Abstract: A variation in the cancellation coefficents of an echo cancelling modem indicate a change in the impedance of a line. In an echo canceller with T spaced samples (i.e. producing a cancellation signal at the data rate) the resolution is too poor to provide satisfactory identification of faults. A set of coefficients is stored and the sampling phase adjusted to produce a second set. This may be repeated producing eight sets in the preferred embodiment. A higher resolution picture of the line impedance is built up allowing comparisons with an earlier or optimum set of values. The test facility may be included in an operating modem providing line test facilities when not transmitting data.

Abstract: A non-linear adaptive filter is described having a linear filter connected in parallel with a non-linear filter. The linear filter provides fast adaption until it has modelled the linear contribution of each coefficient. Thereafter the non-linear filter continues to adapt until the error signal has been reduced to an acceptable level.In a preferred embodiment a plurality of unit delay devices provide taps to sub-processing units each of which is arranged to be adapted according to a linear algorithm during an initial part of a training period, and to a non-linear algorithm after said initial part.The filter has particular use in echo cancellers for data modems.

Abstract: A digital filter comprises a plurality of delay stages 41 such as shift register stages through which an input x(t) is fed. Each of a plurality of individually addressable random access memories 42 is addressed by the output of one of the delay stages and the output from the RAMs 42 are combined. An error signal adapts the values stored within the RAMs. The filter may also be extended such that each RAM receives an address input from more than one delay stage.