How can I determine the x-value of a sudden change in y-values?

I want to determine the start time and end time of that first impulse on the attached graph. I don't want to have to graph every dataset though (I have way too many) so would like to command matlab to do so from a two sets of values I've imported from excel. Does anyone have any suggestions? I'm very new to matlab.

Answer 1

[Star Strider]

It would help to have your data, and more detail about the features of your waveform you want to recover.

The findchangepts (link) or cusum (link) functions could be helpful.

EDIT — If you have R2017b or later, the ischange function and its friends may also work with your data.

[Julia de Lange]

This is the data I have. Basically, I want to locate the first "peak" in the data, and note the times and corresponding voltages that it starts and stops.

[Star Strider]

I am lost.

See if this does what you want:

[D,S,R] = xlsread('Data.xls');

Time = D(:,1);
Voltage = D(:,2);

[pks,locs] = findpeaks(Voltage, 'MinPeakDist',2000);

figure(1)
plot(Time, Voltage)
hold on
plot(Time(locs), Voltage(locs), '*')
hold off
grid

Then adjust the name-value pair arguments in findpeaks to isolate the peaks you want, and select the appropriate ranges of your data. That should at least get you started.

[Julia de Lange]

This definitely help. Is there an easy way to just find the first major peak?

[Star Strider]

That depends on what you define as ‘major’. The name-value pair arguments can help you isolate the peaks you want.

To return only one peak, use the 'NPeaks' argument:

[pks,locs] = findpeaks(Voltage, 'MinPeakDist',2000, 'NPeaks',1);

Here, only the first peak value and its index will be returned.