Abstract: A system for ensuring optimum performance of a modem used in conjunction with a telephone system employing a digital PBX coupled with a two-wire analog trunk includes an echo cancellation circuit having an adaptive filter connected in series circuit with a linear codec between the digital PBX and a two-to-four wire hybrid converter coupled with the two-wire analog telephone trunk. The combination of the echo canceller and the linear codec allows accurate decoding of weak far end modem signals when such signals are summed with a strong near end modem signal echo. The DSP echo canceller removes the very strong near end modem echo and passes the far end modem signal with little distortion, while the linear codec permits accurate recovery of the weak far end signal in the presence of high noise levels.

Abstract: A system is shown for automatically detecting the integrity of telephone subscriber's line connection to a four wire switching system via a hybrid network.

Abstract: A circuit which prepares a conductive path between a ringing voltage generator and a telephone subscriber loop circuit prior to the application of ringing voltage to the conductive path. Photo coupled switching and monitoring circuits are used to coordinate application of ringing voltage with operation of a ring relay which completes the circuit path between the ringing voltage generator and the loop circuit.

Abstract: An automatic gain control circuit for limiting the acoustical power to the receiver portion of a telephone operator's headset. The circuit provides means whereby the maximum level of acoustic energy output a telephone operator is subjected to is controlled to levels well below those where damage to the ear can occur. Features of the invention include the use of diodes in the feedback path of the an amplifier whereby DC currents control the conductivity of the diodes to control the amplifier gain, and a pair of such circuits in phase opposition serve to cancel control current effects on the signal path.