well
! [CC,II] = min(abs([S(ind-1) ; S(ind) ; S(ind+1)]),[],1);
! ind2 = ind + (II-2); %update indices
  
  t0close = t(ind2);
! s0close = S(ind2);
\ No newline at end of file
--- 96,108 ----
  % Addition:
  % Some people like to get the data points closest to the zero crossing,
  % so we return these as well
! % --- hwz 2008-08-26
! % make it work for data starting/ending with a zero crossing!
! ind0 = find(ind > 1 & ind < length(S));
! [CC,II] = min(abs([S(ind(ind0)-1) ; S(ind(ind0)) ; S(ind(ind0)+1)]),[],1);
! II = ind0(II);
! ind2 = ind;
! ind2(ind0) = ind2(ind0) + (II-2); %update indices
  
  t0close = t(ind2);
! s0close = S(ind2);

[First Last]

 

[Nauman Qureshi]

 

[BD Knight]

Following generates error, same as Urs J error:
crossing([8 .1 -3 -5],[],1)

[M. A. Hopcroft]

 

[Yan Cui]

 

[Paulo Emmanuel]

Don't know what to do. I'm kinda new at this.

??? Subscript indices must either be real positive integers or logicals.

Error in ==> crossing at 99
[CC,II] = min(abs([S(ind-1) ; S(ind) ; S(ind+1)]),[],1);

Error in ==> AudioComp at 38
indexLeftZeroCross = crossing(leftFilteredData);

[Patrick Tamellini]

Great, worked ok for me.

[Carlos Martinez]

It doesn't work any more.

[Claire]

doesn't seem to work

Index exceeds matrix dimensions.
Error in crossing (line 77)
t0 = t(ind);