| 摘要 |
<p>PURPOSE:To monitor line interrupt stably by superimposing a pilot signal onto a main signal at a sender side and suppressing the component of the pilot signal at a receiver by calculation. CONSTITUTION:A pilot signal (b) is superimposed on a main signal (a) and sent by a sender side coupling circuit 3 and a receiver side monitors a level change in the pilot signal (b) to discriminate the presence of line interruption. Then a signal whose level and frequency are equal to the pilot signal (b) superimposed on the main signal (a) and whose phase differs therefrom by 180 deg. is obtained and an adder circuit 6 is used to add the obtained signal to the pilot signal (b) or a signal whose level, frequency and phase are equal is subtracted from the pilot signal (b) to obtain a signal (e) not including the pilot signal (b) in the received output signal. Thus, the line interruption is monitored stably without giving effect onto the transmission of the substantial main signal.</p> |
