Spectrogram time axis samples to actual time

Hi. I'm trying to plot the result of a spectrogram like: y-axis frequency and x-axis for time, but instead of time in seconds, it plots samples and I don't know how to convert samples to time. Normally, it would be Sample_number / Sampling_rate, but the original signal has 7813 samples and the spectrogram has only 961. Any ideas? Thank you.

[Rishikesh Shetty]

Same issue here. Did you figure it out?

[Please delete]

Man, it's been 4 years since I asked the question. I think I've managed to solve or at least bypass the problem, but I can't remember how. I'm working in a totally different field now.

Answer 1

[Star Strider]

Include the sampling frequency, and the result is a time-frequency plot (rather than an index-frequency plot). See for example: Spectrogram and Instantaneous Frequency (link). Note that fs must be the fifth input to spectrogram. To input a sample rate and still use the default values of the preceding optional arguments, specify these arguments as empty, [].

[Star Strider]

My pleasure.

I would simply plot the spectrogram and not use imagesc:

spectrogram(signal,window, noverlap, freq,fs,'yaxis')

The spectrogram function produces a surf plot. The imagesc call is inappropriate. If you want to save it as an image, use the saveas (link) function. That will save it with all the visible information, and if you choose to save it as a ‘.fig’ file, will include all the data used to create the plot as well (that being the same as the savefig function).

[Star Strider]

I would simply change the filenames for each spectrogram your function creates by adding a number to the end, for example ‘spectrogram_001.ext’ etc., where ‘.ext’ determines how you want to save it (as .fig, .png, etc.). You can do that with sprintf, possibly saving the number as a persistent (link) variable inside the function, then updating it in each iteration if necessary.

I’m guessing what you’re doing, and my ‘solution’, such as it is, reflects that.

[Rik]

Deleted comments:

Dear @Star Strider, Thank you for your answer. Unfortunately, I was doing this already with no luck. My piece of code is

window=kaiser(128,18);
noverlap=120;
freq=128;
fs=50e10;
sp=spectrogram(signal,window, noverlap, freq,fs,'yaxis');
imagesc(abs(sp))

EDIT: it works if i call

spectrogram(signal,window, noverlap, freq,fs,'yaxis')

but because I integrate it in another function, when I plot it with "imagesc" it doesn't display properly. Any ideas?

How do I do this function-in-function save? Alongside with "spectrogram" being called inside my created function, I'm doing additional processing and I must call this function for 100 signals. It's rather tedious to manually save each figure.