Abstract: A subscriber's telephone installation comprises a plurality of telephone sets coupled in parallel to a subscriber loop so as to provide a voice facility, and a data modern coupled to the subscriber loop so as to provide data facilities at a frequency above that employed for said voice facility. Some or preferably all telephone sets are provided with a respective filter arrangement incorporating series connected inductive and parallel connected capacitive elements for rejecting selected frequencies from the telephone set. The filter also incorporates a switch responsive to current drawn from the line for selectively enabling and disabling the filter arrangement when the telephone set is respectively in its off-hook condition or in its on-hook condition.

Abstract: A circuit for interfacing a subscriber loop to subscriber terminal apparatus is provided with a transformer having a split primary winding, a secondary winding and a balance winding. A constant current source is connected to the balance winding to cause a predetermined amount of current to flow therethrough. A current sink circuit is connected serially with the primary winding for regulating the direct current flowing therethrough to an amount proportional to that flowing through the balance winding whereby the DC flux generated by the current flowing through the primary winding substantially cancels the DC flux generated by the current flowing through the balance winding.

Abstract: An interface circuit for interconnection between a central office line and a telephone apparatus (e.g. telephone set, automatic telephone answering machine, etc.) is disclosed. The interface circuit includes a rectifier which allows the interface circuit to be connected across the central office line, in parallel with other interface circuits, without the use of isolation switches.