A process and system for improving echo cancellation within a transmission network

Claim 2

A process for cancelling residual echo signals in a transmission system wherein echo signals are partially cancelled through use of a FIR filter generating an echo replica, which replica is subsequently subtracted from the signal in the echo path, said process being characterized in that it includes: - measuring the energy (Eout) of so called far end signal; - measuring the energy (Ein) of so called near end signal; -comparing the ratio Ein/Eout of said measured energies to a predetermined threshold value, and stopping the transmission of said near end signal upon the- result of said comparing step.

Claim 3

A process according to claim 1 wherein said threshold is derived from the echo impulse response.

Claim 4

A process according to claim 1 or 2 wherein said echo replica is generated over a delayed far-end signal.

Claim 5

A process according to claim 3 wherein said echo replica is also generated by using a FIR digital filter.

Claim 6

A process according to claim 4 wherein said threshold is derived from the portion of impulse response not considered for the FIR filter.

Claim 7

A voice transmission system for transmitting voice signals between a near end telephone set and a far-end telephone set through a transmission network including bidirectional lines each connected to a couple of unidirectional lines said system also including echo cancellation means synthesizing an echo replica through use of a digital filter fed with the signal received from a far-end telephone sets over a first one of said unidirectional lines and subtracting said replica from the signal over a second one of said unidirectional lines, said transmission system being characterized in that it further includes residual echo cancelling means, said residual echo cancelling means including : - first means connected to said first unidirectional line for measuring the energy of the signal over said first unidirectional line; - second measuring means connected to said second unidirectional line for measuring the energy of the signal from which said replica has been subtracted; - residual echo clipping means connected to said first and second energy measuring means for comparing said measured energies and for deriving therefrom a flag- information, said flag information being used to conditionally stop transmission over said second unidirectional line.

Claim 8

A voice transmission system for transmitting voice signals between a near end telephone set and a far-end telephone set through a transmission network including bidirectional lines each connected to a couple of unidirectional lines said system also including echo cancellation means synthesizing an echo replica through use of a digital filter fed with the signal received from a far-end telephone sets over a first one of said unidirectional lines and subtracting said replica from the signal over a second one of said unidirectional lines, said transmission system being characterized in that it further includes residual echo cancelling means, said residual echo cancelling means including : - first means connected to said first unidirectional line for measuring the energy of the signal over said first unidirectional line; - second measuring means connected to said second unidirectional line for measuring the energy of the signal from which said replica has been subtracted; - residual echo clipping means connected to said first and second energy measuring means for comparing said measured energies and for deriving therefrom a flag- information, said flag information being used to conditionally stop transmission over said second unidirectional line.

Claim 9

A voice transmission system according to claim 6 and wherein said echo replica synthesizing means includes a flat delay line and said first measuring means is connected to said first unidirectional line through said flat delay line.

Claim 10

A voice transmission system according to any one of claims 6 or 7 wherein said first and second energies are repetitively measured over fixed periods of time, said flag information being used to stop signal transmission over time periods at least equal to one of said fixed periods of time.

Claim 11

A voice transmission system according to claim 6 wherein said voice signal is digitally encoded into packets of bits each packet representing the voice signal over said fixed period of time.

Claim 12

A voice transmission system between two telephone sets (T1 and T2) including bidirectional lines respectively connecting Tl and T2 to hybrid transformers, a couple of unidirectional lines connecting each of said hybrid transformers to a processor (10 or 20) wherein digital encodings are performed over the voice signal to derive therefrom packets of bits representing fixed length segments of voice signal, said packets being transmitted between said processors (10 and (20) over a digital transmission network, said processors also including D/A conversion for converting the digitized voice signal into analog form before delivering it to the corresponding telephone set T1 or T2; said processors also each including echo cancelling means including : - a digital filter connected to the D/A input and providing an echo replica output; - subtracting means (24) connected to the A/D output for subtracting said replica from the A/D output; - echo canceller adaptation means (23) connected to compare the D/A input to the subtracting means output and derive therefrom the values which the digital filter coefficient should be set to; said transmission system being characterized in that it further includes means for cancelling residual echoes including : - a first energy measuring means connected to the digital filter delay line for measuring a first energy information; - a second energy measuring means connected to the output of said subtracting means for measuring a second energy information; - residual echo clipping means connected to both energy measuring means and to said echo canceller adaptation means, to derive a residual echo flag; and, - switching means connected to the subtracting means output and controlled by said flag.

Claim 13

A voice transmission system between two telephone sets (T1 and T2) including bidirectional lines respectively connecting Tl and T2 to hybrid transformers, a couple of unidirectional lines connecting each of said hybrid transformers to a processor (10 or 20) wherein digital encodings are performed over the voice signal to derive therefrom packets of bits representing fixed length segments of voice signal, said packets being transmitted between said processors (10 and (20) over a digital transmission network, said processors also including D/A conversion for converting the digitized voice signal into analog form before delivering it to the corresponding telephone set T1 or T2; said processors also each including echo cancelling means including : - a digital filter connected to the D/A input and providing an echo replica output; - subtracting means (24) connected to the A/D output for subtracting said replica from the A/D output; - echo canceller adaptation means (23) connected to compare the D/A input to the subtracting means output and derive therefrom the values which the digital filter coefficient should be set to; said transmission system being characterized in that it further includes means for cancelling residual echoes including : - a first energy measuring means connected to the digital filter delay line for measuring a first energy information; - a second energy measuring means connected to the output of said subtracting means for measuring a second energy information; - residual echo clipping means connected to both energy measuring means and to said echo canceller adaptation means, to derive a residual echo flag; and, - switching means connected to the subtracting means output and controlled by said flag.

Claim 14

A system according to claim 10 wherein said digital filter includes a flat delay line, said system being characterized in that said first energy measuring means is connected to said flat delay line output.