Secondary channel method and apparatus

A secondary data channel is described for use in a four-wire communication system in which there is continuous wire between each of two coupling transformers at a first station and each of two coupling transformers at a second station. In accordance with the invention, the transformer windings at each station that are connected to the transmission line wires are equipped with center taps G that are connected to a secondary channel data set. By these means a secondary data signal can be transmitted from one station to another by applying a signal to the center taps of the two coupling transformers at one station and receiving such signals from the center taps of the two coupling transformers at the other station.