Abstract: In a telephone line feeding arrangement, the line current is monitored by a multi-resistor network (R.sub.p R.sub.s) and the output of the network applied to the inputs of an operational amplifier (A3). The output of this amplifier, which is a voltage whose value depends on the line current, is compared by a comparator (A4) with a reference voltage (V.sub.R) to produce a control voltage for a variable voltage (VVS), the output of which provides line feeding voltage.In an alternative, the monitoring is effected by connecting the two line wires each via a resistor to one of the inputs of an operational amplifier (A6) whose output is a voltage dependent on line voltage. This voltage is applied to an input of another operational amplifier (A7) where it is in effect added to a reference voltage (V.sub.R). The output of this amplifier (A7) is used to control the variable voltage source (VVS), which in turn controls a constant current source (CCS) whose output provides line current.

Abstract: The direct current line feed for a telephone subscriber's line is derived from a variable voltage source connected to the line. In one case, the source is series connected in one leg of the line while in the other case it is connected across the line. In the first case there is a constant current source across the line while in the second case the voltage source is a constant current device. In both cases a pair of matching resistors is connected across the line with the center tap grounded to provide a high resistance balanced termination for the line. In both cases a high impedance monitor is connected across the line to monitor the voltage conditions on the line, and this, via a control circuit controls the voltage source and the separate constant current device if provided. This enables line voltage to be adjusted to take account of line length without introducing undesirably large dissipation.

Abstract: To limit the line current dissipation in a telephone system it is considered desirable, especially in electronic systems, for the line current feed to be constant current. In the present arrangement the constant current supplied to the line is made adjustable in accordance with line conditions, especially the voltage between the line wires. A high impedance monitor is connected across the line to monitor the voltage between the two wires and to produce therefrom a reference voltage which is applied to the constant current generator so as to cause it to generate a current for line feed whose value is determined by the line voltage.

Abstract: A telephone subscriber's line interface for a system in which the telephone instrument and the connections therefrom to the exchange use analogue, while the exchange is digital is described, in which electro-mechanical components are eliminated.To achieve this a microprocessor serves a group of lines on a multiplexed basis, responding to conditions of the line as determined via a high impedance monitor and analogue-to-digital conversion means. It also responds to control inputs from the rest of the exchange. In response to the above inputs it controls line feed, signalling, etc. via digital-to-analogue conversion means and a line driver.

Abstract: Telephone subscriber line circuits and an all-electronic telephone exchange are described wherein Hall effect devices are incorporated to detect line looping, dialing and ring-trip.