The present invention discloses an apparatus and a method for determining the voltage level of a first signal from a playback device having an interface in compliance with an industrial standard. According to the method, first receive the first signal by a signal receiving pin electrically coupled to a signal transmitting pin of the interface. Then output a first output voltage by processing the first signal by the combination of a first zener diode and a first transistor and a second output voltage by processing the first signal by the combination of a second zener diode and a second transistor. Thus the voltage level of the first signal can be determined according to the first output voltage and the second output voltage.