Method and device for adaptive digital cancellation of the echo generated in telephone connections with time-variant characteristics

Claim 2

Procedure for adaptive digital cancellation of the echo generated in telephone connections with characteristics varying rapidly in time and in which is estimated the initial propagation delay between the samples of a signal x(t) coming from a far-end talker and those of an echo signal d(t) produced by reflection of said signal x(t), t being the discretized temporal variable, and in which is accomplished appropriate adaptive digital filtering of said echo signal d(t) and of said signal x(t) the samples of the signal x(t) being delayed for the value of said initial propagation delay and said filtering producing samples d(t) of an estimation signal of the echo signal d(t) subsequently subtracted from a signal affected by an echo obtaining a residual echo signal e(t) characterized in that said adaptive digital filtering is a recursive filtering with adaptive coefficients which obtains said estimated echo samples d̂(t) by multiplication of a vector C N -(t) which includes said adaptive coefficients, the subscript N indicating the size of the vector, and an input vector X N (t) including both the delayed samples of the signal x(t) and the samples of the echo signal d(t); in that said vector of the coefficients C N (t) is updated iteratively, at each iteration adding to the value which it had at a previous iteration the product of said residual echo signal e(t) multiplied by the present value of a gain vector K N (t) obtained by the minimization criterion of the power of said residual echo e(t) and said gain vector adapting the coefficients; and in that each iteration is accomplished in a predetermined number M> of periods of the sampling frequency of the digital signals x(t) and d(t).

Claim 3

Procedure for adaptive digital cancellation of the echo in accordance with claim 1 characterized in that said vector of coefficients C N (t) includes a number N, of adaptive coefficients belonging to a first group (a k ) placed in decreasing time order and a number N 2 of adaptive coefficients belonging to a second group (b k ) placed in decreasing time order and in that said input vector X N(t) includes said number N 1 of said samples of signal x(t) delayed by said initial propagation delay (INDX) placed in decreasing time order and said number N 2 of samples of the echo signal d(t) placed in decreasing time order.

Claim 4

Procedure for adaptive digital cancellation of the echo in accordance with either one of the above claims characterized in that said initial propagation delay of the echo signal d(t) is calculated upon insertion of the canceller and subsequently recalculated only when the level of said residual echo signal e(t) exceeds the appropriately determined threshhold.

Claim 5

Procedure for adaptive digital cancellation of the echo in accordance with any of the above claims characterized in that the present value of said gain vector N (t) is calculated using the following formula (10)- where the symbol IN [...] means the entire part of t/M, τ = M x IN [tlM], indicates a decay factor between 0 and 1, R N N 1 (T-M) indicates an inverse autocorrelation matrix of said input vector X V N ( T -M), T being the transposition operator.

Claim 6

Procedure for adaptive digital cancellation of the echo in accordance with any of the above claims characterized in that the present value of said inverse autocorrelation matrix R N N 1 (t) is calculated recursively using the following expression (11):-

Claim 7

Procedure for adaptive digital cancellation of the echo in accordance with any of the above claims characterized in that said number N 1 of adaptive coefficients (a k ) belonging to said first group is equal to 10 and that said number N 2 of adaptive coefficients (b k ) belonging to said second group is equal to 10.

Claim 8

Procedure for adaptive digital echo cancellation in accordance with any of the above claims characterized in that said number M of periods of the sampling frequency of the digital signals x(t) and d(t) included in an iteration of sthe calculation of said vector C N (t) of coefficients is 3.

Claim 9

Device for adaptive digital cancellation of the echo generated in telephone connections with characteristics varying rapidly in time including (a) a first block (RIT) which estimates the initial propagation delay between the samples of a signal x(t) coming from a far-end talker and those of an echo signal d(t) produced by reflection of said signal x(t), t being the discretized time variable; (b) an adaptive digital filter (FTR) at whose inputs are placed the samples of said echo signal d(t) and the samples of said signal x(t) delayed by the value of said initial propagation delay and at whose output are present samples d̂(t) of an estimation signal of the echo signal d(t); and (c) a subtractor (SOT) which subtracts said samples d(t) from those of said echo signal d(t) obtaining a residual echo signal e(t) characterized in that said adaptive digital filter (FTR) is a recursive filter which includes a first group (a k ) and a second grup (b k ) of adaptive coefficients and calculates the samples d(t) of said estimated echo signal d(t) recursively by multyplying a vector → C N (t) which includes said groups of adaptive coefficients (a k , b k ), the subscript N indicating the size of the vector, for an input vector X N (t) of the digital filter (FTR) which includes said samples of the echo signal d(t) and said delayed voice samples of a far-end talker; in that it includes a second block (ACF) which updates said vector or the coefficients C N (t) iteratively, at each iteration adding to the value which it had at a previous iteration the product of said residual echo signal e(t) and a gain vector K N (t) obtained with the minimization criterion of the power of said residual echo e(t), said gain vector adapting the coefficients; and in that said second block (ACF) accomplishes an updating iteration of the vector of the adaptive coefficients C N (t) in a predetermined number M>1 of periods of the sampling frequency of the digital signals x(t) and d(t).

Claim 10

Device for adaptive digital cancellation of the echo generated in telephone connections with characteristics varying rapidly in time including (a) a first block (RIT) which estimates the initial propagation delay between the samples of a signal x(t) coming from a far-end talker and those of an echo signal d(t) produced by reflection of said signal x(t), t being the discretized time variable; (b) an adaptive digital filter (FTR) at whose inputs are placed the samples of said echo signal d(t) and the samples of said signal x(t) delayed by the value of said initial propagation delay and at whose output are present samples d̂(t) of an estimation signal of the echo signal d(t); and (c) a subtractor (SOT) which subtracts said samples d(t) from those of said echo signal d(t) obtaining a residual echo signal e(t) characterized in that said adaptive digital filter (FTR) is a recursive filter which includes a first group (a k ) and a second grup (b k ) of adaptive coefficients and calculates the samples d(t) of said estimated echo signal d(t) recursively by multyplying a vector → C N (t) which includes said groups of adaptive coefficients (a k , b k ), the subscript N indicating the size of the vector, for an input vector X N (t) of the digital filter (FTR) which includes said samples of the echo signal d(t) and said delayed voice samples of a far-end talker; in that it includes a second block (ACF) which updates said vector or the coefficients C N (t) iteratively, at each iteration adding to the value which it had at a previous iteration the product of said residual echo signal e(t) and a gain vector K N (t) obtained with the minimization criterion of the power of said residual echo e(t), said gain vector adapting the coefficients; and in that said second block (ACF) accomplishes an updating iteration of the vector of the adaptive coefficients C N (t) in a predetermined number M>1 of periods of the sampling frequency of the digital signals x(t) and d(t).

Claim 11

Device for adaptive digital cancellation of the echo in accordance with claim 8 characterized in that said second block (ACF) organizes said vector of the coefficients C N (t) so as to include a number N 1 of adaptive coefficients belonging to said first group (a k ) placed in decreasing temporal order and a number N 2 of adaptive coefficients belonging to said second group (b k ) placed in decreasing temporal order.

Claim 12

Device for adaptive digital cancellation of the echo in accordance with either of claims 8 or 9 above characterized in that it includes a third block (VET) which calculates said input vector X N (t) of the digital filter (FTR) at whose inputs arrive the samples of said echo signal d(t), the samples of said signal x(t) coming from a far-end talker and a signal INDX coming from said block (RIT) which estimates the propagation delay of the echo d(t) and at whose output there is present said vector X N (t) which includes said number N 1 of samples of the signal x(t) placed in decreasing temporal order after being delayed by the value of said initial propagation delay indicated by INDX and said number N 2 of samples of the echo signal d(t) placed in decreasing temporal order.

Claim 13

Device for adaptive digital cancellation of the echo in accordance with any of claims 8 to 10 above characterized in that it includes a fourth block (CNL) which controls the level of said residual echo signal e-(t) and, when said level exceeds an appropriately determined threshhold, imparts a command to said block (RIT) which recalculates the initial propagation delay (INDX) of the echo signal d(t).

Claim 14

Device for adaptive digital cancellation of the echo in accordance with any of claims 8 to 11 above characterized in that it comprises a fifth block (VGK) which receives as an input said input vector X N (t) and a inverse autocorrelation matrix R N N 1 ( T -M) of said vector X N (t) evaluated in an iteration preceding the present one and calculates the present value of said gain vector K N (t) using the following formula (10)- where the symbol IN[...] means the entire part of t/M, r = M x IN[VM], indicates a decay factor between 0 and 1, X N T(τ) being the transposed vector of X N(τ).

Claim 15

Device for adaptive digital cancellation of the echo in accordance with any of claims 8 to 12 above characterized in that it includes a sixth block (MAI) which receives as an input said input vector X N (t) and said gain vector VN(t) and outputs said inverse autocorrelation matrix R N N 1 (t) whose present value is calculated recursively using the following expression (11):-

Claim 16

Device for adaptive digital cancellation of the echo in accordance with any of claims 8 to 13 above characterized in that said number N 1 of adaptive coefficients (a k ) belonging to said first group is equal to 10 and that said number N 2 of adaptive coefficients (b k ) belonging to said second group is equal to 10.

Claim 17

Device for adaptive digital cancellation of the echo in accordance with any of claims 8 to 14 above characterized in that said number M of periods of the sampling frequency of the digital signals x(t) and d(t) included in an iteration of the calculation of said vector C N (t) of coefficients is equal to 3.