FR2515453A1 - INTERFERENCE CANCER - Google Patents

Abstract

The invention relates to techniques for reducing interference in telecommunications. AN ECHO CANCELER CONFORMING TO THE INVENTION IS CAPABLE OF PERFORMING A TWO-STAGE CANCELLATION USING LESS THAN DOUBLE THE NUMBER OF COMPONENTS USED IN A SINGLE STAGE CONFIGURATION. IN PARTICULAR, A SINGLE ECHO REPRODUCTION SIGNAL GENERATOR 18 GENERATES EITHER A PRELIMINARY ECHO REPRODUCTION SIGNAL Y (K) OR A FINAL ECHO REPRODUCTION SIGNAL Y (K), DEPENDING ON THE INPUT SIGNAL WHICH IS APPLIED TO IT, DEPENDING ON THE POSITION OF A SWITCHING DEVICE 24 ACTIVATED BY A CONTROL UNIT 26. APPLICATION TO TELEPHONE NETWORKS.

Description

15453

  The present invention relates to cancellers

jamming.

  Echoes commonly appear in a system

  telecommunications when electrical signals

  in a receive signal path encounter impedance imperfectly matched at a differential junction and are partially reflected to the remote source on a transmit signal path, so that the reflected signal, or echo, is heard at the far end of the transmit channel some time after the original signal is sent When the distance between the speaker and listener increases, the echo takes longer to reach the speaker and, as a result, the echo becomes more embarrassing for the speaker,

less subjectively.

  It is therefore generally attempted to reduce the echoes. An echo control configuration which is described in US Pat. No. 4,005,277 comprises a device actuated by the speech signal which is called an echo canceller. characteristically a certain form of selective attenuation, performed under the influence of the vocal levels in the transmission path, so that the echo which would return by

  elsewhere to the speaker is attenuated Such a configuration

  This is usually satisfactory for terrestrial telecommunication paths, in which the echo delay, corresponding to the round trip delay between the signal transmission and the return of the echo.

  However, in the telecommunication

  satellite delays, transmission delays are much longer and the echo is more awkward and may even interrupt the conversation by chopping the return signal during intervals during which the two correspondents talk, say in

  On the other hand, instead of interrup-

  pre the outbound route, another reduction configuration

  echo canceller, echo canceller

  a signal which is a reproduction of the echo signal, and algebraically subtracts the echo estimate from the signal output, to obtain a signal in which

  echo is canceled Most classical echo cancellers

  Such as the configuration described in US Pat. No. 3,499,999, synthesize the reproduction of the echo using a delay line with adjustable multipliers in an adaptive configuration with downstream reaction, which calls again a transversal filter The multipliers are set automatically by a control signal which is elaborated from the

  difference between the echo and the echo reproduction signal.

  Theoretically, the differential network which is incorporated in the 4-wire / 2-wire interconnection between the signal channels and the transceiver, does not let at most an attenuated signal of 6 d B with respect to the received signal, However, it has been found in practice that a differential network may be poorly connected, or that its associated balancing network may be absent, thus causing a network imbalance condition in which a significant portion Received signal leaks to transmit signal path Echo cancellers of the prior art often confuse this high echo signal with near end speech and, therefore, consider this signal as speech from the near end and they do not cancel

therefore not the echo signal.

  In accordance with the invention, a scrambling canceller for substantially canceling a scrambling signal of a first signal propagating in a transmission channel of a near end transmitter / receiver comprises first means for subtracting from the first signal a scrambling reproduction signal, for

  to produce a signal with interference jammed, means of gen-

  reproduction signal, which responds to an input signal on a receive signal channel of the transceiver and to the canceled interference signal, producing the scrambling reproduction signal, a detector for blocking the adjustment reproduction signal generating means, when a signal from the signal path

15453

  is greater than a predetermined value,

  second means for subtracting a reproduction signal

  preliminary interference with the actual feedback interference signal from the transmitter / receiver, to produce the first signal, and switching means for applying the first signal to the reproduction signal generating means when the cancellation of the interference in the first signal is less than what is necessary, so that the reproduction signal generating means produces the preliminary scrambling reproduction signal, and to apply, instead, the signal to scrambling canceled to the reproduction signal generating means, when the cancellation of the interference in the first signal is at least equal to what is necessary, so that the reproduction signal generating means produces the reproduction signal

jamming.

  In one embodiment of the invention, an echo cancellation process is distributed in two stages

  which respectively provide a first preliminary cancellation

  and a second final cancellation A near-end speech detector responds to the preliminary canceled echo signal produced by the first stage, instead of reacting

  to the actual feedback echo signal, as in the

  Thus, the near-end speech detector is less likely to confuse a signal.

  high echo with the word of the near end.

  The invention makes good use of the fact that, as a single echo canceller adapts at any given instant, many components of an echo canceller

  two floors can be used in common More specifically

  only one transverse filter and one accumulator

  are able to perform the adaptation of the echo canceling signal

  Associated with one or other of the echo canceller stages.

  The invention will be better understood on reading the

  description that will follow of embodiments and by

  referring to the attached drawings in which:

  Figure 1 shows a cancellation configuration

  two-stage echo system according to the invention; and Fig. 2 shows another embodiment

  a two-stage echo cancellation configuration.

  The following description and the drawings that accompany it

  However, the invention can generally be used to reduce any type of signal in the form of an echo canceller at the 2-wire / 4-wire interconnection of a telephone circuit. analogue or digital scrambling appearing for example in radar, satellite, sonar, radio,

  of television, etc. So it should be noted-that the description that

  The following is only an example intended to explain the invention and not to limit it, since it is applicable to any situation in which an interfering signal

is present.

  In theory, one could construct an echo canceller that uses only the correlation properties between an incoming signal x (t) from the far-end

  and the echo return signal y (t), to control the pro-

  adaptation of an incorporated transverse filter, the

  correlation signal 0 (t) which can be defined by the relation

  T (t) fx (t) y (tC) d 2 (1) When a correlation detector is used, the time required for the canceller to obtain a fixed degree of cancellation is determined by the width of the band or bit rate of the incoming signal, the resulting maximum sampling rate of the incoming signal, the relative noise level and the near-end speech When the correlation process is used, the adaptation time

  becomes excessive from the point of view of a tele-

  In the presence of high noise levels and near-end speech characteristic levels, therefore, prior art conventional echo cancellers use a near-end speech detector to block the process. However, these near-end speech detectors can not distinguish between high-level echo signals and near-end speech, which means that echo cancellers of the prior art often operate incorrectly in the presence of high level echo signals. The invention aims to solve this problem by distributing the cancellation process in two stages. FIG. 1 shows an exemplary configuration which illustrates the interference cancellation properties of the invention, in the case of the application to the Specific example of an echo canceller There is basically shown a single transmission terminal for interconnecting a near end transmitter / receiver 10 to a receive signal channel 12 and a transmission signal channel 14 by means of a differential network 16 A balancing network 17 is connected to the differential network 16 in order to achieve an adaptation as good as

  possible with the impedance of the transmitter / receiver 10.

  As shown in Fig. 1, an echo canceller comprises an echo reproduction signal generator 18 which is placed between the receive signal path 12 and the

  transmit signal channel 14, a first combi circuit

  20, a second combination circuit 22, a first

  switching device 24 and a control unit 26.

  The echo reproduction signal generator 18 responds to the input signal x (k) which approaches the transmitter / receiver

  from the near end, 10, on the receive signal path

  12, and discusses samples of the signal x (k) con-

  according to equation (1) to form, as an output signal, an estimate of the echo signal present on the transmit signal path (14), and this estimate is

  called preliminary echo reproduction signal yi (k).

  In a digital form, the reproduction signal generator

  echo ducting may comprise a set of delay elements, multipliers and integrators forming a transversal filter in association with an accumulator, and this generator is thus capable of generating the impulse response of the signal applied to it at the input Ce

  generator is described in more detail in the application

  French patent filed the same day by the application-

  under the title - "Scrambling reduction device".

  From the output of the echo reproduction signal generator 18, the preliminary echo reproduction signal y J (k) is then applied as an input signal to the switching device 24, which is placed in the position shown in Figure 1 at the time of

  start of the process As a result, the reproduction signal

  The preliminary echo pattern y 1 (k) is applied by the

  tif switching 24 to the first combining circuit 20, as the first input signal of this circuit, while

  that the other input signal of the first combination circuit

  its 20 is the real return echo signal y (k) which appears

  The first combining circuit 20 then subtracts the preliminary echo reproduction signal y 1 (k) from the real-return echo signal y (k), to form as one. output signal the difference between these two signals,

  called the preliminary canceled echo signal e 1 (k).

  The preliminary canceled echo signal e 1 (k) propagates on the transmit signal path 14 and is applied as an input signal to the second combining circuit 22 and the switching device 24 In the position shown in FIG. FIG. 1, the device for

  switchover 24 applies the echo signal canceled beforehand.

  ree 1 (k) to the echo reproduction signal generator 18, as an input signal, with the input signal x (k) in accordance with the operation of the generator

  echo reproduction signal 18, the echo cancellation signal

  the preliminary step e 1 (k) performs an adaptation with respect to the input signal x (k), to form an improved value for the preliminary echo reproduction signal y 1 (k), and this adaptation process is described in more detail

  in the aforementioned patent application.

  The pre-canceled echo signal e 1 (k), or

  according to a variant the echo reproduction signal

  min. y (k) (as shown by a dashed connection in FIG. 1), is also applied as an input signal to the control unit 26, which also responds to the input signal x (k) L control unit 26 controls the matching process performed by the echo reproduction signal generator 18, comparing e 1 (k) or y 1 (k) to

  a predetermined value of x (k), and it modifies the posi-

  As a result of the switching device 24 when e 1 (k) or Y 1 (k) exceeds a predetermined threshold, the control unit 26 operates in accordance with one or other of the relationships e 1 (k) Ax (k) or y 1 (k) Bx (k), wherein A or B are selected such that the canceled preliminary echo signal e 1 (k) corresponds to a sufficient cancellation of the signal

  real return echo y (k), so as to allow a func-

  It is to be noted that the control unit 26 may comprise any configuration of a near end detector of the conventional type.

  well known capable of performing threshold detection.

  When the predetermined threshold is exceeded, the uni-

  The control unit 26 actuates the switching device 24, which terminates the first cancellation phase by deleting the preliminary canceled echo signal e 1 (k) as the input signal of the echo reproduction signal generator 18 and suppressing the preliminary echo reproduction signal y 1 (k) as an output signal of the generator 18 The final value of the preliminary echo reproduction signal y 1 (k) is stored in a memory

  alive and remains applied as a signal of

  to the first combination circuit 20, via

  Thus, the first combining circuit 20 continues to produce the preliminary canceled echo signal e 1 (k) which propagates on the track.

  transmission signal 14 to the second combiner circuit.

  At the moment that the switching device 24 is actuated to start the second cancellation phase, the echo reproduction signal generator 18 applies the preliminary echo reproduction signal y 1 (k) to the second echo circuit. 22, er as the second input signal

  of the latter, and by means of the

  The second combining circuit 22 subtracts this echo reproduction signal from the echo signal

  canceled preliminary e 1 (k), to form the echo cancellation signal

  the final e 2 (k) which propagates on the rest of the transmit signal channel 14, returning to the transmitter / receiver

  from the far end (not shown).

  The final canceled echo signal e 2 (k) is also applied, by the other position of the switching device 24 (represented by a dotted line in FIG. 1), as a new input signal for the generator. echo reproduction signal 18, replacing the canceled echo signal e 1 (k) The signal generator

  Echo reproduction 18 is thus adapted to x (k), in relation to

  with e 2 (k), to produce as output signal the final echo reproduction signal y 2 (k) The final echo reproduction signal y 2 (k) is then applied by

  through the other position of the communication device

  24 (represented by a dotted line in FIG. 1), to the second combination circuit 22, as the second input signal of this circuit. The second combination circuit 22 subtracts the signal y 2 (k) from the pre-canceled echo signal e 1 (k), to form a

  updated value for the final canceled echo signal e 2 (k).

  The process described above then continues, and the

  echo signal canceled final e 2 (k) is returned to the gen-

  echo reproducing signal 18 to continuously adapt the final echo sound reproduction signal y 2 (k), for the duration of the presence of the input signal x (k), or until the condition threshold of the control unit 26 is no longer satisfied, as happens in the case where the actual value of y (k) varies slowly over time. More precisely, if at any time during the adaptation of the signal to echo canceled final e 2 (k), e 1 (k) becomes lower than Ax (k) or y 1 (k) becomes lower

  at Bx (k), the control unit 26 again actuates the

  positive switching 24, which again applies the preliminary canceled echo signal e (k) to the generator of

  echo reproduction signal 18, as a signal of

  As previously described, the preliminary canceled echo signal e 1 (k) will be adapted in association with the input signal x (k) until, again, the predetermined threshold of the unity of control 26 is exceeded and the switching device 24 is actuated, so that the final canceled echo signal e 2 (k) is reapplied as an input signal to the echo reproduction signal generator 18 L control unit 26 continues to control the value of e 1 (k) or y 1 (k) as described above and to modify the input signal and the output signal of the reproduction signal generator of echo 18, as necessary in accordance with the predetermined threshold value. The absence of an input signal x (k), or alternatively, the presence of a speech signal from the end

  near, is detected by a speech detector from the

  For example, the presence of near-end speech can be detected by comparing the preliminary echo reproduction signal or the preliminary canceled echo signal y 1 (k) or e 1 (k) with the signal. input x (k) or at

  real return echo signal y (k), using

  dice well known in the art, employed in the confi-

  current end-word detector gurations

  and explained in more detail in the request for

  For example, the near end speech detector 30 can record N past values of the input signal x (k) and determine the existence of near end speech when the present value of the signal at the near end preliminary canceled echo is greater than half of the

  maximum value of the N recorded values of the signal of

  When the near-end speech detector

  actually recognizes the presence of speech from the

  close, a blocking signal is transmitted to the general

  echo reproduction signal 18, which disconnects the echo reproduction signal generator 18 from the transmit signal path 14, thereby preventing any subsequent change of the data recorded in the echo reproduction signal generator 18, and thus allows the near end speech to propagate unmodified to the transmitter / receiver

from the far end.

  Fig. 2 shows another echo canceller in which the control unit 26 and the near-end speech detector 30 are combined into a single detector / control unit 32 which is capable of performing the same functions as the separate components present in Figure 1 In particular, the detector / control unit 32 reacts

  to the input signal x (k), to the previously canceled echo signal

  ree 1 (k) and, in other modes of operation,

  real feedback echo signal y (k) and to the reproducing signal

  preliminary echo y 1 (k) When the detector / control unit 32 operates as a control unit, it compares

  the canceled preliminary echo signal el (k) to the signal of

  trea x (k) as described above in relation to FIG. 1, and modifies the position of the switching device 24 in accordance with the relation e 1 (k) <Ax (k), wherein the threshold value A is chosen at the discretion of the user Alternatively, the detector / control unit 32 may compare the input signal x (k) with the preliminary echo reproduction signal y 1 (k) (as indicated by a dashed connection in FIG. 2),

  and it can operate the switching device 24 in accordance with

  In accordance with the threshold decision yl (k) <Bx (k) In another configuration, the detector / control unit 32 may determine the time derivative of the preliminary canceled echo signal e 1 (k) to form a echo signal variation signal signal canceled by 1 (k) / dt The detector / control unit 32 then compares the echo cancellation signal of 1 (k) / dt with a predetermined constant C, and changes the position of the switching device 24 in accordance with the relation: of 1 (k) dt <C (2) The value of C is chosen so that when the rate of change of e 1 (k) falls below some value,

15453

he

  the detector / control unit 32 considers that a cancellation

  sufficient preliminary echo occurred in his

  functioning as the speech detector of the

  nearer, the detector / control unit 32 blocks the function

  the echo reproduction signal generator 18 in accordance with any of the described methods.

  above in connection with Figure 1.

  The invention realizes a two-stage echo cancellation process, using only one echo reproduction signal generator, instead of using a separate echo reproduction signal generator for each floor, which represents a saving

  to incorporate the device of the invention into a single cir-

  It will furthermore be noted that the invention can be used interchangeably with digital or analog signals, the only difference in structure residing in the internal configuration of the signal generator.

echo reproduction.

  It goes without saying that many modifications can

  can be made to the device described and represented,

  without departing from the scope of the invention.

Claims (8)

  1 Scrambling canceller intended to cancel practically   only a scrambling signal of a first signal propagating on a transmission channel of a near end transmitter / receiver, comprising first means   subtracting a scrambling reproduction signal   in relation to the first signals to produce a canceled interference signal, scrambling reproduction signal generating means responsive to an input signal present on the transmitter receive signal path   receiver and the jammed signal, so that   the interference reproduction signal, and a detection   for generating a jamming reproduction signal generating means when a signal from the transmit signal path is greater than a predetermined value, characterized in that second means (20) for subtracting a preliminary scrambling reproduction signal (yl (k)) by   to the real return scrambling signal (y (k)) pro   from the transmitter / receiver, to produce the first   signal (e 1 (k)), and switching means (24, 26) of   to apply the first signal to the general means of   interference reproduction signal (18) when the cancellation of the interference in the first signal is less than what is necessary, so that the means   generation of scrambling reproduction signal   the preliminary scrambling reproduction signal, and intended to apply the scrambling signal instead.   canceled (e 2 (k)) to the reproduction signal generating means   jamming when the cancellation of the interference in the first signal is at least equal to what is necessary, so that the jamming reproduction signal generating means produces the signal   scrambling reproduction (y 2 (k)).   2 canceller according to claim 1, characterized   in that it includes recording means (28) for   and save a value determined last 15453   preliminary scrambling reproduction signal, and to apply the value determined last to the second   means (20) when the canceled interference signal is applied   to the scrambling reproduction signal generating means. The canceller according to claim 2, characterized in that the recording means consist of a RAM.   4 A canceller according to any one of the claims   1 to 3, characterized in that the switching means responds to the input signal (x (k)) and to the first signal.   applying the first signal (e 1 (k)) to the means of genera-   interference reproduction signal when the first   signal is less than a predefined weighted value   from the input signal, and instead applying the canceled interference signal (e 2 (k)) to the scrambling reproduction signal generating means when the first signal is at least equal to the weighted value predetermined of the input signal.   5 according to any one of the claims   1 to 3, characterized in that the switching means   react to the input signal (x (k)) and the reproduction signal.   preliminary interference ducting (y 1 (k)) by applying the first signal (e 1 (k)) to the signal generating means   interference reproduction when the reproduction signal   preliminary interference ducting is less than a predetermined weighted value of the input signal, and instead applying the canceled scrambled signal (e 2 (k)) to the scrambling reproduction signal generating means when the signal reproduction of   preliminary interference is at least equal to the pon-   predetermined error of the input signal.   6 according to any one of the claims   1 to 3, characterized in that the switching means responds to the first signal (e 1 (k)) by determining the derivative with respect to the time of this signal, in order to establish a speed of variation of the canceled interference signal, for applying the first signal to the scrambling reproduction signal generating means when the rate of change of the canceled scrambled signal is greater than a predetermined value, and for applying, instead, the canceled scrambled signal (e 2 (k )) to the scrambling reproduction signal generation means, when the speed of variation of the canceled scrambled signal is at most equal at the predetermined value.   An inverter according to any one of the claims   1 to 6, characterized in that the detector (30) is responsive to the input signal (x (k)) and the reproduction signal of preliminary interference (yl (k)).   8 according to any one of the claims   1 to 6, characterized in that the detector (30) reacts   to the input signal (x (k)) and the first signal (e 1 (k)).