An automatic zero-crossing signal demodulation and classification device for rapidly identifying an unknown modulation in a signal identifies an unknown modulation in a signal, demodulates differential phase shift keying signals and automatically recognizes certain phase shift keying signals. This is accomplished by eliminating the unknown term fc in differential phase estimation, introducing a symbol rate tracking mechanism, applying hysteresis nonlinearity to eliminate the phase shaping effect and using a weighted average to estimate the phase difference. Better estimates are accomplished by using the hysteretic nonlinear function to detect the zero-crossing points in eliminating the false detecting of the zero-crossing points caused by the additive noise, and calculating differential phase without directly using the center frequency to simplify the estimation process. The present invention also encompasses an automated zero-crossing signal surveillance demodulation and classification device for rapidly identifying an unknown modulation in a signal and a method for automatic zero-crossing demodulation and classification of an unknown modulation signal.