Acoustic echo canceler

Details Acoustic echo canceler Acoustic echo canceler

2000-08-01

2002-08-13

Isen, Forester W. (Department: 2644)

Telephonic communications

Echo cancellation or suppression

Using digital signal processing

C379S406060, C379S406010, C379S406020, C379S406030, C379S406090, C379S406110

Reexamination Certificate

active

06434235

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of echo cancelers generally, and more specifically to acoustic echo cancelers
DESCRIPTION OF THE RELATED ART
A call to a wireless mobile telephone passes through a base station with which the mobile unit is in contact. The speech signals transmitted from the base station of the wireless system may be transmitted from the earpiece or loudspeaker of the mobile unit to the mouthpiece or microphone of the mobile unit. This coupling may result from direct propagation of the sound through the air and/or coupling of the sound through the material of the mobile unit (i.e., the handset) itself. The transmitted mobile signal arrives back at the base station with an amplitude and a time delay that are functions of the medium in which the sound wave propagates, such as the speed of sound, and the distance of the propagation path.
Back at the base station, the acoustically coupled or directly propagated sound adds with the speech from the mobile unit. The coupled sound may have a small effect if the delay time is very short. A long delay produces a distracting, distinct repetition of the caller to the mobile's original speech (an echo). Whether the delay is long or short, the echo produced by such coupling is undesirable.
Echo cancelers are known for use in systems that are prone to echoes, and operate by anticipating the echo that a signal will produce and subtracting the anticipated echo from the signal. The main elements of an echo canceler include an adaptive filter and a subtracter. The adaptive filter correlates a reference signal (presumably a duplicate of the caller's voice signal) with the voice signal returning along the echo path. Over a short period of time, numerical factors are developed that model the echo path. The factors define the impulse response of the echo transmission pathway, which embodies aspects of attenuation and time delay. The reference signal is then multiplied by the impulse response as represented by the factors, producing an echo replica signal that is subtracted from the actual echo signal received along the echo path.
The difference or error is fed back to the adaptive filter. The adaptive filter adjusts its coefficients (or taps) as necessary to minimize the error. The coefficients are initially calculated over a short period at the beginning of a call, and thereafter are refined. Normally, reasonably accurate coefficients are reached promptly. The echo path is typically due to the character and fixed geometry of the signal transmission path. Normally the signal path does not change, and after the coefficients are reached they do not change substantially for the duration of a connection. However it is possible that the transmission path could change (for example, in the case of a hands-free speaker phone in an enclosure with doors or windows that can be opened). It is not possible to predict the echo response, and for these reasons, the echo canceler continuously updates or revises the coefficients, and adaptively cancels the echo by adjusting the coefficients.
FIG. 1
, labeled prior art, shows a typical arrangement in the form of a cordless telephone system
9
that has a base station
9
a
and a mobile unit
9
b
. A sound signal
20
is transmitted from base unit
9
a
via antenna
21
, shown as wireless signal
22
, to a receiving antenna
23
of mobile unit
9
b
. The signal typically contains a voice signal. There is a coupling path
24
, for example due to acoustic coupling between the speaker and microphone of the mobile unit
9
b
. Mobile unit
9
b
transmits the echo signal as a wireless signal
26
, from antenna
25
of the mobile unit
9
b
to antenna
27
of the base station
9
a
. The reference signal
20
′ (which is a duplicate of the original sound signal
20
) and the returning signal
28
are fed to the adaptive filter
29
, which outputs a replica signal
29
a
. The received replica signal
29
a
is subtracted from the echo signal
28
in subtractor
30
, to form a feedback signal
31
that is fed back to the adaptive filter
29
.
The echo canceler in the device shown in
FIG. 1
is associated with the device that produces the echo. Therefore, the device operates substantially for the benefit of the remote correspondent by eliminating the echo produced locally at the mobile unit.
In a simple theoretical arrangement wherein there is a linear relationship between the reference signal and the echo, the returning echo signal might be characterized as a delayed copy of the sound signal and the reference signal, having an amplitude that is proportionately reduced relative to the reference signal. In that case the impulse response of the echo path simply represents a delay. If there are no non-linear operations performed on the sound signal, it is possible to delay the reference signal in the adaptive filter, so as to have the same phase as the echo. The echo response, however, may be more complicated than that.
In a typical cordless telephone system, signals are compressed (encoded) from samples at 64 kilobits per second to eight kilobits per second before being transmitted between the base station and the mobile unit. The compressed data is decompressed (decoded) by the recipient.
FIG. 2
shows the effect of adding encoding and decoding stages into the signal processing path. In
FIG. 2
, a prior art cordless telephone system
10
includes a base station
10
a
and a mobile unit
10
b
. A sound signal
40
is encoded at encoder
41
. The encoded signal
42
is transmitted from base unit
10
a
via antenna
43
, as wireless signal
44
, to a receiving antenna
45
of mobile unit
10
b
. The signal is decoded in decoder
46
and provided to a speaker (not shown). There is an acoustic coupling
47
between the speaker and microphone (not shown) of the mobile unit
10
b
, providing an echo transmission path. The input to the microphone is encoded in encoder
48
. Mobile unit
10
b
transmits the encoded signal
49
as a wireless signal
51
, from antenna
50
of the mobile unit
10
b
to antenna
52
of the base station
10
a
. The signal is decoded at decoder
53
of the base station
10
a
. The reference signal
40
′ (which is a duplicate of the original sound signal
40
) is also fed to the adaptive filter
55
, which outputs a replica signal
55
a
. The received replica signal
55
a
is subtracted from the echo signal
54
in subtractor
56
, to form a feedback signal
58
that is fed back to the adaptive filter
55
.
The encoders
41
,
48
and decoders
46
,
53
perform non-linear operations. The coupled earpiece output is added prior to the second non-linear encoding operation and the second non-linear decoding operation. The speech signals
44
transmitted from the base station to the mobile unit undergo a round of encoding and decoding on the downlink, and the acoustic echo in the mobile unit
10
b
undergoes a second round of encoding and decoding in transmission over the uplink. The acoustic echo in the sound signal
51
transmitted to the base station is thus subjected to a non-linear transformation with respect to the reference signal
40
′. In short, there is no precise delay time associated with the echo transmission path, as is characteristic of a simple acoustic echo transmission path. Standard echo-cancellation techniques, which assume a linear, coherent relationship between the reference signal and the echo, at least at a particular frequency, are not effective.
An improved echo canceler is desired to accommodate this complication.
SUMMARY OF THE INVENTION
The present invention is a method and apparatus for echo cancellation in a system having first and second units in communication with each other, the method including a plurality of encoding and decoding operations performed on a reference signal to form a modified reference signal, the plurality of encoding and decoding operations matching a set of encoding and decoding operations performed on a sound signal by the first unit and the second unit.
In a method according to the in

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