The key of delivering time through telephone channel is measuring time delay. There are two methods of typical time delay measurement in telephone time service: detecting character, detecting carrier phase. Detecting character is easy to implement, but its accuracy is low. While detecting carrier phase can achieve a higher degree of accuracy than measuring character, but the carrier signal is easy to distortion because of the disturbance of noise in telephone channel. In this paper, we put forward a new time delay measurement method based on researching on existing methods, which can achieve high accuracy with strong antiinterference ability, and then can solve problems in current methods. The proposed method makes use of PN code measuring time delay. The PN code first is modulated by sine wave, and then the modulated signal is converted to analog signal. time server transmits the analog signal to the user terminal through telephone channel. Finally, user terminal converts the received analog signal to digital signal and demodulates the digital signal to PN code signal. Because PN code has auto-correlation characteristic, wecan quickly capture the PN code in one second using matched filter. then measure time delay in this way. We have developed a principle prototype according to the proposed method. Different telephone terminal exchange measuring results display: time delay accuracy in telephone channel has increased from 5 milliseconds to less than 100 microseconds.

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