Listen:in facility in telephone terminal - is used with AF dialling systems to allow handset microphone to monitor dialling signals

摘要

The listen-in facility, for a telephone subscriber terminal with AF dialling, has a common break contact in series with its voice circuit and this series combination is connected in parallel with the dialling device with a common make contact to close the transmit loop during dialling. A capacitor (C) bridges the break contact (Tr). A nonlinear impedance (diodes D1, D2) is connected between the connection point between dialing device and make contact (Ta) and the connection point between break contact and voice circuit. The impendance, after the make contact has closed, is low-ohmic before the break contact opens and high ohmic after the break contact has opened.