Patent ID: 6424720
Filing Date: 2002-07-23
Classification: H04M

Abstract:
A method of reducing acoustic echo and adapting sound to space, in a system for digitally transmitting sound signals on P transmission channels between a local site and at least one remote site, each having N microphones and M loudspeakers, N, M, P being integers, it being possible for the values of N and M to differ depending on the sites, whereby:(a) a cumulative distribution function is computed for each microphone signal xi(n) from the local site, i being an integer ranging between 1 and N and n denoting the time rank of the samples, and a cumulative distribution function is computed for each loudspeaker signal zj(n) from the local site, j being an integer ranging between 1 and M; then, for every i, 1&lE;i&lE;N, (b) a first attenuation factor Gmic(i,n) is computed for the microphone signal xi(n) from the local site on the basis of a ratio between the cumulative distribution functions obtained previously; (c) the first attenuation factor Gmic(i,n) is adjusted so as to obtain a second attenuation factor Gâ€²mic(i,n) defined as follows: Gâ€²mic(i,n)=S1(Gmic(i,n)) where S1(Gmic(i,n))=s if Gmic(i,n)&lE;s,S1(Gmic(i,n))=Gmic(i,n) if smic(i,n)<1 and S1(Gmic(i,n))=1 if Gmic(i,n)&gE;1, s being a predetermined minimum threshold which is strictly less than 1; (d) on the basis of the cumulative distribution functions of microphone and loudspeaker signals computed previously, it is determined whether the microphone signal xi(n) is an echo signal only or a signal coming solely from the local site in the case of a first situation or if the microphone signal xi(n) contains components from the local site and other components from the remote site in the case of a second situation; (e) a third attenuation factor Gâ€³mic(i,n) is computed which, in the first situation, is equal to the second attenuation factor Gâ€²mic(i,n) and in said second situation is equal to the second attenuation factor Gâ€²mic(i,n) but in which the minimum threshold s in the computation used to obtain it is increased by a predetermined value; (f) a fourth attenuation factor &Ggr;(i,n) is computed on the basis of a ratio between the cumulative distribution functions of microphone signals; (g) the fourth attenuation factor &Ggr;(i,n) is adjusted in order to obtain a fifth attenuation factor &Ggr;(i,n) defined as follows: &Ggr;â€²(i,n)=S2(&Ggr;(i,n)) where S2(&Ggr;(i,n))=sâ€² if &Ggr;(i,n)&lE;sâ€²,S2(&Ggr;(i,n))=&Ggr;(i,n) if sâ€²S2(&Ggr;(i,n))=1 if &Ggr;(i,n)&gE;1, sâ€² being a predetermined minimum threshold strictly less than 1; (h) the product of the third and fifth attenuation factors Gâ€³mic(i,n) and &Ggr;â€²(i,n) obtained previously is computed so as to obtain a global attenuation factor G*mic(i,n) defined by: G*mic(i,n)=Gâ€³mic(i,n).&Ggr;â€²(i,n); (i) the global attenuation factor G*mic(i,n) is adjusted so as to obtain a weighting factor &bgr;i(n) defined as follows: &bgr;i(n)=S4(G*mic(i,n)) where S4(G*mic(i,n))=sâ€³ if G*mic(i,n)&lE;sâ€³ andS4(G*mic(i,n))=G*mic(i,n) if sâ€³mic(i,n)&lE;1, sâ€³ being a predetermined minimum threshold strictly less than 1; (j) a signal yk(n) is transmitted on each transmission channel, k being an integer between 1 and P, in the form of a linear combination of the weighted microphone signals xi(n), defined as follows: yk&it;(n)=&Sum;i=1N&it;Î±k,i&it;(n)&CenterDot;Î²i&it;(n)&CenterDot;xi&it;(n)where &agr;k.i(n) denotes the predetermined real coding coefficients and &bgr;i(n) denotes the weighting factors obtained previously; then, for every integer j, 1&lE;j&lE;M:(k) a sixth attenuation factor GHP(j,n) is computed for the loudspeaker signal zj(n) from the remote site on the basis of cumulative distribution functions calculated for each transmitted signal yk(n) from the local site; (l) the sixth attenuation factor GHP(j,n) is adjusted so as to obtain a weighting factor &lgr;j(n) defined as follows: &lgr;j(n)=S3(GHP(j,n) where S3(GHP(j,n))=s* if GHP(j,n)&lE;s*,S3(GHP(j,n))=(GHP(j,n) if s*HP(j,n)<1 and S3(GHP(j,n))=1 if GHP(j,n)&gE;1, s* being a predetermined minimum threshold strictly less than 1; (m) the loudspeaker signal zj(n) of the remote site is determined on the basis of a linear combination of the weighted transmitted signals yk(n), defined as follows: zj&it;(n)=Î»j&it;â€ƒ&it;(n)&CenterDot;&Sum;k=1P&it;â€ƒ&it;Î³j.k&it;â€ƒ&it;(n)&CenterDot;yk&it;(n)where &ggr;j.k(n) denotes the predetermined real decoding coefficients and where &lgr;j(n) denotes the weighting factors obtained previously; and(n) the loudspeaker signal zj(n) thus obtained is emitted on the j th loudspeaker of the remote site.