The invention relates to a circuit arrangement for the indication of loop closure during the ringing condition for a subscriber circuit which is connected to the exchange of a digital time multiplex telephone system by way of a subscriber termination circuit comprising a subscriber line interface circuit which, in part, provides the ringing supply and signalling, and a signal processing unit which, in part, provides for analog to digital conversion and digital to analog conversion as well as for filtering. The circuit arrangement includes a changeover switch, by way of which, a signal representative of the subscriber termination line current is applied to the signal processing unit during the ringing condition, in place of the speech signal present during the speech condition. This voltage signal is subjected to analog to digital conversion and filtering as for the speech signal at other times except that the high-pass filtering otherwise employed is avoided, and in that the resulting digital signal is further summed over at least one period of the ringing signal frequency. The sum signal is compared with a reference value which provides a loop closure signal when the reference value is exceeded by the sum signal.
A method and an apparatus for off-hook detection in a subscriber line interface circuit responds to a ringing signal which preferably lacks a DC component. A signal is generated that is a function of the line current through the subscriber line, and the time at which the signal exceeds a predetermined current value is measured. The measured time is compared with a predetermined time value, and an off-hook indication is produced in a processor depending on the comparison.
