Well thank you for educating me on the spectrogram. But is not the spectrogram a digital system, ie software, output? Are you simply saying the program itself does not find it resourceful to reconstruct the analog time-series with complete fidelity? Why wouldn’t it? In theory it should be able to, know?
I had been merely supposing the spectrogram software programming causes such spurious frequencies rather than actual filtering of what is still fundamentally electromagnetic action.