Patent GB-1519295-A (GB)
Title: Echo canceller
IPC: H04B3/23
Filed: 1975-07-02
Abstract: 1519295 Echo suppressors COMPAGNIE INDUSTRIELLE DES TELECOMMUNICATIONS CIT-ALCATEL 2 July 1975 [3 July 1974] 27818/75 Heading H4R In an echo canceller having a transversal filter which produces a synthetic echo by convolution of N samples of a received signal with N weighting filter coefficients, the synthetic echo being subtracted from the real echo, and in which the weighting coefficients are up dated by correlation of the residual echo with the received signal samples, the transversal filter is centred relative the echo path response by providing a delay circuit with multiple output connections in the synthetic echo path and selecting the delay circuit output by comparison of the weighting coefficient value, after the elapse of the transversal filter convergence time, with a predetermined threshold, and switching the tap output to provide that the significant weighting coefficient values occupy positions of lowest rank, or earliest effective time, in the coefficient store. As described with respect to Fig. 2 samples of the signal x(t) on the incoming line 14 are converted to digital form, at 20, and are fed into a store 21 at a sampling interval of T(=125Á seconds). Between each pair of successive sampling instants the last N (= 64) samples are convoluted in 24 with 64 weighting coefficients C 1  to C N  in store 23, the resulting values summed in 25 and subtracted from the echo signal in the transmit path 15. The residual echo e(t) is correlated with the samples of the input signal from store 21 in multiplier 29 to provide adjustment values for the weighting coefficients in store 23. According to the invention the input signal samples are fed into the store 21 via an adjustable delay unit 40 Fig. 3. In order to select the tap from which the input signal samples are applied to the store 21 the weighting coefficient values are compared with a reference value R in comparator 45, the coefficients being split into groups (4) equal to the number of tappings on each section of the delay device 40, where each section has an overall delay # equal to NT, or the total delay of the transversal filter. Simultaneously with the application of a particular group of weighting coefficients to the comparator 45 so a respective one of the gates 46, 47, 48, 49 is enabled by the divider 50, while a further enable signal Tr is applied for the duration of a search period immediately following the filter convergence period so that during the search period store 51 is fed with a signal indicating which groups of weighting coefficients contain values exceeding the reference threshold. A comparator 52 compares the signal stored in store 51 with the signal appearing at the outputs of gates 46 to 49 and only enables entry of a new signal when that new signal corresponds to a lower order of signal than that already stored, i.e. a significant coefficient appears earlier in the store 23. At the end of the search period decoder 53, which receives the state of outputs Ml to M4 of store 23 causes appropriate adjustment of the tapping selector 35 to put an appropriate delay in the signal applied to store 21.