Threshold in my plot.

I have a plot (Amplitude vs time), and I want to obtain the first time determined amplitude appears in my plot. Let's say I have my plot, and the amplitude ranges from -1 to 1, and I decide that the threshold is 0.5, it means that I want to know the time (x-value) that this amplitude was first recorded. How can I do it?

Answer 1

[Image Analyst]

Try this:

% Find where Amplitude first exceeds 0.5:
firstIndex = find(Amplitude > 0.5, 1, 'first');
% Get Amplitude at that index:
amplitudeAtThresh = Amplitude(firstIndex);
% Get time at that index:
timeAtThresh = t(firstIndex);

[Kaique Silva]

Thank you very much, it worked!

[phdcomputer Eng]

@ImageAnalyst I wanted to ask you If this solution is practical for my problem?

I computed the average of distances between each column and other columns in a matrix except itself. (the matrix is nxm size) Then I plotted the distances and I'm trying to empirically obtain the point where the distance values start to decrease but the figure doesn't show this. I attached the figure of distances. I don't know what should I define x-axis and y-axis.

close all;
clc
load lung.mat
a=lung; 
[n,m]=size(a);
For i=1:m
  For j=1:m-1
    If(i==j)
      d=0;
    Else
      d=pdist2(a(:,i),a(:,j),'jaccard');
      s=sum(d)/(m-1);
    End
  End
End
figure,plot(s),title('jaccard distances')

I wanted to know how to define a formula to find the start point of values' decreasing?

Answer 2

[Joseph Cheng]

use the function find and any logical operators to determine the position in the array you are at/above/below the threshold.

amp = -1:.1:1;
time = 1:numel(amp);
thres = .5;
tsample = find(amp==thres);
figure,plot(time,amp,time(tsample(1)),amp(tsample(1)),'rx')

you didn't mention crosses, above, and/or below in your question but at the threshold. so i used the == to determine when you are at the threshold. If there is more to the question about detecting rising and falling edges threshold the signal using the threshold and then use diff() to determine positive or negative transitions.

[Image Analyst]

This will work for powers of 2 or inverse powers of 2. For other floating point numbers you need to use a tolerance if you want to check for "equality", see the FAQ. Also, time is a built-in function so you would not want to use that as a variable name.

[Joseph Cheng]

very true, but is time() a built in function? I air coded the above and just opened matlab to check the doc on time() but found nothing. Which toolbox is it in?

[Image Analyst]

It's in base MATLAB, at least it's in the R2015a version: