Abstract: In a floating battery feed circuit, the voice frequency is normally connected on the subscriber loop side of the blocking capacitor which is used to prevent the DC loop current from flowing in the winding of the associated voice frequency transformer. This capacitor adversely offsets the dial pulse signals. An optical isolator is responsive to the answer control signal from a dial pulse originating (DPO) channel unit and provides a control signal to a switching circuit, which connects the voice frequency circuit to the loop during answer periods and disconnects the voice frequency circuit from the loop during dial pulse signaling.

Abstract: A floating battery feed circuit employs a single primary winding, a pulse width control circuit and a peak detector for providing a controlled current to a subscriber loop. The amplitude of the controlled current is determined by the loop resistance. A comparator circuit has one input connected to the output of a peak detector via a breakdown diode. The second comparator input is connected to a reference voltage. When the voltage from the breakdown diode exceeds a predetermined value the comparator provides an 0 output to indicate an on-hook state. For a peak detector output voltage below said predetermined value, the comparator provides a +5 volt output indicating an off-hook state.

Abstract: Under an off-hook condition a pulse width control circuit is periodically turned on by narrow pulses from a clock source. This initiates current flow from a primary dc source via the single primary winding of a transformer, and through power transistors connected in a Darlington configuration. A ramp voltage is developed across an emitter resistor of one of said power transistors and this voltage is feedback to said pulse width control circuit. When the ramp voltage reaches a predetermined value, the pulse width control circuit turns off the power transistors and the energy stored in the transformer is transferred by the collapsing magnetic field into alternating current in the secondary winding. This alternating current is rectified and filtered to provide loop current to the telephone subscriber loop. The voltage reflected from the secondary to the primary is stored in a peak detector circuit which operates to control the length of time during which the ramp voltage builds to its predetermined amplitude.

Abstract: DC Saturation effects in the VF transformer of a DPT are reduced or eliminated with apparatus diverting cental office battery loop current away from windings of the transformer when a trunk circuit is seized for an off-hook condition of a calling parties telephone handset. A physically smaller transformer having increased inductance may then be employed for providing increased return loss at low frequencies. The apparatus comprises a current amplifier that is electrically connected across tip and ring lines, with a first switch means in its feedback path, and second switch means for AC coupling a transformer winding to the lines. When the trunk is idle for an on-hook condition of the handset, the amplifier is disabled and the lines are terminated with a large resistance.

Abstract: A decoded PAM input signal is coupled through a first polarity switch (which for one polarity of a polarity signal is closed) to the positive input line of a differential operational amplifier and through the series combination of a second polarity switch (which is open for the one polarity) and an input resistor to the negative input line of the differential amplifier. The positive input line is resistively connected to ground through a first bias resistor and is also connected to ground through the series combination of a second bias resistor and a third polarity switch which is also open for the one polarity. A feedback resistor is connected between the output line and the negative input line of the amplifier.