1. A method of calibrating a Viterbi detector, said Viterbi detector comprising at least one noise predictive filter, said method comprising: (a) obtaining noise samples in a training phase; (b) averaging said noise samples; (c) estimating entries of a 3-by-3 conditional noise matrix C [k] defined by C ij [k] =E(n i-3 n j-3 |NRZ condition k); and (d) calculating at least one tap of said at least one noise predictive filter based on said estimated entries.