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

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Jan Ali 2021년 5월 8일
댓글: Bjorn Gustavsson 2021년 5월 12일
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)?
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Asvin Kumar 2021년 5월 12일
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

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### 채택된 답변

Bjorn Gustavsson 2021년 5월 12일
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);
You can then chose whether get power spectral density or power.
HTH
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Bjorn Gustavsson 2021년 5월 12일
Bare hyggelig!

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