# Units of spectrogram and pspectrum questions

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KE BR on 19 Aug 2020
Answered: Frantz Bouchereau on 24 Aug 2020
Hi
I have a time signal as follows: I am trying out "pspectrum" and "spectrogram" to see this signal in frequency-time domain and frequency domain.  The left picture is the result of "pspectrum" and at the right "spectrogram". Now i have some questions regarding the units of the amplitude. With "pspectrum" the unit is expressed as db and with "spectrogram" the unit is expressed as db/Hz. What is the difference between these two units?
My second question and also the most important one is as follows: I need to extract the signal information from these figures, so im using the output arguments with these two. For both functions you get a mxn matrix but both has other values. i want to know what the units are from these two, how can i calculate amplitude in "volts" from these values? The matrix at the top is the result of "pspectrum" and the bottom matrix is the result of "spectrogram". With spectrogram i know i need to use abs(x) to get the amplitude information.  Summarized: i want to know what the units of these two output arguments are and how can i transform them to a unit expressed in "volt"?
PS: the time domain is a sinus with a frequency of 50Hz. The top matrix show the values corresponding with the frequency 50Hz wich is 0.0032. for the bottom matrix row #80 corresponds with 50Hz)
any help would be appreciated!
With kind regards

Frantz Bouchereau on 24 Aug 2020
spectrogram by default plots the magnitude of the power spectral density and hence the units at db/Hz. pspectrum computes and plots power spectrum so the units are simply dB.
pspectrum outputs short-time power spectrum columns in linear units (Watts with respect to a 1 Ohm load).
HTH