 The R Type system was limited to four duo-directional channels using a frequency between 3300 and 5220 cycles per second, which was above normal voice frequency but below the range of other established carrier systems, which used high frequency bands.There was no interference when both normal speech frequency and the carrier frequency operated over the same pair of copper wires.In effect, one pair of copper wires from Townsville to Thursday Island was to carry 3 facilities