Echo cancelling device with phase-roll correction

Claim 1

In a data transmission system connected to a two-wire line portion of a transmission medium over which the transmission of an outgoing signal in analog form which has been converted from an outgoing signal in digital form can be made simultaneously with the reception of an incoming signal in analog form, said system operating in half-duplex mode wherein the incoming signal in analog form is an echo signal produced by said transmission medium in response to the outgoing signal in analog form, an echo cancelling device for generating an estimated echo signal, providing a phase-roll corrected echo signal and subtracting the real component of said phase-roll corrected echo signal from said incoming signal in analog form to form an error signal, said echo cancelling device being characterized in that it includes a phase-roll tracking device comprised of: phase rotating means (58) for rotating the phase of the estimated echo signal by a value X which is the estimated phase-roll value, thereby providing a complex signal representative of said phase-roll corrected echo signal; multiplying means (124) for multiplying said error signal by the imaginary component (120) of said phase-roll corrected echo signal, thereby providing a value Y which is representative of the variation of the phase-roll; a phase variation estimator (128) providing an estimation of the phase-roll variation Z in response to said value Y, said phase variation estimator (128) operating in half duplex mode and including: a gain device (140) which provides a value proportional to said value Y, which is taken as said estimation of the phase-roll variation Z when both said phase-roll corrected echo signal and said error signal are relatively large; and a multiplier (146) for providing the product of the sign bit (142) of the phase-roll corrected echo signal and the sign bit (144) of the error signal, said product being taken as said estimation of the phase-roll variation Z when said phase-roll corrected echo signal and/or said error signal are small; and a phase filter (126) providing said estimated phase-roll value X in response to said estimation of the phase-roll variation Z.