Abstract: A data transmission system using a transmission code in which one data state corresponds to a zero transmission level, such as the alternate mark inversion (AMI) code, includes at each terminal adaptive signal processing circuits such as an echo canceller (402) and/or an adaptive equalizer (409). Each terminal includes a decision circuit (408) an output of which controls a switch (410) which is operative to allow updating of the adaption of the echo canceller (402) and adaptive equalizer (409) only when a data state corresponding to a zero transmission level is detected.

Abstract: A code generator which produces a transmission code in response to a serial binary data stream applied to its input. The transmission code is similar to that know as WAL2 but whereas WAL2 involves a double frequency component when the binary data changes from `1` to `0` or vice versa the transmission code produced by the code generator holds the output signal constant at a level intermediate those corresponding to the binary `1` and `0`. This reduces the high frequency content of the transmitted signal and consequently reduces crosstalk between the channels. The code generator includes a first current source which is connected to a summing network through a first switch, a second current source which is connected to the summing network through a second switch and a control circuit which produces signals on lines to selectively operate the first and second switches.