A communication apparatus for communicating over an electrical power cable, comprising: a transmitter circuit adapted to transmit a communication signal; and a receiver circuit adapted to receive a communication signal from another communication apparatus; wherein the communication apparatus is adapted to transmit and receive communication signals over the power cable both when said power cable is conducting electricity and when said power cable is not conducting electricity, characterised in that the communication signal comprises an amplitude shift keying signal, and wherein the receiver circuit further comprises: an analog to digital converter adapted to sample the communication signal and output digital samples; a filter adapted to filter the digital samples; an amplitude tracker adapted to calculate the magnitude of each of a group of digital samples from the filter; a mark counter adapted to count the number of digital samples within the group having magnitudes equal to or greater than a predetermined threshold; a space counter adapted to count the number of digital samples within the group having magnitudes less than the predetermined threshold; a comparator adapted to output an output signal indicative of whether or not the mark counter exceeds the space counter; and a buffer adapted to store the output of the comparator. a transmitter circuit adapted to transmit a communication signal; and a receiver circuit adapted to receive a communication signal from another communication apparatus; wherein the communication apparatus is adapted to transmit and receive communication signals over the power cable both when said power cable is conducting electricity and when said power cable is not conducting electricity, characterised in that the communication signal comprises an amplitude shift keying signal, and wherein the receiver circuit further comprises: an analog to digital converter adapted to sample the communication signal and output digital samples; a filter adapted to filter the digital samples; an amplitude tracker adapted to calculate the magnitude of each of a group of digital samples from the filter; a mark counter adapted to count the number of digital samples within the group having magnitudes equal to or greater than a predetermined threshold; a space counter adapted to count the number of digital samples within the group having magnitudes less than the predetermined threshold; a comparator adapted to output an output signal indicative of whether or not the mark counter exceeds the space counter; and a buffer adapted to store the output of the comparator. an analog to digital converter adapted to sample the communication signal and output digital samples; a filter adapted to filter the digital samples; an amplitude tracker adapted to calculate the magnitude of each of a group of digital samples from the filter; a mark counter adapted to count the number of digital samples within the group having magnitudes equal to or greater than a predetermined threshold; a space counter adapted to count the number of digital samples within the group having magnitudes less than the predetermined threshold; a comparator adapted to output an output signal indicative of whether or not the mark counter exceeds the space counter; and a buffer adapted to store the output of the comparator.