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Signal-to-noise ratio (SNR) mismatch is found in simulations to have great influence on the performance of low-density parity-check coded magnetic recording channels. While an inappropriate SNR mismatch degrades the performance dramatically, a properly selected optimum SNR mismatch can improve it significantly. In this paper we analyze the causes of this phenomenon and find optimum SNR mismatch values for specific magnetic recording systems with physical impairments such as electronic and media noise as well as erasures, using both density evolution analysis and Monte Carlo simulations. We observed that two characteristics of the probability density function (pdf) of the channel message, namely, the Gaussianity and the variance-to-mean ratio (VMR) have a major effect on the SNR mismatch. Generally speaking, if the channel message is approximately Gaussian-distributed and the VMR is larger than two, a negative SNR mismatch substantially improves the system performance. Numerical results show that for a magnetic recording channel with additive white Gaussian noise (AWGN), the optimum SNR mismatch is about -3 to -2 dB, while for a channel with 10% AWGN and 90% media noise, is about -10 to -8 dB, whether erasures are present or not.