Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04M—TELEPHONIC COMMUNICATION
  • H04M9/00—Arrangements for interconnection not involving centralised switching
  • H04M9/08—Two-way loud-speaking telephone systems with means for conditioning the signal, e.g. for suppressing echoes for one or both directions of traffic
  • H04M9/082—Two-way loud-speaking telephone systems with means for conditioning the signal, e.g. for suppressing echoes for one or both directions of traffic using echo cancellers

Definitions

• the invention relates to digital processing of signals and relates, in particular, to the cancellation of acoustic or electrical echo in sets telecommunications.
• An interesting application of such a system concerns the cancellation of electric echo in a telecommunication terminal or in any type of telecommunication equipment confronted with this problem of electric echo.
• Another particularly interesting application of such a system relates to echo cancellation in a telecommunications station offering hands-free operation.
• the acoustic echo is a major obstacle to the proper functioning of the communication software of a subscriber's terminal equipment. This obstacle is encountered more and more in stations equipped with a so-called "hands-free" function.
• the acoustic echo appears when the signal emitted by the loudspeaker, which corresponds to a signal spoken by a distant speaker, is picked up by the microphone. It then translates, for this remote user, by the restitution, at the level of the listener, of the signal with an offset with respect to the instant of emission. The reduction, or even the cancellation of the echo, is obtained by equipping the stations with echo canceller devices.
• the hands-free mode of operation also poses a dynamic problem for the signals processed, the microphone level then varying over a very wide range. This strong variation in level is mainly due to the distance or proximity of the speaker to the microphone, as well as to the natural variety of speech power between individuals.
• the sound recording system has a finite dynamic, linked to the presence of an analog-digital converter within the digital processing system fitted to the station. It is therefore necessary to adapt the dynamics of the acoustic signal acquisition chain to the envisaged application.
• digital acoustic signal processing systems conventionally use an automatic gain control or AGC device to adapt the strong signal dynamics at the microphone level to the limited dynamics of the sound pickup system.
• This device is intended to keep the level of the output signal constant over a predetermined range of use, thus enabling the conversion of the signal after intervention by the automatic gain control device with good accuracy for small levels and also for levels. forts whose amplitude has been reduced.
• Such devices work relatively efficiently.
• an automatic gain control device does not meet this criterion since it modifies the gain levels applied according to the energy of the input signal. It is however possible to couple an automatic gain control device with an echo canceller device when the transfer function of the automatic gain control device is well known. To do this, the function of reverse transfer of the automatic gain control device, after analog-digital conversion. However, even with full knowledge of the response of the automatic gain control device, coupling may be impossible when using a computing system using a fixed-point architecture to apply compensation to the signal after analog-to-digital conversion. Indeed, for example, when using a fixed point calculation system having a capacity of 16 bits, the output values of the calculation system are necessarily between -32768 and +32767.
• the object of the invention is to overcome the drawbacks of the state of the art and to provide a digital processing system for an acoustic or electrical signal in which it is possible to operate jointly a automatic gain control device and an echo canceller, even when using a fixed-point calculation system.
• a system for digital processing of an acoustic signal which is intended to be placed in a station equipped with a loudspeaker and a microphone between said loudspeaker and said microphone and which includes automatic gain control means associated with an echo canceller.
• This digital processing system includes calculation means adapted to apply a transfer function of the automatic gain control means to the input of the echo canceller so that a signal from the microphone and a signal from reference are each handled by a function comprising the same term corresponding to the transfer function of the automatic gain control means. It therefore becomes possible to factorize the transfer function of the automatic gain control means so that, during the processing of the signal implemented by the system, this function no longer intervenes and is no longer taken into account by the echo canceller algorithm.
• a system for digital processing of an electrical signal said system being intended to be placed in a station equipped with a loudspeaker and a microphone between said loudspeaker and said microphone and comprising automatic gain control means associated with an echo canceller device, characterized in that it includes calculation means adapted to apply a transfer function from the automatic gain control means to the input of the canceller device echo so that the signal from the microphone and the input signal from the echo canceller are each manipulated by a function comprising the same term corresponding to the transfer function of the automatic gain control means.
• one or the other system further comprises multiplier means adapted to perform a multiplication operation between the reference signal and the transfer function of the automatic gain control means.
• the acoustic or electric echo canceller includes a digital filter whose coefficients are calculated so as to minimize the error between the echo and an estimate of the echo.
• FIG. 4 illustrates the structure of a digital electrical echo processing system according to the invention
• FIG. 1 shows an example of a digital processing system for an acoustic signal according to the state of the art, provided with an automatic gain control device and an echo canceller, and for which the transfer function of the automatic gain control device is well known.
• this system comprises a sound acquisition chain 10 and a sound reproduction chain 12 respectively comprising a microphone 14 associated with an analog to digital ADC converter and a speaker 16 associated with a digital converter analog DAC.
• the sound acquisition chain 10 is further provided with an adjustable gain amplifier 18.
• this comprises an automatic gain control module 20 driving the amplifier with adjustable gain 18 so as to adapt the dynamics of the input signal to that of the analog-to-digital converter by keeping the level of the output signal constant over the range of use of the system.
• the CAG 20 module is associated with a multiplier
• FIG. 2 also includes a sound acquisition chain 10 and a sound reproduction chain 12 comprising respectively a microphone 14 and a loudspeaker 16.
• this system comprises an automatic gain control module 20 and an electric echo canceller 24 intended to compensate for the disparities in weakening of the lines.
• the electric echo canceller is also based on the estimation of an impulse response between the signal transmitted on the line and the signal received from the latter.
• FIG. 3 shows the general structure of a digital processing system for acoustic signals according to the invention which makes it possible to overcome the above-mentioned drawbacks.
• this system designated by the general reference numeral 26, is intended to be placed within a telephone set, such as a mobile telephone set for example having a so-called “hands-free” function, between a microphone 27 and a loudspeaker 28. It is essentially intended to ensure processing of the voice signals picked up by the microphone 27 so as to cancel the echoes likely to appear when the microphone 27 picks up the voice signals coming from a speaker remote and reproduced by the speaker 28.
• the system 26 is provided with an echo canceller 29 constituted by a digital filter essentially ensuring a calculation of the estimate of the echo from a corresponding reference signal Ref, in the application.
• the system 26 is provided with automatic gain control means 30 ensuring, in a conventional manner, a processing of the signals picked up by the microphone 27 so as to adapt their dynamics to that of the analog to digital converter ADC which equips the chain. acquisition of which the microphone 27 is a part.
• the automatic gain control means generally comprise a transfer function, the output curve of which as a function of the input has for example a parabola shape so as to roughly maintain constant the output level of the acquisition chain over the operating range of the microphone 27.
• the automatic gain control means control an adjustable gain amplifier 34 to obtain such behavior.
• a similar arrangement is used for canceling the electric echo.
• the digital processing system also includes an echo canceller 29 constituted by a digital filter able to calculate the estimates of the electrical echo from a reference signal
• the CAG transfer function of the automatic gain control means 30 is applied at the input of the echo canceller device 29 by means of a multiplier 36.
• the CAG transfer function of the automatic gain control means 30 is applied at the input of the echo canceller device 29 by means of the multiplier 36 which is also applied the reference signal Ref.
• the echo canceller 29 includes a digital filter whose coefficients are calculated so as to minimize the error between the echo and the estimate of the echo.
• the coefficients are computed recurrently from previously calculated coefficients, from the error between the estimation of the echo and the echo, from the reference signal and from an adaptation step ⁇ , from the relation next :
• Error (t) [Ref (t) x H (t)] x CAG (t) - [Ref (t) x CAG (t)] x H is (t)
• CAG at the input of the echo canceller 29 makes it possible to obtain, in each of the equations defining the echo and the estimate of the echo, a CAG (t) term which can be factorized in the equation defining the error between the echo and estimated echo.
• the solution for canceling the error from the above equation is to make the echo and the estimate of the echo equal.
• This solution is therefore identical to that of a digital processing system in which no AGC function is introduced to the signal from the microphone.
• FIGS. 3 to 8 an example of the result obtained by means of a digital signal processing system according to the invention.
• Figures 5 and 6 show the evolution as a function of time t of a sinusoidal signal Simul. simulation of the automatic gain control device intended to simulate the variable gain at apply to a signal received by the microphone.
• This Simul signal has an amplitude varying between 0.1 and 0.7 in 100 ms.
• Figure 6 is an enlarged view of the curve shown in Figure 5.
• the voice signal picked up by the microphone is shown in Figure 7. As seen in this figure, this signal consists only of echoes E.
• FIG. 9 shows the evolution of the signal processed by means of another signal processing system in which an automatic gain control is applied only to the signal from the microphone, without any other compensation or according to the Figure 1, nor according to Figure 2. We see in this Figure 9 that the echoes E are still present, however attenuated.

Landscapes

• Engineering & Computer Science (AREA)
• Signal Processing (AREA)
• Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=33515370

Family Applications (1)

Country Status (4)

Families Citing this family (3)

Citations (1)

Family Cites Families (4)

• 2003
  • 2003-06-24 FR FR0307635A patent/FR2856869A1/fr active Pending
• 2004
  • 2004-04-27 WO PCT/FR2004/001016 patent/WO2005011248A1/fr active Application Filing
  • 2004-04-27 EP EP04742583A patent/EP1636978A1/de not_active Withdrawn
  • 2004-04-27 US US10/557,839 patent/US20060204020A1/en not_active Abandoned

Patent Citations (1)

Also Published As

Similar Documents

Legal Events