METHOD OF MONITORING THE ON/OFF-HOOK STATUS OF A TELEPHONE SET CONNECTED TO A TELEPHONE NETWORK

摘要

A METHOD AND APPARATUS FOR MONITORING THE ON/OFF-HOOK STATUS OF A TELEPHONE SET In an extension line circuit, an off-hook condition is detected during ringing by superimposing in each ringing cycle the values of the line current existing at two sampling instants exactly one-half cycle apart, and testing whether the sum (or mean value) exceeds a given threshold. When DC current starts flowing due to the set going offhook, the mean value of the two values will pass a threshold. Sample pulses can be derived in common for all line circuits from positive-going zero crossings of the ringing supply signal. The circuit acts as a notch filter centered exactly at the actual frequency of the ringing signal.