MATLAB: The spectrogram have negative values

frequencysinewavespectrogramstft

In this figure we have a exponential sine wave and his spectrogram. My question is why the spectrogram have negative values? (from -50 to -150)

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Assuming he spectrogram function plots the power spectral density (PSD) in decibels. The values are relative, not negative, amplitudes, so -150 dB corresponds to an amplitude of about 3.2E-8.

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MATLAB: Is the output of FFTSHIFT shifted by one when used with the PSDFREQVEC function

This behavior is due to the different conventions used by PSDFREQVEC to specify the frequency vector. FFTSHIFT shifts frequency components 1:512 to 513:1024 and components 513:1024 to 1:512. The zero frequency component occurs at the 513th array index. PSDFREQVEC uses the convention that the zero frequency component is at the 512th index.

To work around this issue, avoid using PSDFREQVEC with FFTSHIFT. The following commands return a frequency vector that is consistent with the FFTSHIFT convention for plotting frequency spectra:

FreqVec2 = linspace(-Fs/2,Fs/2-1,NFFT);
figure,plot(FreqVec2,MX,'ro')
grid on
xlabel('Frequency (Hz)')
ylabel('Amplitude')

You can also use FFTSHIFT itself to construct the frequency vector. The frequency vector can be constructed as follows:

dftFreqVec = (0:NFFT-1)*Fs/NFFT; 
shiftedFreqVec = fftshift(dftFreqVec);

MATLAB: After FFT why there are triangular peaks showing your frequency

The ‘triangle’ is the resolution of the Fourier transform you calculated. If you plotted it on a wider frequency scale (from zero to your Nyquist frequency), it would look like a spike at 60 frequency units.

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Related Solutions

MATLAB: Is the output of FFTSHIFT shifted by one when used with the PSDFREQVEC function

MATLAB: After FFT why there are triangular peaks showing your frequency

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