1. A method of obtaining a desired error rate performance in a point-to-multipoint digital transmission system suffering from error propagation, said system comprising a transmitter unit including a forward error correction encoder (FEC) which generates encoded information provided to an input of a data interleaver having block length N and interleaving depth D, and a data modulator which is coupled to an output of said data interleaver to receive a stream of data symbols, and at least one receiver unit including a decision feedback equalizer (DFE) which includes a feedback filter and provides an input to a data de-interleaver having block length N and interleaving depth D, and a FEC decoder which receives data from an output of said data de-interleaver, said method comprising: periodically adding sequences of known symbols of length at least equal to the length of said feedback filter to the output of said data interleaver, said sequence being removed at said receiver unit prior to passing to said data de-interleaver; establishing the number of data symbols between said periodic sequences to be equal to an integer multiple k of the interleaving depth D, thus limiting error propagation to cause no more than k errors per codeword of block length N; and setting said FEC code to correct at least said k errors per error propagation event.