Electrical audio signal processing systems and devices – Dereverberators
Reexamination Certificate
1999-05-24
2003-04-29
Lee, Ping (Department: 2644)
Electrical audio signal processing systems and devices
Dereverberators
C379S406010
Reexamination Certificate
active
06556682
ABSTRACT:
FIELD OF THE INVENTION
The present invention concerns a method for cancelling multi-channel acoustic echo, as well as a multi-channel acoustic echo canceller.
BACKGROUND OF THE INVENTION
In the domain of transmission of sound signals, in some applications such as “hands free” telephony and teleconference, the acoustic echo is a source of considerable inconvenience. Known devices which counteract the acoustic echo usually comprise adaptive filters, the function of which is to identify and model the impulse response of the acoustic coupling path between the loudspeaker(s) and the microphone(s) of the considered sound signals transmission system.
FIG. 1
illustrates the general structure of a conventional acoustic echo canceller. It is associated with a loudspeaker
10
and a microphone
12
between which there exists an acoustic coupling path or echo path
14
. References
16
and
18
designate respectively the received sound signal and the transmitted sound signal. The echo canceller of
FIG. 1
includes an adaptive filter
20
receiving at its input the received sound signal
16
. The object of the adaptive filter
20
is to estimate through its coefficients the impulse response of the echo path
14
, in order to subtract the echo from the signal received by the microphone
12
. To this end, the output of the adaptive filter
20
is connected to a subtractor
22
which subtracts the signal output by the adaptive filter
20
from the signal received by the microphone
12
. The difference signal obtained at the output from the subtractor
22
supplies an estimation error
24
. The coefficients of the adaptive filter
20
are adjusted over time by an appropriate algorithm which uses estimation error information.
The choice of this algorithm is the determining factor in the performances of the echo canceller. The known echo cancellation methods and devices currently use an algorithm called the normalised stochastic gradient, usually designated by the acronym NLMS. A disadvantage of this algorithm is that its convergence speed depends upon the spectral characteristics of the received sound signal. An object of the present invention is to reduce this dependence.
The problem of the acoustic echo of course also arises in multi-channel sound signals transmission systems, i.e. in systems with several loudspeakers and several microphones, for example two loudspeakers and two microphones in the case of stereophony.
Multi-channel echo cancellation methods and devices are known based upon the same principle as those of single channel cancellation.
FIG. 2
shows by way of an example the partial block diagram of a conventional stereophonic acoustic echo canceller, where, for clarity, only one of the two microphone channels has been shown. The general structure can be easily generalised to that of an acoustic echo canceller with N sound signal channels, where N is any integer. Only the case for N=2 is described in the following. In a similar manner to the echo canceller of
FIG. 1
, the stereophonic echo canceller is applied to received sound signal channels
16
1
,
16
2
and transmitted sound signal channels, only one of which, designated by the reference number
18
1
, is shown. The echo canceller is associated with two loudspeakers
10
1
,
10
2
and two microphones, only one of which, designated by the reference number
12
1
, is shown. There are four echo channels, two of which echo channels (
14
1
,
14
2
) are between the two loudspeakers
10
1
,
10
2
and the first microphone
12
1
and two other echo channels (not shown) are between the two loudspeakers
10
1
,
10
2
and the second microphone. In order to estimate the impulse responses from the various echo channels, an adaptive filter is provided between each loudspeaker channel and each microphone channel. Thus, if the microphone channel
12
1
is considered, it is provided with two adaptive filters
20
1
,
20
2
, which receive respectively as the input the received sound signals
16
1
and
16
2.
The outputs of the adaptive filters
20
1
,
20
2
are supplied as the input to an adder
26
1
. In an echo canceller with N channels where N is any integer, such an adder is provided on each microphone channel. The sum of the output signals of the adaptive filters supplied by the adder
26
1
is subtracted, by a subtractor
22
1
, from the signal received by the microphone
12
1
. The same operation is carried out on each microphone channel. The difference signal obtained as the output of the subtractor
22
1
supplies a common estimation error
24
1
to all the adaptive filters relative to the considered microphone channel; in the example of
FIG. 2
, the estimation error
24
1
is common to the adaptive filters
20
1
and
20
2.
As is the case for mono-channel echo cancellation, the coefficients of the adaptive filters are modified in an iterative manner by an appropriate algorithm, on each microphone channel, from the estimation error obtained.
It has been noted, in multi-channel cancellation, and in particular in stereophonic echo cancellation, that the convergence speed of the adaptation algorithms of the coefficients of the adaptive filters was comparatively smaller than the convergence speed of those algorithms in the case of the mono-channel echo cancellation. It has been shown that this slowing down of the convergence is the result of the fact that the sound signals received by the loudspeakers, designated by the reference numbers
16
1
and
16
2
in
FIG. 2
, are mutually correlated.
The slowing down of the convergence causes several disadvantages. In particular, in a teleconference system, the speakers located in the distant room detect over a longer time the echo of their speech during start up of the system or after an acoustic modification (movement of the listeners for example) in the room where the echo canceller is located. Moreover, in multi-channel echo cancellation, every acoustic modification in the distant room disturbs the convergence of the adaptive filters of the echo canceller, because of the mutual correlation, mentioned previously, between the speech signals received, which causes a reappearance or an increase of the echo level.
On the other hand, it has been observed in practice that when the non-mutually correlated components are present on each microphonic signal, they tend to accelerate the convergence of the multi-channel echo cancellers. An object of the present invention is to use this property of the mutually uncorrelated components to improve the performances of multi-channel echo cancellers, and to improve as a consequence the quality of the communication in the sound signal transmission systems which implement multi-channel echo cancellers. To do that, the general principle of the present invention consists of adding, to the received sound signal channels, mutually uncorrelated auxiliary signals, made inaudible by using some human auditory properties.
SUMMARY OF THE INVENTION
More exactly, the present invention proposes an echo cancellation method on N signal sound channels each having a loudspeaker and an associated microphone, N being an integer greater than or equal to
1
, according to which, on each of the N channels:
(a) a synthetic signal is created having the spectral characteristics of a white noise, the spectrum of this signal extending over several adjacent frequency bands, and this synthetic signal being uncorrelated from the synthetic signals created on the other channels;
(b) for each frequency band, a frequency masking threshold is computed corresponding to the signal associated with the loudspeaker of the considered channel using properties of human auditory perception;
(c) in each frequency band, the level of the synthetic signal is brought to the value of the associated masking frequency threshold, so as to obtain an auxiliary signal;
(d) the auxiliary signal is added to the signal associated with the loudspeaker of the considered channel, the masking frequency thresholds having been previously computed so as to make the auxiliary signal inaudible, and the auxiliary
Gilloire Andre
Turbin Valérie
France Telecom
Gardner Carton & Douglas
Lee Ping
LandOfFree
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