Abstract: A method for decoding a trellis encoded stream d.sub.k by the use of either an 8 state maximum likelihood response decoding mode through a Gaussian channel or a 16 state partial response decoding mode through a partial response channel is disclosed. In response to a mode selection signal FENA, after passing off the trellis encoded stream for the 8 state mode and removing the influence of a segment synchronization signal from the trellis encoded stream for the 16 state mode, all branch metrics with different values in parallel for either the 8 or the 16 state mode. After selecting a predetermined number of branch metrics required for each state among all the branch metrics of either the 8 or the 16 state mode determined based on the FENA and finding the predetermined number of candidate path metrics and determining a survivor metric and a determination vector thereof, a decoded symbol is determined by tracing back for a decoding depth in response to the FENA based on the determination vector of each state.