Carrier telephone system installed between Sydney and Melbourne

This system was inaugurated on September 10, 1925, between Sydney and Melbourne, 450 miles apart. The wires are copper and 600 pounds per mile, the route miles amounting to nearly 600, and three telephone channels are provided with composited duplex telegraphs over each wire of the circuit. Three repeaters are inserted, giving a net efficiency of about 9 T.U. between terminals. Only the carrier and one side band are transmitted. The three two-way channels are obtained by six carriers. For transmission from Sydney to Melbourne 6000, 9000 and 12,000 cycle carriers are used, and in the opposite direction 15,000, 18,000 and 21,000. Upper side bands are used in the latter and lower ones in the former, which minimises crosstalk because of the wider frequency separation between the transmitting and receiving groups. Photographs of the equipment and diagrams of the terminal and repeating station arrangements accompany the paper.