Changing the x-axis from Time instead samples

Hi everybody!

I'm a new user of Matlab, so I really appreciated your help. I'm doing the spectrogram of a chirp signal. Here is how I defined it:

fs = 1000;
t = 0:0.001:1;
y = chirp(t,60,1,20);

Later, I use the spectrogram as below, with a hamming window of 64 samples:

w1 = hamming (64);
spectrogram (y, w1, [], [], fs, 'yaxis');

The point is that when the graph is displayed, the x axis is in samples, and I would like to be in seconds. I try several things but I can´t do it. I atacched a copy of the graph.

So, I want that the x-axis be in time, not in number of samples. Can anybody help me? I apreciated so much,

Thanks!

Answer 1

[Star Strider]

When I run yoiur code in R2020b, the x-axis is labeled ‘Time (ms)’.

To convert them (and the x-axis label) to seconds:

fs = 1000;
t = 0:0.001:1;
y = chirp(t,60,1,20);

w1 = hamming (64);
spectrogram (y, w1, [], [], fs, 'yaxis');
Ax = gca;
xt = Ax.XTick;
Ax.XTickLabel = xt/1000;
Ax.XLabel.String = 'Time (sec)';

.

[Matias Calzada Gianero]

Great! Thanks for your answer. So, I understand that the x-axis is already in mS, not in samples. Am I right? Thank you again!

[Star Strider]

My pleasure!

That is how I understand it, and that is the way it is plotted in R2020b. I believe time and frequency are the default units for spectrogram plots if a sampling frequency is specified. The documentation does not appear to discuss that, at least when I looked through it. If the units are other than time (for example, space), the code here can produce the correct designation for what would otherwise be the time axis label.

[Matias Calzada Gianero]

It makes sense what you say. Thank you so much, you help me a lot!! Thanks for your knowledge and time!! Regards!!

[Star Strider]

As always, my pleasure!

I very much appreciate your compliment!