Telephonic communications – Subscriber line or transmission line interface – Network interface device
Reexamination Certificate
1999-01-07
2001-07-17
Chan, Wing F. (Department: 2643)
Telephonic communications
Subscriber line or transmission line interface
Network interface device
C379S406010
Reexamination Certificate
active
06263078
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to acoustic echo cancellers and more particularly to acoustic echo cancellers as applied to a full-duplex speakerphone device.
DESCRIPTION OF THE RELATED ART
An acoustic echo canceller is an essential component of a full-duplex speakerphone. Unlike half-duplex speakerphones, a full-duplex speakerphone allows near-end and far-end callers to talk simultaneously.
However, performance of a full-duplex speakerphone may suffer from the effects of acoustic echo paths that occur between the loudspeaker and the microphone. The problem manifests itself as audible echo and, possibly, a howlback condition as the echo is re-transmitted and re-amplified between the near-end and far-end speakerphones. Acoustic echo cancellers are designed to eliminate this problem.
With reference to
FIG. 1
, a prior art speakerphone system
10
having a conventional echo canceller
12
is shown. The echo canceller
12
includes an adaptive filter
14
and a subtractor
16
that are connected between a receive path
18
and an output path
20
. On the receive path
18
, a digital-to-analog (D/A) converter
22
, an adjustable amplifier
24
and a loudspeaker
26
are connected in series. The gain provided by the adjustable amplifier
24
is controlled by an analog volume control. On the output path
20
, a microphone
28
, an amplifier
30
, an analog-to-digital (A/D) converter
32
and the subtractor
16
are similarly connected.
When an incoming digital signal from a far-end caller is received by the system
10
, the signal is transmitted through the receive path
18
for conversion by the D/A converter
22
and amplification by the adjustable amplifier
24
. The amplified analog signal is then broadcast into a room by the loudspeaker
26
. Depending on the acoustic characteristics of the room, echo of the broadcast signal is propagated through various echo paths from the loudspeaker
26
to the microphone
28
, such as echo paths
34
a
and
34
b
. The echo may be combined with speech from the near-end caller in the microphone
28
, and transmitted along the output path
20
as an outgoing analog signal. The outgoing signal is first amplified by the amplifier
30
and then converted into a digital format by the A/D converter
32
.
Meanwhile, the adaptive filter
14
samples the original incoming digital signal and performs a convolution step, i.e., a computation of an estimated echo response, using the sampled signal as a reference. The estimated echo response is a predicted acoustic echo response of the system when used in a particular environment. A current estimate of the echo response is utilized to subtract echo components of the outgoing signal. The subtraction, or cancellation, of echo components is performed by the subtractor
16
. After the outgoing signal has been cancelled at the subtractor, the echo-cancelled outgoing signal is transmitted to the far-end caller. In addition, the echo-cancelled outgoing signal is fed back to the adaptive filter
14
as an error signal that is utilized to dynamically adjust filter coefficients that determine echo response.
The quality of the echo cancellation depends on the ability of an adaptation algorithm, which is utilized by the adaptive filter
14
, to accurately model the true echo response. The true echo response is estimated by an adaptation process in which the error signal drives the adaptation algorithm to update the coefficients of the model, so that the error signal is driven toward zero, i.e., the echo-cancelled outgoing signal does not contain any detectable echo residue. However, the acoustic response of a room does not remain constant over time. For example, positional shifts of persons and/or the microphone in the room, or opening and closing of a door change the echo paths. The change in echo paths results in a new echo response that is inaccurately represented by the previously estimated echo model. Until the echo canceller
12
can adapt to the new echo response, a substantial amount of echo may be transmitted to the far-end caller.
U.S. Pat. No. 5,610,909 to Shaw describes a communication system having an echo canceller arrangement that is intended to better compensate for changes in echo paths caused by movements of objects relative to either the microphone or the loudspeaker. The communication system of Shaw includes two echo cancellers that are connected in parallel between a receive path and an output path. An outgoing signal is initially filtered of echo by the first echo canceller. The filtered outgoing signal is then filtered by the second echo canceller. The second echo canceller is designed to adapt much quicker than the first echo canceller. This allows the first echo canceller to cancel echo from slow varying echo paths, while the second echo canceller cancels for echo from fast varying echo paths. That is, echo from different time-varying echo paths is successively removed by the first echo canceller and the second echo canceller.
Another cause for echo response change is an increase in the volume of the acoustic energy from loudspeaker
26
. Often, the volume is controlled after an incoming signal has been converted from a digital signal to an analog signal, as is the case for the echo canceller system
10
. As an example, in a personal computer application, a user may control the volume by manually increasing the signal gain of the signal to the loudspeakers. A sudden increase in the acoustic volume of the loudspeaker causes the magnitude of echoes within the room to change significantly. Until the echo canceller adapts to a different echo response, the sudden increase in volume causes the echo canceller to less effectively remove the echo in the outgoing signal, resulting in a significant amount of echo being transmitted to the far-end caller. The echo due to a volume increase can be quite strong. The initial echo is equal to the difference between the echo estimate and the actual echo. As an example, if the user increased the volume by a factor of four, the amount of echo transmitted to and heard by the far-end caller would be approximately three times the current echo estimate. Furthermore, the amount of time required for the echo canceller to adapt to the updated echo response can be significant. The rate of adaptation can be accelerated with an increase in adaptive filter step-size, but the probability of unstable behavior progressively increases with larger step-sizes.
While the known acoustic echo canceller systems operate well for their intended purposes, what is needed is an acoustic echo canceller system that can quickly adapt in response to sudden changes in the echo response, such as changes induced by variations in acoustic volume, without sacrificing other echo cancelling performance aspects.
SUMMARY OF THE INVENTION
An echo canceller system and a method of filtering an outgoing signal utilize a volume compensator that operates in tandem with a primary adaptive filter to cancel echo due to a change in echo response of the system. In particular, the volume compensator is designed to quickly adapt to a change in acoustic response of a room due to such factors as a sudden increase or decrease in the gain of a signal to a speaker. The echo canceller system may be incorporated into a full-duplex speakerphone or a personal computer system for a full-duplex speakerphone application.
The volume compensator includes a supplementary adaptive filter that operates in conjunction with the primary adaptive filter to cancel echo in an outgoing signal. The primary adaptive filter may be a conventional adaptive filter that utilizes one of many well known adaptive filtering techniques to cancel the echo. As an example, the primary adaptive filter may model the true echo response of the system within a particular room using a linear transfer function for cancelling the echo. The supplementary adaptive filter operates in parallel to the primary adaptive filter to also cancel the echo in the original outgoing signal. The filtered signal having the least amount of echo is selected and t
King Linda J.
Logan Tamara L.
McLaughlin Hugh J.
Chan Wing F.
McHugh Terry
SignalWorks, Inc.
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