4. An apparatus as claimed in claim 1, the apparatus further comprising sync word generator means for generating a sync word for a block of p consecutive code words in the modulated signal, the sync word generator means being adapted to generate said sync word from the following set of available sync words: 11100011111110, 10100011111110, 01100011111110 and 11000100000001, the sync word generator means being adapted to select one of the sync words in dependence upon the coding state.
4. An apparatus as claimed in claim 1, the apparatus further comprising sync word generator means for generating a sync word for a block of p consecutive code words in the modulated signal, the sync word generator means being adapted to generate said sync word from the following set of available sync words: 11100011111110, 10100011111110, 01100011111110 and 11000100000001, the sync word generator means being adapted to select one of the sync words in dependence upon the coding state. 17. An apparatus as claimed in claim 4, wherein the sets of code words belonging to each pair of coding states of the first type are disjunct.