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In this paper, we present techniques for improving the performance of Viterbi detector in the presence of channel nonlinearities and media noise, which are the dominant source of errors in high density magnetic recording channels. Instead of treating these distortions separately, we combine them into a "high-density (HD) noise", which is signal dependent and correlated in nature. To compensate for signal-dependent mean and correlation of HD noise, we modify the branch metrics by subtracting an estimate of the mean from the signals on each branch and optimize the equalization target by minimizing the dominant error event probability, respectively. Simulation results show that these modifications yield significant performance gains.