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Abstract

In a receiver for a loop interface of a digital data transmission system, a received four-level substantially randomized digital signal is sampled and a multilevel slicer compares the samples selectively with a plurality of thresholds to extract the symbols. A timing recovery circuit determines the sampling instant as a function of a precursor sample and a main pulse sample one baud apart, according to the timing function: f(t)=(1/r)h(t)-h(t-T) where 1/T is the baud rate, h(t) and h(t-T) denote the main pulse sample and precursor sample, respectively, r is the ratio between the main pulse sample and the precursor pulse sample, and t is the sampling instant. The timing recovery circuit includes: a timing estimator responsive to baud rate samples before and after slicing, respectively, for deriving a precursor estimate (z.sub.n) of the difference between the actual and desired sampling instants with reference to the ratio between the main sample pulse and the precursor sample one baud earlier; and a generator for generating a baud rate sampling clock signal signal variable in dependence upon the estimate (z.sub.n). The timing estimator derives the timing estimate z.sub.n in accordance with the expression: z.sub.n =(-1).sup.m .multidot.(a.sub.n-1 x.sub.n -a.sub.n x.sub.n-1) where m represents the magnitude bit of the 2B1Q symbol (m=0 for magnitude 1, m=1 for magnitude 3). The slicer may have three thresholds adaptable according to the magnitude of the main cursor.

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