Digital transmission codes: properties of HDB3 and related ternary codes with reference to broadcast signal distribution

An investigation has been carried out to assess the implications for broadcast signal distribution of the use of HDB3 as the proposed customer interface code in the Post Office digital communication network. HDB3 is a three-level transmission code with no d.c. component and with a bound on the maximum number of successive equilevel symbols. These two characteristics are important since a.c. coupling may then be employed in the transmission path and virtually continuous timing information can be extracted from the coded signal. The properties of related transmission codes, including the class of alphabetic (or block) codes were also investigated, in view of the likely adoption of one of these (4B 3T) by the Post Office as a line transmission code at the higher bit rates. The effect of transmission-code digit errors on the decoded binary sequence was studied and suitable error correction schemes are outlined. An experimental HDB3 codec was constructed, primarily to demonstrate the feasibility of such transcoding at bit rates up to the order of 120 Mb/s, using commercially available integrated circuits.