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A maximum-likelihood detector for a class IV partial response magnetic recording system

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Abstract

The use of a maximum-likelihood detector for a class IV partial response magnetic recording system is discussed. In the case of additive white Gaussian noise this detector has a 3 dB better performance than the conventional threshold detector. An implementation of the Viterbi algorithm has been made in TTL logic. A 7-bit analog-to-digital converter is used to digitize the ternary partial response waveform. The detector can be used for bit rates up to 60 Mbits. The performance has been measured on a 20 Mbits experimental digital-video magnetic recording system. In two experiments, reductions of the bit error rate by a factor of 2.9 and 9.3 were achieved, compared with a conventional threshold detector.
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A maximum-likelihood sequence estimator for a digital pulse-amplitude-modulated sequence in the presence of finite intersymbol interference and white Gaussian noise is developed, The structure comprises a sampled linear filter, called a whitened matched filter, and a recursive nonlinear processor, called the Viterbi algorithm. The outputs of the whitened matched filter, sampled once for each input symbol, are shown to form a set of sufficient statistics for estimation of the input sequence, a fact that makes obvious some earlier results on optimum linear processors. The Viterbi algorithm is easier to implement than earlier optimum nonlinear processors and its performance can be straightforwardly and accurately estimated. It is shown that performance (by whatever criterion) is effectively as good as could be attained by any receiver structure and in many cases is as good as if intersymbol interference were absent. Finally, a simplified but effectively optimum algorithm suitable for the most popular partial-response schemes is described.
Partial-Response Signaling
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