Telephonic communications – Echo cancellation or suppression – Using digital signal processing
Reexamination Certificate
2001-03-02
2004-05-11
Tieu, Binh (Department: 2643)
Telephonic communications
Echo cancellation or suppression
Using digital signal processing
C379S406010, C379S406050, C379S406140, C381S066000, C381S071110, C708S322000, C708S323000
Reexamination Certificate
active
06735304
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a circuit using an adaptive filter to estimate an unknown system, and in particular to an adaptive filter suitable for an echo canceller that cancels an echo that may be a problem in the case of long-distance lines having relatively long transmission delay and including devices that couple local two-wire to long-distance four-wire lines.
2. Description of the Related Art
System identification using an adaptive filter is performed by updating the coefficients wi(t) of the adaptive filter to estimate an output signal y(t) corresponding to an input signal x(t) of the system by using a difference e(t)=ŷ(t)−y(t), where ŷ(t) is an output of the active filter.
Taking a learning identification method or normalized LMS (least-mean square) method as an example of system identification method, a conventional echo cancellation method will be described hereafter. Here, it is assumed that an unknown system is an unknown echo path in the case where an echo is created when four-wire lines are converted to two-wire lines. The learning identification method has been described in “Adaptive Filter Theory” by Simon Haykin (Prentice-Hall Information and System Sciences series, 1996).
Consider an adaptive FIR (Finite Impulse Response) filter with N taps, a pseudo echo or echo replica ŷ(t) that is an output of the filter at time instant t is represented by
y
^
(
t
)
=
∑
i
=
1
N
(
wi
(
t
)
*
x
(
t
-
i
)
)
,
(
1
)
where wi(t) is a i-th filter coefficient at time instant t and x(t) is a reference signal at time instant t.
Echo cancellation is achieved by subtracting the pseudo echo ŷ(t) from an echo signal y(t) as follows:
e
(
t
)=
y
(
t
)
−ŷ
(
t
) (2),
where e(t) is a residual signal.
The filter coefficient wi(t) is updated so as to minimize the residual signal e(t) as follows:
wi
(
t
+1)=
wi
(
t
)−&mgr;(
t
)
e
(
t
)
x
(
t−i
) (3) and
&mgr;(
t
)=&mgr;0
/PX
(
t
) (4),
where &mgr;(t) is a step size and &mgr;0 is a positive constant that has an effect on convergence rate and PX(t) is power of a reference signal x(t). The power PX(t) Is represented by
PX
(
t
)
=
∑
i
=
1
N
x
(
t
-
i
)
2
.
(
5
)
As is apparent from the above-described equations (1) through (5), generation of pseudo echo, power calculation of reference signal, and update of filter coefficient need the increasing amount of computation and memory in proportion to N. Further, It is known that the convergence rate of LMS-base algorithm such as the learning identification method is inversely proportional to N (see “Adaptive Filter Theory” by Simon Haykin, pp365-444, Prentice-Hall Information and System Sciences series, 1996).
On the other hand, a dispersive region in impulse response of an echo path is only a part of the impulse response region (see FIG.
7
). To eliminate an echo, it is necessary to estimate such a dispersive region by adjusting N tap filter coefficients of the adaptive filter. However, it is usually unknown when a dispersive region appears and therefore N is determined on the assumption of longest time interval. Since a dispersive region is only a part of the impulse response region, the filter coefficients provide a fixed delay section and a convergence section where coefficients after the dispersive region converge to zero. For the filter coefficients converging to zero, it is not necessary to calculate the equations (1) and (3). Therefore, these steps can be omitted. In other words, waste can be eliminated by dividing the N taps into valid taps to be subjected to the filter calculation and invalid taps not to be subjected to the filter calculation.
There has been disclosed a system estimator having a matrix switch connected between a series of delay elements and a series of coefficient circuits in Japanese Patent Application Unexamined Publication No. 4-245810. The matrix switch is controlled such that the respective positions of the taps are exchanged to intensively allocate valid taps to the dispersive region, resulting in increased convergence rate and reduced memory.
As an improved one, there has been disclosed a system estimator having a matrix switch connected between blocked delay elements and blocked filter circuits in Japanese Patent Application Unexamined Publication No. 10-229324. Hereafter, the blocked structure and operation of this conventional system estimator will be described with reference to
FIGS. 1 and 2
.
Referring to
FIG. 1
, echo cancellation is achieved by a subtracter
904
subtracting a pseudo echo ŷ(t) that is an output of an adaptive filter from an echo signal y(t), that is, e(t) =y(t)−ŷ(t), as described before. An adaptation controller
911
updates the filter coefficients of the adaptive filter so as to minimize the residual signal e(t).
The adaptive filter is provided with n input signal blocks
909
.
1
through
909
.n, m filter circuits
910
.
1
through
910
.m, a matrix switch
905
that connects the n input signal blocks
909
.
1
through
909
.n to the n filter circuits
910
.
1
-
910
.m under control of a controller
906
. An input signal x(t) Is divided into the n input signal blocks
909
.
1
through
909
.n, each block including k input signals. Each of the m filter circuits
910
.
1
-
910
.m inputs k signals through the matrix switch 905 and generates k filter coefficients corresponding to respective ones of the k signals.
The controller
906
monitors m blocks of filter coefficients to control the matrix switch
905
so as to select m blocks from n blocks and connect the selected blocks to respective ones of the m filter circuits.
Hereinafter, an input signal block that is connected to an appropriate one of the filter circuits
910
.
1
through
910
.m by the matrix switch
905
is called “valid block”. On the other hand, an input signal block that is not connected to any of the filter circuits by the matrix switch
905
is called “invalid block”. Such a switching operation of valid and invalid blocks, that is, selection operation of taps, is performed by the controller
906
as follows:
Selection 1: selecting one of the valid blocks that Is to switch from valid to invalid: and
Selection 2: selecting one of the invalid blocks that is to switch from invalid to valid.
FIG. 2
shows the functional configuration of the controller
906
for performing the above selection operation: Selection 1 and Selection 2.
In the processing of Selection 1, a minimum coefficient power block detector
813
detects the filter number BN and the position information POS of a block connected to the filter circuit providing the minimum coefficient power. The detected position information POS is stored in a shift register (or FIFO)
814
to append it to a queue stored in the shift register
814
,
In the processing of Selection 2, a maximum coefficient power block detector
820
detects the filter number BN and the position information POS of a block connected to the filter circuit providing the maximum coefficient power. The decision section
817
inputs the position information of a candidate from the shift register
814
and the position information corresponding to the filter circuit providing the maximum coefficient power from the maximum coefficient power block detector
820
and determines whether the candidate should switch from invalid to valid. For example, the determination is made depending on whether a difference between the position of the candidate and the position of the filter circuit providing the maximum coefficient power is smaller than a predetermined value (L). When it is determined that the candidate should switch from invalid to valid, the decision section
817
outputs the block number BN and the position information POS to a controller
815
to control the matrix switch
905
. Contrarily, when it is determined that the candidate should not switch from invalid to valid, the decision section
817
outputs the po
Foley & Lardner
NEC Corporation
Pham Tuan A
Tieu Binh
LandOfFree
Adaptive filter and echo canceller using the same does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Adaptive filter and echo canceller using the same, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Adaptive filter and echo canceller using the same will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3244256