How to plot actual amplitude on the colorbar using STFT (spectrogram)

I am using built in spectrogram function and it automatically plots the power/frequency (dB/Hz) values on the colorbar on the right.

Does anyone know how to plot actual amplitude/magnitude of a signal (A or V) on the colorbar using STFT (spectrogram)?

Thanks in advance,

[Asvin Kumar]

I'm not really sure why you want to do that. Could you clarify your use case? That will help you get a better answer from the community.

The colorbar in the spectrogram corresponds to the values in the plot. And the values in the plot are derived from applying STFT on the time domain signal. So, these values are in the frequency domain. They're no longer in the time domain. It only makes sense for the colorbar to show power/frequency values.

[Jan Ali]

In order to present the signal actual amplitude. Otherwise, what is the reference for power/frequency? power/frequency is more confusing.

[Asvin Kumar]

I'm not sure if that's possible but I can say that replacing the colorbar range with Amplitude values would not be meaningful. That would be like comparing apples with oranges. This is because the plot that you get from spectrogram is actually information about the same signal in the frequency domain.

Power/frequency shows the amount of energy per unit time for each frequency. You might recall from Parseval's theorem that energy in time domain is equal to the energy in frequency domain. I would recommend reading more about Power Spectral Density (PSD) if that's confusing. Here's a wikipedia article to get you started: Spectral Density

Answer 1

[Bjorn Gustavsson]

If you read the help for spectrogram you'll find this:

    [...]  = spectrogram(...,SPECTRUMTYPE) uses the window scaling
    algorithm specified by SPECTRUMTYPE when computing the power spectral
    density matrix P.
    SPECTRUMTYPE can be set to 'psd' or 'power':
       'psd'   - returns the power spectral density.
       'power' - scales each estimate of the PSD by the equivalent noise
                bandwidth of the window (in Hz).  Use this option to
                obtain an estimate of the power at each frequency.
    The default value for SPECTRUMTYPE is 'psd'.

So just call spectrogram with its outputs and plot yourself:

[S,F,T,P] = spectrogram(your,inputs,sampling,frequency,and,window,etc);
pcolor(T,F,P),shading flat,colorbar

You can then chose whether get power spectral density or power.

HTH

[Jan Ali]

Tusen takk Bjørn! Setter stor pris på det :)

[Bjorn Gustavsson]

Bare hyggelig!