Patent GB-1380944-A (GB)
Title: Echo canceller having two echo path models
IPC: H04B3/23
Filed: 1972-05-18
Abstract: 1380944 Echo suppressors NIPPON ELECTRIC CO Ltd 18 May 1972 [19 May 1971 13 Aug 1971] 23502/72 Heading H4R  An echo suppressor for cancelling the echo signal appearing from the actual echo path comprises self-adaptive means provided with a first echo path model approximating the characteristics of said echo path and further comprises a second echo path model approximating said characteristics, means for comparing the models, and means responsive to the results of comparison for substituting one of said echo path models for the other. The second echo path model is included to overcome problems that arise in known echo suppressors employing self adaptive echo path models when both parties on a telephone connection are talking simultaneously. Fig. 1 shows the general arrangement of one embodiment in which the echo suppressor bridges an incoming one way path 11 and an outgoing one way path 12 of a four-wire line 13 connected to a two-wire line 14 through a hybrid circuit 15, which delivers in practice a fraction of the incoming signal to the outgoing path 12 as an echo signal. The echo suppressor comprises a first echo path model unit 21 connected to the incoming path 11 for modifying or processing the incoming received signal x in conformity with a first echo path model or a first set of parameters h1 contained therein to # produce a first synthesized echo signal y 1 ; a first combining circuit 22 for combining the first synthesized signal with the outgoing received signal y to produce a first combined signal e 1 ; and means symbolically depicted by a connection 23 for adjusting the first set of parameters h 1  in compliance with the outgoing received signal y and the first combined signal e 1 . A second echo path model unit 26 is connected to the incoming one-way path 11 for modifying the incoming received signal x in conformity with a second set of parameters h 2  contained therein to produce a second syn- # thesized echo signal y 2 ; a second combining circuit 27 interposed in the outgoing one-way path 12 for combining the second synthesized signal with the outgoing received signal y to produce a second combined signal e 2  which is delivered through the outgoing path 12 to the remote end as the outgoing transmitted signal; an echo path model comparing circuit 28 supplied with the first and second combined signals for producing a command signal when the short-time average of the former is smaller by a predetermined threshold than that of the latter; and a switching circuit or gate 29 responsive to the command signal for substituting the contents of the first echo path model unit 21 for the contents of the second echo path model unit 26. In this arrangement the signal e 2  supplied to the outgoing path is that produced by the second echo path model which is not self adaptive and has its characteristics h 2  altered to those, h 1  of the first echo path model where the latter results in a first combined signal e 1  which is freer of echo, and therefore on average lower in amplitude, than the second combined signal, or outgoing signal, e 2 . Hence when the parameters h 1  of the first echo path model are distorted due to a non-echo signal in the outgoing signal e 2 , the second echo path model previously derived from the first echo path model continues to be used until the first echo path model again yields a signal e 1  which is freer of echo than the signal e 2 . In a second embodiment (Fig. 5, not shown) the parameters of the second echo path model can be written into the first echo path model and vice versa according to which model is giving the better results. Either the first or second combined signals e 1  or e 2  can then be used as the outgoing signal. It is to be noted that the purpose of having the first echo path model self adaptive is to allow for changes in the transmission properties of the hybrid 15. When there is no speech to be transmitted from the hybrid 15, the first echo path model should self adapt to yield a zero outgoing signal e 2  (e 1 ). Two detailed arrangements for use as the echo path model arrangements of Fig. 1 are described. The first (Fig. 3, not shown) is based on sampling the incoming signal x on line 11 and the outgoing-received signal y on line 12 in successive time intervals, and involves converting all speech to digital form before performing the various steps necessary for producing the signals e 1 , e 2  which are then time samples in digital form, a final digital-analogue converter yielding the outgoing signal. Shift registers are then used for storing the echo path model characteristics h 1 , h 2  in digital form. The second arrangement (Fig. 4, not shown) is based on the frequency content of the signals and involves fourier transforming successive samples of the signals. The characteristics h 1 , h 2  are then a non-linear conversion of the fourier transformers of the first combined signal e 1 . A full description of the arrangement of Figs. 3 and 4 is given in the Specification; examples of a suitable comparing circuit 28 are also described. U.S. Specifications 3,465,106, 3,499,999 and 3,500,000 are referred to.