In a method of modulating/demodulating a signal, a transmission signal is generated from an input data signal including data information by combining a plurality of input shift clock signals, which are modulated based on input clock signal during an interval period of the input clock signal, during 2 interval periods of the input clock signal. An output clock signal is detected and an output clock signal is generated from the transmission signal by combining a plurality of output shift clock signals, demodulated based on the output clock signal during an interval period of the output clock, during 2 interval periods of the output clock signal, to restore the data information.