| 摘要 |
A receiver for a digital data transmission system comprises a sampling device for sampling digital data signals received from a subscriber loop. A precursor equalizer connected to the output of the sampling device equalizes, in effect filters, the sampled digital data signal. A timing recovery circuit connected to the output of the precursor equalizer controls the sampling device in dependence upon the filtered digital data signal. The precursor equalizer has a difference equation such that the equalized digital data signal has at least one zero-crossing occurring substantially one baud before the main cursor of each pulse. For twisted wire pair subscriber loops, the precursor equalizer may have a difference equation: w(n)= <IMG> which produces a single precursor zero-crossing. Alternatively, the precursor equalizer may have a difference equation: w(n)= <IMG> which produces two precursor zero crossings, y(n) being the input to the filter, w(n) its output, ho the main cursor height before equalization and h-1 the precursor height before equalization. The latter precursor equalizer may be modified by multiplying each of the coefficients (h-1/h0)2 and h-1/h0 by a factor .delta. = <IMG> to define the zero crossing more precisely. A (1-Z-1) filter may be provided ahead of the precursor equalizer. 38
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