The system uses a 3B2T line code, i.e. one in which sets of three or triplets of binary digits are converted into pairs or duplets of ternary elements. All nine of the possible ternary duplets are used, in such a way that the same ternary duplet does not occur twice consecutively. This is done in one version by using the ninth duplet 00 as a repeat indicator, and in the other version by a conversion process in which each conversion operation takes into account the result of the preceding operation.