GB2341066A

GB2341066A - Adaptive echo canceller removes non-linear signals

Info

Publication number: GB2341066A
Application number: GB9907912A
Authority: GB
Inventors: Dieter Schulz
Original assignee: Mitel Corp
Current assignee: Microsemi Semiconductor ULC
Priority date: 1998-08-20
Filing date: 1999-04-07
Publication date: 2000-03-01
Also published as: CA2245411A1; GB9907912D0

Abstract

An adaptive echo canceller with a limiter 9,11 introduced into the reference path for modeling a non-linear limiting or clipping effect introduced by a CODEC. The adaptive echo cancellation algorithm is not required to compensate for the non-linearity and therefore converges quickly to provide accurate echo cancellation. The echo canceller may be used in speaker phones characterized by large loop gains which can quickly drive the CODECs beyond their fixed dynamic ranges. The limiter can be replaced by vocoders.

Description

2341066 ECHO CANCELLER WITH COhfPENSATI FOR CODEC LM-TING EFFECTS

FIELD OF TBE INVENTION

This invention relates in general to echo cancellation in telecommunications applications, and more particularly to an echo canceller which compensates for limiting effects introduced by CODECs in the transmit and receive paths.

BACKGROUND OF THE INVENTION

Echo is a phenomenon in which a delayed and distorted version of an original signal is reflected back to its source. In general, echoes are caused by the reflection of signals from the boundaries of discontinuities in a transmission medium. In the transmission of telephone signals, an echo occurs whenever the transmitted signal encounters an impedance mismatch in a circuit. Such echoes cause impairment of the fidelity of the speech signals and are often found to be annoying to telephone users.

Network echo cancellers are devices designed to remove these undesirable echoes in telephone transmission. The basic approach utilized in such prior art echo cancellers is to synthesize a replica of the echo and to subtract it from the total received signal. An efficient algorithm to obtain the estimated echo path is the well known Least Mean Square (LMS) adaptive algorithm.

It is common to include a CODEC (coder-decoder) in one or both of the transmit or receive paths between which an echo canceller is connected. ITU-T Recommendation G. 165 describes the quantization effect of G.712 CODECs on the residual error level when operating within the dynamic range of the CODEC and indicates that non-linear effects (in this case quantization) have a detrimental effect on the echo cancelling capabilities of an echo canceller. To counter the increased residual error G. 165 recommends the use of NLPs (non-linear processors).

2 In standard network echo cancellers the signal is not normally of sufficiently high amplitude that it would saturate the dynamic range of the echo canceller (although this may happen rarely with loud speech signals). However, where CODECs are used in speaker phones the problem is more pronounced since the acoustic echo canceller and line echo canceller are adjacent and have a high loop gain. Clipping or limiting non-linearities may be introduced since the dynamic range of the fixed point digital signal processors (DSPs) which implement the echo cancellation algorithm is typically wider than that of the CODEC (e.g. 16 bit dynamic range for the DSP versus 13 bits for a g-Law CODEC). If a signal is applied to the CODEC which exceeds its dynamic range, the signal is "clipped" or limited to the maximum range of the CODEC. This limited signal represents a non- linearity in the echo path which a traditional prior art adaptive echo canceller is unable to converge. Thus, on start-up, it is very common for the CODECs to be driven into saturation, resulting in nonconvergence and therefore even more saturation.

is One approach to addressing the problem of clipping introduced by CODECs in the echo path is to scale the signal. However, this has a detrimental effect on signalto-noise ratio, especially for lower level signals.

SUMMARY OF THE INVENTION

According to the present invention, a lin-fiter is introduced into the reference path of the echo canceller for modeling the limiting or clipping effect introduced by the CODEC. Consequently, the adaptive echo cancellation algorithm is not required to compensate for the non-linearity and therefore converges quickly to provide accurate echo cancellation. The system of the present invention is particularly advantageous when applied to speaker phones characterized by large loop gains which can quickly drive the CODECs beyond their fixed dynamic ranges.

BRIEF DESCRIPTION OF THE DRAWINGS

3 A detailed description of the preferred embodiment and of the prior art is provided herein below with reference to the following drawings, in which:

Figure I is a block diagram of a prior art echo canceller, and

Figure 2 is a block diagram of an echo canceller with compensation for CODEC limiting effects, according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODINIENT

Figure I shows an adaptive echo canceller according to the prior art. A reference signal (which can be received, for example, from a telephone line) is applied to an input of echo canceler I and to the echo path 3 (which can be a network echo path resulting from line impedance mismatch, or an acoustic echo path from speaker to microphone). The echo canceller models an estimation of the echo introduced by the echo path 3 using the well known LMS algorithm (although other adaptive algorithms may be used), and subtracts the echo signal from the received signal (which contains the undesirable echo)via a subtractor 4. Provided that the transfer function of the model of the echo path provided by echo canceler I is identical to the transfer function of the echo path 3, the error signal becomes zero and the echo canceler I converges to the correct transfer function, resulting in perfect echo cancellation. However, as discussed above, LMS algorithms are not capable of converging in the presence of non-linear effects such as may be caused by signal clipping, rattling of phone keys, frequency shifts, etc.

According to the present invention, an improved echo canceler is provided which compensates for the non-linearities introduced by signal clipping due to CODEC overflow.

With reference to Figure 2, an echo canceler is shown according to the present invention. In addition to the adaptive echo canceller I and echo path 3, a transmit CODEC 5 and receive CODEC 7 are shown. Clipping non-linearities are typically 4 introduced by the transmit CODEC for the reasons discussed above. To compensate for this clipping non-linearity, a limiter 9 is introduced in the reference path. This limiter bounds the signal to the dynamic range of the CODEC 5, thereby modeling the non-linearity introduced by clipping in the echo path.

If the echo path 3 has zero gain or less, then the CODEC 7 on the receive side introduces no additional clipping non-linearities. However, if the echo path has a positive gain, then the receive CODEC 7 can also be driven into clipping, even if the transmit signal is within the dynamic range of the transmit CODEC 5. The clipping effect of the receive CODEC 7 can therefore be compensated by bounding the estimated echo signal prior to being applied to subtractor 4.

1A-Law CODECs have a dynamic range of 13 bits. Thus the signals applied to CODEC 5 are bounded by 8192, so that any applied signal having amplitude in excess -of 8192 becomes clipped. Consequently the reference signal has to be limited to 8192 by limiter 9. Likewise, for a p-Law CODEC 7, limiter I I must bound the estimated echo signal to 8192.

Similarly, since A-Law CODECs have a dynamic range of 4096, the range of limiters 9 and 11, when compensating for non-linearities introduced by A-Law CODECs 5 and 7, has to be 4096.

Alternative embodiments and variations of the invention are possible.

For example, when speech vocoders are used in place of the CODECs 5 and 7, non-linearities introduced into the echo path by such vocoders can be compensated by replacing the limiters 9 and I I with vocoding blocks, resulting in better performance of the echo canceler. In fact the concepts of the present invention can be applied to any application where echo canceflers (network or acoustic) are needed and where well defined non-linearities are introduced in the echo path.

Also, as an alternative to the configuration of Figure 2, it is contemplated that the limiter may be located in the main signal path before the node where the reference signal and CODEC signal are split.

Furthermore, whereas the embodiment discussed herein uses limiters as a first order approximation of the dynamic range, it is contemplated that digital signal processing can be used to model the transfer function of the CODEC, as a replacement to using limiters. Limiters do not take into account the non-linear quantization effect of the CODEC transfer function. The advantage of limiting is that it is simple to implement and provides a low cost solution. Modelling the transfer function requires a forward transform to the companded domain and an inverse transform back to the linear domain. For u-Law this requires u-Law companding and expanding in place of the limiter of the preferred embodiment.

All such alternatives and variations are believed to be within the sphere and scope of the invention as defined by the claims appended hereto.

6

Claims

I CLAIM:

I. An echo canceller for canceling echo of a reference signal from a source signal received from an echo path, wherein said echo path contains at least one source of non-linearity, comprising:

a compensator for receiving and modifying said reference signal so as to introduce said non-linearity to said reference signal; an adaptive filter connected to said compensator for producing a replica of said echo containing said non-linearity; and a subtractor connected to said adaptive filter for subtracting said replica of said echo from said source signal thereby yielding a regenerated source signal without said echo.
2. The echo canceller of claim 1, further comprising an additional compensator connected to said adaptive filter for receiving and modifying said replica of said echo to introduce a finther non-linearity modeled on an additional source of non-linearity in said echo path.
3. The echo canceller of claim 1, wherein said adaptive filter comprises a Least Mean Square (LMS) filter.
4. The echo canceller of claim 1, wherein said at least one source of nonlinearity is a transmit CODEC characterized by a predetermine dynamic range, and said compensator is a limiter for bounding said reference signal to said predetermined dynamic range.
5. The echo canceller of claim 2, wherein said additional source of nonlinearity is a receive CODEC characterized by a predetermine dynamic range, and said 7 additional compensator is a limiter for bounding said replica of said echo to said predetermined dynamic range.
6. The echo canceller of claim 4 or 5, wherein said CODEC is a i-Law CODEC and said dynamic range is 8192.
7. The echo canceller of claim 4 or 5, wherein said CODEC is a A-Law CODEC and said dynamic range is 4096.
8. The echo canceller of claim 1, wherein said at least one source of nonlinearity is a transmit CODEC characterized by a predetermine dynamic range, and said compensator is a digital signal processor for modelling the transfer function of said transmit CODEC.
9. The echo canceller of claim 1, wherein said at least one source of nonlinearity is a receive CODEC characterized by a predetermine dynamic range, and said compensator is a digital signal processor for modelling the transfer function of said receive CODEC.
10. A method for canceling echo of a reference signal from a source signal received from an echo path, wherein said echo path contains at least one source of non-linearity, comprising the steps of- receiving and modifying said reference signal so as to introduce said non- linearity to said reference signal; receiving said reference signal with introduced non-linearity and producing a replica of said echo containing said non-linearity; and subtracting said replica of said echo from said source signal thereby yielding a regenerated source signal without said echo.