Four-wire communication line circuit and conference network comprising such circuits

摘要

A four-wire communication line circuit for two or more two-wire subscriber lines, comprising a pair of amplifiers respectively connected in the line pairs of the four-wire line for each direction of transmission and a transmission direction detection circuit connected to each line pair. The detection circuit generates a control signal from the voltage signals transmitted on the line pairs, and which is dependent on the direction in which the net energy of such voltage signals is transmitted over the two-wire subscriber lines. The control signal is applied to control circuitry which switches the gain factors of the line amplifiers between a high and a low value, as determined by the direction of net energy transmission. When used in a conference network, the four-wire communication line circuit includes circuitry for establishing a higher threshold level at which the amplifier gain factors will be switched to a high level for transmission than the threshold level at which they are switched to a high value for reception.