Abstract: A capacitor cancellation method and apparatus for use in an interface circuit having a transformer blocking capacitor. The method includes sensing a voltage across the transformer blocking capacitor and generating a cancellation signal to compensate for the effect of the transformer blocking capacitor. The apparatus includes a sensor to sense a differential voltage across the transformer blocking capacitor to develop a capacitor signal and an amplifier to amplify the capacitor signal to obtain a cancellation signal.

Abstract: An amplifier having an improved output current drive capability includes an input stage and an output stage. An input of the output stage is operatively coupled to an output of the input stage. The amplifier further includes a current regeneration circuit operatively coupled to the input of the output stage in a feedback arrangement, the current regeneration circuit feeding back a current to the output circuit in accordance with a predetermined scale factor, the fed back current being proportional to an input current supplied to the output stage. The input current supplied to the output stage is dynamically adjustable by the current regeneration circuit in response to an input current requirement at the output stage.

Abstract: A capacitor cancellation method and apparatus for use in an interface circuit having a transformer blocking capacitor. The method includes sensing a voltage across the transformer blocking capacitor and generating a cancellation signal to compensate for the effect of the transformer blocking capacitor. The apparatus includes a sensor to sense a differential voltage across the transformer blocking capacitor to develop a capacitor signal and an amplifier to amplify the capacitor signal to obtain a cancellation signal.

Abstract: An amplifier having an improved output current drive capability includes an input stage and an output stage. An input of the output stage is operatively coupled to an output of the input stage. The amplifier further includes a current regeneration circuit operatively coupled to the input of the output stage in a feedback arrangement, the current regeneration circuit feeding back a current to the output circuit in accordance with a predetermined scale factor, the fed back current being proportional to an input current supplied to the output stage. The input current supplied to the output stage is dynamically adjustable by the current regeneration circuit in response to an input current requirement at the output stage.

Abstract: Quantization noise, introduced into data transmission when analog signals are translated into PCM code using the logarithmic conversion of &mgr;-law or A-law rules necessary to accommodate the transmission of voice signals, limits the maximum attainable speed of data transmission. However, when the PCM code signals represent data (rather than voice signals), linear conversion of analog data signals into the PCM code would avoid such logarithmic quantization noise. To signal the translating codec that a digital modem call is being made, the digital modem allocates one or more of the least significant bit positions of the code representing the 2100 Hz answer-tone-with-phase-reversal (specified in ITU-T Recommendation G. 165) to send a repetitive pattern “P1” to signal to the associated codec that a digital modem connection has been made.

Abstract: The present invention is directed to an integrated circuit comprising a voltage source and a voltage sensing circuit having a switch circuit for sensing a switch voltage and a switch current; a voltage comparison circuit connected to the switch circuit for comparing the switch voltage to a limiting voltage; and a current limiting circuit connected to the switch circuit and the voltage comparison circuit, which is capable of limiting the switch current when the switch voltage reaches or exceeds the limiting voltage. In a preferred embodiment of the invention the switch circuit is a bi-directional switch circuit and the voltage comparison circuit incorporates a series of Zener diodes, wherein the current limiting circuit is a current foldback circuit.

Abstract: Automatic testing a group of line circuits on a smart line card served by a central switch over a PCM bus is localized by using the microprocessor/DSP that provides codec functions for the group of line circuits. Tests such as DC overhead that are not performable by the central switch over the PCM bus are performed by the microprocessor/DSP when informed by the central switch that at least one of the channels is not processing voice signals. The microprocessor/DSP controls test switches on the line card to set-up various test terminations for the idle circuit as well as to selectively control the gain of the idle channel's amplifiers. The microprocessor/DSP synthesizes test tones and executes test tone detection processes within the time slot intervals accorded by the central switch to the idle channel.

Abstract: A reference voltage source has at least one transistor having a base, emitter and collector electrode operating in an active state that has an inherent voltage drop between its base and emitter electrodes to generate a low noise core voltage which is applied to the input of an amplifier to set the amplifier operating point, and thereby the amplifier output, which is the reference voltage. The reference voltage is temperature compensated by providing the amplifier with a feedback loop, or by altering the core voltage applied to the amplifier, or by altering the gain of the amplifier. Several transistors can be connected in series and their base to emitter voltage drops added to change the core voltage value.

Abstract: A line circuit whose circuit components are selectively controllable by a microprocessor/DSP to provide normal BORSCHT functions or to be selectively tested without need for an electromechanical relay to isolate the line circuit from the loop conductors. The conventional line transformer is advantageously replaced by a loop current feed resistor network and either a high voltage or a lower voltage battery may be connected to provide loop current to the resistor network through a current regulator circuit and battery switch circuit. In normal operation, an ac feedback path, which includes the line receive amplifier, increases the terminating impedance above the dc value of the feed resistors so the loop termination resistor network may match the characteristic impedance of the loop at the audio frequencies employed.

Abstract: A line circuit provides BORSCHT functions and supplies the customer loop through a feed resistor network and current limiter either from a high voltage battery to provide for maximum signaling range or from a lower voltage battery to conserve power. The current limiter operates under the control of a microprocessor/DSP which selectively applies or denies power to isolate or include components for testing and controls the switching in and out of transistors to control the scaling factor between the channel width-to-length ratios of the current mirror's input and output FET transistors according to the type of test or operating condition dictated by the microprocessor while the input reference current to the current mirror is maintained constant.

Abstract: A subscriber line circuit which truly implements all of the BORSCHT functions and which does not rely on an external ringing generator to provide the ringing voltage when needed is disclosed. This implementation is achieved by recognizing that supervisory and ringing signals occur in some sort of sequence and, therefore, do not have to be generated simultaneously. Based on this recognition, we provide a circuit which truly implements all of the BORSCHT functions and includes a power section that generates each of the supervisory signals, e.g., -48 volts and -24 volts, and alerting signal, e.g., ringing signal, in turn and as required.