Abstract: The invention provides a telephone line circuit in which the likelihood of exposing circuit devices in an associated switching facility to potentially damaging currents or voltages is greatly reduced. A repeat coil includes a primary winding for connection to a subscriber loop circuit. Direct loop current is maintained in the primary winding and the resulting d-c flux is compensated for by a regulator including a balancing winding. The core material in the repeat coil is saturable and limited in bulk to that sufficient to transmit normal a-c voice signals across the line circuit. Signals of a magnitude which cause the core material to saturate are thereby impeded in transmission across the line circuit.

Abstract: Disclosed is a line circuit providing the normal line circuit functions and key system control functions within an exchange using time division switching. The line circuit may be used interchangeably for key telephones or non-key telephones. In the former case, the circuit provides the well-known key telephone features for multiple line access at each station of a plurality of stations. Each line circuit provides electronic signalling controls, such as lamp flash indicating a call awaiting an answer, lamp wink indicating a line is on a hold condition, and the fully lit line busy indication, together with audible controls for a called line, and also provides an inverted flash signal at the line circuit for trouble or problem situations.

Abstract: A telecommunications system using TDM principles for signalling and speech path usage plus PAM principles for speech transmission. Each function circuit such as a trunk, link or register, etc. is allocated a basic time slot or channel for initiation of its function when indicated. The active period of each function circuit is continued during the period of three succeeding time slots. In this way, up to four function circuits or channel units may be active during a finite period of time.In the system, a line is scanned at the end of each complete scan of all channels, thereby lessening the complexity of associating a line and a function circuit. The system shown uses the dual highway principle for its speech path and provision is made in each line circuit for switching onto both highways.