Abstract: 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 different equation: ##EQU1## which produces a single precursor zero-crossing. Alternatively, the precursor equalizer may have a difference equation: ##EQU2## which produces two precursor zero crossings, y(n) being the input to the filter, w(n) its output, h.sub.0 the main cursor height before equalization and h.sub.

Abstract: In order to deal with echoes having exceptionally long tails, such as are generated by 2B1Q signals in ISDN systems, an echo canceller comprises a first echo cancelling means, conveniently a transversal filter, operative over a first number of baud periods, and filter means, conveniently an infinite impulse response filter, operative over a later number of baud periods. The filter means responds to each symbol of the transmitted signal to generate a pulse response which decays exponentially according to a predetermined function. The echo canceller further comprises means for subtracting the outputs of the first echo cancelling means and the filter means, respectively, from the received signal. The filter means may have more than one tap, enabling the exponential decay function to be adjusted to give a closer approximation to the shape of the echo tail.