Telephonic communications – Echo cancellation or suppression
Reexamination Certificate
2001-05-03
2004-03-02
Tieu, Binh (Department: 2643)
Telephonic communications
Echo cancellation or suppression
C379S379000, C379S388030, C379S381000, C379S391000, C379S413020, C379S413020
Reexamination Certificate
active
06700976
ABSTRACT:
BACKGROUND AND PRIOR ART
This invention relates to a system for noise suppression, particularly a noise canceler capable of cancelling background noise in a voice signal that is intermingled with noise, an accompanying method and a transceiver.
In many cases, noise corrupts a voice (speech) signal and hence the quality of recognition of the voice signal significantly. An example for such noise is background noise intermingled with the voice signal acquired by a microphone, a hand-free phone, a handset or the like.
It is important to recognize voice in a noisy environment, e.g. a construction site, a sport club, a Karaoke room, a hands-free communication system in a vehicle, especially a car, a helicopter, a tank or the like. Furthermore, noise suppression is useful in a live reporting system, a public addressing system or the like.
The recognition of voice can be done by an automatic voice recognition system or by at least one human listener.
The undesirable background noise can be of different sources. For example, making telephone calls out of a driving car, the driving noise, especially the noise of the engine, is a dynamically varying kind of noise that results in poor recognition of the voice, particularly in a hands-free speaking environment of the car. The addressee permanently hears a contaminated acoustic signal, in which the voice of the driver is included but difficult to understand. As a consequence, the driver has to speak up or take the handset of the telephone, which binds his attention to the handset and not the traffic—a very undesirable effect.
Moreover, there are lots of sites which need better recognition of voice and/or better understanding because of a noisy background. Some sites, additional to the above mentioned scenarios, are: airplanes, helicopters, airports, trains, buses, train stations, bus stops, construction sites, highways, streets or the like.
In [1] a concept and basic approach for adaptive noise cancellation are given. It can be used to eliminate background noise and improve a signal-to-noise-ratio (SNR). Therefore, a main input containing a corrupted signal and a reference input containing noise correlated in some unknown way with the primary noise are used. This reference input is adaptively filtered and subtracted from the main input to obtain the signal estimate. Adaptive filtering before subtraction allows the treatment of inputs that are deterministic or stochastic, stationary or time variable. Wiener solutions are developed to describe asymptotic adaptive performance and output SNR for stationary stochastic inputs, including single and multiple reference inputs. These solutions show that, when the reference input is free of signal and certain other conditions are met, noise in the main input can be essentially eliminated without signal distortion. In this case, the canceler behaves as a linear, time-invariant system, with the adaptive filter converging on a dynamic rather than a static solution.
A kind of adaptive noise canceler with adaptive cross-talk filters can be found in some articles and patents. [1] is used as a basis for improvement to eliminate cross-talk between voice and noise signals from a main input and a reference input (see [2], [3] and [4] for further details).
In [5] a different configuration of a cross-talk adaptive noise canceler is developed by splitting the adaptive cross-talk filter into a pre-filter section and an adaptive filter section.
Document [6] discloses a noise canceler utilizing four adaptive filters and a signal-to-noise power ratio estimator to do cross-talk noise cancellation. Furthermore, adjustment of the step sizes of the two main adaptive cross-talk filters is provided to incorporate a better tracking ability while the wanted voice signal does not exist. On the other hand, a smaller residual noise is achieved while the wanted voice signal is present.
FIG. 1
shows a noise canceler as disclosed in [6]. This noise canceler includes a main input
1
(first microphone) to obtain a main signal x
1
(
n
), a reference input
2
(second microphone) to obtain a reference signal x
2
(
n
), a signal output
5
, adaptive filters
3
and
6
, adders
4
and
7
, delay circuits
8
and
9
, a signal-to-noise power ratio estimator
10
and a step size output circuit
11
. The signal-to-noise power ratio estimator
10
is made up of adaptive filters
12
and
13
, adders
14
and
15
, power mean circuits
16
,
17
,
18
and
19
, and Dividers
20
and
21
.
The main signal x
1
(
n
) is delayed by the delay circuit
8
by D samples to turn out a delayed main signal. This delayed main signal is applied to the subtracter
4
. On the other hand, the reference signal x
2
(
n
) is delayed by the delay circuit
9
by D samples to turn out a delayed reference signal that is applied to the subtracter
7
. The adaptive filter
3
operates to estimate a noise signal included in the main signal x
1
(
n
) while the adaptive filter
6
operates to estimate a desired signal included in the reference signal x
2
(
n
). To allow the filter
3
to estimate the noise signal y
1
(
n
), the desired signal y
2
(
n
) estimated by filter
6
is subtracted from the reference signal x
2
(
n
) by the subtracter
7
, and the resulting noise signal e
2
(
n
) is input to the filter
3
. Likewise, the noise signal y
1
(
n
) estimated by the filter
3
is subtracted from the main signal x
1
(
n
), and the resulting desired signal e
1
(
n
) is input to filter
6
. For this purpose, the two filters
3
and
6
are cross-coupled, as illustrated.
Now, for the adaptive filter
3
to estimate the noise signal y
1
(
n
) in the main signal accurately, it is necessary to increase the amount of updating of the filter coefficient when the desired signal of the main signal obstructing the estimation is smaller than the noise signal to be estimated. Conversely, when the desired signal of the main signal is greater than the noise signal, it is necessary to reduce the above amount because the signal obstructing the estimation is greater than the noise signal.
On the other hand, for the adaptive filter
6
to estimate the desired signal of the reference signal accurately, it is necessary to increase the amount of updating of the filter coefficient when the noise signal contained in the reference signal obstructing the estimation is smaller than the desired signal. Conversely, when the noise signal of the reference signal is greater than the desired signal, it is necessary to reduce the above amount because the signal obstructing the estimation is greater than the desired signal.
The coefficient for each adaptive filter can be controlled to meet the above requirement by controlling the step size of the adaptive filters.
It is a significant disadvantage of the system disclosed in [6] that the noise canceler as shown in
FIG. 1
comprises a signal-to-noise power ratio estimator
10
with two additional adaptive filters
12
and
13
. The computations of the noise canceler are increased due to these filters
12
and
13
. Moreover, the adaptive filters
12
and
13
embody fixed step sizes affecting an inflexible voice and noise estimation.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an acoustic noise canceler to be able to achieve a good noise cancellation with reduced computational effort.
Furthermore, it is an object of the invention to provide a noise suppression system and an accompanying method.
More specifically, it is an object of the invention to provide an adaptive cross-talk noise canceler which comprises two cross-coupled adaptive filters with adjustable step sizes for updating the coefficients of the filters.
Other objects, features and advantages according to the present invention will be presented in the following detailed description of the illustrated embodiments when read in conjunction with the accompanying drawings.
These objects of the present invention are achieved by the features of the independent claims. Additiona
Lan Hui
Zhang Ming
Nanyang Technological University
Pham Tuan
Rutan & Tucker
Tieu Binh
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