Peak value on plot graph

y=a1(:,1); 
chan1= y-mean(a1); 
fs=128;
d=1/fs; 
t=[0:length(chan1)-1]*d;
fs=fft(chan1,128); 
pp=fs.*conj(fs)/128; 
ff=(2:9)/128/d; 
figure(4); 
plot(ff,pp(2:9)); 
title('Power Spectrum');
xlabel('Frequency(Hz)');
ylabel('Power Spectrum Density(µv)');
grid on;

How to find peak value on y-axis without using Data Cursor tools. Thanks in advance.

Answer 1

[Star Strider]

If you have R2017b or later, you can use the islocalmax function.

If you have the Signal Processing Toolbox, you can use the findpeaks function.

[Reff]

I tried to use findpeaks on y-axis by findpeaks(pp(2:9)), unfortunately the graph shift -1 at x-axis and -1000 on y-axis. Am I missing something?

[Star Strider]

If you are using:

[pks,locs] = findpeaks(pp(2:9))

you will need to add 1 to the ‘locs’ that findpeaks returns to get the actual indices.

Example —

x = 1:9;
pp = [50 100 1800 300 2500 1500 3000 3500 5800];
[pks,locs] = findpeaks(pp(2:9));

figure
plot(x, pp)
hold on
plot((locs+1), pks, 'r*')
hold off

Note that my code uses ‘(locs+1)’ in the plot call to refer to them correctly.

[Reff]

Hi Strider,

It works well. Really aprreciate it. Thanks.

[Star Strider]

As always, my pleasure!