PURPOSE: To prevent the malfunctions and to shorten the response time with a ring trip circuit which is used for a subscriber line circuit, by deleting a call signal of 16Hz and then applying it to an LPF having a high cutoff frequency.
CONSTITUTION: A signal applied to a terminal IN is divided and some of these divided signals are applied directly to an adder circuit 18 with the rest signals applied to the circuit 18 with a half cycle delay given from a delay circuit 17. For added signals, the signals of the start and end half cycle parts of a call signal are not deleted and remain with addition of DC components. Some of these signals are applied to a switch 21 via comparators 19 and 20 having reference voltages Vref-2 and Vref-3 and a NAND circuit 22. Thus the call signal of 16Hz is deleted and smoothed by an LPF23 to be applied to a comparator 15. The DC voltage produced in an off-hook mode has a higher level than the Vref-1 and therefore the off-hook detection signal is dent to outside from the comparator 15 through a terminal OUT.
ENDO YUKIO
TOKIWA KOJI
