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WO2000074361A1 - Annulation d'echo residuel - Google Patents

Annulation d'echo residuel Download PDF

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Publication number
WO2000074361A1
WO2000074361A1 PCT/EP2000/004665 EP0004665W WO0074361A1 WO 2000074361 A1 WO2000074361 A1 WO 2000074361A1 EP 0004665 W EP0004665 W EP 0004665W WO 0074361 A1 WO0074361 A1 WO 0074361A1
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WO
WIPO (PCT)
Prior art keywords
signal
echo
positive
error signal
threshold levels
Prior art date
Application number
PCT/EP2000/004665
Other languages
English (en)
Inventor
Jim Rasmusson
Original Assignee
Telefonaktiebolaget Lm Ericsson (Publ)
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Telefonaktiebolaget Lm Ericsson (Publ) filed Critical Telefonaktiebolaget Lm Ericsson (Publ)
Priority to AU52155/00A priority Critical patent/AU5215500A/en
Publication of WO2000074361A1 publication Critical patent/WO2000074361A1/fr

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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 present invention relates to the reduction or cancellation of acoustic echoes, and more particularly to the reduction or cancellation of acoustic echoes occurring in a hands-free arrangement, such as in a telephone configured for hands-free operation.
  • a cellular telephone's microphone might be mounted on the sun visor, while the loudspeaker may be a dash-mounted unit, or may be one that is associated with the car's stereo equipment.
  • the loudspeaker may be a dash-mounted unit, or may be one that is associated with the car's stereo equipment.
  • a cellular telephone user may carry on a conversation without having to hold the cellular unit or its handset.
  • personal computers often have microphones and loudspeakers mounted, for example, in a monitor in relatively close proximity to each other.
  • a hands-free arrangement One problem with a hands-free arrangement is that the microphone tends to pick up sound from the nearby loudspeaker, in addition to the voice of the user of the hands-free equipment (the so-called "near-end user"). This is also a problem in some non-hands-free devices, such as handheld mobile telephones, which are becoming smaller and smaller. (Because of the small size, a mobile telephone's microphone cannot entirely be shielded from the sound emitted by its loudspeaker.) This sensing by the microphone of sound generated by the loudspeaker can cause problems in many types of applications.
  • FIG. 1 An exemplary "hands-free" mobile telephone, having a conventional echo canceler in the form of an adaptive filter arrangement, is depicted in FIG. 1.
  • a hands-free communications environment may be, for example, an automotive interior in which the mobile telephone is installed. Such an environment can cause effects on an acoustic signal propagating therein, which effects are typically unknown. Henceforth, this type of environment will be referred to throughout this specification as an unknown system H(z).
  • the microphone 105 is intended for detecting a user's voice, but may also have the undesired effect of detecting audio signals emanating from the loudspeaker 109. It is this undesired action that introduces the echo signal into the system.
  • Circuitry for reducing, if not eliminating, the echo includes an adaptive filter 101, such as an adaptive Finite Impulse Response (FIR) or Infinite Impulse Response (IIR) filter, an adaptation unit 103, such as a least mean square (LMS) cross correlator, and a subtractor 107.
  • an adaptive filter 101 such as an adaptive Finite Impulse Response (FIR) or Infinite Impulse Response (IIR) filter
  • an adaptation unit 103 such as a least mean square (LMS) cross correlator
  • LMS least mean square
  • a subtractor 107 a subtractor 107.
  • the adaptive filter 101 generates an echo estimate signal 102, which is commonly referred to as a ⁇ signal.
  • the echo estimate signal 102 is the convolution of the far-end signal 112, and a sequence of m filter weighting coefficients (b j ) of the filter 101 (See Equation 1).
  • x(n) is the input signal
  • m is the number of weighting coefficients
  • n is the sample number
  • the adaptive filter 101 When the weighting coefficients are set correctly, the adaptive filter 101 produces an impulse response that is approximately equal to the response produced by the loudspeaker 109 within the unknown system H(z).
  • the echo estimate signal 102 generated by the adaptive filter 101 is subtracted from the incoming digitized echo signal 126, to produce an echo filtered microphone signal, also referred to as an error signal e(ri) because its energy is entirely attributable to the error in the echo estimate signal 102 when the only sound reaching the microphone 105 is attributable to the sound emanating from the loudspeaker 109:
  • any echo response from the unknown system H(z), introduced by the loudspeaker 109, is removed from the digitized echo signal 126 by the subtraction of the echo estimate signal 102. Consequently, when the digitized echo signal 126 also includes signal components attributable to the intended near-end user, the resulting difference signal should ideally include only these near-end user components.
  • the number of weighting coefficients (henceforth referred to as "coefficients") required for effectively canceling an echo will depend on the application. For handheld phones, fewer than one hundred coefficients may be adequate. For a hands-free telephone in an automobile, about 200 to 400 coefficients will be required. A large room may require a filter utilizing over 1000 coefficients in order to provide adequate echo cancellation.
  • the effectiveness of the echo canceler is directly related to how well the adaptive filter 101 is able to replicate the impulse response of the unknown system H(z).
  • This is directly related to the set of coefficients, h v maintained by the filter 101.
  • a well-known coefficient adaptation scheme is the Least Mean Square (LMS) process, which was first introduced by Widrow and Hoff in 1960, and is frequently used because of its efficiency and robust behavior.
  • LMS Least Mean Square
  • the update information produced by the LMS process e(n) • x( ⁇ ) is used to determine the value of a coefficient in a next sample.
  • the expression for calculating a next coefficient value h j (n + l) is given by:
  • x(n) is an nth sample of the digitized input signal 112
  • hAn is a filter weighting coefficient
  • i designates a particular coefficient
  • m is the number of coefficients
  • n is the sample number
  • ⁇ is a step or update gain parameter.
  • the LMS method is very common in real-time applications due to its efficiency and simplicity . It produces information in incremental portions each of which portions may have a positive or a negative value.
  • the information produced by the LMS process can be provided to the adaptive filter 101 to update the filter's coefficients.
  • the conventional echo cancellation circuit includes a filter adaptation unit 103 in the form of an LMS cross correlator for providing coefficient update information 104 to the filter 101.
  • the filter adaptation unit 103 monitors the error signal e(n) that represents the digitized echo signal 126 minus the echo estimate signal 102 generated by the filter 101.
  • the echo estimate signal 102 is generated, as described above, with the use of update information 104 provided to the adaptive filter 101 by the filter adaptation unit 103.
  • the coefficients, h t , of the adaptive filter 101 accumulate the update information 104 as shown by Eq. (3).
  • the signals generated for further processing may very often still include echo components. This may occur, for example, because the adaptive filter has not yet converged to a fully adapted state, or even after such convergence whenever the unknown environment H(z) changes, thereby requiring the adaptation process to be repeated.
  • the presence of strong echo signal components in the signal can cause degraded or even faulty operation of the down-stream processing components, since these echo signal components may be mistaken for near-end speech. For these reasons, particular applications may set requirements for echo return loss enhancement (ERLE).
  • ERLE echo return loss enhancement
  • the typical (linear) echo cancellation arrangement described above has problems fulfilling the ERLE requirements, which are quite high for delayed systems such as the Global System for Mobile communications (GSM) and the Digital Advanced Mobile Phone System (DAMPS).
  • the required ERLE figure can be as high as 40-50 dB in these cases.
  • With good audio components it might be possible to achieve 30-35 dB ERLE by means of the linear echo canceler alone.
  • To substantially eliminate the remainder, referred to as the residual echo some extra processing is needed. This is called residual echo suppression.
  • Residual echo suppression is especially difficult because one is dealing with the non-linear components of the echo. This means that linear methods cannot be applied (at least not in a straightforward manner). Instead, non-linear methods tend to be better at this task.
  • a technique that has been applied to the problem of residual echo suppression is the use of a center clipper, which "clips" out a slice of the signal around the middle or zero level. An example of this technique is illustrated in FIGS. 2a, 2b and 3.
  • FIG. 2a an exemplary signal 201 is plotted as a function of time prior to application of center clipping. For center clipping, respective positive and negative threshold levels 203, 205 are defined.
  • center clipper on speech signals has to be done with great care because the clipping process removes low level signals in a very crude manner. If the center clip threshold levels are not held as low as possible, the distortion that is introduced by the clipping process rapidly dominates the resulting signal.
  • U.S. Patent No. 5,475,731 which is hereby incorporated herein by reference, discloses an arrangement that uses the echo estimate signal 102 ( ⁇ ) to control a center clipper.
  • a block diagram of this arrangement is shown in FIG. 4.
  • the echo estimate signal 102 ( ⁇ ) is generated as described above, and is subtracted from the digitized echo signal 126 (u) to generate an error signal e( ⁇ ) that includes the residual echo components. Even though the echo estimate signal 102 does not completely cancel the echo (i.e. , some residual echo energy is present in the error signal e(ri)), the echo estimate signal 102 does provide an accurate indication of when and at what magnitude the echo appears .
  • This dynamically changing information is supplied to a threshold calculator 401, which generates therefrom positive and negative thresholds 203, 205.
  • the threshold calculator 401 may be for example, an envelope, or level detector.
  • the threshold values may be determined by:
  • K is a constant that may be in the range from 0.1 to 0.5.
  • a center clipper 403 which generates a center-clipped output signal from the supplied error signal e( ).
  • the center clipper 403 may be replaced by an attenuator, whose degree of attenuation is dynamically controlled by information derived from the echo estimate signal 102.
  • center clipping While the purpose of the center clipping (or attenuation) is to remove the echo residuals, this approach suffers from a drawback in that it introduces quite a bit of distortion into the signal. This is due to the fact that center clipping cuts out not only the residual echo, but also some wanted parts of the signal. Background noise sounds are also especially distorted after being subjected to center clipping.
  • the use of attenuation instead of center clipping does not really improve matters, since it also degrades the wanted parts of the signal. Consequently, attenuation should also be avoided.
  • reducing an echo in an electrical audio signal includes converting a first acoustic signal into the electrical audio signal; generating an echo estimate signal from a loudspeaker signal; and generating an error signal that is a difference between the electrical audio signal and the echo estimate signal. A residual echo contained in the echo estimate signal is then reduced by decorrelating the error signal.
  • decorrelation is applied to the error signal only when a magnitude of the error signal is in a range defined by positive and negative threshold levels.
  • the positive and negative threshold levels are determined as a function of the echo estimate signal.
  • the positive and negative threshold levels may be determined by detecting an envelope of the echo estimate signal.
  • the positive and negative threshold levels are determined periodically, thereby providing dynamic adjustment.
  • decorrelation is accomplished by differentiating the error signal to reduce the residual echo contained in the echo estimate signal.
  • FIG. 1 is a block diagram of a conventional hands-free transceiver that includes an acoustic echo canceler in the form of an adaptive filter arrangement;
  • FIG. 2a is a graph of an exemplary signal plotted as a function of time prior to application of center clipping
  • FIG. 2b is a graph depicting the resulting signal after application of center clipping in accordance with conventional techniques
  • FIG. 3 is a graph depicting the transfer function of a center clipper in accordance with conventional techniques
  • FIG. 4 is a block diagram of an arrangement that uses an echo estimate signal ( ⁇ ) to control a center clipper for the purpose of reducing a residual echo in accordance with conventional techniques;
  • FIG. 5 is a block diagram of an apparatus that applies decorrelation to reduce a residual echo in accordance with one aspect of the invention
  • FIG. 6 is a block diagram of an exemplary embodiment of a decorrelator for use in the invention
  • FIG. 7 is a graph depicting the transfer function of a selective decorrelator, in accordance with one aspect of the invention
  • FIG. 8a is a graph of an exemplary time-domain signal prior to application of decorrelation
  • FIG. 8b is a graph depicting the resulting signal after application of selective decorrelation in accordance with one aspect of the invention.
  • FIG. 9 is a block diagram of an exemplary embodiment of a selective decorrelator in accordance with one aspect of the invention.
  • decorrelation is applied to the error signal, e(n), in order to decorrelate the echo residuals, that is, to decrease the autocorrelation characteristic of the echo residuals.
  • This will cause the residual echoes, which are essentially non-linearly distorted speech, to sound like noise, thereby avoiding the detrimental affect that they would otherwise have on the far-end user.
  • decorrelation of the desired near-end speech signal is avoided by selectively applying decorrelation to substantially only those parts of the error signal, e(n), where the echo residuals are estimated to reside.
  • e(n) the error signal
  • a dynamic determination is made of where the echo residuals are estimated to reside, so that the residual echo cancellation will dynamically adapt to changing conditions.
  • the invention could also be embodied as a computer-readable storage medium having stored therein the program instructions for effecting the invention.
  • Computer readable storage media include, but are not limited to, volatile and non- volatile Random Access Memories (RAM), magnetic storage media (e.g., magnetic disk, diskette or tape) and optical storage media (e.g. , compact disk read only memory (CD ROM)).
  • RAM volatile and non- volatile Random Access Memories
  • magnetic storage media e.g., magnetic disk, diskette or tape
  • optical storage media e.g. , compact disk read only memory (CD ROM)
  • CD ROM compact disk read only memory
  • the primary echo cancellation functions are performed by an adaptive filter 101, LMS cross correlator 103 (or other type of filter adaptation device, such as a recursive least squares or normalized LMS filter) and subtr actor 107 arranged to operate as in the prior art circuits depicted in FIGS. 1 and 4.
  • the resultant error signal, e( ⁇ ) generated by the conventional arrangement includes unacceptable levels of residual echo components.
  • the error signal e( ⁇ ) is supplied to a decorrelator 503.
  • decorrelation causes speech signals to sound like noise, which does not have the disturbing effect associated with echo signals.
  • the decorrelator 503 can be implemented in any of a number of ways, including but not limited to implementation as a module that takes the derivative of the input signal (i.e., the output of the decorrelator 503 can be a signal that is de(n) . proportional to — — ). dn
  • a simple embodiment of the decorrelator 503 is illustrated by the block diagram of FIG.
  • the error signal e(n) is supplied directly to an input of a subtractor 603 and also to an input of a delay device 601.
  • the output of delay device 601 is a delayed error signal 605, which is a delayed version of its input.
  • the delayed error signal 605 is then supplied by the delay device 601 as the subtrahend to the subtractor 603.
  • the output of the subtractor 603 is a signal that is proportional to the derivative of the error signal e( ⁇ ).
  • operation of the residual echo canceler can be further improved by accounting for the fact that the simple decorrelator illustrated in FIG. 6 will decorrelate the wanted components contained in the error signal e( ⁇ ) as well as the residual echo components.
  • the strategy applied is to determine a center slice in the error signal, e( ⁇ ), that will be decorrelated. Those portions of the error signal, e( ⁇ ), whose magnitude falls outside of the center slice are left unaffected.
  • the threshold levels that define the center slice can be dynamically changed as a function of the echo estimate signal 102.
  • FIG. 7 A center slice is defined as an region in which the magnitude of an input signal lies between a positive threshold 701 and a negative threshold 703. Outside of this region, the signal is permitted to pass unchanged. However, when the input signal falls within the center slice region, it is subjected to decorrelation as described above. This process is further illustrated in FIGS. 8a and 8b. In FIG. 8a, an exemplary signal 801 is graphed in the time domain. The result of selective decorrelation of the signal 801 is shown in FIG. 8b.
  • the selectively decorrelated signal 803 is identical to the exemplary signal 801 for those regions where the exemplary signal 801 either exceeds a positive threshold 701 or falls below a negative threshold 703.
  • the slope of the lines representing transfer function in these regions need not be 1:1, as illustrated. In other embodiments, the slope could be greater than 1 (amplification) or less than 1 (attenuation), as required by the particular application.
  • the output signal is a decorrelated version of the exemplary signal 801 , as depicted by the shaded region 805 of the graph.
  • the positive and negative thresholds 701, 703 could be static. In this case, appropriate values should be selected at the time that the residual echo canceler is designed. Suitable threshold values will permit the wanted voice signal to pass substantially unaffected by decorrelation, while applying decorrelation to signal energy levels at which the residual echo most likely appear.
  • the exemplary residual echo canceler depicted in FIG. 5 further includes a decorrelator threshold calculator 501.
  • the decorrelator threshold calculator 501 may be for example, an envelope, or level detector.
  • the threshold values may be determined by:
  • K is a constant that may be in the range from 0 to 1.
  • the decorrelator 503 needs to include means for making use of the threshold information.
  • An exemplary embodiment of a decorrelator 503 that operates in this fashion is shown in FIG. 9.
  • An error signal, e( ⁇ ) is supplied as an input to the decorrelator 503.
  • This signal is supplied directly to an input of a subtractor 903 and also to an input of a delay device 901.
  • the output of delay device 901 is a delayed error signal 907, which is a delayed version of its input.
  • the delayed error signal 907 is then supplied by the delay device 901 as the subtrahend to the subtractor 903.
  • the output of the subtractor 903 is a signal that is proportional to the derivative of the error signal e( ⁇ ).
  • This signal is supplied to a first input of a multiplexor (MUX) 909.
  • MUX multiplexor
  • the error signal e(n) is also supplied directly to a second input of the MUX 909.
  • the purpose of the MUX is to selectively output either the unchanged error signal e(ri), or the signal that is proportional to the derivative of the error signal e(n).
  • a control signal for the MUX 909 is generated by a comparison circuit 911, which receives as inputs: the error signal e( ⁇ ), the positive threshold 701 and the negative threshold 703. In this embodiment, the output of the comparison circuit 911 is asserted whenever the magnitude of the error signal e(n) is in the range falling between the positive and negative thresholds 701, 703.
  • This signal when asserted, selects the signal that is proportional to the derivative of the error signal e( ⁇ ) to appear at the output of the MUX 909.
  • the output signal from the comparison circuit 911 is not asserted, the output of the MUX 909 is the unchanged error signal e( ⁇ ).
  • the output of the MUX 909 (which is also the output from the decorrelator 503) selectively supplies either an unchanged error signal e( ⁇ ) or a decorrelated signal as a function of whether the error signal e( ) is outside the center slice defined by the positive and negative thresholds 701, 703.
  • the above-described invention provides improved methods and apparatus for reducing residual echos.
  • Benefits of the invention include improved background noise quality compared to the use of a center clipper, because where a center clipper cuts out the signal entirely, the decorrelator only decorrelates. Decorrelated noise sounds like noise with a small change in character. The background noise quality is one of the most important quality parameters in contemporary full- duplex hands-free designs.
  • Another benefit of the invention is that the residual echoes, which are essentially non-linearly distorted speech signals, will also sound like noise after decorrelation.
  • inventive methods and apparatuses described herein can be applied in any hands-free microphone-loudspeaker arrangement to reduce the residual echoes of the loudspeaker signal present in the microphone signal.
  • Such arrangements include, but are not limited to, hands-free telephones (including desk and mobile telephones such as vehicular hands-free telephones) and computer-based hands-free arrangements.
  • the inventive methods and apparatuses described herein can also be applied advantageously in non-hands-free arrangements that nonetheless suffer from acoustic echo problems, such as hand-held mobile telephones.

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  • Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)

Abstract

La présente invention permet de réduire un signal audio électrique en convertissant un premier signal acoustique en ce signal audio électrique, en produisant un signal d'estimation d'écho à partir d'un signal de haut-parleur, et en produisant un signal d'erreur qui représente la différence entre le signal audio électrique et le signal d'estimation d'écho. On réduit ensuite l'écho résiduel contenu dans le signal d'estimation d'écho par une décorrélation du signal d'erreur, que l'on peut effectuer en différenciant le signal d'estimation d'écho par rapport au temps. Afin d'éviter la décorrélation des composantes désirées du signal d'erreur (par exemple, les composantes audio associées à un utilisateur proche), on peut n'effectuer la décorrélation du signal d'erreur que lorsque l'amplitude de ce dernier se trouve dans une plage définie par des seuils positif et négatif. Les seuils positif et négatif peuvent être déterminés en fonction du signal d'estimation d'écho ; ils peuvent par exemple être dérivés d'une enveloppe du signal d'estimation d'écho. On peut déterminer des seuils positif et négatif de façon périodique, ce qui permet un réglage dynamique de ces derniers.
PCT/EP2000/004665 1999-05-27 2000-05-23 Annulation d'echo residuel WO2000074361A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU52155/00A AU5215500A (en) 1999-05-27 2000-05-23 Residual echo suppression

Applications Claiming Priority (2)

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US32046899A 1999-05-27 1999-05-27
US09/320,468 1999-05-27

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WO2000074361A1 true WO2000074361A1 (fr) 2000-12-07

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Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5274705A (en) * 1991-09-24 1993-12-28 Tellabs Inc. Nonlinear processor for an echo canceller and method
US5475731A (en) * 1994-01-07 1995-12-12 Ericsson Inc. Echo-canceling system and method using echo estimate to modify error signal

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5274705A (en) * 1991-09-24 1993-12-28 Tellabs Inc. Nonlinear processor for an echo canceller and method
US5475731A (en) * 1994-01-07 1995-12-12 Ericsson Inc. Echo-canceling system and method using echo estimate to modify error signal

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