How can i get frames of a signal using peaks as reference?

I created some data to simulate a signal, then i calculate the mean and std of this signal and kept only with is up to a define treshold, then i fin the peaks in this new data, now i want to take the first peak detected and the 500ms after, then repeat this proceding with a new peak (not included in the last window) and save this frames in some array.

Anyone knows how can i abord this problem?

Thanks

This is my code

numberofdata = 1000000;
for i = 1:numberofdata;
    datos = [randn(1,4000)] >0.5;
    signal(i) = sum(datos); %this is my artificial data
end 
threshold=mean(signal)+std(signal)*2.5; 
Noisefree=find(signal>umbral);
Noisefree=signal(Noisefree); %just the values up

burst=findpeaks(Noisefree)

Answer 1

[Sourav Bairagya]

“findpeaks” function returns the values of the peaks as well as their locations. Hence, you can store both information in two arrays as follows:

[pks, locs]=findpeaks(Noisefree) 

Now, pks(1) and locs(1) will give the value and location of the first peak respectively.

Using this information, you can define the first 500ms time frame which starts from the location of the first peak and can extract the necessary information from your signal “Noisefree” and save those in some array for further processing.

arr=Noisefree(locs(1):locs(1)+499);

Here, I am assuming the signal data is spaced at an interval of 1ms. You should change according to the original spacing.

For extracting next frames starting from other peaks which were not included in previous window, you can follow this logic written in this code snippet:

endfrm = locs(1)+499;                         %End point of 1st frame 
start = locs(1)+500;                          %Probable Starting point of 2nd frame 
 
for j = 2:numel(locs)                         %Loop will run until last peak location turns up 
 
    if(locs(j)>endfrm)                        %Checking whether this peak is already in the previous window or not  
 
       endfrm=locs(j)+499;                    %If new peak is found, then compute endpoint of the new frame 
 
       if(endfrm>numel(Noisefree))            %If end-point exceeds the size of the signal, then limit it to the end index of the signal 
           
          endfrm=numel(Noisefree); 
        
       end 
 
     arr=Noisefree(locs(j):y);                %Extract the desired frame and save 
 
    end 
    %%Do your required operations on arr 
end 

Thus, you can extract and save frames from the signal using peaks as per your preferences.

Here “numel” function is used to compute the total number of elements in “locs”.

For more information about “findpeaks” follow this link:

https://www.mathworks.com/help/signal/ref/findpeaks.html

[Perla González Pereyra]

Thank you so much! Really.

Now i have only one little question: to save every frame in a matrix (one frame per row) what would be the indexing?

Thank u, again.

[Sourav Bairagya]

You can first initialize a matrix M with the first frame extracted.

M = arr1; %arr1 is the first frame starting from 1st peak

Then, inside the for loop, you can append the new frames as rows to it.

for j = 2:numel(locs)                         %Loop will run until last peak location turns up 
 
    if(locs(j)>endfrm)                        %Checking whether this peak is already in the previous window or not  
 
       endfrm=locs(j)+499;                    %If new peak is found, then compute endpoint of the new frame 
 
       if(endfrm>numel(Noisefree))            %If end-point exceeds the size of the signal, then limit it to the end index of the signal 
           
          endfrm=numel(Noisefree); 
        
       end 
 
     arr=Noisefree(locs(j):y);                %Extract the desired frame and save 
      
     M = [M; arr];
     
    end 
    %%Do your required operations on arr 
end 

[Perla González Pereyra]

Thank you very much, I was missing one line in the first code ^^ '

P.S. I really appreciate the time and the detailed explanation. It helped me a lot.