Abstract: Apparatus, and a related method for its operation, for transmitting binary data at high speed through unshielded twisted pairs of conductors intended for a much lower data speed and bandwidth. By using a combination of demultiplexing into multiple data channels, encoding the binary signals into a modified three-level duobinary code, and filtering the signals before transmission, data rates as high as 125 megabits per second can be transmitted over four unshielded twisted pairs of conductors. Two illustrative forms of the invention are disclosed, one of which uses two channels for transmission over two twisted pairs in each direction of transmission. The other form of the invention uses four data channels, to further reduce the transmitted bandwidth in each channel, and four twisted pairs of conductors, for transmission in both directions by means of hybrid circuits interconnecting the transmitter and receiver functions at each end of the channels.