Abstract: An improved impedance canceller circuit, for use with two-wire transmission systems of the type encountered in telephony, is adapted to be interfaced with the wires and operates in a manner to effectively eliminate or cancel an unwanted impedance across the wires. The impedance canceller includes a current source, and essentially supplies on the wires all of the current that would otherwise be required by the unwanted impedance in response to a metallic voltage applied across the wires by a signal source, so that the signal source does not supply such current and the unwanted impedance appears as an open circuit to the metallic voltage. At the same time, the impedance canceller provides common mode rejection to longitudinal voltages on the wires.

Abstract: An improved impedance generator circuit, for use with two-wire transmission systems of the type encountered in telephony, is adapted to be interfaced with the wires and operates in a manner to effectively generate a desired positive or negative impedance across the wires. The impedance generator includes a current source, and generates a positive impedance by drawing from the wires all of the current that would otherwise be required by the positive impedance, or a negative impedance by providing on the wires all of the current that would otherwise be provided by the negative impedance, in response to a metallic voltage applied across the wires by a signal source. At the same time, the impedance generator provides common mode rejection to longitudinal voltages on the wires.

Abstract: An improved current source circuit is adapted for use with two-wire transmission systems of the type encountered in telephony, and has excellent common mode noise rejection. In its various embodiments, the current source provides an interface for coupling signals with a telephone pair, operates to cancel or negate the effect of unwanted impedances on the pair, generates a selected impedance across the pair, and functions as an electronic hybrid circuit for interfacing between a two and a four-wire telephone transmission facility, all while affording common mode rejection to noise on the line. In addition, the various embodiments of the current source are combined into an improved terminating set for a two to four-wire telephone transmission facility.

Abstract: A processor for an echo canceller generates an estimate of an actual echo on an echo path and applies the same to a subtractor circuit in the path to cancel the echo. To generate the echo estimate, the processor multiplies A-law digitally encoded samples of signals which cause the echo by A-law digitally encoded samples of impulse responses of the echo path and sums a plurality of the products. The particular manner of multiplying the A-law samples produces a product which is a precise linear representation of the product of the linear equivalents of the samples, and no errors or approximations occur in the multiplication or in generation of the echo estimate.

Abstract: A residual echo suppressor for an echo canceller compares the levels of the uncancelled signal and the residual signal. If the level of the residual signal has a selected proportionality to the level of the uncancelled signal, it is assumed that the residual signal consists solely of echo and it is suppressed to zero. If the residual signal fails the proportionality test, which indicates that it may contain speech, it is transmitted in its entirety. To prevent sporadic transmissions of residual signal which fails the proportionality test, but which might not contain speech, a hysteresis circuit blocks transmission of the residual signal upon the same continuously failing the proportionality test for a selected time, at the end of which time the residual signal is assumed to contain speech and is transmitted.

Abstract: An improved echo suppressor for use at intermediate points in a telephone transmission facility, whereat both speech signals and single frequency signaling tones may simultaneously be present, provides voice-switched echo control without affecting network signaling. In one mode of operation, during periods of critical signaling tone transmission and while high level or augmented tone is present, such as during dial pulsing, the echo suppressor is automatically disabled and completely transparent to the tone, whereby the signaling tone may pass therethrough completely without distortion. In another mode of operation, during periods of continuous low level signaling tone, the tone is blocked from an attenuation control portion of the echo suppressor and is reinserted into the transmission facility thereafter, irrespective of the presence or absence of speech energy in the facility.