using combinations of direct currents of different amplitudes or polarities over line conductors or combination of line conductors

Full Title

Details of selecting apparatus or arrangements > Electrical details > Signalling arrangements; Manipulation of signalling currents (multiplex systems providing for calling or supervisory signals H04J1/14, H04J3/12) > using combinations of direct currents of different amplitudes or polarities over line conductors or combination of line conductors