The signal is the same. We decide to accept as analog or digital singal based on input circuits. Digital Signal is decoded based on VIH and VOL. All voltages above VIH are considered as Logic High and all voltages below VOL are considered logic low. Analog signals go to an OPAMP and then to an ADC where the signal is decoded as bits of the ADC. You can even say that Digital signal is an output of a 1 bit ADC.