Telephonic communications – Echo cancellation or suppression – Combined diverse function
Reexamination Certificate
1999-05-20
2003-01-21
Isen, Forester W. (Department: 2644)
Telephonic communications
Echo cancellation or suppression
Combined diverse function
C379S406010, C379S406020, C379S406080, C379S406090, C379S406100, C379S406060
Reexamination Certificate
active
06510224
ABSTRACT:
BACKGROUND
The present invention relates to the processing of speech signals in a communications system, and more particularly to the enhancement of near-end speech in a signal that includes the near-end speech combined with an echo of far-end speech.
In the field of telecommunications, such as with speaker phones and in cellular telephony, it is often desirable to allow a user to operate communication equipment without requiring the continued occupation of one or more of the user's hands. This can be an important factor in environments, such as automobiles, where a driver's preoccupation with holding telephone equipment may jeopardize not only his or her safety, but also the safety of others who share the road. Freedom of use one's hands for something other than holding a microphone is useful in other applications as well, such as with internet communication by means of a personal computer, speech recognition by a computer, or with audio-visual presentation systems.
To accommodate these important needs, so-called “hands-free” equipment has been developed, in which microphones and loudspeakers are mounted within the hands-free environment, thereby obviating the need to hold them. For example, in an automobile application, 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. With components mounted in this fashion, a cellular phone user may carry on a conversation without having to hold the cellular unit or its handset. Similarly, personal computers often have microphones and loudspeakers mounted, for example, in a monitor in relatively close proximity to each other.
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. For example, in communications equipment, delays introduced by the communications system as a whole can cause the sound from the loudspeaker to be heard by the individual on the other end of the call (the so-called “far-end”) as an echo of his or her own voice. Such an echo degrades audio quality and its mitigation is desirable. A similar problem can exist, for example, in automated systems that synthesize speech through a loudspeaker, and include voice recognition components for recognizing and responding to spoken commands or other words sensed by the microphone. In such applications, the presence of an echo of synthesized speech in the microphone signal can severely degrade the performance of the speech recognition components. Solutions for ameliorating such echoes include utilizing an adaptive echo cancellation filter or an echo attenuator.
As a representative example of hands-free equipment in general, 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) filter, an adaptation unit
103
, such as a least mean square (LMS) cross correlator, and a subtractor
107
. In operation, the adaptive filter
101
generates an echo estimate signal
102
, which is commonly referred to as a û signal. The echo estimate signal
102
is the convolution of the far-end signal
112
, and a sequence of m filter weighting coefficients (h
i
) of the filter
101
(See Equation 1).
u
^
(
n
)
=
∑
i
=
0
m
-
1
h
i
x
(
n
-
i
)
(
1
)
where:
x(n) is the input signal,
m is the number of weighting coefficients, and
n is the sample number.
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 microphone signal
126
(designated u(n) in Eq. 2), to produce an error signal e(n) (see Eq. 2)
e
(
n
)=
u
(
n
)−
û
(
n
) (2)
Ideally, any echo response from the unknown system H(z), introduced by the loudspeaker
109
, is removed from the digitized microphone signal
126
by the subtraction of the echo estimate signal
102
. Typically, 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.
It can be seen that 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, in turn, is directly related to the set of coefficients, h
i
, maintained by the filter
101
.
It is advantageous to provide a mechanism for dynamically altering the coefficients, h
i
, to allow the adaptive filter
101
to adapt to changes in the unknown system H(z). In a car having a hands-free cellular arrangement, such changes may occur when a window or car door is opened or closed. 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. As applied to the echo cancellation problem, the LMS process is a stochastic gradient step method which uses a rough (noisy) estimate of the gradient, g(n)=e(n)
x
(n), to make an incremental step toward minimizing the energy of an echo signal in a microphone signal, e(n), where
x
(n) is in vector notation corresponding to an expression
x
(n)=[x(n)x(n−1)x(n−2) . . . x(n−m+1)]. The update information produced by the LMS process e(n)
x
(n) is used to determine the value of a coefficient in a next sample. The expression for calculating a next coefficient value h
1
(n+1) is given by:
h
i
(n+1)=
h
i
(
n
)+&mgr;
e
(
n
)
x
(
n−i
),
i=
0
. . . m−
1 (3)
where
x(n) is the digitized input signal,
(h
i
) is a filter weighting coefficient,
i designates a particular coefficient,
m is the number of coefficients,
n is the sample number, and
&mgr; is a step or update gain parameter.
The LMS method produce 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 a filter to update the filter's coefficients.
Referring back to
FIG. 1
, the conventional echo cancellation circuit includes a filter adaptation unit
103
in the form of an LMS cross correlator for providing coefficient update information to the filter
101
. In th
Christensson Nils
Philipsson John
Burns Doane Swecker & Mathis L.L.P.
Isen Forester W.
Singh Ramnandan
Telefonaktiebolaget L M Ericsson
LandOfFree
Enhancement of near-end voice signals in an echo suppression... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Enhancement of near-end voice signals in an echo suppression..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Enhancement of near-end voice signals in an echo suppression... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3036383