How to sample data at set intervals and then plot?

I am currently trying to create code that samples ECG data at certain intervals and extracts the maximum value between each interval. I believe my code does this as the ouput A returns the peaks of the data. However when i try and then plot these peaks it instead plots it as the sample number. E.g. one of my peaks is 257 in amplitude but the graph plots it as the 257th sample instead of the sample corresponding to the max value. I have attched an example of what my code plots. Any help is greatly appreciated!

ECG = load('ECG.csv');


Fs = 350;
frame_duration = 0.5;
frame_len = frame_duration*Fs;
N = length(ECG);
num_frames = floor(N/frame_len);

Time = (0:length(ECG)-1)/Fs; %Number of samples divided by frquency
Amp = ECG(:,1); %ECG Amplitude
hold on
for k = 1:num_frames
    frame = ECG((k-1)*frame_len + 1: frame_len*k);
    
    max_val = max(frame);
    
    if (max_val > 100)
       A = max_val
       figure 
    plot(Time,Amp,Time(A),Amp(A),'r*')
    end
    hold off
end

[KALYAN ACHARJYA]

Ensure that A must be vector (array).

Answer 1

[Image Analyst]

You're giving the max value as the index when what you should be getting from max() is the index. And you don't need A at all.

Try

    [max_val, indexAtMax] = max(frame);    
    if (max_val > 100)
        hFig = figure 
        plot(Time,Amp, Time(indexAtMax),Amp(indexAtMax),'r*')
        hFig.WindowState = 'maximized'; % Maximize the window, if desired.

[ImperialStar]

Unfortunately its still plotting each max value as a point on the X axis rather than the Y. Do know what I could be doing wrong please?

[Image Analyst]

Your index is wrong. Try this:

% Read data.
ECG = csvread('ECG.csv');
Fs = 350;
frame_duration = 0.5;
frame_len = frame_duration*Fs;
N = length(ECG);
num_frames = floor(N/frame_len);
Time = (0:length(ECG)-1)/Fs; %Number of samples divided by frquency
Amp = ECG(:,1); %ECG Amplitude
plot(Time, Amp, 'b-', 'LineWidth', 2);
xlabel('Time', 'FontSize', 20);
ylabel('Entire ECG', 'FontSize', 20);
hold on
threshold = 100;
line(xlim, [threshold, threshold], 'Color', 'r', 'LineWidth', 2);
grid on;
for k = 1:num_frames
	index1 = (k-1)*frame_len + 1;
	index2 = frame_len*k;
	frame = Amp(index1 : index2);
	fprintf('Extracted frame #%d, from index %d to %d.\n', ...
		k, index1, index2);
	
	[max_val, indexAtMax] = max(frame);
	originalIndex = indexAtMax + index1 - 1;
	fprintf('  Max of %f occurs at index %d of cropped signal, or %d of original signal.\n', ...
	max_val, indexAtMax, originalIndex);
	if (max_val > threshold)
		% Bring up a new figure.
		figure
		plot(Time,Amp, Time(originalIndex),Amp(originalIndex),'r*')
		hold on
		line(xlim, [threshold, threshold], 'Color', 'r', 'LineWidth', 2);
		xlabel('Time', 'FontSize', 20);
		ylabel('Entire ECG', 'FontSize', 20);
		grid on;
		hFig.WindowState = 'maximized'; % Maximize the window, if desired.
	end
	hold off
end

[ImperialStar]

This is brilliant thank you. It has also helped me realise how to sort out my various other scripts so it was a big help. Much appreciated.

Answer 2

[David Hill]

ECG = load('ECG.csv');
Fs = 350;
frame_duration = 0.5;
frame_len = frame_duration*Fs;
N = length(ECG);
num_frames = floor(N/frame_len);
Amp = ECG(:,1)'; %ECG Amplitude
Amp = Amp(1:num_frames*frame_len);
Time = (0:length(Amp)-1)/Fs;
amp = reshape(Amp,frame_len,num_frames);
[mamp,idx] = max(amp);
Idx=(find(mamp>100)-1)*frame_len + idx;
plot(Time,Amp,Time(Idx),mamp(Idx),'r*');

[ImperialStar]

Unfortunately this returns an error of "Matrix dimensions must agree".

[David Hill]

ECG = load('ECG.csv');
Fs = 350;
frame_duration = 0.5;
frame_len = frame_duration*Fs;
N = length(ECG);
num_frames = floor(N/frame_len);
Amp = ECG(:,1)'; %ECG Amplitude
Amp = Amp(1:num_frames*frame_len);
Time = (0:length(Amp)-1)/Fs;
amp = reshape(Amp,frame_len,num_frames);
[mamp,idx] = max(amp);
Idx=(find(mamp>100)-1)*frame_len + idx;
plot(Time,Amp,Time(Idx),Amp(Idx),'r*');

This should work. Could not test without the ECG.csv file.