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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to method and apparatus for noise canceling
and noise reducing by attenuating unwanted ambient noise from reaching the
eardrum and canceling background acoustic noise received from a boom
microphone or directional microphone, when used with a headset or boom
headset or the like.
The invention further relates to an active noise reduction system for use
in headsets, particularly in the earphone vicinity where the system
utilizes a sensor microphone to detect unwanted, background noise. This
noise signal outputted by the sensor microphones is processed by
electro-acoustical means to produce an inverted signal so that a quiet
zone is created in an acoustical waveguide located between the output
transducer, and the eardrum. Therefore the desired original audio signal
is not disturbed by noise when transmitted to the ear of the user. The
acoustical waveguide absorbs any sound returning to the microphone from
the ear (preventing feedback) and deadens any sound returning from the
microphone to the ear.
This invention also relates to a noise cancellation apparatus, for use with
a telephone handset or a boom microphone or directional microphones or the
like, where the system utilizes two microphones, a first microphone for
receiving sound comprised of speech and background noise, and a second
microphone for receiving sound comprised of substantially background
noise, with the means for subtracting the second signal from the first
signal.
The microphone in the noise cancellation system of the present invention
utilizes a two terminal system, in which the output audio signal comprised
of speech and the power support input used to drive the system are
transmitted on one terminal and the second terminal is grounded.
The noise cancellation apparatus of the present invention also relates to a
directional microphone used in a far-field microphone device having the
ability to accept acoustical sounds in certain directions better than in
other directions.
The noise cancellation and noise reduction system of the present invention
may be enhanced by the inclusion of an automatic audio microphone
transmission feature, a sidetone feature to transmit a portion of the
signal to the earcup of the speaker, and a feature to convert an active
noise cancellation microphone to a standard omni-directional microphone by
removing voice microphone from the circuit, and the increasing the gain of
the noise microphone amplifier. This enhancement allows all audio from
external surroundings to be transmitted to the earcup of the speaker by
increasing the sidetone channel gain without the addition of any other
microphone elements.
2. Description of the Prior Art
As is to be appreciated, in numerous situations, the presence of background
acoustic noise is undesirable. As an example, consider the situation in
which an operator is attempting to conduct a telephone conversation from a
telephone or such similar device located in a noisy area. In this
situation, loud acoustic background noise is received by a microphone in
the handset of the telephone and converted to an electrical signal which
is supplied to the telephone(s) of the person(s) having the conversation
with the operator and is converted thereat to an acoustic signal. As a
result, the person to whom the operator is communicating constantly hears
the loud background noise. Further, when the person is speaking, such
speech is combined with the background noise and, as such, may be
difficult for the other person(s) to understand. As a result, the operator
may have to shout into the microphone of the telephone. Furthermore, the
signal representing the background noise is also supplied from the
microphone in the operator's handset to the speaker in the operator's
handset as sidetone. Thus, the operator also constantly hears the
background noise from the speaker in the operator's handset and, when the
other person is speaking, may impair the understanding thereof.
As another example, consider the situation in which a pilot who is
operating a helicopter or the like wishes to communicate with another
person by way of radio frequency (RF) communication. In this situation,
the pilot typically speaks into a so-called boom microphone or boom
headset Which is coupled to a radio transmitting/receiving device
whereupon the speech is converted into RF signals which are transmitted to
a second receiving/transmitting device and converted therein to speech so
as to be heard by the other person(s). As with the above situation of a
telephone located in a noisy area, the loud background noise from the
helicopter is received and converted into an electrical signal by the boom
microphone or headset device and thereafter supplied to the receiving
device. As a result, the person(s) communicating with the pilot hears the
loud background noise. This may be particularly annoying when the pilot
leaves the radio transmitting/receiving device in the "ON", (the hot mike)
position while operating the helicopter.
As yet another example, consider voice verification and/or recognition
systems into which an operator must speak for access, for instance to a
physical facility or, to operate a computer or automatic teller machine.
Background noise can prevent access (no recognition or verification due to
background noise) or can provide false access by false verification.
In an attempt to reduce background noise so as to improve performance of a
telephone or a boom microphone or headset or the like located in a noisy
environment or the like, pressure gradient microphones may be utilized.
Basically, a pressure gradient microphone responds to the difference in
pressure at two closely spaced points. When used in an environment where
the pressure gradient of the background noise is isotropic, the electrical
signal produced by the pressure-gradient microphone due to such background
noise is effectively zero. However, in most actual situations, the
pressure gradient of the background noise is not isotropic and, as a
result, in these situations, the performance of the pressure-gradient
microphone is adversely affected. Additionally, since voice or speech
propagates in more than one direction, the electrical signal produced by
the microphone which corresponds thereto is often degraded. Thus, even if
a pressure gradient microphone is utilized in either a telephone handset
or a boom microphone, the desired amount of background noise cancellation
may not be sufficient and the performance may not be adequate.
Furthermore, since two opposite sides of a pressure-gradient microphone
respond to acoustic pressure, as previously mentioned, the handset of an
existing telephone would have to be substantially modified so as to enable
these two sides of the microphone to respond to the acoustic pressure.
Moreover, as a result of using such a microphone in a telephone handset,
the electrical signals produced therefrom should be amplified. Thus, to
replace the conventional microphone in a telephone handset of an existing
telephone with a pressure-gradient microphone would typically necessitate
replacing the handset with a new handset and, as such, would be relatively
expensive.
As an alternative to using pressure-gradient microphones, an acoustic
feed-back type system may be utilized. Such a system normally includes
compensation filters which are used to equalize the transfer function of
the output transducers. Since the characteristics of the speakers are
tightly controlled by these filters, the cost of the filters is relatively
high. As a result, such acoustic feed-back systems are typically
relatively expensive.
Many microphones used with noise cancellation and noise reduction apparatus
are inherently nondirectional or omnidirectional, such as the
electrostatic, piezoelectric, magnetic and carbon microphones. With
omnidirectional small microphones, at low frequencies there is sufficient
diffraction of sound around the microphone so that diaphragm motion is
insensitive to the direction of the sound. At high frequencies, and
correspondingly shorter wavelengths, the microphone becomes acoustically
larger and shows a preference for sound arriving perpendicular to the
diaphragm. Thus, the smaller in size of the microphone, the higher in
frequency its behavior remains omnidirectional. Hence, the omnidirectional
microphones are small compared to the wavelength and the microphone case
shields the rear side of the diaphragm from receiving certain sound waves
at different angles. As a result, these prior art microphones are referred
to as pressure microphones since pressure is a scaler, and not a vector
quantity. Thus, a directional microphone response able to increase the
sensitivity of sound in a far-field region from a variety of directions is
desired for a microphone device in an active noise cancellation system.
That is, to achieve a directional microphone response by adding the
outputs of the omnidirectional pattern and bidirectional or "figure-eight"
pattern, and then simply adjusting the amplitude and phase of the summed
output signal to produce the desired pattern. The figure-eight pattern is
also known as a cosine pattern and is mathematically expressed a p=COS
.theta., in polar coordinates. In directional microphones, distance is a
factor. The distance factor measures how much farther away from a source a
directional microphone may be used, relative to an omnidirectional
pattern, and still preserve the same ratio of direct to reverberant
pickup. Thus, the prior art has failed to provide a directional microphone
in an active noise reduction apparatus based on the omni-directional
patterns and the cardioid patterns where the sound pressures arriving at a
determined point are added vectorially.
In devising the circuitry for an active noise cancellation apparatus for
use with a boom microphone device or a directional microphone device
comprising at least two microphones, it is known to use a three terminal
microphone configuration. That is, a noise cancellation system having two
or more microphones connected to an amplifier, for example, requires
circuitry having three terminals: a power supply input terminal, an audio
signal output terminal, and a ground terminal. In an effort to reduce the
complexity and cost of the noise cancellation system utilized in the
microphone, or boom microphone or the like which optionally may be used
with a headset of the noise reduction apparatus, a two terminal microphone
configuration is desired. It is desired to have a microphone configuration
where the DC voltage supplied from a power supply is inputted on the same
terminal as the AC audio signal outputted from the microphones, whereby
the AC signal is superimposed on the DC signal. Thus, the prior art has
failed to provide a two terminal microphone configuration for use in an
active noise cancellation apparatus, where the power and signal are
superimposed on the first terminal and the second terminal is grounded
In yet a further attempt to reduce background noise so as to improve the
intelligibility of electro-acoustic communication using headsets with a
microphone, a technique has been developed, called active noise reduction
that utilizes a sensor microphone placed between the speaker and the ear
in the sound field of the speaker, and which senses the background noise
and programs audio. With this active type headphone device, a negative
feedback loop is used whereby the electrical signals converted from the
external noises by a microphone unit are fed back in a reverse phase for
reducing the noise in the vicinity of the headphone unit. A feedback
circuit utilizing a closed loop system as shown in the prior art provides
a "quiet zone" between the speaker and the ear which eliminates the
background noise. This is because in a noisy environment, the ear will
detect not only the output of the speaker, but also the background noise.
Reference is made to the following documents providing a closed loop active
noise reduction system, which documents are hereby incorporated by
reference:
U.S. Pat. No. 2,972,018 to Hawley et al.
U.S. Pat. No. 3,098,121 to Wadsworth
U.S. Pat. No. 4,833,719 to Carme et al.
U.S. Pat. No. 5,138,664 to Kimura et al.
Japanese Patent Abstract No. 3-161999 to Saeki.
The above-referenced patents illustrate a variety of noise canceling
devices. For instance, Hawley et al. relates to a noise reduction system
for earphones having a plastic casing located between the speaker and the
microphone; Wadsworth provides an earphone having a microphone located on
top of the headband; Carme et al. is directed to an earphone having a
hollow annular part located between the speaker and the microphone; Kimura
et al. calls for a noise reduction headphone having a cup member located
between a speaker and a microphone; and Saeki relates to a noise canceling
headphone having a microphone located between two oppositely facing
loudspeakers.
However, there exist various disadvantages in the conventional active noise
reduction systems. The prior active noise cancellation systems, for
instance, utilize closed loop-type circuits governed by the associated
equations:
##EQU1##
where P=output
S=standard audio signal
H.sub.1 =high pass filter
H.sub.2 =speaker at headset
N=noise component
B=variable gain/phase control
The conventional closed loop noise reduction system is not ideal as a very
large direct transmission gain (1+BH1H2) is required in order to reduce
the noise component (N) to zero at the output (P). This system suffers
from the problem of instability. This creates drawback of oscillation,
i.e., squealing due to the unstable loop conditions caused by variations
in the transfer function of the speaker, feedback microphone and acoustic
cavity containing these elements and user headgear. The degree of noise
cancellation generated by the conventional closed loop noise reduction
device, at any frequency, is directly related to the direct transmission
gain at that frequency. However, the higher the gain the more susceptible
the device is to instability.
The conventional active noise reducing headphone device also has the
drawback that when mechanical vibrations such as impact, frictional
induced vibrations from connecting cords, user jaw movement induced
vibrations etc., are transmitted to the noise feedback microphone, these
vibrational noises are converted to electrical signals by the microphone.
These signals are amplified and cause instability and other non-linear
effects, for example, audio interruption, loud noises or pressure surges.
Another drawback of conventional active noise reducing headphone devices is
the complexity added to the device to avoid canceling the desired audio
signal, which signal is inputted as an electrical signal. The desired
audio signal (S) of the conventional device is input into two summing
nodes to create the signal transmitted to the user's ear. The first
summing node adds the negative feedback microphone signal to the desired
input audio signal. But, in a conventional closed loop feedback device,
the signal feedback from the microphone contains the desired audio signal
as well as the ambient noise signal which is desired to be canceled. This
feedback signal is subtracted from the desired input audio signal to
create the anti-noise signal, with zero desired audio signal content.
Then, a second summing node is used to add the desired audio signal back
into the loop so it can be transmitted to the output transducer. This
method of generating the desired audio signal adds complexity and cost to
the conventional noise reducing device. The additional summing node
processing in the conventional device also increases chances of creating
distortion in the desired audio signal as well as increasing the
possibility of instability.
In addition, various other prior art headphone configurations have been
developed for creating an active noise reduction device, where the input
and output transducers are positioned in relation to the ear, such as the
following three documents, which are incorporated by reference:
U.S. Pat. No. 5,134,659 to Moseley.
U.S. Pat. No. 5,117,461 to Moseley.
U.S. Pat. No. 5,001,763 to Moseley.
Moseley ('659) relates to a noise canceling system for headphones having a
baffle, two speakers, and two microphones wherein the baffle serves to
impede noise from traveling directly from a noise source to the input
transducer by forcing the noise to travel a longer distance around the
baffle and through a foam barrier. Moseley ('461) is directed to an
electroacoustic function including noise cancellation for use with
headbands having a microphone mounted on the headband to face in same
direction of the ear canal. Moseley ('763) relates to a noise cancellation
system for headbands having a speaker, microphone, and a baffle.
Thus, in general, the Moseley patents are concerned with the location of
the speaker, being the output transducer, and the microphone, which is
input transducer. In fact, the patents require that the speaker and
microphone be in the same plane or substantially aligned in the same
plane. Also, the patents teach that the processed signal output is
substantially in the same time domain as the original acoustic wave, that
is the signal is in phase.
In contrast to the Moseley patents, the present invention is not per se
concerned with the alignment of the speaker and microphone in the same
plane (although such alignment need not be explicitly excluded). The
output transducer and microphone utilized in the open loop active noise
reduction of the present invention may be perpendicular, tangential, or in
any other location out of the same plane (as well as in the same plane).
The present invention provides a noise reduction system having the
capability to transmit the original input audio signal to the speaker
without the readdition of the input audio signal. This is because the
sensor microphone, which is the control action of the open loop, is so
disposed from the audio signal, that the audio signal is not detected by
the pickup or sensor microphone. That is, in the open loop system of the
present invention, the original desired audio signal is transmitted to the
speaker independent of the ambient noise detected by the microphone. In
addition, in the present invention an acoustical material can be located
between the output transducer and the eardrum of the user to create an
acoustical waveguide for the transducer by coupling the audio signal to
the ear of the user. The acoustical material located between the output
transducer and microphone acts as an acoustic filter to decrease the open
loop gain by placing an acoustical impediment in the path of the pickup
microphone and the output transducer. The acoustical material isolates the
desired original inputted audio signal from the noise detected and
canceled by the pickup microphone. The background noise signal detected by
the pick-up microphone is inverted through electric-acoustical processing
means producing an anti-noise signal, which signal is transmitted to the
acoustical waveguide to create a quiet zone. This quiet zone is located
between the output transducer and the eardrum of the user.
Thus, the prior art has failed to provide a relatively low-cost means for
reducing background noise to an acceptable level for use with
communication systems or the like, and a cost-effective means for enabling
existing audio communication systems to reduce background noise to an
acceptable level.
OBJECTS AND SUMMARY OF THE INVENTION
An object of the present invention is to provide an active noise
cancellation apparatus and an active noise reduction apparatus to create a
noise reducing system which overcomes the problems associated with the
prior art.
More specifically, it is an object of the present invention to provide an
active noise cancellation apparatus and active noise reduction apparatus
which reduce background noise to an acceptable level.
Another object of the present invention is to provide noise reduction
apparatus for use with a headset device and boom microphone or to provide
a noise cancellation microphone device or the like.
It is still another object of the present invention to provide noise
reduction and cancellation apparatus and an active noise reducing system
as aforementioned which is relatively inexpensive.
It is yet another object of the present invention to provide a relatively
low-cost noise reduction and cancellation apparatus for use with
telecommunication systems which is operable with standard available
on-line power.
Another object of the present invention is to provide an enhanced active
noise cancellation and noise reduction headset by adding a talk thru
feature, which enables the user to hear the microphone audio signals as
well as the external audio from the surrounding environment, without the
physical addition of any other microphone elements. The object of the
present invention is to have an active noise cancellation and noise
reduction headset where all the audio from external area is transmitted to
the earcup of the speakers by increasing the gain of the sidetone channel.
This active noise cancellation microphone of the present invention is
converted to a standard omni-directional microphone by removing the voice
microphone from the electronics and increasing the gain of the noise
microphone amplifier.
A still further object of the present invention is to provide a relatively
low-cost noise cancellation apparatus which is readily adaptable to
handsets of existing communication systems and which is operable with
standard available on-line power.
A yet further object of the present invention is to provide a relatively
low-cost noise reduction apparatus for use with audio communication
systems which enables the user to selectively amplify a received signal
or, which may be used in a boom microphone with a headset or, which may be
used as a noise canceling microphone.
In many applications as described herein, microphones with
other-than-omnidirectional characteristics are desired. Such microphones
reject signals from certain directions and thus yield an improvement of
the signal-to-noise ratio. The directional microphones based on summation
scheme, which is that of the present invention, may depend on the
algebraic combinations of the sound pressure signals with phase
differences which are exclusively due to the electronics of the system. As
opposed to gradient-type microphones, the directivity of such microphones
is dependent on the ratio of linear dimensions to wavelength.
When two or more microphones are fed into the same amplifier, it is
possible that signals from a sound source at distance from the microphones
may arrive at the microphones 180.degree. out of phase, canceling each
other. Therefore, it is an object of the present invention to ensure the
omni-directional and directional microphones are phased properly.
It is also an object of this invention that the first and second
microphones arranged at a predetermined angle and/or distance with
subtraction apparatus disclosed herein can also be used in the area of
ambient noise cancellation for microphones in acoustic surveillance or
telemetry or even directional microphones such as directional microphones
with sidelobes.
Accordingly, is an object of the present invention to provide a low cost
microphone for use in a noise cancellation system with
other-than-omnidirectional characteristics.
It is a further object of the present invention to provide a controllable
variety of directivity patterns with a microphone based on the magnitude
and phase lobe construction.
It is yet another object of the present invention to provide a directional
microphone by adding vectorially at a determined point the sound pressures
arriving at that point from all simple sources.
It is still another object of the present invention to provide a
two-terminal microphone system, including the directional microphone as
aforementioned, in an active noise cancellation environment, which allows
the audio output signal to be superimposed on the voltage input signal at
the same terminal.
Another object of the invention to provide a novel active noise reduction
apparatus for use in headsets due to its simplicity and low cost circuitry
by positioning elements in an open loop system.
It is object of the present invention to provide a noise reduction
apparatus in which the ambient noise is attenuated in a regular manner
without being degraded by mechanical or vibration induced microphone
signals.
It is another object of the present invention to provide an active noise
reducing system comprised of a headset, handset or the like with a boom
microphone or directional microphone or the like which is unconditionally
stable due to its open loop configuration.
It is further object of the present invention to reduce the power required
by the noise reduction apparatus by coupling the electro-acoustic
transducer efficiency.
It is further object of the present invention to reduce the complexity
and/or cost of the active noise reduction circuit by employing a method of
combining the desired audio signal and the anti-noise signal to the output
transducer in a single summing node.
It is further object of the present invention in a noise reducing system to
reduce anti-noise processing induced distortion of the desired electrical
input signal which is converted to an acoustic signal and transmitted to
the ear in a noise reduction system.
Another object of the noise reduction apparatus involves a sensor or pickup
microphone placed behind or in front of the output transducer, and outside
of the sound field and the plane of the speaker, so that the microphone
detects only the background noise by utilizing of the acoustical material,
which performs dual functions.
It is a further object of this invention to provide an acoustical material
as an acoustic filter when positioned over a microphone, and as an
acoustic waveguide when placed between the output transducer and ear of
the user.
It is the microphone that is the control action of the system, the
microphone is independent of the inputted audio signal, the desired
output. A resilient acoustical waveguide is preferably positioned between
the speaker/microphone and the ear to create a quiet zone. This waveguide
is preferably more than just the usual rubber sponge which is commonly
provided on earphones for comfort purposes. One type of such material is
called "Slo-Flo" foam and it is of such a density and construction so as
to define a noise-free response and to deaden any sound reflections
returning to the microphone, acting as an acoustical filter, from the
listener's face and/or ear; whereas the prior art uses a negative feedback
of the signal from the microphone, no such feedback is produced in the
present invention. Instead, an open-loop arrangement is utilized, wherein
there is no need to add another audio signal, but the original input audio
signal is transmitted to the speaker, as the signal has not been disturbed
by the open loop system.
It is important to understand the distinctions between a conventional
closed-loop reduction apparatus and the novel open loop reduction
apparatus of the present invention.
An open loop system of the present invention is one in which the control
action is independent of the output or desired result. A closed loop
system is one in which the control action is dependent on the output. The
key term in these definitions is control action. Basically, the term
refers to the actuating signal of the system, which in turn represents the
quantity responsible for activating the system to produce a desired
output. In the case of the open loop system, the input command is the sole
factor for providing the control action, whereas for a closed loop system,
the control action is provided by the difference between the input command
and the corresponding output.
To complete the comparison of the closed loop versus open loop operation,
certain performance characteristics of each system is as follows: open
loop systems have two outstanding features, namely, the ability to perform
a function being determined by calibration and simplicity in construction,
for instance because the problems of instability are not incurred. For
closed loop systems, a noteworthy feature is the ability to faithfully
reproduce the input owing to the feedback, since the actuating signal is a
function of the deviation of the output from the input; this control
action forces the actuating signal almost at zero. A major disadvantage of
this feedback factor is that it is responsible for one of the greatest
difficulties in using a closed loop systems, namely the tendency to
oscillate.
The active noise reduction apparatus as well as the noise cancellation
apparatus can be used in any telecommunication systems that are used in
flight (e.g., helicopter or airplane) or in other settings such as
telephones, or voice recognition and/or verification systems for instance,
for access to a physical facility or to a computer (either via direct or
indirect interface or via telephone lines) or to an automatic teller
machine or, in other recognition and/or verification systems.
The noise cancellation apparatus comprises: a housing having first
microphone means for receiving a first acoustic sound composed of speech
originating from an operator operating said apparatus and background
noise, and for converting said first acoustic sound to a first signal, and
second microphone means arranged at a predetermined angle .phi. in close
proximity with respect to said first microphone means for receiving a
second acoustic sound composed of substantially said background noise and
for converting said second acoustic sound to a second signal; and means
for subtracting the second signal from the first signal so as to obtain a
signal representing substantially said speech. The two terminal transducer
for use in the noise cancellation apparatus for reducing background noise
comprises: a plurality of microphones connected to an amplifier means of
the noise cancellation apparatus; the amplifier means for receiving audio
signals from the microphone having a first terminal and a second terminal
wherein the second terminal is grounded; a voltage means inputting a DC
signal on the first terminal; a transistor means connected to the first
terminal for receiving an AC signal from the microphones; means for
superimposing the AC signal onto the DC signal on the first terminal;
means for filtering the AC signal from the DC signal, so the DC signal
powers the amplifier means; and means for outputting the AC signal
generated by the microphones at th | | |
