MATLAB: PLotting matlab noisy signal

plot

Suppose we have some noise signal that's a function of time.
Suppose at t = pi/2, 3pi/4, 5pi/4 I want to mark where exactly, at these time points, the signal is. How would you do this? Is there a toolbox I can use?
If not, what would I have to do to manually input the values of 'y' for each corresponding 't' ?

Best Answer

Do you know the sampling interval for the time signal? Suppose the sampling interval is pi/100.
 t = 0:pi/100:40;
 x = cos(pi/10*t)+0.5*randn(size(t));
 plot(t,x)
Then you know where pi/2, 3pi/4 etc are.
t(50) is pi/2 and x(50) is the signal value at t=pi/2
plot(t,x); hold on
plot(t(50),x(50),'r^','markerfacecolor',[0 0 0],'markersize',10);
plot(t(75),x(75),'r^','markerfacecolor',[0 0 0],'markersize',10);
and so on.
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