Abstract: An electronic ringing generator circuit with provision for power efficient current limiting and preservation of sinusoidal or near sinusoidal output waveform under overload conditions. A pulsewidth modulated DC-DC power convertor produces supply voltages for a linear class B power amplifier which has a low frequency low level sinusoidal signal applied to its input through a first controllable attenuator. The DC-DC convertor's reference input is also driven through a second controllable attenuator which is coordinated with the first controllable attenuator. The attenuators are operated in such a way that if the amplifier's output current exceeds a predetermined value, both the input signal and amplifier supply voltages are reduced in a coordinated manner so that simultaneously the output AC waveshape is preserved and the amplifier continues to operate at or very near optimum efficiency. The amplifier may be operated to produce a sinusoidal output (crest factor =1.

Abstract: An amplifier for use with low level pulse code modulation (PCM) nonreturn-to-zero (NRZ) signals. The amplifier has a high, but controllable gain and does not have a DC shift at its output as long as there is at least one zero or one one bit in each of frame PCM NRZ signals. The amplifier includes a feedback circuit to control the bias of one of the two stages of amplification when zeroes are being received. The amplifier also includes an AGC circuit which reduces the gain when a high amplitude one is received.

Abstract: A circuit for the non-linear limiting of the metallic current flowing in telephone line conductors. The circuit senses the metallic current flowing in the conductors. If that current exceeds a predetermined limit, the circuit provides from a controlled and variable voltage source a voltage across the conductors whose amplitude is reduced from the essentially constant amplitude voltage provided across the conductors when the current does not exceed the limit. The reduction in voltage amplitude is sufficient to maintain the metallic current at the limit.

Abstract: A system for use in a digital loop carrier (DLC) which allows low frequency (sub-audible) tones such as those used with alarm systems to be transmitted from one end of the DLC to the other end even though the DLC has been designed to reject signasl below 200 Hz. The system may be added to an already installed DLC or included with the DLC prior to installation. At the end of the DLC from which the low tone is to be transmitted, the system controls the amplitude of the tone so that it can be encoded for transmission within a predetermined window. The DLC allows such low frequencies to be transmitted through the window while still rejecting 60 Hz signals (and its lower order harmonics). The controlled low tone and the voice signals are combined for transmission to the other end. At the other end, the decoded low tone is amplified by the system and then recombined with the decoded voice signals for use therein.

Abstract: A compensated circuit for use in interfacing a transmission device to a cable facility. The circuit includes a transformer one winding of which is connected to the cable facility and the other winding of which is connected through an impedance generating device to the transmission device. The circuit provides a predetermined one of a number of selectable impedances to the cable facility. The impedance generating device allows the desired impedance to be elected and includes a negative resistance generating circuit which responds to the current flowing in the loop formed by the winding and the impedance generating device.