Abstract: A local battery feed circuit for telephone station sets comprises two half circuit portions substantially symmetric about an imaginary line midway between the TIP and RING telephone lines. Each half circuit includes a current drive amplifier including a pair of up-down emitter follower transistors and a driver transistor. A coupling capacitor and matched resistors provide common mode interference reduction without degrading differential mode signals. Power supply noise is substantially eliminated by transistors in each half circuit arranged to produce an a.c. current equal and opposite to that generated from the power source.

Abstract: A line circuit is disclosed employing a transformer having tip and ring windings and a third balancing winding designed to be serially connected in circuit with the tip and ring windings and poled so as to balance out their DC magnetization. The AC components of longitudinal current flowing through the tip and ring windings are prevented from flowing through the balancing winding by a capacitor bypass which offers a low impedance path compared to that of the balancing winding. This capacitor is prevented from short circuiting the AC voice frequency voltages induced by transformer action in the balancing winding by driving the balancing winding from the collector terminal of a transistor biased to remain out of saturation and which, therefore, exhibits a high impedance to changes in collector voltage. The resistance in the emitter circuit of this transistor determines the level of DC current delivered through the balancing winding to the tip and ring windings and the telephone line.

Abstract: A line transformer circuit in which the DC component of magnetization produced in the tip and ring windings by the battery supplying loop current to the telephone set microphone may be neutralized by an oppositely-poled, serially-connected balancing winding is disclosed. One end of the balancing winding is connected to the supply battery and is, therefore, at AC ground. The other end is kept at a point of high AC impedance to ground by the collector of a transistor so that audio frequency voltages induced in the balancing winding cannot load the transformer. The AC currents in the tip and ring windings are provided with a virtual ground return path by means of a differential amplifier controlled Darlington pair in circuit between the tip and ring windings and the balancing winding. The AC components of the Darlington pair collector current are bypassed to ground by a shunt capacitor at the junction of the Darlington pair collector terminal and the third winding.

Abstract: A time-sampled port circuit is shown in which the charges impressed by the telephone set on the shunt capacitor of a low-pass filter associated with the port are passively sampled for transmission to the other port circuits, but which capacitor is impulse driven at a repetition rate equal to twice the cut off frequency of the filter with speech samples received from the other port circuits. By employing passive sampling in the transmitting leg instead of actual charge transfer, as in conventional resonant transfer design, the series inductor normally required in resonant transfer circuits may be omitted with the result that the port circuit switches may be integrated on one chip together with the digital circuitry for encoding and decoding the amplitude samples. Further, all need for amplifier gain adjustment in the port circuit is eliminated by applying different amplitude send and receive ramp signals to the encoder and decoder circuits.

Abstract: A read only memory (ROM) patching arrangement is disclosed for providing valid output information whenever ROM word locations containing invalid information are addressed. The disclosed arrangement uses a plurality of small capacity PROMs as a decoder to detect the receipt of each address word representing a defective ROM locaton. Upon each detection of a defective address, the decoder temporarily inhibits the output of the ROM and causes a small auxiliary memory to output valid program information as a substitute for that in the defective ROM location. A counter is used as a supplemental addressing source for both the decoder and the auxiliary memory. This increases the patching capability by subdividing the decoder and the auxiliary memory into 2.sup.n segments where n is the number of bits supplied by the counter.

Abstract: A conference port for permitting simultaneous voice communication among a plurality of local stations and an external or central office line is shown. The stations may be part of a time division PBX or may be arranged as a time-sampled key telephone system having access to the same line. The conference port includes a first operational amplifier for summing the station port voltages on a capacitor during an interval that is a predetermined fraction of the resonant transfer interval. This first, or quasi-resonant, interval is sufficient to reduce the initial voltage on each of the station port sampling capacitors to a predetermined fraction of their initial level. At the conclusion of this interval the voltage on the summing capacitor in the conference port is multiplied by an appropriate factor in a second operational amplifier and is applied back simultaneously to all of the conference ports together with the signal incoming over the external or central office line.

Abstract: A nonlocking key telephone station is arranged to display full line status information when the station is on-hook and for normally illuminating only the key lamp associated with the line in use at the station when the set is off-hook. To permit a user to discern the full status of the lines terminating at the set when the set is off-hook, circuitry responsive to the depression of one of the keys at the station is provided for displaying full line status information.

Abstract: A line circuit, interconnecting a subscriber telephone line and a port circuit, includes shunt battery feed resistors of equal impedance together with resistors in series with the subscriber line and a further resistor connected to a point intermediate one battery feed resistor and the adjacent series line resistor. Each series line resistor has an impedance value equal to half the impedance value of the further resistor and forms a pad configuration that provides longitudinal balance and bilateral impedance matching. Stability declines, created by the bidirectional gain of an active hybrid used to overcome the insertion loss of the pad, are offset by stability improvements created through a reduced sensitivity to impedance variations in the subscriber line. Numerous other advantages result from the use of this pad configuration such as a reduction in size, weight and cost.

Abstract: An electronic key telephone system is disclosed in which connections among the station sets and telephone lines are made in a central time division switching network remote from the station sets. Each station set and line is equipped with a port circuit having an individual shift register for defining the time slot interval during which a connection may take place. To assign a time slot to a port circuit, the main controller interrogates the network to find an idle time slot and then furnishes that time slot number to the network controller. In order to save central memory and to increase system reliability, the main activity memory which stores an indication of which ports are or should be interconnected in the network need not contain any reference to the time slot number assigned for the particular connection. In order periodically to verify that actual network connections reflect the state of the activity memory, two types of audits are performed.

Abstract: In pulse amplitude modulated time division switching systems, noise and crosstalk often result because the time division switches used to connect individual channels to a common bus are not perfect open circuits when they are in the off-state. The switches in the off-state possess a small parasitic capacitance which couples energy on the common bus to the channel resulting in a noise signal during time slots which are not associated with the channel. A method is disclosed for substantially reducing crosstalk and noise induced by stray capacitances without increasing the complexity of the time division switch. Each pulse on the common bus is converted into a bipolar pulse in which positive and negative portions have equal average values. In addition, the timing of each of the time division switches is modified to extract the desired signal by sampling the first portion of the bipolar pulse in the appropriate time slot for that channel, and rejecting the second portion of the bipolar pulse.