How to analyze part of a signal?

Hello, I have created a sinusoidal signal, but I only want to find a peak value between 'x' to 'y' seconds of the signal. How would I set a limit so that only the data of the sinusoidal signal is analyzed in MATLAB between 'x' to 'y' seconds?

I have tried to use the function "xlim", but I think that is only for setting limits on a graph.

I hope this is not to broad of a question, but any help would be greatly appreciated.

Answer 1

[dpb]

Index into the result vector based on values of time; if you have a fixed timestep then it's simply T/dt intervals past T0 or the delta is (Tend-Tstart)/dt points.

Then, "Use the colon, Luke!"

ymax=max(y(idxStart:idxEnd));

where you've just computed the two indices above.

Alternatively, there is (beginning R2015a, anyway) a timeseries class that has the ability to retrieve data by time and methods to manipulate data that potentially could be simpler coding. I've not 'spearminted with it to date so no real firsthand data with to really recommend for/against the implementation; just know it exists.

[Anonymous45]

How are you defining idxStart and idxEnd? Is idxStart = x and idxEnd = y?

[dpb]

As outlined, it's simply Tstart/dt and Tend/dt for the two times assuming 0 origin (with a +1 correction for the 1-based array indexing vs 0 time origin).

>> dt=0.001; t=0:dt:0.1;  % arbitrary time series definition
>> tStart=0.035;          % and a time inside there to find
>> idxStart=tStart/dt+1
idxStart =
  36
>> t(idxStart)
ans =
  0.0350
>>

Or, of course, you can search for it within the time series...

>> idxStart=find(t>=tStart,1)
idxStart =
  36
>>

This has the advantage of not needing an exact match if there's just a little rounding error or you want a time not identically in the time series but close.

[Anonymous45]

I have another question if you don't mind me asking. Say I have two separate signals. For signal 1 (m1) I find the highest peak using

indexmax1 = find(max(m1) == m1)
xmax1 = er1(indexmax1)

Then I want to take that max value "xmax1(1,1)" and make it into a range xmax1(1,1)-5 to xmax1(1,1)+5.

Then I want look at signal 2 and find the peak value between xmax1(1,1)-5 to xmax1(1,1)+5. How would you limit MATLAB to focus on signal 2 within that range in order to find the peak?

[dpb]

I'm not sure who x is here; the abscissa or ordinate for the time series? Where does m1 come from in the above; why are you searching for a specific value?

You may want to study

doc findpeaks  % it has many options for peak-finding with conditions

[Anonymous45]

I am sorry for not being more clear and perhaps I should have just started this forum with my code to begin. This is the code that I am work with and the two signals I am using. I am trying to take the peak/max value of "signal_1", then create a range: max_value_x-component - 0.1e+9 to max_value_x-component + 0.1e+9.

Then for signal_5 I want to find the x and y coordinates of the peak/max value between the rang: max_value_x-component - 0.1e+9 to max_value_x-component + 0.1e+9.

 %
h = 1.00E-12;
t = 0:h:40E-9;
A = 1;
A2 = 1.1111111;
A3 = 2.2222222;
A4 = 10;
A5 = 20;
f=3E9;
nfft = 2^(nextpow2(length(t))+5);

 %Graphed signal_1
signal = cos(2*pi*f*t).*(A.*(heaviside(t)-heaviside(t-39E-9)));
snrdB = 0;
noise = 10^(-snrdB/20)*randn(size(signal));
sn = noise+signal;
y1 = fft(sn,nfft);
m1 = abs(y1);
er1 = (0:length(y1)-1)*(1/h)/length(y1);
figure (1)
subplot(2,1,1)
plot(t,sn)
title('Graph of signal_1')
xlabel('Time') % x-axis label
ylabel('Amplitude') % y-axis label
subplot(2,1,2)
plot(er1,m1)
grid on
title('Graph of signal_1 Fourier Transform')
xlabel('Frequency') % x-axis label
ylabel('Amplitude') % y-axis label

 %Graphed signal_5
signal_5 = [cos(2*pi*f*t).*(A5.*(heaviside(t)-heaviside(t-1E-9)))]+[cos(2*pi*f*t).*(A5.*(heaviside(t-4E-9)-heaviside(t-5E-9)))];
snrdB = 0;
noise = 10^(-snrdB/20)*randn(size(signal_5));
s5n = noise+signal_5;
y5 = fft(s5n,nfft);
m5 = abs(y5);
er5 = (0:length(y5)-1)*(1/h)/length(y5);
indexmax = find(max(m5) == m5)
xmax = er5(indexmax)
ymax = m5(indexmax)
figure (5)
subplot(2,1,1)
plot(t,s5n)
title('Graph of signal_5')
xlabel('Time') % x-axis label
ylabel('Amplitude') % y-axis label
subplot(2,1,2)
plot(er5,m5)
grid on
title('Graph of signal_5 Fourier Transform')
xlabel('Frequency') % x-axis label
ylabel('Amplitude') % y-axis label

Answer 2

[AJ Woodson]

If I have signal X for say the if want to take a portion of that signal you could do something like b = X(beginPoint:DesiredEnd)