Electrical audio signal processing systems and devices – Acoustical noise or sound cancellation – Counterwave generation control path
Reexamination Certificate
2000-08-18
2004-01-27
Mei, Xu (Department: 2644)
Electrical audio signal processing systems and devices
Acoustical noise or sound cancellation
Counterwave generation control path
C381S071110, C381S071100
Reexamination Certificate
active
06683960
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active noise control apparatus, and in particular to an active noise control apparatus for transmitting, from a loud speaker, a secondary noise (pseudo noise) synthesized so as to have the same amplitude as and the opposite phase to a primary noise (noise to be controlled) and to be suppressed small, and for canceling the controlled noise by acoustically overlapping therewith the secondary noise.
2. Description of the Related Art
FIG. 30
shows a prior art example of an active noise control apparatus of a feedforward type. In this example, after a noise flowing through a duct
200
toward an outlet on the right side is detected as a signal x
j
(j: sample time index) by a noise detecting microphone
201
, the signal x
j
is assumed to change into a noise g
j
by the time when the signal reaches an error detecting microphone
202
.
In this duration, a noise control filter
220
synthesizes a secondary noise G
j
with the detected noise x
j
and a coefficient vector H
j
from a coefficient updating circuit
240
. Obviously when G
j
=g
j
, the noise is canceled, so that an outputted noise from the outlet of the duct
200
becomes small.
The coefficient updating circuit
240
updates the coefficient of the noise control filter
220
in order that a signal E
j
outputted as a sum of a secondary noise −G
j
, to which a phase inversion is performed by multiplying “−1” at a multiplier
205
and which is transmitted from a loud speaker
203
, and the noise g
j
from the error detecting microphone
202
may become a minimum.
The secondary noise −G
j
outputted from the loud speaker
203
feeds back to the noise detecting microphone
201
through the duct
200
. At this time, a feedback control filter
210
is inserted in order to intercept the feedback path of the duct
200
leading to the noise detecting microphone
201
from the loud speaker
203
to prevent an occurrence of a howling.
Also, an error path filter
230
is a filter for simulating a characteristic of an error path leading to the coefficient updating circuit
240
from the output terminal of the multiplier
205
through the loud speaker
203
and the error detecting microphone
202
, and is used for a coefficient update of the noise control filter
220
.
At that time, it is necessary for the error path filter
230
to accurately simulate the error path.
From the structure of the prior art example shown in
FIG. 30
, it is seen that the noise g
j
is canceled when the coefficient of the noise control filter
220
is updated in order that the sum of the secondary noise −G
j
, to which the phase inversion is performed at the multiplier
205
and which is transmitted from the loud speaker
203
, and the noise g
j
, i.e. the output E
j
from the error detecting microphone
202
may become a minimum. The most typical algorithm applied to the coefficient updating circuit
240
which updates the coefficient vector H
j
is a Filtered-x NLMS method applying a general LMS (Least Mean Square) method, which is expressed by the following equation:
H
j
+
1
=
H
j
+
μ
E
j
X
j
&LeftDoubleBracketingBar;
X
j
&RightDoubleBracketingBar;
2
Eq
.
(
1
)
where, &mgr; is a constant called “step gain”, E
j
is the output of the error detecting microphone
202
, and X
j
is a vector expressed by the following Eq.(2) composed of “I” number of elements X
j
, X
j−1
, . . . X
j(I−1)
expressed by X
j
(
1
), X
j
(
2
), . . . , X
j
(I) and obtained by the output X
j
of the error path filter
230
, which simulates the characteristic of the error path leading to the coefficient updating circuit
240
from the output of the multiplier
205
through the error detecting microphone
202
, being retraced up to the past I−1 sampling periods.
X
j
=[X
j
(
1
)
X
j
(
1
), . . . ,
X
j
(
I
)] Eq.(2)
Also, the noise control filter
220
and the error path filter
230
are composed of a non-recursive type. In the following description, the number of taps in the filters
220
and
230
are expressed by “I” and “M” for the convenience's sake.
Assuming the Filtered-x NLMS method is a coefficient updating algorithm, the following coefficient of the error path filter
230
:
C=[
C(
1
)
C
(
2
)
. . .
C(
M
)
]
Eq.(3)
is required to be an estimated value approximated, with a high accuracy, to the following impulse response of the error path:
C=[C
(
1
)
C
(
2
) . . .
C
(
M
)] Eq.(4)
Generally, the coefficient of the error path filter
230
in
FIG. 30
is fixed and the calculation thereof is performed by the arrangement of providing, before starting the active noise control of
FIG. 30
, a white noise generated by a white noise generation circuit
250
, as shown in
FIG. 31
, to the loud speaker
203
and the error path filter
230
, providing the outputs of the error detecting microphone
202
and the error path filter
230
to a subtracter
251
, and providing the difference output to the coefficient updating circuit
240
.
Obviously, the problem of such an error path filter is that the white noise is outputted from a white noise generator
130
through the loud speaker to the outside of the duct upon the calculation of the coefficient. In spite of a temporary occurrence, it is not preferable that a noise of another kind is outputted from the active noise control apparatus in that way.
It is also a problem that the coefficient thus calculated of the error path filter
230
is hereafter to be fixed upon the active noise control as shown in FIG.
30
. This is natural because the change of the characteristic within the duct
200
after the calculation can be fully expected.
In fact, it is known that when the coefficient of the noise control filter
220
is updated by using the calculation result to decrease the noise at the position of the error detecting microphone
202
, a reflection position of the noise moves from the outlet end of the duct
200
to the position of the error detecting microphone
202
to change the acoustic characteristic within the duct
200
so that the error path filter
230
fails to accurately simulate the above-mentioned error path.
This simulating operation with a lowered accuracy may have a bad influence on the accuracy maintenance of the coefficient of the noise control filter
220
, whereby a sufficient quantity of the noise reduction can not be obtained, and besides the noise control operation becomes unstable. The fact that the acoustic characteristic within the duct changes along with the decrease of the estimation error of the coefficient of the noise control filter
220
indicates that the coefficient correction of the error path filter
230
is required to be repeatedly performed with the active noise control being kept operated.
For estimating the coefficient of the error path filter
230
during the active noise control, a method using a circuit arrangement shown in
FIG. 32
is known.
Namely, the white noise generated by the white noise generation circuit
250
is added to the secondary noise −G
j
from the multiplier
205
at an adder
252
to be outputted from the loud speaker
203
, so that the coefficient of the error path filter
230
is updated in order that the output of the subtracter
251
may become a minimum by applying thereto the coefficient updating circuit
240
which is not operating or has become available by not performing (by fixing) the coefficient update of the noise control filter
220
, different from the example of FIG.
30
.
In this arrangement, since the coefficient of the error path filter
230
gives the impulse response of the error path whose characteristic has been changed by the noise control at the time when the output of the subtracter
251
has become a minimum, the coefficient at this time has only to be used at the circuit of FIG.
30
.
However, in such a circuit arrangement, as mentioned above, the overlap of the white noise outputted from the l
Fujii Kensaku
Ohga Juro
Fujitsu Limited
Katten Muchin Zavis & Rosenman
Mei Xu
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