Abstract: Interface circuitry is provided between a pair of lines, on the one hand, and an input to and output from other equipment on the other hand. The interface circuitry will commonly be used between `tip` and `ring` lines of a telephone connection and office telephone equipment but is not limited to the telephone art. The interface equipment provides independently controllable and adjustable impedances to `longitudinal` and transverse voltage changes on the pair of lines, means for transferring signals from the pair of lines to the output line and means for transferring signals from the input line to the pair of lines.

Abstract: The addition of two digitized audio signals, each of which has been non-linearly compressed and PCM encoded, is accomplished by converting only one of the signal samples (preferably the smaller) to an exponential-format value which may be linearly added to the other.