the multilevel data encoding and modulation technique uses a pair of complementary logic sets . in its most basic form , the sets are binary sets each containing a line level for a logical one and a line level for a logical zero for a total of four logic levels . the encoding technique requires a polar change in the line level after every bit . an optional fifth level may be used in order to skew the frequency or to enable automatic gain control circuitry to ensure consistent level discrimination . the encoding technique may be used in a bipolar device , or a bias level may be applied to the signal for unipolar transmission . the encoding technique involves inverting the polarity of alternating bits , filtering out all odd harmonics , transmitting and receiving the waveform , and decoding the demodulated waveform by comparing the absolute value of the half - cycle peak - to - peak voltage gain to a predetermined table .