Telephonic communications – Echo cancellation or suppression – Using digital signal processing
Reexamination Certificate
1999-09-14
2004-02-17
Tieu, Binh (Department: 2747)
Telephonic communications
Echo cancellation or suppression
Using digital signal processing
C379S406080, C379S406110, C370S291000, C375S233000
Reexamination Certificate
active
06694020
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to acoustic echo cancellation and more specifically to stereophonic acoustic echo cancellation.
BACKGROUND OF THE INVENTION
The evolution of teleconferencing to a more lifelike and transparent audio/video medium depends upon, among other things, the evolution of teleconferencing audio capabilities. The more realistic the sound, the more lifelike a teleconference will be and the more people and businesses will use it. Some present-day teleconferencing systems have already evolved to the point of including high-fidelity audio systems (100-7000 Hz bandwidth). These systems provide a significant improvement over older telephone systems (200-3200 Hz bandwidth). However, such high fidelity systems are by no means the limits of audio evolution in teleconferencing.
Spatial realism is highly desirable for audio/video teleconferencing. This is because of the need of a listener to follow, for example, a discussion among a panel of dynamic, multiple, and possibly simultaneous talkers. The need for spatial realism leads to consideration of multi-channel audio systems in teleconferencing, which, at a minimum, involves two channels (i.e., stereophonic).
Many present-day teleconferencing systems have a single (monophonic) full-duplex audio channel for voice communication. These systems, which range from simple speaker-phones to modern video teleconferencing equipment, typically employ acoustic echo cancelers (AECs) to remove undesired echos that result from acoustic coupling. This coupling results when sound, emitted from the teleconference loudspeaker (in response to a signal from a remote location), arrives at the teleconference microphone in the same room. The microphone generates a signal in response to this sound (i.e., this echo). This microphone signal is then transmitted to the remote location. An AEC employs an adaptive filter to estimate the impulse response from the loudspeaker to the microphone in a room in which an echo occurs and to generate a signal which is subtracted from the receiver signal to cancel that echo electrically. Like monophonic teleconferencing, high-quality stereophonic teleconferencing requires AEC. (See, e.g., M. M. Sondhi and D. R. Morgan, “Acoustic echo cancellation for stereophonic teleconferencing,”
Proc. IEEE ASSP Workshop Appls. Signal Processing Audio Acoustics,
1991, which is incorporated herein by reference).
Stereophonic AEC presents a problem which does not exist in the monophonic context. In monophonic teleconferencing systems, a single adaptive filter is used to estimate a single impulse response from the loudspeaker to the microphone in the room experiencing an echo. There is only one impulse response to estimate because there is only one loudspeaker and one microphone in the room. As the adaptive filter impulse response estimate approaches the true impulse response of the room, the difference between these responses approaches zero. Once their difference is very small, the effects of echo are reduced. The ability to reduce echo is independent of the signal from the loudspeaker, since the real and estimated impulse responses are equal (or nearly so) and both the room (with its real impulse response) and the adaptive filter (with its estimated impulse response) are excited by the same signal.
In multi-channel stereophonic teleconferencing systems, multiple (e.g., two) adaptive filters are used to estimate the multiple (e.g., two) impulse responses of the room. Each adaptive filter is associated with a distinct acoustic path from a loudspeaker to a microphone in the receiving room. Rather than being able to independently estimate the individual impulse responses of the room, conventional stereophonic AEC systems derive impulse responses which have a combined effect of reducing echo. This limitation on independent response derivation is due to the fact that the AEC system can measure only a single signal per microphone. This signal is the sum of multiple acoustic signals arriving at a single microphone through multiple acoustic paths. Thus, the AEC cannot observe the individual impulse responses of the room. The problem with deriving impulse response estimates based on the combined effect of reduced echo is that such combined effect does not necessarily mean that the actual individual impulse responses are accurately estimated. When individual impulse responses are not accurately estimated, the ability of the AEC system to be robust to changes in the acoustic characteristics of the remote location is limited, commonly resulting in undesirable lapses in performance. (See, e.g., M. M. Sondhi, D. R. Morgan, and J. L. Hall, “Stereophonic Acoustic Echo Cancellation—An Overview of the Fundamental Problem,” IEEE Signal Processing Lett., Vol. 2, No. 8, August 1995, pp. 148-151, which is incorporated herein by reference.)
FIG. 1
presents a schematic diagram of a conventional stereophonic (two-channel) AEC system in the context of stereo teleconferencing between two locations. A transmission room
1
is depicted on the right of the figure. Transmission room
1
includes two microphones
2
,
3
which are used to pick up signals from an acoustic source
4
(e.g., a speaking person) via two acoustic paths that are characterized by the impulse responses g
1
(t) and g
2
(t). (For clarity of presentation, all acoustic paths are assumed to include the corresponding loudspeaker and/or microphone responses.) Output from microphones
2
,
3
are stereophonic channel source signals x
2
(t) and x
1
(t), respectively. These stereophonic channel source signals, x
2
(t) and x
1
(t), are then transmitted via a telecommunications network (such as a telephone or an ATM network) to loudspeakers
11
,
12
in a receiving room
10
(shown on the left). For convenience, this direction will herein be termed the upstream direction and transmissions in the opposite direction, i.e., from room
10
to room
1
, will be termed the downstream direction. The terms upstream and downstream are intended to have no particular connotation other than to differentiate between two directions. Loudspeakers
11
,
12
are acoustically coupled to microphone
14
in receiving room
10
via the paths indicated with impulse responses h
1
(t) and h
2
(t). These are the paths by which acoustic echo signals arrive at microphone
14
.
The output of the microphone
14
is signal y(t), which is a signal representing acoustic signals in the receiving room impinging on the microphone. These acoustic signals include the acoustic echo signals. Loudspeakers
11
,
12
are also acoustically coupled to microphone
13
by other acoustic paths. For clarity of presentation, however, only the coupling to microphone
14
and AEC with respect to its output will be discussed.
Further, those of ordinary skill in the art will recognize that the analysis concerning AEC for the output of microphone
14
is applicable to the output of microphone
13
as well. Similarly, those skilled in the art will recognize that AEC as performed for the outputs of microphones
13
and
14
in receiving room
10
also may be advantageously performed for the outputs of microphones
2
and
3
in transmitting room
1
, wherein the functions of receiving room
10
and transmitting room
1
are swapped.
If nothing were done to cancel the acoustic echo signals in receiving room
10
, these echoes would pass back to loudspeaker
5
in transmission room
1
(via microphone
14
and the telecommunications network) and would be circulated repeatedly, producing undesirable multiple echoes, or even worse, howling instability. This, of course, is the reason that providing AEC capability is advantageous.
Conventional AECs typically derive an estimate of the echo with use of a finite impulse response (FIR) filter with adjustable coefficients. This “adaptable” filter models the acoustic impulse response of the echo path in the receiving room
10
.
FIG. 1
generally illustrates this technique with use of AEC
20
using two adaptive FIR filters
16
,
15
having impulse responses, ĥ
1
(t) and &h
Agere Systems Inc.
Synnestvedt & Lechner LLP
Tieu Binh
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