a method for 90 ° phase - shift modulation and demodulation in a circuit having a pll . a phase - shift of 90 ° between a main and an additional signal is provided . the method comprises the steps of : modulating by selecting a specified position and inserting a pilot signal , giving the phase - shift of 90 ° to an image carrier wave and generating a modulated signal ; and demodulating by receiving the modulated signal and demodulating the main and additional signals having phase - shift of 90 ° to each other , generating a frequency locked to the transmitted signal and controlling an output of a voltage controlled oscillator and reproducing the main and additional signals of step . by inserting the pilot signal into the overscanned interval of the horizontal line of the standard image signal and using it again as the reference signal of the remote pll , the phase of the image carrier waves between the transmitter and receiver are locked to the same reference , so that phase error due to the 90 ° phase - shift modulation and demodulation is removed and the signal attenuation and crosstalk are prevented .