Digital echo canceller

Claim 1

A digital echo canceller (1) with a receive path (2) between a receive input (RI) and a receive output (RO), and a send path between a send input (SI) and a send output (SO), said echo canceller being used for cancelling an adaptive echo signal at the send input (SI) which has occurred in response to a receive input signal applied to the receive input (RI), said echo canceller including: - a first filter combination comprising: - a first digital filter (9) with a programmable filter coefficient memory (9(1)) for generating in response to the receive input signal (X(k)) a first replica signal (ê(k)) which is an estimate of the echo signal, and - a second, adaptive digital filter (11) with a filter coefficient memory (12(1)) for generating in response to the receive input a second replica signal (1 a (k)) which is an estimate of the echo signal, the second digital filter (11) further including means for forming an error signal (r a (k)) that is representative of the difference between a signal applied to the send input and the second replica signal, and a adaptation processor (13) for adaptively modifying filter coefficients in response to the receive input signal and the error signal and applying the modified filter coefficients to the filter coefficient memory (12(1)) of this second digital filter; - means (10) for forming a send output signal as the difference between the signal applied to the send input and the first replica signal; - controllable gate means (15) for selectively applying the modified filter coefficients to the programmable filter coefficient memory of the first digital filter; and - control means (16) for blockwise determining the respective levels of the error signal and the send output signal, and for generating in response to the levels thus found a control signal for the gate means which depends in a predetermined manner on differences between the respective levels, characterized in that - in the said filter combination the second digital filter (11) is a frequency domain block-adaptive filter having a block length of N' components and having, for each signal block m a number of N' frequency-domain filter coefficients W(p;m) with p = 0,1,2,...,N'-1, and the first digital filter (9) is a time-domain programmable digital filter having a number of N time-domain filter coefficients w(i;m) with i= 0,1,2,...,N-1, with N' exceeding N; and - the echo canceller further includes transforming means (20), cascaded with the gate means, for forming the N time-domain filter coefficients w(i;m) required in the first digital filter as components of an N'-point Discrete Orthogonal Transform of a block of N' frequency-domain filter coefficients W(p;m).