How change the x axis from second to hour:min:sec?

Dears,

I would like to know how I convert the x axis values showed in seconds to the format HH:MM:SS. The data that I read has more than 1 hour of time and the axis show the values in seconds, being that I know the hour of beginning of axis.

I will be very grateful.

Regards, Guilherme de Melo

Answer 1

[Walter Roberson]

If you are using datetime objects for the x axis, set the Format property of the objects.

If you are using datenum, use datetick() and indicate the format style you want.

[Guilherme de Melo]

How I apply this function in a variable with x values in seconds? I have only the initial time and after the values in seconds.

[Walter Roberson]

Example:

times_in_seconds = sort( randi(250000, 1, 100) );
corresponding_data = sort( rand(size(times_in_seconds)) );

times_in_days = times_in_seconds / (24 * 60 * 60);

plot(times_in_days, corresponding_data);
datetick('x', 'HH:MM')

[Guilherme de Melo]

To plot the figure that I before sent you, I use this command:

timeaxis = [0:npts-1]/Fs;figure;plot(timeaxis, signal);

So I should apply it like this:

timeaxis = [0:npts-1]/Fs;figure;plot(timeaxis, signal);times_in_seconds = sort( randi(250000, 1, 100) );corresponding_data = sort( rand(size(times_in_seconds)) );times_in_days = times_in_seconds / (24 * 60 * 60);plot(times_in_days, corresponding_data);datetick('x','HH:MM:SS');

?

thanks,

[Walter Roberson]

timeaxis = [0:npts-1]/Fs;
timeaxis_days = timeaxis ./ (24 * 60 * 60);
figure();
plot(timeaxis_days, signal);
datetick('x','HH:MM:SS');

[Guilherme de Melo]

Very thank you.

If I have the start time of the data (12:59:27.858), is possible plot the x axis as a continuation of that hour?

Other question: Do you know how I make a spectrogram Frequency/Time with this signal?

[Walter Roberson]

start_time = datenum('12:59:27.858','hh:MM:ss.fff');
timeaxis_days = timeaxis ./ (24 * 60 * 60) + start_time;

And see https://www.mathworks.com/help/signal/ref/spectrogram.html

[Guilherme de Melo]

Hello,

I applied the command

specgram(signal,4000,'5');

and the figure in attached was generated. Do you know how I plot the x axis in HHMMSS too?

About the other figure, it show separately the hour value for hourly. I would like to plot the value with interval de minute to minute when I zoom in the figure. Is this possible?

[Walter Roberson]

For the spectogram would it be okay if the labels changed but the data cursor still gave relative seconds?

Automatic adjustment of date tick scale is a bit of a nuisance.

If you have R2016b or later then for the signal plots, I would recommend switching to datetime objects for the times. I do not know at the moment whether it is possible to use datetime for spectogram though.

[Guilherme de Melo]

Yes, the X label of spectrogram is in seconds and, the Y label I think that is frequency, but I still not understand how define a frequency band when I plot the spectrogram. Do you know how define? I would like use a bandpass filter between 0 and 20 Hz, but I dont understood how use this function:

https://www.mathworks.com/help/dsp/ref/fdesign.bandpass.html

My matlab version is R2016a.

[Walter Roberson]

I do not know what specgram(signal,4000,'5'); does. specgram was replaced in R14SP2 in March 2005, and Mathworks no longer has any documentation about what it did.

[Guilherme de Melo]

And the 'spectrogram()', do you know use it?

[Walter Roberson]

Signal processing is not something I do much of, so I am somewhat rusty on the theory, but I have dug into spectrogram on occasion to figure out whether particular parts were configurable.

[Guilherme de Melo]

No problem. Very thank you for all help.