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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a noise reducing microphone apparatus and, in
particular, to such an apparatus for reducing noise components in
microphone outputs.
2. Description of the Prior Art
Most of microphones are configured to convert changes in sound pressure of
an acoustic wave to mechanical vibration of a diaphragm and to activate an
electro-acoustic transducer system on the basis of the vibration.
Therefore, if a factor affects the diaphragm when sound is picked up by
the microphone, a noise is produced.
If the factor is wind, a noise by wind (hereafter referred to as a wind
noise) is produced, and if the factor is vibration, a noise by vibration
(hereafter referred to as a vibration noise) is produced.
There are, for example, the following existing techniques for reducing a
wind noise:
(1) the use of a windscreen
(2) the use of an electro-acoustic high pass filter
(3) the use of an arrangement exploiting a non-directional property in low
sound ranges
There are, for example, the following existing techniques for reducing a
vibration noise:
(1) the use of a vibration isolating mechanism
(2) the use of a non-directional microphone element
(3) an analog noise-canceling method
The above-mentioned existing techniques for reducing a wind noise involve
the following problems:
(1) In the case where a windscreen is used, in general, as the outer
dimension of the windscreen increases and as the distance between the
microphone and the inner wall of the windscreen increases, a wind noise
decreases. However, the size of the microphone apparatus increases.
(2) Since a wind noise mainly consists of low band components, it is
certainly effective for reducing the wind noise by using a high pass
filter. However, since low band components of the sound itself are also
cut in addition to the wind noise, the sound pickup quality is decreased.
(3) With a non-directional microphone, in comparison with a directional
microphone, the level of a wind noise decreases more. Practically,
however, because of the effect of a casing surrounding the microphone, the
noise is not decreased to a sufficiently low level by employing an
"arrangement exploiting a non-directional property in low sound ranges".
Therefore, under the present circumstances where both smaller dimension of
a microphone and higher sound pickup quality of the microphone are
desired, more reduction of a wind noise is difficult with only the
existing techniques. This also applies to a vibration noise.
On the other hand, as a technique for eliminating a noise incorporated into
a signal, adaptive noise cancelling is known (B. Widrow et al. "Adaptive
noise cancelling: principles and applications" Proc. IEEE, vol. 63, no.
12, pp. 1692-1716, Dec. 1975.).
According to the technique, it is necessary to supply noise components
which are strongly correlated with a noise to be eliminated as a reference
input signal. However, it is very difficult in a small apparatus to supply
only noises such as a wind noise as a reference input which is received
from the same direction as necessary sounds.
OBJECTS AND SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a noise reducing
microphone apparatus that can be small-scaled and can reliably eliminate a
wind noise, a vibration noise, and so on.
According to an aspect of the invention, there is provided a noise reducing
microphone apparatus having an adaptive noise canceller which has a
primary input and a reference input and in which the reference input
signal is subtracted from the primary input through an adaptive filter and
the adaptive filter is adaptively controlled by an output signal resulted
from the subtraction, comprising:
a pair of microphone units disposed in proximate locations; and
subtracting means for performing subtractions of outputs from the pair of
microphone units,
wherein an output from one of the microphone units is supplied as the
primary input signal of the adaptive noise canceller and a differential
output from the pair of microphone units is supplied as the reference
input signal of the adaptive noise canceller.
Outputs from a pair of microphones disposed in proximate locations
originally include an audio signal component and a noise component (for
example, noise component caused by wind). These outputs from the
microphones undergo subtraction. As a result, the output from one of the
microphones includes the audio signal component and the noise component
and a differential output from the pair of the microphones include only a
noise component. The output including the audio component and the noise
component is used as the primary input while the differential output
including only the noise component is used as the reference input.
The reference input is adaptively processed to equalize with the noise
component in the primary input. The adaptively processed reference input
is subtracted from the primary input. As a result, only the noise
component is canceled from the primary input, and the audio signal
component is output in the original form.
The above, and other, objects, features and advantages of the present
invention will become readily apparent from the following detailed
description thereof which is to be read in connection with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an embodiment of the invention;
FIG. 2 is a block diagram of an arrangement of an adaptive filter;
FIG. 3 is a diagram showing the frequency spectrum of a wind noise
component;
FIG. 4 is a diagram showing the rate of correlation of wind noise
components picked up by a pair of microphones;
FIG. 5 is a diagram showing an example of a differential output of the wind
noise components picked up by the pair of microphones;
FIG. 6 is a waveform diagram showing the noise reducing effects;
FIG. 7 is a block diagram showing a first modification of the embodiment;
FIG. 8 is a block diagram of a second modification of the embodiment;
FIG. 9 is a block diagram of another embodiment of the invention; and
FIG. 10 is a block diagram of a modification of another embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the invention are explained below with reference to FIGS. 1
to 10.
FIGS. 1 to 8 are views illustrating an embodiment of the invention.
A pair of microphones 1 and 2 disposed in close locations detect ambient
sound together with a wind noise, and output it in the form of an
electrical signal. Since the microphones 1 and 2 are disposed in close
locations, the same sound and wind noise are detected, and they are output
in the form of electrical signals. FIG. 3 shows an example of a frequency
spectrum of a wind noise component included in the outputs from the
microphones 1 and 2. It is known from FIG. 3 that the wind noise mainly
consists of low band components.
The microphones 1 and 2 may be oriented in the same direction or,
alternatively, they may be oriented in the opposite directions if the
distance between the microphones 1 and 2 is within the wavelength defined
by the frequency of a desired signal. An electrical signal output from the
microphone 1 is supplied to an A/D converter 3 while an electrical signal
output from the microphone 2 is supplied to an A/D converter 4.
The A/D converters 3 and 4 convert the electrical signals supplied from the
microphones 1 and 2 to digital signals. The digital signal converted by
the A/D converter 3 is used as a primary input expressed by (S+n). The
digital signal converted by the A/D converter 4 is expressed by (S+(n*)).
In the digital signals, S represents the audio signal component while n
and (n*) represents the wind noise component. The noise component n has an
additive property while the noise component (n*) is correlative with the
noise component n in the primary input (S+n).
The primary input (S+n) is supplied to a delay circuit 7 provided in an
adaptive noise canceler 6. The primary input (S+n) is also supplied to an
adder 5. In addition, an output of the A/D converter 4 is supplied to the
adder 5.
The adder 5 adds the primary input (S+n) to the output of the A/D converter
4 attached with a negative sign, that is, ›-(S+(n*))!. Since the audio
signal components S have sufficiently long wavelengths, they have
substantially the same phase in the near place. Therefore, the audio
signal components S are eliminated by executing subtraction. Accordingly,
a reference input expressed by (n-(n*)) is created.
Explained below is creation of the reference input (n-(n*)).
FIG. 4 shows an example of coherence of the wind noise component generated
in the pair of microphones 1 and 2. It has been known, as shown in FIG. 4,
that, in general, wind noise components produced in two acoustic terminals
represent a low correlation even if the terminals are proximately located.
Therefore, a difference between outputs from the microphones 1 and 2 does
not become zero, and creation of the reference input (n-(n*)) is possible.
FIG. 5 shows a frequency spectrum of the reference input (n-(n*)). The
reference input (n-(n*)) is supplied to an adaptive filter 9 in the
adaptive noise canceler 6.
The delay circuit 7 in the adaptive noise canceler 6 outputs the primary
input (S+n) after a delay of a predetermined time. The amount of the delay
is equivalent to a time delay required for computation for adaptive
processing or to a time delay in the adaptive filter 9, and so on, and can
be set adequately in accordance with the arrangement of a system. The
primary input (S+n) which has passed the delay circuit 7 is supplied to an
adder 8.
The adder 8 executes addition of the output from the delay circuit 7 and a
signal Y attached with a negative sign and output from the adaptive filter
9 which will be described later. The signal Y, as explained later, is a
component analogous to the noise component n in the primary input (S+n).
Therefore, the signal Y, which is a component analogous to the noise
component n, is subtracted from the primary input (S+n) by the adder 8,
and the audio signal component S remains. In other words, the noise
component n in the primary input (S+n) is minimized.
The audio signal component S is supplied to a D/A converter 10 and also fed
back to the adaptive filter 9. The audio signal component S expressed in
the form of a digital signal is converted to an analog signal by the D/A
converter 10, and it is taken out from a terminal 11.
FIG. 6 shows a result of noise reduction by the foregoing embodiment. FIG.
6 illustrates the main input (S+n), that is, the output from the
microphone 1, shown by a solid line, and a system output, that is, the
output from the adaptive noise canceler 6, by a broken line. A sine wave
of 500 Hz which is a pseudo representation of the audio signal component S
is added.
It is known from FIG. 6 that the decrease of the level of the signal
(broken line in FIG. 6), which is the output from the adaptive noise
canceler 6, is remarkable as compared with the level of the noise
component n (solid line in FIG. 6) in the output from the microphone 1. It
is also known that the sine wave of 500 Hz maintains its level regardless
of the presence or absence of the adaptive noise canceler 6.
Explained below is operation of the adaptive filter 9 of the adaptive noise
canceler 6.
The adaptive filter 9 creates the signal Y as a component analogous to the
noise component n in the primary input (S+n). That is, its filtering
characteristic is automatically adjusted from time to time so that the
output from the adaptive noise canceler 6 resembles the audio signal
component S in the primary input (S+n).
An adaptive linear coupler of an FIR filter type shown in FIG. 2 is used as
the adaptive filter 9. In the construction of FIG. 2, DL1 to DLL denote
delay circuits, and MP1 to MPL denote coefficient multipliers. Reference
numeral 16 refers to an adder, and 15 and 17 to input/output terminals.
›Z.sup.-1 ! in the delay circuits DL1 to DLL represents a delay of a unit
sampling time, and W.sub.nk supplied to the coefficient multipliers MP1 to
MPL represents a weighting coefficient. If the weighting coefficient
W.sub.nk is fixed, the filter behaves as a normal FIR digital filter.
Explained below is an algorithm for adaptively activating the adaptive
filter 9. Although various algorithms may be used for computation in the
adaptive filter 9, the following explanation is directed to LMS (least
mean square), which is practical and often used because of a relatively
less amount of computation:
If an input vector X.sub.k is expressed by:
X.sub.k =›X.sub.k X.sub.k-1 X.sub.k-2 . . . X.sub.k-L !
an output Y.sub.k from the adaptive filter 9 is given by:
##EQU1##
Let an output from the delay circuit 7 be d.sub.k, then its differential
output ›residual output! is:
.epsilon..sub.k =d.sub.k -X.sub.k.sup.T W.sub.k
By the LMS (least mean square) method, renewal of the weighting vector
W.sub.k is performed in accordance with the following equation:
W.sub.k+1 =W.sub.k +2.mu..epsilon..sub.k X.sub.k
.mu. in the foregoing equation is a gain factor determining the speed and
stability of the adaptation, which is so called a step gain.
By renewing the weighting vector from time to time as explained above, the
device behaves to minimize the output power of the system. This operation
is explained below in a formulated manner. When the delay circuit 7 is
disregarded for simplification, the differential output .epsilon. from the
adder 8 is:
.epsilon.=S+n-Y
An expected value of square of (.epsilon.) is expressed by:
E›.epsilon..sup.2 !=E›S.sup.2 !+E›(n-Y).sup.2 !+2E›S(n-Y)!
Since S is not correlative with n and Y, in the above equation,
E›S(n-Y)!=0
Therefore, the expected value E›.epsilon..sup.2 ! of square of (.epsilon.)
is expressed by:
E›.epsilon..sup.2 !=E›S.sup.2 !+E›(n-Y).sup.2 !
Although the adaptive filter 9 is adjusted to minimize E›.epsilon..sup.2 !,
E›S.sup.2 ! is not affected. As a result,
Emin›.epsilon..sup.2 !E›S.sup.2 !+E›(n-Y).sup.2 !
Since E›S.sup.2 ! is not affected, minimization of E›.epsilon..sup.2 !
means minimization of E›(n-Y).sup.2 !. Therefore, the output Y of the
adaptive filter 9 is an optimum estimated value of least square of ›n!.
When E›(n-Y).sup.2 ! is minimized, E›(.epsilon.-S).sup.2 ! is also
minimized because ›.epsilon.-S=n-Y!. Therefore, minimization of the entire
output power by adjusting the adaptive filter 9 is equivalent to making
the differential output .epsilon. be an optimum estimated value of least
square of the audio signal component S.
The differential output .epsilon., in general, includes a certain amount of
noise component in addition to the audio signal component S. Since the
noise component output is defined by (n-Y), minimization of
E›(.epsilon.-Y).sup.2 ! is equivalent to maximization of signal-to-noise
ratio of the output.
FIG. 7 shows a first modification of the foregoing embodiment. The first
modification is based on the frequency spectrum of a wind noise component
being concentrated in low bands. Circuits elements common to those in the
foregoing embodiment are labeled with the same reference numerals, and
their redundant explanation is omitted.
The first modification is different from the foregoing embodiment in that a
line 23 connecting the output of the microphone 1 to the terminal 11 is
provided and that a high pass filter 22 is interposed in the line 23.
Further, low pass filters 21 are interposed between the microphones 1, 2
and the A/D converters 3, 4, when necessary. The low pass filter 21 may be
interposed between the terminal 11 and the D/A converter 10 in the output
site of the system, and the other terminal of the line 23 may be coupled
between the low pass filter 21 and the terminal 11.
This arrangement makes it possible to obtain an audio signal component S
which is mixture of a low band audio signal component S.sub.L, in which
the wind noise component has been reduced by the adaptive noise canceler
6, and a high band audio signal component S.sub.H, which is obtained from
the microphone 1 through the high pass filter 22 and from which the wind
noise component has been cut. The other arrangements, their operations and
effects are equal to those of the foregoing embodiment, and their
redundant explanation is omitted.
FIG. 8 shows a second modification of the foregoing embodiment. The second
modification is different from the foregoing embodiment in that the adder
5 is replaced by an analog adder 25 and that the analog adder 25 is
located between the microphones 1, 2 and the A/D converters 3, 4. That is,
a reference input is in an analog form. The other arrangements, their
operations and effects are equal to those of the foregoing embodiment.
Elements common to the foregoing embodiment are therefore labeled with the
same reference numerals, and their redundant explanation is omitted.
According to the embodiment, the primary input (S+n) and the reference
input (n-(n*)) are created on the basis of the outputs from the pair of
microphones 1 and 2 disposed in close locations. In the adaptive filter 9,
the signal Y analogous to the noise component n in the primary input (S+n)
is created on the basis of the reference input (n-(n*)). By subtracting
the signal Y from the primary input (S+n) by the adder 8, the noise
component n is canceled, and the audio signal component S is output.
Therefore, by using a pair of normal microphones 1 and 2, a wind noise
component can be canceled without using a windscreen. In addition, since
the microphones 1 and 2 are disposed in close locations, the embodiment
contributes to scale reduction of the apparatus. In regard of cancellation
of a wind noise component, since no electroacoustic high pass filter is
required, deterioration of the sound pickup quality is prevented.
Moreover, since the adaptive noise canceler 6 is used, the characteristic
of the adaptive filter 9 is automatically renewed, regardless of changes
in the wind noise characteristic (for example, level or spectral
distribution, and so on), and the wind noise component can be reduced in a
stable manner.
FIGS. 9 and 10 show another embodiment. The embodiment is different from
the foregoing embodiment in that not only a wind noise but also a
vibration noise caused by vibrations are taken into consideration. That
is, as shown in FIG. 9, there are provided a vibration sensor 31 for
detecting vibrations and an A/D converter 32 for converting an analog
output from the vibration sensor 31 into a digital signal. The adder 5
shown in the foregoing embodiment is replaced by an adder 33 which can
perform addition and subtraction of three inputs. Elements common to those
of the foregoing embodiment are labeled with the same reference numerals,
and their redundant explanation is omitted.
Outputs from the microphones 1 and 2 respectively include an audio signal
component S and a noise component including a wind noise and a vibration
noise.
An electrical signal output from the microphone 1 is supplied to the A/D
converter 3 and converted into a digital signal by the A/D converter 3. As
a result, a primary input is created. The primary input is supplied to the
delay circuit 7 in the adaptive noise canceler 6. The primary input is
also supplied to the adder 33.
An electrical signal output from the microphone 2 is supplied to the A/D
converter 4 and converted into a digital signal by the A/D converter 4.
The digital signal is supplied to the adder 33.
A vibration component detected by the vibration sensor 31 is converted into
a digital signal by the A/D converter 32. The digital signal is supplied
to the adder 33.
The adder 33 adds outputs from the A/D converters 3 and 32 to the output
from the A/D converter 4 attached with a negative sign. As a result of the
addition and subtraction, the audio signal component S is eliminated, and
a noise component consisting of the wind noise and the vibration noise is
created for use as a reference input. After this, a signal Y is created on
the basis of the reference input. The signal Y is subtracted from the
primary input by the adder 8, which results in canceling the noise
component consisting of the wind noise and the vibration noise, and the
audio signal component S is output.
Excepting that the noise component consists of the wind noise and the
vibration noise and that both the wind noise and the vibration noise can
be canceled, the other arrangements, their operations and effects of
another embodiment are equal to those of the foregoing embodiment, and
their redundant explanation is omitted.
FIG. 10 shows a modification of another embodiment. This modification is
different from another embodiment in that the adder 33 is replaced by an
analog adder 35 and that the analog adder 35 is located between the
microphone 2 and the A/D converter 4.
Since the other arrangements, their operations and effects are equal to
those of another embodiment and the second modification of the foregoing
embodiment, common elements are labeled with the same reference numerals,
and their redundant explanation is omitted. Although not illustrated, the
same arrangements as those of the first modification of the foregoing
embodiment may be employed in another embodiment.
Another embodiment has, in addition to those of the foregoing embodiment,
the arrangement in which vibrations are detected by the vibration sensor
31, and the vibration component detected by the vibration sensor 31 is
supplied to the adder 33. Therefore, the reference input consisting of the
wind noise and vibration noise is created. On the basis of the reference
input, the adaptive filter 9 creates the signal Y analogous to the noise
component in the primary input. When the signal Y is subtracted from the
primary input by the adder 8, the noise component is canceled, and the
audio signal component S is output.
Therefore, in addition to the effects of the foregoing embodiment, another
embodiment can cancel the vibration noise component, and can realize an
excellent sound pickup quality with a single processing system without
preparing different processing systems for different kinds of noises.
Another embodiment has been explained as being directed to a noise
component consisting of a wind noise and a vibration noise. However, it is
not limited to this, but may target only a vibration noise.
The noise reducing device shown in any of the embodiments is applicable to
various kinds of recording systems. For example, it is applicable to a
small-scaled portable video camera apparatus to detect and eliminate
vibrations caused by a user, vibrations caused by mechanical systems, and
so on, in addition to a wind noise. Further, the pair of microphones 1 and
2 used in the embodiments may be either directional or non-directional.
Having described specific preferred embodiments of the present invention
with reference to the accompanying drawings, it is to be understood that
the invention is not limited to those precise embodiments, and that
various changes and modifications may be effected therein by one skilled
in the art without departing from the scope or the spirit of the invention
as defined in the appended claims.
The noise reducing microphone apparatus according to the invention has the
effect that a wind noise component can be cancelled without using a
windscreen. Close positional relationship between the pair of microphones
contributes to scale reduction of the apparatus. Because of no
electro-acoustic high pass filter or the like being required,
deterioration of the sound pickup quality is prevented.
Further, the use of the adaptive noise canceler gives the effect that the
characteristic of the adaptive filter is automatically renewed, regardless
of a change in the nature of a wind noise (for example, level or spectral
distribution, etc.), and the wind noise component is stably reduced.
In addition, a vibration noise component can be canceled. Further, an
excellent sound pickup quality can be realized with a single processing
system without using different processing systems for different kinds of
noises.
* * * * *
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