Abstract: A subscriber line interface circuit receives and twice inverts currents from a 2-wire subscriber line and returns the currents to the subscriber line so longitudinal input impedance is low. Any differential input current in the subscriber lines is bypassed by a capacitor, so differential input impedance is very high. The circuit also sends a signal current from one system having one ground to another system having a different ground through one current path, while sending another current, having a phase opposite and amplitude equal to the signal current, through another path. This effectively isolates the two systems having different grounds.

Abstract: In a calling signal transmitting apparatus, a ring trip equipment, including a ring trip detecting circuit for detecting a ring trip signal in response to an off-hook operation of the telephone set during a signal state of an intermittent ringing signal and a loop detecting circuit for detecting a loop signal in response to an off-hook operation of the telephone set during a nonsignal state of the intermittent ringing signal. Also included are a logical OR gate circuit, for providing, as a status signal, a logical sum of the ring trip signal and the loop signal, and a supervising circuit for supervising the status signal.

Abstract: Battery feed circuit for a pair of subscriber lines includes a voltage drive circuit with an operational amplifier having a predetermined output impedance determined by an alternating current terminal impedance for the pair of subscriber lines. Both voice signals and induced noise signals are terminated by the voltage drive circuit. The voltage drive circuit is connected in parallel with an electronic inductance circuit, between one of the subscriber lines and ground or a direct current voltage supply.

Abstract: A battery feed circuit for a subscriber line includes first and second feed circuit parts, each supplying a constant current to a corresponding subscriber line. The first and second feed circuit parts are provided with at least one of a common mode noise suppressor and a power source noise suppressor. The common mode noise suppressor includes first and second reference voltage output circuits and an intermediate voltage output circuit to produce the voltage between the first and second subscriber lines and superpose the same onto the first and second reference voltages. The power source noise suppressor functions to cancel the power source noise voltage at the output, of the first and second feed circuit parts.

Abstract: A battery feed circuit for a two-wire telephone subscriber line is miniaturized by an electronic integrated circuit without a transformer. The electronic battery feed circuit is composed of first and second operational amplifiers, output resistors, a DC voltage feedback loop, an AC signal feedback loop and a switching power supply. The switching power supply means is to control the power source voltage of the first operational amplifier, corresponding to the length of the subscriber line loop and the status of a telephone set in the loop. Low power consumption of the circuit is attained as follows. Most of power loss of the circuit can be separated as between the amplifiers and the output resistors. The loss of the output resistors are reduced by using small resistances, and the loss of the second amplifier is reduced by using a small DC bias. The loss of the first amplifier is reduced by setting its power source voltage to several volts lower than output voltage of the switching power supply.

Abstract: In the case where a two-wire circuit forming a trunk or a subscriber line of the two-wire system is converted by a hybrid circuit to a four-wire circuit of sending and receiving paths to form a transmission line between subscribers, if an impedance variation on the two-wire side is large, a leak from sending path to receiving path occurs and is amplified to cause singing. To eliminate the singing, a circuit for detecting the impedance fluctuation of the two-wire side is inserted in the two-wire circuit, and switching means inserted in the four-wire circuit is activated by an impedance detecting signal of the detecting circuit to electrically open the four-wire circuit, thereby preventing the singing phenomena. This method does not involve the use of an expensive attenuator which is troublesome to design for the prevention of singing.