# How to find valley amplitude position of individual signals?

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krishna kireeti on 21 Apr 2017
Commented: Star Strider on 22 Apr 2017
hey guys, In the given picture(consider only the upper subplot), is it possible to find the position of maximum negative peak for every signal out there? please help me asap!

Star Strider on 21 Apr 2017
I would use the findpeaks function on the negative of each signal. So the negative peaks would be:
[pks,locidx] = findpeaks(-signal); % Peaks & Peak Indices In Vector
pks = -pks; % Correct Amplitudes Of Peaks
krishna kireeti on 22 Apr 2017
don't we any other method for this, because I need to plot the peaks(positive and negative peaks in a graph) if we do like this we cannot plot both the peaks at the same time.
Star Strider on 22 Apr 2017
Yes, you can plot both peaks at the same time. You need two different calls to findpeaks, one to find the positive peaks (the ‘usual’ way), and one to find the negative peaks (negating the argument first, then negating the result). Then, use the hold command to do the plotting.
Example
[pos_pks,pos_loc] = findpeaks(signal,x);
[neg_pks,neg_loc] = findpeaks(-signal,x);
figure(1)
plot(x, signal, '-b')
hold on
plot(pos_loc, pos_pks, '^r')
plot(neg_loc, -neg_pks, 'vg')
hold off
grid
legend('Data', 'Peaks', 'Valleys')
NOTE This is UNTESTED CODE. It should work.