Acoustic echo canceler with a peak impulse response detector

Details Acoustic echo canceler with a peak impulse response detector Acoustic echo canceler with a peak impulse response detector

1999-08-13

2003-06-17

Tieu, Binh (Department: 2643)

Telephonic communications

Echo cancellation or suppression

Using digital signal processing

C379S406010, C379S406160

Reexamination Certificate

active

06580794

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to echo cancelers and more specifically to an acoustic echo canceler for loudspeaking telephones and the like for canceling acoustic echoes produced as a result of acoustic coupling between a loudspeaker and a microphone.
2. Description of the Related Art
Japanese Laid-Open Patent Specification 07-58673 discloses an acoustic echo canceler for loudspeaking telephones or teleconferencing systems.
FIG. 1
illustrates a prior art digital acoustic echo canceler
102
incorporated in a loudspeaking telephone, which is connected via a hybrid circuit
101
to a telephone exchange line
100
. Analog speech signal from the telephone line
100
is converted to a digital signal by an analog-to-digital converter
110
and applied to an adaptive filter
112
for producing a digital replica of an acoustic echo, or “pseudo-echo”. The digital speech signal is also applied to a digital-to-analog converter
111
for driving a loudspeaker
104
with an analog speech signal amplified by an amplifier
103
. The original sound signal is reconstructed by the loudspeaker
104
and part of the sound energy is coupled to a microphone
105
, amplified by amplifier
106
and converted to digital form by an analog-to-digital converter
113
. This talker's voice would be returned to the source as an echo. This echo signal is applied to a digital subtractor
114
, where it is cancelled with the echo replica supplied from the adaptive filter
112
.
Adaptive filter
112
includes an input buffer
120
which serves as a tapped delay line for the remote speech signal to produce a series of successively delayed digital tap signals. These tap signals are supplied to tap-gain multipliers
121
where they are weighted by respective tap-gain coefficients from tap-gain update circuits
123
and then summed together in an adder
122
. Tapgain multipliers
121
and the adder
122
form a convolutional integrator. Each update circuit
123
includes a multiplier (correlator)
124
, an adder
125
and a unit delay element
126
. Multiplier
124
of each update circuit is used to detect the correlation between the corresponding tap signal and a residual echo which appears at the output of subtractor
114
. Adder
125
provides summation of the correlation output of multiplier
124
with the output of the delay element
126
to produce a tap-gain coefficient.
Since the echo cancelling effect by the adaptive filter is not sufficient to completely remove the acoustic echo, the residual echo is detected by a level detector
115
. A gain controller
116
is responsive to the detected level of the residual echo for controlling a gain adjustment circuit
117
, which is interposed in the circuit coupling the output of subtractor
114
to the hybrid circuit
101
via a digital-to-analog converter
118
. When the amplitude of the output of subtractor
114
is lower than some critical level, the level detector
115
directs the gain controller
116
to control the gain adjustment circuit
117
to reduce its gain, so that the acoustically coupled residual echo is prevented from returning to the talker.
However, one disadvantage of the prior art is that the amount of computations involved in the gain adjustment circuit
117
is substantial. Another disadvantage is that, since the gain adjustment is determined exclusively by the residual echo, the range of adjustment is still insufficient to cover large residual echoes.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an acoustic echo canceler which does not require time-consuming, complex computations.
Another object of the present invention is to provide an acoustic echo canceler capable of canceling large residual echoes.
According to a first aspect, the present invention provides an acoustic echo canceler for a speech communication system including a loudspeaker and a microphone acoustically coupled with the louspeaker through a feedback path. The acoustic echo canceler comprises an adaptive filter including a tapped delay line for receiving a remote speech signal to produce a sequence of successively delayed tap signals and a convolutional integrator for respectively weighting the tap signals with filter coefficients and summing the weighted tap signals to produce an acoustic echo replica of the remote speech signal, the adaptive filter updating the filter coefficients with a residual echo and restricting the filter coefficients to those which correspond to an echo component of the remote speech signal and restricting the tap signals to those which correspond to the restricted filter coefficients. The output signal of the microphone is combined in a subtractor with the acoustic echo replica, producing the residual echo. An attenuator is provided for receiving an output signal of the subtractor. Comparator circuitry compares the output signal of the subtractor with the restricted tap signals of the adaptive filter for increasing the attenuation of the attenuator when the restricted tap signals are greater in magnitude than the output signal of the subtractor and decreasing the attenuation when the restricted tap signals are smaller in magnitude than the output signal of the subtractor.
According to a second aspect, the present invention provides an acoustic echo canceler for a speech communication system including a loudspeaker and a microphone acoustically coupled with the louspeaker through a feedback path. The acoustic echo canceler comprises an adaptive filter including a tapped delay line for receiving a remote speech signal to produce a sequence of successively delayed tap signals and a convolutional integrator for respectively weighting the tap signals with filter coefficients and summing the weighted tap signals to produce an acoustic echo replica of the remote speech signal, the adaptive filter updating the filter coefficients with a residual echo and restricting the filter coefficients to those which correspond to an echo component of the remote speech signal and restricting the tap signals to those which correspond to the restricted filter coefficients. The output signal of the microphone is combined in a subtractor with the acoustic echo replica, producing the residual echo. A first attenuator is provided for suppressing small amplitude components of the residual echo and a second attenuator receives the output signal of the first attenuator. Comparator circuitry compares the output signal of the first attenuator with the restricted tap signals of the adaptive filter for increasing the attenuation of the second attenuator when the restricted tap signals are greater in magnitude than the output signal of the first attenuator and decreasing the attenuation of the second attenuator when the restricted tap signals are smaller in magnitude than the output signal of the first attenuator


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