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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a microphone apparatus.
2. Description of the Related Art
With a so-called camcorder, a lightweight television camera with an
incorporated video cassette recorder, for example, sound around an object
is recorded while the object is being pictured. In recording the sound,
the microphone is designed so that only the sound coming from the
direction of the object is recorded. That is, the camcorder is provided
with a directional microphone that picks up the sound coming into the
front of the camcorder.
One example of a microphone apparatus of this type is known as a "gun
microphone." This microphone is provided, as shown in FIG. 1, with a pipe
2 extending from a diaphragm 1. The pipe 2 is provided with many
through-holes 3 in its side wall, providing directionality so that the
microphone is highly sensitive to a sound coming from its front and long
along the center line of the pipe 2, or the opposite side of the diaphragm
1.
To be more specific, as shown in FIG. 1A, acoustic waves coming from the
front of the microphone (the right-hand in the figure) have the same path
length to the diaphragm 1 whether they arrive at it from the top of the
pipe 2 or any one through-hole 3, so that they arrive in the same phase to
be added together.
In contrast, as shown in FIG. 1B, acoustic waves coming from a side of the
pipe 2 through different through-holes 3 differ in phase because their
path lengths from the through-holes, or incident positions, to the
diaphragm 1 are different. Likewise, as shown in FIG. 1C, an acoustic wave
coming from the backside of the microphone arrives via different
through-holes 3 at the diaphragm 1, causing a phase difference in the
acoustic wave, or an incident signal. A plurality of holes 3 in the pipe 2
are arranged so that incident acoustic signals weaken each other. The
microphone shown in FIG. 1 has a directionality in which sensitivity is
low to acoustic waves coming from the side or back of the pipe.
Thus, the gun microphone as shown in FIG. 1 provides a directional
microphone having a high sensitivity to an acoustic wave coming from the
front of the microphone.
However, as described above, this microphone requires a pipe 2, which is
long, thereby increasing the microphone's external dimensions.
Additionally, this unidirectional microphone has a high sensitivity only to
acoustic waves coming from the front of the microphone, providing fixed,
inflexible directionality. This makes it difficult to record not only
sound coming from the desired direction of source, but also sound coming,
for example, from the sides of the camcorder.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a microphone
apparatus which is compact in size and easily provides desired
directionality.
In carrying out the invention and according to one aspect thereof, there is
provided a microphone apparatus comprising a first microphone 11 (this and
other reference characters below are identified in the accompanying
drawings) for recording a desired sound, a second microphone having
directionality in which sensitivity in the direction of the desired sound
is low, an adaptive filter means 24 to which a sound signal is supplied
from the second microphone, and a subtracting means 15 for subtracting an
output signal of the adaptive filter means 24 from a sound signal of the
first microphone 11, wherein the adaptive filter means 24 is adjusted to
minimize an output power of the subtracting means 15.
If the directions in which sounds to be recorded come are different, it
indicates that their sources are different and correlation between them is
often low. In the above-mentioned novel constitution, directionality of
the second microphone 21 is low in sensitivity in the direction of the
desired sound. Therefore, correlation is low between a sound signal from
the second microphone 21 and a sound signal from the first microphone 11.
If the sound signal from the second microphone 21 is assumed to be noise,
the microphone apparatus according to the invention has a constitution of
an adaptive noise reduction system. In this system, when the output power
of the subtracting means is minimized, the sound signal of the second
microphone 21 is removed from the sound signal of the first microphone 11,
providing only a desired sound from the first microphone 11 as an output
sound signal. The adaptive noise reduction system is disclosed in U.S.
Patent application Ser. No. 07/680,408 for example.
That is, the microphone apparatus according to an invention has the
adaptive noise reduction system which makes a distinction between desired
sound and noise depending on sound arrival direction wherein the
directionality of the second microphone 21 is arranged to make the system
mainly sensitive to the arrival direction of desired sound.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A-1C are a diagram illustrating an example of a prior-art microphone
apparatus;
FIG. 2 is a block diagram of an embodiment of the microphone apparatus
according to the invention;
FIG. 3 is a diagram illustrating an example of directionalities of the
first and second microphones;
FIG. 4 is a diagram illustrating an example of an adaptive filter circuit
of FIG. 2;
FIG. 5A-5C are diagram describing the operation of the microphone apparatus
according to the invention;
FIG. 6 is a diagram illustrating another example of the directionalities of
the first and second microphones;
FIG. 7 is a diagram illustrating still another-example of the
directionalities of the first and second microphones;
FIG. 8 is a diagram explaining an example of constituting the microphone
with a plurality of microphone units;
FIG. 9 is a diagram illustrating the example of constituting the microphone
with a plurality of microphone units; and
FIG. 10 is a diagram illustrating another example of a part of the
constitution of FIG. 9.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
For a general understanding of the features of the present invention,
references are made to the embodiment of the microphone apparatus
according to the invention as shown in FIG. 2.
Referring to FIG. 2, reference numeral 11 is a main input microphone for
recording a desired sound and reference numeral 21 is a reference input
microphone for picking up sound coming from a direction to be removed from
the recording. In this example, the arrival direction of desired sound is
mainly a direction indicated by an arrow AR in FIG. 3, or a direction from
up to down (hereinafter referred to as the front direction). This setup is
intended to implement a microphone apparatus which generally does not pick
up any sound coming from a direction (hereinafter referred to as a rear
direction) opposite to the front direction.
In the above-mentioned example, the main input microphone 11 is constituted
by an omnidirectional microphone as shown in FIG. 3, while the reference
input microphone 21 is constituted by a unidirectional microphone which is
mainly sensitive to the rear direction, not to the front direction or the
desired sound arrival direction as shown in FIG. 3.
A sound signal picked up by the main input microphone 11 and converted into
an electrical signal is fed to an A-D converter 13 through an amplifier 12
to be converted into a digital equivalent which is fed to a subtracting
circuit 15 through a delay circuit 14.
A sound signal picked up by the reference microphone 21 and converted into
an electrical signal is fed to an A-D converter 23 through an amplifier 22
to be converted into a digital equivalent which is fed to an adaptive
filter circuit 24. The output signal of the adaptive filter circuit 24 is
fed to the subtracting circuit 15. The output signal of the subtracting
circuit 15 is fed back to the adaptive filter circuit 24 and, at the same
time, converted into an analog signal by a D-A converter 16 to be fed to
an output pin 17.
It should be noted that the sound signal may be output without passing it
through the D-A converter 16, or the signal may be output in digital form.
The delay circuit 14 is provided to compensate a time delay required by
the adaptive filter circuit 24 for adaptive processing and a propagation
time in the filter.
The adaptive filter circuit 24 controls so that a reference input sound
signal approximates a sound signal other than that coming from the front
direction included in a main input sound signal, as will become apparant.
Consequently, if there is no correlation between a desired sound signal in
the sound signal picked up by the main input microphone 11 and a sound
signal other than that coming from the front direction, the sound signal
picked up by the reference input microphone 21 is subtracted by the
subtracting circuit 15 from the sound signal picked up by the main input
microphone, making the subtracting circuit 15 put out only the desired
sound signal.
In other words, the above-mentioned setup provides an adaptive noise
reduction system to which the output sound signal of the main input
microphone 11 is supplied as a main input and the output sound signal of
the reference input microphone 21 is supplied as a reference input. This
system operates as follows.
The main input sound signal from the A-D converter 13 is obtained by adding
the desired sound signal s coming from the direction of arrow AR or the
front direction to the sound signal n0 coming from the rear direction
(hereinafter referred to as a noise) which is supposed to have no
correlation with the main input sound signal. On the other hand, letting
the reference input sound signal from the A-D converter 23 be n1, then, as
seen from the above description, this reference input sound signal n1 has
correlation with the noise n0, not with the desired sound signal. An
adaptive processing algorithm makes the adaptive filter circuit 24 filter
the reference input sound signal n1 to output a signal y and controls the
adaptive filter circuit 24 so that a subtraction error e from the
subtracting circuit 15 is minimized.
Here, suppose that s, n0, and n1 are statistically stationary and their
average is 0, then an output is:
e=s+n0-y
Because there is no correlation between s and n0 and between s and y, an
expected value obtained by squaring this result becomes as follows:
##EQU1##
The adaptive filter circuit 24 is adjusted to minimize E [e.sup.2 ]. At
this time, E [s.sup.2 ] is not affected;
Emin [e.sup.2 ]=E [s.sup.2 ]+Emin [(n0-y).sup.2 ]
That is, minimizing E [e.sup.2 ] in turn minimizes E [(n0-y).sup.2 ],
making the output y of the adaptive filter circuit 24 equal to an
estimator of the noise n0. And an expected value of the output from the
subtracting circuit 15 becomes only the desired signal. In other words,
adjusting the adaptive filter circuit 24 to minimize a total output power
is equal to making the subtracting output e be a least square estimator of
the desired sound signal s.
Referring to FIG. 4, one embodiment of the adaptive filter circuit 24 is
exemplarily shown by using the algorithm of so-called LMS (Least Mean
Square).
As shown in FIG. 4, an adaptive linear coupler 300 of FIR filter type is
used in this example. This linear coupler comprises a plurality of delay
circuits DL1, DL2, . . . DLm (m is a positive integer) respectively having
a delay time Z.sup.-1 of unit sampling time, multipliers MX0, MX1, . . .
MXm for multiplying an output signal of each of the delay circuits DL1,
DL2, . . . DLm by the input signal n1, and an adder 310 for adding outputs
of the multipliers MX0 through MXm. An output of the adder 310 is
equivalent to y shown in FIG. 2.
A weight to be supplied to the multipliers MX0 through MXm is formed based
on the residual signal e coming from the subtracting circuit 15 in an LMS
computing circuit consisting of a microcomputer for example. An algorithm
to be executed in the LMS computing circuit 320 is as follows:
As shown in FIG. 4, let an input vector X.sub.k at time k be:
X.sub.k 32 [x.sub.0k x.sub.1k x.sub.2l . . . x.sub.mk ].sup.T
and an output be y.sub.k and the weight be w.sub.jk (j=0, 1, 2, . . . m),
then a relation between input and output is an shown in equation (1).
##EQU2##
If a weight vector W.sub.k at time k is defined as
W.sub.k =[w.sub.0k w.sub.1k w.sub.2k . . . w.sub.mk ].sup.T
then, the relation between input and output is given as
Y.sub.k =X.sub.k T.multidot.W.sub.k
Let a desired response be d.sub.k, then an error e.sub.k with the output is
represented as follows:
##EQU3##
With the LMS technique, the weight vector is updated by the following
relation:
W.sub.k+1 =W.sub.k +2.mu..multidot.e.sub.k .multidot.X.sub.k
where, .mu. is a step gain for determining adaptivity speed and stability.
Thus, the sound signal mainly consisting of the desired sound signal, with
the noise removed, appears on the output pin 17.
Meanwhile, to reduce the noise in the main input by using the reference
input by means of the adaptive processing as described above, there should
be no correlation between desired sound and reference noise as mentioned
above. For this reason, conventional adaptive noise reduction systems of
this type take such measures as preventing reception of a desired sound in
a reference input by sound-proofing the reference input microphone or
placing it as near a noise source as possible to separate it from a main
input microphone. However, these measures make the systems large and
inconvenient to move around.
In contrast, the present invention makes the distinction between desired
sound and noise depending on the sound arrival direction. And it is so
arranged that the main input microphone 11 has a directionality (including
non-directionality) in which a sound coming from the desired sound arrival
direction may be picked up and the reference input microphone 21 has a
directionality in which there is no or little sensitivity in the desired
sound arrival direction, thereby providing no correlation between the
desired sound in the sound picked up by the main input microphone 11 and
the noise picked up the reference input microphone 21.
Therefore, the present invention may only consider the directionalities of
the main input microphone and the reference input microphone. This makes
it possible to place both microphones in proximity, resulting in a compact
implementation as compared with the conventional microphone systems.
The constitution according to the present invention adequately eliminates
the noise signal from the main input, making it possible to easily
implement a microphone system having directionality in which there is no
or little sensitivity in the noise arrival direction. FIG. 5 illustrates
an effect brought about by an experimental system based on this example.
To be specific, in the above-mentioned experimental system, the main input
microphone 11 is placed in front of the reference input microphone 21,
both placed along the desired sound arrival direction indicated by the
arrow AR, as shown in FIG. 3. For a sound pickup operation, a
sinusoidal-wave signal of 1 kHz for example is introduced in the arrow AR
direction as a desired sound and a sinusoidal-wave signal of 600 Hz is
introduced in a direction 30 degrees to the rear side as a noise.
In this example, sensitivity of the omnidirectional main input microphone
is 0 dB and that of the reference input microphone 21 is -20 dB to a sound
coming from the front side, 0 dB to a sound coming from the rear side, and
-0.7 dB to a sound coming from a direction 30 degrees to the rear side.
An input waveform on the main input microphone 11 is a composite of the 1
kHz and 600 Hz sinusoidal waves as shown in FIG. 5A. An output sound
waveform appearing on the output pin 17 is as shown in FIG. 5B, which
approximates an ideal output sinusoidal wave of 1 kHz as shown in FIG. 5C,
proving the effect of the microphone apparatus according to the present
invention.
FIG. 6 and FIG. 7 respectively illustrate directional characteristics of
the main input microphone 11 and the reference input microphone 21 of
another embodiment of the present invention. In these examples, like the
above-mentioned example, the main input microphone 11 is placed in front
of the reference input microphone 21, both placed along the desired sound
arrival direction indicated by the arrow AR.
In the example of FIG. 6, the main input microphone 11 is unidirectional
and placed with its most sensible side in the front direction. The
reference input microphone is also unidirectional and is placed with its
most sensible side in the rear direction for example. In other words, the
reference input microphone 21 has a low sensibility in the desired sound
arrival direction and a high sensitivity in the rear direction or noise
arrival direction.
Consequently, the example of FIG. 6 also may implement a microphone
apparatus that outputs only a desired sound. In this example, if a noise
signal arrives at an angle between the rear direction and about 90 degrees
to it, a noise level in the main input becomes low because the sensitivity
of the main input microphone 11 is low at that angle. Therefore, the main
input microphone 11 itself contributes to noise reduction to some extent.
In the example of FIG. 7, the noise arrival direction is limited to around
90 degrees to the desired sound arrival direction and the sensitivity of
the reference input microphone 21 is made high in a direction 90 degrees
to the arrow AR direction. In this example, the reference input microphone
21 is bidirectional. As with the example of FIG. 6, the main input
microphone 11 is unidirectional and is placed so that its sensitivity
becomes highest in the desired sound arrival direction. The main input
microphone 11 may also be non-directional in this example.
The above-mentioned examples use single microphone units having the
discussed directional characteristics for the main input microphone 11 and
the reference input microphone 21. For these microphones, a plurality of
microphone units may also be used to implement respective microphones
having desired directionality.
Implementation of a unidirectional microphone system by using two
non-directional microphone units will be described as follows by referring
to FIG. 8 and FIG. 9.
Referring to FIG. 8, the non-directional microphone units 30 and 31 are
spaced by a distance d. As shown in FIG. 9, an output sound signal of the
microphone unit 30 is fed to a subtracting circuit 32 through an amplifier
not shown. Likewise, an output sound signal of the microphone unit 31 is
fed to the subtracting circuit 32 through an amplifier not shown and a
filter 33. In this example, the filter 33 comprises a resistor 34 and a
capacitor 35. Now, let resistance of the resistor 34 be R1 and capacity of
the capacitor 35 be C1, then R1 and C1 are set so that a relation shown
below is established:
Cl.multidot.Rl=d/c
where c stands for acoustic velocity.
Then, in this example, an output of the subtracting circuit 32 is sent as
an output sound signal to the output pin 37 through a frequency
characteristic correcting circuit 36 such as an integrator for flattening
the frequency characteristic of the signal. As will appear, this frequency
characteristic correcting circuit 36 is provided as required.
The microphones in this example operate as follows. As shown in FIG. 8, let
outputs of two microphone units 30 and 31 be P0 and P1 where a sound
source is located at angle--to the direction in which the two microphone
units are arranged and a sound arrives from the source at each microphone
unit, then output P1 is:
P1=P0.epsilon.-j.omega.(d/c)cos.theta.
where--w is an angular frequency.
The output of the microphone unit 31 is fed to the subtracting circuit 32
through the filter 33, so that an output signal Pa of the subtracting
circuit 32 is as given by equation (2):
##EQU4##
In the equation (2), A indicates a filter function of the filter 33, and
j.omega. d/c<<1.
In the equation (2), if equation (3) below is satisfied, the output Pa is
unidirectional:
##EQU5##
That is, if the equation (3) is satisfied, the equation (2) becomes:
Pa=P0.multidot.j.omega.(d/c)(1+cos.theta.)
making the output Pa unidirectional to angle .theta..
Meanwhile, in the above-mentioned example, the filter function A of the
filter 33 is represented by
A=1/(1+j.omega.Cl.multidot.Rl)
and is configured to be Cl.multidot.Rl)
and is configured to be Cl.multidot.Rl=d/c, so that
A=1/(1+j.omega.d/c)
Therefore, it is clear from the equation (3) that the microphone units in
the embodiment of FIG. 8 are unidirectional, provided that, however,
frequency characteristics of these microphone units are going upward to
the right (that is, the higher the frequency, the greater the response).
In this example, the frequency characteristic correcting circuit 36 is
provided to flatten this characteristic.
It should be noted that, in the example of FIG. 9, the filter 33, the
subtracting circuit 32, and the frequency characteristic correcting
circuit 36 may also be implemented by a digital filter or a program
(software).
For example, the filter 33 may be constituted by a digital filter
comprising an adder 41, a delay circuit 42, and a transfer function A
feedback amplifier 43 as shown in FIG. 10.
Although the microphone apparatus according to the present invention has
been described as applied to the microphone unit for the camcorder, the
present invention is also applicable to any microphone systems, including
a stand-alone microphone unit, a microphone for a professional-use video
camera, and an instrumentation microphone.
It should also be noted that, although, in the above-mentioned example, the
adaptive filter circuit 24 is constituted by a digital circuit to make the
entire system, digital, the filter circuit 24 may also be constituted by
an analog circuit to make the entire system analog. It is also possible to
make only the filter circuit 24 digital in an analog system.
Thus, according to the present invention, simply modifying the directional
characteristics of the first and second microphones may implement a
microphone system having desired directional characteristics. Further
substituting the second microphone with a microphone having a different
directional characteristic may change the directional characteristic of
the entire microphone system, thus providing wide freedom in
implementation of the directional characteristics. These features allow
the embodiments to be used in a variety of applications, bringing about a
remarkable practical effect.
Additionally, according to the present invention, the first and second
microphones may be placed in proximity to each other and they need not be
provided with a special shape such that of a gun microphone, thereby
providing a compact, easy-to-transport implementation.
While preferred embodiments of the invention have been described using
specific terms, such description is for illustrative purpose only, and it
is to be understood that changes and variations may be made without
departing from the spirit or scope of the appended claims.
* * * * *
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